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Fabrication and Characterization of Compound Semiconductor Sensors for Pressure, Gas, Chemical, and Biomaterial Sensing

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INGEST IEID E20110330_AAAANB INGEST_TIME 2011-03-30T23:08:10Z PACKAGE UFE0012990_00001
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FABRICATION AND CHARACTERIZATI ON OF COMPOUND SEMICONDUCTOR SENSORS FOR PRESSURE, GAS, CHEMICAL, AND BIOMATERIAL SENSING By BYOUNG SAM KANG A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2005

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Copyright 2005 by Byoung Sam Kang

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To our LORD, Jesus Christ

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iv ACKNOWLEDGMENTS I would like to thank God, our Father, for a ll of the graces He gave me while I was studying here in Gainesville. The past years have been among the most crucial years in my life, because He led me to meet invaluab le people I could not se e in other places, and they served as Barnabas changing Gainesville into Gilgal. I am sorry I can not list all of them here. First of all, I really appreciate my su pervisory committee chair, Dr. Fan Ren for technical and financial support throughout this study. He gene rously gave me lots of heartwarming advice professionally and also personally. He willingly guided me with better ideas when I had difficulty in solving th e problems. I am also indebted to my other supervisory committee members: Steve Peart on, David Norton, and Jason Weaver. Their significant contributions to th is work are greatly appreciat ed. During collaboration with Dr. Pearton’s and Dr. Norton’s groups, I ha d the opportunity to develop interpersonal skills, emphasize efforts, and pursue goals. I am honored to have been associated with such an eminent committee. I would also like to thank Dr. S.N.G. Chu (Multiplex Inc. South Plainfield, New Jersey) for helping me with stress analysis of the hetero interfaces. Many thanks go to my colleagues and coworkers in the Department of Chemical Engineering: Jeff LaRoche, Ben Luo, Suku Kim, Jihyun Kim, Rishabh Mehandru, Soohwan Jang, Hungta Wang, Traivis Anderson, Jau Jiun. Special thanks go to Kwang Hyun Baik, Rohit, Sang Yoon Han, and Huck Soo Yang in Materials Science and Engineering department for their various di scussion and helping in dry etch. System

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v maintenance and good friendship of Brent Gila are gratefully acknowledged. I had many happy discussions with him about work. His optimistic attitude could make him a good and kind professor someday soon. I thank my family for their love and support. I especially thank our parents, I really appreciate the enormous love they have s hown me. I hope I can be as good a parent for my daughter as they have been. I hope this dissertation, in some sm all way, repays them for their love which I can never forget. I thank my wife, Jinah, who helps me to concentrate this work by taking care of all other works including our daughter, Dahyoung. This dissertation would no t easy without their consistent supports. Half of this work is her achievement. Finally, I thank valuable pastors, He e-Young Sohn, and Joong-Soo Lee for guiding me to learn the immeasurable depth of love in Jesus Christ. I pray that their consistent love and whole dedication to our God Fa ther will not be changed forever.

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vi TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF FIGURES.........................................................................................................viii ABSTRACT.....................................................................................................................xi ii CHAPTER 1 INTRODUCTION........................................................................................................1 2 BACKGROUND AND LI TERATURE REVIEW......................................................4 2.1 Historical Review...................................................................................................4 2.2 Background.............................................................................................................7 2.2.1 AlGaN/GaN High Electron Mob ility Transistors (HEMTs)........................7 2.2.2 ZnO based Chemical Sensor......................................................................11 3 PRESSURE SENSOR USING PI EZOELECTRIC POLARIZATION.....................15 3.1 Introduction...........................................................................................................15 3.2 Effect on External Strain on th e Conductivity of AlGaN/GaN HEMTs..............16 3.2 Pressure induced Changes in th e Conductivity of AlGaN/GaN HEMTs.............24 3.3 Capacitance Pressure Sensor Based on GaN HEMT on Si Membrane................31 4 CATALYST BASED GAS SENSOR FOR HYDROCARBON GASES..................38 4.1 Introduction...........................................................................................................38 4.2 AlGaN/GaN based MOS Diode Hydrogen Gas Sensor.......................................39 4.3 Hydrogen Induced Reversible Changes in Drain Current in Sc2O3/AlGaN/GaN HEMTs...................................................................................45 4.4 Comparison of MOS and Schottky W/Pt -GaN Diodes for Hydrogen Detection.52 4.5 Detection of C2H4 Using Wide Bandgap Semiconductor Sensors.......................59 4.6 Hydrogen and Ozone Gas Sensing Using Multiple ZnO Nanorods.....................68 5 CHEMICAL SENSOR FOR PO LYMERS AND POLAR LIQUIDS.......................75 5.1 Introduction...........................................................................................................75 5.2 Gateless AlGaN/GaN HEMT Response to Block Co-Polymers..........................76

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vii 5.3 pH Measurements with Single ZnO Na norod Integrated with a Microchannel...82 6 BIOSENSORS FOR BIOMATERIALS AND CELL GROWTH.............................89 6.1 Introduction...........................................................................................................89 6.2 Detection of Halide Ions with AlGaN/GaN HEMTs............................................91 6.3 Electrical Detection of Immobilized Proteins with Ungated AlGaN/GaN HEMTs...................................................................................................................98 6.4 Use of 370 nm Light for Select ive Area Fibroblast Cell Growth.......................103 7 SUMMARY AND FUTURE WORKS....................................................................114 7.1 Pressure Sensor...................................................................................................114 7.2 Gas Sensor..........................................................................................................115 7.3 Chemical Sensor.................................................................................................117 7.4 Bio Sensor...........................................................................................................118 APPENDIX A STRAIN CALCULATION OF A CI RCULAR MEMBRANE OF ALGAN/GAN UNDER DIFFERENTIA L PRESSURE...................................................................121 A.1 Strains in the Films............................................................................................121 A.2 Electrical Polarization a nd Piezoelectric Effects...............................................123 A.3 2DEG and Conductance of HEMT....................................................................124 LIST OF REFERENCES.................................................................................................126 BIOGRAPHICAL SKETCH...........................................................................................138

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viii LIST OF FIGURES Figure page 1-1 Semilog plot of intrinsi c carrier concentration versus inverse temperature for Si, GaAs, GaN (left). Performance of Al GaN/GaN based power amplifier as compared to GaAs and SiC based devices (right)......................................................2 2-1 Sheet carrier concentration in the 2DEG channel of AlGaN/GaN HEMT induced by the piezoelectric polarization as a function of Al concentration...........................9 2-2 Piezoelectric (PE) and spontaneous ( SP) polarization effects in Ga face or Nface AlGaN/GaN heterostructures...........................................................................10 2-3 Crystal structure of wurtzite ZnO.............................................................................12 2-4 Energy bandgap of several, III-V and II-IV compound semiconductors as a function of lattice constant.......................................................................................13 3-1 Layer structures of two-terminal nonmesa(top) and mesa(middle) devices, and top view photo-micrograph of fabricated two-terminal devices with different channel lengths(bottom)...........................................................................................16 3-2 Pressure sensor package: experiment al setup to detect I-V characteristics connected to the BNC cable according to various mechanical stresses (top) and mechanical stressor with cantilever (bottom)...........................................................18 3-3 Strain induced by AlGaN on GaN fo r the un-relaxed and partially relaxed AlGaN layer as a function of Al c oncentration (top) and sheet carrier concentration induced by the piezoelect ric polarization as a function of Al concentration (bottom).............................................................................................19 3-4 Effect of tensile or compressive st ress on the conductivity of the AlGaN/GaN HEMT with mesa etching (top) and without mesa etching (bottom).......................22 3-5 Circular membrane of AlGaN/GaN on a Si substrate fabricated by etching a circular hole in the substrate (left). A deflection of the membrane away from the substrate due to differentia l pressure on the two sides of the membrane produces a tensile strain in th e membrane (right)....................................................................25 3-6 Device structure with a finger patte rned device on the HEMT membrane..............27

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ix 3-7 SEM micrographs of via through the Si wafer(left)and cross sectional view of a via hole(right)...........................................................................................................28 3-8 IDS-VDS characteristics at 25C from AlGaN/ GaN HEMT membrane as a function of applied pressure...................................................................................................28 3-9 Channel conductivity of the AlGaN/ GaN HEMT membrane as a function of differential pressure..................................................................................................29 3-10 Device structure (top) and SEM micr ograph of AlGaN/GaN circular membrane on a Si substrate fabricated by etching a circular hole in the substrate(bottom)......32 3-11 Top view of HEMT capacitance pressu re sensor(top) and capacitance as a function of pressure for different diaphragm radii (bottom)....................................34 3-12 Capacitance change as a functi on of radius of the AlGaN/GaN HEMT membrane over the pressure range from -1 to +9.5 bar...........................................35 3-13 Capacitance change as a functi on of radius of the AlGaN/GaN HEMT membrane over the pressure range from -1 to +9.5 bar...........................................36 4-1 Cross-sectional schematic of co mpleted MOS diode on AlGaN/GaN HEMT layer structure (top) and plan-view photograph of device(bottom).........................40 4-2 Forward I-V characteristics of MOS-HE MT based diode sensors of two different dimensions at 25C measured under pure N2 or 10%H2 /90%N2 ambient..............42 4-3 Time response at 25C of MOS-HEMT based diode forward current at a fixed bias of 2V when switching the ambient from N2 to 10%H2 /90%N2 for periods of 10, 20 or 30 seconds and then back to pure N2........................................................43 4-4 Time response at 25C of MOS-HEMT based diode forward current at a fixed bias of 2V for three cycles of switching the ambient from N2 to 10%H2 /90%N2 for periods of 10 (top) or 30 (bottom)......................................................................44 4-5 Photograph of MOS HEMT hydrogen sensor..........................................................47 4-6 IDS-VDS characteristics of MOS-HEMT measured at 25oC under pure N2 ambient or in 10% H2/90%N2 ambient..................................................................................48 4-7 Change in drain-source current for measurement in N2 versus 10%H2 /90%N2 ambient, as a function of gate voltage (top) and correspond ing transconductance at a fixed drain-source voltage of 3V(bottom).........................................................49 4-8 Time dependence of drain-source current when switching from N2 to 1%H2 /99% N2 ambient and back again. The top show s different injection times of the H2/N2, while the bottom shows the revers ibility of the current change...................51

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x 4-9 The W/Pt Schottky diode (t op) and MOS diode (bottom).......................................53 4-10 Photograph of packaged gas sensor.........................................................................53 4-11 Forward I-V characteristics at 300 oC(top) or 500 oC(bottom) from the Schottky and MOS diodes in pure N2 and 10% H2 /90% N2...................................................54 4-12 Measurement temperature dependence of forward I-V characteristics of the Schottky (top) and MOS (bottom) ..........................................................................55 4-13 Temperature dependence of tu rn-on voltage(top) and on-state resistance(bottom)....................................................................................................57 4-14 Change in forward current when measuring in 10% H2 /90% N2 relative to pure N2 at 3 or 3.5 V in both the Schottky and MOS diodes...........................................58 4-15 Schematic of AlGaN/GaN MOS di ode (top) and bulk ZnO Schottky diode structure (bottom).....................................................................................................61 4-16 Forward I-V characteristics of MOS-HEMT based diode sensor at 400C measured under pure N2 or 10% C2H4 /90% N2 ambients.......................................62 4-17 Change in MOS diode forward current at fixed forward bias of 2.5V(top) or at fixed current(bottom)................................................................................................64 4-18 I-V characteristics at 50C (top) or 1 50 C (bottom) of Pt/ZnO diodes measured in different ambients.................................................................................................65 4-19 Change in current at a fixed bias (top) or change in voltage at fixed current (bottom)....................................................................................................................67 4-20 TEM of ZnO nanorod...............................................................................................69 4-21 SEM image of ZnO multiple nanoro ds (top) and the pattern contacted by Al/Pt/Au electrodes (bottom)...................................................................................70 4-22 I-V characteristics at different temp eratures of ZnO multiple nanorods measured in either N2 or 10 % H2 in N2 ambient.....................................................................71 4-23 Change in current measured at 0.1 V for measurement in either N2 or 10%H2 in N2 ambients..............................................................................................................72 4-24 I-V characteristics at 25C of ZnO multiple nanorods measured in either N2 OR 3% O3 in N2..............................................................................................................73 4-25 Time dependence of current at 1V bi as when switching back and forth from N2 to 3% O3 in N2 ambients...........................................................................................74 5-1 Layout of gate HEMT structure (t op) and device cross-section (bottom)...............82

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xi 5-2 Structure of block co-polymer, comp osed of different portions of PS and PEO (top) and chemical formula for PS and PEO (bottom).............................................83 5-3 Drain I-V characteristic s for the air, PS and PEO....................................................84 5-4 Drain I-V characteristics for the different concentration of PS-PEO block copolymer.................................................................................................................85 5-5 Drain IV characteristics of c opolymers with different composition........................86 5-6 Schematic (top) and scanning electr on micrograph (bottom) of ZnO nanorod with integrated microchannel (4m width)..............................................................89 5-7 I-V characteristics of ZnO nanorod after wire-bonding, measured either with or without UV (365nm) illumination............................................................................90 5-8 Change in current (top) or conducta nce (bottom) with pH (from 2-12) at V = 0.5V..........................................................................................................................9 1 5-9 Relation between pH and conductance of ZnO nanorod either with or without UV (365nm) illumination.........................................................................................92 6-1 Schematic of HEMT structure for detect ion of halide ions (top) and plan view photomicrograph of completed device usi ng a 20 nm Au film in the gate region (bottom)....................................................................................................................92 6-2 Time dependence of current change in Au-gated HEMTs upon exposure to NaF (top) or NaCl (bottom) solutions with different concentration................................93 6-3 Current change as a function of con centration for HEMTs with or without the Au gate.....................................................................................................................95 6-4 Change in current as a function of time as a Au-gated HEMT is exposed to water. In the case of thiol modification of the gate, no response was observed......96 6-5 AC measurement of change in current in gateless HEMT exposed to water. The applied bias of 500 mV wa s modulated at 11 Hz.....................................................97 6-6 Structure of APS (3-Aminopropyl ) triethoxysilane (top left), surface functionalization before ch emical modification (top right). A schematic diagram of the gateless HEMT whose surface is functionalized by chemical modification in the gate region (bottom).......................................................................................99 6-7 Fluorescent photographs of biotinylated probes on chemically treated GaN surface (left) and non-biotinated surface (right).....................................................100

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xii 6-8 SEM micrographs of amine-functionaliz ed GaN surfaces reacted with biotin NHS and then blocked and exposed to eith er BSA-coated (left) or streptavidin (right) nanoparticles...............................................................................................101 6-9 Change in HEMT drain-source current as a result of interaction between Biotin (Sulfo-NHS-Biotin) and streptavidine introduced to the gateless HEMT surface.102 6-10 Patterns formed on glass slides coated with 50 nm TiO2.......................................104 6-11 Optical spectrum of a GaN light emitt ing diode at an operating current of 4 mA (top). The current-voltage characterist ics of the GaN based LED (bottom)..........105 6-12 Effect of UV light on growth of Fibroblast cells on gl ass slices and TiO2 coated glass slides..............................................................................................................107 6-13 Simulated reflectivity of 370 nm UV light with different substrates.....................108 6-14 Fibroblast cell growth on patterned TiO2 coated glass slide with the UV light illumination (top) and without UV light illumination (bottom).............................110 6-15 A set-up of fibroblast cell on patterned TiO2 coated glass slides under different UV intensity illumination.......................................................................................111 6-16 Simulated reflectivity of 370 nm UV light with six TiO2/SiO2 dielectric mirror stacks......................................................................................................................112

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xiii Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy FABRICATION AND CHARACTERIZATI ON OF COMPOUND SEMICONDUCTOR SENSORS FOR PRESSURE, GAS, CHEMICAL, AND BIOMATERIAL SENSING By Byoung Sam Kang December 2005 Chair: Fan Ren Major Department: Chemical Engineering GaN-based diodes and high electron mobility transistors (HEMTs) for pressure, gas, liquid and biological sensing were fabr icated and characterized. A novel metal oxide, ZnO was also evaluated as a potential el ectronic device for chemical sensing applications. The devices presented herein showed high sensitivity and capability of operation in harsh environmental conditions such as high temperature and pressure. The AlGaN/GaN HEMTs grown on (0001) sa pphire substrates show polarizationinduced two dimensional electron gas (2DE G) at the AlGaN/Ga N hetero-interface. Linear dependence of the 2DEG channel c onductance on external strain was observed using a cantilever beam in a bending configuration. To overcom e the rigidity of sapphire substrates in applying external stress, a micro pressure se nsor using a 150 m diameter thin flexible AlGaN/GaN circular membrane with an interdigitat ed-finger device on a (111) Si substrate was demonstrated. The measured pressure sensitivity was 7.1 10-2

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xiv mS/bar, which was two orders of magnitude larg er than that of a cantilever beam pressure sensor. Pt gated AlGaN/GaN HEMT-based meta l-oxide semiconductor (MOS) diodes and field effect transistors (F ETs) were demonstrated for detecting hydrocarbon gases, followed by a comparison between MOS a nd W/Pt Schottky-based GaN diodes for hydrogen sensing. Changes in current, when sensors exposed to hydrogen on the Pt-gated AlGaN/GaN HEMT were approximately an orde r of magnitude larger than that of Pt /GaN Schottky diodes and 5 times larger than Sc2O3/AlGaN/GaN MOS diodes. For the liquid sensors, gateless AlGaN/ GaN HEMTs showed large changes in source-drain current on exposing the gate region to polar li quids and block copolymers. The polar nature of these chemicals leads to a change of surface charge in the gate region on the HEMT, producing a change in surface potential at the semiconductor/liquid interface. For biomaterials de tection, the gate region was chemically modified with aminopropyl silane. As streptavidin was in troduced to the biotin-functionalized gate region, the drain-source current showed a clear decrease of 4 A, which shows interaction between antibody and antigen. A Schottky diode was fabricated on a ZnO th in film and showed higher sensitivity to hydrogen (5 ppm). A single ZnO nanorod F ET-based sensor was also demonstrated. Conductivity of the single nanorod sensor decr eased linearly when th e pH value of the solution varied from 2 to 12.The measured se nsitivity was 8.5 nS/pH in the dark and 20 nS/pH under UV (365 nm) illumination, showing tremendous potential for sensing applications.

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1 CHAPTER 1 INTRODUCTION The sensor industry has grown rapidly in recent years. The non-military world market for sensors exceeded expectations with US $42.2 billion in 2003 and it is expected to reach US $54 billion by 2008 [Mar 04]. This rapid growth of the sensor market increases the need to develop chemical sensor technology in applications such as chemical reactor processing, factory au tomation, industrial monitoring, automotive industry, computers, robotics, a nd telecommunications [Wil05]. In practice, the sensing elements must be relatively small in size, robust, and should not require a large sensing sample volume [Hun01, Liu04]. Due to the development of fabrication and processing techniques, a Si-b ased chemical sensing microsystem has the advantage of producing microsize structures in a highly uniform and geometrically well defined manner [Mad97]. While Si has proven to be the primary contestant in the microsensor market, there is an ever-growing need for devices operating at conditions beyond the limits of silicon. Silicon based micr o-sensors can not be operated in harsh environments such as in high temperature, pr essure, and chemically corrosive ambients. Wide bandgap electronics and sensors ba sed on GaN can be operated at elevated temperatures (600 C) where conventional Si-based de vices cannot function, being limited to < 350 C. This is because of the low intrinsi c carrier concentration of wide-bandgap energy semiconductors at high temperatur e, as shown in Figure 1-1(left).

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2 2 Figure 1-1. Semilog plot of intr insic carrier concentration ve rsus inverse temperature for Si, GaAs, GaN (left). Performance of AlGaN/GaN based power amplifier as compared to GaAs and SiC based devices (right). The ability of GaN-based materials to f unction in high temperature, high power and high flux/energy radiation conditions will enab le large performance enhancements in a wide variety of spacecraft, satellite, mi ning, automobile, nuclear power, and radar applications. One additional attractive attribute of GaN is that sensors based on these materials could be integrated with high-temperature electronic devices on the same chip. AlGaN/GaN heterojunction based high elec tron mobility transistors (HEMTs) have demonstrated extremely promising results fo r the use as power devices in many analog applications due to the high sheet carrier concentration, electron mobility in the two dimensional electron gas (2DEG) channel and hi gh saturation velocity, as illustrated in Figure 1-1 (right). Without a ny surface passivation, the sheet carrier concentration of the polarization-induced 2DEGs confined at interfaces of AlGaN/GaN HEMT becomes sensitive to any manipulation of surface ch arge. However, the nature of ambient sensitivity for the unpassivated nitride used to build micro-sensors able to detect applied strain and surface polarity change by polar li quids or toxic gas and harmful cancer cell

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3 3 exposure to the surface of HEMTs. In addi tion, sensors fabricated from these wide bandgap semiconductors could be readily inte grated with solar blind UV detectors or high temperature, high power electronics on the same chip [Kim00a, Ris94, Lut99]. Another wide bandgap semiconductor, ZnO proposed for the sensing applications has several fundamental advantages. It ha s higher free-exciton binding energy (60 meV), and more resistance to radiation damage, piezo electricity and transparency. It has been used effectively as sensing material ba sed on near surface modification of charge distribution with certain su rface absorbed species. Anot her benefit of oxide-based semiconductors is that they do not rely on speci fic catalytic metals for chemical detection [Yun05]. Instead, they exploit the change in near surface conductivity most likely due to the adsorption of chemical species. Most of the resistive gas sens ors that employ surface conductivity change have been metal oxide semiconductors (MOS) such as ZnO, SnO2, In2O3, MoO3, WO3, and titanium substrated chromi um oxide (CTO). The ZnO-based MOS-based diodes and field eff ect transistors have been demonstrated and they can be used with a wide range of chemicals us ing fractional surface conductivity by adsorption and desorption.

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CHAPTER 2 BACKGROUND AND LITERATURE REVIEW 2.1 Historical Review The AlGaN/GaN high electron mobility tr ansistors (HEMTs) have demonstrated extremely promising results for use in broad band power amplifiers in wireless base station applications, due to the high sheet ca rrier concentration, high electron mobility in the two dimensional electron gas (2DEG) ch annel, and high saturation velocity [Mor99, Eas02, Tar02, Zha00, Zha01, Joh02, Kou02, Pea 99]. The high electr on sheet carrier concentration of nitride HEMT s is induced by piezoelectric pol arization of the strained AlGaN layer and spontaneous polari zation [Kou02, Kan03, Che95, Ned98, Amb00]. Polarization induced piezoelect ric properties play an im portant role in AlGaN/GaN heterostructures. The high electron sheet carrier concentratio n in the strained AlGaN/GaN layer suggests the possibility that nitride HEMTs may be excellent candidates for sensing applications includi ng pressure, chemical, gas, and biological sensors. For pressure sensor applications, only a few basic studies reported piezo effect related cantilever beam s [Str03, Dav04, Wu05]. Strittmatter et al. [Str03] reported that capacitive strain can be sensed with GaN me tal insulator semiconductor (MIS) diode but for better performance, high qua lity surface oxide/GaN interf ace was needed. Davies et al. [Dav04] first demonstrated the feasib ility of free standing GaN cantilevers on Si substrates. Both dry and wet etch processes we re used to remove the Si substrate under the GaN but any measured strain data us ing this device was not shown. Wu and Singh

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5 5 [Wu05] examined potential for the strain sensor using a BaTiO3 piezoelectric semiconductor field effect transistor. Two cla sses (Si and GaN) of heterostructures for stress sensing were used and high sensitivity could be acquired using a very thin piezoelectric layer. However, direct app lication of AlGaN/GaN HEMTs structure for pressure sensors is not widely studied, especially for high pressure sensing. Gas sensors have been fabricated on a number of semiconductors using catalytic metals as the gate in the me tal insulator semiconductor (MIS) or as the metal contact in Schottky diodes [You82, Lun86, Rye87]. Various field effect transistors based on silicon have been developed for hydrogen gas sens ing [Lun89]. But silicon based sensors are limited to operation in environments of below 250oC, prohibiting them from being used as hydrocarbon detectors or for other appli cations requiring high temperature operation. Because hydrocarbon gases should be decomposed by the catalytic metals and hydrogen atoms diffuse to the device interface, it is presumed that a dipole forms, lowering the effective work function of the metal and changing electrical characteri stics of the devices. Baranzanhi et al. [Bar95] demonstrated ga s sensitivity of Pt gated SiC transistors operating up to 500oC but the SiC Schottky diodes have displayed poor thermal stability. Pd silicides were observed at temperatures as low as 425oC when Pd was used as Schottky metal [Hun95, Che96]. Luther et al. [Lut99] first demonstrated Pt-GaN gas sensor for hydrogen and propane at high temperature (200-400oC). The Pt–GaN gas sensor showed faster response for hydrocar bons and enhanced sensitivity at higher temperatures (500oC). After that, Schalwig et al. [S ch01] showed gas sensors for the exhausted lean burn engines using Pt-GaN and Pt-HEMTs. The device performance was investigated at high temperatures (200-600oC) and it was shown that a HEMT based gas

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6 6 sensor was more sensitive than GaN diodes but detailed analysis of sensitivity differences for GaN diodes vs HEMTs was left as future work. Since the first demonstration of a flui d monitoring sensor based on AlGaN/GaN hetero structures by Neuberger [Neu01], the application of AlGaN/GaN HEMTs as liquid sensors has been a subject of intense researc h. Neuberger et al. sugge sted that the sensing mechanism for chemical absorbates origin ated from compensation of the polarization induced bound surface charge by interaction with polar molecules in the fluids. The time dependence of changes in source-drain curr ent of gateless HEMTs exposed to polar liquids (isopropanol, acetone methanol) with different dipole moments using GaN/AlGaN hetero-interfaces was reported. In pa rticular, it was shown that it is possible to distinguish liquids with different polariti es. Steinhoff et al. [Ste 03a] suggested that the native oxide on the nitride surface was responsible for the p H sensitivity of the response of gateless GaN based heterostructure transist ors to electrolyte solutions. It was shown that the linear response of a nonmetallized GaN ga te region using different p H valued electrolyte solutions and sensitivit y with a resolution better than 0.05 p H from p H = 2 to p H = 12. Chaniotakis et al. [Cha05] showed that the GaN surface in teracts selectively with Lewis acids, such as sulphate (SO4 2-) and hydroxide (OH-) ions using impedance spectra. It was also shown that gallium f ace GaN was considerably reactive with many Lewis bases, from water to thiols and or ganic alcohols without any metal oxide and nitrides A novel metal oxide, ZnO has numerous at tractive characteri stics for gas and chemical sensors [Kan05a, Kan05b, He o04, Loo01, Nor04]. The bandgap can be increased by Mg doping. The ZnO has been used effectively as gas se nsor material based

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7 7 on near surface modification of charge distribu tion with certain surface absorbed species. In addition, it is attrac tive for biosensors given that Zn and Mg are essential elements for neurotransmitter production and enzyme functioning [Ste05, Gur98]. The ZnO is attractive for forming various types of nanorods, nanowires, and nanotubes [Hua01, Li04, Kee04, Kin02, Liu03, Par03a, Ng03, Hu03, Par03b, Heo02, He03, Zhe01, Lyu03, Zha03, Pan01, Lao03]. The large surface area of the nanorods makes them attractive for gas and chemical sensing, and the abil ity to control their nucleati on sites makes them candidates for high density sensor arrays. 2.2 Background 2.2.1 AlGaN/GaN High Electron M obility Transistors (HEMTs) One of the most outstanding advantages of the GaN is the availability of AlGaN/GaN heterostructures. The type I band alignment between AlGaN and GaN has been shown to form a potential well and a 2-dimensional electron gas (2DEG) at the heterointerface [Hen95]. When these materials are brought into contact, thermal equilibrium requires alignment of their respective Fermi levels (EF). This induces conduction (Ec) and valence (Ev) band bending in both the AlGaN and GaN layers and can cause the GaN conduction band at the interface to drop below EF, as illustrated in Figure 2-1. Since the Fermi level can be viewed as an electrochemical potential for electrons, majority electrons will accumulate in the narrow gap material just below the heterointerface to fill the quasi tr iangular potential well between EC and EF. With the heterointerface on one side and a potential barrie r on the other, electrons in the 2DEG are only free to move in along the plane of the interface. Modulati on doped field effect transistors (MODFETs) are a class of heterost ructures FET that use selective barrier

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8 8 doping to spatially separate ionized donors from the electrons in the 2DEG, leading to an increase in channel mobility. Figure 2-1. Simplified view of modulation doping with an en larged view of energy band diagram illustrating formation of 2-di mensional electron gas at AlGaN/GaN heterointerface. For this reason, these devices are also know as high electron mobility transistors, or HEMTs.Unlike conventional III-V based HEMT s, such as AlGaAs/GaAs HEMTs, there is no dopant in the typical nitride based HE MT structure and all the layers are undoped. The carriers in the two di mensional electron (2DEG) gas channel is induced by piezoelectric polarization of the strained AlGaN layer a nd spontaneous polarization, which are very large in wurtzite III-nitrides. Carrier concentration > 1013 cm-3 in the 2DEG, which is 5 times larger than that in AlGaAs/GaAs material system, can be routinely obtained. The porti on of carrier concentration induced by the piezoelectric effect is around 45-50%. This makes nitride HEMTs are excellent candidates for pressure sensor and piezoelectric-related applications.

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9 9 The piezoelectric polarization induced sheet carrier concentration of undoped Gaface AlGaN/GaN can be calculated by using Equation 2-1. ) ( ) ( ) ( ) ( ) ( ) (2 0x E x E x e e d x e x x nC F b d s (2-1) where (x) is piezoelectric polarization, x is the Al concentration in AlxGa1-xN, (x) is the dielectric constant, dd is the AlGaN layer thickness, e b is the Schottky barrier of the gate contact on AlGaN, EF is the Fermi level and Ec is the conduction band discontinuity between AlGaN and GaN. 1E111E121E13 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 No relaxation Partial relaxation X : Al mole fractionCarrier concentration induced by PE(cm-2) Figure 2-2. Sheet carrier concentration in the 2DEG channel of AlGaN/GaN HEMT induced by the piezoelectric polarizati on as a function of Al concentration As shown in Figure 2-2, the sheet carrier concentration induced by the piezoelectric polarization is a strong functi on of Al concentration. In th e case of a partially relaxed AlGaN strain layer, there is also a maxi mum sheet carrier con centration around Al = 0.35. If an external stress can be applied to AlGaN/GaN material system, the sheet carrier

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10 10 concentration can be changed significantly and de vices fabricated in this fashion could be used in sensor-related applications. The changes in the two dimensional (2 D) channel of AlGaN/GaN HEMTs are induced by spontaneous and piezo electric polarization, which are balanced with positive charges on the surface. Figure 2-3 shows sche matic diagrams of the direction of the spontaneous and piezoelectric polarization in both Ga and N face wurtzite GaN crystals [Amb00]. For the Ga-face AlGaN/GaN HEMTs structur e, at the surface of a relaxed GaN buffer layer or a strained AlxGa1-xN barrier as well as at the interfaces of a AlxGa1xN/GaN heterostructure, the total polariza tion changes abruptly, causing a fixed two dimensional polarization sheet charge given by AlGaNAlGaNAlGaN SP AlGaN pzPPP (2-2) where, ), 1 ( 019 0 ) 1 ( 034 0 090 0 ) ( x x x x x PSP AlGaN (2-3) ) 1 ( 0402 0 0583 0 ) ( x x x x PPZ AlGaN (2-4) Figure 2-3. Piezoelectric (PE) and spontaneous (SP) polariza tion effects in Ga face or Nface AlGaN/GaN heterostructures. + Ga face N face PSP+ PPEPSPSPSP+ PPE

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11 11 Therefore, the sheet charge density in the 2D channel of AlGaN/GaN HEMT is extremely sensitive to its ambient. Numerous gr oups have demonstrated the feasibility of AlGaN/GaN hetero-structures based hydroge n detectors with extremely fast time response and capable of operati ng at high temperature (500-800oC), eliminating bulky and expensive cooling systems [Amb02, Sch01, Eic01, Sch01a, Stu02, Eic03, Kim03a, Kim03b, Kim03c, Amb03]. In addition, ga teless AlGaN/GaN HEMTs show a strong dependence of source/drain current on the pol arity and concentration of polar solutions [Ste03b]. There have also been re cent reports of the investigati on of the effect of external strain on the conductivity of an AlGaN/GaN hi gh electron mobility transistor [Kan03]. 2.2.2 ZnO based Chemical Sensor ZnO has numerous attractive characteristic s for gas and chemical sensors [Kan05a, Kan05b, Heo04, Loo01, Nor04] ZnO is a direct bandgap semiconductor normally form in hexagonal (wurtzite) crystal st ructure like GaN, with lattice parameters a = 3.25 and c = 5.12 . The Zn atoms are tetrahedrally coor dinated with four O atoms, where the Zn – d electrons hybridize with the O p-electrons. Al ternating Zn and O layers form the crystal structure shown in Figure 2-4. Compared with GaN in Table 2-1 [Str00], it has direct bandgap energy of 3.37 eV, which makes it transparent in visible light a nd operates in the UV blue wavelengths. The exciton binding energy ~60 meV for ZnO, as compared to GaN ~25meV; the higher exciton binding energy enhan ces the luminescence efficiency of light emission. In the past, ZnO has been used in its pol ycrystalline form in applications ranging from piezoelectric transducers [Kad92] to va ristors [Lou80, Ver00], and as transparent conducting electrodes [Pet79]. Recent improvement s in the growth of high quality, single crystalline ZnO in both bulk and epitaxial form s has revived interest in this material

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12 12 [Pet79]. Especially, ZnO is a piezoelectric, transparent wide bandgap semiconductor used in surface acoustic wave devices. Figure 2-4. Crystal struct ure of wurtzite ZnO. Table 2.1 Physical properties of GaN and ZnO. GaN ZnO Bandgap 3.39 eV, direct 3.37 eV, direct Crystal structure (Wurtzite) a = 3.189 , c = 5.206 (c/a = 1.632) a = 3.250 , c = 5.205 (c/a = 1.602) Mobility 1000 cm2/Vs (e) 30 cm2/Vs (h) 200 cm2/Vs (e) 5-50 cm2/Vs (h) Effective mass m* = 0.20m0 (e) m* = 0.80m0 (h) m* = 0.24m0 (e) m* = 0.59m0 (h) Saturation velocity 2.5107 cm/s 3.2107 cm/s Exciton binding energy 28 meV 60 meV Energy gaps of several common semiconduc tors are given in Figure 2-4 as a function of lattice constant The approximate boundaries of the visible spectrum are shown as is the nature of the energy transi tion for each material. From the figure, it is c a

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13 13 noted that group III-nitrides and II oxides have bandgap from red to shorter UV wavelengths. The bandgap can be increased by Mg doping. ZnO has been effectively used as a gas sensor material based on the near-surface modification of charge distribution with certain surface-abso rbed species [Kan93]. 2.62.83.03.23.43.63.84.0 1 2 3 4 5 6 7 8 Energy bandgap (eV)Lattice constant a(A) Figure 2-4. Energy bandgap of several, II I-V, and II-IV compound semiconductors as a function of lattice constant. In addition, it is attractive for biosenso rs given that Zn and Mg are essential elements for neurotransmitter production and enzyme functioning [Mil98, Oga04]. ZnO is attractive for forming va rious types of nanorods, nanow ires and nanotubes [Hua01, Li04, Kin02, Liu03, Par03a, Ng03, Hu03b, Pa r03b, Heo02, Nor04, Poo03, He03, Wu00, Zhe01, Lyu01, Zha03b, Par03c, Yao02, Pan01, Lao03]. Compared with bulk materials, a significant characteristic of nanostructures is their high su rface to volume ratio. ZnO nanowires have the same crystal structures as bulk structure, confirmed by X-ray diffraction (XRD) and transmission electr on microscopy (TEM) [Pan01, Roy03, Li02]. o

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14 14 Therefore, all the bulk properties are still preserved for the nanowires and ZnO nanorods are very promising for a wide va riety of sensor applications.

