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Process Development for ZnO-Based Devices

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PROCESS DEVELOPMENT FOR ZnO-BASED DEVICES By KELLY PUI SZE IP 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

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Copyright 2005 by Kelly Pui Sze Ip

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To my family and all the special people in my life

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iv ACKNOWLEDGMENTS I would like to thank my advisor, Prof. Stephen J. Pearton, the most integral part of my graduate studies, for all the opportunities, guidance, motivation and support. I would also like to thank my committee members, Prof. Cammy R. Abernathy, Prof. David P. Norton, Prof. Fan Ren and Prof. Rajiv Singh, for their time, expertise and evaluation. I thank Prof. Ren for introducing me to the world of semiconductors by welcoming me into his research group when I was an undergraduate student. I am grateful for his advice and encouragement that have helped me grow professionally and personally. I would like to thank members of the research groups of Prof. Pearton, Prof. Ren and Prof. Abernathy for their assistance a nd friendship, especially Kyu-Pil Lee, Kwang Baik, Ben Luo, Jihyun Kim, Rishabh Meha ndru, Jeff LaRoche, Brent Gila, Andrea Onstine, Jennifer Hite, Jerry Thaler and Rachael Frazier, and countless others who have made graduate school enjoyable. I appreci ate Prof. Norton and collaborators from his group, Young-Woo Heo and Yuanjie Li, for developing and growing the ZnO films used in this dissertation. I appreciate the valuable experience from the internship opportunities at Sandia National Laboratory’s Compound Semiconducto r Research Laboratory (CSRL) made possible by Randy Shul, my technical advisor, Kent Schubert, my technical manager, and Regan Stinnet, from the MESA Institute. It was a pleasure to work with such dedicated researchers, especially Randy, Albert Baca, Mark Overberg, Carlos Sanchez and

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v Stephanie Jones. I would like to thank them and the members of CSRL for their training and assistance. Most importantly, I express my deepest gratitude to family members and friends in my life who have helped to shape me into the person I am today, professionally and personally.

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vi TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES.............................................................................................................ix LIST OF FIGURES...........................................................................................................xi ABSTRACT....................................................................................................................xvi i CHAPTER 1 INTRODUCTION........................................................................................................1 2 BACKGROUND..........................................................................................................5 Properties of ZnO.........................................................................................................5 Growth......................................................................................................................... .6 Bulk.......................................................................................................................6 Thin Film...............................................................................................................8 Donors and Acceptors...........................................................................................8 Processing Techniques..................................................................................................9 Dry Plasma Etching...............................................................................................9 Ion Implantation..................................................................................................13 Rapid Thermal Annealing...................................................................................14 Characterization Techniques......................................................................................15 Atomic Force Microscopy...................................................................................15 Auger Electron Spectroscopy..............................................................................15 X-ray Photoelectron Spectroscopy......................................................................15 Electrical Measurements.....................................................................................16 Photoluminescence..............................................................................................16 Rutherford Backscattering Spectrometry/Channeling.........................................17 Scanning Electron Microscopy............................................................................17 Secondary Ion Mass Spectrometry......................................................................18 Stylus Profilometry..............................................................................................18 3 INDUCTIVELY COUPLED ETCHING OF ZnO.....................................................27 Introduction.................................................................................................................27 Experimental Methods................................................................................................27

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vii Results and Discussion...............................................................................................28 Summary.....................................................................................................................30 4 HYDROGEN INCORPORATIO N, DIFFUSIVITY AND EVOLUTION IN ZnO..39 Introduction.................................................................................................................39 Experimental Method.................................................................................................39 Results and Discussion...............................................................................................41 Summary.....................................................................................................................45 5 OHMIC CONTACTS TO ZnO..................................................................................54 N-type ZnO..................................................................................................................54 Introduction.........................................................................................................54 Bulk ZnO.............................................................................................................56 Experimental methods..................................................................................56 Results and discussion..................................................................................57 Summary......................................................................................................58 Thin Film n-ZnO.................................................................................................59 Experimental methods..................................................................................59 Results and discussion..................................................................................60 Summary......................................................................................................63 p-ZnMgO Thin Film...................................................................................................63 Introduction.........................................................................................................63 Experimental Methods.........................................................................................64 Results and Discussion........................................................................................65 Summary..............................................................................................................67 6 SCHOTTKY CONTACTS TO ZnO..........................................................................85 Introduction.................................................................................................................85 Thin Film ZnO............................................................................................................86 Experimental Methods.........................................................................................86 Results and Discussion........................................................................................87 Summary..............................................................................................................89 Bulk ZnO....................................................................................................................89 Experimental Methods.........................................................................................89 Results and Discussion........................................................................................90 Summary..............................................................................................................97 7 PN JUNCTION DIODE...........................................................................................126 Introduction...............................................................................................................126 Experimental.............................................................................................................127 Results and Discussion.............................................................................................128 Summary...................................................................................................................129 8 CONCLUSIONS......................................................................................................134

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viii APPENDIX A OHMIC CONTACT TABLES.................................................................................138 B SCHOTTKY CONTACT TABLES.........................................................................144 LIST OF REFERENCES.................................................................................................152 BIOGRAPHICAL SKETCH...........................................................................................163

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ix LIST OF TABLES Table page 2-1. Electrical properties of GaN and ZnO.......................................................................19 2-2. Basic physical properties of ZnO. .............................................................................19 2-3. Properties of bulk ZnO grown by pressurized melt growth technique......................20 5-1. Ohmic contacts ZnO and their respective specific contact resistance from published works........................................................................................................68 5-2. Sheet resistance, transfer length and specific contact resistance of annealed Au or Au/Ni/Au contacts on p-ZnMgO...................................................................68 6-1. Sc hottky contacts to ZnO and their respective barrier height and ideality factor from published works....................................................................................98 6-2. Metal work function and ideal barrier heights for ZnO (electron affinity: 4.1 eV).......................................................................................................................99 6-3. Ideality factor, saturation current density and barrier height for Pt contacts measured at temperatures between 303-473K on n-type ZnO, both before and after annealing at 300C.The contact diameter was 50 m in all cases...................99 6-4. Carbon atomic concentrations on the ZnO surface before and after ozone treatment.................................................................................................................100 6-5. Summary of XPS data for O-related species before and after ozone cleaning..................................................................................................................100 6-6. Summary of electrical characteristics for W-based contacts after 700C anneals....................................................................................................................100 6-7. Concentration of elements detected on the surface (in Atom%) for W2B and W2B5 contacts..................................................................................................101 Table 6-8. Concentration of elements detected on the surfaces (in Atom%) for CrB2 contacts...................................................................................................................101 Table 7-1. Characteristics of p-ZnMgO/ n-ZnO junctions measured on diodes with diameter 50 m......................................................................................................130

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x A-1. Ohmic contacts to ZnO.................................................................................139 B-1. Schottky contact s to thin film ZnO...............................................................145 B-2. Schottky contacts on bulk n-type ZnO .........................................................148

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xi LIST OF FIGURES Figure page 2-1. Crystal structure of wurtzite ZnO. .............................................................................20 2-2. Photographs of ZnO substrates grown by pressurized melt growth technique: (a) 2-inch diameter wafer, (b) boules and wafers of various diameters and (c) 1 cm2 pieces.................................................................................................21 2-3. Pulsed laser deposition system..................................................................................22 2-4. ICP reactor. ...............................................................................................................23 2-5. Electric and ma gnetic fields inside the reactor. ........................................................23 2-6. Chemical etching process. (a) Generation of reactive species. (b) Diffusion of reactive neutrals to surface. (c) Adsorption of reactive neutrals to surface. (d) Chemical reaction with surface. (e) Desorption of volatile byproducts. (f) Diffusion of byproducts into bulk gas............................................24 2-7. Physical etching process. (a) Generation of reactive species. (b) Acceleration of ions to the surface. (c ) Ions bombard surface. (d) Surface atoms are ejected from the surface.....................................................................................24 2-8. Combination of chemical and physical etching process. (a) Generation of reactive species. (b) Diffusion of reactive neutrals to surface. (b) Ion bombardment to surface. (c) Adsorption of reactive neutrals to surface. (d) Chemical reaction with surface. (e) Desorption of volatile byproducts. (f) Diffusion of byproducts into bulk gas......................................................................25 2-9. Ion implantation system............................................................................................25 2-10. Principle of AFM. ...................................................................................................26 2-11. The Auger process: (a) isolated atom, (b) inner core level electron dislodged, leaving behind a vacancy, (c) an outer level electron fills the vacancy and releases excess energy and (d) the excess energy ejects an Auger electron......26 3-1. Etch rates of ZnO as a function of rf chuck power in ICP CH4/H2/Ar or Cl2/Ar discharges. The dc self-bia s on the cathode is also shown..........................32

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xii 3-2. Etch rate of ZnO in CH4/H2/Ar or Cl2/Ar plasmas as a function of the average ion kinetic energy (plasma potential of 25 V minus the measured dc bias voltage).....................................................................................................................32 3-3. PL spectra at 300K from ZnO before and after CH4/H2/Ar etching at different rf chuck powers, shown on both linear (a) and log (b) scales...................33 3-4. Room temperature, deep-level PL emission from ZnO etched in ICP CH4/H2/Ar discharges at different rf chuck powers. The data are shown on both energy (a) and wavelength (b) scales.......................................................................34 3-5. AFM scans of ZnO before and after ICP CH4/H2/Ar etching at different rf chuck powers. The z-scale is 150nm/div................................................................35 3-6. RMS roughness of ZnO surfaces etched in ICP CH4/H2Ar discharges at different rf chuck powers.........................................................................................36 3-8. SEM micrographs of features etched into ZnO using a CH4/H2/Ar plasma. The photoresist mask has been removed..................................................................38 4-1. SIMS profiles of 2H implanted into ZnO (100 keV, 1015 cm-2) before and after annealing at different te mperatures (5 min anneals)........................................47 4-2. RBS spectra of bulk, single-crystal ZnO before and after 100 keV 1H+ implantation to a dose of 1016 cm-2..........................................................................47 4-3. PL spectra at 300K of ZnO implanted with 2H+ ions (100 keV, 1015 cm-2) as a function of post-implanted annea ling temperature (5 min anneals)..................48 4-4. SIMS profiles of 2H in ZnO exposed to deuterium plasmas for 0.5 h at different temperatures..............................................................................................48 4-6. SIMS profiles of 2H in ZnO exposed to deuterium plasma for 0.5 h at 200 C and then annealed at 400 C or 500 C for 5 mins..........................................49 4-7. Percentage of retained 2H implanted into ZnO (100 keV, 1015 cm-2) as a function of annealing temperature (5 min anneals). The inset shows the data on a log scale....................................................................................................................50 4-8. Donor concentration profiles in ZnO before and after plasma exposure and after subsequent annealing................................................................................50 4-9. PL spectra from 2H plasma exposed ZnO.................................................................51 4-10. Detailed band edge and deep level emission PL spectra from 2H plasma exposed ZnO............................................................................................................52

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xiii 4-11. 300K PL spectra from 2H implanted ZnO, as a function of subsequent anneal temperature...................................................................................................53 5-1. Schematic diagram (a) and secondary electron image (b) of circular TLM pattern on ZnO substrate..........................................................................................69 5-2. Specific contact resistance as a function of anneal temperature for Ti/Al/Pt/Au contacts on n-ZnO. Solvent chemical cleaning or H2 plasma exposure of the surface prior to metalli zation was compared with the case of depositing the metal on the as-received surface.......................................................70 5-3. Secondary electron image (a) and AES depth profile (b) of as-deposited Ti/Al/Pt/Au contact on ZnO.....................................................................................71 5-5. AES surface scans of Ti/Al/Pt/Au contacts on ZnO after annealing at 250 C (a), 350 C (b), 450 C (c) or 600 C (d).........................................................73 5-6. AES depth profiles of Ti/Al/Pt/Au contacts on ZnO after annealing at 250 C (a), 350 C (b), 450 C (c) or 600 C (d).........................................................74 5-7. Carrier mobility and resistivity of epi-ZnO as a function of post-growth anneal temperature...................................................................................................75 5-8. Schematic diagram (a), SEM (b) and microsope image (c) of the linear TLM ohmic contact pads on ZnO mesa...................................................................76 5-9. Carrier concentration of epiZnO and specific contact resistance of asdeposited ohmic contacts measured at 30 C versus post-growth anneal temperature...............................................................................................................77 5-10. Specific contact resistance as a function of measurement temperature for samples with various carrier concentrations. The solid symbols represent measurements prior to ohmic contact anneal. The corresponding open symbols denote measurements after 200 C, 1 min anneal in N2 ambient..............................77 5-11. Specific contact resistance versus measurement temperature of asdeposited ohmic contact measured at 30 C, and after annealing at 200 C, 1 min measured at 30 C and 200 C...................................................................................78 5-12. ln(cT) versus 1000/T for samples with various carrier concentrations. The solid symbols represent measurements prior to ohmic contact anneal. The corresponding open symbols denote measurements after 200 C, 1 min anneal in N2 ambient................................................................................................................78 5-13. AES surface scans (a) and surface scans (b) of Ti/Al/Pt/Au ohmic contacts to ZnO after annealing at 200 C................................................................79

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xiv 5-14. I-V characteristics from Au/Ni/Au (a) or Au (b) contacts on p-type ZnMgO before and after annealing at 600C...........................................................80 5-15. AES depth profiles from Au/Ni/Au contacts on p-type ZnMgO before (a) and after (b) annealing at 600C..............................................................................81 5-16. AES depth profiles from Au contacts on p-type ZnMgO before (a) and after (b) annea ling at 600C.....................................................................................82 5-17. AES surface scans from as-deposited Au (a) and after annealing at 600C (b) or as-deposited Au/Ni/Au (c) and after annealing at 600C (d)..............83 5-18. Cross-section TEM micrograph of Au/Ni/Au contact after annealing at 600C........................................................................................................................84 6-1. Plan view optical micrographs of contacts before (a) and after (b) 300C annealing. The Pt Schottky contacts are the inner circles, while the Ti/Al/Pt/Au Ohmic contacts are the outer rings.........................................................................102 6-2. Forward I-V characteristics for as-deposited or 300C annealed Pt contacts on n-type ZnO, for two different measurement temperatures................................103 6-3. Reverse I-V characteristics for as-deposited or 300C annealed Pt contacts on n-type ZnO, for two different measurement temperatures................................103 6-4. Pt barrier height on n-type ZnO as a function of measurement temperature for as-deposited and 300C contacts......................................................................104 6-5. AFM scans of ZnO surfaces over 1 m2 area, either before (a) or after (b) UV ozone cleaning.................................................................................................105 6-6. AFM scans from the samples of Figure 6-5, showing the RMS roughness values......................................................................................................................106 6-7. XPS survey spectra of ZnO before (a) and after (b) UV ozone cleaning. ..............107 6-8. XPS spectra from the region of O-bonded transitions, before (a) and after (b) UV ozone cleaning...........................................................................................108 6-9. I-V characteristic from Pt/Au contacts on ZnO without any ozone cleaning prior to metal deposition..........................................................................109 6-10. Forward (a) and reverse (b) I-V characteristics from Pt/Au contacts on ozone cleaned ZnO.................................................................................................110 6-11. I-V characteristics from W/Pt/Au contacts on ZnO both as-deposited (a) and after annealing at 700C (b)............................................................................111

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xv 6-12. AES depth profiles of W/Pt/Au contacts both as-deposited (a) and after 700 C annealing (b)...............................................................................................112 6-13. Secondary electron images of W2B/Au (a, c) or W2B/Pt/Au (b,d) contacts on ZnO, either as-d eposited (a, b) or after 600 C annealing (c, d).........113 6-14. I-V characteristics from W2B/Au (a) or W2B/Pt/Au (b) contacts on ZnO, as a function of annealing temperature..................................................................114 6-15. Barrier height (a), apparent ideality factor (b) and saturation current density (c) from W2B/Pt/Au Schottky contacts on n-type ZnO, as a function of anneal temperature.................................................................................................115 6-16. AES surface scans of W2B/Au (a, c) or W2B/Pt/Au (b, d) contacts on ZnO, either as-deposited (a, b) or after 600 C annealing (c, d).............................116 6-17. AES depth profiles of W2B/Au (a, c) or W2B/Pt/Au (b,d) contacts on ZnO, either as-deposited (a, b) or after 600 C annealing (c, d).............................117 6-18. Secondary electron images of W2B5/Pt/Au contacts on ZnO, either asdeposited (a) or after 600 C annealing (b).............................................................118 6-19. I-V characteristics from W2B5/Pt/Au contacts on ZnO, as a function of annealing temperature............................................................................................119 6-20. AES surface scans of W2B5/Pt/Au contacts on ZnO, either as-deposited (a) or after 600 C annealing (b).............................................................................120 6-21. AES depth profiles of W2B5/Pt/Au contacts on ZnO, either as-deposited (a) or after 600 C annealing (b).............................................................................121 6-22. Optical microscopy photos of CrB2 /Pt/Au on ZnO either as-deposited (a) or after annealing at 500 (b), 600 (c) or 700C (d)...........................................122 6-23. I-V characteristics from CrB2Pt/Au contacts on ZnO, as a function of annealing temperature............................................................................................123 6-24. AES surface scans of Cr2B/Pt/Au contacts on ZnO, either as-deposited (a) or after 600 C annealing (b).............................................................................124 6-25. AES depth profiles of Cr2B/Pt/Au contacts on ZnO, either as-deposited (a) or after 600 C annealing (b).............................................................................125 7-1. Schematic of ZnMgO/ZnO p-n junction structure..................................................130 7-2. I-V characteristics at 30C of ZnMgO/ZnO p-n junctions using Pt/Au as the ohmic contact to p-type ZnMgO......................................................................131

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xvi 7-3. Reverse I-V characteristics of ZnMgO/ZnO p-n junctions using Pt/Au as the ohmic contact on the p-type ZnMgO, as a function of the measurement temperature.............................................................................................................132 7-4. Measurement temperature dependence of the reverse breakdown voltage in ZnMgO/ZnO p-n junctions using Pt/Au as the ohmic contact on p-type ZnMgO...................................................................................................................133

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xvii 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 PROCESS DEVELOPMENT FOR ZnO-BASED DEVICES By Kelly Pui Sze Ip August 2005 Chair: Stephen J. Pearton Major Department: Materials Science and Engineering Our study focused on process development for ZnO-based devices. Our intent was to achieve practical plasma etching processes, to understand the role of hydrogen in ZnO and to optimize ohmic and Schottky contacts to ZnO. We also demonstrated p-n diode using ZnO/ZnMgO heterostructure. Two different plasma chemistries, Cl2/Ar and CH4/H2/Ar, for etching ZnO were examined. Methane-based chemistry is able to achieve practical etch rates and high fidelity anisotropic pattern transfer. Hydrogen introduced into ZnO by ion implantation and plasma exposure was investigated. Hydrogen incorporation depths of >25 m were obtained in bulk, single-crystal ZnO during exposure to 2H plasmas for 30 min at 300 C and completely removed by subsequent annealing at 500-600 C. Ohmic contacts to both n-type and p-type ZnO were studied. Ti/Al/Pt/Au metallization was considered for ohmic contacts to thin film and bulk n-type ZnO, whereas Au/Ni/Au and Ni/Au were used on the p-type thin film. Schottky contacts to n-

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xviii type PLD thin film and bulk ZnO were investigated. The Pt contacts on thin film ZnO show rectifying behavior with a barrier height at 25oC of 0.61eV. This barrier height is reduced significantly (to 0.42 eV) after annealing at 300C. Several metals, including Pt W, W2B, W2B5 and CrB2, were considered for contacts to n-type, bulk ZnO. For Pt contacts, the UV ozone treatment produces a ch ange from ohmic behavior to rectifying behavior with Schottky barrier height of ~0.7 eV. The W, W2B, W2B5, and CrB2 metals deposited by sputtering on ZnO produce non-rectifying contacts in the as-deposited state, but these convert to rectifying upon anneali ng at 500-600C when a Pt diffusion barrier between the metal and the Au overlayer is used to prevent dissociation of the ZnO. We also reported the electrical characteristics of Zn0.9Mg0.1O/ZnO p-n junctions grown by pulsed laser deposition on bulk, single-crystal ZnO substrates. Acceptable rectification in the junctions required growth of an n-type ZnO buffer on the ZnO substrate before growth of the p-type, phosphorus-doped Zn0.9Mg0.1O. Without this buffer, the junctions showed very high leakage current.

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1 CHAPTER 1 INTRODUCTION Since the invention of the first semiconductor transistor in 1947 by the scientists of Bell Labs, the semiconductor industry has grown incessantly; fabricating faster, smaller, more powerful devices while manufacturing in larger volume at lower costs. Although the first semiconductor transistor was made from germanium (Ge), silicon (Si) became the semiconductor of choice because Ge has low melting point that limits high temperature processes and because of the lack of a naturally occurring germanium oxide to prevent the surface from electrical leakage. Due to the maturity of its fabrication technology, silicon continues to dominate commercial markets in discrete devices and integrated circuits for computing, power switching, data storage and communication. For high-speed and optoelectronic devices such as high-speed integrated circuits and laser diodes, gallium arsenide (GaAs) is the material of choice. It exhibits superior electron transport properties and special optical properties. GaAs has higher carrier mobility and higher effective carrier velocity than Si, allowing faster devices. GaAs is a direct bandgap semiconductor, whereas Si is indirect; making GaAs better suited for optoelectronic devices. However, physi cal properties required for high-power, high-temperature electronics and UV/blue light emitter applications are beyond the limits of Si and GaAs. It is essential to investigate alternative materials and their growth and processing techniques in order to achieve these devices [1,2]. Wide bandgap semiconductors exhibit inherent properties such as larger bandgap, higher electron

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2 mobility and higher breakdown field strength. Therefore, they are suitable for high-power, high-temperature electronic devi ces and short-wavelength optoelectronics. Zinc oxide is a direct, wide bandgap semiconductor material with many promising properties for blue/UV optoelectronics, tran sparent electronics, spintronic devices and sensor applications [3-9]. ZnO has been commonly used in its polycrystalline form for over a hundred years in a wide range of applications: facial powders, ointments, sunscreens, catalysts, lubricant additives, paint pigmentation, piezoelectric transducers, varistors, and transparent conducting electrodes [10-14]. Its research interest has waxed and waned as new prospective applications revive interest in the material, but the applications have been limited by the technology available at the time. The first use of ZnO for its semiconductor properties was for detectors in buildyour-own radio sets in the 1920s. A thin copper wire, known as a “cat’s whisker,” is placed in contact with sensitive spots on a ZnO crystal. The metal/semiconductor junction allows current to flow only in one direction, converting the incoming radio waves from alternating current to direct current in the radio circuit. In 1957, the New Jersey Zinc Company published the book entitled “Zinc Oxide Rediscovered” to promote the material’s “frontier” properties (semiconductor, luminescent, catalytic, ferrite, photoconductive, and photochemical) and applications [15]. Research focused mainly on growth, characterization and applications th at do not require single crystals such as varistors, surface acoustic wave devices and transparent conductive films. Recent improvements in the growth of high quality, single crystalline ZnO in both bulk and epitaxial forms has renewed interest in this material. Originally, research efforts in ZnO growth were intended for gallium nitride (GaN) epitaxy. GaN is another wide,

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3 direct bandgap semiconductor that has been the focus of intensive research for high-power, high-frequency electronics that can operate at elevated temperatures and UV/blue optoelectronics. The lack of a native substrate has led to a search for suitable choices of substrate in other materials, including sapphire, silicon carbide (SiC) and ZnO [16]. The work of Look and colleagues [18] played a major role in reviving interest in ZnO research. The group began studying ZnO as a substrate for GaN epitaxy in the 1990s and realized the properties and potential of ZnO itself. They also organized the First Zinc Oxide Workshop in 1999 that brought together researchers from all over the world to disseminate their findings and to exchange ideas. Furthermore, Look and colleagues published the first convincing results of carefully characterized p-type ZnO homoepitaxial film grown by molecular beam epitaxy (MBE), a critical step in achieving p-n junctions for light-emitting devices. Subsequent ZnO Workshops, in 2002 and 2004, further encouraged research on ZnO [17-19]. As a wide bandgap semiconductor (Eg = 3.37 eV), ZnO is a candidate material for blue/UV optoelectronics, including light-emitti ng diodes, lasers, and detectors. These shorter wavelengths enable higher storage density in high-density optical storage systems, since storage density is inversely proportional to the wavelength squared. Also, ZnO blue or UV light-emitters could be employed in white solid-state lighting by using them to excite phosphors. In addition, ZnO is transparent to visible light and can be made into transparent transistors for active optical circuitry for color displays. Other applications include communications, biologi cal detectors, and gas sensors [3].

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4 Significant efforts in the last few years have been aimed at controlling conductivity and improving crystal quality. However, to fully realize ZnO devices, material and process development issues must be overcome. Our motivation was to develop practical plasma etching processes, to understand the role of hydrogen in ZnO and to optimize ohmic and Schottky contacts to ZnO.

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5 CHAPTER 2 BACKGROUND Properties of ZnO ZnO has many attractive characteristics for electronic and optoelectronic devices. Its electrical properties are compared to those of GaN in Table 2-1 [ 3,20,21]. It has direct bandgap energy of 3.37 eV, which makes it transparent in visible light and operates in the UV to blue wavelengths. The exciton binding energy is ~60 meV for ZnO, as compared to GaN ~25 meV; the higher exciton binding energy enhances the luminescence efficiency of light emission. The room-temperature electron Hall mobility in single-crystal ZnO is ~200 cm2 V-1, slightly lower than that of GaN, but ZnO has higher saturation velocity. ZnO exhibits better radiation resistance than GaN for possible devices used in space and nuclear applications [22]. ZnO can be grown on inexpensive substrate, such as glass, at relatively low temperatures. Nanostructures, such as nanowires and nanorods, have been demonstrated [23, 24]. These structures are ideal for detection applications due to its large surface area-to-volume ratio. Recent works shows ferromagnetism in ZnO by doping with transition metal, e.g. Mn, with practical Curie temperatures for spintronic devices [25]. One attractive feature of ZnO is the ability to bandgap tuning via divalent substitution on the cation site to form heterostructures. Bandgap energy of ~3.0 eV can be achieved by doping with Cd2+ [26], while Mg2+ increases the bandgap energy to ~4.0eV [27]. ZnO has a hexagonal wurtzite crystal structure, with lattice parameters a = 3.25 and c = 5.12 . The Zn atoms are tetrahedrally coordinated with four O atoms, where the

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6 Zn d-electrons hybridize with the O p-elect rons. Bonding between the Zn atoms and O atoms is highly ionic, due to the large difference in their electronegative values (1.65 for Zn and 3.44 for O). Alternating Zn and O layers form the crystal structure (Figure 2-1) [21]. Basic physical properties of ZnO are shown in Table 2-2 [3, 20, 21]. Some of the values compiled in the table remain uncertain; the disparity originates from the inhomogeneity of the materials. Robust, reproducible p-type ZnO remains elusive, thus the hole mobility and effective masses are still in debate. Crystal defects, such as dislocations, may contribute to variation in thermal conductivity, as observed in GaN. Growth Bulk Bulk, single-crystal ZnO substrates up to 2-inches in diameter are commercially available. ZnO inherently has n-type conductivity. Good quality, bulk, single-crystal ZnO substrates are primarily grown by one of the three methods: seeded vapor phase technique [19], melt growth technique [2931] or hydrothermal technique [32-37]. Hydrothermal solution growth has been limited to research laboratories until recently while both seeded sublimation growth and melt growth have produced 2-inch diameter wafers that are commercially available (Figure 2-2) [32]. The 1 1 cm2 substrates we were purchased from Eagle-Picher Technologies, LLC (Joplin, MO) and Cermet, Inc. (Atlanta, GA). Seeded sublimation growth, also known as seeded vapor growth or chemical vapor transport, is the technique used by Eagl e-Picher Technologies, LLC (Joplin, MO, ZnO division is now ZN Technologies in Brea, CA). High-purity ZnO powder is formed by Zn vapor and O2 and is heated to 1100 C inside a nearly closed horizontal tube. The H2

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7 carrier gas is used to transport the vapor to a cooler region of the tube where the reaction Zn (g) + H2O (g) ZnO (s) + H2 (g) is predicted to occur on a single-crystal seed. This process is capable of producing 2-inch diameter, 1-cm thick crystal in about 150 to 175 h [19]. Melt growth involves heating polycrystalline ZnO powder into a molten state and allowing to crystallize into a single-crystal state. ZnO grown by the melt growth method tends to have a large amount of defects (ZnO1-x) due to decomposition of ZnO at high temperatures. In a pressurize melt growth method that utilizes a modified Bridgman configuration (developed by Cermet, Inc., Atlanta, GA), an overpressure of oxygen prevents reduction of the lattice. The growth apparatus consists of a water-cooled crucible, such that a portion of the ZnO charge is solidified along the crucible wall (in order to maintain same composition as the melt and to avoid contamination from the crucible). The heat source uses radio frequency to induce eddy current to melt the charge into the molten phase [29-31]. The hydrothermal growth technique involves heating precursors of nutrient and mineralizer aqueous solution in a platinum-lined capsule inside an autoclave. The nutrient consists of high-purity sintered ZnO polycrystal powder, and the mineralizer solution is composed of LiOH and KOH [34] or Li2CO3, KOH and [35]. Single crystal seed provides a template for growth as ZnO is precipitated from the solution. Typical growth temperature and pressure are about 300 to 400C and 80 to 100 MPa. The growth rate of the hydrothermal growth technique is relatively slow (about 0.2 mm per day). The size of the crystal grown by this technique is usually limited by the need for a Pt-liner to