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15 CHAPTER 3 PRESSURE SENSOR USING PIEZOELECTRIC POLARIZATION 3.1 Introduction There are a number of applications in the automotive, aerospace, and industrial fields for robust miniaturized pressure sens ors. A number of di fferent semiconductors systems have been used to make piezore sistive sensors [Ko99, Ned98, Wu97, You04, Dad94]. Especially, polarization induced piezoelectric propert ies play a very important role in strained AlGaN/GaN heterostru ctures. The high electron sheet carrier concentration in the AlGaN/GaN layer sugge sts that nitride HEMTs can be used as excellent pressure sensors. Several research groups have reported pi ezo effect related GaN pressure sensors [Str03, Dav04, Wu05]. Strittmatte r et al. [Str03] reported that capacitive st rain can be sensed with GaN metal insulator semiconducto r (MIS) diode but for better performance, high quality surface oxide film on the GaN laye r was needed. Davies et al. [Dav04] first demonstrated the feasibility of the free standing GaN cantilevers on Si substrates but any measured strain data using this device wa s not shown. Wu and Singh [Wu05] examined the potential for the strain sensor using BaTiO3 piezoelectric semiconductor field effect transistor. Two classes (Si and GaN) of hetero structures were used for stress sensing and showed that high sensitivity can be acqui red using a very thin piezoelectric BaTiO3 layer. However, the direct application of AlGaN/Ga N HEMTs structure for pressure sensors is not widely studied, especially for high pressure sensing.

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16 In this chapter, a thorough discussion of pressure sensor us ing AlGaN/GaN HEMT structure will be given. In Sec tion, 3.2, the effect of extern al strain on the sheet resistance of the two dimensional electron gas cha nnel in AlGaN/GaN HEMTs grown on sapphire will be presented for the cantilever beam. In Section 3.3, AlGaN/GaN membrane pressure sensor fabricated on Si substrat e will be analyzed, which can overcome the rigidity of sapphire substrate in applying ex ternal stress. In Sec tion 3.4, the capacitive pressure sensor which is less sensitive to variations in contact resistance will be discussed. 3.2 Effect on External Strain on the Conductivity of AlGaN/GaN HEMTs Two terminal high electron mobility Al0.25Ga0.75N/GaN devices used with simple bonding test were used to to study the effect of external strain on the conductivity on the sheet resistance of the two-dimensional elect ron gas channel in an AlGaN/GaN HEMT. The HEMT structures consisted of a 3 m thick undoped GaN buffer, 30 thick Al0.25Ga0.75N spacer, 270 thick Si-doped Al0.3Ga0.7N cap layer. The epi-layers were grown on sapphire substrate by metal organi c chemical vapor deposition (MOCVD). Two sets of samples were fabricated; one with me sa definition and the other without the mesa as shown in Figure 3-1(top). Mesa isolation was performed with an Inductively Coupled Plasma (ICP) etching with Cl2/Ar based discharges at –90 V dc self-bias, ICP power of 300 W at 2 MHz and a process pressure of 5 mTorr. 100 100 m2 ohmic contacts separated with gaps of 5, 10, 20, 50 and 100 m like a transmission-line-method (TLM) pattern consisted of e-beam deposited Ti/Al/Pt/Au patterned by lift-off and annealed at 850 C, 45 sec under flowing N2. Plated Au was subsequently deposited on the ohmic metal pads for wire bonding on the samples as shown in Figure 3-1(bottom).

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17 Figure 3-1. Schematic diagrams of layer st ructures of two-terminal non-mesa(top) and mesa(middle) devices, and top view photo-micrograph of fabricated twoterminal devices with different channel lengths(bottom). The devices were fabricated on half of 2” wafer, sawed into 2 mm wide stripes and wire bonded on the test feature. The dc charac teristics were obtained from measurements on an Agilent 4156C parameter analyzer. Figur e 3-2 illustrates the setup for measuring the effect of external stra in on the conductivity of 2DEG channel of the nitride HEMT.

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18 Lucite blocks secure the sample and PCB boa rd for testing. The contact pads were connected to the PCB board, which had BN C connectors on the end for signal outputs, with 1 mil thick gold wire. A high precision si ngle axis traverse was used to bend the sample. Sheet charges in the AlGaN/GaN high el ectron mobility transistors (HEMTs) are induced by spontaneous polarization and pi ezoelectric polarization [Kou02, Ras02, Amb00, Che95]. Wurtzite GaN and AlGaN are tetrahedral semiconductors with a hexagonal Bravais lattice with f our atoms per unit cell. The mi sfit strain insi de a film is measured against its relaxed state. In the misf it strain calculation, the strain is calculated against the relaxed films, ao, i.e. ) ( ) ( ) ( x a x a x ao o misfit (3-1) where a(x) is the lattice constants of AlxGa1-xN and ao(x) = (aGaN – aAlNx) = (3.189 0.077x) [Amb99]. Here a(x) ao(x) is chosen because the AlGaN film is always under tension due to a(x) aGaN and misfit will always be positive. Having defined the elastic strain in the film, a partially relaxed film pa rameter should be defined. For a perfectly coherent film, a(x) = aGaN or x x x x a x a ao o GaN misfit024 0 189 3 077 0 077 0 189 3 077 0 ) ( ) (. max (3-2) For a partially relaxed film, the ratio of strain comparing to the un-relaxed state defines the degree of strain in the film, ) ( ) ( ) (. maxx a a x a x a x So GaN o misfit misft (3-3) Hence, the degree of relaxation is then given by

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19 ) ( ) ( ) ( 1 x a a x a a x S x ro GaN GaN (3-4) For around 300 of AlGaN layer on the top of GaN, r(x) was measured by Ambacker [Amb00]. 0 0 x 0.38 r(x) = 3.5x-1.33 0.38 x 0.67 (3-5) 1 0.67 x 1 Lucite To BNC Lucite PCB Cantilever HEMT High precision single axis traverse Cantilever Figure 3-2. Schematic diagram of a pressure se nsor package: experimental setup to detect I-V characteristics conne cted to the BNC cable according to various mechanical stresses (top) and mechanical stressor with cantilever (bottom).

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20 0481216 0.0 0.2 0.4 0.6 X : Al mole fractionStrain(x103) No relaxation Partial relaxation 1011101210130.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 X : Al mole fractionCarrier concentration induced by PE(cm-2) No relaxation Partial relaxation Figure 3-3 Strain induced by AlGaN on GaN for the un-relaxed and partially relaxed AlGaN layer as a function of Al c oncentration (top) and sheet carrier concentration induced by the piezoelect ric polarization as a function of Al concentration (bottom)

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21 The piezoelectric polarization, for partially relaxed strained layer can be expressed by modifying the Equation 3-3 subtr acting the portion of relaxation 33 13 33 31 0 0) ( 1 2 C C e e a a a x r xGaN (3-6) where e31 and e33 are the piezoelectric coefficients and C13 and C33 are the elastic constants. Figure 3-3 (left) plots the relationship between Al c oncentration of AlGaN and strain induced by AlGaN on GaN for the unrelaxed and partially relaxed conditions. Unlike the linear model for the un-relaxed condition, there is a maximum strain around Al concentration of 0.35. The piezoelect ric polarization induced sheet carrier concentration of undoped Ga-face AlGaN/GaN can be calculated by following equation [Asb97]: ) ( ) ( ) ( ) ( ) ( ) (2 0x E x E x e e d x e x x nC F b d s (3-7) where (x) is the dielectric constant, dd is the AlGaN layer thickness, e b is the Schottky barrier of the gate contact on AlGaN, EF is the Fermi level and Ec is the conduction band discontinuity between AlGaN and GaN. In the case of applying external tensile strain on the HEMT sample, a increase of conductivity was observed. The mesa depth was around 500 , which is below the AlGaN/GaN interface (300 AlGaN layer on 3 m GaN layer). As illustra ted in Figure 3-3 (top), the sheet carrier concentration induced by the piezoelectri c polarization is a st rong function of Al concentration. In the case of a partially relaxed AlGaN strain layer, there is also a maximum sheet carrier concentration around Al = 0.35. If an external stress can be applied to AlGaN/GaN material system, the sheet carrier concentration can be changed

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22 significantly and devices fabricated in this fashion could be used in sensor-related applications. In order to study the effect of external strain on the conductivity of the HEMT material systems, transmission line patterns were fabricated with a mesa of 500 . Elastic bending of an analogous system, GaAs grown on Si wafers, has been carefully studied using Stony’s equation [Sto09, Chu98] The curvature and wafer bowing for different thickness of GaAs grown on Si was modeled [Chu98]. A pure bending of a single beam was used in this work to estim ate the strain, since the HEMT structure, around 3 m, is much thinner that that of sa pphire substrate, 200 m and the degree of deflection (maximum deflection is around 2.2 mm) is much shorter than the length of the beam, 27 mm. The strain, xx, of the bending can be estimated from the single beam with thickness of t and unit width. The tensile strain near the top surface of the beam is simply given by xx = td/ L2 (3-8) where t is the sample thickness, d is the deflection and L is the length of the beam. Figure 3-4 (top) shows the eff ects of external tensile and compressive strain on the conductivity of AlGaN/GaN HEMT sample with mesa. Therefore, the AlGaN layer sits above the beam and the external stress appl ied on the beam should not change the strain of the AlGaN layer. As a result, applying a tensile stress on the beam would pull apart the GaN atoms only and not affect the AlGaN. The tota l strain on the AlGaN/GaN interface and piezoelectric should increase, as observed in Fi gure 3-4 (top). In the case of applying a

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23 compressive stress on the beam, the GaN at oms were pushed together and reduced the total strain at the AlGaN/GaN interface and the resultant conductivity. 02468101214 7.75 7.80 7.85 7.90 7.95 8.00 8.05 Conductivity(mS)Compressive or tensile stress(x106) Tensile Compressive 012345 8.50 8.55 8.60 8.65 Conductivity(mS)Compressive or tensile stress(x106) Tensile Compressive Figure 3-4. The effect of tensile or co mpressive stress on the conductivity of the AlGaN/GaN HEMT with mesa etchin g (top) and without mesa etching (bottom).

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24 For the non-mesa devices fabricated on the same wafer in a different area, Figure 34(bottom) shows the strain dependen ce of with a channel length of 10 m. A reversal of strain sensitivity indeed ha s observed indicating a larger strain relaxation in the AlGaN/GaN layer. The changes in conductance of the channel of Al0.25Ga0.75N/GaN high-electronmobility transistor structures during applica tion of both tensile and compressive strain were measured relatively large. For fixed Al mole fraction, the changes in conductance were roughly linear over the range up to 1.4 x 107 N.m-2, with coefficients for planar devices of + 9.8 x 10-12 S-N–1-m2 for tensile stress and – 1.05 x 10 -11 S-N-1-m2 for compressive stress. For mesa-isolated structures, the coefficients were smaller due to the reduced effect of the AlGaN st rain, with values of –1.2 x10 –12 S-N-1-m2 for tensile stress and + 1.97 x10 –12 S-N-1-m2 for compressive stress. The large changes in conductance demonstrate that simple AlGaN/GaN heterost ructures are promising for pressure and strain sensor applications. In summary, we have demonstrated the effect of the external strain on the piezoelectric polarization of Al GaN/GaN material systems. This relatively large effect may have application in stra in and pressure sensors. 3.2 Pressure induced Changes in the Conductivity of AlGaN/GaN HEMTs AlGaN/GaN high electron mobility transi stors (HEMTs) show a strong dependence of source/drain current on the piezoelectric polarization induced tw o dimensional electron gas (2DEG). The spontaneous and piezoelectric polarization induced surface and interface charges can be used to develop very sensitive but robust sensors for the detection of pressure changes. The change s in the conductance of the channel of a

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25 AlGaN/GaN High Electron Mobili ty Transistor (HEMT) memb rane structure fabricated on a Si substrate were measured during the application of both tensile and compressive strain through changes in the ambient pressure. The piezoelectric polarization induced sheet carrier concentration of undoped Gaface AlGaN/GaN can be calculated by fo llowing equation [Amb00, Amb99, Lu01]: ) ( ) ( ) ( ) ( ) ( ) (2 0x E x E x e e d x e x x nC F b d s (3-9) where (x) is the dielectric constant, dd is the AlGaN layer thickness, e b is the Schottky barrier of the gate contact on AlGaN, EF is the Fermi level and Ec is the conduction band discontinuity between AlGaN and GaN. The sheet carrier concentration induced by the piezoelectric polarization is a strong function of Al concen tration. If an external stress can be applied to AlGa N/GaN material system, the sh eet carrier concentration can be changed significantly and devi ces fabricated in this fashion could be used in sensorrelated applications. Figure 3-5. Circular membrane of AlGaN/GaN on a Si substrate fabricated by etching a circular hole in the substrate (left). A deflection of the membrane away from the substrate due to differential pressu re on the two sides of the membrane produces a tensile strain in the membrane (right).

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26 The sensors used to monitor the different ial pressure are made of a circular membrane of AlGaN/GaN on a Si substrate by etch ing a circular hole in the substrate, as illustrated in Figure 3-5(left). A deflection of the membrane away from the substrate due to differential pressure on the two sides of the membrane produces a tensile strain in the membrane, as shown in Figure 3-5(right). The differential piezoelectric responses of AlGaN and GaN layers creates a space charge which induces 2DEG at the AlGaN/Ga N interface. The concen tration of 2DEG is expected to be directly correlated with the te nsile strain in the membrane and hence with the differential pressure. The radi al strain is given by [Sto09, Chu98], r = (S-D)/D = (2 R – 2Rsin )/(2Rsin ) = /sin 1 (3-10) The total tensile force around the edge of the circular membrane is, T = DtGaNr = D tGaN [EGaN/ (1)] r (3-11) where tGaN is the film of thickness, D is the diameter of the via hole, r = [EGaN/ (1)] r, EGaN is the Young’s modulus and is the Poisson’s ratio of the GaN film. The component of T along z direction is balan ced by the force on the membrane due to a differential pressure Pi P0, where Pi and P0 are the inside and outside pressure respectively. Hence Tsin = (Pi P0 ) D2/4 and the radial strain, r, in the nitride film can be expressed as a function of the differential pressure Pi P0. ( sin ) = (Pi P0 )[(1)D]/(4EGaNtGaN) (3-13) where EGaN is the Young’s modulus, D is the diameter of the via hole, tGaN is the film of thickness and is the Poisson’s ratio of the GaN film. If is measured, the differential pressure Pi P0. can be estimated with Equation 3-13. We also derive the relationship between conductance, of AlGaN/GaN HEMT and radial strain, r.

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27 = ( rAlGaN) + ( fGaN ) r (3-14) where ( rAlGaN) = s{1/[1 + ( 0 (x)/tAlGaN e2)h2/4 m*(x)]}{| PSP| –|eeff | AlGaN rAlGaN – [ 0 (x)/tAlGaN e] [e b(x) – Ec(x)]} (3-15) and ( fGaN ) = s{1/[1 + ( 0 (x)/tAlGaN e2 )h2/4 m*(x)]}{ |eeff (GaN)| |eeff (AlGaN)| } (3-16) where s is the mobility of 2DEG, 0 is the electric permitivity, (x) = 9.5 – 0.5x is the relative permitivity, e b(x) = 0.84 + 1.3x (eV) is th e Schottky barrier height, eeff = (e31e33)C13/ C33, h is Plank constant, e is th e electron charge, m*(x) ~ 0.228me. By monitoring the conductance of the HEMT on membrane, the pressure difference, Pi P0, can be obtained. The detail deivations of above equations are listed in the Appendix. Figure 3-6. Schematic diagram of device stru cture with a finger patterned device on the HEMT membrane. The HEMTs were grown by metalorganic chemical vapor deposition on 100 mm (111) Si substrates at Nitrone x Corporation. The structures consisted of an (Al,Ga)Nbased transition layer, ~0.8 m undoped GaN buffer, and 300 undoped AlGaN barrier layer. Mesa isolation was performed with an Inductively Coupled Plasma (ICP) etching with Cl2/Ar based discharges at –90 V dc self-b ias, ICP power of 300 W at 2 MHz and a

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28 process pressure of 5 mTorr. Ti/Al/Pt/Au ba sed inter-digitated finge r pattern separated by 4 m was formed with e-beam deposition and standard lift-off shown in Figure 3-6. The fingers were annealed at 850 C, 45 sec under flowing N2. Plated Au was subsequently deposited on the ohmic metal pads for wire bonding on the samples. Via holes were fabricated from the back side of the Si substrate and stopping on the GaN layer using ICP etching with SF6/Ar. The etch sel ectivity is more th an 1000:1. 2000 of AuSn was deposited on the backside of the sample and a glass slice. A RD automation flip-chip bonder was used to bond the glass slic e and the sample at 400 C to seal off the via holes. Figure 3-7 shows scanning electron micr oscopy (SEM) photos of the via through the Si wafer(left) and cross sect ional view of the via(right). Figure 3-7. SEM micrographs of via through the Si wafer(left) and cross sectional view of a via hole(right) The dc current-voltage(I-V) characteristic s were obtained from measurements on an Agilent 4156C parameter analyzer while the device was measured at 25oC under either Si Substrate GaN

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29 vacuum(10mTorr) or pressure (40-200psi) cond itions. Figure 3-8 shows the drain-source I-V characteristics from the membrane HEMT structure as a function of the ambient pressure. This current increases with incr easing pressure and decreases under vacuum conditions. -3-2-10123 -12 -8 -4 0 4 8 12 -0.4-0.20.00.20.4 -3 -2 -1 0 1 2 3 IDS(mA)VDS(V) IDS(mA)VDS(V) 10 mT atmosphere 40 psi 100 psi 200 psi Figure 3-8. IDS-VDS characteristics at 25C from Al GaN/GaN HEMT membrane as a function of applied pressure. The resulting channel conductance derived from this data is shown as a function of differential pressure in Figur e 3-9. In the case of app lied positive pressure, which corresponds to compressive strain induced in the HEMT laye rs, the conductivity decreases with a coefficient of -7.1x10-2 mS/bar. For the case of applied negative pressure (vacuum), the conductivity shows a posi tive coefficient of the same value within experimental error, given the limited data fo r vacuum conditions. These trends are similar to those observed with actual bending of HEMT samples on a cantilever beam to produce tensile or compressive strain [Kan03], but exhibit sensitivitie s to the induced tensile or compressive strain of almost two orders of ma gnitude larger. This is due to the absence of

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30 the thick sapphire substrate that is present in the cantilever struct ures. The new membrane structures are particularly sensitive to changes in differential pressure. 02468101214 4.6 4.8 5.0 5.2 5.4 5.6 Conductivity(mS)Pressure difference, |Po-Pi|(bar) Vacuum(tensile strain) Pressure(compressive strain) Figure 3-9.Channel conductivity of the AlGa N/GaN HEMT membrane as a function of differential pressure. AlGaN/GaN high electron mobility transi stors (HEMTs) show a strong dependence of source/drain current on the piezoelectric polarization induced tw o dimensional electron gas (2DEG). The spontaneous and piezoelectric polarization induced surface and interface charges can be used to develop very sensitive but robust sensors for the detection of pressure changes. The change s in the conductance of the channel of a AlGaN/GaN High Electron Mobili ty Transistor (HEMT) memb rane structure fabricated on a Si substrate were measured during the application of both tensile and compressive strain through changes in th e ambient pressure. The conductivity of the channel shows a linear change of –(+)7.1x10-2 mS/bar for application of compressive(tensile) strain. The AlGaN/GaN HEMT membrane-based sensors appear to be promising for pressure sensing applications.

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31 In summary, an AlGaN/GaN HEMT memb rane on Si shows large changes in channel conductivity as a result of changes in ambient pressure. These structures appear promising for use in integrated sensors in which the HEMTs can also be used for gas, chemical and biological detection combined with on-chip transmission of the data. 3.3 Capacitance Pressure Sensor Based on GaN HEMT on Si Membrane The AlGaN/GaN high-electron-mobility tr ansistors (HEMTs) show a strong dependence of the conductance of the channel when a membrane structure fabricated on a Si substrate was measured during changes in the ambient pressure [Kan04c, Kan03, Pea04a]. However, one drawback of piezoresi stive sensors is that contact resistance changes significantly with temperature and may mask the changes in sensor signal from actual pressure changes [You04]. By sharp contrast, capacitive pressure sensors are less sensitive to variations in contact resistan ce and in addition, sens ors based on AlGaN/GaN HEMTs could be readily integrated with o ff-chip wireless communication chips that eliminate additional wiring capacitance. AlGaN/GaN high electron mobility transistors (HEMTs) have demonstrated extremely prom ising results for use in broad-band power amplifiers in wireless base station applica tions [Zha01, Tar02, Zha03a, Shu98]. The high electron sheet carrier concen tration of nitride HEMTs is induced by piezoelectric polarization of the st rained AlGaN layer and spontane ous polarization [Amb00, Amb99, Asb97], suggesting that nitride HEMTs are excellent candidates for robust pressure sensing. In this part, Circular AlGaN/GaN diaphragms fabr icated with radii 200-600 m on Si substrates show linear changes in capacitanc e over a range of applied pressure and that the sign of the capacitance change is reversed when vacuum is applied to the diaphragm.

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32 The sensors used to monitor the different ial pressure are made of a circular membrane of AlGaN/GaN HEMT on a Si subs trate. The membrane is fabricated by etching a circular hole in the substrate, as shown schematically in Figure 3-10 (top). A scanning electron microscope (SEM) cross-sectio nal view of an actual device is shown at the bottom of Figure 3-10 Figure 3-10. Schematic diagram of device structure (top) and SEM micrograph of AlGaN/GaN circular membrane on a Si substrate fabricated by etching a circular hole in the substrate(bottom). Si Substrate GaN Si Teflon

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33 A deflection of the membrane away from th e substrate due to differential pressure on the two sides of the membrane produces a te nsile strain in the membrane. This leads to a change in the piezo-induced two dime nsional electron gas (2DEG) density at the AlGaN/GaN interface. This in turn affects the capacitance of th e HEMT diaphragm. The carrier density is therefore directly correlated with the tensile strain in the membrane and hence with the differential pressure. We have previ ously calculated the radial strain, r, in the membrane as a function of the differential pressure Pi P0 [Kan04c] by employing a modified Stoney analysis [Sto09, Chu98]. By monitoring th e conductance of the HEMT on membrane, the pressure difference, Pi P0, can be obtained. The HEMTs were grown by metal-organi c chemical vapor deposition on 100 mm (111) Si substrates at Nitrone x Corporation. The structures consisted of an (Al,Ga)Nbased transition layer, ~0.8 m undoped GaN buffer, and 300 undoped AlGaN barrier layer. Mesa isolation was pe rformed with an Inductively Coupled Plasma (ICP) etching with Cl2/Ar based discharges at –90 V dc self-b ias, ICP power of 300 W at 2 MHz and a process pressure of 5 mTorr. Ti/Al/Pt/Au based inter-digitated fi nger pattern separated by 4 m was formed with e-beam deposition and standard lift-off. The fingers were annealed at 850 C, 45 sec under flowing N2. Plated Au was subsequently deposited on the ohmic metal pads for wire bonding on the samples. Ohmic contact for the silicon used 1000 Al deposited by using sputter and annealed in nitrogen at 300C. Via holes were fabricated from the back side of th e Si substrate and st opping on the GaN layer using ICP etching with SF6/Ar. The etch selectivity is more than 1000:1. 2000 of AuSn was deposited on the backside of the sample and a glass slice.

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34 0246810 12 16 20 24 28 32 36 Capacitance(pF)Pressure(bar) R=600 m R=400 m R=280 m R=200 m Figure 3-11. Top view of HEMT capacitance pr essure sensor(top) a nd capacitance as a function of pressure for different diaphragm radii (bottom). The teflon bonding spin coating on the silicon wafer employed liquid Teflon (CYTOP CTL-809M, from Bellex International Corp.) at 5000 rpm to a thickness of ~ 5000 and then flipchip bonding (RD automati on flip-chip bonder) the fabricated device

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35 using a mechanical force of 1000g between th e upper and lower chucks for 10 minutes and finally baking for 1 hour at 200C to solidify the Teflon to seal off the via holes. Figure 3-11(top) shows a top view of a co mpleted HEMT capacitive pressure sensor. The capacitance of the HEMT diaphrag m structures were obtained from measurements on an Agilent 4156C parameter an alyzer while the device was measured at 25oC under either vacuum (-1 bar) or pressure (+9.5 bar) conditions. Figure 3-11(bottom) shows the capacitance from the membrane HEMT structure as a function of the ambient pressure, for different membrane radii. This capacitance increases with increasing pressure and decreases under vacuum conditions, due to corresponding changes in the carrier density in the 2DEG. -20246810 -1 0 1 2 3 Capacitance change(pF)Pressure(bar) R=600 m R=400 m R=280 m R=200 m Figure 3-12. Capacitance change as a func tion of radius of the AlGaN/GaN HEMT membrane over the pressure range from -1 to +9.5 bar.

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36 The resulting capacitance change derived from this data is shown as a function of pressure in Figure 3-12. In the case of a pplied positive pressure, which corresponds to compressive strain induced in the HEMT la yers, the capacitance increases in a linear fashion over the range between 0 and +1 bar with a sensitivity of 0.86pF/bar for a 600 m radius membrane. For the case of applied negative pressure (vacuum), the conductivity shows a sensitivity of the same valu e within experimental error to a vacuum of -0.5 bar. Within the linear range, the devi ces exhibited a hysteresis of <0.4%. Outside these pressure limits the sensor has reduced sensitivity due to the device geometry. The sensitivity could be increased by having a sh allower via depth, obtained by thinning the Si substrate. Figure 3-13. Capacitance change as a func tion of radius of the AlGaN/GaN HEMT membrane over the pressure range from -1 to +9.5 bar These trends are similar to those observe d with actual bending of HEMT samples on a cantilever beam to produce tensile or compressive strain [Kan03], but exhibit much 150300450600 1 2 3 Capacitance change(pF)Radius( m)

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37 larger sensitivities to the induced tensile or compressive strain. This is due to the absence of the thick sapphire substrate that is present in the cantilever structures, as we also reported for the piezo-conductance memb rane sensors previously [Kan04c]. Figure 3-13 shows the capacitance change as a function of radius of the AlGaN/GaN HEMT membrane at a fixed pressu re of +9.5 bar. The capacitance of the channel displays a change of 7.19 +/-0.45x10-3 pF/ m. The sensor characteristics measured at this same pressure on several different days showed a maximum capacitance variation of 0.07pF, corresponding to a sensi ng repeatability of ~0.15 bar. The high temperature characteristics still need to be esta blished, but in this case will be limited by the thermal stability of the ohmic contacts. R ecent reports have shown that some contact metallization on GaN HEMTs are stable for extended periods at 500oC [Sel04]. In summary, an AlGaN/GaN HEMT memb rane on Si shows large changes in capacitance as a result of changes in ambient pr essure. This approach is less sensitive to contact resistance variations with temperatur e than the previous conductance sensors. The sensors can also be readily integrated with conventional HEMTs or Si circuitry to provide off-chip wireless transmission of pressure data.

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38 CHAPTER 4 CATALYST BASED GAS SENSOR FOR HYDROCARBON GASES 4.1 Introduction Gas sensors have been fabricated on a number of semiconductors using catalytic metals as the gate in the me tal insulator semiconductor (MIS) or as the metal contact in Schottky diodes [You82, Lun86, Rye87]. Various field effect transistors based on silicon have been developed by several groups fo r hydrogen gas sensing [Lun89]. But the silicon based sensors are limited to opera tion in environments of below 250oC, prohibiting them from being used as hydrocarbon detectors or for other applic ations requiring high temperature operation. Because hydrocarbon gases should be decomposed by the catalytic metals and hydrogen atoms diffuse to the device interface, it is presumed that a dipole forms, lowering the effective work f unction of the metal a nd changing electrical characteristics of the devices. Baranzanhi et al. [Bar95] demonstrated gas sensitive Pt gated SiC transistors operating up to 500oC but the SiC Schottky diodes have displayed poor thermal stability and formation of Pd silicides has been observed at temperatures as low as 425oC when Pd was used as Schottky metal [Hun95, Che96]. Luth er et al. [Lut99] fi rst demonstrated PtGaN gas sensor for hydrogen and propa ne at high temperature (200-400oC). It was also exhibited that Pt–GaN gas se nsor showed faster response for hydrocarbons and enhanced sensitivity at higher temperatures (500oC) After that, Schalwig et al. [Sch01] showed gas sensors for the exhausted lean burn engine s using Pt-GaN and Pt-HEMTs. The device performance at high temperature (200-600oC) was investigated. It was also shown that a

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39 HEMT based gas sensor was more sensitive th an GaN diodes but the detailed anal ysis of sensitivity difference between GaN diodes and HEMTs was left as future work. In this chapter, extensive discussion of all the processes including fabrication, measurement, and characterization of gas sensor devices using wide bandgap semiconductor diodes and transistors will be given. In Section 4.2, the higher performance of AlGaN/GaN MOS diode for hydr ogen gas sensor will be given compared with Schottky GaN diode. In Section 4.3, hydrogen reversible changes in drain and source current in the transistor based gas sens or will be given. In addition, the reason for higher sensitivity of this struct ure operated with gain will al so be discussed. In Section 4.4, a direct comparison of MOS and Scho ttky W/Pt-GaN diode for hydrogen detection will be given. In Section 4.5, the method for detecting ethylene (C2H4), which causes problems because of its strong double bond s and hence the difficulty in dissociating it at modest temperature will be investigated us ing wide band gap semiconductor. In Section 4.6, nanotechnology driven sensor for hydroge n and ozone detecti on will be discussed using multiple ZnO nanorods. 4.2 AlGaN/GaN based MOS Diode Hydrogen Gas Sensor Simple GaN Schottky diodes exhibit str ong changes in current upon exposure to hydrogen containing ambients [Ste03a, Ne u01, Sch01, Eic01, Sch02a, Stu02, Eic03, Kim03a]. The effect is thought to be due to a lowering of the effec tive barrier height as molecular hydrogen catalytically cracks on th e metal gate and atomic hydrogen diffuses to the interface between the me tal and GaN, altering interfacial charge. Steinhoff et al. [Ste03a] founded that it was necessary to have a native oxide present between semiconductor and the gate metal in order to see significant current changes. Thus, it is

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40 desirable to specifically incorporate an oxide into GaN-based diodes or HEMTs in order to maximize the hydrogen detection response. Figure 4-1.Cross-sectional schematic of completed MOS diode on AlGaN/GaN HEMT layer structure (top) and plan-v iew photograph of device(bottom). Gas sensors based on MOS diode on AlGaN/GaN high electron mobility transistor(HEMT) layer structure are of inte rest, because HEMTs are expected to be the first GaN electronic device that is commerciali zed, as part of next generation radar and wireless communication systems. These structures have much higher sensitivity than Schottky diodes on GaN layer, because they are true transistors and therefore operate

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41 with gain. In addition, th e MOS-gate version of the HEMT has significantly better thermal stability than a metal-gate structure [Kha01, Pal00, Sim00, Kou02, Sim02] and is well-suited to gas sensing. When exposed to changes in ambient, ch anges in the surface potential will lead to large changes in channel current. HEMT layer structures were grown on C-plane Al2O3 substrates by Metal Organic Chemical Vapor Deposition (MOCVD). The layer structure included an initial 2 m thick undoped GaN buffer followed by a 35nm thick unintentionally doped Al0.28Ga0.72N layer. The sheet carrier concentration was ~1 1013 cm-2 with a mobility of 980 cm2/V-s at room temperature Mesa isolation was achieved with 2000 plasma enhanced chemical vapor deposited SiNx. The ohmic contacts was formed by lift-off of e-beam deposited Ti(200)/Al(1000)/Pt(400)/Au(800). The contacts were annealed at 850 C for 45 sec under a flowing N2 ambient in a Heatpulse 610T system. 400 Sc2O3 was deposited as a gate dielectric throu gh a contact window of SiNx layer. Before oxide deposition, the wafer was exposed to ozone for 25 minutes It was then heat in-situ at 300 C cleaning for 10mins inside the growth chamber. 100 Sc2O3 was deposited on AlGaN/GaN by rf plasma-activated MBE at 100 C using elemental Sc evaporated from a standard effusion all at 1130 C and O2 derived from an Oxford RF plasma source [Gil01, Kim00b, Kim00c]. 200 Pt Schottky cont act was deposited on the top of Sc2O3. Then, final metal of e-beam deposited Ti/Au (300/1200 ) interconnection contacts was employed on the MOS-HEMT diodes. Figure 4-1 shows a schematic (top) and photograph (bottom) of the completed device. The devices were bonded to electrical feed-through and exposed to different gas ambient in an environm ental chamber [Kim03a, Kim03b, Kim03c].

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42 0.00.51.01.52.02.53.0 0 5 10 15 20 Current(mA)Biased Voltage(V) R30 10% H2 R10 N2 R30 10% H2 R10 N2 Figure 4-2. Forward I-V characteristics of MOS-HEMT based diode sensors of two different dimensions at 25C measured under pure N2 or 10%H2 /90%N2 ambient ;R30-diode with 30 m radius R10 diode with 10 m radius. Figure 4-2 shows the forward current-voltage (I-V) characteristics at 25 C of the MOS-HEMT diode both in pure N2 and in a 10%H2 / 90%H2 atmosphere. At a given forward bias, the current increa ses upon introduction of the H2, through a lowering of the effective barrier height. The H2 catalytically decomposes on the Pt metallization and diffuses rapidly though the underlying oxide to the interface where it forms a dipole layer [Eic03]. At 2.5V forward bias the change in forward current upon introduction of the hydrogen into the ambient is ~6mA or equivalen tly 0.4V at a fixed current of 10mA. This is roughly double the detection sensitivity of comparable GaN Schottky gas sensors tested under the same conditions [Kim03c], confirming that the MOS-HEMT based diode

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43 has advantages for applications requiring the ability to detect combustion gases even at room temperature. 050100150200 6.7 6.8 6.9 7.0 7.1 7.2 10sec 20sec 30sec Current(mA)Time(sec) Figure 4-3. Time response at 25C of MOS-HEMT based diode forward current at a fixed bias of 2V when switching the ambient from N2 to 10%H2 /90%N2 for periods of 10, 20 or 30 seconds and then back to pure N2 As the detection temperature is increase d, the response of the MOS-HEMT diodes increases due to more efficient cracking of the hydrogen on the metal contact. The threshold voltage for a MOSFET is given by [Shu90] 5 0) 4 ( 2i S B D B FB TC eN V V (4-1) where, VFB is the voltage required for flat band conditions, B the barrier height, e the electronic charge and Ci the Sc2O3 capacitance per unit area. In analogy with results for MOS gas sensor s in other materials systems [Ste03a], the effect of the introduction of the atomic hydrogen into th e oxide is to create a dipole layer at the oxide/semiconductor interface that will screen some of the piezo-induced

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44 charge in the HEMT channel. To test the time response of the MOS diode sensors, the 10%H2/90%N2 ambient was switched into the chambe r through a mass flow controller for periods of 10, 20 or 30 seconds and then switched back to pure N2. 050100150200250300 10.2 10.4 10.6 10.8 Figure 4-4. Time response at 25C of MOS-HEMT based diode forward current at a fixed bias of 2V for three cycles of switching the ambient from N2 to 10%H2 /90%N2 for periods of 10 (top) or 30 (bo ttom) seconds and then back to pure N2

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45 Figure 4-3 shows the time dependence of fo rward current at a fixed bias of 2V under these conditions. The res ponse of the sensor is rapi d(<1 sec), with saturation taking almost the full 30 seconds. U pon switching out of the hydrogen–containing ambient, the forward current decays exponentia lly back to its initial value. This time constant is determined by the volume of the test chamber and the flow rate of the input gases and is not limited by the response of the MOS diode itself. Figure 4-4 shows the time response of the forward current at fixed bi as to a series of gas injections into the chamber, of duration 10secs each (top) or 30 secs each (bottom). The MOS diode shows good repeatability in its changes of current and the ability to cy cle this current in response to repeated introductions of hydr ogen into the ambient. Once again, the response appears to be limited by the mass tran sport of gas into and out of the chamber and not to the diffusion of hydrogen through the Pt/Sc2O3 stack. In conclusion, AlGaN/GaN MOS-HEMT di odes appear well-suited to combustion gas sensing applications. The changes in fo rward current are approximately double those of simple GaN Schottky diode gas sensors te sted under similar conditions and suggest that integrated chips involving gas sensor s and HEMT-based circuitry for off-chip communication are feasible in the AlGaN/GaN system. 4.3 Hydrogen Induced Reversible Changes in Drain Current in Sc 2 O 3 /AlGaN/GaN HEMTs It has been observed that AlGaN/Ga N metal oxide semiconductor (MOS) diodes utilizing Sc2O3 as the gate dielectric have appr oximately double the sensitivity for hydrogen detection than Pt/GaN Schottky di odes [Kan04a]. The use of a true MOS transistor should be even more effective becau se of the current gain in the 3-terminal device. The reversible hydrogen-induced changes in drain-source current were

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46 investigated using Sc2O3/AlGaN/GaN High Electron Mobili ty Transistors (HEMTs). The current changes are significantly larger (a factor of 5) than observed for Sc2O3/AlGaN/GaN MOS diodes exposed under th e same conditions. The response time of the HEMTs is limited by the mass transfer characteristics of the hydrogen-containing gas ambient into the test chamber. These devi ces can be used as sensitive combustion gas sensors, but the results also point out the susceptibility of the HEMTs to changes in current depending on the composition of the am bient in which they are being operated. The HEMT layer structures were grown on C-plane Al2O3 substrates by Metal Organic Chemical Vapor Depos ition (MOCVD). The layer stru cture included an initial 2 m thick undoped GaN buffer followed by a 35nm thick unintentionally doped Al0.28Ga0.72N layer. The sheet carrier concentration was ~1 1013 cm-2 with a mobility of 980 cm2/V-s at room temperature. Mesa isol ation was achieved by using an inductive coupled plasma system with Ar/Cl2 based discharges. The ohmic contacts was formed by lift-off of e-beam deposited Ti(200)/Al( 1000)/Pt(400)/Au(800). The contacts were annealed at 850 C for 45 sec under a flowing N2 ambient in a Heatpulse 610T system. 100 Sc2O3 was deposited as a gate dielectri c through a contac t window of SiNx layer. Before oxide deposition, the wafer was exposed to ozone for 25 minutes. It was then heat in-situ at 300 C cleaning for 10mins inside the growth chamber. The Sc2O3 was deposited by rf plasma-activated MBE at 100 C using elemental Sc evaporated from a standard effusion all at 1130 C and O2 derived from an Oxford RF plasma source [Gil01, Kim00b]. 200 Pt Schottky cont act was deposited on the top of Sc2O3. Then, final metal of e-beam deposited Ti/Au (300/1200) interconnection contacts was employed on the MOS-HEMTs.

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47 Figure 4-5. Photograph of MOS HEMT hydrogen sensor Figure 4-5 shows photograph (top) and a cro ss sectional schematic (bottom) of the completed device. The gate dimension of the device is 1 50 m2. The devices were bonded to electrical feed-through an d exposed to either pure N2 or 10%H2/90% N2 ambients in an environmental chamber in which the gases were introduced through electronic mass flow controllers. Figure 4-6 shows the MOS-HEMT drain-source current voltage (IDS-VDS) characteristics at 25oC measured in both the pure N2 or 10%H2/90% N2 ambients. The current is measurably larger in the latter case as would be expected if the hydrogen

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48 catalytically dissociates on the Pt contact and diffuses through the Sc2O3 to the interface where it screens some of the piezoinduced channel charge [Eic03]. 012345 0 5 10 15 20 25 30 35 40 IDS(mA)VDS(V) N2(VG=0 to -6V) 10% H2(VG=0 to -6V) Figure 4-6. IDS-VDS characteristics of MOS-HEMT measured at 25oC under pure N2 ambient or in 10% H2/90%N2 ambient. This is a clear demonstration of th e sensitivity of AlGaN/GaN HEMT dc characteristics to the presence of hydrogen in the ambient in which they are being measured. The use of less efficient catalytic metals as the gate metallization would reduce this sensitivity, but operation at elevated te mperatures would increase the effect of the hydrogen because of more efficient dissociation on the metal contact. Figure 4-7 shows the measured change in drain-source current for measurement in the two different ambients as a function of gate voltage for different drain-source voltages. The maximum change in this current is ~3 mA(150 mA/mm), which is approximately a factor of 5 larger than obtained for the same bias conditions in Sc2O3/AlGaN/GaN MOS diodes exposed under the same conditions [Kan04a] and an

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49 order of magnitude larger than the changes in forward current in simple Pt/GaN Schottky diodes in the same test chamber. -6-5-4-3-2-10 0.5 1.0 1.5 2.0 2.5 3.0 IDS-I0(mA)VG(V) VDS=3V VDS=4V VDS=5V -6-5-4-3-2-10 0 20 40 60 80 100 120 140 N2 10% H2VDS = 3V Figure 4-7. Change in drain-sour ce current for measurement in N2 versus 10%H2 /90%N2 ambient, as a function of gate voltage (top) and corresponding transconductance at a fixed dr ain-source voltage of 3V.

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50 This shows the advantage of using the 3-terminal device structure, with its attendant current gain. This approach is particularly advantageous when small concentrations of hydrogen must be detected over a broad range of temperatures. The MOS-HEMT is more thermally stable than a conventional metal gate HEMT and will be usable at higher temperatures. The change in drain-source curr ent tracked the device transconductance, as shown at the botto m of Figure 4-7. The shift in peak transconductance when hydrogen is present in the ambient is consistent with an increase in the total channel charge. Figure 4-8 shows some of the recovery characteristics of the MOS-HEMTs upon cycling the ambient from N2 to 1%H2/ 99% N2 While the change in drain-source current is almost instantaneous (< 1 sec),the recovery back to the N2 ambient value is of the order of 20 secs. This is controlled by the mass transport characteristics of the gas out of the test chamber, as demonstrated by changing the tota l flow rate upon switching the gas into the chamber. Given that the current change upon introduction of the hydrogen is rapid, the effective diffusivity of the atomic hydrogen through the Sc2O3 must be greater than 4 x10-12 cm2/V.s at 25C. Note the complete reversibility of the drain-source current for repeated cycling of the ambient. In conclusion, Sc2O3/AlGaN/GaN MOS-HEMTs show a marked sensitivity of their drain-source current to the pr esence of hydrogen in the measur ement ambient. This effect is due to the dissociation of the molecular hydrogen on the Pt gate contact, followed by diffusion of the atomic species to the oxi de/semiconductor interface where it changes the piezo-induced channel charge. The MOS-HEMTs show larger change s in current than

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51 their corresponding MOS-diode or Schottky diode counterparts a nd show promise as sensitive hydrogen detectors. 05101520253035 16.2 16.3 16.4 16.5 16.6 16.7 16.8 IDS(mA)Time(sec) 7 sec 5 sec 3 sec 1 sec 1% H2 in N2 020406080100120140160 16.2 16.3 16.4 16.5 16.6 16.7 16.8 1% H2 in N2 IDS(mA)Time(sec) at VG = -3 V Figure 4-8. Time dependence of drainsource current when switching from N2 to 1%H2 /99% N2 ambient and back again. The top s hows different injection times of the H2/N2, while the bottom shows the reversibility of the current change.

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52 4.4 Comparison of MOS and Schottky W/ Pt-GaN Diodes for Hydrogen Detection For GaN Schottky diodes, there is evidence th at the presence of an interfacial oxide increases the magnitude of th e change in barrier height [Sch02b] and hence the current flowing at a given bias on the device. In support of that finding, we have previously observed that AlGaN/GaN metal-oxide semiconductor(MOS) diodes utilizing Sc2O3 as the gate dielectric have appr oximately double the sensitivity for hydrogen detection than Pt/GaN Schottky diodes [Kan04a]. An add itional key requirement in some sensor applications such as long-term spaceflight is the need for very good reliability of the contact metal on the semiconductor. We have found that W shows little reaction with GaN to temperatures in excess of 700oC [Col96, Col97, Cao98] and when used as a bilayer with Pt, can also provide measurable sensitivity for H2 detection. The response time of both types of sensor is limited by the ma ss transfer characteristics of the hydrogencontaining gas ambient into the test chamber. Approximately 6 m of n-GaN was grown on sapphire substrates by Metal Organic Chemical Vapor Deposition. Ohmic contacts was formed by lift-off of Ti/Al/Pt/Au, annealed at 500 C. In some cases, MOS structures were formed by deposition of 100 Sc2O as a gate dielectric th rough a contact window of SiNx .Before oxide deposition, the wafer was exposed to ozone for 25 minutes It was then heat in-situ at 300 C cleaning for 10mins inside the growth chamber. The Sc2O3 was deposited by rf plasma-activated MBE at 100 C using elemental Sc evaporated from a standard effusion all at 1130 C and O2 derived from an Oxford RF plasma source [Gil01, Kim00b]. 200 of W was deposited on both types of samples by spu ttering, followed by e-beam evaporation of 150 of Pt. The Pt Schottky contacts were formed by lift-off.

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53 Figure 4-9.Schematic of both the W/Pt Scho ttky diode (top) and MOS diode (bottom). Figure 4-10. Photograph of packaged gas sensor

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54 0123450.1 1 10 Current(mA)Voltage(V) at 300oC N2 GaN 10% H2 GaN N2 GaN with oxide 10% H2 GaN with oxide 012345 0.1 1 10 Current(mA)Voltage(V) at 600 oC N2 GaN 10% H2 GaN N2 GaN with oxide 10% H2 GaN with oxide Figure 4-11.Forward I-V characteristics at 300 oC(top) or 500 oC(bottom) from the Schottky and MOS diodes in pure N2 and 10% H2 /90% N2 Figure 4-9 shows a cross-sectional sche matic of the completed devices. The devices were bonded to electrical feed -through and exposed to either pure N2 or

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55 10%H2/90% N2 ambients in an environmental chamber in which the gases were introduced through electronic mass flow controllers. 012345 0.1 1 10 Current(mA)Voltage(V) T25N T25H T100N T100H T200N T200H T300N T300H T400N T400H T500N T500H T600N T600H 012345 0.1 1 10 Current(mA)Voltage(V) T25N T25H T100N T100H T200N T200H T300N T300H T400N T400H T500N T500H T600N T600H Figure 4-12. Measurement temperature dependen ce of forward I-V characteristics of the Schottky (top) and MOS (bottom) diodes in both pure N2 and 10% H2 /90% N2

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56 Figure 4-10 shows a photograph of a typical packaged de vice. Figure 4-11 shows the forward I-V characteristic s of both the Schottky and MOS diodes at both 300(top) and 600 C (bottom) measured in both the pure N2 or 10%H2/90% N2 ambients. The current is measurably larger in the latter case as w ould be expected if th e hydrogen catalytically dissociates on the Pt contact and di ffuses through the W (and the through the Sc2O3 in the case of the MOS diodes) to the interface wh ere it screens some of the piezo-induced channel charge [Sve99]. The decrease in e ffective barrier height was obtained from fitting the forward I-V characteristics to th e relation for the thermionic emission over a barrier ) exp( ) exp( .2 *nkT eV kT e T A Jb F (4-2) where J is the current density, A* is the Richardson’s constant for n-GaN, T the absolute temperature, e the electronic charge, b the barrier height, k Boltzmann’s constant n the ideality factor and V the applied voltage. From the data, the decrease in b in the presence of 10%H2 in the ambient was 30-50mV over the range of temperatures investigated here. This is a clear demonstration of the sensitiv ity of GaN diode dc ch aracteristics to the presence of hydrogen in the ambient in whic h they are being measured. Operation at elevated temperatures should increase the effect of the hydroge n because of more efficient dissociation on the metal contact. Figure 4-12 shows the forward I-V characte ristics over the entir e temperature range investigated, measured in both pure N2 and 10%H2 /90%H2 .The turn-on voltage, VF, of the diodes, defined as the voltage at which the forward current is 1A.cm-2, and given by the relation F ON B F FJ R n T A J e nkT V ) ln(2 * (4.3)

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57 decreases with temperature for both types of diodes due to the lowering of the effective energy barrier for electrons 0100200300400500600 0.25 0.50 0.75 1.00 1.25 VT(V)Temperature(oC) GaN GaN with oxide 0100200300400500600 0.005 0.010 0.015 0.020 0.025 Ron(cm2)Temperature(oC) GaN GaN with oxide Figure 4-13.Temperature dependence of turn-on voltage(top) and on-state resistance(bottom) for the GaN Schottky and MOS diodes. The on-state resistance ,RON, derived from the relation

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58 C S M B ONR W s E V R ) / 4 (3 2 (4.4) where, is the GaN permittivity, the carrier mobility, S and WS are substrate resistivity and thickness, and RC is the contact resistance ,increas ed with measurement temperature for both types of diodes, as shown in Figur e 4-13.The MOS-HEMT is more thermally stable than a conventional metal gate HEMT and should be usable at higher temperatures. 0100200300400500600 0.0 0.5 1.0 1.5 2.0 Current(mA)Temperature(oC) GaN at 3V GaN at 3.5V GaN with oxide at 3V GaN with oxide at 3.5V Figure 4-14.Change in forward cu rrent when measuring in 10% H2 /90% N2 relative to pure N2 at 3 or 3.5 V in both the Schott ky and MOS diodes ,as a function of the measurement temperature Figure 4-14 shows the change in forward curre nt at fixed bias of 3 V for both types of diodes when 10% H2 is introduced into the N2 ambient, as a function of temperature. The use of the MOS diode structure provide s a much broader temperature window in which the detected change in current is above 1mA.This clearly an advantage in terms of using the sensors over a wide range of temperatures without the need for an on-chip heater.

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59 The recovery characteristic s of both types of diodes upon cycling the ambient from N2 to 1%H2/ 99% N2 showed a rapid (< 1 sec) change of current upon introduction of the hydrogen into the ambient, while the recovery back to the N2 ambient value took of the order of 20 secs. This is cont rolled by the mass transport characteristics of the gas out of the test chamber, as demonstrated by changi ng the total flow ra te upon switching the gas into the chamber. Given that the current change upon introducti on of the hydrogen is rapid, the effective diffusivity of the atomic hydrogen through the Sc2O3 must be greater than 4 x10-12 cm2/V.s at 90oC. There was complete reversibility of the current for repeated cycling of the ambient. In conclusion, Pt/W/GaN MOS and Schott ky diodes show a marked sensitivity of their forward current to the presence of hydrogen in the measurement ambient. This effect is due to the dissoci ation of the molecular hydrogen on the Pt gate contact, followed by diffusion of the atomic species to the oxide/semiconductor interface where it changes the piezo-induced channel charge. The MOS-diode shows a wider range of temperatures in which it shows large cha nges in current than the Schottky diode counterparts and shows promise as sensitive hydrogen detectors. 4.5 Detection of C 2 H 4 Using Wide Bandgap Semiconductor Sensors Currently, there is a strong interest in the development of wide bandgap semiconductor gas sensors for applications in cluding detection of combustion gases for fuel leak detection in spa cecraft, automobiles and aircra ft, fire detectors, exhaust diagnosis and emissions from industrial pr ocesses [Vas98, Sav00, Lol00, Con02, Arb93, Hun02, Che96, Eke98, Sve99, Hun01, Che98, Tob97, Bar95, Cas98]. Of particular interest are methods fo r detecting ethylene (C2H4), which offers problems because of its strong double bonds and hence the difficulty in dissociating it at modest temperatures.

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60 Wide bandgap semiconductors such as GaN and ZnO are capable of operating at much higher temperatures than more conventional semiconductors such as Si. Diode or fieldeffect transistor structures fabricated in these materials are sensitive to gases such as hydrogen and hydrocarbons [Tom03, Mit03, Wol03, Ju03a, Lin01, Rao00, Mit98, Cha02]. Ideal sensors have the ability to di scriminate between different gases and arrays that contain different metal oxides (eg.SnO2, ZnO, CuO, WO3) on the same chip can be used to obtain this result [Tom03]. The ga s sensing mechanism s uggested include the desorption of adsorbed surface oxygen a nd grain boundaries in poly-ZnO [Mit03], exchange of charges between adsorbed ga s species and the ZnO surface leading to changes in depletion depth [Wol03] and ch anges in surface or grain boundary conduction by gas adsorption/desorption [Kim03c]. A nother prime focus should be the thermal stability of the detectors, since they are expe cted to operate for long periods at elevated temperature. MOS diode-based sensors have si gnificantly better thermal stability than a metal-gate structure and also sensitivity than Schottky diodes on GaN. In this work, we show that both AlGaN/GaN MOS diodes and P t/ZnO bulk Schottky diod es are capable of detection of low concentrations(10%) of ethylene at temperatures between 50-300 C(ZnO) or 25-400C(GaN). AlGaN/GaN laye r structures were grown on C-plane Al2O3 substrates by Metal Organic Chemical Vapor Deposition (MOCVD). The layer structure included an initial 2 m thick undoped GaN buffer followed by a 35nm thick unintentionally doped Al0.28Ga0.72N layer. The sheet carrier concentration was ~1 1013 cm-2 with a mobility of 980 cm2/V-s at room temperature. Device isolation was achieved with 2000 plasma enhanced chemical vapor deposited SiNx. The ohmic contacts was formed by lift-off of e-beam deposited Ti (200)/Al(1000)/Pt(400)/Au(800). The

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61 contacts were annealed at 850 C for 45 sec under a flowing N2 ambient in a Heatpulse 610T system. 400 Sc2O3 was deposited as a gate dielec tric through a contact window of SiNx layer. Before oxide deposition, the wafe r was exposed to ozone for 25 minutes. It was then heat in-situ at 300 C cleaning for 10mins inside the growth chamber. 100 Sc2O3 was deposited on AlGaN/GaN by rf plasma-activated MBE at 100 C using elemental Sc evaporated from a standard effusion all at 1130 C and O2 derived from an Oxford RF plasma source [Gil01, Kim00b]. Figure 4-15. Schematic of AlGaN/GaN MOS diode (top) and bulk ZnO Schottky diode structure (bottom)

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62 200 Pt Schottky contact was deposited on the top of Sc2O3. Then, final metal of e-beam deposited Ti/Au (300/1200) interc onnection contacts was employed on the MOS-HEMT diodes. Figure 4-15 (top)shows a schematic of the completed device. The bulk ZnO crystals from Cermet, Inc. showed electron co ncentration of 9 1016 cm-3 and the electron mobility of 200 cm2/V.s. at room temperature from van der Pauw measurements. The back(O-f ace) of the substrates were deposited with full area Ti (200 )/Al (800 )/Pt (400 )/Au (800 ) by e-beam evaporation. After metal deposition, the samples were annealed in a Heatpulse 610 T system at 200 C for 1 min in N2 ambient. The front face was deposited with plasma-enhanced chemical vapor deposited SiNx at 100C and windows opened by wet etch ing so that a thin (20nm) layer of Pt could be deposited by e-beam evapora tion. After final metal of e-beam deposited 0.00.51.01.52.02.53.0 0 2 4 6 8 10 10% C2H4 N2Measured at 400oC Figure 4-16. Forward I-V characteristics of MOS-HEMT based diode sensor at 400C measured under pure N2 or 10% C2H4 /90% N2 ambients

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63 Ti/Au (300/1200) interconn ection contacts was depos ited, the devices were bonded to electrical feed-thr oughs and exposed to different gas ambients in an environmental chamber while the diode cu rrent-voltage (I-V) characteristics were monitored. Figure 4-15 (bottom) shows a sche matic of the complete d device. Figure 4-16 shows the forward diode current-voltage (I-V) characteristic s at 400C of the Pt/Sc2O3/AlGaN/GaN MOS-HEMT diode both in pure N2 and in a 10% C2H4/90%N2 atmosphere. At a given forward bias, the current increases upon introduction of the C2H4. Hydrogen either decomposed from C2H4 in the gas phase or chemisorbed on the Pt Schottky contacts then decomposed and released hydrogen. The hydrogen diffused rapidly though the Pt metalliz ation and the underlying oxide to the interface where it forms a dipole layer [Amb03] and lowered the effective barrier height. Figure 4-17 shows both the change in current at fixed bias (top) and change in voltage at fixed current (bottom) as a func tion of temperature for the MOS diodes when switching from a 100 % N2 ambient to 10% C2H4/90% N2. As the detection temperature is increase d, the response of the MOS-HEMT diodes increases due to more efficient cracking of the hydrogen on the metal contact. Note that the changes in both current and voltage are quite la rge and readily detected. In analogy with results for MOS gas sensor s in other materials systems [Eic03], the effect of the introduction of the atomic hydroge n into the oxide is to create a dipole layer at the oxide/semiconductor interface that will sc reen some of the piezo-induced charge in the HEMT channel.

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64 0100200300400 0 100 200 300 400 Current Change at 2.5 V(uA)Temperature(oC) 100200300400 0 30 60 90 120 Voltage change(mV)Temperature(oC) 5 mA 10 mA 15 mA Figure 4-17. Change in MOS diode forward curr ent at fixed forward bias of 2.5V(top) or at fixed current(bottom) as a function of temperature for measurement in 100%N2 or 10% C2H4/90% N2.

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65 -0.30.00.30.60.91.21.5 0 10 20 30 40 At 50 oC N2 10 % Ethylene 20 % EthyleneCurrent(mA)Voltage(V) -0.30.00.30.60.91.2 -10 0 10 20 30 40 At 150 oC N2 10 % Ethylene 20 % EthyleneCurrent(mA)Voltage(V) Figure 4-18. I-V characteristic s at 50C (top) or 150 C (bottom) of Pt/ZnO diodes measured in different ambients. The time constant for response of the diodes was determined by the mass transport characteristics of the gas into the volume of the test chamber and was not limited by the response of the MOS diode itself.

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66 Figure 4-18 shows the I-V characteristics at 50 and 150C of the Pt/ZnO diode both in pure N2 and in ambients containing various concentrations of C2H4. At a given forward or reverse bias, the current in creases upon introduction of the C2H4, through a lowering of the effective barrier height. One of the ma in mechanisms is once again the catalytic decomposition of the C2H4 on the Pt metallization, followed by diffusion to the underlying interface with the ZnO. In conventional semiconductor gas sensors, the hydrogen forms an interfacial dipole layer th at can collapse the Schottky barrier and produce more ohmic-like behavior for the Pt contact. The recovery of the rectifying nature of the Pt contact was many orders of magnitude longer than fo r Pt/GaN or Pt/SiC diodes measured under the same conditions in the same chamber. For measurements over the temperature range 50-150C, the activation energy for recove ry of the rec tification of the contact was estimated from the change in fo rward current at a fixed bias of 1.5V. This was thermally activated thr ough a relation of the type IF =IO exp(-Ea/kT) with a value for Ea of ~0.25 eV, comparable for the value of 0. 17 eV obtained for the diffusivity of atomic deuterium in plasma exposed bulk ZnO. This i ndicates suggests that at least some part of the change in current upon hydrogen gas e xposure is due to in-diffusion of hydrogen shallow donors that increase the effective dop ing density in the near-surface region and reduce the effective barrier height. The changes in current at fixed bias or bias at fixed current were larger for the ZnO diodes than for the AlGaN/GaN MOS diodes because of this additional detection mechanism, as shown in Figure 4-19. Note that the changes in these parameters are approximately an order of magnitude larger at 150C. However the ZnO diodes wee not thermally stable above ~300C due to direct reaction of the Pt with the ZnO surface.

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67 255075100125150 0 3 6 9 12 15 10 % Ethylene at 1.2 V 10 % Ethylene at -0.5 V 20 % Ethylene at 1.2 V 20 % Ethylene at -0.5 VCurrent change(mA)Temperature(oC) 255075100125150 0 100 200 300 400 10 % Ethylene at 10 mA 10 % Ethylene at 20 mA 20 % Ethylene at 10 mA 20 % Ethylene at 20 mAVoltage change(mV)Temperature(oC) Figure 4-19.Change in current at a fixed bias (top) or change in voltage at fixed current (bottom) as a function of measurement te mperature in different percentages of C2H4/N2 ambients In conclusion, AlGaN/GaN MOS-HEMT diodes and bulk ZnO Schottky diodes appear well-suited to detection of C2H4. The former have a larger temperature range of sensitivity, but the absolute changes in voltage or current are larger with the ZnO diodes.

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68 The introduction of hydrogen shallow donors into the near-surface region of the ZnO is a plausible mechanism for the non-recovery of th e I-V characteristics at room temperature. 4.6 Hydrogen and Ozone Gas Sensing Using Multiple ZnO Nanorods ZnO nanowires and nanorods are attracting attention for use in gas, humidity and ultra-violet (UV) detectors [Wan04a, Kee04]. ZnO is attractive for a broad range of applications in thin film form [Loo01, Wra 99, gor99, Kri02, Lia01, Kuc02, Pea04b], but the ability to make arrays of nanorods with large surface area which has been demonstrated with a number of different growth methods has gr eat potential for new types of sensors that operate with low pow er requirements [Hua01, Li04, Kin02, Liu03, Par03a, Ng03, Hu03b, Par03b, Heo02, Nor04, Poo03, Heo03, Wu00, Zhe01, Lyu01, Zhe03b, Par03c, Yao02, Pan01, Lao03, Sun03]. A large variety of ZnO one-dimensional structures have been demonstrated [Wan04b] The large surface area of the nanorods and bio-safe characteristics of ZnO makes them attractive for gas and chemical sensing and biomedical applications, and the ability to control their nucleation sites makes them candidates for micro-lasers or memory arrays. To date, most of the work on ZnO nanostructures has focused on the synthesis me thods [Wan04b] and there have been only a few reports of the electrical characteristics [Lia01, Kuc02, Pea04b, Hua01, Li04]. The initial reports show a pronounced sensitivity of the nanowire conductivity to ultraviolet illumination and the presence of oxygen in the measurement ambient [Wan04a, Kee04]. In this chapter, the gas ambient dependen ce of current-voltage characteristics of multiple ZnO nanorods prepared by site-sel ective Molecular Beam Epitaxy (MBE) will be investigated. These structures can read ily detect a few percent of ozone in N2 at room temperature and are able to dete ct hydrogen beginning at around 112oC. Over a limited range of partial pressures of O3(POZONE) in the ambient gas, the conductance G of the

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69 sensor at fixed bias voltage decr eased according to the relation G=(GO + A(POZONE)0.5)1,where A is a constant and GO the resistance in N2.The nanorods begin to show detectable sensitivity to hydrogen in the measurement ambient at around 112oC. Figure 4-20. TEM of ZnO nanorod The growth of the nanorods has been described in detail [Heo02, Nor04]. Discontinuous thin films (~100 ) of e-b eam evaporated Ag were deposited on p-Si (100) wafers terminated with native oxide. ZnO nanorods we re deposited by MBE with a base pressure of 5x10-8 mbar using high purity ( 99.9999%)Zn metal and an O3/O2 plasma discharge as the source chemicals. The Zn pressure was varied between 5x10-7 and 5x10-8 mbar, while the beam pressure of the O3/O2 mixture was varied between 5 x 10 -6 and 5x10-4 mbar .The growth time was ~2 h at 400 C. The typical lengt h of the resultant nanorods was ~ 2 m, with typical diameters in the range of 15 – 30 nm. Selected area diffraction patterns showed the nanorods to be single-crystal. Figure 4-20 shows a transmission electron microgra ph of a single ZnO nanorod. The nanorods were heated in hydrogen at 300oC to ensure they were conducting. They were released from the substrate by dissolution of the Ag catalyst and then transferred to SiO2-coated Si substrates. Ebeam lithography was used to pattern sputtere d Al/Ti/Au electrodes contacting both ends 20 nm

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70 of multiple nanorods. The separation of the electrodes was ~3 um. A scanning electron micrograph of the completed device is shown in Figure 4-21. Figure 4-21. SEM image of ZnO multiple na norods (top) and the pattern contacted by Al/Pt/Au electrodes (bottom) Au wires were bonded to the contact pad for current –voltage (I-V) measurements performed over the range 25-150 C in a range of different ambients (,N2,3 % O3 in N2 or 10%H2 in N2). Figure 4-22 shows the I-V characteristics from the multiple nanorods measured in either N2 or 10H2/90% N2 ambients at different temperat ures. At room temperature there

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71 is no detectable change in current but the pr esence of the hydrogen in the ambient can be detected beginning at ~112oC.The reversible chemisorption of reactive gases at the surface of these metal oxides can produce a large and reversible variation in the conductance of the material [Tom03]. -0.4-0.20.00.20.4 -8.0x10-8-4.0x10-80.0 4.0x10-88.0x10-8 Current(A)Voltage(V) 22oC N2 22oC H2 112oC N2 112oC H2 194oC N2 194oC H2 Figure 4-22. I-V characteristics at differe nt temperatures of ZnO multiple nanorods measured in either N2 or 10 % H2 in N2 ambient The gas sensing mechanism suggested incl ude the desorption of adsorbed surface oxygen and grain boundaries in poly-ZnO [M it03], exchange of charges between adsorbed gas species and the ZnO surface lead ing to changes in depletion depth [Mit98] and changes in surface or grain boundary conduction by gas adsorption/desorption [Cha02]. The detection mechanis m is still not firmly established in these devices and needs further study. When detecting hydrogen with the same types of rectifiers, we have observed changes in current consistent with changes in the near -surface doping of the ZnO, in which hydrogen introduces a shallow donor state. The nanorods also showed a

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72 strong photoresponse to above bandgap UV li ght(366nm).The photoresponse was fast and indicates that the changes in conductivity due to injection of carriers is bulk-related and not due to surface effects [Kee04]. 050100150200 0.0 4.0x10-98.0x10-91.2x10-81.6x10-8 Current difference(A)Temperature(oC) Figure 4-23. Change in current measured at 0.1 V for measurement in either N2 or 10H2 in N2 ambients Figure 4-23 shows the difference in current at fixed bias of 0.1V for measurement in N2 versus 10%H2/90% N2 as a function of the measurement temperature. The change in current is still in the nA range at 112oC but increases with temperature and is about 16nA at 200oC.These changes are readily detected by conventional ammeters. However, the inability to detect hydrogen at room temp erature means that an on-chip heater would still be needed for any practic al application of ZnO nanorods for detection of combustion gases. The nanorods were more sensitive to the presence of ozone in the measurement ambient. Figure 4-24 shows the room temperature I-V characteristics from the multiple nanorods measured in pure N2 or 3% O3 in N2.

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73 -3-2-10123 -3 -2 -1 0 1 2 3 Current(A)Voltage(V) N2 3% Ozone Figure 4-24. I-V characteristics at 25C of Zn O multiple nanorods measured in either N2 OR 3% O3 in N2 The changes in current are much larger than for the case of hydrogen detection. Over a relatively limited range of O3 partial pressures in the N2 ambient, the conductance increased according to G =(GO + A(PO3)0.5)-1 where A is a constant and GO the resistance in pure N2 ambient. This is a similar dependen ce to the case of CO detection by SnO2 conduction sensors, in which the effective c onductance increased as the square root of CO partial pressure. The gas sensitivity can be calculated from the difference in conductance in O3-containing ambients, divided by the conductance in pure N2, i.e.(GN2 GO)/GN2.At 25 C ,the gas sensitivity was 18 % for 3%O3 in N2 Figure 4-25 shows the time dependence of ch ange in current at a fixed voltage of 1V when switching from back and forth from N2 to 3% O3 in N2 ambients. The recovery time constant is long(>10 mins) so that the na norods are best suited to initial detection of ozone rather than to determining the actual time dependence of change in concentration. In the latter case a much faster recovery ti me would be needed. The gas sweep-out times

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74 in our test chamber are relatively short (~a fe w secs) and therefore the long recovery time is intrinsic to the nanorods. 02004006008001000 0.804 0.808 0.812 0.816 0.820 0.824 Ozone exposed Current Change(A)Time(sec) Current Figure 4-25.Time dependence of current at 1V bias when switching back and forth from N2 to 3% O3 in N2 ambients ZnO nanorods appear well-suited to detection of O3. They are sensitive at temperatures as low as 25C for percent levels of O3 in N2. The recovery characteristics are quite slow at room temperature, indicat ing that the nanorods can be used only for initial detection of ozone. The nanorods are also sensitive to detection of hydrogen at more elevated temperatures(>~100oC) but are not sensitive at room temperature .The ZnO nanorods can be placed on cheap transparent substrates such as glass, making them attractive for low-cost sensing applications.

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75 CHAPTER 5 CHEMICAL SENSOR FOR POLYMERS AND POLAR LIQUIDS 5.1 Introduction Since the first demonstration of a flui d monitoring sensor based on AlGaN/GaN hetero structures by Neuberger [Neu01], the application of AlGaN/GaN HEMTs as liquid sensors has been a subject of intense research Neuberger et al. have suggested that the sensing mechanism for chemical absorbates originated from the compensation of the polarization induced bound surface charge by in teraction with polar molecules in the fluids. The time dependence of changes in source-drain current of gateless HEMTs exposed to polar liquids (isopropanol, acetone methanol) with different dipole moments using GaN/AlGaN hetero-interf aces was also reported. In particular, it was possible to distinguish liquids with different polarities. Steinhoff et al. suggested that the na tive oxide on the nitride surface was responsible for the p H sensitivity of the response of gateless GaN based heterostructure transistors to electrolyte solutions [Ste03a]. The linear response of nonmetallized GaN gate region using different p H valued electrolyte solutions and sensitivity with a resolution better than 0.05 p H from p H = 2 to p H = 12 were shown. Chaniotakis et al. [Cha05] showed that the GaN surface interact s selectively with Lewis acids, such as sulphate (SO4 2-) and hydroxide (OH-) ions using impedance spectra. It was also shown that gallium face GaN was considerably reac tive with many Lewis bases, from water to thiols and organic alcohols wit hout any metal oxide and nitrides.

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76 A novel metal oxide, ZnO has numerous at tractive characteri stics for gas and chemical sensors [Kan05a, Kan05b, Heo04, L oo01, Nor04]. ZnO is also attractive for forming various types of nanorods, nanowir es, and nanotubes [Hua01, Li04, Kee04, Kin02, Liu03, Par03a, Ng03, Hu03, Par03b, Heo02, He03, Zhe01, Lyu03, Zha03, Pan01, Lao03]. The large surface area of the nanor ods makes them attractive for gas and chemical sensing, and the ability to contro l their nucleation sites makes them candidates for high density sensor arrays. In this chapter, a study of two different gate less transistors will be given. In Section 5.2, the response of block copolymers on th e gate area of AlGaN/GaN HEMT will be will be discussed. In Section 5.3, the details of pH measurements with single ZnO nanorods integrated with a mi cro-channel will be given. 5.2 Gateless AlGaN/GaN HEMT Response to Block Co-Polymers The epi structure and processing sequence ha ve been described in detail previously [Meh04]. In brief, The HEMT structure wa s grown by metal organic chemical vapor deposition at 1040 C on c-plane Al2O3 substrates. The layer stru cture consisted of a low temperature GaN nucleation layer, a 3 m undoped GaN buffer and a 30 nm Al0.3Ga0.7N undoped layer. The sheet carrier density in the channel was ~81012 cm-2 with a 300 K electron mobility of 900 cm2/Vs. The ungated HEMT was fabricated using Cl2/Ar inductively coupled plasma etching for mesa isolation and lift-off of e-beam deposited Ti/Al/Pt/Au subsequently annealed at 850 C for 30 s to lower the Ohmic contact resistance. Silicon Nitride was used to encap sulate the source/drain regions, with only the gate region open to allow the polar liquids to reach across the surface. A schematic of the device is shown in Figure 5-1(top), plan vi ew layout with a cross-sectional view at the bottom of Figure 5-1. The source-drain curr ent-voltage characteristics were measured at

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77 25 C using an Agilent 4156C parameter analy zer with the gate regi on exposed either to air or various concentrations of the bl ock co-polymers and individual polymers Figure 5-1. Schematic layout of gate HEMT structure (top) and device cross-section (bottom). The block copolymers are composed di fferent proportions of the individual polymers, PS and PEO. The configuration a nd chemical symbols of the block copolymer are illustrated in Figure 5-2. The block co-pol ymers and individual polymers used in this work were dissolved in the benzyl alcohol.