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8 prevent impurity contamination. Large area substrates (50 50 15 mm3 in size) have been reported recently [34]. Thin Film ZnO thin films are grown by various techniques, including molecular beam epitaxy (MBE), chemical vapor deposition (CVD), pulsed laser ablation, atomic layer deposition (PLD), reactive sputtering and spray pyrolysis [36-46]. Both n-type and p-type materials have been reported. ZnO thin films can be grown on a wide range of substrates: sapphire, Si, GaN, and even inexpensive materials such as glass. ZnO can be grown at relatively low temperatures as compared to other wide bandgap semiconductors. Inexpensive substrate and low-temperature growth make devices feasible to manufacture and lower the cost of the final product. The ZnO films we used were developed and grown using pulsed laser deposition by Prof. Norton and his research group (Dr. Y.-W. Heo and Y. Li) [45, 46]. The PLD system is shown in Figure 2-3. The main advantages of PLD growth are the ability to achieve epitaxial growth at low temperatures and the ease of controlling and altering the film composition by modifying the solid targets and background gases. Inside a vacuum chamber, a high intensity laser is used to vaporize material from a solid target, forming a highly directional plume. The solid target sources are made by pressing and sintering high purity-material powder. Background gases can be introduced to form specific reactions inside the plume. The material is transferred and deposited onto a heated substrate several centimeters away. Donors and Acceptors As-grown, nominally undoped ZnO exhibits n-type conduction. This behavior can be attributed to several possible sources: the presence of hydrogen [47, 48], Zn

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9 interstitials or O vacancies [49, 50]. These intrinsic defects act as shallow donors and reside approximately 0.01 to 0.05 eV below the conduction band. In addition, n-type conduction can be controlled by excess Zn atoms or by adding Al, Ga, or In dopants. A major obstacle is p-type doping in ZnO. High quality p-type ZnO has been elusive due to asymmetrical doping limits that are common in wide bandgap semiconductors, which are either n-type or p-type, but not both [49-51]. ZnTe, for instance, can be doped p-type with relative ease, but n-type is difficult to achieve [52]. Shallow acceptors for practical p-type conduction are difficult to achieve since dopants tend to form deep acceptor levels instead of shallow acceptor levels and do not contribute significantly to hole conduction. Several mechanisms offer insight to the origin of asymmetric doping limits: low solubility of dopant in host material, strong lattice relaxations that force the dopant energy level deep within the bandgap and the presence of native defects or dopant atoms in interstitial sites that compensate for substitutional impurity levels by forming deep level traps. Various p-type dopants (including Cu, Li, Ag, N, P, Sb, As) and co-doping of group III (Ga, Al, In) with N have been investigated [53]. Many groups have reported p-type conduction; however, the materials often exhibit ambiguous conduction and are not consistently reproducible. As research continues in refining growth methods and optimizing growth conditions, the material will become more robust over time. Processing Techniques Dry Plasma Etching Etching refers to the crucial integrated circuits (IC) fabrication process of transferring a pattern by removing specified areas. Wet chemical etching was widely used in manufacturing until the 1960s. Even though this technique is inexpensive, the feature size is limited to about 3 microns. The isotropic etching results in sloped

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10 sidewalls and undercutting of the mask material. As feature dimension decreases to microns and submicrons and device density per chip increases, anisotropic etching is necessary. Dry etching techniques using gase s as primary etch medium were developed to meet this need. In addition to anisotropic pattern transfer, dry etching provides better uniformity across the wafer, higher reproducibility, smoother surface morphology, and better control capability than wet chemical etching. Three general types of dry etching include plasma etching, ion beam milling, and reactive ion etch (RIE) [54, 55]. Inductively coupled plasma (ICP) etching was used in our study and is discussed in detail. ICP etching is a dry etching technique in which high-density plasmas are formed in a dielectric vessel encircled by inductive coils (Figures 2-2 and 2-3). When an rf power is applied to the coil, commonly referred to as the ICP source power, the time-varying current flowing through the coil creates a magnetic flux along the axis of the cylindrical vessel. This magnetic flux induces an electric field inside the vacuum vessel. The electrons are accelerated and collide with the neutral operating gas, causing the gas molecules to be ionized, excited or fragmented, forming high-density plasma. The electrons in circular path are confined and only have a small chance of being lost to the chamber walls; thus the dc self-bias remains low. The plasma generated as described above consists of two kinds of active species: neutrals and ions. The material to be etched sits on top of a small electrode that acts as parallel plate capacitor along with the chamber as the second electrode. When an rf power, also known as electrode power or chuck power, is applied to the sample stage, the electrons in the plasma accelerates back and forth in the plasma from the changes in the sinusoidal field. Since electrons have

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11 much lighter mass compared to the other species in the plasma, they respond more rapidly to the frequency change than the other species. As the electrons impinge on chamber surfaces, the chamber becomes slightly negative relative to the plasma. The surface area of the chamber is larger than the sample stage, thus the negative charge is concentrated on the sample stage. This bias attracts the ions toward the sample, bombarding the surface to remove material. In an ICP system, the plasma density and the ion energy and are effectively decoupled in order to achieve uniform density and energy distributions and maintain low ion and electron energy low. This enables ICP etching to reduce plasma damage while achieving fast etch rates. The plasma generated as described above consists of two kinds of active species: neutrals and ions. Neutrals are chemically reactive and etch the material by chemical reactions, while ions are usually less reactive and are responsible for removing material by physically bombarding the sample surface. The kinetic energy of the ions is controlled by electrode bias. The electron density and ion density are equal on average, but the density of neutrals, known as the plasma density, is typically higher. Anisotropic profiles are obtained by superimposing an rf bias on the sample to independently control ion energy and by using low pressure conditions to minimize ion scattering and lateral etching. The plasma is neutral but is positive relative to the electrode. It appears to glow due the ion excitation from the electron movements. The recombination of charges at the boundary surfaces surrounding the plasma creates a charge depletion layer, also known as a sheath, dark space or dark region, resulting in diffusion of carriers to the boundaries. The diffusion of electrons is faster than ions initially; thus an excess of positive ions is

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12 left in the plasma and assumes plasma potential (Vp) with respect to the grounded walls. The plasma and substrate potentials generate drift current to enhance the ion motions and hinder the electron motions until steady-state condition is achieved. The difference in electron and ion mobility also generates a sheath near the powered electrode. The dark region, a small region in the plasma immediately above the sample, keeps the electrons away due to the negatively charged electrode. The powered electrode reaches a self-bias negative voltage (Vdc) with respect to the ground. Even though the voltage drop controls the ion bombardment energy across the plasma sheath, it is difficult to measure; therefore, it is common to monitor the Vdc. Note that the dc bias is not a basic parameter and is characteristic to a particular piece of equipment. Etching is accomplished by interaction of the plasma with the substrate. There are three basic etching mechanisms: chemical etch process, physical etch process, and a combination of both chemical and physical et ching process (Figures 2-5, 2-6 and 2-7). The chemical etch process is the chemical reaction that etches the substrate when active species (neutrals) from the gas phase are absorbed on the surface material and react with it to form a volatile product. The chemical etch rate is limited by the chemical reaction rate or diffusion rate that depends on the volatility of the etch products since undesorbed products coat the surface and prevent or hinder further reactions. Chemical etching is a purely chemical process therefore etches isotropically, or equally in all directions. Physical process, also known as sputtering, occurs when ions impinge normal to the substrate surface. If the ions have sufficiently high energy, atoms, molecules or ions are ejected from the substrate surface to achieve a vertical etch profile. The etch rate of sputtering is slow, and the surface is often damaged from the ion bombardment. A

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13 combination of both chemical and physical et ching process, also known as energy-driven, ion-enhanced mechanism, takes advantage of the effect of ion bombardment in the presence of reactive neutral species. The energetic ions damage the surface and leave the surface more reactive toward incident neutrals, leading to removal rates that exceed the sum of separate sputtering and chemical etching. This process produces very fast etch rates and anisotropic profile; therefore, it is desirable in high fidelity pattern transfer. Ion Implantation Ion implantation (Figure 2-8) is a physical process that introduces dopants by means of high-voltage bombardment to achieve desired electrical properties in defined areas with minimal lateral diffusion. Inside a vacuum chamber, a filament is heated to a sufficiently high temperature where electrons ar e created from the filament surface. The negatively charged electrons are attracted to an oppositely charged anode in the chamber. As the electrons travel from the filament to the anode, they collide and create positively charged ions from the dopant source molecules. The ions are separated in a mass analyzer, a magnetic field that allows the passage of the desired species of positive ions with specific characteristic arc radius based upon ion mass. The selected ions are accelerated in an acceleration tube and then focused into a small diameter or several parallel beams. The beam is scanned onto the wafer surface, and the ions physically bombard the wafer. The ions enter the surface and come to rest below the surface as they lose their energy through nuclear interactions and coulombic interactions, resulting in Gaussian distribution concentration profile [56]. During implantation, the collisions with high-energy ions cause crystal damage to the wafer, leading to poor electrical characteris tics. In most cases, the carrier lifetime and mobility decrease drastically. Also, only a small fraction of the implanted ions are

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14 located in substitutional sites and contribute to carrier concentration. Annealing is needed to repair the crystal damage and to activate the dopants. To determine the depth and damage profile, Rutherford Backscattering and Channeling (RBS/C) analytical technique is employed. Annealing process and RBS/C will be further discussed in the subsequent sections. Rapid Thermal Annealing Annealing is a thermal process used for repairing the ion implantation damage, diffusing dopants and alloying metal contacts. After ion implantation, annealing is employed to repair the crystal damages caused by the high-energy ion bombardment that degrade carrier lifetime and mobility. Since the majority of the implanted dopants reside in the interstitial sites, the as-implanted materials have poor electrical properties. Annealing provides thermal energy for the dopants to migrate to the substitutional sites and contribute to the carrier concentration [57, 58]. Traditionally, tube furnaces were used for annealing after ion implantation. However, furnace annealing causes the implanted atoms to diffuse laterally and requires relatively long anneal time. Rapid thermal annealing was developed in order to overcome these drawbacks. Rapid thermal annealing (RTA) utilizes radiation heating from arc lamps or tungsten-halogen lamps to heat the wafer in an inert atmosphere such as N2 or Ar. It can attain higher temperature at a shorter time period than a conventional tube furnace, and the overall anneal time is relatively short, usually taking seconds as compared to several minutes to hours in a conventional tube furnace. RTA allows uniform heating and cooling that reduces thermal gradients that can lead to warping and stress-induced defects, enabling more dense design and fewer failures due to dislocations.

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15 Characterization Techniques Atomic Force Microscopy Atomic force microscopy (AFM) (Figure 2-7) employs a microscopic tip on a cantilever that deflects a laser beam depending on surface morphology and properties through an interaction between the tip and the surface. The signal is measured with a photodetector, amplified and converted into an image display on a cathode ray tube. Depending on the type of surface, AFM can be performed in contact mode and tapping mode. Auger Electron Spectroscopy Auger electron spectroscopy (AES) determines the elemental composition of the few outermost atomic layers of materials. A focused beam of electrons with energies from 3 keV to 30 keV bombards the surface of a specimen. The core-level electrons are ejected from approximately 1 m within the sample, resulting in a vacancy in the corelevel. As the atom relaxes, an outer-level electron fills the core vacancy and releases excess energy, which in turn, ejects an outer electron, known as an Auger electron. This process is illustrated in Figure 2-8. The kinetic energy of the Auger electrons is characteristic of each element, with the ex ception of hydrogen and helium. Therefore, by measuring the energies of the Auger electrons, the near-surface composition of a specimen can be identified. In addition, AES can provide compositional depth profile from relative intensities of the elements present if the system is equipped with an ion gun to sputter away material [59]. X-ray Photoelectron Spectroscopy X-ray photoelectron spectroscopy (XPS), al so known as electron spectroscopy for chemical analysis (ESCA), provides similar information as AES. Instead of impinging

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16 the sample surface with an electron beam, XPS utilizes a monoengergetic x-ray beam to cause electrons to be ejected, usually two to 20 atomic layers deep. The variation of the kinetic energies of the ejected electrons identifies the elements present and chemical states of the elements [59]. Electrical Measurements Current-voltage (I-V) measurements were taken to characterize the electrical properties of the contacts. These measurements are performed on a semiconductor parameter analyzer connected to a micromani pulator probe station. For vertical diodes, the input voltage will be applied to a highly conductive copper disk on which the samples were mounted on the backside with silver paste. Photoluminescence Photoluminescence (PL) is an analytical technique that provides information about the optical properties of a substrate. A light source, such as He-Cd, Ar and Kr lasers, with energy larger than the bandgap energy of the semiconductor being studied, generates electron-hole pairs within the semiconductor. The excess carriers can recombine via radiative and non-radiative recombination. Photoluminescence, the light emitted from radiative recombination, is detected. The wavelength associated with the different recombination mechanism is measured. The luminescence from excitons, electrons and holes bound to each other, is observed only at low temperatures in highly pure materials. As the temperature increases, the exciton breaks up into free carriers from the thermal energy. Increase in doping also causes the dissociation of excitons under local electric fields. Under these conditions, the electrons and holes recombine via the band-to-band process. Since some of the electrons may not lie at the bottom of the conduction band, their recombination and

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17 holes will produce a high-energy tail in the luminescence spectrum. On the other hand, the band-to-band recombination will yield a sharp cutoff at the wavelength corresponding to the band gap of the material [59]. Rutherford Backscattering Spectrometry/Channeling Depth profile of implanted ions and damages can be obtained by the Rutherford Backscattering Spectrometry/Channeling (RBS/C) technique, which measures the energy distribution of the backscattered ions from the implanted sample surface at a specific angle. The energy of the backscattered ion is determined by the mass of the atomic nucleus and the depth at which the elastic collisons take place. A beam of high-energy ions impacts the surface of the specimen. The angle of the analyzing ions affects the penetration depth. If the ions are injected parallel to the crystal axis of the specimen, they penetrate considerably deeper than if injected randomly, due to the lower stopping power from channeling. Deeper penetration results in higher backscattered ions yield. The displacement of an atom, either as host or impurities, from the crystal lattice also increases the backscattering yield. Therefore, the distribution of displaced atoms that are caused by the radiation damage from ion implantation can be measured by increasing the backscattered ion yield [59]. Scanning Electron Microscopy Scanning electron microscopy (SEM) generates images from electrons instead of light. A beam of electron is produced and accelerated from an el ectron gun. The electron beam passes through a series of condenser and objective lenses, which focus the electron beam. A scanning coil moves the beam across the specimen surface. The electron beam interacts with the specimen, and electrons from the surface interaction volume, such as backscattered, secondary, characteristic x-ray continuous x-ray, and Auger, are emitted.

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18 The signals are collected, amplified and converted to a cathode ray tube image. Depending on the specimen and the equipment setup, the contrast in the final image provides information on the specimen composition, topography and morphology. The main advantages of using electrons for image formation are high magnification, high resolution and large depth of fields [59]. Secondary Ion Mass Spectrometry Secondary ion mass spectrometry (SIMS) is a highly sensitive chemical characterization technique. Primary ions, such as Cs+, O2 +, Oand Ar, bombard the specimen in an ultra high vacuum environment, sputtering away secondary ions from the specimen surface. A small fraction of the ejected atoms are ionized either positively or negatively, and they are called secondary electrons. The composition of the surface is determined by the secondary electrons that ar e individually detected and tabulated using a mass spectrometer, as a function of their mass-to-charge ratio. There are two modes of SIMS, static or dynamic. In the static mode, a low primary-ion flux <1014cm-2 is used, leaving the specimen surface relatively undisturbed. The majority of secondary ions originate in the top one or two monolayers of the samples. The dynamic mode monitors the selected secondary ion intensities as a function of the sputtering time, resulting in a concentration versus depth profile. The depth resolution of this technique ranges from 5 to 20 nm [59]. Stylus Profilometry Stylus profilometry is used to measure the topographical features of a specimen surface, such as roughness, step height, widt h and spacing. A probe, or stylus, contacts the surface of the specimen and follows height variation as it scans across the surface. The height variations are converted into electrical signals, providing a cross-sectional

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19 topographical profile of the specimen. In this work, the etch rate was calculated by the depth, as measured by the profilometer, over a specified period of time. Table 2-1. Electrical properties of GaN and ZnO. Property GaN ZnO Direct bandgap energy (eV) 3.4 3.4 Electron mobility (cm2/Vs) 220 200 Hole mobility (cm2/Vs) 10 5.50 Electron effective mass 0.27 m0 0.24 m0 Hole effective mass 0.80 m0 0.59 m0 Exciton binding energy (meV) 28 60 Table 2-2. Basic physical properties of ZnO. Property Value Lattice parameters at 300 K (nm) a0: 0.32495 c0: 0.52069 Density (g cm-3) 5.606 Stable phase at 300 K Wurtzite Melting point (C) 1975 Thermal conductivity 0.6, 1-1.2 Linear thermal expansion coefficient a0: 6.5 10-6 c0: 3.0 10-6 Static dielectric constant 8.656 Refractive index 2.008, 2.029 Energy bandgap (eV) Direct, 3.37 Intrinsic carrier concentration (cm-3) <106 Max n-type doping: n ~ 1020 Max p-type doping: p ~ 1017 Exciton binding energy (meV) 60 Electron effective mass 0.24 Electron Hall mobility, n-type at 300 K (cm2V-1s-1) 200 Hole effective mass 0.59 Hole Hall mobility, p-type at 300 K (cm2V-1s-1) 5 50

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20 Table 2-3. Properties of bulk ZnO grow n by pressurized melt growth technique. Temperature (K) Resistivity ( cm) Density (cm 3) Mobility (cm2 V-1 s-1) 296 9.430 10 2 5.045 1017 131 78 5.770 10 1 3.640 1016 298 Figure 2-1. Crystal structure of wurtzite ZnO.

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21 (a) (b) (c) Figure 2-2. Photographs of ZnO substrates grown by pressurized melt growth technique: (a) 2-inch diameter wafer, (b) boules and wafers of various diameters and (c) 1 cm2 pieces.

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22 Figure 2-3. Pulsed laser deposition system. EFFUSION CELLS SUBSTRATE HEATER RHEED SCREEN ION GAUGE e-GUN Zn Mg O RF PLASMA dopant

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23 Figure 2-4. ICP reactor. Figure 2-5. Electric and magnetic fields inside the reactor. InducedEField Rf Current Magnetic Field ~ Plasma Powered electrode Sample Gas distribution 2 MHz Power supply 13.56 MHz Rf power Alumina chamber ~ Gas outlet

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24 Figure 2-6. Chemical etching process. (a) Generation of reactive species. (b) Diffusion of reactive neutrals to surface. (c) Adso rption of reactive neutrals to surface. (d) Chemical reaction with surface. (e) Desorption of volatile byproducts. (f) Diffusion of byproducts into bulk gas. Figure 2-7. Physical etching process. (a) Generation of reactive species. (b) Acceleration of ions to the surface. (c) Ions bombard surface. (d) Surface atoms are ejected from the surface. (a) (b) (c) (d) Sample Negatively biased (a) (b) (c) (d) (e) (f) + Electron Reactive neutral Ion Substrate atom +

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25 Figure 2-8. Combination of chemical and phys ical etching process. (a) Generation of reactive species. (b) Diffusion of reactive neutrals to surface. (b) Ion bombardment to surface. (c) Adsorption of reactive neutrals to surface. (d) Chemical reaction with surface. (e) Desorption of volatile byproducts. (f) Diffusion of byproducts into bulk gas. Figure 2-9. Ion implantation system. (a) (b1) (c) (d) (e) (f) (b2) Acceleration Tube Ion Source Mass Analyser Scanner Focus Wafer

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26 Figure 2-10. Principle of AFM. Figure 2-11. The Auger process: (a) isolated atom, (b) inner core level electron dislodged, leaving behind a vacancy, (c) an outer level electron fills the vacancy and releases excess energy and (d) the excess energy ejects an Auger electron. AFM t i p Photodetec Laser beam Specimen surface Electron Vacancy Auger Electron

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27 CHAPTER 3 INDUCTIVELY COUPLED ETCHING OF ZNO Introduction Etching process is a crucial step in device fabrication to form features and patterns. Numerous wet etchants have been reported for ZnO, including NH4Cl, HNO3/HCl and HF [6,60-63]; however, little are known about its dry etching characteristics and the associated mechanisms and effects on the optical properties of the material. Some initial results have appeared on plasma etching of sputter-deposited thin films [62-65], while plasma-induced damage from high ion density Ar or H2 discharges was found to increase the conductivity of the near surface of similar samples and lead to improved n-type ohmic contact resistivities [66]. In this section, the etching characteristics of high-quality, bulk single-crystal ZnO in inductively-coupled plasmas (ICP) of either CH4/H2/Ar or Cl2/Ar, which are two common chemistries for II-VI and III-V compound semiconductors [67], were studied. The etching mechanism was investigated by varying the ion impact energy and by examining the effects on both the luminescence efficiency and near-surface stoichiometry of the ZnO. Experimental Methods The bulk, wurtzite (0001) ZnO crystals from Eagle-Picher Technologies were nominally undoped (n ~ 8 1016 cm-3, mobility 190 cm2/Vs at 300K from Hall measurements). Photoresist masked and unmasked samples were exposed to CH4/H2 /Ar (3/8/5 sccm) or Cl2/Ar (10/5 sccm) discharges in a Unaxis 790 ICP reactor

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28 (St. Petersburg, FL). The 2 MHz power applied to the ICP source was held constant at 500W, and the rf (13.56MHz) chuck power was varied from 50 – 300W. The source power controls the ion density while the rf chuck power affects the ion energy. Over the rf power range investigated, the dc self-bias on the sample electrode varied from –75V to –344 V for Cl2/Ar and –91 V to –294 V for CH4/H2/Ar. Etch rates were obtained from stylus profilometry measurements while PL spectra were obtained at 300K using He-Cd laser excitation. AES surface scans were used to determine the near-surface stoichiometry, and SEM was used to examine the anisotropy. Results and Discussion Figure 3-1 shows the ZnO etch rates in both chemistries as a function of rf power. Figure 3-2 displays the ZnO etch rate in both types of plasma chemistry as a function of the substrate bias, Vb. The x-axis is plotted as the square root of the average ion energy, which is the plasma potential, ~25V for this particular equipment, minus the dc self-bias. A commonly accepted model for an etching process occurring by ion-enhanced sputtering in a collision-cascade process predicts the etch rate will be proportional to E0.5-ETH 0.5, where E is the ion energy and ETH is the threshold energy [68]. Therefore, a plot of etch rate versus E0.5 should be a straight line with an x-intercept equal to ETH. In the case of CH4/H2/Ar, which produced the faster etch rates, the value of ETH is ~96 eV. The Cl2/Ar data would indicate negative activation energy, but this is an artifact of the complexity of the ion energy distribution in that chemistry, as reported in detail previously [69-72]. The fact that CH4/H2/Ar exhibits an ion-assisted etch mechanism is consistent with the moderate vapor pressure for the expected group II etch product, namely (CH3)2Zn, with a vapor pressure of 301 mTorr at 20 C [64, 73] and the high bond strength of ZnO. To form the etch product, the Zn-O bonds must first be broken by ion

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29 bombardment. The ZnCl2 etch product has a lower vapor pressure (1 mTorr at 428 C) [65], which explains the slower etch rates for this chemistry. Given the higher etch rates for CH4/H2/Ar, the remainder of the study is focused on this plasma chemistry. Figure 3-3 shows 300K PL spectra from the samples etched in CH4/H2/Ar as a function of rf chuck powers. The overall PL intensity is decreased from both the band edge (3.2 eV) and the deep-level emission bands (2.3 – 2.6 eV). There is a decrease of approximately a factor of 4 even for the low bias condition, which shows that ZnO is susceptible to ion-induced damage during plasma etching. In etching real device structures, it would be necessary to minimize both ion energy and ion flux toward the end of the process in order to minimize lattice damage. Figure 3-4 shows a close-up of the deep level emission spectra. The intensity of these transitions is also decreased by the etch process. An increase in band edge intensity and suppression of the deep-level emission reported for H2 plasma exposure of ZnO [64, 66] were not observed, suggesting that the Ar ion bombardment component dominates during CH4/H2/Ar etching at room temperature. At this stage, it is not clear if this is a result of hydrogen incorporation, or simply a decrease in overall intensity from the introduction of non-radiative centers. Figure 3-5 shows AFM surface scans taken before and after etching with CH4/H2/Ar at different rf chuck powers. The surface morphology depends on the incident ion energy and most likely results from differences in the removal rates of Zn and O etch products. Figure 3-6 shows the measured root-mean-square (RMS) roughness measured over 5 5 m2 areas, as a function of the rf chuck power. The roughness goes through a minimum at 200 W rf power, corresponding to an ion energy of ~251 eV. At

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30 this incident ion energy, the surface roughness is approximately the same as for the unetched control material. Under these conditions the etch rate is ~2000 min-1, which is a practical value for most device processing applications. The near-surface stoichiometry of the ZnO was unaffected by CH4/H2/Ar etching, as determined by AES. A smooth surface morphology is a good indicator that both Zn and O are removed equally during etching. Figure 3-7 shows AES surface scans before and after etching for 3 mins at the 200W rf chuck power (-167eV average ion energy) condition. Approximately 60 of ZnO was removed by Ar ion sputtering in the AES analysis chamber prior to analysis to remove adventitious carbon and other atmospheric contaminants. The Zn-to-O ratio is identical within experimental error and demonstrates that the CH4/H2/Ar plasma chemistry is capable of equi-rate removal of the Zn and O etch products during ICP etching. Given this result and the fact that the etching occurs through an ion-assisted mechanism, smooth, anisotropic pattern transfer is expected. Figure 3-8 shows SEM micrographs of features etched in ZnO using a photoresist mask and CH4/H2/Ar discharge. The vertical sidewalls are an i ndication that the etch products are volatile only with additional ion-assistance. In addition, the etched field shows only a slight degree of roughening, consistent with the fact that the surface retains its stoichiometry. The sidewall striations on the bottom figure are due to initial photoresist sidewall roughness, but one can observe excellent high fidelity pattern transfer into the ZnO. Summary The etch mechanism for ZnO in plasma chemistries of CH4/H2/Ar and Cl2/Ar have been investigated. For both chemistries the etch rate increases with ion energy as predicted from an ion-assisted chemical s puttering process. Smooth, anisotropic pattern

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31 transfer in ZnO can be achieved with ICP CH4/H2/Ar discharges. Under optimum conditions, the etched surface is smooth and stoichiometric under these conditions and the etching proceeds by an ion-driven mechanism with a threshold ion energy of ~96eV. ICP Cl2/Ar discharges also produce practical etch rates for ZnO but are slower than with CH4/H2/Ar due to low volatility of the ZnCl2 etch product. The luminescence intensity from the ZnO is significantly degraded by the dry etching by creating non-radiative defects.

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32 Figure 3-1. Etch rates of ZnO as a function of rf chuck power in ICP CH4/H2/Ar or Cl2/Ar discharges. The dc self-bias on the cathode is also shown. 101214161820 0 500 1000 1500 2000 2500 3000 Etch Rate (/min)Square Root (25+Vb) Cl2/Ar Chemistry CH4/H2/Ar Chemisty Figure 3-2. Etch rate of ZnO in CH4/H2/Ar or Cl2/Ar plasmas as a function of the average ion kinetic energy (plasma potential of 25 V minus the measured dc bias voltage).

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33 2.02.53.03.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 50W rf 100W rf 300W rf 200W rf Control PL Intensity (Arb.)Energy (eV) (a) 2.02.53.03.5 0.01 0.1 1 Control 50W rf 100W rf 300W rf 200W rfLog PL Intensity (Arb.)Energy (eV) (b) Figure 3-3. PL spectra at 300K from ZnO before and after CH4/H2/Ar etching at different rf chuck powers, shown on both linear (a) and log (b) scales.

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34 2.02.5 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 200W rf 300W rf 100W rf 50W rf Control PL Intensity (Arb.)Energy (eV) (a) 450500550600650 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 300W rf 200W rf 100W rf 50W rf Control PL Intensity (Arb.)Wavelength (nm) (b) Figure 3-4. Room temperature, deep-level PL emission from ZnO etched in ICP CH4/H2/Ar discharges at different rf chuck powers. The data are shown on both energy (a) and wavelength (b) scales.

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35 Figure 3-5. AFM scans of ZnO before and after ICP CH4/H2/Ar etching at different rf chuck powers. The z-scale is 150nm/div. Control50W rf 300W rf 200W rf 100W rf

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36 050100150200250300 2 4 6 8 Control ZnO CH4/H2/Ar 500W ICP Power RMS Roughness (nm)rf Power (W) Figure 3-6. RMS roughness of ZnO surfaces etched in ICP Ch4/H2Ar discharges at different rf chuck powers.

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37 Figure 3-7. AES surface scan before (a) and after (b) CH4/H2/Ar etching. The spectra was taken at a depth of ~60 by first sputtering briefly with an Ar+ beam. Kinetic Energy (eV) dN(E) Min: -4667Max: 3827 50250450650850105012501450165018502050 Kinetic Energy (eV) dN(E) Min: -5937Max: 5248 50250450650850105012501450165018502050 (a) ZnO control O C Zn dN(E) dN(E) 50 450 850 1250 1650 2050Kinetic Energy (eV) O C Zn 50 450 850 1250 1650 2050Kinetic Energy (eV) ( b ) ZnO etched

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38 Figure 3-8. SEM micrographs of features etched into ZnO using a CH4/H2/Ar plasma. The photoresist mask has been removed.