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78 The charges in the two-dimensional (2 D) channel of AlGaN/GaN HEMTs are induced by spontaneous and piezo electric polarization, which are balanced with positive charges on the surface. Figure 5-2. Structure of block co-polymer, co mposed of different portions of PS and PEO (top) and chemical formula for PS and PEO (bottom). The induced sheet carrier concentrati on of undoped Ga-face AlGaN/GaN can be calculated by following equation [Asb97]: ) ( ) ( ) ( ) ( ) ( ) (2 0x E x E x e e d x e x x nC F b d s (5-1) where 0 is the electric permittivity, (x) = 9.5-0.5x is the relative permittivity, x is the Al mole fraction of AlxGa1-xN, dd is the AlGaN layer thickness, e b is the Schottky barrier of the gate contact on AlGaN (e b(x) = 0.84 + 1.3x(eV)), EF is the Fermi level (EF(x) =

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79 [9he2ns(x)/16 0 (x) (8m*(x))]2/3+h2ns(x)/4 m*(x)), Ec is the conduction band (Eg(x) = 6.13x+3.42(1-x)-x(1-x)(eV)), and Ec is the conduction band discontinuity between AlGN and GaN ( Ec(x) = 0.7[Eg(x)-Eg(0)]). Note that m*(x) is 0.228Therefore the sheet charge density in the 2D channel of AlGaN/GaN HEMT is extremely sensitive to its ambient. The adsorption of polar molecule s on the surface of GaN affects in the surface potential and device characteristics. As illust rated in Figure 5-3, nitride HEMT exposed to different ambients and drain I-V characteristics were affected significantly. At 40 V of drain bias voltage, drain current reduced fo r 25 and 50% for devices exposed to PE and PS solution, respectively. 010203040 0 2 4 6 8 10 IDS(mA)VDS(V) air PEO PS Figure 5-3. Drain I-V characteristics for the air, PS and PEO(weight concentration of the polymers are CPEO = 0.0387mg/ml and CPS = 0.3781mg/ml). Due to large gate dimension (20 150 m2) induced parasitic resistance, the drain current did not reach the saturation at 40. If the device dimension reduced, larger changes of drain current should be expected.

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80 010203040 0 2 4 6 8 10 IDS(mA)VDS(V) air solvent C4 C3 C2 C1 0.00.20.40.60.81.01.2 2.0 2.5 3.0 3.5 4.0 Current(mA)Concentration of R11(mg/mL) at 20V Figure 5-4. Drain I-V characteristics for th e different concentration of PS-PEO block copolymer (molecular weight: 58.8 kg/mol, 71% of PS and 29% PEO) and concentration of the copolymer solutions are following C1= 1.0917 mg/ml, C2 = 0.7612 mg/ml, C3 = 0.5504 mg/ml, C4 = 0.08734 mg/ml (top). The drain current reduction as a function of copolymer concentration (bottom).

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81 The dipole moments of et hylene and styrene monomer are 1.89 and 0.62, respectively [CRC97]. The dipole moment of ethylene monomer is three times larger than that of styrene monomer; however, the e ffect of PEO solutions on drain current of nitride HEMT is only half of the PS styrene. 010203040 0 2 4 6 8 10 IDS(mA)VDS(V) air R5 R11 Figure 5-5. Drain IV characteristics of copolymers with different composition(R5 : MW=66.6kg/mol, 53% of PS and 47% of PEO and R11 : MW 58.8 kg/mol, 71% of PS and 29% of PEO). This could be due to PEO is extremely lin ear and dipole is along the polymer chain. The 20 150 m2 gate opening is much larger than the individual monomer in the PEO chain. Therefore the dipole effect on the device is very locally within the big gate opening and some of the net surface charges induced by the PEO were cancelled out. In the case of PS, the polymer chain is very bul ky and not very linear the net net dipole induce charge may be higher than that of PE O, therefore, larger drain current changes were observed. If the gate dimension redu ces to the size of i ndividual monomer, the complete opposite results may be obtained.

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82 The concentration of the block copolymer also affected the changes of the drain current, as showed in Figure 5-4. The mol ecular weight of the copolymer is 58.8kg/mole and it contains 71% of PS and 29% PEO. The concentration of the copolymer was varied from 1.0917 mg/ml to 0.08734 mg/ml. As th e copolymer concentr ation increased, the drain current reduced more. The current reduc tion verses the copolymer concentration is not quite linear as showed in Figure 5-4 (botto m), this could be due to the high parasitic resistance of the devices and the saturation currents were not reached. Copolymer with similar molecular weight, but different com positions also had significant impact on the drain IV characteristics. As shown in Figure 5-5, the copolymer with large percentage of PS reduced more drain current, which was c onsistent with results in Figure 5-3. Gateless AlGaN/GaN HEMTs show a strong dependence of source/drain current on the polarity and concentration of polymer solu tions. This suggests the possibility of functionalizing the surface for application as biosensors, especially given the excellent stability of the GaN and AlGaN surfaces whic h should minimize degradation of adsorbed cells [Eic03]. 5.3 pH Measurements with Single ZnO Nanorod Integrated with a Microchannel The electrical response of ZnO nanorod surfaces to variations of the pH in electrolyte solutions introduced via an inte grated microchannel was analyzed. The ioninduced changes in surface pot ential are readily measured as a change in conductance of the single ZnO nanorods and suggest that thes e structures are very promising for a wide variety of sensor applications. The preparation of the nanorods has been de scribed in detail previously [Heo02] .In brief, discontinuous Au droplet s were used as the catalyst for ZnO nanorods growth and formed by annealing e-beam evaporated Au th in films (~100 ) on p-Si (100) wafers at

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83 700 C. ZnO nanorods were deposited by MBE with a base pressure of 5x10-8 mbar using high purity (99.9999%) Zn metal and an O3/O2 plasma discharge as the source chemicals. The Zn pressure was varied between 4x10-6 and 2x10-7 mbar, while the beam pressure of the O3/O2 mixture was varied between 5x10 -6 and 5x10-4 mbar The growth time was ~2 h at 400 ~ 600 C. The typical length of the resultant nanorods was 2 ~ 10 m, with typical diameters in the range of 30 – 150 nm. Selected area diffraction patterns showed the nanorods to be single-crystal. They were released from the substrate by sonication in ethanol and then transferred to SiO2-coated Si substrates. E-beam lithography was used to pattern sputtered Al/P t/Au electrodes contacting both ends of a single nanorod. The separation of the electrodes was ~3.7 m. Au wires were bonded to the contact pad for current –voltage (I-V) measurements to be performed .An integrated microchannel was made from SYLGARD@ 184 polymer from DO W CORNING. After mixing this silicone elastomer with curing ag ent using a weight ratio of 10 : 1 and mixing with for 5 min ,the solution was evacuated fo r 30 min to remove re sidual air bubbles. It was then applied to the alre ady etched Si wafer(channel length, 10-100um) in a cleaned and degreased container to make a molding pa ttern. Another vacuum de-airing for 5 min was used to remove air bubbles, followed by curing for 2 hours at 90 C. After taking the sample from the oven, the film was peeled from the bottom of the container. Entry and exit holes in the ends of th e channel were made with a small puncher(diameter less than 1mm) and the film immediately applied to the nanorod sensor. The pH solution was applied using a syringe auto pipette (220ul). A scanning electron micrograph of the completed device is shown in Figure 5-6.

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84 Prior to the pH measurements, we used pH 4, 7, 10 buffer solutions from Fisher Scientific to calibrate the electrode and the m easurements at 25 C were carried out in the dark or under UV illumination from 365 nm light using an Agilent 4156C parameter analyzer to avoid parasitic effects. The pH solution made by the titration method using HNO3, NaOH and distilled water. The electr ode was a conventional Acumet standard Ag/AgCl electrode. Figure 5-6. Schematic (top) and scanning el ectron micrograph (bottom) of ZnO nanorod with integrated microchannel (4m width)

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85 -0.4-0.20.00.20.4 -8.0x10-8-4.0x10-80.0 4.0x10-88.0x10-81.2x10-7 IDS(A)VDS(V) non UV UV(365nm) Figure 5-7. I-V characteristic s of ZnO nanorod after wire-bo nding, measured either with or without UV (365nm) illumination. Figure 5-7 shows the I-V characteristics from the single ZnO nanorod after wire bonding, both in the dark and under UV illumina tion. The nanorods show a very strong photoresponse. The conductivity is greatly increased as a resu lt of the illumination, as evidenced by the higher current. No effect was observed for illumination with below bandgap light. The photoconduction appears pr edominantly to originate in bulk conduction processes with only a minor surface trapping component [Kan05e]. The adsorption of polar molecules on th e surface of ZnO affects the surface potential and device characteristics. Figure 5-8( top) shows the current at a bias of 0.5V as a function of time from nanorods exposed fo r 60s to a series of solutions whose pH was varied from 2-12. The current is significantly reduced upon exposure to thes e polar liquids as the pH is increased. The corresponding nanorod conducta nce during exposure to the solutions is

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86 shown at the bottom of Figure 5-8.The data in Figure 3 shows that the HEMT sensor is sensitive to the concentration of the pola r liquid and therefore could be used to differentiate between liquids into which a sm all amount of leakage of another substance has occurred. 0100200300400500600 0.0 4.0x10-88.0x10-81.2x10-71.6x10-7 12 11 10 9 8 7 6 5 4 3 2 IDS(A)Time(sec) non UV UV(365nm) 0100200300400500600 0 50 100 150 200 250 300 12 11 10 9 8 7 6 5 4 3 2 Conductance(nS)Time(sec) non UV UV(365nm) Figure 5-8. Change in current (top) or conductance (bottom) with pH (from 2-12) at V = 0.5V.

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87 23456789101112 0 50 100 150 200 250 300 Conductance(nS)pH non UV UV(365nm) Figure 5-9. Relation between pH and conducta nce of ZnO nanorod either with or without UV (365nm) illumination. Figure 5-9 shows the conductance of the na norods in either the dark or during UV illumination at a bias of 0.5V for different pH values. The nanorods exhibit a linear change in conductance between pH 2-12 of 8.5nS/pH in the dark and 20nS/pH when illuminated with UV (365nm) light .The nanorod s show stable operation with a resolution of ~0.1 pH over the entire pH range ,showi ng the remarkable sensitivity of the HEMT to relatively small changes in c oncentration of the liquid. There is still much to understand about th e mechanism of the current reduction in relation to the adsorption of the polar liquid mo lecules on the ZnO surface. It is clear that these molecules are bonded by van der-Waals t ype interactions and that they screen surface change that is induced by polarization in the ZnO. Different chemicals are likely to exhibit degrees of interact ion with the ZnO surface. Steinhoff et. al. [Ste03a] found a linear response to changes in the pH ra nge 2-12 for ungated GaN-based transistor

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88 structures and suggested that the native metal oxide on the semiconductor surface is responsible. ZnO has the advantage that it is already a metal oxide and the nanorods can be produced cheaply and transferred to any substrate. In summary, ZnO nanorods show dramatic changes in conductance upon exposure to polar liquids. Bonding of polar liquid mo lecules appear to a lters the polarizationinduced positive surface change, leading to ch anges in the effective carrier density and hence the drain-source current in biased na norods. This suggests the possibility of functionalizing the surface for application as biosensors, especially given the excellent biocompatibility of the ZnO surfaces which should minimize degradation of adsorbed cells.

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89 CHAPTER 6 BIOSENSORS FOR BIOMATERIALS AND CELL GROWTH 6.1 Introduction Biologically modified field e ffect transistors (BioFETs), either at conventional or nano-dimensions, have the poten tial to directly detect biochemical interactions in aqueous solutions for a wide variety of bi osensing applications [Fri02, Ing05, Fen05, Kan05f, Ste05]. To enhance the practicality of bioFETs, the device must be sensitive to biochemical interactions on their surface, th e surface must be functionalized to probe specific biochemical interactions and the de vice must be stable in aqueous solutions across a range of p H and salt concentrations. The AlGaN/GaN high electron mobility transistors (HEMTs) are attractive for these applications, since it forms a high electron sheet carrier concentration channel induced by piezoelectric polarizat ion of the strained AlGaN layer [Amb02, Sch01, Eic01, Sch02a, Stu02, Ste03b, Ste05]. The conducting channel of AlGaN/GaN base d HEMT is very close to the surface (< 35 nm) and extremely sensitive to the ambient, which should enhance detection sensitivity. It has been shown that it is possible to distinguish liquids with different polarities and to quantitatively measure pH over a broad rang e [Ste03a]. Steinhoff et al. [Ste05]. recently reported the recording of cell action potentials from a layer of rat heart muscle cells cultivated directly on the ga te region of a AlGa N/GaN HEMT. The AlGaN surface is chemically inert in electrolyte solutions, leading to a low linear drift and has a sensitivity of ~57mV/pH ( cf SiO 2 with 32-40 mV/pH). In additi on there is the possibility

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90 of integration with blue nitr ide light-emitting diodes for directed cell growth [Yao04] and off-chip communications circuitry. Controlled pattern cell growths, which can be used to spatially control the development of cells and neurons may provide new approaches to the study of surfacedirected growth, intercellular communicat ion, and organogenesis. Self-assembled monolayer of organic functionalities has been used previously to pattern neuron cell growth [Spa94, Ste92, Spa89]. UV and blue li ght have been used in inhibiting plant cell growth and photo-catalyzed oxi dation reactions to kill micr o-organisms in the air and water [Blo97, Gos97, Ono98]. Recent developments in high intensity blue and UV light emitting diodes provide a potentially improve d light source for controlled environment plant growth applications such as in vitro micropropagation a nd biologically based advanced life support for space missions and ot her application for inhibiting cell growths [Kat90]. Blue LEDs, but not gr een or red LEDs, inhibited growth of retinal pigment epithelial cells, aortic endothelial cells and fibroblasts in vitro [O ha02]. A wavelength of 470 nm blue light was used to inhibit the gr owth of B16 Melanoma cells, which showed the potential for the application of UV radiation for cancer treatment. In this chapter, a detail study for the transi stor with chemically modified gate, i.e. Au deposited gate for halide ions, antigen attached for antibody will be given. In addition, selective cell growth method using UV LED also will be given. In Section 6.2, the method of detecting halide ions using Au deposited gate AlGaN/GaN HEMT will be analyzed. In Section 6.3, electrical detecti on of immobilized proteins with chemically modified gate on AlGaN/GaN HEMT will be i nvestigated. In Section 6.4 the effect of UV illumination for selective area patterned cell growth will be discussed.

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91 6.2 Detection of Halide Ions with AlGaN/GaN HEMTs The most common application for AlGaN/Ga N High Electron Mobility Transistors (HEMTs) is expected to be in broa d-band power amplifiers in base station applications [Tar02, Gre00, Mis02, Zha03c, Lee02a, Bi n02, Moo01, Wei03]. The high electron sheet carrier concentration of nitride HEMTs is induced by piezoelectric polarization of the strained AlGaN layer and spontaneous polariza tion is very large in wurtzite III-nitrides [Neu01, Amb02, Sch01]. This suggests that ni tride HEMTs are also excellent candidates for sensor and piezoelectric-related applica tions. Gated and gateless HEMT structures have demonstrated the ability to perform as combustion gas sensors, strain sensors and also chemical detectors [Sch02a, Stu02, Kan04b, Ste03b, Ste05, Hil96]. In most cases, the application of some extern al change in surface conditio ns changes the piezoelectricinduced carrier density in the channel of the HEMT, which in turn alters the drain-source or gate current. Several groups have shown the strong sensitivity of AlGaN/GaN heterostructures to ions, polar liquids, hydrogen gas and even biological materials [Kan04c, Pea04a, Ste03b, Ste05, Hil96]. In particular it has been shown that it is possible to distinguish liquid with different polarities and to quantitatively measure pH over a broad range [Ste03a]. The HEMT su rface is chemically in ert in electrolyte solutions, leading to a low linear drift and has a sensitivity of ~57mV/pH ( cf SiO2 with 32-40 mV/pH). Moreover there is the prospect of ready integration with blue nitride light-emitting diodes for directed cell growth and off-chip communications circuitry. In this chapter, we studied the effect of exposing the gate region to halide ions (NaF and NaCl) both with and without a thin Au layer present on the gate. Changes in the source-drain current in the tens of micr o-Amp range are observed relative to the value

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92 measured in air or water. The drain-sour ce current change observed is a logarithmic function of the concentration and reinfo rces the notion that AlGaN/GaN HEMT structures are very promising for a wide va riety of chemical sensor applications. Figure 6-1.Schematic of HEMT structure for de tection of halide ions (top) and plan view photomicrograph of completed device us ing a 20 nm Au film in the gate region (bottom). gate area exposed(Au:2nm) ohmic contact metal final metal SiNx 10 m

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93 0204060801001201.113 1.116 1.119 1.122 1.125 1.128 Current(mA)Time(sec) water 10 uM 100 uM 1 mM 10 mM 100 mM 1 M 0204060801001201.115 1.120 1.125 1.130 1.135 1.140 1.145 Current(mA)Time(sec) water 10 uM 100 uM 1 mM 10 mM 100 mM 1 M Figure 6-2.Time dependence of current ch ange in Au-gated HEMTs upon exposure to NaF (top) or NaCl (bottom) solutions w ith different concentration. Water was also used as a reference. In each case the aqueous solutions were introduced to the HEMT surface at ~20 secs.

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94 The HEMT structures consisted of a 3 m thick undoped GaN buffer, 30 thick Al0.3Ga0.7N spacer, 220 thick Si-doped Al0.3Ga0.7N cap layer. The epi-layers were grown by rf plasma-assisted Molecular Beam Epitaxy on the thick GaN buffers produced on sapphire substrates by metal organic chemical vapor deposition (MOCVD). Mesa isolation was performed with an Inductively Coupled Pl asma (ICP) etching with Cl2/Ar based discharges at –90 V dc self-bias, ICP power of 300 W at 2 MHz and a process pressure of 5 mTorr. 100 100 m2 Ohmic contacts separated with gaps of 10 m consisted of e-beam deposited Ti/Al/Pt/Au pa tterned by lift-off and annealed at 850 C, 45 sec under flowing N2. Silicon nitride was used to encapsulate the source/drain regions, with only the gate region open to allow the polar liquids to across the surface. In some cases, we deposited a thin (~2 nm) Au film on the gate region, to facilitate chemical binding of the different solutions we invest igated. A schematic cross-section of the device is shown at the top of Figure 6-1, wh ile a scanning electron micrograph plan view of the completed device using the Au film on th e gate is shown at the bottom of Figure 61. The source-drain current-voltage characte ristics were measured at 25C using an Agilent 4156C parameter analy zer with the gate region expos ed either to air, ~3mm2 of water, or 10M-1M solutions of NaF or Na Cl. Both DC and AC measurements were performed, with the latter a pproach used to prevent side electrochemical reactions. Figure 6-2 shows the time dependence of current change in Au-gated HEMTs upon exposure to NaF (top) or NaCl (bottom) solu tions with different c oncentration. Water was also used as a reference. The addition of the Au gate increased the detection sensitivity and the consistency for detection of the halide ions. The NaCl leads to the opposite polarity of current change relati ve to NaF due to the highly adsorbed Clions,

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95 which creates postitive image charge on the Au film for charge neutrality, induces a higher positive charge on the AlGaN surface, and finally increases the piezo-induced charge density in the HEMT channel. Past measurements on adsorption of halides onto Au surfaces have shown Clto be rapidly and strongly adsorbed, while Fis only weakly attached [Ign97, Bon99]. 1x10-51x10-410-310-210-11001.11 1.12 1.13 1.18 1.19 1.20 Current(mA)Concentration(M) gateless NaF gateless NaCl Au gate NaF Au gate NaCl Figure 6-3.Current change as a function of c oncentration for HEMTs with or without the Au gate Figure 6-3 shows the current change as a function of the concentration for HEMTs with or without the Au gate. The presence of the Au gate leads to a logarithmic dependence of current (measured at an appl ied bias of 500 mV) on the concentration for both NaCl and NaF. By sharp contrast, the gateless devices show lower changes, with no clear dependence on the concen tration, suggesting non-specifi c adsorption of the halide ions.

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96 Another advantage of the Au is that it has a chemical affinity with thiol, forming a self-assembled monolayer [Kim04, Chu01]. Long chain alkanethiols, adsorb onto the gold films and form ordered monolayer films due to specific inte raction of gold with sulfur group in thiol [Chu01]. 0204060801001.115 1.120 1.125 1.130 Water Air Current(mA)Time(sec) thiol modification w/o modification Figure 6-4.Change in current as a function of time as a Au-gated HEMT is exposed to water. In the case of thiol modification of the gate, no response was observed. In this study, 1-octadecanethiol (HS(CH2)17CH3 ODT) was used. The Au film coated with ODT is hydrophobic due to the hyd rophocity of the thiol. The ODT coated Au gated devices no longer showed a response to the addition of water to the gate region, indicating that the entire ga te area was totally covered w ith the hydrophobic thiols which blocked the adsorption of ions from water (Fig ure 6-4). This is an important first step towards the goal of using AlGaN/GaN HEMT s as fast, robust electrical-based bio sensors. Well established thiol chemistr y can be employed to immobilize various

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97 functional groups on the Au film, which can recognize chemicals and biomaterials such as DNA, protein, and virus [Zha02, Koh04, Lee02b, Smi03, Cui01]. Figure 6-5.AC measurement of change in curr ent in gateless HEMT exposed to water. The applied bias of 500 mV was modulated at 11 Hz. Finally to confirm that side electroche mical reactions were not playing a role during the DC measurements of current, we used AC measurem ents of current to confirm the changes observed during in troduction of the aqueous so lutions. Figure 6-5(left) shows an example for introduction of water in to the gate region, while the 500mV bias was modulated at 11Hz. The resulting peak cu rrent is plotted at th e right of Figure 6-5 and confirms the result obtained from the DC data. In conclusion, AlGaN/GaN ungated HEMT s show dramatic changes in drainsource current upon exposure to polar liquids in the gate region. Bonding of polar liquid molecules appear to alters the polarizatio n-induced positive surface change, leading to changes in the channel carrier density and hence the drain-source current. The results show the potential of AlGaN/GaN transistor structures for a variety of chemical and biological sensing applications. 0204060-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 Current(mA)Time(min) -505101520253035404550551.18 1.19 1.20 1.21 1.22 Water Air Current(mA)Time(sec) gateless

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98 6.3 Electrical Detection of Immobilized Proteins with Ungated AlGaN/GaN HEMTs The HEMT surface with receptors to sens e the binding of target molecules was functionalized in aqueous solutions. Conj ugation or hydridization events of solution molecules with the molecules attached to the gate region in the HEMT change the conductivity of the HEMT channel by changing the charge distribution in the attached molecules and consequently the charge on th e nitride surface. First we treat the AlGaN with aminopropyl silane (APS). The AlGaN fo rms a thin oxide layer when exposed to oxygen, which can be reacted with the A PS to functionalize the surface with amine groups. By covalently attaching the antigen biotin to the surface of the HMET gate region, the device can be made sensitive to the conjugation of the antibody protein streptavidin. The immobilization of the protein on these sites was detected as a change in HEMT drain-source current. The HEMT structures consisted of a 3 m thick undoped GaN buffer, 30 thick Al0.3Ga0.7N spacer, 220 thick Si-doped Al0.3Ga0.7N cap layer. The epi-layers were grown by rf plasma-assisted Molecular Beam Epitaxy on the thick GaN buffers produced on sapphire substrates by metal organic chemical vapor deposition (MOCVD). Mesa isolation was performed with an Inductively Coupled Pl asma (ICP) etching with Cl2/Ar based discharges at –90 V dc self-bias, ICP power of 300 W at 2 MHz and a process pressure of 5 mTorr. 100 100 m2 Ohmic contacts separated with gaps of 10 m consisted of e-beam deposited Ti/Al/Pt/Au pa tterned by lift-off and annealed at 850 C, 45 sec under flowing N2. Silicon nitride was used to encapsulate the source/drain regions, with only the gate region open to al low the liquid solutions to cross the surface. A schematic cross-section of the device is s hown at the bottom of Figure 6-6. The source-

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99 drain current-voltage charac teristics were measured at 25C using an Agilent 4156C parameter analyzer with the gate region expos ed. In addition, we used GaN epi films on sapphire substrates grown in a similar fashi on to the HEMT structures as templates for studying the attachment of the amine groups. Figure 6-6. Structure of APS (3-Aminopr opyl) triethoxysilane (top left), surface functionalization before chemical m odification (top right). A schematic diagram of the gateless HEMT whose surface is functionalized by chemical modification in the gate region (bottom). The GaN thin films on sapphire were treated with 1% APS (3-Aminopropyl) triethoxysilane in water for 24 hours to func tionalize the surface with amine groups and then washed with ethanol and water. The chem ical structure of the APS is shown at the top of Figure 6-6, along with a schematic of the functionalized surface. To confirm the modification worked, we first reacted th e amine surface with amine-reactive TMR GaN AlGaN gate region exposed ohmic contact SiNx 2DEG Sapphire

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100 (Tetramethylrhodamine) and Fluorescein [Dra04, Hic94]. There was a strong signal with repeating dark lines observed. However, th e GaN surface reflects enough light that the excitation laser light was reflected and read as emission, making accu rate characterization of the surface impossible. The excitation a nd emission of organic dye s are close together and we could not find filters sharp enough to block all the excitation. Figure 6-7. Fluorescent photogra phs of biotinylated probes on chemically treated GaN surface (left) and non-biotinated surface (right). To overcome the problem of GaN refl ectivity, the organometallic fluorophore Rubpy was selected because of its large St okes shift which separates excitation and emission. Since Rubpy does not have func tionalized versions, we used Rubpy-doped silica nanoparticles labeled with strept avidin to confirm the nitride surface functionalization. A control test was developed by labeling the fluorescent nanoparticles with the protein bovine serum albumin (BSA ), which as no affinity for biotin or biotinated surfaces. The amine-functiona lized GaN was further reacted with NHydroxysulfosuccinimidobiotin (sulfo-NHS-bioti n) and then blocked and exposed to the Rubpy nanoparticles. Sulfo-NHS-biotin enables simple and efficient biotin labeling of antibodies, proteins and any other primary amine-containing macromolecules in solution

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101 or on a surface. Specific labeling of cell su rface proteins is another common application for these uniquely water-soluble and membrane impermeable reagents. Biotin is a small naturally occurring vitamin that binds with hi gh affinity to avidin and streptavidin proteins. Because it is so small (244 Da), biotin can be conjugated to many proteins without altering their biologi cal activities. The differen ce in the emission of the biotinated GaN with streptavidin nanopart icles and biotinated GaN with BSA coated nanoparticles is large. Non-bi otinated surfaces showed almost no emission, as illustrated in Figure 6-7. Figure 6-8.SEM micrographs of amine-functi onalized GaN surfaces reacted with biotin NHS and then blocked and exposed to eith er BSA-coated (left) or streptavidin (right) nanoparticles. When the same area was viewed in the scanning electron microscope (SEM), the difference in the nanoparticle concentration was significant, as shown in Figure 6-8. There were still a large numb er of non-specifically bound na noparticles after washing. We attribute this to the adhesion of the ne gatively charged silica particles with the positively charged amine groups that did not re act with NHS-biotin. Nevertheless, the

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102 contrast in concentration is significant, so we believe that we are modifying the surface successfully. 0200400600800 1.732 1.736 1.740 1.744 IDS(mA)Time(sec) Figure 6-9. Change in HEMT drain-source current as a result of interaction between Biotin (Sulfo-NHS-Biotin) and streptavidine introduced to the gateless HEMT surface. To detect the immobilization of proteins on the surface of the HEMT, we used a similar surface modification procedure, w ith attachment of amine groups, followed by exposure to biotin solution and then streptavid in protein. Labeled pr oteins were purified from unlabeled proteins using immobilized stre ptavidin and avidin affinity gels After 24 hours of APS chemical modification by imme rsing the device into a 1% aqueous APS solution and washing with water and etha nol, the HEMT was dipped into a 5mg/2ml biotin solution at pH 7.4 (sodium phosphate buffer). After 65 seconds the source-drain current of the nitride HEMT decreased 5 A due to the bonding between the amine group and biotin. After waiting for 10 minutes the nitride HEMT was removed from the biotin

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103 solution, followed by a three times rinse with buffer solution and dipped into the 5mg/ml streptavidin solution around 645 sec. As shown in Figure 6-9, the HEMT showed another reduction (~4 A) in drain-source current, sugges ting that the protein has been immobilized on the functionalized surface, whic h explains that the current in the gate channel can be depleted by the interaction be tween biotin and streptavidin molecules on the gate area. We also found that current ch ange has not been detected for the HEMT without chemical modification on the gate su rface. This change in current is easily detectable and was reproducible upon cl eaning the surface and remeasuring. In summary, we have shown that thr ough a chemical modification sequence, the gate region of an AlGaN/GaN HEMT structure can be func tionalized for detection of streptavidin proteins. This el ectronic detection approach is a significant step towards integration with microfluidic channels sp ecially designed for th e detection of DNA hybridization. 6.4 Use of 370 nm Light for Sel ective Area Fibroblast Cell Growth There is a strong interest in controlled pa ttern cell growths, which can be used to spatially control the development of cells and neurons. Such methods may provide new approaches to the study of surface-directed growth, intercellular communication, and organogenesis or be used to control the ali gnment of individual ce lls with transducer elements in biosensors and implant. Self-assembled monolayers of organic functionalities have been used previously to pattern neur on cell growth [Spa94, Ste92, Spa89]. UV and blue light have been used in inhibiting plant cell growth and photocatalyzed oxidation reactions to kill micro-or ganisms in the air and water [Blo97, Gos97, Ono98]. Recent developments in high intensity blue and UV light emitting diodes provide

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104 a potentially improved light source for controll ed environment plant growth applications such as in vitro micropropagation and biologi cally based advanced life support for space missions and other application for inhibiting cell growths [K at90]. Blue LEDs, but not green or red LEDs, inhibited gr owth of retinal pigment epith elial cells, aortic endothelial cells and fibroblasts in vitro [Oha02]. A wa velength of 470 nm blue light was used to inhibit the growth of B16 Melanoma cells, whic h showed the potential for the application of UV radiation for cancer treatment. Figure 6-10. Patterns formed on glass slides coated with 50 nm TiO2. Patterns were generated by etching off TiO2 in the patterns; squares with the dimension of 100 100 m2 and circles with diameter of 20 and 40 m. In this chapter, we present a novel tec hnique for selective-area patterned cell growth using UV illumination combining with a TiO2 thin film based reflection coating. The effects of UV dose and different growth su bstrates on the fibroblast cell growth were investigated. Simulations of the reflected 370 nm UV light from the glass slice or TiO2 coated glass slice were performed. Simulation of more complicated TiO2/SiO2 mirror stacks was also carried out to obtain higher UV light reflectivity to provide even higher

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105 resolution patterned growth and to lower the intensity of the light needed to initiate growth. 300350400450500 0 20 40 60 80 Relative power intensityWave length(nm) -101234 0 10 20 30 40 50 Current(mA)Voltage(V) Figure 6-11. Optical spectrum of a GaN light emitting diode at an operating current of 4 mA (top). The current-voltage characteristics of the GaN based LED (bottom).

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106 The TiO2 film growth on glass slices was pe rformed in a reactive radio frequency (RF) magnetron sputter deposition system equipped with a load-lock for substrate exchange. A quartz lamp heater provided for substrate heating up to 750C. Two-inch diameter Ti sputtering targets were used The target-to-substrate distance was approximately 15 cm. The base pressure of th e deposition system was on the order of 5 10-8 Torr. The substrates were cleaned in tric hloroethylene, acetone, and ethanol prior to mounting on the sample platen with Ag pain t. The electronic grade oxygen gas was provided through a mass flow control valve. The structure of the film was determined using x-ray diffraction vi a a Cu K-alpha source. A 55 nm thick blanket layer of TiO2 was deposited on glass slides for all the TiO2 coated samples. Circular and square patterns on the TiO2 coated samples were generated with standard photolithogra phy and etched with Ar/Cl2 based discharges in an inductively coupled plasma system using resist as the et ch mask. As shown in Figure 6-10, the TiO2 was readily etched off and created faithful repl ication of the circular and square patterns from the mask; square with the dimension of 100 100 m2 and circles with diameter of 40 and 20 m. Prior to the cell growth, the glass s lides with patterned and without TiO2 films were cleaned and sterilized with th e following procedure: washed with DI water, sprayed with acetone for 3 mins under a pressure of 40 psi, exposed to O2 plasma in a Techtronic parallel plate reactive ion etch ing system for 1 min to rem ove the carbon residue, dipped into a 5% HCl solution for 1 min, rinsed with DI water, ultrasonic cleaned for 10 min. in DI water and sterili zation of the samples at 300 C for 2 hours.

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107 0.00.20.40.60.81.0 0 20 40 60 80 100 120 glass TiO2/glass Cell Number (% of Control)Power Density of UV Illumination (mW/cm2) Figure 6-12. Effect of UV light on growth of Fibroblast cells on glass slices and TiO2 coated glass slides Ledtronics high bright GaN based light emitting diodes (LEDs) were used as the UV exposure source during some of the cell grow ths. Six of the LEDs were packaged in a Teflon disk spaced 1 cm from each other. The LEDs were elec trically connected together and clamped with a 500 serial resistor to limit the output power. An HP5550 power supply was used to bias the diodes. The spectral distribution and current-voltage characteristics of the UV LEDs are shown in Figure 6-11. The UV spectral distribution of the LED was measured with a Verity optical emission spectrometer and the LED peak emission wavelength of 382 nm was obtained. The LED turn-on voltage was 1.2 V and was biased at 3.6 V during some of the cell growths. The power densities of the LEDs were measured with Newport 840-C optical power meter. The Chinese hamster lung fibroblast V79 ce lls (CCL-93) from the American Type Culture Collection (ATCC Rockville, MD) we re used as the test bed for these experiments. The cells were cultured in Du lbecco's Modified Eagle's Medium (DMEM;

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108 Gibco BRL, Grand Island, NY) supplemented w ith 5% fetal bovine serum(FBS, Sigma) and 1% of penicillin (Sigma), in a humidified chamber of 5% CO2 and 95% air at 36.5 oC. After 48 hours incubation, the DMSO enzyme was injected to make the cell float from the bottom of the flask. The cell susp ension was centrifuged for 5 min in 300 earth gravity force for the separation. 0.125 mL of cell suspension was taken out to count cell density with a hemocytometer. 300350400450500 0 20 40 60 80 100 glass TiO2/glass ReflectanceWavelength (nm) Figure 6-13. Simulated reflectivity of 370 nm UV light with different substrates The effect of UV light on the fibroblast ce ll growth is shown in Figure 6-12. For this set of experiments, the cells were gr own on petri dishes under constant UV exposure at different power densities and the dishes were not coated with TiO2. Adjustment of the power density was achieved by moving the dish away from the LED source and the UV power density for each location of the dish were measured with Newport 840-C optical power meter. The number of cells for the UV exposed samples was expressed as a

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109 percentage of the number of cells in the dark control in Figure 6-12. The fibroblast cell growth in blue light was signi ficantly reduced as the light intensity increased. The cell was barely grown for the power > 0.8 mW/cm2 of UV exposure in this set of experiments. The effect of UV illumination for the cell growth on the glass slides with TiO2 coating is also shown in Figure 6-12. In this experiment, glass s lides coated with 55 nm TiO2 were placed in petri dishes along with glass slices without TiO2 coating. The reference samples without the TiO2 coating showed the same trend as the first experiment, as illustrated in Figure 6-12 for the control slices. However, the samples with TiO2 coating showed much higher percentages of cell growth under the same UV exposures. We attribute this phenomenon to the fact that most of the UV light shining on the TiO2 coated sample is actually reflected back due to the refractive index difference between TiO2 and glass. As illustrated in Figure 6-13, we performed simulations of the reflectivities of 382 nm UV light from both a glass slide and a glass slide coated with 55 nm TiO2 using the TFcal simulator. For the gl ass slide, there was only around 8% of the 382 nm light reflected while there was 68% of the 382 nm UV light reflected from the TiO2 coated glass slide. Therefore the fibr oblast cells could grow on the TiO2/glass substrates under the conditions of UV exposure, while they w ould not grow on the bare glass slide. We used this TiO2 coating technique to control the cell growth patterns. As shown in Figure 6-14 (top), a patterned TiO2/glass substrate with 100 100 m2 squares and circles with diameters of 40 and 20 m was used for cell growth. No cell growth was observed on the glass (square and circular dark areas), which received the higher dose of UV illumination. The cell grew well on the top of TiO2 (gray area), which reflected most of the UV light.