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39 CHAPTER 4 HYDROGEN INCORPORATION, DIFFUSI VITY AND EVOLUTION IN ZNO Introduction There is particular interest in the properties of hydrogen in ZnO, because of the predictions from density functional theory a nd total energy calculations that it should be a shallow donor [47, 74-77]. The generally observed n-type conductivity, therefore, may at least in fact be explained by the presence of residual hydrogen from the growth ambient, rather than to native defects such as Zn interstitials or O vacancies. Some experimental support for the predicted observations of its muonium counterpart [48, 78] and from electron paramagnetic resonance of single-crystal samples [79]. There have been many other studies on the effects of hydrogen on th e electrical and optical properties of ZnO [80-88], but no detailed studies have been performed on the thermal stability and diffusion behavior of hydrogen introduced by ion implantation and by plasma exposure. In this section, the retention of hydrogen by ion implantation and plasma exposure in single-crystal, bulk ZnO as a function of annealing temperature is presented. The effects of the implantation on both the crystal quality and optical properties of the material were also examined. Changes in the electrical and optical properties of the plasma-exposed ZnO are also discussed. Experimental Method Bulk, wurtzite (0001) ZnO crystals from Eagle-Picher Technologies of grade I quality were employed for all experiments. The samples were nominally undoped with

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40 as-received n-type carrier concentrations of ~1017 cm-3 and a room temperature mobility of 190 cm2/V s. To investigate the thermal stability of implanted hydrogen, either 2H+ or 1H+ ions were implanted into the Zn faces of the samples. In the latter case, implantation was performed with a 1.7 MV tandem accelerator (NEC, 55DH-4, Middleton, WI) at 25 C with 100 keV H+ ions using a beam flux of ~1.31013 cm-2s-1 to a dose of either 1015 or 1016 cm-2. During implantation, samples were tilted by ~7 relative to the incident ion beam to minimize channeling. After implantation, these samples were characterized by RBS/C using the same accelerator with 1.8 MeV 4He+ ions incident along the [0001] direction and backscattered at ~168 relative to the incident beam direction. The RBS/C spectra were accumulated for long enough that the random yield at the depth of the bulk defect peak corresponded to ~4000 counts. The 2H+ implantation was also performed at energy of 100keV to a dose of 1015 cm-2. Annealing was performed for 5 mins at 500-700 C under flowing N2 rapid thermal annealing furnace with the samples in a faceto-face configuration. These samples were examined by photoluminescence at 300K using a He-Cd laser and by SIMS. The latter was performed using a Cs+ ion beam with 14.5 keV energy and 24 incident angle. To study the thermal stability and diffusion behavior of hydrogen introduced by plasma exposure, the samples were exposed to 2H plasmas at temperatures of 100-300 C in a Plasma Therm 720 series reactor operating at 900 mTorr with 50 W of 13.56 MHz power. Some of these samples were subsequently annealed at temperatures up to 600 C under flowing N2 ambients for 5 mins. SIMS measurements were used to obtain the deuterium profiles as a function of plasma exposure or annealing temperature. The

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41 electrical properties of some of the samples were examined by electrochemical capacitance-voltage measurements using a 0.2 M NaOH/0.1 M EDTA electrolyte as the rectifying contact. Finally, optical proper ties were measured using photoluminescence spectroscopy at variable temperatures, with a He-Cd laser as the excitation source. Results and Discussion Figure 4-1 shows the SIMS profiles of implanted 2H as a function of subsequent annealing temperature. The effects of the annealing is an evolution of 2H out of the ZnO crystal, with the remaining deuterium atoms of each temperature decorating the residual implant damage. The peak in the as-implanted profile occurred at 0.96 m, in good agreement with the projected range from Transfer-of-Ion-in-Matter (TRIM) simulations. The thermal stability of the implanted 2H is considerably lower in ZnO than in GaN [89], where temperatures of ~900 C are needed to remove deuterium to below the detection limit (~3 1015 cm-3) of SIMS and this suggests that slow-diffusing H2 molecules or larger clusters do not form during the ann eal. Since conventional out-diffusion profiles were not observed, it was impossible to estimate a diffusion coefficient for the 2H in ZnO. The results reported here are consistent with an implant-damaged trap-controlled release of 2H from the ZnO lattice for temperatures >500 C. RBS/C showed that implantation of 1H, even at much higher doses (1016 cm-2), did not affect the backscattering yield near the ZnO surface (Figure 4-2). However, there was a slight but detectable increase in scattering peak deeper in the sample, in the region where the nuclear energy loss profile of 100keV H+ is a maximum. The RBS/C yield at this depth was ~6.5% of the random level before H+ implantation and ~7.8% after implantation to the dose of 1016 cm-2.

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42 While the structural properties of the Zn O were minimally affected by the hydrogen (or deuterium) implantation, the optical properties were severely degraded. Room temperature cathodoluminescence showed that even for a dose of 1015 cm-2 1H+ ions, the intensity of near-gap emission has reduced by more than 3 orders of magnitude as compared to control values. This is due to the formation of effective non-radiative recombination centers associated with ion-beam produced defects. Similar results were obtained from PL measurements. Figure 4-3 shows the 300K spectra from the 2H+ implanted samples annealed at different temperatures. The band-edge luminescence is still severely degraded even after 700 C anneals where the 2H has been completely evolved from the crystal. This indicates that point defect recombination centers are still controlling the optical quality under these conditions. Kucheyev et al. [90] have found that resistance of ZnO can be increased by about 7 orders of magnitude as a result of trap introduction by ion irradiation. Figure 4-4 shows SIMS profiles of 2H in plasma exposed ZnO, for different sample temperatures during the plasma treatment. The profiles follow those expected for diffusion from a constant or semi-infinite source, i.e. Dt X erfc C t x CO4 (2-1) where C(x,t) is the concentration at a distance x for diffusion time t Co is the solid solubility and D is the diffusivity of 2H in ZnO [91]. The incorporation depths of 2H are very large compared to those in GaN or GaAs under similar conditions, where depths of 1-2 m are observed [89, 92]. The rapid diffusion of the hydrogen suggests that it diffuses as an interstitial, with little trapping by the lattice elements or by defects or impurities. The position of H in the lattice after immobilization has not yet been

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43 determined experimentally, but from theory the lowest energy states for H+ is at a bond-centered position forming an O-H bond, while for H2 the anti-bonding Zn site is most stable [47]. Using a simple estimate of the diffusivity D from D = X2/4t and where X is the distance at which 2H concentration has fallen to 5 1015 cm-3 (Figure 4-4), we can estimate the activation energy for diffusion from the data shown in Arrhenius form (Figure 4-5). The extracted activation energy, Ea, is 0.17 12 eV for 2H in ZnO. Note that the absolute diffusivities of 1H would be ~40% larger because of the relationship for diffusivities of isotopes, i.e. 2 / 1 1 2 2 1 H H H HM M D D (2-2) The small activation energy is consistent with the notion that the atomic hydrogen diffuses in interstitial form. Figure 4-6 shows SIMS profiles of a ZnO sample exposed to 2H plasma of 0.5 h at 200 C, then annealed for 5 mins under N2 at different temperatures. There is significant loss of 2H even after a short anneal at 400 C, with virtually all of it evolved out of the crystal by 500 C. This is also in sharp contrast to 2H in GaN, where much higher temperatures ( 800 C) are needed to evolve the deuterium out of the sample [89, 92] To compare this data to the thermal stability of 2H incorporated by direct implantation, Figure 4-7 shows the percentage of 2H remaining as a function of annealing temperature for incorporation by either plasma exposure or implantation. The 2H concentrations were obtained by integrating the area under the curves in the SIMS data. The 2H is slightly more thermally stable in the latter case, most likely due to trapping at

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44 residual damage in the ZnO carried by the nuclear stopping process. It is evident that the thermal stability of implanted deuterium is not that high with 12% of the initial dose retained after 500 C anneals and ~0.2% after 600 C anneals. Lavrov et al. [93] have identified two hydrogen-related defects in ZnO, by using local vibrational mode spectroscopy. The H-I center consists of a hydrogen atom at the bond centered site, while the H-II center contains two inequivalent hydrogen atoms bound primarily to two oxygen atoms. Figure 4-8 shows donor concentration profiles in the ZnO before and after plasma exposure and following subsequent annealing. The 2H plasma treatment causes an increase in donor concentration, consistent with past reports. In that case, the effect was attributed to hydrogen passivation of compensating acceptor impurities present in the as-grown ZnO epitaxial layers [81]. An alternative explanation is that the hydrogen induces a donor state and thereby increases the free electron concentration [47]. Subsequent annealing reduces the carrier density to slightly below the initial value in the as-received ZnO, which may indicate that it contained hydrogen as a result of the growth process. It is important to note that the n-type conductivity probably arises from multiple impurity sources [3, 22, 94] and cannot be unambiguously assign all of the changes to the presence of hydrogen. Figure 4-9 shows the PL spectrum from a plasma treated sample as a function of measurement temperature. The sample shows strong band-edge luminescence and a small deep-level band (~2.6 eV). Past reports have shown that the efficiency of bandedge emission was increased by plasma hydr ogenation of various types of ZnO [82]; however, the degree of improvement depended on the impurity and defect concentration

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45 in the original samples [82, 84]. No significant difference in the intensity or shape of the PL spectra as a result of plasma hydrogenation of our samples was observed in these experiments. More detail on the measurement temperature dependence of the band-edge and deep-level emissions from the plasma deuterat ed ZnO (Figure 4-10). As expected and as reported previously [84], the bandedge intensity increases significantly as the temperature is lowered and the deep level emission is quenched. The overall intensity of the plasma treated ZnO remains much higher than the material hydrogenated by direct implantation of protons or deuterons. Figure 4-11 shows 300K PL spectra from ZnO after 2H implantation at a dose of 1015 cm-3, followed by annealing at different temperatures. The implantation step severely degrades the band edge intensity, and even annealing at 700 C where all of the 2H has been evolved from the ZnO, the intensity remains about 2 orders of magnitude lower than in the unimplanted material. Summary Hydrogen is found to exhibit a very rapid diffusion in ZnO when incorporated by plasma exposure, with D of 8.7 10-10 cm2/VS at 300 C. The low activation energy for diffusion is indicative of interstitial motion. All of the plasma-incorporated hydrogen is removed from the ZnO by annealing at 500 C. When the hydrogen is incorporated by direct implantation, the thermal stability is somewhat higher, due to trapping at residual damage. Optical degradation is more severe when the hydrogen is incorporated by implantation then by plasma exposure. The photoluminescence intensity does not completely recover even after annealing to the point where all the hydrogen is essentially removed. The free electron concentration increases after plasma hydrogenation,

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46 consistent with the small ionization energy predicted for H in ZnO [47] and the experimentally measured energy of 60 10 meV for muonium in ZnO [48]. The electrical activity and rapid diffusivity of H or ZnO must be taken into account when designing device fabrication processes such as deposition of dielectrics using SiH4 as a precursor or dry etching involving use of CH4/H2/Ar plasmas since these could lead to significant changes in near-surface conductivity.

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47 0.51.01.52.0 101510161017101810191020 700 C 600 C 500 C As-implanted2H 1015 cm-2 ZnO 5 min anneals H Concentration (atoms/cm3)Depth (m) Figure 4-1. SIMS profiles of 2H implanted into ZnO (100 keV, 1015 cm-2) before and after annealing at different temperatures (5 min anneals). 0.40.60.81.01.21.41.6 0 200 400 600 RBS YieldEnergy (MeV) virgin 11016 cm2Depth () 100 keV 1H ZnO 100007500500025000 Figure 4-2. RBS spectra of bulk, singl e-crystal ZnO before and after 100 keV 1H+ implantation to a dose of 1016 cm-2.

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48 2.02.53.03.5 10-310-210-1100101 As-implanted 500 C 700 C 600 C Control PL Intensity (Arb.)Energy (eV) Figure 4-3. PL spectra at 300K of ZnO implanted with 2H+ ions (100 keV, 1015 cm-2) as a function of post-implanted annealing temperature (5 min anneals). 051015202530 1E15 1E16 1E17 1E18 200C 100C 300C2H plasma treatment Concentration (atoms/cc)Depth (m) Figure 4-4. SIMS profiles of 2H in ZnO exposed to deuterium plasmas for 0.5 h at different temperatures.

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49 1.52.02.53.0 1E-11 1E-10 1E-9 2H plasma exposure in bulk ZnO D (cm2/Vs)1000/T (K) Figure 4-5. Arrhenius plot of diffusivity for 2H in ZnO. 0510152025 1E15 1E16 1E17 1E18 No anneal2H plasma 200C Post treatment anneal 500C, 5min 400C, 5min Concentration (atoms/cc)Depth (m) Figure 4-6. SIMS profiles of 2H in ZnO exposed to deuterium plasma for 0.5 h at 200C and then annealed at 400C or 500C for 5 mins.

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50 0100200300400500600700 0 20 40 60 80 100 0200400600 1 10 100 % 2H RemainingAnneal Temperature (C)100 keV 2H ZnO 5 min anneals % 2H RemainingAnneal Temperature (C) Figure 4-7. Percentage of retained 2H implanted into ZnO (100 keV, 1015 cm-2) as a function of annealing temperature (5 min anneals). The inset shows the data on a log scale. 0.00.10.20.3 1x10172x10173x10174x1017 Dopant Concentration (cm-3)Depth (m) 2H Plasma 200C + 600C Anneal 2H Plasma 200C As Grown Figure 4-8. Donor concentration profiles in ZnO before and after plasma exposure and after subsequent annealing.

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51 2.53.03.5 10-310-210-1100101 6 K 50 K 100 K 200 K 300 K Intensity (a.u.)Energy (eV) Figure 4-9. PL spectra from 2H plasma exposed ZnO.

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52 2.42.52.62.7 0.01 6K 50K 100K 200 K 300K Intensity (a.u.)Energy (eV)3.153.203.253.303.353.403.45 0.01 0.1 1 6K 50K 100K 200K 300K Intensity (a.u.)Energy (eV) Figure 4-10. Detailed band edge and deep level emission PL spectra from 2H plasma exposed ZnO.

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53 1.52.02.53.03.5 0.004 0.006 0.008 0.010 0.012 0.014 500 C No anneal 700 C 600 C 700C Anneal 600C Anneal 500C Anneal No Anneal PL Intensity (Arb.)Energy (eV) Figure 4-11. 300K PL spectra from 2H implanted ZnO, as a function of subsequent anneal temperature.

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54 CHAPTER 5 OHMIC CONTACTS TO ZnO N-type ZnO Introduction Ohmic contacts are essential to connect electronic devices to allow the current to flow in and out. For ohmic contact, the ideal contact resistance is minimal and negligibly small, in which current and voltage relationship is linear and obeys ohms law. In order to attain devices with acceptable characteristics, high quality ohmic contacts with low specific contact resistance are imperative. The achievement of acceptable device characteristics relies heavily on developing low specific contact resistance ohmic metallization schemes. Table 5-1 summarizes the results of reported by other groups [95-116]. A more detailed table is located in Appendix A. Various metallization schemes and results for ohmic contact to n-ZnO have been reported. Specific contact resistance of 3 10-4 cm2 was reported for Pt-Ga contacts [98, 99]. Ti/Au ohmic contacts on Al-doped epitaxial layers resulted in specific contact resistances of 2 10-4 cm2 after 300 C anneal [100, 101] and 4.3 10-5 cm2 with plasma surface treatment prior to metal deposition [102]. Non-alloyed Al on epitaxial n-type ZnO yielded sp ecific contact resistance of 2.5 10-5 cm2 [114]. The specific contact resistance of Ta/A u on n-ZnO epitaxial layer was 4.3 10-6 cm2 [115]. For Al/Pt ohmic contacts to Al-doped ZnO (n ~ 8 1018 cm-3), the specific contact resistances of nonalloyed ohmic contacts and after 300 C anneal were 1.2 10-5 cm2

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55 [106] and 2 10-6 cm2 [105], respectively. Lowest specific contact resistance of 9.0 10-7 cm2 was reported for Ti/Al on n+ZnO with n ~ 1.7 1018 cm-3 after 300 C anneal [116]. The usual approaches involve surface cleaning to reduce barrier height or increase the effective carrier concentration of th e surface through preferential ion of oxygen. Minimum contact resistance generally occurs for post-deposition annealing temperatures of 200 C to 300 C on doped samples treated so as to further increase the near-surface carrier concentration. In this section, the annealing temperature dependence of contact resistance and morphology for Ti/Al/Pt/Au contacts on high quality, bulk ZnO substrates are reported. Two different surface cleaning procedures we re employed, although it was found that in general the as-received surface produced the lowest specific contact resistances. In addition, the effects carrier concentration on resistance of Ti/Al/Pt/Au contacts to n-type ZnO thin film were also investigated Previous reports have shown that Au, Ni, Pt and Ni/Au metallurgy can be used as ohmic metallization on thin films of p-type ZnO or ZnMgO [109, 111]. The lowest reported specific contact resistance of 1.7 10-4 cm2 after annealing at 600C in air was achieved for Ni/Au on P-doped ZnO produced by sputtering [109]. More work is needed to establish the contact formation mechanism as well as the intermixing and morphology of the contact metallurgy during th e anneal process. A study of Au and Au/Ni/Au contacts to p-type ZnMgO grown by pulsed laser deposition is presented.

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56 Bulk ZnO Experimental methods Bulk, wurtzite (0001) crystals obtained from Cermet, Inc. were used for all experiments. The samples were nominally undoped (n ~ 1017 cm-3) and the Znterminated surface was used in all cases. A circular transmission line method (C-TLM) pattern was created by lift-off of e-beam deposited Ti /Al / Pt /Au (200/400/200/800 ) on the front surface of the samples. Three di fferent types of surface preparation wee used prior to the metal deposition, namely the as-received condition, sequential cleaning in acetone, tricholorethylene, and methanol (3 minutes in each solvent), and finally, a oneminute exposure to an inductively-coupled plasma of H2 (5 mTorr, 100 W rf chuck power, 300 W surface power) in a PlasmaTherm 790 reactor (St. Petersburg, FL). The latter treatment is predicted to increase the near-surface doping through introduction of hydrogen donors. A schematic diagram and micrographs of a completed sample are shown in Figure 5-1. The contact pad spacing varied from 5 to 45 m. Samples were annealed at temperatures up to 600C for 1 min under flowing N2. The specific contact resistance, c, was derived from the circular TLM based on measurements on Agilent 4156 Precision Semiconductor Parameter Analyzer (Palo Alto, CA), while the interdiffusion of the metal layers was examined by AES on a Physical Electronics 660 Scanning Auger Microprobe (Chanhassen, MN) with a 10 keV, 1 A beam at 30 from the sample normal. Depth profiling during AES analysis was achieved by sputtering with a 3 keV Ar ion beam at a current of 2 A rastered over a 3 mm2 area. The sputter rate for the metals was in the range of 90 to 200 / minute. Optical and secondary electron images of the analysis areas were also obtained.

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57 Results and discussion The specific contact resistance, c, was obtained form the circular TLM measurements with the relationships ) / ( ) / ( ) / ( ) / ( ln 21 1 1 1 1 1 T T O T T O T O O O T O S TL R K L R K R L L R I L R I R L R R R R (5-1)2 T S CL R (5-2) where RT is the total resistance, RS is the sheet resistance, R1 is the outer radius of the annular gap, RO is the inner radius of the annular gap, IO, I1, KO, and K1 are the modified Bessel functions, LT is the transfer length, and c is the specific contact resistance [117]. The resulting data is shown as a function of annealing temperature (Figure 5-2). The minimum C values were obtained after 250C anneals, with the as-deposited samples exhibiting non-ohmic behavior. The C values for anneals at 350C and then slightly decrease before again showing non-ohmic characteristics after higher temperature (600C) processing. Figure 5-3 shows a secondary electron image of the as-deposited contact on ZnO, along with the AES depth profile. The surface morphology is excellent and there is no intermixing of the individual metal layers. The metal morphology started to roughen after 350C anneals as shown in the secondary electron images (Figure 5-4). As will be seen later from the AES data, the Al diffuses out towards the surface and begins to oxidize, while the Au diffuses in through the Pt layer. For Ti/Au contacts on ZnO, Kim et al. [101] found formation of TiO2 (srilankite and rutile) phase and TiO at the semiconductor-metal interface even in as-deposited samples. After annealing at 300C, the reaction of Ti and O was more complete while the Au began to form Ti-Au phases

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58 such as TiAu2 and Ti3Au by 500C. The surface morphology of the contacts in our work does not degrade significantly above 350C, even though the intermetallic reactions are more prevalent. AES surface scans from samples annealed at 250C, 350C, 450C and 600C are shown in Figure 5-5. Adventitious carbon is present on all samples, but Al is not detected on the surface until the 350C anneal. The amount of Al on the surface increases with increasing anneal temperature, and the presence of Al on the surface is likely to be accompanied by the associated oxidation of the metal. After 600C anneals, even Pt is detected on the surface, confirming the reacted nature of the contact after this treatment. The associated AES elemental depth profiles are shown in Figure 5-6. The 250C annealed sample shows some interfacial reacti on of Ti with the ZnO to form Ti-O phases, as reported previously [100, 101] and some interaction between Pt and Al, in-diffusion of Au and more significant formation of Ti-O phases at the semiconductor-metal interface. The dissociation of the ZnO and the further intermixing of the metal increases at higher temperatures leading to the completely intermixing contract after annealing at 600C. Summary A minimum specific contact resistance of 6 10-4 cm2 was obtained for Ti/Al/Pt/Au ohmic contacts on undoped (n ~ 1017 cm-3) bulk ZnO substrates. The contacts do not show ohmic behavior in the as-deposited state and reached their minimum resistance after 250C annealing. Higher processing temperatures led to a degraded contact resistance and reaction of the metal layers with each other and of Ti reaction with the ZnO. Solvent cleaning or H2 plasma exposure did not improve contact

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59 resistance in our case relative to the as-received state. Future work should focus on the doping dependence of specific contact resistance and on finding more thermally stable contact metallurgies for ZnO. Thin Film n-ZnO Experimental methods The phosphorus-doped ZnO epitaxial films in this study were grown by pulsed laser deposition (PLD) on single crystal (0001) Al2O3 substrate, using a ZnO: P0.02 target and a KrF excimer laser ablation source. The laser repetition rate and laser pulse energy density were 1 Hz and 3 Jcm-2, respectively. The films were grown at 400C in an oxygen pressure of 20 mTorr. The samples were annealed in the PLD chamber at temperatures ranging from 425 to 600C in O2 ambient (100 mTorr) for 60 min. The resulting film thickness ranged from 350 nm to 500 nm. Four-point van der Pauw Hall measurements were performed. Figure 5-7 shows the carrier mobility and the resistivity of the films after post-growth anneal at va rious temperatures. The conductivity of the ZnO films was n-type in all cases. Increasing post-growth anneal temperature decreases the carrier mobility and increases the resisitivity. Transmission line method (TLM) patterns, consisting of 100 m2 contact pads and gap spacings varying from 5 to 80 m, were created by dry etching of mesa and lifting off of e-beam evaporated metals. The samples were etched in an Unaxis 790 reactor in a CH4/Ar/H2 plasma with gas flows of 3, 5, and 8 sccm, respectively, under 5 mTorr pressure, 200 W rf power, and 500 W ICP power for 3 min. The samples were then deposited with Ti/Al/Pt/Au (200/400/200/800 ) by e-beam evaporation. After metal deposition, the samples were annealed at 200C for 1 min in N2 ambient. Schematic

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60 diagram and secondary electron image of the ohmic contact pads on a ZnO mesa are shown in Figure 5-8. The specific contact resistance, c, was derived from the TLM-based measurements on Agilent 4156C Precision Semiconductor Parameter. The measurement temperature was regulated with a Wentworth Labs Tempchuck TC-100 (Brookfield, CT), ranging from 30C to 200C. The inter-diffusion of the metal layers was examined by AES on a Physical Electronics 660 Scanning Auger Microprobe with a 10 keV, 1 A beam at 30 from the sample normal. Depth profiling during AES analysis was achieved by sputtering with a 3 keV Ar ion beam at a current of 2 A rastered over a 3 mm2 area. The sputter rate for the metals was in the range of 90 to 200 / minute. Results and discussion Current and voltage information obtained from electrical measurements are curve fitted with the corresponding equations to determine the specific contact resistance. For linear TLM, the total resistance, Rs, and specific contact resistance, c, are given by W L RRRSCT2 (5-3) S C CR WR22 (5-4) where RC is the contact resistance, Rs is the sheet resistance, L is the distance between two pads, W is the width of the pad. The carrier concentration in the ZnO films and the specific contact resistance of as-deposited contacts on these films, measured at room temperature as a function of post-growth ann eal temperatures (Figure 5-9). The carrier

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61 concentration ranged from 7.5 1015 cm-3 after 600 C post-growth anneal to 1.5 1020 cm-3 for the unannealed sample. The as-deposited ohmic contacts with the lowest specific contact resistance (8.7 10-7 cm2) were obtained for the sample with the highest carrier concentration, as expected. Figure 5-10 compares the specific contact resistance of the Ti/Al/Pt/Au contacts both before and after annealing at 200 C, for measurement temperatures up to 480K. At the lower doping levels, there is little variation in contact resistance versus measurement temperature in the range we measured. In addition, in most cases, the 200C anneal did not improve contact resistance significantly. The lowest c was achieved for the 200C annealed contact on the n=2.4 1018 cm-3 ZnO subsequently measured at 200C. The data as a function of both carrier concentration in the ZnO, before and after 200C annealing and for measurement temperatures of 30 or 200C are compiled in Figure 5-11. The lowest specific contact resistance in the as-deposited (unannealed) samples was 8.7 10-7 cm2 in the ZnO with the highest carrier concentration 1.5 1020 cm-3. The lowest specific contact resistances in the 200C annealed samples were 3.9 10-7 cm2 and 2.2 10-8 cm2 obtained in samples with carrier concentrations of 6.0 1019 cm-3 measured at 30 C and 2.4 1018 cm-3 measured at 200 C, respectively. The low specific contact resistance in the highest carrier concentration sample (1.5 1020 cm-3) may be explained by the tunneling process, which is given by D Bn S CN m *2 exp ~ (5-5)

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62 where q is the electronic charge, Bn the barrier height, S the semiconductor permittivity, m* the effective mass, the Plancks constant, and ND the donor density. The strong influence on doping is attributable to the ND -1/2 term. Another possible transport mechanism is thermionic emission, in which specific contact resistance is given by kT q T qA kBn C exp* (5-6) where k is the Boltzmanns constant, A* the Richardson constant, and T the measurement temperature. The ln ( cT) dependence on (1/T) for the thermionic emission is illustrated in Figure 5-12. For the sample with carrier concentration of 2.4 1018 cm-3, Bn extracted from the ln ( cT) versus 1/T plot was 0.21 eV before 200C anneal and 0.29 eV after this anneal. The energy required for thermionic emission is greater after annealing, even though a decrease in specific contact resistance is observed. Finally, Figure 5-13 shows AES depth prof iles of the Ti/Al/Pt/Au contact after annealing at 200 C. The initially sharp interfaces between the different metals are degraded by reactions occurring, especially between the Ti and the ZnO to form Ti-O phases and between the Pt and Al. The O appear s to diffuse outward while the Pt diffuses inward. The as-deposited contacts showed very sharp interfaces between the different metals and between the Ti and the ZnO. Anneals at 600 C almost completely intermixed the contact metallurgy. These results are similar to the same contact metallization on bulk ZnO as discussed above. Low thermal stability of both ohmic and Schottky contacts on ZnO appears to be a significant problem in this materials system [118], and there is an apparent need to investigate refractory metals with better thermal properties if applications such as high

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63 temperature electronics or lasers operating at high current densities are to be realized. The relatively low thermal stability of these metals on ZnO is in sharp contrast to the stability of the same metal system on GaN [119, 120]. Summary The lowest specific contact resistance of 8.7 10-7 cm2 for nonalloyed ohmic contact was achieved for ZnO with an n-type carrier concentration of 1.5 1020 cm-3. In the 200C annealed samples, the minimum specific contact resistances of 3.9 10-7 cm2 and 2.2 10-8 cm2 were obtained in samples with carrier concentrations of 6.0 1019 cm-3 measured at 30 C and 2.4 1018 cm-3 measured at 200 C, respectively. The low specific contact resistance in the high carrier concentration sample may be explained by the tunneling mechanism. For the sample with carrier concentration of 2.4 1018 cm-3, Bn extracted from the ln ( cT) versus 1/T plot based on the thermionic emission mechanism are 0.21 eV before anneal and 0.29 eV after anneal. AES revealed Ti-O interfacial reactions and intermixing between Al and Pt layers. In summary, specific contact resistances in the range 10-7-10-8 cm2 were obtained for Ti/Al/Pt/Au contacts on heavily n-type ZnO thin films, even in the as-deposited state. However, the contacts show significant changes in morphology even for low temperature (200 C) anneals, and this suggests that more thermally stable contacts schemes should be investigated. p-ZnMgO Thin Film Introduction There is increasing interest in the ZnMgO/ZnO/ZnCdO system for solid-state lighting applications [121-123]. The larger exciton binding energy relative to GaN,

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64 commercial availability of large area ZnO substrates, ability to grow at lower epitaxial growth temperatures and availability of wet etchants suggest that there may be advantages relative to the nitrides in some of these applications. Most of the recent interest has derived from advances in p-type doping [18, 113, 124-143] and demonstrations of a variety of nZnO/p-AlGaN and nZnO/p-ZnO/ScMgAlO4 electroluminescent devices have followed [111, 144-148]. Substantial improvement is necessary to establish robust p-type doping, which often exhibits very low mobilities and poor optical properties [124, 125]. Recent reports also show it may revert to n-type conductivity over a few days at room temperature [147]. In addition to achieving stable and high hole concentrations, work is also needed to develop low resistance p-ohmic contacts. Previous reports have shown that Au, Ni, Pt and Ni/Au metallurgy can be used as ohmic metallization on thin films of p-type ZnO or ZnMgO [111, 148]. The lowest reported specific contact resistance of 1.7 10-4 cm2 after annealing at 600C in air was achieved for Ni/Au on P-doped ZnO produced by sputtering [148]. More work is also needed to establish the contact formation mechanism as well as the intermixing and morphology of the contact metallurgy during the anneal process. In this section, we report on a study of Au and Au/Ni/Au contacts to p-type ZnMgO grown by pulsed laser deposition. In both cases, the as-deposited contacts are rectifying and the transition to ohmic behavior is associated with out-diffusion of Zn from the ZnMgO. Experimental Methods The phosphorus-doped (Zn0.9Mg0.1)O epitaxial films were grown by pulsed laser deposition (PLD) on c-plane sapphire substrates. The target was fabricated using high-purity ZnO (99.9995%) and MgO (99.998%), mixed with P2O5 (99.998%) as the