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110 As is clearly shown in Figure 6-14 (top), extr emely sharp edge definitions were obtained down to < 4 m. The gaps between squares are 16, 8, 4 and 2 m. With this TiO2 coating on glass technique, the cells can be controllably grown on the desired areas or specific patterns. Same substrate without the UV light exposure th e cells grew well on both glass and TiO2 surface, as illustrated in Figure 6-14 (bottom). Figure 6-14. Fibroblast cell growth on patterned TiO2 coated glass slide with the UV light illumination (top) and without UV light illumination (bottom). We have also confirmed the patterned growth under UV illumination with a set of four patterned TiO2 coated substrates, as illustrated in Figure 6-15. Four glass slices

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111 coated with patterned TiO2 were placed in a petri dish The LEDs are 1 cm above the petri dish. The estimated power density of the LEDs for the glass slice 1, 2, 3 and 4 are 2, 0.8, 0.8, and 0.05 mW/cm2. There was no cell growth on slice 1 for both TiO2 and glass area due to high dose of UV illumination. For Slice, almost no e ffect of UV was observed on the cell growth and the cells grew on both TiO2 and glass area. The cell growths on the slice 2 and 3 were very similar to the patterned growth as illustrated in Figure 6-15. The cells were selectively grown on the TiO2 coated area but not on the gl ass area. With, this TiO2 coating on glass technique, the cells can be controllably grown on the desired areas or specific patterns. Figure 6-15. A set-up of fibr oblast cell on pa tterned TiO2 coated glass slides under different UV intensity illumination. TiO2/SiO2 dielectric stacks have been widely used in 850 nm vertical cavity surface emitting lasers(VCSELs). The cavity length of the VCSEL is much shorter that of ridge

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112 waveguide lasers. Therefore the reflectivitie s of mirrors for VCSEL have to be better than 99.5%. As shown in Figure 6-13, there is a reflectivity of 68% for 55 nm of TiO2 coating used in our cell growth experime nt, which is not optimized. If the TiO2/SiO2 dielectric stacks are used in the experiment the effect of the UV light on the cell growth will be further reduced. With 5 TiO2/SiO2 dielectric stacks, the reflectivity can reach 98% and as shown in Figure 6-16, a reflect ivity of >99% can be obtained with 6 TiO2/SiO2 dielectric stacks. 200300400500600 0 20 40 60 80 100 ReflectanceWave length(nm) Figure 6-16. Simulated reflectivity of 370 nm UV light with six TiO2/SiO2 dielectric mirror stacks We have studied the effect of 370 nm UV light on fibroblast cell growth. The UV light inhibited the cell growth By employing a thin TiO2 film on the glass substrate for cell growth, most of the UV light was reflec ted from the substrate and allowed the cells to grow. Combining the techniques of TiO2 coating, standard photolithographic patterning and etching of TiO2, produces a novel method for selective area cell growth.

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113 A 20 m resolution of pattern cell growth was achieved. With optimization of TiO2 thickness and number of TiO2/SiO2 stacks, better resolution and less intensity of UV light for selective area cell growth can be expected.

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114 CHAPTER 7 SUMMARY AND FUTURE WORKS 7.1 Pressure Sensor AlGaN/GaN HEMTs grown on (0001) sapphire were studied in terms of the changes in the conductance of two dimensi onal electron gas (2DEG) with changes in external strain. By using a two terminal non mesa device on a sapphire beam, a simple beam bending experiment confirmed the predicted strain dependence of the channel conductance. The changes in the co nductance of th e channel of Al0.25Ga0.75N/GaN HEMTs were roughly linear ove r the range up to 1.4 x 107 Nm-2, with coefficients for planar devices of + 9.8 x 10-12 S N–1 m2 for tensile stress and – 1.05 x 10 -11 S N-1 m2 for compressive stress. For mesa-isolated structures the coefficients were smaller due to the reduced effect of the AlGaN laye r strain, with values of –1.2 x10 –12 S N-1 m2 for tensile stress and + 1.97 x10 –12 S N-1 m2 for compressive stress. A Micro membrane pressure sensor used to monitor the differential pressure was demonstrated with a circular membrane of AlGaN/GaN on a (111) Si substrate by etching a circular hole in the Si subs trate. The conductivity of the channel was measured in terms of the changes in the conductance between two inter digit contact fingers with changes in pressure. The dc current-voltage characteristics were measured at 25oC under either vacuum (10 m Torr) or pressurized (200 ps i) conditions. For the application of compressive (tensile) stress, the changes in the conductance of the channel showed a linear change of -(+)7.1 10-2 mS/bar (7.1 10-10 S N-1 m2), which was two orders of magnitude larger than that of can tilever beam pressure sensors.

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115 The circular AlGaN/GaN diaphragms were fabricated with radii of 200 – 600 m on Si substrates. The changes in capacitance ove r a range of applied pressure were linear and were reversed when vacuum is applie d to the diaphragm. The capacitance of the channel displayed a change of 7.190.4510-3 pF/m as a function of the radius of the membrane at fixed pressure of 9.5 bar a nd exhibited a linear characteristic response between -0.5 and +1 bar with a sensitivity of 0.86 pF/bar for a 600 m radius membrane. The hysteresis was 0.4% in the linear range. Future efforts in this area should involve different gas environments in the pressure sensor system. The catalytic Schottky gate between the inter dig it fingered source and drain pressure sensor will make the previ ous pressure sensor wo rk in different gas ambients. Additional improvements in AlGaN/ GaN material growth, device design, and processing techniques may extend the se nsitivity range well beyond this. 7.2 Gas Sensor The characteristics of Sc2O3/AlGaN/GaN metal-oxide semiconductor diodes as hydrogen gas sensors were demonstrated. At 25 C, a change in forward current of ~ 6mA at a bias of 2V was obtained in respon se to a change in ambient from pure N2 to 10% H2/ 90% N2. This is approximately double the change in forward current obtained in Pt/GaN Schottky diodes measured under the same cond itions .The mechanism of the change in forward gate current appears to be formation of a dipole layer at th e oxide/GaN interface that screens some of the piezo-induced ch annel charge. The MOS-diode response time was limited by the mass transport of gas into the test chamber and not by the diffusion of atomic hydrogen through the metal/oxide stack. Pt contacted AlGaN/ GaN HEMTs with Sc2O3 gate dielectrics showed reversible changes in drain-source current upon exposure to H2-containing ambients, even at room

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116 temperature. The changes in current (as high as 3mA for relatively low gate voltage and drain-source voltage) were approx imately an order of magnitude larger than for Pt /GaN Schottky diodes and a factor of 5 larger than Sc2O3/AlGaN/GaN metal-oxide semiconductor(MOS) diodes exposed under th e same conditions. This showed the advantage of using a transistor structure in which the gain produces larger current changes upon exposure to hydrogen-containing am bients. The increase in current was the result of a decrease in eff ective barrier height of the MO S gate of 30-50mV at 25C for 10%H2/90%N2 ambients relative to pure N2 and is due to catalyti c dissociation of the H2 on the Pt contact, followed by diffusion to the Sc2O3 /AlGaN interface. The characteristics of Sc2O3/AlGaN/GaN metal-oxide semiconductor (MOS) diodes and Pt/ZnO Schottky diodes as detectors of C2H4 were analyzed. At 25C, a change in forward current of ~ 40 A at a bias of 2.5V was obtained in response to a change in ambient from pure N2 to 10% C2H4/ 90% N2. The current changes were almost linearly proportional to the testing temperature and r eached around 400 A at 400 C. The mechanism of the change in forward gate current appears to be formation of a dipole layer at the oxide/AlGaN inte rface that screens some of the piezo-induced channel charge at the AlGaN/GaN interface. The ZnO diodes showed no detectable change in current when exposed to ethylene at 25C but exhib ited large changes (up to 10 m A) at higher temperatures. In these diodes the detecti on mechanism appeared to also involve introduction of hydrogen donors into the near-s urface region of the ZnO, increasing the effective doping level unde r the rectifying contact. ZnO nanorods grown by site selective Molecular Beam Epitaxy (MBE) showed current-voltage characteristics that were sens itive to the presence of hydrogen or ozone in

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117 the measurement ambient for temperatures as low as ~112 C for H2 or room temperature for O3. The sensitivity to hydrogen increased sh arply with temperature, and multiple nanorods contacted at both ends by ohmi c electrodes showed currents of ~10-8A at 200 C and a differential current change of ~18% when changing from a pure N2 ambient to 10% H2 in N2. The nanorods were able to detect sm all concentrations (3% by flow) of O3 in N2, with changes in current of ~10-7 A at 25C.The sensitivity was 18% for O3 at room temperature. All devices fabricated for this study employed Pt/Au or Pd/Au Schottky gate metallization. Different Schottky metals can be tried for detecting different gases including hydrogen with different sensitivities. Multiple transistors with different gate metals in one array gas sensor platform ma y be an extremely sensitive monitor for a mixture of different gases in ambient condi tion. The device in the gas sensor system connected with Au wire bond c ould not stand more than 700oC in the heating furnace. For high temperature measurement, extra de velopment in design and fabrication of a ceramic or quartz based device package will be needed. 7.3 Chemical Sensor Gateless AlGaN/GaN HEMT structures exhi bited large changes in source-drain current upon exposing the gate region to vari ous block co-polymer solutions. The polar nature of some of these polymer chains lead s to a change of surf ace charges in the gate region on the HEMT, producing a change in surface potential at the semiconductor/liquid interface. Different gate modulation in Al GaN/GaN HEMT was exhibited according to the different kinds, concentration and structur es of polymers based on solvent and air. Single ZnO nanorods with ohmic contacts at either end exhibited large changes in current upon exposing the surface region to polar liquids introduced through an integrated

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118 microchannel .The polar nature of the electr olyte introduced led to a change of surface charges on the nanorod, producing a change in surface potential at the semiconductor /liquid interface. The nanorods exhibited a linear change in conductance between pH 212 of 8.5nS/pH in the dark and 20nS/pH when illuminated with UV (365nm) light .The nanorods showed stable operation with a resolution of ~0.1 pH over the entire pH range. Compared with the gas sensor, which e xhibited sharp reproducible response without memory effects, gateless AlGaN/ GaN HEMT for polar liquid needed some settling time to remove residual effect for exact data acquisition. As shown in Chapter 5, the laminar flowed buffer solutions (pH 212) could be easily e xposed upon the nanorod surface in tiny amounts through micro channe ls using a syringe pump without any memory effects. Future works relating to exposing polar liquids on the gate area should involve micro channels, but PDMS adhesion on the device and sealing at the end of microchannel still should be improved for bette r performance of micro chemical sensing devices. Even though the single ZnO nanorod de vice had higher sensitivity due to its large surface to volume ratio, it required a number fabrication steps to be ready for testing. Therefore, pre-aligned multiple ZnO nanorods grown in the MBE by fixing the initial catalyst position may be more favorable for easy fabrication and multiple productions in the future. 7.4 Bio Sensor AlGaN/GaN HEMTs both with and without an Au gate were found to exhibit significant changes in channel conductance upon exposing the gate region to various halide ions. The polar nature of the halide ions lead to a chan ge in surface charge in the gate region of the HEMT, producing a change in surface potential at the semiconductor/liquid interface. HEMTs with Au-gate electrode not only doubled the

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119 sensitivity of the change in channel conducta nce as compared to gateless HEMT, but also showed the opposite conductance behavior. When anions adsorbed on the Au, they produced a counter charge for electrovalence. These anions dragged some counter ions from the bulk solution or created an image positive charge on the metal for the required neutrality. The HEMTs can be used as sensors for a range of chemicals through appropriate modification with thiol chemistry on the Au surface. Ungated AlGaN/GaN structures were f unctionalized in the gate region with aminopropyl silane. This served as a binding layer to the AlGaN surface for attachment of fluorescent biological pr obes. Fluorescence microscopy showed that the chemical treatment creates sites for specific absorption of probes. Biotin was then added to the functionalized surface to bind with high affin ity to streptavidin proteins. The HEMT drain-source current showed a clear decrease of 4 A as this protein was introduced to the surface, showing the promise of this all-el ectronic detection approach for biological sensing The effect of 370 nm UV light on fibroblast cell growth was exhibited. At this wavelength, the UV light produced a strong in hibition of the cell growth. By employing a thin TiO2 film on the glass template for cell growth, most of UV light was reflected from the substrate and allowed the cells to grow. The TiO2 thin film could be patterned with standard photolithography followed by et ching. Employing this technique, a 4 m resolution of patterned cell growth was achieved. Based on the simulation results of using TiO2/SiO2 mirror stacks, 99.5% of UV light can be reflected from the TiO2/SiO2 coated area and even higher resolution and a lower intensity of UV light for selective area cell growth can be achieved.

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120 Some studies of cell adhesion on GaN a nd AlN are underway. With a coating of fibronectin below the cells, bio cells can be adhered very well to the III nitride surface and survive in a culture medium for a couple of days. So far very little is known about the interaction of living cell tissue with the III ni tride surface, and a systematic investigation of this topic is being performed currently. Ev entually, it will be necessary to integrate a lipid bilayer membrane onto the surface of the gateless HEMT devices in order to measure single ion currents associated with high sensitivity ion channel detection.

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121 APPENDIX A STRAIN CALCULATION OF A CI RCULAR MEMBRANE OF ALGAN/GAN UNDER DIFFERENTIAL PRESSURE The proposed pressure sensors will be made of a circular memb rane of AlGaN/GaN on a SiC substrate by etching a circular hol e in the substrate. A deflection of the membrane away from the substrate due to di fferential pressure on the two sides of the membrane produces a tensile strain in th e membrane. The differential piezoelectric response of AlGaN and GaN laye rs creates a space charge which induces 2DEG at the AlGaN/GaN interface. The concentration of 2DEG is expected to be directly correlated with the tensile strain in the membrane and hence with the differential pressure. A high electron mobility transistor at the center of the membrane senses the 2DEG such that a change in the conductance of th e transistor measures the differential pressure. To analyze the transducer response, we need to calcul ate the piezoelectric i nduced polarization in each film by taking into acc ount of all the stresses in the AlGaN/GaN structure. A.1 Strains in the Films The radial strain is given by r = (S-D)/D = (2 R – 2Rsin )/(2Rsin ) = /sin 1 (A-1) and the corresponding stress is r = [EGaN/ (1)] r (A-2) where EGaN is the Youngs modulus and is the Poisson’s ratio of the GaN film. The total tensile force around the edge of the circular membrane is, T = DtGaNr = D tGaN [EGaN/ (1)] r (A-3)

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122 where tGaN is the film of thickness. The compone nt of T along z direction is balance by the force on the membrane due to a differential pressure Pi P0, where Pi and P0 are the inside and outside pressure respectively. Hence Tsin = (Pi P0 ) D2/4 or DtGaN [EGaN/ (1)] ( sin ) = (Pi P0 ) D2/4 (A-4) which gives ( sin ) = (Pi P0 )[(1)D]/(4EGaNtGaN) (A-5) Equations (A.1) and (A.5) gi ves the relationship between the radial strain in the film r and the differential pressure Pi P0. The total stress in a film can be separated into two parts, i.e. = m + a (A-6) where m is the misfit stress from the adjacent layers and a is the applied stress due to bending of the structure. The corre sponding strains in each layer is = m + a (A-7) These elastic strains lead to the piezoele ctric induced polarization in the layers. A difference in the polarizations of the adjacen t layers results a space charge, which induces a 2DEG at the AlGaN/GaN interface. To calcula te the strain in each layer, we start with the misfit strain components. Since the misfit strain in an epitaxial layer is calculated against its relaxed lattice constant, if a(x) is the in-plane lattice constant of AlxGa1-xN, the misfit strain is given by (x) = [a(x) a0(x)]/a0(x) (A-8) where, a0(x) = 3.189 – 0.077x () is the theoretica l relaxed lattice cons tant. Note that (x) > 0, the misfit strain in AlGaN is always tensile. Therefore the misfit strain in GaN is compressive. From the force balance equation between the two layers,

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123 EGaN| 2|tGaN = EAlGaN| (x)|tAlGaN (A-9) where EGaN and EAlGaN are the Youngs modulus of the f ilms, the compressive strain in GaN is given by 1 = (EAlGaNtAlGaN/ EGaNtGaN) (x) (A-10) The applied strain in a circular thin f ilm membrane deflected by a differential pressure from the two sides of the membrane is given by Equations (A-1) and (A-5), i.e. r = /sin – 1, and ( sin ) = (Pi P0 )[(1)D]/(4EGaNtGaN) (A-11) Here we only consider GaN as the membrane, we will refine the calculation later to include the relatively thin AlGaN layer. For small deflection of the membrane where < 15 , the following approximation can be used, sin 3/3! (A-12) Substituting Equation (A-12) into Equation (A-11), we have = [3(1)D(Pi P0 )/(2EGaNtGaN)]1/3, r = [3(1)D(Pi P0 )/(2EGaNtGaN)]1/3 / sin [3(1)D(Pi P0 )/(2EGaNtGaN)]1/3– 1 (A-13) On the other hand, if we can m easure the deflection of the film Equation (A-11) calculates the strain r, which in turn gives the differential pressure. A.2 Electrical Polarization and Piezoelectric Effects The total electric polarization P in AlxGa1-xN and GaN consist of the spontaneous polarization PSP and the piezoelectric polarization PPE, i.e., PAlGaN = PSP (AlGaN) + PPE (AlGaN) = | PSP (AlGaN) | |eeff (AlGaN)|[ (x) + r] (A.14) where, eeff = e31 e33 C13/ C33, and PGaN = PSP (GaN) + PPE (GaN) = |PSP(GaN)| |eeff(GaN)|[-(EAlGaNtAlGaN/EGaNtGaN) (x)+ r] (A-15)

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124 respectively. The difference in the electric polarization is then, P = PAlGaN PGaN = |PSP(AlGaN)| |eeff(AlGaN)|[ (x)+ r] + |PSP(GaN)| + |eeff(GaN)| [-(EAlGaNtAlGaN/ EGaNtGaN) (x) + r] (A-16) Since the AlGaN/GaN interface usually c ontains misfit dislocations, the misfit strain in AlGaN is partially relaxed. To take it into account, (x) can be expressed as [a0(x) – aGaN] rAlGaN /a0(x) = AlGaN rAlGaN, where rAlGaN is the fraction of un-relaxed mismatch in AlxGa1-xN layer. To simplify Equation (A-16), we have, P = | PSP| |eeff| AlGaNrAlGaN | eeff| r, (A-17) where, | PSP| = | PSP (AlGaN) | | PSP (GaN) | where | PSP (AlGaN) | > | PSP (GaN) |, |eeff | = |eeff (AlGaN) | + |eeff (GaN) | (EAlGaNtAlGaN/ EGaNtGaN), | eeff | = |eeff (AlGaN) | |eeff (GaN) |. A.3 2DEG and Conductance of HEMT The corresponding 2DEG concentration at the AlxGa1-xN/GaN interface is given by ns(x) = P/e [ 0 (x)/tAlGaN e2][e b(x) + EF(x) – Ec(x)] (A-18) where 0 is the electric permitivity, (x) = 9.5 – 0.5x is the relative permitivity, e b(x) = 0.84 + 1.3x (eV) is the Schottky barrier height, EF(x) = [9 he2ns(x)/16 0 (x) (8m*(x))]2/3 + h2ns(x)/4 m*(x) is the Fermi levels, and Ec = 0.7[Eg(x) – Eg (0)|, where Eg (x) = 6.13x+3.42(1-x)x(1-x) (eV). Note that m*(x) ~ 0.228me. Since EF(x) is a function of ns(x), to further simplify equation (A-13), we can drop the 1st term in EF(x) for less than 5% error in ns(x), then we have ns(x) = {1/[1 +( 0 (x)/tAlGaNe2)h2/4 m*(x)]}(1/e){| PSP| |eeff | AlGaN rAlGaN [ 0 (x)/tAlGaNe][e b(x) – Ec(x)] | eeff | r } (A-19) The conductance of the device is given by = e sns, where s is the mobility of 2DEG. Substituting equation (A.15) for ns, we obtain

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125 = ( rAlGaN) + ( fGaN ) r, where (A-20) ( rAlGaN) = s{1/[1 + ( 0 (x)/tAlGaN e2)h2/4 m*(x)]}{| PSP| –|eeff | AlGaN rAlGaN – [ 0 (x)/tAlGaN e] [e b(x) – Ec(x)]} (A-21) and ( fGaN ) = s{1/[1 + ( 0 (x)/tAlGaN e2 )h2/4 m*(x)]}{ |eeff (GaN)| |eeff (AlGaN)| } (A-22) Finally, this device involves strain in the radial direction, the difference in the effective piezoelectric coefficients between Ga N and AlGaN in the radi al direction can be different from using the published values. Howe ver, if the mobility of 2DEG is know, the difference in effective piezoelectric coeffici ents between GaN and AlGaN in the radial direction can be calculated from the measured conductance vs differential pressure of the devices.

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138 BIOGRAPHICAL SKETCH Byoung Sam Kang was born on August 2nd, 1971, in Gwang Ju, South Korea. He graduated with a bachelor’s de gree in Chemical Engineering from Po Hang Institute of Science and Technology (POTECH) in Decem ber 1995 and then he earned Master of Science in Chemical Engineeri ng at POSTECH in December 1997. From 1997 to 2002, he worked at fuel cell research group at Korea Electric Power Research Institute (KEPRI) in Daejeon, South Korea, as technical staff for fuel cell system design and analysis. On November 28th, 1998, he married his wife, Jinah and they had a beautiful daughter named Dahyoung on January 23rd, 2001. In August 2002, he enrolled at the Univer sity of Florida in the Department of Chemical Engineering. From the spring of 2003, he has pursued a PhD degree under the guidance of Dr. Fan Ren. His main research involved various sens ors using piezoelectric polarization based wide bandgap electronic devices. He is co -author of approximately 50 journal and conference papers dealing with compound semi conductor sensing devices. He graduated from the University of Fl orida with a doctoral degree in Chemical Engineering in December 2005.


Permanent Link: http://ufdc.ufl.edu/UFE0012990/00001

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Title: Fabrication and Characterization of Compound Semiconductor Sensors for Pressure, Gas, Chemical, and Biomaterial Sensing
Physical Description: Mixed Material
Copyright Date: 2008

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Source Institution: University of Florida
Holding Location: University of Florida
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Permanent Link: http://ufdc.ufl.edu/UFE0012990/00001

Material Information

Title: Fabrication and Characterization of Compound Semiconductor Sensors for Pressure, Gas, Chemical, and Biomaterial Sensing
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
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FABRICATION AND CHARACTERIZATION OF COMPOUND SEMICONDUCTOR
SENSORS FOR PRESSURE, GAS, CHEMICAL, AND BIOMATERIAL SENSING


















By

YOUNG SAM KANG


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


2005

































Copyright 2005

by

Byoung Sam Kang

































To our LORD, Jesus Christ















ACKNOWLEDGMENTS

I would like to thank God, our Father, for all of the graces He gave me while I was

studying here in Gainesville. The past years have been among the most crucial years in

my life, because He led me to meet invaluable people I could not see in other places, and

they served as Barnabas changing Gainesville into Gilgal. I am sorry I can not list all of

them here.

First of all, I really appreciate my supervisory committee chair, Dr. Fan Ren for

technical and financial support throughout this study. He generously gave me lots of

heartwarming advice professionally and also personally. He willingly guided me with

better ideas when I had difficulty in solving the problems. I am also indebted to my other

supervisory committee members: Steve Pearton, David Norton, and Jason Weaver. Their

significant contributions to this work are greatly appreciated. During collaboration with

Dr. Pearton's and Dr. Norton's groups, I had the opportunity to develop interpersonal

skills, emphasize efforts, and pursue goals. I am honored to have been associated with

such an eminent committee. I would also like to thank Dr. S.N.G. Chu (Multiplex Inc.

South Plainfield, New Jersey) for helping me with stress analysis of the hetero interfaces.

Many thanks go to my colleagues and coworkers in the Department of Chemical

Engineering: Jeff LaRoche, Ben Luo, Suku Kim, Jihyun Kim, Rishabh Mehandru,

Soohwan Jang, Hungta Wang, Traivis Anderson, Jau Jiun. Special thanks go to Kwang

Hyun Baik, Rohit, Sang Yoon Han, and Huck Soo Yang in Materials Science and

Engineering department for their various discussion and helping in dry etch. System









maintenance and good friendship of Brent Gila are gratefully acknowledged. I had many

happy discussions with him about work. His optimistic attitude could make him a good

and kind professor someday soon.

I thank my family for their love and support. I especially thank our parents, I really

appreciate the enormous love they have shown me. I hope I can be as good a parent for

my daughter as they have been. I hope this dissertation, in some small way, repays them

for their love which I can never forget. I thank my wife, Jinah, who helps me to

concentrate this work by taking care of all other works including our daughter,

Dahyoung. This dissertation would not easy without their consistent supports. Half of this

work is her achievement.

Finally, I thank valuable pastors, Hee-Young Sohn, and Joong-Soo Lee for guiding

me to learn the immeasurable depth of love in Jesus Christ. I pray that their consistent

love and whole dedication to our God Father will not be changed forever.
















TABLE OF CONTENTS



A C K N O W L E D G M E N T S ................................................................................................. iv

LIST OF FIGURES ..................................................... .......... ................ viii

A B S T R A C T .......................................... .................................................. x iii

CHAPTER

1 INTRODUCTION ............... ................. ........... ................. ... ..... 1

2 BACKGROUND AND LITERATURE REVIEW ...............................................4

2 .1 H isto rical R ev iew ......................................................................... .......... .. .. .4
2.2 B background ...................................................................... ...................... ...... 7
2.2.1 AlGaN/GaN High Electron Mobility Transistors (HEMTs)........................7
2.2.2 ZnO based Chem ical Sensor ....................................... ......................... 11

3 PRESSURE SENSOR USING PIEZOELECTRIC POLARIZATION................ 15

3 .1 In tro d u ctio n .............................................. .. .. .. .......... ......................... ...............15
3.2 Effect on External Strain on the Conductivity of AlGaN/GaN HEMTs ..............16
3.2 Pressure induced Changes in the Conductivity of AlGaN/GaN HEMTs .............24
3.3 Capacitance Pressure Sensor Based on GaN HEMT on Si Membrane ..............31

4 CATALYST BASED GAS SENSOR FOR HYDROCARBON GASES ..................38

4 .1 Intro du action ............................. ............. ..... ................ ................ 3 8
4.2 AlGaN/GaN based MOS Diode Hydrogen Gas Sensor ......................................39
4.3 Hydrogen Induced Reversible Changes in Drain Current in
Sc203/AlGaN/GaN HEM Ts................................................ .............................. 45
4.4 Comparison of MOS and Schottky W/Pt-GaN Diodes for Hydrogen Detection.52
4.5 Detection of C2H4 Using Wide Bandgap Semiconductor Sensors .....................59
4.6 Hydrogen and Ozone Gas Sensing Using Multiple ZnO Nanorods ...................68

5 CHEMICAL SENSOR FOR POLYMERS AND POLAR LIQUIDS .....................75

5.1 Introduction .......................... ... ... ......... ................ ................ .......... 75
5.2 Gateless AlGaN/GaN HEMT Response to Block Co-Polymers ........................76









5.3 pH Measurements with Single ZnO Nanorod Integrated with a Microchannel ...82

6 BIOSENSORS FOR BIOMATERIALS AND CELL GROWTH .............................89

6 .1 Introdu action ................... .. ................... ........... ............ ............... 89
6.2 Detection of Halide Ions with AlGaN/GaN HEMTs............................................91
6.3 Electrical Detection of Immobilized Proteins with Ungated AlGaN/GaN
H E M T s ................................................................................ ................ 9 8
6.4 Use of 370 nm Light for Selective Area Fibroblast Cell Growth.......................103

7 SUMMARY AND FUTURE WORKS .............. .............................................114

7.1 Pressure Sensor ..................................... .................. ............... 114
7.2 G as Sensor ............................................................... ... ... ......... 115
7 .3 C hem ical Sen sor ................................................................... ......... .... 117
7 .4 B io S en so r.................................................................. 1 18

APPENDIX

A STRAIN CALCULATION OF A CIRCULAR MEMBRANE OF ALGAN/GAN
UNDER DIFFERENTIAL PRESSURE.................. ........................................121

A 1 Strains in the Film s ............... .......... .................... ............... ........ ......121
A.2 Electrical Polarization and Piezoelectric Effects..............................................123
A.3 2DEG and Conductance of HEMT .......................................... ...............124

L IST O F R E FE R E N C E S ....................................................................... .................... 126

BIOGRAPH ICAL SKETCH .............................................................. ............... 138















LIST OF FIGURES


Figure page

1-1 Semilog plot of intrinsic carrier concentration versus inverse temperature for Si,
GaAs, GaN (left). Performance of AlGaN/GaN based power amplifier as
compared to GaAs and SiC based devices (right) ....................................................2

2-1 Sheet carrier concentration in the 2DEG channel of AlGaN/GaN HEMT induced
by the piezoelectric polarization as a function of Al concentration...........................9

2-2 Piezoelectric (PE) and spontaneous (SP) polarization effects in Ga face or N-
face AlGaN/GaN heterostructures. ........................................ ....... ............... 10

2-3 Crystal structure of wurtzite ZnO................... ...... .... .. .................12

2-4 Energy bandgap of several, III-V, and II-IV compound semiconductors as a
function of lattice constant. ........................................ ....................................... 13

3-1 Layer structures of two-terminal non-mesa(top) and mesa(middle) devices, and
top view photo-micrograph of fabricated two-terminal devices with different
channel lengths(bottom ). ................................................ ............................... 16

3-2 Pressure sensor package: experimental setup to detect I-V characteristics
connected to the BNC cable according to various mechanical stresses (top) and
m echanical stressor with cantilever (bottom)............................................... 18

3-3 Strain induced by AlGaN on GaN for the un-relaxed and partially relaxed
AlGaN layer as a function of Al concentration (top) and sheet carrier
concentration induced by the piezoelectric polarization as a function of Al
concentration (bottom ) .............................................. .... .. .. .. ............ 19

3-4 Effect of tensile or compressive stress on the conductivity of the AlGaN/GaN
HEMT with mesa etching (top) and without mesa etching (bottom)....................22

3-5 Circular membrane of AlGaN/GaN on a Si substrate fabricated by etching a
circular hole in the substrate (left). A deflection of the membrane away from the
substrate due to differential pressure on the two sides of the membrane produces
a tensile strain in the membrane (right) ........................................................25

3-6 Device structure with a finger patterned device on the HEMT membrane..............27









3-7 SEM micrographs of via through the Si wafer(left)and cross sectional view of a
v ia h o le(rig h t) ...................................... ............................................. 2 8

3-8 IDs-VDs characteristics at 25C from AlGaN/GaN HEMT membrane as a function
of applied pressure. ........................ .... .................. ... ...... ... .... ........... 28

3-9 Channel conductivity of the AlGaN/GaN HEMT membrane as a function of
differential pressure .................. ............................. ........ ... ........ .... 29

3-10 Device structure (top) and SEM micrograph of AlGaN/GaN circular membrane
on a Si substrate fabricated by etching a circular hole in the substrate(bottom) ......32

3-11 Top view of HEMT capacitance pressure sensor(top) and capacitance as a
function of pressure for different diaphragm radii (bottom)...................................34

3-12 Capacitance change as a function of radius of the AlGaN/GaN HEMT
membrane over the pressure range from -1 to +9.5 bar. ........................................35

3-13 Capacitance change as a function of radius of the AlGaN/GaN HEMT
membrane over the pressure range from -1 to +9.5 bar ........................... ........36

4-1 Cross-sectional schematic of completed MOS diode on AlGaN/GaN HEMT
layer structure (top) and plan-view photograph of device(bottom) .......................40

4-2 Forward I-V characteristics of MOS-HEMT based diode sensors of two different
dimensions at 250C measured under pure N2 or 10%H2 /90%N2 ambient ............42

4-3 Time response at 250C of MOS-HEMT based diode forward current at a fixed
bias of 2V when switching the ambient from N2 to 10%H2 /90%oN2 for periods of
10, 20 or 30 seconds and then back to pure N2 ............................. ...................43

4-4 Time response at 250C of MOS-HEMT based diode forward current at a fixed
bias of 2V for three cycles of switching the ambient from N2 to 10%H2 /90%oN2
for periods of 10 (top) or 30 (bottom ) ........................................... ............... 44

4-5 Photograph of MOS HEMT hydrogen sensor............ .............................. 47

4-6 IDs-VDs characteristics of MOS-HEMT measured at 250C under pure N2 ambient
or in 10% H 2/90% N 2 am bient. ............................................................................ 48

4-7 Change in drain-source current for measurement in N2 versus 10%H2 /90%N2
ambient, as a function of gate voltage (top) and corresponding transconductance
at a fixed drain-source voltage of 3V(bottom). .................. ................................49

4-8 Time dependence of drain-source current when switching from N2 to 1%H2
/99% N2 ambient and back again. The top shows different injection times of the
H2/N2, while the bottom shows the reversibility of the current change...................51









4-9 The W/Pt Schottky diode (top) and MOS diode (bottom). .....................................53

4-10 Photograph of packaged gas sensor .............................................. ............... 53

4-11 Forward I-V characteristics at 300 oC(top) or 500 oC(bottom) from the Schottky
and MOS diodes in pure N2 and 10% H2 /90% N2......................................... 54

4-12 Measurement temperature dependence of forward I-V characteristics of the
Schottky (top) and M O S (bottom ) ........................................................................ 55

4-13 Temperature dependence of turn-on voltage(top) and on-state
resistance(bottom ). ........................................... ............... .... ....... 57

4-14 Change in forward current when measuring in 10% H2 /90% N2 relative to pure
N2 at 3 or 3.5 V in both the Schottky and MOS diodes .......................................58

4-15 Schematic of AlGaN/GaN MOS diode (top) and bulk ZnO Schottky diode
structure (bottom ) ......................................... ............... .. ........ .... 61

4-16 Forward I-V characteristics of MOS-HEMT based diode sensor at 4000C
measured under pure N2 or 10% C2H4/90% N2 ambients .....................................62

4-17 Change in MOS diode forward current at fixed forward bias of 2.5V(top) or at
fixed current(bottom ) .................................................................... .. ...................64

4-18 I-V characteristics at 500C (top) or 150 OC (bottom) of Pt/ZnO diodes measured
in different ambients ........... ......... .... ... .. ....... ... ... ...... .............. 65

4-19 Change in current at a fixed bias (top) or change in voltage at fixed current
(b o tto m ) ......................................................................................................6 7

4-20 TEM of ZnO nanorod ......................................................... ................ 69

4-21 SEM image of ZnO multiple nanorods (top) and the pattern contacted by
Al/Pt/Au electrodes (bottom ) ............................................................................70

4-22 I-V characteristics at different temperatures of ZnO multiple nanorods measured
in either N 2 or 10 % H2 in N 2 ambient .......................................... ............... 71

4-23 Change in current measured at 0.1 V for measurement in either N2 or 10%H2 in
N 2 am bients ........................................... ........................... 72

4-24 I-V characteristics at 25C of ZnO multiple nanorods measured in either N2 OR
3 % 0 3 in N 2 ....................................................................................... 7 3

4-25 Time dependence of current at IV bias when switching back and forth from N2
to 3% 0 3 in N 2 am bients.............................................................................. ..... 74

5-1 Layout of gate HEMT structure (top) and device cross-section (bottom) ..............82









5-2 Structure of block co-polymer, composed of different portions of PS and PEO
(top) and chemical formula for PS and PEO (bottom) .........................................83

5-3 Drain I-V characteristics for the air, PS and PEO ........................................... 84

5-4 Drain I-V characteristics for the different concentration of PS-PEO block
copolym er................................................................................................85

5-5 Drain IV characteristics of copolymers with different composition......................86

5-6 Schematic (top) and scanning electron micrograph (bottom) of ZnO nanorod
with integrated microchannel (4pm width).......................... ...................... 89

5-7 I-V characteristics of ZnO nanorod after wire-bonding, measured either with or
without UV (365nm) illumination............................ ............... 90

5-8 Change in current (top) or conductance (bottom) with pH (from 2-12) at V=
0 .5V ............................................................................... 9 1

5-9 Relation between pH and conductance of ZnO nanorod either with or without
UV (365nm) illumination .............................. ................ ................... 92

6-1 Schematic of HEMT structure for detection of halide ions (top) and plan view
photomicrograph of completed device using a 20 nm Au film in the gate region
(b o tto m ) .......................................................................... 9 2

6-2 Time dependence of current change in Au-gated HEMTs upon exposure to NaF
(top) or NaCl (bottom) solutions with different concentration.. ............................93

6-3 Current change as a function of concentration for HEMTs with or without the
A u g ate ........................................................................... 9 5

6-4 Change in current as a function of time as a Au-gated HEMT is exposed to
water. In the case of thiol modification of the gate, no response was observed. .....96

6-5 AC measurement of change in current in gateless HEMT exposed to water. The
applied bias of 500 mV was modulated at 11 Hz............................................... 97

6-6 Structure of APS (3-Aminopropyl) triethoxysilane (top left), surface
functionalization before chemical modification (top right). A schematic diagram
of the gateless HEMT whose surface is functionalized by chemical modification
in the gate region (bottom ). ..... ........................... ....................................... 99

6-7 Fluorescent photographs of biotinylated probes on chemically treated GaN
surface (left) and non-biotinated surface (right).....................................................100









6-8 SEM micrographs of amine-functionalized GaN surfaces reacted with biotin
NHS and then blocked and exposed to either BSA-coated (left) or streptavidin
(right) nanoparticles. ..................................................................... ...................10 1

6-9 Change in HEMT drain-source current as a result of interaction between Biotin
(Sulfo-NHS-Biotin) and streptavidine introduced to the gateless HEMT surface. 102

6-10 Patterns formed on glass slides coated with 50 nm TiO2............. .....................104

6-11 Optical spectrum of a GaN light emitting diode at an operating current of 4 mA
(top). The current-voltage characteristics of the GaN based LED (bottom). .........105

6-12 Effect of UV light on growth of Fibroblast cells on glass slices and TiO2 coated
glass slides .................................. ................... ... ........ ............... 107

6-13 Simulated reflectivity of 370 nm UV light with different substrates.....................108

6-14 Fibroblast cell growth on patterned TiO2 coated glass slide with the UV light
illumination (top) and without UV light illumination (bottom) .............................110

6-15 A set-up of fibroblast cell on patterned TiO2 coated glass slides under different
U V intensity illum nation .................................................... ......... ............... 111

6-16 Simulated reflectivity of 370 nm UV light with six TiO2/SiO2 dielectric mirror
stacks ..................................... ................................ .......... 112















Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

FABRICATION AND CHARACTERIZATION OF COMPOUND SEMICONDUCTOR
SENSORS FOR PRESSURE, GAS, CHEMICAL, AND BIOMATERIAL SENSING

By

Byoung Sam Kang

December 2005

Chair: Fan Ren
Major Department: Chemical Engineering

GaN-based diodes and high electron mobility transistors (HEMTs) for pressure,

gas, liquid and biological sensing were fabricated and characterized. A novel metal oxide,

ZnO was also evaluated as a potential electronic device for chemical sensing

applications. The devices presented herein showed high sensitivity and capability of

operation in harsh environmental conditions such as high temperature and pressure.

The AlGaN/GaN HEMTs grown on (0001) sapphire substrates show polarization-

induced two dimensional electron gas (2DEG) at the AlGaN/GaN hetero-interface.

Linear dependence of the 2DEG channel conductance on external strain was observed

using a cantilever beam in a bending configuration. To overcome the rigidity of sapphire

substrates in applying external stress, a micro pressure sensor using a 150 [m diameter

thin flexible AlGaN/GaN circular membrane with an interdigitated-finger device on a

(111) Si substrate was demonstrated. The measured pressure sensitivity was 7.1 x 10-2









mS/bar, which was two orders of magnitude larger than that of a cantilever beam pressure

sensor.

Pt gated AlGaN/GaN HEMT-based metal-oxide semiconductor (MOS) diodes and

field effect transistors (FETs) were demonstrated for detecting hydrocarbon gases,

followed by a comparison between MOS and W/Pt Schottky-based GaN diodes for

hydrogen sensing. Changes in current, when sensors exposed to hydrogen on the Pt-gated

AlGaN/GaN HEMT were approximately an order of magnitude larger than that of Pt

/GaN Schottky diodes and 5 times larger than Sc203/AlGaN/GaN MOS diodes.

For the liquid sensors, gateless AlGaN/GaN HEMTs showed large changes in

source-drain current on exposing the gate region to polar liquids and block copolymers.

The polar nature of these chemicals leads to a change of surface charge in the gate region

on the HEMT, producing a change in surface potential at the semiconductor/liquid

interface. For biomaterials detection, the gate region was chemically modified with

aminopropyl silane. As streptavidin was introduced to the biotin-functionalized gate

region, the drain-source current showed a clear decrease of 4 [A, which shows

interaction between antibody and antigen.

A Schottky diode was fabricated on a ZnO thin film and showed higher sensitivity

to hydrogen (5 ppm). A single ZnO nanorod FET-based sensor was also demonstrated.

Conductivity of the single nanorod sensor decreased linearly when the pH value of the

solution varied from 2 to 12.The measured sensitivity was 8.5 nS/pH in the dark and 20

nS/pH under UV (365 nm) illumination, showing tremendous potential for sensing

applications.














CHAPTER 1
INTRODUCTION

The sensor industry has grown rapidly in recent years. The non-military world

market for sensors exceeded expectations with US $42.2 billion in 2003 and it is

expected to reach US $54 billion by 2008 [Mar04]. This rapid growth of the sensor

market increases the need to develop chemical sensor technology in applications such as

chemical reactor processing, factory automation, industrial monitoring, automotive

industry, computers, robotics, and telecommunications [Wil05].

In practice, the sensing elements must be relatively small in size, robust, and should

not require a large sensing sample volume [HunOl, Liu04]. Due to the development of

fabrication and processing techniques, a Si-based chemical sensing microsystem has the

advantage of producing microsize structures in a highly uniform and geometrically well

defined manner [Mad97]. While Si has proven to be the primary contestant in the micro-

sensor market, there is an ever-growing need for devices operating at conditions beyond

the limits of silicon. Silicon based micro-sensors can not be operated in harsh

environments such as in high temperature, pressure, and chemically corrosive ambients.

Wide bandgap electronics and sensors based on GaN can be operated at elevated

temperatures (6000C) where conventional Si-based devices cannot function, being limited

to < 3500C. This is because of the low intrinsic carrier concentration of wide-bandgap

energy semiconductors at high temperature, as shown in Figure 1-1(left).










Temperature C)
400300 200 100 25 0
1 0- 1
Si 10 Small periphery
E GGaN/AIGaN HEMTs
10 -- --- atX-band(8- 12 GHz)


S10 best SiC
U 4-

1 10"10 2 best GaAs


1.5 2.0 2.5 3.0 3.5 4.0 1995 1996 1997 1998 1998 1999
1000 /T (K') Date


Figure 1-1. Semilog plot of intrinsic carrier concentration versus inverse temperature for
Si, GaAs, GaN (left). Performance of AlGaN/GaN based power amplifier as
compared to GaAs and SiC based devices (right).

The ability of GaN-based materials to function in high temperature, high power and

high flux/energy radiation conditions will enable large performance enhancements in a

wide variety of spacecraft, satellite, mining, automobile, nuclear power, and radar

applications. One additional attractive attribute of GaN is that sensors based on these

materials could be integrated with high-temperature electronic devices on the same chip.

AlGaN/GaN heterojunction based high electron mobility transistors (HEMTs) have

demonstrated extremely promising results for the use as power devices in many analog

applications due to the high sheet carrier concentration, electron mobility in the two

dimensional electron gas (2DEG) channel and high saturation velocity, as illustrated in

Figure 1-1 (right). Without any surface passivation, the sheet carrier concentration of the

polarization-induced 2DEGs confined at interfaces of AlGaN/GaN HEMT becomes

sensitive to any manipulation of surface charge. However, the nature of ambient

sensitivity for the unpassivated nitride used to build micro-sensors able to detect applied

strain and surface polarity change by polar liquids or toxic gas and harmful cancer cell









exposure to the surface of HEMTs. In addition, sensors fabricated from these wide

bandgap semiconductors could be readily integrated with solar blind UV detectors or

high temperature, high power electronics on the same chip [KimOOa, Ris94, Lut99].

Another wide bandgap semiconductor, ZnO proposed for the sensing applications

has several fundamental advantages. It has higher free-exciton binding energy (60 meV),

and more resistance to radiation damage, piezoelectricity and transparency. It has been

used effectively as sensing material based on near surface modification of charge

distribution with certain surface absorbed species. Another benefit of oxide-based

semiconductors is that they do not rely on specific catalytic metals for chemical detection

[Yun05]. Instead, they exploit the change in near surface conductivity most likely due to

the adsorption of chemical species. Most of the resistive gas sensors that employ surface

conductivity change have been metal oxide semiconductors (MOS) such as ZnO, SnO2,

In203, MoO3, WO3, and titanium substrated chromium oxide (CTO). The ZnO-based

MOS-based diodes and field effect transistors have been demonstrated and they can be

used with a wide range of chemicals using fractional surface conductivity by adsorption

and desorption.














CHAPTER 2
BACKGROUND AND LITERATURE REVIEW

2.1 Historical Review

The AlGaN/GaN high electron mobility transistors (HEMTs) have demonstrated

extremely promising results for use in broad band power amplifiers in wireless base

station applications, due to the high sheet carrier concentration, high electron mobility in

the two dimensional electron gas (2DEG) channel, and high saturation velocity [Mor99,

Eas02, Tar02, ZhaOO, ZhaOl, Joh02, Kou02, Pea99]. The high electron sheet carrier

concentration of nitride HEMTs is induced by piezoelectric polarization of the strained

AlGaN layer and spontaneous polarization [Kou02, Kan03, Che95, Ned98, AmbOO].

Polarization induced piezoelectric properties play an important role in AlGaN/GaN

heterostructures. The high electron sheet carrier concentration in the strained

AlGaN/GaN layer suggests the possibility that nitride HEMTs may be excellent

candidates for sensing applications including pressure, chemical, gas, and biological

sensors.

For pressure sensor applications, only a few basic studies reported piezo effect

related cantilever beams [Str03, Dav04, Wu05]. Strittmatter et al. [Str03] reported that

capacitive strain can be sensed with GaN metal insulator semiconductor (MIS) diode but

for better performance, high quality surface oxide/GaN interface was needed. Davies et

al. [Dav04] first demonstrated the feasibility of free standing GaN cantilevers on Si

substrates. Both dry and wet etch processes were used to remove the Si substrate under

the GaN but any measured strain data using this device was not shown. Wu and Singh









[Wu05] examined potential for the strain sensor using a BaTiO3 piezoelectric

semiconductor field effect transistor. Two classes (Si and GaN) of heterostructures for

stress sensing were used and high sensitivity could be acquired using a very thin

piezoelectric layer. However, direct application of AlGaN/GaN HEMTs structure for

pressure sensors is not widely studied, especially for high pressure sensing.

Gas sensors have been fabricated on a number of semiconductors using catalytic

metals as the gate in the metal insulator semiconductor (MIS) or as the metal contact in

Schottky diodes [You82, Lun86, Rye87]. Various field effect transistors based on silicon

have been developed for hydrogen gas sensing [Lun89]. But silicon based sensors are

limited to operation in environments of below 250C, prohibiting them from being used

as hydrocarbon detectors or for other applications requiring high temperature operation.

Because hydrocarbon gases should be decomposed by the catalytic metals and hydrogen

atoms diffuse to the device interface, it is presumed that a dipole forms, lowering the

effective work function of the metal and changing electrical characteristics of the devices.

Baranzanhi et al. [Bar95] demonstrated gas sensitivity of Pt gated SiC transistors

operating up to 5000C but the SiC Schottky diodes have displayed poor thermal stability.

Pd silicides were observed at temperatures as low as 425C when Pd was used as

Schottky metal [Hun95, Che96]. Luther et al. [Lut99] first demonstrated Pt-GaN gas

sensor for hydrogen and propane at high temperature (200-400C). The Pt-GaN gas

sensor showed faster response for hydrocarbons and enhanced sensitivity at higher

temperatures (500C). After that, Schalwig et al. [SchO1] showed gas sensors for the

exhausted lean burn engines using Pt-GaN and Pt-HEMTs. The device performance was

investigated at high temperatures (200-600C) and it was shown that a HEMT based gas









sensor was more sensitive than GaN diodes but detailed analysis of sensitivity differences

for GaN diodes vs HEMTs was left as future work.

Since the first demonstration of a fluid monitoring sensor based on AlGaN/GaN

hetero structures by Neuberger [NeuO 1], the application of AlGaN/GaN HEMTs as liquid

sensors has been a subject of intense research. Neuberger et al. suggested that the sensing

mechanism for chemical absorbates originated from compensation of the polarization

induced bound surface charge by interaction with polar molecules in the fluids. The time

dependence of changes in source-drain current of gateless HEMTs exposed to polar

liquids isopropanoll, acetone, methanol) with different dipole moments using

GaN/AlGaN hetero-interfaces was reported. In particular, it was shown that it is possible

to distinguish liquids with different polarities. Steinhoff et al. [Ste03a] suggested that the

native oxide on the nitride surface was responsible for the pH sensitivity of the response

of gateless GaN based heterostructure transistors to electrolyte solutions. It was shown

that the linear response of a nonmetallized GaN gate region using different pH valued

electrolyte solutions and sensitivity with a resolution better than 0.05 pH from pH = 2 to

pH = 12. Chaniotakis et al. [Cha05] showed that the GaN surface interacts selectively

with Lewis acids, such as sulphate (S042-) and hydroxide (OH-) ions using impedance

spectra. It was also shown that gallium face GaN was considerably reactive with many

Lewis bases, from water to thiols and organic alcohols without any metal oxide and

nitrides

A novel metal oxide, ZnO has numerous attractive characteristics for gas and

chemical sensors [Kan05a, Kan05b, Heo04, LooOl, Nor04]. The bandgap can be

increased by Mg doping. The ZnO has been used effectively as gas sensor material based









on near surface modification of charge distribution with certain surface absorbed species.

In addition, it is attractive for biosensors given that Zn and Mg are essential elements for

neurotransmitter production and enzyme functioning [Ste05, Gur98]. The ZnO is

attractive for forming various types of nanorods, nanowires, and nanotubes [Hua01, Li04,

Kee04, Kin02, Liu03, Par03a, Ng03, Hu03, Par03b, Heo02, He03, Zhe01, Lyu03, Zha03,

PanO1, Lao03]. The large surface area of the nanorods makes them attractive for gas and

chemical sensing, and the ability to control their nucleation sites makes them candidates

for high density sensor arrays.

2.2 Background

2.2.1 AlGaN/GaN High Electron Mobility Transistors (HEMTs)

One of the most outstanding advantages of the GaN is the availability of

AlGaN/GaN heterostructures. The type I band alignment between AlGaN and GaN has

been shown to form a potential well and a 2-dimensional electron gas (2DEG) at the

heterointerface [Hen95]. When these materials are brought into contact, thermal

equilibrium requires alignment of their respective Fermi levels (EF). This induces

conduction (Ec) and valence (Ev) band bending in both the AlGaN and GaN layers and

can cause the GaN conduction band at the interface to drop below EF, as illustrated in

Figure 2-1.

Since the Fermi level can be viewed as an electrochemical potential for electrons,

majority electrons will accumulate in the narrow gap material just below the

heterointerface to fill the quasi triangular potential well between Ec and EF. With the

heterointerface on one side and a potential barrier on the other, electrons in the 2DEG are

only free to move in along the plane of the interface. Modulation doped field effect

transistors (MODFETs) are a class of heterostructures FET that use selective barrier









doping to spatially separate ionized donors from the electrons in the 2DEG, leading to an

increase in channel mobility.







AlGaN GaN AIGaN






EF- -

2-DEG

AIGaN GaN

Figure 2-1. Simplified view of modulation doping with an enlarged view of energy band
diagram illustrating formation of 2-dimensional electron gas at AlGaN/GaN
heterointerface.

For this reason, these devices are also know as high electron mobility transistors, or

HEMTs.Unlike conventional III-V based HEMTs, such as AlGaAs/GaAs HEMTs, there

is no dopant in the typical nitride based HEMT structure and all the layers are undoped.

The carriers in the two dimensional electron (2DEG) gas channel is induced by

piezoelectric polarization of the strained AlGaN layer and spontaneous polarization,

which are very large in wurtzite III-nitrides. Carrier concentration > 1013 cm-3 in the

2DEG, which is 5 times larger than that in AlGaAs/GaAs material system, can be

routinely obtained. The portion of carrier concentration induced by the piezoelectric

effect is around 45-50%. This makes nitride HEMTs are excellent candidates for pressure

sensor and piezoelectric-related applications.










The piezoelectric polarization induced sheet carrier concentration of undoped Ga-

face AlGaN/GaN can be calculated by using Equation 2-1.



n, e dde 2


where o(x) is piezoelectric polarization, x is the Al concentration in AlxGal-xN, E(x) is the

dielectric constant, dd is the AlGaN layer thickness, epb is the Schottky barrier of the gate

contact on AlGaN, EF is the Fermi level and AEc is the conduction band discontinuity

between AlGaN and GaN.



0.7 .

0.6 -

o 0.5

M 0.4 No relaxation
4--_S I I t
D--- Partial relaxation
00.3
E
0.2

X 0.1

0.0 **** .
1Ell 1E12 1E13

Carrier concentration induced by PE(cm-2)


Figure 2-2. Sheet carrier concentration in the 2DEG channel of AlGaN/GaN HEMT
induced by the piezoelectric polarization as a function of Al concentration

As shown in Figure 2-2, the sheet carrier concentration induced by the piezoelectric

polarization is a strong function of Al concentration. In the case of a partially relaxed

AlGaN strain layer, there is also a maximum sheet carrier concentration around Al =

0.35. If an external stress can be applied to AlGaN/GaN material system, the sheet carrier









concentration can be changed significantly and devices fabricated in this fashion could be

used in sensor-related applications.

The changes in the two dimensional (2D) channel of AlGaN/GaN HEMTs are

induced by spontaneous and piezoelectric polarization, which are balanced with positive

charges on the surface. Figure 2-3 shows schematic diagrams of the direction of the

spontaneous and piezoelectric polarization in both Ga and N face wurtzite GaN crystals

[AmbOO].

For the Ga-face AlGaN/GaN HEMTs structure, at the surface of a relaxed GaN

buffer layer or a strained AlxGal-xN barrier as well as at the interfaces of a AlxGajl

xN/GaN heterostructure, the total polarization changes abruptly, causing a fixed two

dimensional polarization sheet charge cr, given by
OAIG PAG p + p1. (2-2)
cAlGaN AlGaN 1AlGaN AlGaN (2-2)

where, PAs (x) = -0.090x 0.034(1 x)+ 0.019x(1- x), (2-3)

P~AzG (x) = -0.0583x + 0.0402x(1 x). (2-4)

Ga face N face




O Ga Ps+ PPE Ps+ PPE
N G L




S PSt t PS



Figure 2-3. Piezoelectric (PE) and spontaneous (SP) polarization effects in Ga face or N-
face AlGaN/GaN heterostructures.









Therefore, the sheet charge density in the 2D channel of AlGaN/GaN HEMT is

extremely sensitive to its ambient. Numerous groups have demonstrated the feasibility of

AlGaN/GaN hetero-structures based hydrogen detectors with extremely fast time

response and capable of operating at high temperature (500-800C), eliminating bulky

and expensive cooling systems [Amb02, SchOl, EicOl, SchOla, Stu02, Eic03, Kim03a,

Kim03b, Kim03c, Amb03]. In addition, gateless AlGaN/GaN HEMTs show a strong

dependence of source/drain current on the polarity and concentration of polar solutions

[Ste03b]. There have also been recent reports of the investigation of the effect of external

strain on the conductivity of an AlGaN/GaN high electron mobility transistor [Kan03].

2.2.2 ZnO based Chemical Sensor

ZnO has numerous attractive characteristics for gas and chemical sensors [Kan05a,

Kan05b, Heo04, LooOl, Nor04] ZnO is a direct bandgap semiconductor normally form in

hexagonal (wurtzite) crystal structure like GaN, with lattice parameters a = 3.25 A and c

= 5.12 A. The Zn atoms are tetrahedrally coordinated with four O atoms, where the Zn -

d electrons hybridize with the O p-electrons. Alternating Zn and O layers form the crystal

structure shown in Figure 2-4.

Compared with GaN in Table 2-1 [StrOO], it has direct bandgap energy of 3.37 eV,

which makes it transparent in visible light and operates in the UV blue wavelengths. The

exciton binding energy -60 meV for ZnO, as compared to GaN -25meV; the higher

exciton binding energy enhances the luminescence efficiency of light emission.

In the past, ZnO has been used in its polycrystalline form in applications ranging

from piezoelectric transducers [Kad92] to varistors [Lou80, VerOO], and as transparent

conducting electrodes [Pet79]. Recent improvements in the growth of high quality, single

crystalline ZnO in both bulk and epitaxial forms has revived interest in this material









[Pet79]. Especially, ZnO is a piezoelectric, transparent wide bandgap semiconductor used

in surface acoustic wave devices.


* Zn

0o


a


Figure 2-4. Crystal structure of wurtzite ZnO.

Table 2.1 Physical properties of GaN and ZnO.
GaN

Bandgap 3.39 eV, direct

Crystal structure a = 3.189 A, c = 5.206 A
(Wurtzite) (c/a= 1.632)
1000 cm2/V-s (e)
Mobility 30 cm2/V-s (h)

Sm m* = 0.20mo (e)
Effective mass 0m (
m* = 0.80mo (h)
Saturation velocity 2.5 x 107 cm/s
Exciton binding energy 28 meV


ZnO

3.37 eV, direct

a = 3.250 A, c = 5.205 A
(c/a = 1.602)
200 cm2/Vs (e)
5-50 cm2/Vs (h)
m* = 0.24mo (e)
m* = 0.59mo (h)
3.2x107 cm/s
60 meV


Energy gaps of several common semiconductors are given in Figure 2-4 as a

function of lattice constant. The approximate boundaries of the visible spectrum are

shown as is the nature of the energy transition for each material. From the figure, it is






13


noted that group III-nitrides and II oxides have bandgap from red to shorter UV

wavelengths. The bandgap can be increased by Mg doping. ZnO has been effectively

used as a gas sensor material based on the near-surface modification of charge

distribution with certain surface-absorbed species [Kan93].




MgO
7

> AIN
5




5 5- violet
SCdO
c 2 -
LU ......... ............... ............. ............ re d
,1 ,I I I
2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0
Lattice constant a(A)

Figure 2-4. Energy bandgap of several, III-V, and II-IV compound semiconductors as a
function of lattice constant.

In addition, it is attractive for biosensors given that Zn and Mg are essential

elements for neurotransmitter production and enzyme functioning [Mil98, Oga04]. ZnO

is attractive for forming various types of nanorods, nanowires and nanotubes [HuaO1,

Li04, Kin02, Liu03, Par03a, Ng03, Hu03b, Par03b, Heo02, Nor04, Poo03, He03, WuOO,

Zhe01, LyuOl, Zha03b, Par03c, Yao02, PanOl, Lao03]. Compared with bulk materials, a

significant characteristic of nanostructures is their high surface to volume ratio. ZnO

nanowires have the same crystal structures as bulk structure, confirmed by X-ray

diffraction (XRD) and transmission electron microscopy (TEM) [PanOl, Roy03, Li02].









Therefore, all the bulk properties are still preserved for the nanowires and ZnO nanorods

are very promising for a wide variety of sensor applications.














CHAPTER 3
PRESSURE SENSOR USING PIEZOELECTRIC POLARIZATION

3.1 Introduction

There are a number of applications in the automotive, aerospace, and industrial

fields for robust miniaturized pressure sensors. A number of different semiconductors

systems have been used to make piezoresistive sensors [Ko99, Ned98, Wu97, You04,

Dad94]. Especially, polarization induced piezoelectric properties play a very important

role in strained AlGaN/GaN heterostructures. The high electron sheet carrier

concentration in the AlGaN/GaN layer suggests that nitride HEMTs can be used as

excellent pressure sensors.

Several research groups have reported piezo effect related GaN pressure sensors

[Str03, Dav04, Wu05]. Strittmatter et al. [Str03] reported that capacitive strain can be

sensed with GaN metal insulator semiconductor (MIS) diode but for better performance,

high quality surface oxide film on the GaN layer was needed. Davies et al. [Dav04] first

demonstrated the feasibility of the free standing GaN cantilevers on Si substrates but any

measured strain data using this device was not shown. Wu and Singh [Wu05] examined

the potential for the strain sensor using BaTiO3 piezoelectric semiconductor field effect

transistor. Two classes (Si and GaN) of heterostructures were used for stress sensing and

showed that high sensitivity can be acquired using a very thin piezoelectric BaTiO3 layer.

However, the direct application of AlGaN/GaN HEMTs structure for pressure sensors is

not widely studied, especially for high pressure sensing.









In this chapter, a thorough discussion of pressure sensor using AlGaN/GaN HEMT

structure will be given. In Section, 3.2, the effect of external strain on the sheet resistance

of the two dimensional electron gas channel in AlGaN/GaN HEMTs grown on sapphire

will be presented for the cantilever beam. In Section 3.3, AlGaN/GaN membrane

pressure sensor fabricated on Si substrate will be analyzed, which can overcome the

rigidity of sapphire substrate in applying external stress. In Section 3.4, the capacitive

pressure sensor which is less sensitive to variations in contact resistance will be

discussed.

3.2 Effect on External Strain on the Conductivity of AlGaN/GaN HEMTs

Two terminal high electron mobility Al0.25Gao.75N/GaN devices used with simple

bonding test were used to to study the effect of external strain on the conductivity on the

sheet resistance of the two-dimensional electron gas channel in an AlGaN/GaN HEMT.

The HEMT structures consisted of a 3[tm thick undoped GaN buffer, 30A thick

Al0.25Gao.75N spacer, 270A thick Si-doped Al0.3Gao.7N cap layer. The epi-layers were

grown on sapphire substrate by metal organic chemical vapor deposition (MOCVD). Two

sets of samples were fabricated; one with mesa definition and the other without the mesa

as shown in Figure 3-1(top). Mesa isolation was performed with an Inductively Coupled

Plasma (ICP) etching with C12/Ar based discharges at -90 V dc self-bias, ICP power of

300 W at 2 MHz and a process pressure of 5 mTorr. 100 x 100 im2 ohmic contacts

separated with gaps of 5, 10, 20, 50 and 100 [m like a transmission-line-method (TLM)

pattern consisted of e-beam deposited Ti/Al/Pt/Au patterned by lift-off and annealed at

850 C, 45 sec under flowing N2. Plated Au was subsequently deposited on the ohmic

metal pads for wire bonding on the samples as shown in Figure 3-1(bottom).











ohmic
Contact
.. ....' m etal


final
metal

AIGaN

GaN


ohmic
contact
metaI

2DEG


A203 Su bstrate


Figure 3-1. Schematic diagrams of layer structures of two-terminal non-mesa(top) and
mesa(middle) devices, and top view photo-micrograph of fabricated two-
terminal devices with different channel lengths(bottom).

The devices were fabricated on half of 2" wafer, sawed into 2 mm wide stripes and

wire bonded on the test feature. The dc characteristics were obtained from measurements

on an Agilent 4156C parameter analyzer. Figure 3-2 illustrates the setup for measuring

the effect of external strain on the conductivity of 2DEG channel of the nitride HEMT.


final
metal


AIGaN
2DEG
GaN


A1203 Substrate


ohmic contact


.. .. .









Lucite blocks secure the sample and PCB board for testing. The contact pads were

connected to the PCB board, which had BNC connectors on the end for signal outputs,

with 1 mil thick gold wire. A high precision single axis traverse was used to bend the

sample.

Sheet charges in the AlGaN/GaN high electron mobility transistors (HEMTs) are

induced by spontaneous polarization and piezoelectric polarization [Kou02, Ras02,

AmbOO, Che95]. Wurtzite GaN and AlGaN are tetrahedral semiconductors with a

hexagonal Bravais lattice with four atoms per unit cell. The misfit strain inside a film is

measured against its relaxed state. In the misfit strain calculation, the strain is calculated

against the relaxed films, ao, i.e.

a(x)-ao(x)
misfit = () (3-1)
a, (x)

where a(x) is the lattice constants of AlxGal-xN and ao(x) = (aGaN aAINx)A = (3.189 -

0.077x)A [Amb99]. Here a(x) ao(x) is chosen because the AlGaN film is always under

tension due to a(x) < aGaN and misfit will always be positive. Having defined the elastic

strain in the film, a partially relaxed film parameter should be defined. For a perfectly

coherent film, a(x) = aGaN or

amax a, (x) 0.077x 0.077x 0.024x (3-2)
e G 0 7 = 0.024x (3-2)
max sfit ao(x) 3.189-0.077 3.189


For a partially relaxed film, the ratio of strain comparing to the un-relaxed state

defines the degree of strain in the film,


S(x)= msf -a(x)- a(x) (33)
Emax misfit aGa ao (x)


Hence, the degree of relaxation is then given by










r(x)=l 1


S(x)= aG a(x)
aG a o(x)


(3-4)


For around 300 A of AlGaN layer on the top of GaN, r(x) was measured by

Ambacker [Amb00].


0 < x < 0.38


r(x) =


3.5x-1.33 0.38 < x <0.67


0.67

High precision
single axis traverse


Cantilever


Figure 3-2. Schematic diagram of a pressure sensor package: experimental setup to detect
I-V characteristics connected to the BNC cable according to various
mechanical stresses (top) and mechanical stressor with cantilever (bottom).


(3-5)













0.6
O

0.4


E
0.2

X
0.0







0.7

0.6


0 4 8 12

Strain(xl03)


1011 1012 1013


Carrier concentration induced by PE(cm-2)


Figure 3-3 Strain induced by AlGaN on GaN for the un-relaxed and partially relaxed
AlGaN layer as a function of Al concentration (top) and sheet carrier
concentration induced by the piezoelectric polarization as a function of Al
concentration (bottom)


- -No relaxation
---- Partial relaxation









The piezoelectric polarization, for partially relaxed strained layer can be expressed

by modifying the Equation 3-3 subtracting the portion of relaxation


a(x)= 2(1-r(x)) aGay a e3 e33 13 (3-6)


where e31 and e33 are the piezoelectric coefficients and C13 and C33 are the elastic

constants.

Figure 3-3 (left) plots the relationship between Al concentration of AlGaN and

strain induced by AlGaN on GaN for the un-relaxed and partially relaxed conditions.

Unlike the linear model for the un-relaxed condition, there is a maximum strain around

Al concentration of 0.35. The piezoelectric polarization induced sheet carrier

concentration of undoped Ga-face AlGaN/GaN can be calculated by following equation

[Asb97]:


n (x) = )eA -(x) + E(x)- A (x)) (3-7)
e d,e

where E(x) is the dielectric constant, dd is the AlGaN layer thickness, epb is the Schottky

barrier of the gate contact on AlGaN, EF is the Fermi level and AEc is the conduction

band discontinuity between AlGaN and GaN. In the case of applying external tensile

strain on the HEMT sample, a increase of conductivity was observed.

The mesa depth was around 500 A, which is below the AlGaN/GaN interface (300

A AlGaN layer on 3 rm GaN layer). As illustrated in Figure 3-3 (top), the sheet carrier

concentration induced by the piezoelectric polarization is a strong function of Al

concentration. In the case of a partially relaxed AlGaN strain layer, there is also a

maximum sheet carrier concentration around Al = 0.35. If an external stress can be

applied to AlGaN/GaN material system, the sheet carrier concentration can be changed









significantly and devices fabricated in this fashion could be used in sensor-related

applications.

In order to study the effect of external strain on the conductivity of the HEMT

material systems, transmission line patterns were fabricated with a mesa of 500 A.

Elastic bending of an analogous system, GaAs grown on Si wafers, has been carefully

studied using Stony's equation [Sto09, Chu98]. The curvature and wafer bowing for

different thickness of GaAs grown on Si was modeled [Chu98]. A pure bending of a

single beam was used in this work to estimate the strain, since the HEMT structure,

around 3 rim, is much thinner that that of sapphire substrate, 200 rim, and the degree of

deflection (maximum deflection is around 2.2 mm) is much shorter than the length of the

beam, 27 mm. The strain, exx, of the bending can be estimated from the single beam with

thickness oft and unit width. The tensile strain near the top surface of the beam is simply

given by

xx = td/ L2 (3-8)

where t is the sample thickness, d is the deflection and L is the length of the beam.

Figure 3-4 (top) shows the effects of external tensile and compressive strain on the

conductivity of AlGaN/GaN HEMT sample with mesa. Therefore, the AlGaN layer sits

above the beam and the external stress applied on the beam should not change the strain

of the AlGaN layer.

As a result, applying a tensile stress on the beam would pull apart the GaN atoms

only and not affect the AlGaN. The total strain on the AlGaN/GaN interface and

piezoelectric should increase, as observed in Figure 3-4 (top). In the case of applying a











compressive stress on the beam, the GaN atoms were pushed together and reduced the

total strain at the AlGaN/GaN interface and the resultant conductivity.




8.05

8.00 -

5" 7.95
E

7.90 0 O Tensile
SCompressive

7.85

7.80

7.75 ...
0 2 4 6 8 10 12 14

Compressive or tensile stress(x106)


8.65




8.60
E
.I-

8.55


8.50
O


8.50


0 1 2 3 4 5

Compressive or tensile stress(x106)



Figure 3-4. The effect of tensile or compressive stress on the conductivity of the
AlGaN/GaN HEMT with mesa etching (top) and without mesa etching
(bottom).









For the non-mesa devices fabricated on the same wafer in a different area, Figure 3-

4(bottom) shows the strain dependence of with a channel length of 10 |tm. A reversal of

strain sensitivity indeed has observed indicating a larger strain relaxation in the

AlGaN/GaN layer.

The changes in conductance of the channel of Al0.25Gao.75N/GaN high-electron-

mobility transistor structures during application of both tensile and compressive strain

were measured relatively large. For fixed Al mole fraction, the changes in conductance

were roughly linear over the range up to 1.4 x 10 N.m-2, with coefficients for planar

devices of+ 9.8 x 10-12 S-N 1-m2 for tensile stress and 1.05 x 10 -1 S-N_ -m2 for

compressive stress. For mesa-isolated structures, the coefficients were smaller due to the

reduced effect of the AlGaN strain, with values of -1.2 xlO 12 S-N1-m2 for tensile stress

and + 1.97 xl0 12 S-N -m2 for compressive stress. The large changes in conductance

demonstrate that simple AlGaN/GaN heterostructures are promising for pressure and

strain sensor applications.

In summary, we have demonstrated the effect of the external strain on the

piezoelectric polarization of AlGaN/GaN material systems. This relatively large effect

may have application in strain and pressure sensors.

3.2 Pressure induced Changes in the Conductivity of AlGaN/GaN HEMTs

AlGaN/GaN high electron mobility transistors (HEMTs) show a strong dependence

of source/drain current on the piezoelectric polarization induced two dimensional electron

gas (2DEG). The spontaneous and piezoelectric polarization induced surface and

interface charges can be used to develop very sensitive but robust sensors for the

detection of pressure changes. The changes in the conductance of the channel of a









AlGaN/GaN High Electron Mobility Transistor (HEMT) membrane structure fabricated

on a Si substrate were measured during the application of both tensile and compressive

strain through changes in the ambient pressure.