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65 doping agent. The phosphorus doping level in the target was 2 at.%. The addition of Mg shifts the conduction band edge to higher energy, reducing the n-type background carrier concentration. The phosphorus introduces an acceptor level at ~0.25 eV from the valence band [146]. The ZnO growth chamber base pressure was 410-6 Torr. An undoped ZnO buffer layer (~50 nm) was initially deposited at 400C and 20 mTorr oxygen partial pressure before the growth of P-doped (Zn0.9Mg0.1)O films at a substrate temperature of 500C under oxygen partial pressure of 150 mTorr. The total film thickness was 400 to 600 nm. Hall measurements showed a 25C hole concentration of 2.7 1016 cm-3, a mobility of 8 cm2 /Vs and a resistivity of 35 cm. The films exhibited good crystallinity with c-axis orientation. Contact metallurgy of either Au(1000) or Au/Ni/Au (200/200/800 ) was deposited by electron-beam evaporation and pattern ed by resist lift-off to form a circular transmission line pattern (C-TLM) with inner-outer ring spacings of 5 to 45 m. The samples were annealed for 1 min at 600C in N2 and the current-voltage (I-V) characteristics recorded on an Agilent 4156 parameter analyzer. AES depth profiling was performed on a Physical Electronics 660 Scanning Auger Microprobe. The electron beam conditions were 10keV, 1 A beam current at 30 from sample normal. For depth profiling, the ion beam conditions were 3 keV Ar+, 2.0 A, 3 mm2 raster, with measured sputter rate of 110 /minute. Cross-sec tion transmission electron microscopy (TEM) was also performed on the annealed samples. Results and Discussion The as-deposited contacts showed rectifying behavior for both Au and Au/Ni/Au, but the I-V characteristics became ohmic for annealing at 600C (Figure 5-14). Previous

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66 work reported ohmic behavior with specific contact resistance of ~ 8 10-3 cm2 in as-deposited Ni/Au contacts on p-type ZnO of higher initial hole concentration (1018 cm-3) than used in this experiment [148]. The contact behavior observed here are similar to Ni/Au contacts on p-GaN, which also has relatively low hole densities due to the high ionization energy of the acceptor dopant. The electrical properties of our contacts to p-ZnMgO before and after 600C annealing are compiled in Table 5-2. The specific contact resistance of 7.6x10-6 cm2 is the lowest value reported for any contact to p-ZnO or ZnMgO. Au alone also produces a low specific contact resistance, indicating that Ni is not necessary to achieve good ohmic contacts, though it does help produce a lower contact resistance. Lim et al. [148] reported a value of 1.7 10-4 cm2 for Ni/Au on sputtered p-ZnO, where the minimum specific contact resistance was achieved for 600C anneals in air. In their case, annealing was found to produce NiO and Au3Zn phases that may play a role in the current conduction mechanism. Figure 5-15 shows the AES depth profiles from the Au/Ni/Au contacts before and after annealing. A part of the Ni layer has outdiffused through the Au overlayer even in the as-deposited case and most likely becomes oxidized on the surface. After annealing at 600C, the flat profiles for the Ni and Au concentrations indicate that they have completely reacted. By sharp contrast, the Au-only contacts show little change in the Au profile (Figure 5-16) after annealing. In both cases however, there is diffusion of Zn through the metallization after annealing,as shown by the AES surface scans in Figure 5-17. The formation of Zn vacancies in the ZnMgO would increase the near-surface hole concentration and improve contact resistance. This has also been suggested by Lim et al. [144] as a contributing factor in the c onduction mechanism in their Ni/Au contacts on

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67 p-ZnO. The fact that Au-only contacts pr oduce low contact resistance suggests that Au acts to enhance Zn vacancy formation, perhaps through formation of the Au3Zn phases. When Ni is also present, it may react to form Ni-Zn phases as well as NiO, both of which may play a role in achieving low contact resistance. Figure 5-18 shows a cross-section TEM of the annealed Au/Ni/Au contact on p-ZnMgO. The morphology of the contact becomes roughened, consistent with the intermixing observed in the AES depth profile s. Different grains with dimensions of order 100 nm are visible in the metal layer. It remains to be seen if this will be an issue in terms of uniformity of contact properties and current density in device structures such as light-emitting diodes, though generally, the contact dimensions on such devices are very large (>100 m) relative to the grain size. Summary Both Au and Au/Ni/Au are found to provide low specific contact resistance on lightly doped p-ZnMgO after annealing at 600C. In both cases, the as-deposited contacts are rectifying and the transition to ohmic behavior is associated with outdiffusion of Zn from the ZnMgO. A minimum specific contact resistance of 7.6 10-6 cm2 was obtained with Au/Ni/Au, which is about a factor of 3 lower than for pure Au contacts annealed at the same temperature.

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68 Table 5-1. Ohmic contacts ZnO and their respective specific contact resistance from published works. Description Metal Lowest c ( cm2) Ref (a) n – type n-ZnO bulk E-beam Ti/Au 5 10-5 95 n-ZnO bulk E-beam In 7 10-1 96 n-ZnO epi on Al2O3 Ti/Au 1.5 10-5 97 n-ZnO epi on Al2O3 FIB direct write Ga-Pt 3.1 10-4 98, 99 n-ZnO epi on Al2O3 E-beam Ti/Au 2 10-4 100, 101 n-ZnO epi on Al2O3 E-beam Ti/Au 4.3 10-5 102 n-ZnO epi on Al2O3 E-beam Re/Ti/Au 1.7 10-7 103 n-ZnO epi on Al2O3 E-beam Ti/Al 9.0 10-7 104 n-ZnO epi on Al2O3 E-beam Al/Pt 2 10-6 105, 106 n-ZnO epi on Al2O3 E-beam Ru 2.1 10-5 107 n-ZnO epi on Al2O3 E-beam Al 8 10-4 108 n-ZnO epi on Al2O3 E-beam Al 2.5 10-5 114 n-ZnO epi on Al2O3 E-beam Ta/Au 5.4 10-6 115 n-ZnO epi on Al2O3 E-beam Ti/Al 9 10-7 116 (b) p type p-ZnO epi on Al2O3 E-beam Ti/Au 1.72 10-4 109 p-ZnO epi on Al2O3 E-beam Pt/ITO 7.7 10-4 110 p-ZnMgO epi on glass E-beam Ti/Au or Ni/Au 3 10-3 111 p-ZnMgO epi on glass E-beam Ni/Au 4 10-5 112 p-ZnO on SiC E-beam In/Au, Ti/Au, or Ni/Au --113 Table 5-2. Sheet resistance, transfer lengt h and specific contact resistance of annealed Au or Au/Ni/Au contacts on p-ZnMgO. Rs ( / ) Lt ( m) Rc ( cm-2) Au (600 C annealed) 7.5 103 0.57 2.5 10-5 Au/Ni/Au (600 C annealed) 1.7 103 0.68 7.6 10-6

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69 Bulk n-ZnO MetalsCross-sectional view of circular TLM R1RO Bulk n-ZnO MetalsCross-sectional view of circular TLM R1RO (a) (b) Figure 5-1. Schematic diagram (a) and secondary electron image (b) of circular TLM pattern on ZnO substrate.

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70 250300350400450 2.0x10-34.0x10-36.0x10-38.0x10-31.0x10-2 Specific Contact Resistivity (cm2)Anneal Temperature (C) As Received H2 Plasma Solvent Figure 5-2. Specific contact resistance as a function of anneal temperature for Ti/Al/Pt/Au contacts on n-ZnO. Solvent chemical cleaning or H2 plasma exposure of the surface prior to metalli zation was compared with the case of depositing the metal on the as-received surface.

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71 (a) 01000200030004000 0 20 40 60 80 100 C Zn O Ti Al Pt Au Atomic Concentration (%)Depth () (b) Figure 5-3. Secondary electron image (a) and AES depth profile (b) of as-deposited Ti/Al/Pt/Au contact on ZnO.

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72 Figure 5-4. Secondary electron images of Ti/Al/Pt/Au contacts on ZnO after annealing at 250 C (a), 350 C (b), 450 C (c) or 600 C (d). (a) (b) (c) (d)

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73 Figure 5-5. AES surface scans of Ti/Al/P t/Au contacts on ZnO after annealing at 250 C (a), 350 C (b), 450 C (c) or 600 C (d). Energy (eV) -2000 500 1000 1500 2000 -8000 -6000 -4000 0 2000 4000 Energy (eV) O Au Ni Ni Au Au Au Au Ti Na dN(E)/dE C Atomic % C 48.9 Au 43.0 O 2.9 Ni 2.4 Na 2.4 Ti 0.5 -1 0.5 500 1000 1500 2000 -1.5 -0.5 0 1 1.5 x 10 Energy (eV)dN ( E ) /dE C O F Al Au Au Au Au Al Atomic % O 34.4 Al 28.1 C 19.9 Au 16.9 F 0.8 500 1000 1500 2000 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 x 10 Energy (eV) dN ( E ) /dE C O Al Zn Au Al Pt Pt Pt Atomic % O 40.7 Al 3.2 C 10.8 Au 9.6 Pt 6.5 Zn1.1 500 1000 1500 2000 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 x 104 dN(E)/dE C O F Al Au Au Atomic % C 29.0 O 26.6 Al 24.7 Au 18.3 F 1.5 (a) (b) (c) (d)

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74 Figure 5-6. AES depth profiles of Ti/Al/P t/Au contacts on ZnO after annealing at 250 C (a), 350 C (b), 450 C (c) or 600 C (d). 01000200030004000 0 20 40 60 80 100 (b) 350CTi Au C O Al PtAtomic Concentration (%)Depth () Zn01000200030004000 0 20 40 60 80 100 (d) 600CO Al Ti C Zn Pt Au Depth ()Atomic Concentration (%) 01000200030004000 0 20 40 60 80 100 (a) 250CTi Pt Al Au O Zn C Atomic Concentration (%)Depth () 01000200030004000 0 20 40 60 80 100 (c) 450CC Zn O Ti Al Pt AuAtomic Concentration (%)Depth ()

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75 0100200300400500600 5 10 15 20 Mobility Resistivity Post-growth Anneal Temperature (C)Carrier Mobility (cm2/V-s)10-310-210-1100101102103 Resistivity (-cm) Figure 5-7. Carrier mobility and resistivity of epi-ZnO as a function of post-growth anneal temperature.

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76 Semiconductor film Metal Pads Semiconductor film Metal Pads (a) (b) (c) Figure 5-8. Schematic diagram (a), SEM (b) and microsope image (c) of the linear TLM ohmic contact pads on ZnO mesa.

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77 0100200300400500600 1015101610171018101910201021 Carrier Concentration c measured at 30 C Post-growth Anneal Temperature (C)Carrier Concentration (cm-3)10-710-610-510-410-3 Specific Contact Resistance (-cm2) Figure 5-9. Carrier concentration of epiZnO and specific contact resistance of asdeposited ohmic contacts measured at 30 C versus post-growth anneal temperature. 280320360400440480 10-810-710-610-510-410-3 As-deposited 200 C Anneal 7.5 1015 cm-33.2 1017 cm-32.4 1018 cm-36.0 1019 cm-31.5 1020 cm-3 Specific Contact Resistance (-cm2)Measurement Temperature (K) Figure 5-10. Specific contact resistance as a function of measurement temperature for samples with various carrier concentrations. The solid symbols represent measurements prior to ohmic contact anneal. The corresponding open symbols denote measurements after 200 C, 1 min anneal in N2 ambient.

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78 101510161017101810191020102110-710-610-510-410-3 Specific Contact Resistance (-cm2)Carrier Concentration (cm-3) As-deposited 200C anneal, measured at 30 C 200C anneal, measured at 200C Figure 5-11. Specific contact resistance versus measurement temperature of as-deposited ohmic contact measured at 30 C, and after annealing at 200 C, 1 min measured at 30 C and 200 C. 2.02.22.42.62.83.03.23.4 -12 -10 -8 -6 -4 -2 0 2.4 1017 cm-36.0 1019 cm-31.5 1020 cm-37.5 1017 cm-33.2 1017 cm-3ln (cT)1000/T (1/K) Figure 5-12. ln( cT) versus 1000/T for samples with various carrier concentrations. The solid symbols represent measurements prior to ohmic contact anneal. The corresponding open symbols denote measurements after 200 C, 1 min anneal in N2 ambient.

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79 0100020003000 0 20 40 60 80 100 O Zn Ti Al Pt Au C Atomic Concentration (%)Sputter Depth () (b) Figure 5-13. AES surface scans (a) and surf ace scans (b) of Ti/Al/Pt/Au ohmic contacts to ZnO after annealing at 200 C. Counts per second Kinetic Energy (eV) 2500 500 1000 1500 -6000 -4000 -2000 0 4000 C O Au Au Au Au 0 2000 Atomic % C 50.3 Au 47.2 Ni 1.5 2000 Au Ni Au (a)

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80 -5-4-3-2-1012345 -0.04 -0.02 0.00 0.02 0.04 As-deposited 600 C Annealed Au/Ni/Au/Zn0.9Mg0.1O:P0.02Current (A)Bias (V) (a) -5-4-3-2-1012345 -0.04 -0.02 0.00 0.02 0.04 600 C Annealed As-depositedAu/Zn0.9Mg0.1O:P0.02 Current (A)Bias (V) (b) Figure 5-14. I-V characteristics from Au/Ni/Au (a) or Au (b) contacts on p-type ZnMgO before and after annealing at 600C.

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81 05001000150020002500 0 20 40 60 80 100 Au/Ni/Au/Zn0.9Mg0.1O:P0.02 As-depositedMg Ni C O Zn Au Atomic Concentration (%)Sputter Depth () (a) 05001000150020002500 0 20 40 60 80 100 Ni O Zn C Mg Au Au/Ni/Au/Zn0.9Mg0.1O:P0.02 600C Annealed Atomic Concentration (%)Sputter Depth () (b) Figure 5-15. AES depth profiles from Au/N i/Au contacts on p-type ZnMgO before (a) and after (b) annealing at 600C.

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82 0500100015002000250030003500 0 20 40 60 80 100 Au/Zn0.9Mg0.1O:P0.02 As-depositedO Zn Mg C Au Atomic Concentration (%)Sputter Depth ()(a)0500100015002000250030003500 0 20 40 60 80 100 Au/Zn0.9Mg0.1O:P0.02 600C Annealed C Zn Mg O Au Atomic Concentration (%)Sputter Depth ()(b) Figure 5-16. AES depth profiles from Au c ontacts on p-type ZnMgO before (a) and after (b) annealing at 600C.

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83 Figure 5-17. AES surface scans from as-deposited Au (a) and after annealing at 600C (b) or as-deposited Au/Ni/Au (c) and after annealing at 600C (d). 500 1000 1500 2000 -8000 -6000 -4000 -2000 0 2000 4000 Kinetic Energy (eV) C O A u A u A u A u A u A u S S Atomic % C1 54.4 Au4 43.5 O1 1.2 S1 0.8Au on ZnMgO As deposited Surface survey Atomic % C 54.4 Au 43.5 O 1.2 S 0.8 Au As-deposited 0 -4000 8000 4000 Counts p er secon d 500 1000 1500 2000 10000 -8000 -6000 -4000 -2000 0 2000 4000 6000 050481111.spe Kinetic Energy (eV) C O Z n A u A u A u A u A u A u A u A u Atomic % C1 54.4 Au4 36.6 O1 4.6 Zn1 4.4Au on ZnMgO Annealed @600C Surface survey Au 600 C Atomic % C 54.4 Au 36.6 O 4.6 Zn 4.4 0 10000 6000 6000 500 1000 1500 2000 500 1000 1500 2000 Kinetic Energy (eV)Kinetic Energy (eV) 500 1000 1500 2000 -8000 -6000 -4000 -2000 0 2000 4000 050481201.spe Kinetic Energy (eV) C O S N i N i N i A u A u A u A u A u A u Atomic % C1 49.1 Au4 42.5 Ni3 4.8 O1 2.0 S1 1.6Au/Ni/Au/ZnMgO As deposited Surface survey 2000 Au/Ni/Au As-deposited 0 -4000 8000 4000 Counts p er secon d 500 1000 1500 Kinetic Energy (eV) Atomic % C 40.1 Au 42.5 N i 4.8 O 2.0 S 1.6 500 1000 1500 2000 -8000 -6000 -4000 -2000 0 2000 4000 050481211.spe Kinetic Energy (eV) C O S S N i N i N i A u A u A u A u A u Z n Atomic % C1 50.4 Au4 26.4 Ni3 13.5 O1 5.1 Zn1 2.7 S1 2.0Au/Ni/Au/ZnMgO Annealed @600C Surface survey Atomic % C 50.4 Au 26.4 N i 13.5 O 5.1 Zn 2.7 S 2.0 0 -4000 8000 4000 500 1000 1500 2000 Kinetic Energy (eV) Au/Ni/Au 600 C Au Au Au Au O C Au S Au Au Au Au Au Au Au O S Au C N i Au C Au Zn O Au Au O N i N i S Au (c) (d) (b)

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84 Figure 5-18. Cross-section TEM micrograph of Au/Ni/Au contact after annealing at 600C.