The piezoelectric polarization induced sheet carrier concentration of undoped Ga-

face AlGaN/GaN can be calculated by following equation [AmbOO, Amb99, Lu01]:


n (x) C (X) (e (x )+ EF(, ()C (x)) (3-9)
n, e dde

where E(x) is the dielectric constant, dd is the AlGaN layer thickness, epb is the Schottky

barrier of the gate contact on AlGaN, EF is the Fermi level and AEc is the conduction

band discontinuity between AlGaN and GaN. The sheet carrier concentration induced by

the piezoelectric polarization is a strong function of Al concentration. If an external

stress can be applied to AlGaN/GaN material system, the sheet carrier concentration can

be changed significantly and devices fabricated in this fashion could be used in sensor-

related applications.

Circular membrane of AIGaN/GaN
P / S


PI Si substrate

__ glass


\ kR
*w


Z
P P P>P,

Figure 3-5. Circular membrane of AlGaN/GaN on a Si substrate fabricated by etching a
circular hole in the substrate (left). A deflection of the membrane away from
the substrate due to differential pressure on the two sides of the membrane
produces a tensile strain in the membrane (right).


_ D .


-----









The sensors used to monitor the differential pressure are made of a circular

membrane of AlGaN/GaN on a Si substrate by etching a circular hole in the substrate, as

illustrated in Figure 3-5(left). A deflection of the membrane away from the substrate due

to differential pressure on the two sides of the membrane produces a tensile strain in the

membrane, as shown in Figure 3-5(right).

The differential piezoelectric responses of AlGaN and GaN layers creates a space

charge which induces 2DEG at the AlGaN/GaN interface. The concentration of 2DEG is

expected to be directly correlated with the tensile strain in the membrane and hence with

the differential pressure. The radial strain is given by [Sto09, Chu98],

Sr = (S-D)/D = (20R 2RsinO)/(2RsinO) = 0/sinO 1 (3-10)

The total tensile force around the edge of the circular membrane is,

T = 7[DtGaNGr = 7[D tGaN [EGaN/ (1- V)] Sr (3-11)

where tGaN is the film of thickness, D is the diameter of the via hole, or = [EGaN/ (1- v)] Sr,

EGaN is the Young's modulus and v is the Poisson's ratio of the GaN film. The

component of T along z direction is balanced by the force on the membrane due to a

differential pressure Pi Po, where Pi and Po are the inside and outside pressure

respectively. Hence TsinO = (Pi Po) t2D2/4 and the radial strain, Sr, in the nitride film can

be expressed as a function of the differential pressure Pi Po.

(0 sin 0) = (Pi Po)[(1- v)D]/(4EGaNtGaN) (3-13)

where EGaN is the Young's modulus, D is the diameter of the via hole, tGaN is the film of

thickness and v is the Poisson's ratio of the GaN film. If 0 is measured, the differential

pressure Pi Po. can be estimated with Equation 3-13. We also derive the relationship

between conductance, u, of AlGaN/GaN HEMT and radial strain, Sr.









o = a( rAlGaN) + P( fGaN ) x Sr (3-14)

where a(rAiGaN)= ts{ 1/[l + (g0s(x)/tAIGaN e2)h2/4x7m*(x)]} {- APspI
-leeff |ASAlGaN AlGaN [FO(x)/tA1GaN e] [e4b(x) AEc(x)] } (3-15)

and P( fGaN ) = s{ 1/[l + (sos(X)/tA1GaN e2 )h2/47m*(x)]} { eeff (GaN)|
eeff(AlGaN) } (3-16)

where [ts is the mobility of 2DEG, So is the electric permitivity, s(x) = 9.5 0.5x is the

relative permitivity, e4b(x) = 0.84 + 1.3x (eV) is the Schottky barrier height, eeff = (e31-

e33)C13/ C33, h is Plank constant, e is the electron charge, m*(x) 0.228me. By

monitoring the conductance of the HEMT on membrane, the pressure difference, Pi Po,

can be obtained. The detail deivations of above equations are listed in the Appendix.









HEMT

ICP etch
Si (SFS)
AuSn
glass

Figure 3-6. Schematic diagram of device structure with a finger patterned device on the
HEMT membrane.

The HEMTs were grown by metalorganic chemical vapor deposition on 100 mm

(111) Si substrates at Nitronex Corporation. The structures consisted of an (Al,Ga)N-

based transition layer, -0.8 tm undoped GaN buffer, and 300A undoped AlGaN barrier

layer. Mesa isolation was performed with an Inductively Coupled Plasma (ICP) etching

with C12/Ar based discharges at -90 V dc self-bias, ICP power of 300 W at 2 MHz and a









process pressure of 5 mTorr. Ti/Al/Pt/Au based inter-digitated finger pattern separated by

4 itm was formed with e-beam deposition and standard lift-off shown in Figure 3-6.

The fingers were annealed at 850 C, 45 sec under flowing N2. Plated Au was

subsequently deposited on the ohmic metal pads for wire bonding on the samples. Via

holes were fabricated from the back side of the Si substrate and stopping on the GaN

layer using ICP etching with SF6/Ar. The etch selectivity is more than 1000:1. 2000A of

AuSn was deposited on the backside of the sample and a glass slice. A RD automation

flip-chip bonder was used to bond the glass slice and the sample at 400 OC to seal off the

via holes.

Figure 3-7 shows scanning electron microscopy (SEM) photos of the via through

the Si wafer(left) and cross sectional view of the via(right).





















Figure 3-7. SEM micrographs of via through the Si wafer(left)and cross sectional view of
a via hole(right)

The dc current-voltage(I-V) characteristics were obtained from measurements on

an Agilent 4156C parameter analyzer while the device was measured at 250C under either










vacuum(lOmTorr) or pressure (40-200psi) conditions. Figure 3-8 shows the drain-source

I-V characteristics from the membrane HEMT structure as a function of the ambient

pressure. This current increases with increasing pressure and decreases under vacuum

conditions.



12 .
--- 10 mT
8 atmosphere
40 psi
4 100 psi
200 psi

Eo
-- -4



-3-04 -02 00 02 04
-12 1 V IA)
-3 -2 -1 0 1 2 3
VDS()



Figure 3-8. IDs-VDs characteristics at 25C from AlGaN/GaN HEMT membrane as a
function of applied pressure.

The resulting channel conductance derived from this data is shown as a function of

differential pressure in Figure 3-9. In the case of applied positive pressure, which

corresponds to compressive strain induced in the HEMT layers, the conductivity

decreases with a coefficient of -7. 1x102 mS/bar. For the case of applied negative

pressure (vacuum), the conductivity shows a positive coefficient of the same value within

experimental error, given the limited data for vacuum conditions. These trends are similar

to those observed with actual bending of HEMT samples on a cantilever beam to produce

tensile or compressive strain [Kan03], but exhibit sensitivities to the induced tensile or

compressive strain of almost two orders of magnitude larger. This is due to the absence of










the thick sapphire substrate that is present in the cantilever structures. The new membrane

structures are particularly sensitive to changes in differential pressure.



5.6

5.4 0Vacuum(tensile strain)
5.4 -- Pressure(compressive strain)

) 5.2\.

S 5.0
.-.
0 4.8 -
o m

4.6

0 2 4 6 8 10 12 14
Pressure difference, IPo-P,(bar)



Figure 3-9.Channel conductivity of the AlGaN/GaN HEMT membrane as a function of
differential pressure.

AlGaN/GaN high electron mobility transistors (HEMTs) show a strong dependence

of source/drain current on the piezoelectric polarization induced two dimensional electron

gas (2DEG). The spontaneous and piezoelectric polarization induced surface and

interface charges can be used to develop very sensitive but robust sensors for the

detection of pressure changes. The changes in the conductance of the channel of a

AlGaN/GaN High Electron Mobility Transistor (HEMT) membrane structure fabricated

on a Si substrate were measured during the application of both tensile and compressive

strain through changes in the ambient pressure. The conductivity of the channel shows a

linear change of -(+)7. 1x102 mS/bar for application of compressive(tensile) strain. The

AlGaN/GaN HEMT membrane-based sensors appear to be promising for pressure

sensing applications.









In summary, an AlGaN/GaN HEMT membrane on Si shows large changes in

channel conductivity as a result of changes in ambient pressure. These structures appear

promising for use in integrated sensors in which the HEMTs can also be used for gas,

chemical and biological detection combined with on-chip transmission of the data.

3.3 Capacitance Pressure Sensor Based on GaN HEMT on Si Membrane

The AlGaN/GaN high-electron-mobility transistors (HEMTs) show a strong

dependence of the conductance of the channel when a membrane structure fabricated on a

Si substrate was measured during changes in the ambient pressure [Kan04c, Kan03,

Pea04a]. However, one drawback of piezoresistive sensors is that contact resistance

changes significantly with temperature and may mask the changes in sensor signal from

actual pressure changes [You04]. By sharp contrast, capacitive pressure sensors are less

sensitive to variations in contact resistance and in addition, sensors based on AlGaN/GaN

HEMTs could be readily integrated with off-chip wireless communication chips that

eliminate additional wiring capacitance. AlGaN/GaN high electron mobility transistors

(HEMTs) have demonstrated extremely promising results for use in broad-band power

amplifiers in wireless base station applications [ZhaOl, Tar02, Zha03a, Shu98]. The high

electron sheet carrier concentration of nitride HEMTs is induced by piezoelectric

polarization of the strained AlGaN layer and spontaneous polarization [AmbOO, Amb99,

Asb97], suggesting that nitride HEMTs are excellent candidates for robust pressure

sensing.

In this part, Circular AlGaN/GaN diaphragms fabricated with radii 200-600 tm on

Si substrates show linear changes in capacitance over a range of applied pressure and that

the sign of the capacitance change is reversed when vacuum is applied to the diaphragm.









The sensors used to monitor the differential pressure are made of a circular

membrane of AlGaN/GaN HEMT on a Si substrate. The membrane is fabricated by

etching a circular hole in the substrate, as shown schematically in Figure 3-10 (top). A

scanning electron microscope (SEM) cross-sectional view of an actual device is shown at

the bottom of Figure 3-10


-- Ohmic contact
(Ti/AI/Pt/Au)

ICP etch
-- Silicon Substrate
Teflon


Ohmic contact(AI)


I I


Figure 3-10. Schematic diagram of device structure (top) and SEM micrograph of
AlGaN/GaN circular membrane on a Si substrate fabricated by etching a
circular hole in the substrate(bottom).


-- ----------- - - ----------- ------ -----


Si





Teflon Si Substrate









A deflection of the membrane away from the substrate due to differential pressure

on the two sides of the membrane produces a tensile strain in the membrane. This leads

to a change in the piezo-induced two dimensional electron gas (2DEG) density at the

AlGaN/GaN interface.

This in turn affects the capacitance of the HEMT diaphragm. The carrier density is

therefore directly correlated with the tensile strain in the membrane and hence with the

differential pressure. We have previously calculated the radial strain, ;r, in the membrane

as a function of the differential pressure Pi Po [Kan04c] by employing a modified Stoney

analysis [Sto09, Chu98]. By monitoring the conductance of the HEMT on membrane,

the pressure difference, Pi Po, can be obtained.

The HEMTs were grown by metal-organic chemical vapor deposition on 100 mm

(111) Si substrates at Nitronex Corporation. The structures consisted of an (Al,Ga)N-

based transition layer, -0.8 tm undoped GaN buffer, and 300A undoped AlGaN barrier

layer. Mesa isolation was performed with an Inductively Coupled Plasma (ICP) etching

with C12/Ar based discharges at -90 V dc self-bias, ICP power of 300 W at 2 MHz and a

process pressure of 5 mTorr. Ti/Al/Pt/Au based inter-digitated finger pattern separated

by 4 rm was formed with e-beam deposition and standard lift-off. The fingers were

annealed at 850 C, 45 sec under flowing N2. Plated Au was subsequently deposited on

the ohmic metal pads for wire bonding on the samples. Ohmic contact for the silicon

used 1000A Al deposited by using sputter and annealed in nitrogen at 3000C. Via holes

were fabricated from the back side of the Si substrate and stopping on the GaN layer

using ICP etching with SF6/Ar. The etch selectivity is more than 1000:1. 2000A of AuSn

was deposited on the backside of the sample and a glass slice.
































36 -
m----m-m-m-m-m--m---

32 +- R=600 tm
-0-- R=400 im
L28 R=280 tm
S-v-- R=200 im
S24
O0O o--O0-0-0-0-0
S20 -DO

0 16
v --v-v-v--v--v--v--v
12 I i i ,
0 2 4 6 8 10
Pressure(bar)



Figure 3-11. Top view of HEMT capacitance pressure sensor(top) and capacitance as a
function of pressure for different diaphragm radii (bottom).

The teflon bonding spin coating on the silicon wafer employed liquid Teflon

(CYTOP CTL-809M, from Bellex International Corp.) at 5000 rpm to a thickness of-

5000A and then flipchip bonding (RD automation flip-chip bonder) the fabricated device









using a mechanical force of 1000g between the upper and lower chucks for 10 minutes

and finally baking for 1 hour at 2000C to solidify the Teflon to seal off the via holes.

Figure 3-1 l(top) shows a top view of a completed HEMT capacitive pressure sensor.

The capacitance of the HEMT diaphragm structures were obtained from

measurements on an Agilent 4156C parameter analyzer while the device was measured at

25C under either vacuum (-1 bar) or pressure (+9.5 bar) conditions. Figure 3-1 l(bottom)

shows the capacitance from the membrane HEMT structure as a function of the ambient

pressure, for different membrane radii. This capacitance increases with increasing

pressure and decreases under vacuum conditions, due to corresponding changes in the

carrier density in the 2DEG.





2 -- "--



0---



g 9 1 R=600 pm
2 o R=400 Im

1 R=280 pm
O C R=-00 ,m,
-2 0 2 4 6 8 10

Pressure(bar)



Figure 3-12. Capacitance change as a function of radius of the AlGaN/GaN HEMT
membrane over the pressure range from -1 to +9.5 bar.
membrane over the pressure range from -i to +9.5 bar.










The resulting capacitance change derived from this data is shown as a function of

pressure in Figure 3-12. In the case of applied positive pressure, which corresponds to

compressive strain induced in the HEMT layers, the capacitance increases in a linear

fashion over the range between 0 and +1 bar with a sensitivity of 0.86pF/bar for a 600

lm radius membrane. For the case of applied negative pressure (vacuum), the

conductivity shows a sensitivity of the same value within experimental error to a vacuum

of-0.5 bar. Within the linear range, the devices exhibited a hysteresis of <0.4%. Outside

these pressure limits the sensor has reduced sensitivity due to the device geometry. The

sensitivity could be increased by having a shallower via depth, obtained by thinning the

Si substrate.






LL
-. 3 -


c

U 2
0
(D
6'B
U


(U


150 300 450 600
Radius(pm)



Figure 3-13. Capacitance change as a function of radius of the AlGaN/GaN HEMT
membrane over the pressure range from -1 to +9.5 bar

These trends are similar to those observed with actual bending of HEMT samples

on a cantilever beam to produce tensile or compressive strain [Kan03], but exhibit much









larger sensitivities to the induced tensile or compressive strain. This is due to the absence

of the thick sapphire substrate that is present in the cantilever structures, as we also

reported for the piezo-conductance membrane sensors previously [Kan04c].

Figure 3-13 shows the capacitance change as a function of radius of the

AlGaN/GaN HEMT membrane at a fixed pressure of +9.5 bar. The capacitance of the

channel displays a change of 7.19 +/-0.45x10-3 pF/[tm. The sensor characteristics

measured at this same pressure on several different days showed a maximum capacitance

variation of 0.07pF, corresponding to a sensing repeatability of -0.15 bar. The high

temperature characteristics still need to be established, but in this case will be limited by

the thermal stability of the ohmic contacts. Recent reports have shown that some contact

metallization on GaN HEMTs are stable for extended periods at 5000C [Sel04].

In summary, an AlGaN/GaN HEMT membrane on Si shows large changes in

capacitance as a result of changes in ambient pressure. This approach is less sensitive to

contact resistance variations with temperature than the previous conductance sensors. The

sensors can also be readily integrated with conventional HEMTs or Si circuitry to provide

off-chip wireless transmission of pressure data.














CHAPTER 4
CATALYST BASED GAS SENSOR FOR HYDROCARBON GASES

4.1 Introduction

Gas sensors have been fabricated on a number of semiconductors using catalytic

metals as the gate in the metal insulator semiconductor (MIS) or as the metal contact in

Schottky diodes [You82, Lun86, Rye87]. Various field effect transistors based on silicon

have been developed by several groups for hydrogen gas sensing [Lun89]. But the silicon

based sensors are limited to operation in environments of below 250C, prohibiting them

from being used as hydrocarbon detectors or for other applications requiring high

temperature operation. Because hydrocarbon gases should be decomposed by the

catalytic metals and hydrogen atoms diffuse to the device interface, it is presumed that a

dipole forms, lowering the effective work function of the metal and changing electrical

characteristics of the devices.

Baranzanhi et al. [Bar95] demonstrated gas sensitive Pt gated SiC transistors

operating up to 5000C but the SiC Schottky diodes have displayed poor thermal stability

and formation of Pd silicides has been observed at temperatures as low as 425C when Pd

was used as Schottky metal [Hun95, Che96]. Luther et al. [Lut99] first demonstrated Pt-

GaN gas sensor for hydrogen and propane at high temperature (200-400C). It was also

exhibited that Pt-GaN gas sensor showed faster response for hydrocarbons and enhanced

sensitivity at higher temperatures (500C) After that, Schalwig et al. [SchOl] showed gas

sensors for the exhausted lean bum engines using Pt-GaN and Pt-HEMTs. The device

performance at high temperature (200-600C) was investigated. It was also shown that a









HEMT based gas sensor was more sensitive than GaN diodes but the detailed analysis of

sensitivity difference between GaN diodes and HEMTs was left as future work.

In this chapter, extensive discussion of all the processes including fabrication,

measurement, and characterization of gas sensor devices using wide bandgap

semiconductor diodes and transistors will be given. In Section 4.2, the higher

performance of AlGaN/GaN MOS diode for hydrogen gas sensor will be given compared

with Schottky GaN diode. In Section 4.3, hydrogen reversible changes in drain and

source current in the transistor based gas sensor will be given. In addition, the reason for

higher sensitivity of this structure operated with gain will also be discussed. In Section

4.4, a direct comparison of MOS and Schottky W/Pt-GaN diode for hydrogen detection

will be given. In Section 4.5, the method for detecting ethylene (C2H4), which causes

problems because of its strong double bond s and hence the difficulty in dissociating it at

modest temperature will be investigated using wide band gap semiconductor. In Section

4.6, nanotechnology driven sensor for hydrogen and ozone detection will be discussed

using multiple ZnO nanorods.

4.2 AlGaN/GaN based MOS Diode Hydrogen Gas Sensor

Simple GaN Schottky diodes exhibit strong changes in current upon exposure to

hydrogen containing ambients [Ste03a, NeuOl, SchOl, EicOl, Sch02a, Stu02, Eic03,

Kim03a]. The effect is thought to be due to a lowering of the effective barrier height as

molecular hydrogen catalytically cracks on the metal gate and atomic hydrogen diffuses

to the interface between the metal and GaN, altering interfacial charge. Steinhoff et al.

[Ste03a] founded that it was necessary to have a native oxide present between

semiconductor and the gate metal in order to see significant current changes. Thus, it is









desirable to specifically incorporate an oxide into GaN-based diodes or HEMTs in order

to maximize the hydrogen detection response.


Schottky contact metal


final metal-

Ohmic
contact metal


Sc2O3
SiNX


Figure 4-1.Cross-sectional schematic of completed MOS diode on AlGaN/GaN HEMT
layer structure (top) and plan-view photograph of device(bottom).

Gas sensors based on MOS diode on AlGaN/GaN high electron mobility

transistor(HEMT) layer structure are of interest, because HEMTs are expected to be the

first GaN electronic device that is commercialized, as part of next generation radar and

wireless communication systems. These structures have much higher sensitivity than

Schottky diodes on GaN layer, because they are true transistors and therefore operate









with gain. In addition, the MOS-gate version of the HEMT has significantly better

thermal stability than a metal-gate structure [Kha0l, Pal00, Sim00, Kou02, Sim02] and is

well-suited to gas sensing. When exposed to changes in ambient, changes in the surface

potential will lead to large changes in channel current.

HEMT layer structures were grown on C-plane A1203 substrates by Metal Organic

Chemical Vapor Deposition (MOCVD). The layer structure included an initial 2tlm thick

undoped GaN buffer followed by a 35nm thick unintentionally doped A10.28Ga0.72N layer.

The sheet carrier concentration was 1 x1013 cm-2 with a mobility of 980 cm2/V-s at room

temperature

Mesa isolation was achieved with 2000 A plasma enhanced chemical vapor

deposited SiNx. The ohmic contacts was formed by lift-off of e-beam deposited

Ti(200A)/Al(1000A)/Pt(400A)/Au(800A). The contacts were annealed at 850 C for 45

sec under a flowing N2 ambient in a Heatpulse 610T system. 400A Sc203 was deposited

as a gate dielectric through a contact window of SiNx layer. Before oxide deposition, the

wafer was exposed to ozone for 25 minutes. It was then heat in-situ at 300 OC cleaning

for 10mins inside the growth chamber. 100 A Sc203 was deposited on AlGaN/GaN by rf

plasma-activated MBE at 100 OC using elemental Sc evaporated from a standard effusion

all at 1130 C and 02 derived from an Oxford RF plasma source [GilOl, KimOOb,

KimOOc]. 200 A Pt Schottky contact was deposited on the top of Sc203. Then, final

metal of e-beam deposited Ti/Au (300A/1200A) interconnection contacts was employed

on the MOS-HEMT diodes. Figure 4-1 shows a schematic (top) and photograph (bottom)

of the completed device. The devices were bonded to electrical feed-through and exposed

to different gas ambient in an environmental chamber [Kim03a, Kim03b, Kim03c].






42




20 .

R3010% H2
15 R10 N2
-o-R30 10% H .
R10 N2 0
E lo 0* 0


0 O
5 moo 0

E 10 i II


0.0 0.5 1.0 1.5 2.0 2.5 3.0
Biased Voltage(V)



Figure 4-2. Forward I-V characteristics of MOS-HEMT based diode sensors of two
different dimensions at 250C measured under pure N2 or 10%H2 /90%N2
ambient ;R30-diode with 30 rm radius, R10 diode with 10 rm radius.

Figure 4-2 shows the forward current-voltage (I-V) characteristics at 25 OC of the

MOS-HEMT diode both in pure N2 and in a 10%H2/ 90%H2 atmosphere. At a given

forward bias, the current increases upon introduction of the H2, through a lowering of the

effective barrier height. The H2 catalytically decomposes on the Pt metallization and

diffuses rapidly though the underlying oxide to the interface where it forms a dipole layer

[Eic03].

At 2.5V forward bias the change in forward current upon introduction of the

hydrogen into the ambient is -6mA or equivalently 0.4V at a fixed current of 10mA. This

is roughly double the detection sensitivity of comparable GaN Schottky gas sensors

tested under the same conditions [Kim03c], confirming that the MOS-HEMT based diode










has advantages for applications requiring the ability to detect combustion gases even at

room temperature.



72 i i ,


71 lsec
-- 20sec
T 30sec
S70 -


6 69


68 -


67 I I I
0 50 100 150 200
Time(sec)



Figure 4-3. Time response at 250C of MOS-HEMT based diode forward current at a fixed
bias of 2V when switching the ambient from N2 to 10%H2 /90%oN2 for periods
of 10, 20 or 30 seconds and then back to pure N2

As the detection temperature is increased, the response of the MOS-HEMT diodes

increases due to more efficient cracking of the hydrogen on the metal contact. The

threshold voltage for a MOSFET is given by [Shu90]


V = FB + 2B+(4eND )05 (4-1)
C,

where, VFB is the voltage required for flat band conditions, qb the barrier height, e the

electronic charge and C, the Sc203 capacitance per unit area.

In analogy with results for MOS gas sensors in other materials systems [Ste03a],

the effect of the introduction of the atomic hydrogen into the oxide is to create a dipole

layer at the oxide/semiconductor interface that will screen some of the piezo-induced







44


charge in the HEMT channel. To test the time response of the MOS diode sensors, the

10%H2/90%N2 ambient was switched into the chamber through a mass flow controller for

periods of 10, 20 or 30 seconds and then switched back to pure N2.


50 100 150 200
Tin e(sec)


*i U
U
F'

'C
10.2r" i I


* U
U
U
U



hK


10.2 r' .- I I I ,I
0 50 100 150 200
Tin e(sec)


250 300


Figure 4-4. Time response at 250C of MOS-HEMT based diode forward current at a fixed
bias of 2V for three cycles of switching the ambient from N2 to 10%H2
/90%N2 for periods of 10 (top) or 30 (bottom) seconds and then back to pure
N2


10.6 F


10.4 k


10.2
0


II

* -
El U





,


250 300


10.8


10.6k


10.4 I


. .. :, :. 1









Figure 4-3 shows the time dependence of forward current at a fixed bias of 2V

under these conditions. The response of the sensor is rapid(
taking almost the full 30 seconds. Upon switching out of the hydrogen-containing

ambient, the forward current decays exponentially back to its initial value. This time

constant is determined by the volume of the test chamber and the flow rate of the input

gases and is not limited by the response of the MOS diode itself. Figure 4-4 shows the

time response of the forward current at fixed bias to a series of gas injections into the

chamber, of duration 10secs each (top) or 30 secs each (bottom). The MOS diode shows

good repeatability in its changes of current and the ability to cycle this current in

response to repeated introductions of hydrogen into the ambient. Once again, the

response appears to be limited by the mass transport of gas into and out of the chamber

and not to the diffusion of hydrogen through the Pt/Sc203 stack.

In conclusion, AlGaN/GaN MOS-HEMT diodes appear well-suited to combustion

gas sensing applications. The changes in forward current are approximately double those

of simple GaN Schottky diode gas sensors tested under similar conditions and suggest

that integrated chips involving gas sensors and HEMT-based circuitry for off-chip

communication are feasible in the AlGaN/GaN system.

4.3 Hydrogen Induced Reversible Changes in Drain Current in Sc2O3/AlGaN/GaN
HEMTs

It has been observed that AlGaN/GaN metal oxide semiconductor (MOS) diodes

utilizing Sc203 as the gate dielectric have approximately double the sensitivity for

hydrogen detection than Pt/GaN Schottky diodes [Kan04a]. The use of a true MOS

transistor should be even more effective because of the current gain in the 3-terminal

device. The reversible hydrogen-induced changes in drain-source current were









investigated using Sc203/AlGaN/GaN High Electron Mobility Transistors (HEMTs). The

current changes are significantly larger (a factor of 5) than observed for

Sc203/AlGaN/GaN MOS diodes exposed under the same conditions. The response time

of the HEMTs is limited by the mass transfer characteristics of the hydrogen-containing

gas ambient into the test chamber. These devices can be used as sensitive combustion gas

sensors, but the results also point out the susceptibility of the HEMTs to changes in

current depending on the composition of the ambient in which they are being operated.

The HEMT layer structures were grown on C-plane A1203 substrates by Metal

Organic Chemical Vapor Deposition (MOCVD). The layer structure included an initial

2[tm thick undoped GaN buffer followed by a 35nm thick unintentionally doped

Al0.28Gao.72N layer. The sheet carrier concentration was 1 x 1013 cm-2 with a mobility of

980 cm2/V-s at room temperature. Mesa isolation was achieved by using an inductive

coupled plasma system with Ar/C12 based discharges. The ohmic contacts was formed by

lift-off of e-beam deposited Ti(200A)/Al(1000A)/Pt(400A)/Au(800A). The contacts

were annealed at 850 C for 45 sec under a flowing N2 ambient in a Heatpulse 610T

system. 100A Sc203 was deposited as a gate dielectric through a contact window of SiNx

layer. Before oxide deposition, the wafer was exposed to ozone for 25 minutes. It was

then heat in-situ at 300 C cleaning for 10mins inside the growth chamber. The Sc203

was deposited by rf plasma-activated MBE at 100 OC using elemental Sc evaporated from

a standard effusion all at 1130 OC and 02 derived from an Oxford RF plasma source

[GilOl, KimOOb]. 200 A Pt Schottky contact was deposited on the top of Sc203. Then,

final metal of e-beam deposited Ti/Au (300A/1200A) interconnection contacts was

employed on the MOS-HEMTs.



























Schottky contact
metal
final metal

ohmic S203
ohmic SiN
contact metal SiNx
AIGaN

GaN


Al203 Substrate




Figure 4-5. Photograph of MOS HEMT hydrogen sensor

Figure 4-5 shows photograph (top) and a cross sectional schematic (bottom) of the

completed device. The gate dimension of the device is 1 x 50 im2. The devices were

bonded to electrical feed-through and exposed to either pure N2 or 10%H2/90% N2

ambients in an environmental chamber in which the gases were introduced through

electronic mass flow controllers.

Figure 4-6 shows the MOS-HEMT drain-source current voltage (IDs-VDS)

characteristics at 250C measured in both the pure N2 or 10%H2/90% N2 ambients. The

current is measurably larger in the latter case as would be expected if the hydrogen






48


catalytically dissociates on the Pt contact and diffuses through the Sc203 to the interface

where it screens some of the piezo-induced channel charge [Eic03].



40
35 -- N2(VG=0 to -6V)
--10% H2(VG=0 to -6V)
30
25














Figure 4-6. IDs-VDs characteristics of MOS-HEMT measured at 25C under pure N2
S20







ambient or in 10% H90%N ambient.
5
0-
0 1 2 3 4 5
VDS(V)



Figure 4-6. IDS-VDS characteristics of MOS-HEMT measured at 25oC under pure N2
ambient or in 10% H2/90%N2 ambient.

This is a clear demonstration of the sensitivity of AlGaN/GaN HEMT dc

characteristics to the presence of hydrogen in the ambient in which they are being

measured. The use of less efficient catalytic metals as the gate metallization would reduce

this sensitivity, but operation at elevated temperatures would increase the effect of the

hydrogen because of more efficient dissociation on the metal contact.

Figure 4-7 shows the measured change in drain-source current for measurement in

the two different ambients as a function of gate voltage for different drain-source

voltages. The maximum change in this current is -3 mA(150 mA/mm), which is

approximately a factor of 5 larger than obtained for the same bias conditions in

Sc203/AlGaN/GaN MOS diodes exposed under the same conditions [Kan04a] and an







49


order of magnitude larger than the changes in forward current in simple Pt/GaN Schottky

diodes in the same test chamber.




3.0


/ \
2.5 -

2.0 -

o 1.5
VDS3V
1.0- VDSV=4V
S/DS =5V
0.5 -

-6 -5 -4 -3 -2 -1 0

VG(V)






140

120

100 -
E
E 80 I

60
i/ -N2
S40 10% H2
VDS = 3V
20 -

0 I I I I I I I
-6 -5 -4 -3 -2 -1 0
V (V)



Figure 4-7. Change in drain-source current for measurement in N2 versus 10%H2 /90%N2
ambient, as a function of gate voltage (top) and corresponding
transconductance at a fixed drain-source voltage of 3V.









This shows the advantage of using the 3-terminal device structure, with its

attendant current gain. This approach is particularly advantageous when small

concentrations of hydrogen must be detected over a broad range of temperatures. The

MOS-HEMT is more thermally stable than a conventional metal gate HEMT and will be

usable at higher temperatures. The change in drain-source current tracked the device

transconductance, as shown at the bottom of Figure 4-7. The shift in peak

transconductance when hydrogen is present in the ambient is consistent with an increase

in the total channel charge.

Figure 4-8 shows some of the recovery characteristics of the MOS-HEMTs upon

cycling the ambient from N2 to 1%H2/ 99% N2. While the change in drain-source current

is almost instantaneous (< 1 sec),the recovery back to the N2 ambient value is of the order

of 20 secs.

This is controlled by the mass transport characteristics of the gas out of the test

chamber, as demonstrated by changing the total flow rate upon switching the gas into the

chamber. Given that the current change upon introduction of the hydrogen is rapid, the

effective diffusivity of the atomic hydrogen through the Sc203 must be greater than 4

x10-12 cm2/V.s at 25C. Note the complete reversibility of the drain-source current for

repeated cycling of the ambient.

In conclusion, Sc203/AlGaN/GaN MOS-HEMTs show a marked sensitivity of their

drain-source current to the presence of hydrogen in the measurement ambient. This effect

is due to the dissociation of the molecular hydrogen on the Pt gate contact, followed by

diffusion of the atomic species to the oxide/semiconductor interface where it changes the

piezo-induced channel charge. The MOS-HEMTs show larger changes in current than







51


their corresponding MOS-diode or Schottky diode counterparts and show promise as

sensitive hydrogen detectors.


16.8

16.7

16.6

16.5

16.4

16.3

16.2








16.8

16.7

16.6

16.5

16.4

16.3

16.2


I I I I I I I
0 5 10 15 20 25 30 35

Time(sec)






1% H2 in N2


*





' m
M


0 20 40 60

Time(sec)


80 100 120 140 160

at VG = -3 V


Figure 4-8. Time dependence of drain-source current when switching from N2 to 1%H2
/99% N2 ambient and back again. The top shows different injection times of
the H2/N2, while the bottom shows the reversibility of the current change.


o --- 7 sec
v-- 5 sec
-- 3 sec
1 sec
y 1%Hin N
0 2 2









4.4 Comparison of MOS and Schottky W/Pt-GaN Diodes for Hydrogen Detection

For GaN Schottky diodes, there is evidence that the presence of an interfacial oxide

increases the magnitude of the change in barrier height [Sch02b] and hence the current

flowing at a given bias on the device. In support of that finding, we have previously

observed that AlGaN/GaN metal-oxide semiconductor(MOS) diodes utilizing Sc203 as

the gate dielectric have approximately double the sensitivity for hydrogen detection than

Pt/GaN Schottky diodes [Kan04a]. An additional key requirement in some sensor

applications such as long-term spaceflight is the need for very good reliability of the

contact metal on the semiconductor. We have found that W shows little reaction with

GaN to temperatures in excess of 700C [Col96, Col97, Cao98] and when used as a bi-

layer with Pt, can also provide measurable sensitivity for H2 detection. The response time

of both types of sensor is limited by the mass transfer characteristics of the hydrogen-

containing gas ambient into the test chamber.

Approximately 6[tm of n-GaN was grown on sapphire substrates by Metal Organic

Chemical Vapor Deposition. Ohmic contacts was formed by lift-off of Ti/Al/Pt/Au,

annealed at 5000C. In some cases, MOS structures were formed by deposition of 100A

Sc20 as a gate dielectric through a contact window of SiNx .Before oxide deposition, the

wafer was exposed to ozone for 25 minutes. It was then heat in-situ at 300 OC cleaning

for 10mins inside the growth chamber. The Sc203 was deposited by rf plasma-activated

MBE at 100 C using elemental Sc evaporated from a standard effusion all at 1130 C

and 02 derived from an Oxford RF plasma source [GilOl, KimOOb]. 200 A of W was

deposited on both types of samples by sputtering, followed by e-beam evaporation of

150A of Pt. The Pt Schottky contacts were formed by lift-off







53



Schottky contact metal(W/Pt)


Schottky contact metal(W/Pt)


final metal-

Ohmic
contact metal
(Ti/AI/Pt/Au)


-Sc0O
-SIN,


GaN


AIO, substrate


Figure 4-9.Schematic of both the W/Pt Schottky diode (top) and MOS diode (bottom).


Figure 4-10. Photograph of packaged gas sensor


SiN,
























0 1 2 3 4
Voltage(V) at 3000C


Voltage(V) at 600 C


Figure 4-11.Forward I-V characteristics at 300 oC(top) or 500 oC(bottom) from the
Schottky and MOS diodes in pure N2 and 10% H2 /90% N2

Figure 4-9 shows a cross-sectional schematic of the completed devices. The

devices were bonded to electrical feed-through and exposed to either pure N2 or







55


10%H2/90% N2 ambients in an environmental chamber in which the gases were

introduced through electronic mass flow controllers.


10


E
1
(D

0

0.1












10


0 1 2 3
Voltage(V)


4 5


0 1 2 3 4 5
Voltage(V)



Figure 4-12. Measurement temperature dependence of forward I-V characteristics of the
Schottky (top) and MOS (bottom) diodes in both pure N2 and 10% H2 /90%
N2.









Figure 4-10 shows a photograph of a typical packaged device. Figure 4-11 shows

the forward I-V characteristics of both the Schottky and MOS diodes at both 300(top) and

600 C (bottom) measured in both the pure N2 or 10%H2/90% N2 ambients. The current is

measurably larger in the latter case as would be expected if the hydrogen catalytically

dissociates on the Pt contact and diffuses through the W (and the through the Sc203 in the

case of the MOS diodes) to the interface where it screens some of the piezo-induced

channel charge [Sve99]. The decrease in effective barrier height was obtained from

fitting the forward I-V characteristics to the relation for the thermionic emission over a

barrier


J, = A.T2 exp(- ) exp( e (4-2)
kT nkT

where Jis the current density, A* is the Richardson's constant for n-GaN, Tthe absolute

temperature, e the electronic charge, ob the barrier height, k Boltzmann's constant, n the

ideality factor and V the applied voltage. From the data, the decrease in 4b in the presence

of 10%H2 in the ambient was 30-50mV over the range of temperatures investigated here.

This is a clear demonstration of the sensitivity of GaN diode dc characteristics to the

presence of hydrogen in the ambient in which they are being measured. Operation at

elevated temperatures should increase the effect of the hydrogen because of more

efficient dissociation on the metal contact.

Figure 4-12 shows the forward I-V characteristics over the entire temperature range

investigated, measured in both pure N2 and 10%H2 /90%H2 .The turn-on voltage, VF, of

the diodes, defined as the voltage at which the forward current is 1A.cm-2, and given by

nkT J
the relation VF ln( )+ nB +RN -J (4.3)
e A T







57


decreases with temperature for both types of diodes due to the lowering of the effective

energy barrier for electrons


1.25


1.00


0.75
I-
> 0.50


0.25


0.025


0.020

E
0.015



0.010


0.005


0 100 200 300 400 500 600

Temperature(oC)


S- GaN 0
--- GaN with oxide o






0
0
0



0 100 200 300 400 500 600

Temperature(oC)


Figure 4-13.Temperature dependence of turn-on voltage(top) and on-state
resistance(bottom) for the GaN Schottky and MOS diodes.

The on-state resistance ,RON, derived from the relation


- --GaN
--- GaN with oxide






58


RoN = (4V,2 / ls-. EM3)+ p ..W + Rc (4.4)

where, s is the GaN permittivity, t the carrier mobility, S and Ws are substrate resistivity

and thickness, and Rc is the contact resistance ,increased with measurement temperature

for both types of diodes, as shown in Figure 4-13.The MOS-HEMT is more thermally

stable than a conventional metal gate HEMT and should be usable at higher temperatures.



2.0 .


1.5


E 1.0o


S0.5 GaN at 3V
S/-*- GaN at 3.5V
0 GaN with oxide at 3V
0.0 -y- GaN with oxide at 3.5V

0 100 200 300 400 500 600
Temperature(oC)



Figure 4-14.Change in forward current when measuring in 10% H2 /90% N2 relative to
pure N2 at 3 or 3.5 V in both the Schottky and MOS diodes ,as a function of
the measurement temperature

Figure 4-14 shows the change in forward current at fixed bias of 3 V for both types

of diodes when 10% H2 is introduced into the N2 ambient, as a function of temperature.

The use of the MOS diode structure provides a much broader temperature window in

which the detected change in current is above ImA.This clearly an advantage in terms of

using the sensors over a wide range of temperatures without the need for an on-chip

heater.









The recovery characteristics of both types of diodes upon cycling the ambient from

N2 to 1%H2/ 99% N2 showed a rapid (< 1 sec) change of current upon introduction of the

hydrogen into the ambient, while the recovery back to the N2 ambient value took of the

order of 20 secs. This is controlled by the mass transport characteristics of the gas out of

the test chamber, as demonstrated by changing the total flow rate upon switching the gas

into the chamber. Given that the current change upon introduction of the hydrogen is

rapid, the effective diffusivity of the atomic hydrogen through the Sc203 must be greater

than 4 x10-12 cm2/V.s at 90C. There was complete reversibility of the current for

repeated cycling of the ambient.

In conclusion, Pt/W/GaN MOS and Schottky diodes show a marked sensitivity of

their forward current to the presence of hydrogen in the measurement ambient. This

effect is due to the dissociation of the molecular hydrogen on the Pt gate contact,

followed by diffusion of the atomic species to the oxide/semiconductor interface where it

changes the piezo-induced channel charge. The MOS-diode shows a wider range of

temperatures in which it shows large changes in current than the Schottky diode

counterparts and shows promise as sensitive hydrogen detectors.

4.5 Detection of C2H4 Using Wide Bandgap Semiconductor Sensors

Currently, there is a strong interest in the development of wide bandgap

semiconductor gas sensors for applications including detection of combustion gases for

fuel leak detection in spacecraft, automobiles and aircraft, fire detectors, exhaust

diagnosis and emissions from industrial processes [Vas98, SavOO, Lol00, Con02, Arb93,

Hun02, Che96, Eke98, Sve99, HunOl, Che98, Tob97, Bar95, Cas98]. Of particular

interest are methods for detecting ethylene (C2H4), which offers problems because of its

strong double bonds and hence the difficulty in dissociating it at modest temperatures.









Wide bandgap semiconductors such as GaN and ZnO are capable of operating at much

higher temperatures than more conventional semiconductors such as Si. Diode or field-

effect transistor structures fabricated in these materials are sensitive to gases such as

hydrogen and hydrocarbons [Tom03, Mit03, Wo103, Ju03a, LinOl, Rao00, Mit98,

Cha02]. Ideal sensors have the ability to discriminate between different gases and arrays

that contain different metal oxides (eg.SnO2, ZnO, CuO, WO3) on the same chip can be

used to obtain this result [Tom03]. The gas sensing mechanism suggested include the

desorption of adsorbed surface oxygen and grain boundaries in poly-ZnO [Mit03],

exchange of charges between adsorbed gas species and the ZnO surface leading to

changes in depletion depth [Wol03] and changes in surface or grain boundary conduction

by gas adsorption/desorption [Kim03c]. Another prime focus should be the thermal

stability of the detectors, since they are expected to operate for long periods at elevated

temperature. MOS diode-based sensors have significantly better thermal stability than a

metal-gate structure and also sensitivity than Schottky diodes on GaN. In this work, we

show that both AlGaN/GaN MOS diodes and Pt/ZnO bulk Schottky diodes are capable of

detection of low concentrations(10%) of ethylene at temperatures between 50-300

C(ZnO) or 25-4000C(GaN). AlGaN/GaN layer structures were grown on C-plane A1203

substrates by Metal Organic Chemical Vapor Deposition (MOCVD). The layer structure

included an initial 2[tm thick undoped GaN buffer followed by a 35nm thick

unintentionally doped A10.28Ga0.72N layer. The sheet carrier concentration was 1 x1013

cm-2 with a mobility of 980 cm2/V-s at room temperature. Device isolation was achieved

with 2000 A plasma enhanced chemical vapor deposited SiNx. The ohmic contacts was

formed by lift-off of e-beam deposited Ti(200A)/Al(1000A)/Pt(400A)/Au(800A). The










contacts were annealed at 850 C for 45 sec under a flowing N2 ambient in a Heatpulse

610T system. 400A Sc203 was deposited as a gate dielectric through a contact window of

SiNx layer. Before oxide deposition, the wafer was exposed to ozone for 25 minutes. It

was then heat in-situ at 300 C cleaning for 10mins inside the growth chamber. 100 A

Sc203 was deposited on AlGaN/GaN by rf plasma-activated MBE at 100 OC using

elemental Sc evaporated from a standard effusion all at 1130 OC and 02 derived from an

Oxford RF plasma source [GilOl, KimOOb].



Schottky contact
metal
final metal

ohmic Sc203
contact metal SiNx
AIGaN
GaN


Al203 substrate





Schottky contact final metal
metal(Pt/Au)



Oxide
film(SiNx)
Bulk ZnO





Ohmic contact
metal(Ti/AI/Pt/Au)


Figure 4-15. Schematic of AlGaN/GaN MOS diode (top) and bulk ZnO Schottky diode
structure (bottom)









200 A Pt Schottky contact was deposited on the top of Sc203. Then, final metal of

e-beam deposited Ti/Au (300A/1200A) interconnection contacts was employed on the

MOS-HEMT diodes. Figure 4-15 (top)shows a schematic of the completed device.

The bulk ZnO crystals from Cermet, Inc. showed electron concentration of 9 x

1016 cm-3 and the electron mobility of 200 cm2/Vs. at room temperature from van der

Pauw measurements. The back(O-face) of the substrates were deposited with full area Ti

(200 A)/Al (800 A)/Pt (400 A)/Au (800 A) by e-beam evaporation. After metal

deposition, the samples were annealed in a Heatpulse 610 T system at 200 OC for 1 min

in N2 ambient. The front face was deposited with plasma-enhanced chemical vapor

deposited SiNx at 1000C and windows opened by wet etching so that a thin (20nm) layer

of Pt could be deposited by e-beam evaporation. After final metal of e-beam deposited




10 -
.-0 10% C2H4
8 mN2
::C Measured at 4000C
E=
6-


4o

2 2-

0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
D bde Voltage(V)


Figure 4-16. Forward I-V characteristics of MOS-HEMT based diode sensor at 4000C
measured under pure N2 or 10% C2H4/90% N2 ambients









Ti/Au (300A/1200A) interconnection contacts was deposited, the devices were

bonded to electrical feed-throughs and exposed to different gas ambients in an

environmental chamber while the diode current-voltage (I-V) characteristics were

monitored. Figure 4-15 (bottom) shows a schematic of the completed device. Figure 4-16

shows the forward diode current-voltage (I-V) characteristics at 4000C of the

Pt/Sc203/AlGaN/GaN MOS-HEMT diode both in pure N2 and in a 10% C2H4/90%N2

atmosphere.

At a given forward bias, the current increases upon introduction of the C2H4.

Hydrogen either decomposed from C2H4 in the gas phase or chemisorbed on the Pt

Schottky contacts then decomposed and released hydrogen. The hydrogen diffused

rapidly though the Pt metallization and the underlying oxide to the interface where it

forms a dipole layer [Amb03] and lowered the effective barrier height.

Figure 4-17 shows both the change in current at fixed bias (top) and change in

voltage at fixed current (bottom) as a function of temperature for the MOS diodes when

switching from a 100 % N2 ambient to 10% C2H4/90% N2.

As the detection temperature is increased, the response of the MOS-HEMT diodes

increases due to more efficient cracking of the hydrogen on the metal contact. Note that

the changes in both current and voltage are quite large and readily detected.

In analogy with results for MOS gas sensors in other materials systems [Eic03], the

effect of the introduction of the atomic hydrogen into the oxide is to create a dipole layer

at the oxide/semiconductor interface that will screen some of the piezo-induced charge in

the HEMT channel.














<

L 300
c\J

Q 200
c




C 0
0 100


200 300 400


Temperature(oC)


100 200 300 400


Temperature(C)



Figure 4-17. Change in MOS diode forward current at fixed forward bias of 2.5V(top) or
at fixed current(bottom) as a function of temperature for measurement in
100%N2 or 10% C2H4/90% N2.














At 50 C
N2 e
- 10 % Ethylene
20 % Ethylene





e l s u l7 \l vlw lr-


-0.3 0.0 0.3 0.6
Voltage(V)


-0.3 0.0 0.3 0.6
Voltage(V)


0.9 1.2 1.5


0.9 1.2


Figure 4-18. I-V characteristics at 500C (top) or 150 OC (bottom) of Pt/ZnO diodes
measured in different ambients.

The time constant for response of the diodes was determined by the mass transport

characteristics of the gas into the volume of the test chamber and was not limited by the

response of the MOS diode itself.









Figure 4-18 shows the I-V characteristics at 50 and 1500C of the Pt/ZnO diode both

in pure N2 and in ambients containing various concentrations of C2H4. At a given forward

or reverse bias, the current increases upon introduction of the C2H4, through a lowering of

the effective barrier height. One of the main mechanisms is once again the catalytic

decomposition of the C2H4 on the Pt metallization, followed by diffusion to the

underlying interface with the ZnO. In conventional semiconductor gas sensors, the

hydrogen forms an interfacial dipole layer that can collapse the Schottky barrier and

produce more ohmic-like behavior for the Pt contact. The recovery of the rectifying

nature of the Pt contact was many orders of magnitude longer than for Pt/GaN or Pt/SiC

diodes measured under the same conditions in the same chamber. For measurements over

the temperature range 50-1500C, the activation energy for recovery of the rectification of

the contact was estimated from the change in forward current at a fixed bias of 1.5V. This

was thermally activated through a relation of the type IF =Io exp(-Ea/kT) with a value for

Ea of -0.25 eV, comparable for the value of 0.17 eV obtained for the diffusivity of atomic

deuterium in plasma exposed bulk ZnO. This indicates suggests that at least some part of

the change in current upon hydrogen gas exposure is due to in-diffusion of hydrogen

shallow donors that increase the effective doping density in the near-surface region and

reduce the effective barrier height.

The changes in current at fixed bias or bias at fixed current were larger for the ZnO

diodes than for the AlGaN/GaN MOS diodes because of this additional detection

mechanism, as shown in Figure 4-19. Note that the changes in these parameters are

approximately an order of magnitude larger at 1500C. However the ZnO diodes wee not

thermally stable above -3000C due to direct reaction of the Pt with the ZnO surface.


























25 50 75 100

Temperature(oC)


25 50 75 100

Temperature(oC)


125 150


125 150


Figure 4-19.Change in current at a fixed bias (top) or change in voltage at fixed current
(bottom) as a function of measurement temperature in different percentages of
C2H4/N2 ambients

In conclusion, AlGaN/GaN MOS-HEMT diodes and bulk ZnO Schottky diodes

appear well-suited to detection of C2H4. The former have a larger temperature range of

sensitivity, but the absolute changes in voltage or current are larger with the ZnO diodes.


400


E 300

C
(U 200
-c
0
U,
U 100
O









The introduction of hydrogen shallow donors into the near-surface region of the ZnO is a

plausible mechanism for the non-recovery of the I-V characteristics at room temperature.

4.6 Hydrogen and Ozone Gas Sensing Using Multiple ZnO Nanorods

ZnO nanowires and nanorods are attracting attention for use in gas, humidity and

ultra-violet (UV) detectors [Wan04a, Kee04]. ZnO is attractive for a broad range of

applications in thin film form [LooOl, Wra99, gor99, Kri02, LiaOl, Kuc02, Pea04b], but

the ability to make arrays of nanorods with large surface area which has been

demonstrated with a number of different growth methods has great potential for new

types of sensors that operate with low power requirements [Hua01, Li04, Kin02, Liu03,

Par03a, Ng03, Hu03b, Par03b, Heo02, Nor04, Poo03, Heo03, WuOO, Zhe01, LyuO1,

Zhe03b, Par03c, Yao02, PanOl, Lao03, Sun03]. A large variety of ZnO one-dimensional

structures have been demonstrated [Wan04b] The large surface area of the nanorods and

bio-safe characteristics of ZnO makes them attractive for gas and chemical sensing and

biomedical applications, and the ability to control their nucleation sites makes them

candidates for micro-lasers or memory arrays. To date, most of the work on ZnO nano-

structures has focused on the synthesis methods [Wan04b] and there have been only a

few reports of the electrical characteristics [LiaOl, Kuc02, Pea04b, HuaOl, Li04]. The

initial reports show a pronounced sensitivity of the nanowire conductivity to ultraviolet

illumination and the presence of oxygen in the measurement ambient [Wan04a, Kee04].

In this chapter, the gas ambient dependence of current-voltage characteristics of

multiple ZnO nanorods prepared by site-selective Molecular Beam Epitaxy (MBE) will

be investigated. These structures can readily detect a few percent of ozone in N2 at room

temperature and are able to detect hydrogen beginning at around 112C. Over a limited

range of partial pressures of O3(PozoNE) in the ambient gas, the conductance G of the









sensor at fixed bias voltage decreased according to the relation G=(Go + A(POZONE) .5)-

,where A is a constant and Go the resistance in N2.The nanorods begin to show

detectable sensitivity to hydrogen in the measurement ambient at around 112C.















Figure 4-20. TEM of ZnO nanorod

The growth of the nanorods has been described in detail [Heo02, Nor04].

Discontinuous thin films (-100 A) of e-beam evaporated Ag were deposited on p-Si

(100) wafers terminated with native oxide. ZnO nanorods were deposited by MBE with a

base pressure of 5x10-8 mbar using high purity (99.9999%)Zn metal and an 03/02 plasma

discharge as the source chemicals. The Zn pressure was varied between 5x10-7 and 5x10-8

mbar, while the beam pressure of the 03/02 mixture was varied between 5 x 10 -6 and

5x10-4 mbar .The growth time was -2 h at 400 oC. The typical length of the resultant

nanorods was 2 |tm, with typical diameters in the range of 15 30 nm. Selected area

diffraction patterns showed the nanorods to be single-crystal. Figure 4-20 shows a

transmission electron micrograph of a single ZnO nanorod. The nanorods were heated in

hydrogen at 300C to ensure they were conducting. They were released from the substrate

by dissolution of the Ag catalyst and then transferred to SiO2-coated Si substrates. E-

beam lithography was used to pattern sputtered Al/Ti/Au electrodes contacting both ends






70


of multiple nanorods. The separation of the electrodes was -3 um. A scanning electron

micrograph of the completed device is shown in Figure 4-21.




























,l ,'... 1
AIIPIu ~ $~tY


Figure 4-21. SEM image of ZnO multiple nanorods (top) and the pattern contacted by
Al/Pt/Au electrodes (bottom)

Au wires were bonded to the contact pad for current -voltage (I-V) measurements

performed over the range 25-150 C in a range of different ambients (,N2,3 % 03 in N2 or

10%H2 in N2).

Figure 4-22 shows the I-V characteristics from the multiple nanorods measured in

either N2 or 10H2/90% N2 ambients at different temperatures. At room temperature there


nn I re a ar~cs~










is no detectable change in current but the presence of the hydrogen in the ambient can be

detected beginning at -112C.The reversible chemisorption of reactive gases at the

surface of these metal oxides can produce a large and reversible variation in the

conductance of the material [Tom03].



I I I I
-U- 220C N,2
8.0x10 8 --220C H,
1120C N, 2
4.0x10-8 -y--1120CH2 / .
1940C N 2
S -4- 194C H2
S. 0.0


-4.0x10- ..

-8.0x108 -

-0.4 -0.2 0.0 0.2 0.4
Voltage(V)



Figure 4-22. I-V characteristics at different temperatures of ZnO multiple nanorods
measured in either N2 or 10 % H2 in N2 ambient

The gas sensing mechanism suggested include the desorption of adsorbed surface

oxygen and grain boundaries in poly-ZnO [Mit03], exchange of charges between

adsorbed gas species and the ZnO surface leading to changes in depletion depth [Mit98]

and changes in surface or grain boundary conduction by gas adsorption/desorption

[Cha02]. The detection mechanism is still not firmly established in these devices and

needs further study. When detecting hydrogen with the same types of rectifiers, we have

observed changes in current consistent with changes in the near -surface doping of the

ZnO, in which hydrogen introduces a shallow donor state. The nanorods also showed a









strong photoresponse to above bandgap UV light(366nm).The photoresponse was fast

and indicates that the changes in conductivity due to injection of carriers is bulk-related

and not due to surface effects [Kee04].





1.6x10 -
0 8
L 1.2x10 -
S.-
)(D
8.0x10l -

c 4.0x10-9

0 0 .0

0 50 100 150 200

Temperature(C)



Figure 4-23. Change in current measured at 0.1 V for measurement in either N2 or 10H2
in N2 ambients

Figure 4-23 shows the difference in current at fixed bias of 0. 1V for measurement

in N2 versus 10%H2/90% N2 as a function of the measurement temperature. The change

in current is still in the nA range at 1120C but increases with temperature and is about

16nA at 200C.These changes are readily detected by conventional ammeters. However,

the inability to detect hydrogen at room temperature means that an on-chip heater would

still be needed for any practical application of ZnO nanorods for detection of combustion

gases. The nanorods were more sensitive to the presence of ozone in the measurement

ambient. Figure 4-24 shows the room temperature I-V characteristics from the multiple

nanorods measured in pure N2 or 3% 03 in N2.







73





3
-U- N,
2
2 -- 3% Ozone



0



O-2-

-3 -

-3 -2 -1 0 1 2 3
Voltage(V)



Figure 4-24. I-V characteristics at 25C of ZnO multiple nanorods measured in either N2
OR 3% 03 in N2

The changes in current are much larger than for the case of hydrogen detection.

Over a relatively limited range of 03 partial pressures in the N2 ambient, the conductance

increased according to G =(Go + A(Po3)05)-1, where A is a constant and Go the resistance

in pure N2 ambient. This is a similar dependence to the case of CO detection by SnO2

conduction sensors, in which the effective conductance increased as the square root of

CO partial pressure. The gas sensitivity can be calculated from the difference in

conductance in O3-containing ambients, divided by the conductance in pure N2, i.e.(GN2 -

Go)/GN2.At 25 C ,the gas sensitivity was 18 % for 3%03 in N2

Figure 4-25 shows the time dependence of change in current at a fixed voltage of

1V when switching from back and forth from N2 to 3% 03 in N2 ambients. The recovery

time constant is long(>10 mins) so that the nanorods are best suited to initial detection of

ozone rather than to determining the actual time dependence of change in concentration.

In the latter case a much faster recovery time would be needed. The gas sweep-out times







74


in our test chamber are relatively short (-a few sees) and therefore the long recovery time

is intrinsic to the nanorods.



0.824 ,

-m- Current

,, 0.820 -

CD 0.816 -

t-
O 0.812


0.808 Ozone exposed


0.804 -i i
0 200 400 600 800 1000
Time(sec)



Figure 4-25.Time dependence of current at IV bias when switching back and forth from
N2 to 3% 03 in N2 ambients

ZnO nanorods appear well-suited to detection of 03. They are sensitive at

temperatures as low as 250C for percent levels of 03 in N2. The recovery characteristics

are quite slow at room temperature, indicating that the nanorods can be used only for

initial detection of ozone. The nanorods are also sensitive to detection of hydrogen at

more elevated temperatures(>-100C) but are not sensitive at room temperature .The

ZnO nanorods can be placed on cheap transparent substrates such as glass, making them

attractive for low-cost sensing applications.














CHAPTER 5
CHEMICAL SENSOR FOR POLYMERS AND POLAR LIQUIDS

5.1 Introduction

Since the first demonstration of a fluid monitoring sensor based on AlGaN/GaN

hetero structures by Neuberger [Neu01], the application of AlGaN/GaN HEMTs as liquid

sensors has been a subject of intense research. Neuberger et al. have suggested that the

sensing mechanism for chemical absorbates originated from the compensation of the

polarization induced bound surface charge by interaction with polar molecules in the

fluids. The time dependence of changes in source-drain current of gateless HEMTs

exposed to polar liquids isopropanoll, acetone, methanol) with different dipole moments

using GaN/AlGaN hetero-interfaces was also reported. In particular, it was possible to

distinguish liquids with different polarities.

Steinhoff et al. suggested that the native oxide on the nitride surface was

responsible for the pH sensitivity of the response of gateless GaN based heterostructure

transistors to electrolyte solutions [Ste03a]. The linear response of nonmetallized GaN

gate region using differentpH valued electrolyte solutions and sensitivity with a

resolution better than 0.05 pH from pH = 2 to pH = 12 were shown. Chaniotakis et al.

[Cha05] showed that the GaN surface interacts selectively with Lewis acids, such as

sulphate (SO42-) and hydroxide (OH-) ions using impedance spectra. It was also shown

that gallium face GaN was considerably reactive with many Lewis bases, from water to

thiols and organic alcohols without any metal oxide and nitrides.









A novel metal oxide, ZnO has numerous attractive characteristics for gas and

chemical sensors [Kan05a, Kan05b, Heo04, Loo0l, Nor04]. ZnO is also attractive for

forming various types of nanorods, nanowires, and nanotubes [Hua01, Li04, Kee04,

Kin02, Liu03, Par03a, Ng03, Hu03, Par03b, Heo02, He03, Zhe01, Lyu03, Zha03, PanO1,

Lao03]. The large surface area of the nanorods makes them attractive for gas and

chemical sensing, and the ability to control their nucleation sites makes them candidates

for high density sensor arrays.

In this chapter, a study of two different gateless transistors will be given. In Section

5.2, the response of block copolymers on the gate area of AlGaN/GaN HEMT will be

will be discussed. In Section 5.3, the details of pH measurements with single ZnO

nanorods integrated with a micro-channel will be given.

5.2 Gateless AlGaN/GaN HEMT Response to Block Co-Polymers

The epi structure and processing sequence have been described in detail previously

[Meh04]. In brief, The HEMT structure was grown by metal organic chemical vapor

deposition at 1040 OC on c-plane A1203 substrates. The layer structure consisted of a low

temperature GaN nucleation layer, a 3 ptm undoped GaN buffer and a 30 nm Al0.3Gao.7N

undoped layer. The sheet carrier density in the channel was -8x 1012 cm-2 with a 300 K

electron mobility of 900 cm2/Vs. The ungated HEMT was fabricated using C12/Ar

inductively coupled plasma etching for mesa isolation and lift-off of e-beam deposited

Ti/Al/Pt/Au subsequently annealed at 850 C for 30 s to lower the Ohmic contact

resistance. Silicon Nitride was used to encapsulate the source/drain regions, with only

the gate region open to allow the polar liquids to reach across the surface. A schematic of

the device is shown in Figure 5-1(top), plan view layout with a cross-sectional view at the

bottom of Figure 5-1. The source-drain current-voltage characteristics were measured at










25 C using an Agilent 4156C parameter analyzer with the gate region exposed either to

air or various concentrations of the block co-polymers and individual polymers



contact









Final metal
gate area
exposed


-ohmic
\contact metal
m esa




gate area
final metal exposed SiNx

--_ohmic
AIGaN contact metal

GaN


A12zO Substrate





Figure 5-1. Schematic layout of gate HEMT structure (top) and device cross-section
(bottom).

The block copolymers are composed different proportions of the individual

polymers, PS and PEO. The configuration and chemical symbols of the block copolymer

are illustrated in Figure 5-2. The block co-polymers and individual polymers used in this

work were dissolved in the benzyl alcohol.







The charges in the two-dimensional (2D) channel of AlGaN/GaN HEMTs are
induced by spontaneous and piezoelectric polarization, which are balanced with positive
charges on the surface.


{1CHz2


/ CH2 o4


PS(Polystyrene) PEO(Polyethyleneoxide)


Figure 5-2. Structure of block co-polymer, composed of different portions of PS and PEO
(top) and chemical formula for PS and PEO (bottom).
The induced sheet carrier concentration of undoped Ga-face AlGaN/GaN can be
calculated by following equation [Asb97]:

n,(x) = C(X)- ( (e(x)+E(x)) (5-1)
e dde )

where S0 is the electric permittivity, E(x) = 9.5-0.5x is the relative permittivity, x is the Al
mole fraction of AlxGal-xN, dd is the AlGaN layer thickness, epb is the Schottky barrier of
the gate contact on AlGaN (e4b(x) = 0.84 + 1.3x(eV)), EF is the Fermi level (EF(x) =


:d






79


[9he2ns(x)/16Fo0(x)/(8m*(x))]2/3+h2ns(x)/47sm*(x)), Ec is the conduction band (Eg(x)=

6.13x+3.42(1-x)-x(1-x)(eV)), and AEc is the conduction band discontinuity between

A1GN and GaN (AEc(x) = 0.7[Eg(x)-Eg(0)]). Note that m*(x) is 0.228Therefore the sheet

charge density in the 2D channel of AlGaN/GaN HEMT is extremely sensitive to its

ambient. The adsorption of polar molecules on the surface of GaN affects in the surface

potential and device characteristics. As illustrated in Figure 5-3, nitride HEMT exposed

to different ambients and drain I-V characteristics were affected significantly. At 40 V of

drain bias voltage, drain current reduced for 25 and 50% for devices exposed to PE and

PS solution, respectively.



10

-- air No
8 0-PEO ,
PS m*E
6 -mE ooooooooo


0000

2- 0
OC o


0 10 20 30 40

VDs(V)



Figure 5-3. Drain I-V characteristics for the air, PS and PEO(weight concentration of the
polymers are CPEo = 0.0387mg/ml and Cps = 0.3781mg/ml).

Due to large gate dimension (20 x 150 .im2) induced parasitic resistance, the drain

current did not reach the saturation at 40. If the device dimension reduced, larger

changes of drain current should be expected.































0 10 20

VDS(V)


4.0 -



3.5



3.0



2.5



2.0
0.0 0.2 0.4

Concentration


0.6 0.8 1.0

of R11(mg/mL) at


Figure 5-4. Drain I-V characteristics for the different concentration of PS-PEO block
copolymer (molecular weight: 58.8 kg/mol, 71% of PS and 29% PEO) and
concentration of the copolymer solutions are following Ci= 1.0917 mg/ml, C2
= 0.7612 mg/ml, C3 = 0.5504 mg/ml, C4 = 0.08734 mg/ml (top). The drain
current reduction as a function of copolymer concentration (bottom).


-- air
S-0-solvent
C4
S C3
C2
-- C1


30 40


1.2

20V


I I


I I I I I I I I I


No


I'
a;37
a
a
a-










The dipole moments of ethylene and styrene monomer are 1.89 and 0.62,

respectively [CRC97]. The dipole moment of ethylene monomer is three times larger

than that of styrene monomer; however, the effect of PEO solutions on drain current of

nitride HEMT is only half of the PS styrene.



10 ..
--air
8 R5 No
R 11 ME N"
6 .E 0 00.0000 0

< 1 2000
E 4 oo

2 -

0 '-

0 10 20 30 40

VDS(V)



Figure 5-5. Drain IV characteristics of copolymers with different composition(R5 :
MW=66.6kg/mol, 53% of PS and 47% of PEO and R11 : MW 58.8 kg/mol,
71% of PS and 29% of PEO).

This could be due to PEO is extremely linear and dipole is along the polymer chain.

The 20 x 150 iLm2 gate opening is much larger than the individual monomer in the PEO

chain. Therefore the dipole effect on the device is very locally within the big gate

opening and some of the net surface charges induced by the PEO were cancelled out. In

the case of PS, the polymer chain is very bulky and not very linear, the net net dipole

induce charge may be higher than that of PEO, therefore, larger drain current changes

were observed. If the gate dimension reduces to the size of individual monomer, the

complete opposite results may be obtained.









The concentration of the block copolymer also affected the changes of the drain

current, as showed in Figure 5-4. The molecular weight of the copolymer is 58.8kg/mole

and it contains 71% of PS and 29% PEO. The concentration of the copolymer was varied

from 1.0917 mg/ml to 0.08734 mg/ml. As the copolymer concentration increased, the

drain current reduced more. The current reduction verses the copolymer concentration is

not quite linear as showed in Figure 5-4 (bottom), this could be due to the high parasitic

resistance of the devices and the saturation currents were not reached. Copolymer with

similar molecular weight, but different compositions also had significant impact on the

drain IV characteristics. As shown in Figure 5-5, the copolymer with large percentage of

PS reduced more drain current, which was consistent with results in Figure 5-3.

Gateless AlGaN/GaN HEMTs show a strong dependence of source/drain current on

the polarity and concentration of polymer solutions. This suggests the possibility of

functionalizing the surface for application as biosensors, especially given the excellent

stability of the GaN and AlGaN surfaces which should minimize degradation of adsorbed

cells [Eic03].

5.3 pH Measurements with Single ZnO Nanorod Integrated with a Microchannel

The electrical response of ZnO nanorod surfaces to variations of the pH in

electrolyte solutions introduced via an integrated microchannel was analyzed. The ion-

induced changes in surface potential are readily measured as a change in conductance of

the single ZnO nanorods and suggest that these structures are very promising for a wide

variety of sensor applications.

The preparation of the nanorods has been described in detail previously [Heo02] .In

brief, discontinuous Au droplets were used as the catalyst for ZnO nanorods growth and

formed by annealing e-beam evaporated Au thin films (-100 A) on p-Si (100) wafers at









700 C. ZnO nanorods were deposited by MBE with a base pressure of 5x10-8 mbar

using high purity (99.9999%) Zn metal and an 03/02 plasma discharge as the source

chemicals. The Zn pressure was varied between 4x10-6 and 2x107 mbar, while the beam

pressure of the 03/02 mixture was varied between 5x10 -6 and 5x10-4 mbar The growth

time was -2 h at 400 600 C. The typical length of the resultant nanorods was 2 10

|tm, with typical diameters in the range of 30 150 nm. Selected area diffraction patterns

showed the nanorods to be single-crystal. They were released from the substrate by

sonication in ethanol and then transferred to SiO2-coated Si substrates. E-beam

lithography was used to pattern sputtered Al/Pt/Au electrodes contacting both ends of a

single nanorod. The separation of the electrodes was -3.7 |tm. Au wires were bonded to

the contact pad for current -voltage (I-V) measurements to be performed .An integrated

microchannel was made from SYLGARD@ 184 polymer from DOW CORNING. After

mixing this silicone elastomer with curing agent using a weight ratio of 10 : 1 and mixing

with for 5 min ,the solution was evacuated for 30 min to remove residual air bubbles. It

was then applied to the already etched Si wafer(channel length, 10-100um) in a cleaned

and degreased container to make a molding pattern. Another vacuum de-airing for 5 min

was used to remove air bubbles, followed by curing for 2 hours at 90 OC. After taking the

sample from the oven, the film was peeled from the bottom of the container. Entry and

exit holes in the ends of the channel were made with a small puncher(diameter less than

1mm) and the film immediately applied to the nanorod sensor. The pH solution was

applied using a syringe auto pipette (2-20ul). A scanning electron micrograph of the

completed device is shown in Figure 5-6.









Prior to the pH measurements, we used pH 4, 7, 10 buffer solutions from Fisher

Scientific to calibrate the electrode and the measurements at 25 C were carried out in the

dark or under UV illumination from 365 nm light using an Agilent 4156C parameter

analyzer to avoid parasitic effects. The pH solution made by the titration method using

HNO3, NaOH and distilled water. The electrode was a conventional Acumet standard

Ag/AgCl electrode.


Oectrode
(AIUPt/Au


Nanowire


Microchcann e


Figure 5-6. Schematic (top) and scanning electron micrograph (bottom) of ZnO nanorod
with integrated microchannel (4ipm width)












1.2xl 0-7 1 1
1.2x107I I I

non UV
8.0x108 -0 UV(365nm) o

4.0x10-8 -


0
< U0 0_ 0__, O --


-4.0x108 -

-8.0x1 08
-8.0x108 I I I
-0.4 -0.2 0.0 0.2 0.4

VDs(V)



Figure 5-7. I-V characteristics of ZnO nanorod after wire-bonding, measured either with
or without UV (365nm) illumination.

Figure 5-7 shows the I-V characteristics from the single ZnO nanorod after wire

bonding, both in the dark and under UV illumination. The nanorods show a very strong

photoresponse. The conductivity is greatly increased as a result of the illumination, as

evidenced by the higher current. No effect was observed for illumination with below

bandgap light. The photoconduction appears predominantly to originate in bulk

conduction processes with only a minor surface trapping component [Kan05e].

The adsorption of polar molecules on the surface of ZnO affects the surface

potential and device characteristics. Figure 5-8(top) shows the current at a bias of 0.5V

as a function of time from nanorods exposed for 60s to a series of solutions whose pH

was varied from 2-12.

The current is significantly reduced upon exposure to these polar liquids as the pH

is increased. The corresponding nanorod conductance during exposure to the solutions is







86


shown at the bottom of Figure 5-8.The data in Figure 3 shows that the HEMT sensor is

sensitive to the concentration of the polar liquid and therefore could be used to

differentiate between liquids into which a small amount of leakage of another substance

has occurred.


1.6x10-7


1.2x10-7


8.0x10-'


4 0 x 1 0 0- 8 6


0.0
0 100 200 300 400 500 600
Time(sec)


0 100 200 300 400 500 600
Time(sec)


Figure 5-8. Change in current (top) or conductance (bottom) with pH (from 2-12) at V
0.5V.