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85 CHAPTER 6 SCHOTTKY CONTACTS TO ZNO Introduction To realize UV light emitters/detectors or field effect transistors, it is necessary to develop reliable, reproducible doping methods and ohmic and rectifying contacts to this material in order to exploit these device opportunities. Table 6-1 summarizes the results of works other researchers have done, including most recent materials published after the completion of this work [95, 111, 118, 149-163]. A more detailed table is located in Appendix B. Previous reports have shown low reactive metals such as Au, Ag and Pd form rectifying contacts on n-type ZnO with Schottky barrier heights in the range of 0.6-0.8 eV [95, 111, 118, 149-163]. However, the trend in barrier heights did not correlate with the metal work functions, sugges ting that either intrinsic surface states or residual surface contamination is playing a role in determining the electrical properties of the contacts. In addition, the thermal stability of Au was extremely poor, with degradation occurring even at 60C [118, 149, 151, 153, 160-163]. The metal work functions and calculated ideal barrier heights for several potential candidates are listed in Table 6-2 [164]. Various surface cleaning procedures prior to depositing contacts on ZnO have been reported, such as etching in concentrated phosphoric or nitric acid or organic solvent rinsing [95, 118, 152-155, 158, 162]. In most cases the reported ideality factors are high, indicating the presence of transport mechanisms such as tunneling, the impact of interface states or the influence of deep recombination centers.

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86 There is a need to study the effect of su rface cleaning on the electrical properties of contacts on ZnO. More thermally stable metallization schemes are also desirable for ZnO. Potential candidates for more thermally stable metallization include the use of tungsten, which has been explored previously for both GaN and SiC. Other groups have reported the use of both TiC and TiW as th ermally stable contacts on SiC [165-167], but they have not yet been applied to ZnO. Another potential set of contact materials is based on borides. For example, the high temperature material W2B5 has a reasonable electrical resistivity (19 ) and in hexagonal crystalline form has a lattice constant of 2.982. Another potential set of contact materials is based on borides. For example, the high temperature material CrB2 has a reasonable electrical resistivity (21 ) and in hexagonal crystalline form has a lattice constant of 2.969 . In this chapter, the measurement and annealing temperature dependence of Pt Schottky contacts on thin film n-type ZnO is reported. The effect of UV ozone cleaning on the properties of both Pt and W contacts on bulk n-type ZnO is investigated. Schottky contacts using W2B, W2B5, and CrB2 on bulk n-type ZnO were also examined. Thin Film ZnO Experimental Methods The phosphorus-doped ZnO epitaxial films in this study were grown by pulsedlaser deposition (PLD) on single crystal (0001) Al2O3 substrate, using a ZnO:P0.02 target and a KrF excimer laser ablation source. The laser repetition rate and laser pulse energy density were 1 Hz and 3 Jcm-2, respectively. The films were grown at 400 C in an oxygen pressure of 20 mTorr. The samples were annealed in the PLD chamber at 600 C in O2 ambient (100 mTorr) for 60 min. The film thickness was 500 nm. Four-point van

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87 der Pauw Hall measurements were performed to obtain the carrier concentration and mobility in the films. The carrier concentration was 7.5 1015 with corresponding mobility of 6 cm2/Vs. Ohmic contacts of Ti/Al/Pt/Au were patterned by lift-off of e-beam evaporated films onto the ZnO. Circular contacts of 50 m diameter of Pt/Au (200) were then deposited by e-beam evaporation. I-V-T measurements were performed with a hot chuck and an HP4156 parameter analyzer. Some of the samples were annealed at temperatures up to 300C for 1 min under a N2 ambient. This did not have any visible impact on the Pt metallurgy but did cause a reacted appearance on the Ti/Al/Pt/Au contacts (Figure 6-1). Results and Discussion Figure 6-2 shows the forward I-V characteristics from the Pt Schottky diodes both as-deposited and after 300C anneals, as a function of measurement temperature. The forward current increases with increasing temperature. The saturation currents IS at each temperature (T) were extracted from the relation nkT IR V e I IS Sexp for V>>kT/e (6-1) where V is the applied voltage, RS the series resistance, e is the electronic charge and n is the ideality factor. The extracted values in terms of saturation current density, JS, and ideality factor are shown in Table 6-3. At room temperature for the as-deposited contact, the saturation current density was 1.5 10-4 Acm-2 and the ideality factor was 1.7. The saturation current can also be represented as ) exp(2 *kT e T AA IB S (6-2)

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89 similar to values reported previously for Au and Ag on n-type ZnO [95, 111, 118, 149-163]. It should also be noted that in th e early stage of developing a new materials system it is common to have a wide range of values reported for barrier heights due to the presence of surface defects, interfacial layers and material inhomogeneities. Summary Pt contacts on ZnO show rectifying beha vior with a barrier height at 25oC of 0.61 eV.This barrier height is reduced signifi cantly (to 0.42 eV) after annealing at 300C. It is desirable to investigate additional candi dates for rectifying contacts on n-type ZnO to determine if larger barrier heights can be achieved. Bulk ZnO Experimental Methods The samples were (0001) undoped bulk ZnO crystals from Cermet, Inc. The samples were epiready, one-side-Zn-face-polished by the manufacturer. The room temperature electron concentration and mobility established by van der Pauw measurements were ~1017 cm-3 and 190 cm2/V.s, respectively. To study the effects of UV ozone treatment (UVOCS UV Ozone Cleaning System Model 70606B, Montgomeryville, PA) prior to Schottky metal deposition, samples we exposed for 30 mins at room temperature and then characterized with AFM using tapping mode, over an 1 m 1 m area. XPS/ESCA measurements in a Perkin-Elmer PHI 5100 system (Boston, MA) were also performed. The contact study was performed by depositing a full backside ohmic contact of Ti/Al/Pt/A u (200/800/400/800 )by e-beam evaporation. This was annealed at 200C, 1 min, N2 ambient. The subsequent contacts were deposited either with or without 30 min UV ozone tr eatments. The Pt Schottky contact was patterned by standard lift-off photolithography after e-beam evaporation of

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88 where A** is the Richardson’s constant for ZnO, A the diode area and B is the Schottky barrier height. The theoretical value of A** for n-ZnO is 32 A cm-2K-2 [2]. From the I-V-T characteristics, we extracted a value for A** of 61.2 Acm-2K-2, but there is a large uncertainty (~ 60%) in this due to narrow range of measurement temperatures from which we had to extrapolate to obtain the estimated Richardson’s constant. Figure 6-3 shows the reverse I-V characteristics from the diodes as a function of temperature. The reverse leakage depends on both bias and temperature. From a moderately doped sample of the type studied here, thermionic emission is expected to be the dominant leakage current mechanism [169]. According to image-force barrier height lowering, this leakage current density, JL can be written as kT J JB S Lexp (6-3) where B is the image-force barrier height lowering, given by 2 / 14 S MeEwhere EM is the electric field strength at the metal/semiconductor interface and S is the permittivity. The experimental dependence of JL on bias and temperature is stronger than predicted from equation (6-3). The large bandgap of ZnO makes the intrinsic carrier concentration in a depletion region very small, suggesting that contributions to the reverse leakage from generation in the depletion region are small. Therefore, the additional leakage must originate in contributions from other mechanisms such as thermionic field emission or surface leakage. From the measured I-V characteristics at each temperature we were able to extract the B values, as shown in Figure 6-4. The barrier height at 25oC was 0.61 0.04 eV,

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90 Pt/Au (200/1000 ). The refractory metal contacts were also patterned by standard liftoff photolithography after deposition by sputtering in a Kurt J. Lesker CMS-18 system (Pittsburg, PA). The following metal stacks were examined: W/Pt/Au (720/200/1000 ), W2B5 or W2B/Pt/Au (500/0-200/800) or CrB2/Pt/Au (500/200/1000 ) All of the metals were deposited by Ar plasma-assisted rf sputtering at pressure of 7 mTorr and rf (13.56 MHz) powers of 400 W. The contacts were patterned by liftoff and annealed at 500-1000 C for 1 min in a flowing N2 ambient in a RTA furnace. In each case the samples were characterized with an HP 4156 Semiconductor Parameter Analyzer and AES Perkin-Elmer PHI 660 Scanning Auger Microscope after post-sputtering anneals up to 700C, under a N2 ambient for 1 min. Results and Discussion The overall roughness of the ZnO surface impr oved slightly after ozone treatment. Figure 6-5 shows the morphology of the bulk ZnO before (a) and after (b) a 30 min ozone clean. As shown in Figure 6-6, the rms roughness and average roughness of the samples before ozone treatment were 0.248 nm and 0.347 nm, respectively. After 30 min ozone treatment, the rms roughness to 0.162 nm, and the average roughness was 0.322 nm.The main effect of the ozone exposure is expected to desorption of surface C contamination through conversion to volatile CO and CO2 products. To examine this in more detail, XPS measurements were carried out. Figure 6-7 shows the surface surveys before (a) and after (b) ozone exposure for 30 min, with a much higher concentration of C on the untreated sample. The binding energies were calibrated by taking the C 1s peak at 284.6eV as a reference.500 eV Ar+ sputtering was used for depth profiling. Carbon atomic concentrations were calculated using the standard sensitivity factors and are tabulated in Table 6-4, as a function of Ar sputter

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91 time. The average surface concentration of C was decreased significantly from 29.5 at.% to 5.8 at.% as a result of the ozone treatment. Given the approximate Ar sputter rate of ~60 /min, it is seen that this decrease is predominantly a near-surface effect. The O 1s spectra was resolved into two components, O1 and O2, for no ozone and 30 min ozone treated bulk ZnO on the surface and after 1 min and 2 min of Ar+ sputtering. The O1 line is due to O in the form of Zn-O, while the O2 line is due to O in the form of O-H and related species. Figure 6-8 shows these spectra for the before and after ozone cases. There is a decrease in the O2 component relative to the O1 component. Table 6-5 summarizes the XPS data as a function of Ar sputter time. The chemical data discussed above suggest that the ozone exposure removes C-related contamination from the ZnO surface. This process also had a major influence on the electrical characteristics of the contacts deposited on these surfaces. The current-voltage (I-V) characteristics for the Pt metallization were measured on 50 m diameter contacts. The sample without ozone treatment resulted in linear I-V characteristics, as shown in Figure 6-9. By sharp contrast, the sample treated with ozone exhibited rectifying behavior (Figure 6-10). In the latter case, the barrier height derived from the forward characteristics from the relation for the thermionic emission over a barrier ) exp( ) exp( .2 *nkT eV kT e T A Jb F (6-4) where J is the current density, A* is the Richardsons constant for n-ZnO, T the absolute temperature, e the electronic charge, b the barrier height, k Boltzmanns constant n the ideality factor and V the applied voltage. From the data, b was obtained as 0.70 eV for the as-deposited Pt on the ozone cleaned surface. The ideality factor was ~1.5, suggesting

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92 transport mechanisms other than thermionic emission, such as recombination. The saturation current density was 6.17 x10-6 Acm-2. The transition from ohmic to rectifying behavior as a result of the ozone cleaning is due to the removal of a defective surface layer that contains C contamination and that pr events electrical contact of the Pt with the ZnO surface. The bottom part of the Figure 6-10 shows the reverse I-V characteristics from the diodes. In the presence of image-force induced barrier lowering, the reverse current can be expressed by equation 6-2. The reverse current shown in Figure 6-10 (b) is several orders of magnitude higher than expected from this relation and indicates other current transport mechanisms are present. By sharp contrast to the results for Pt contacts on ZnO, the W contacts displayed non-rectifying behavior for as-sputtered, 250 C and 500 C annealed conditions, independent of the use of ozone cleaning. However, as shown in Figure 6-11, after subsequent annealing at 700C, both uncleaned and ozone-exposed samples showed rectifying characteristics. Table 6-6 summarizes the electrical properties of these contacts. The barrier heights are significantly lower than for the Pt contacts, with maximum values of ~0.49 eV for the ozone-cleaned samples. The need for postdeposition annealing shows that the initial damage from the sputter process itself dominates the electrical characteristics of the as-deposited diodes. In addition, the postdeposition annealing produced some intermixing of the contact metallurgy, as shown in Figure 6-12. After 700 C anneal, the depth profiles show that Zn diffuses out to the Au-Pt interface. The ozone exposure had no appare nt effect of the thermal stability of the contacts.

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93 The ozone cleaning produces a conversion of the Pt contacts from ohmic to rectifying behavior, while the W contacts are ohmic as-deposited independent of the surface cleaning. W contacts are found to be stable up, to ~700C. Figure 6-13 shows the secondary electron images of W2B/Au (a) or W2B/Pt/Au (b) contacts on ZnO, either as-deposited (c) or after 600C annealing (d). In all cases, the morphology remains essentially featureless. The scratches on the images at lower left are due to the contact probes used for the I-V measurements. The I-V characteristics from W2B/Au (a) or W2B/Pt/Au (b) contacts on ZnO are shown in Figure 6-14, as a function of annea ling temperature. The contacts are all ohmic as-deposited, and only in the case of the samples with Pt interlayer rectification is observed for anneal temperatures above 500 C. After 900C anneals in both types of contacts, ohmic behavior with higher resistance than the as-deposited case is observed. For the contacts showing rectifying behavior, the Schottky barrier height was obtained by fitting the forward characteristics to the relation for thermionic emission over a barrier as mentioned previously. The data extracted from this analysis is summarized in Figure 6-15 as a function of anneal temperature. The ideality factor was >2 in all cases, suggesting transport mechanisms other than thermionic emission, such as recombination and surface leakage. The barrier heights are all ~0.4 eV. The work function of W2B is not known, but we can estimate the value of the expected barrier height from the data in Table 6-2. It would be expected to be in the range 0.35-0.45 eV, from the values for the endpoint compositions of pure W or B. Figure 6-16 shows AES surface scans of W2B/Au (a) or W2B/Pt/Au (b) contacts on ZnO, either as-deposited (c) or after 600C annealing (d). Carbon, oxygen, and gold were

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94 detected on all of the as-received ZnO samples. Sulfur was detected on the as deposited sample and zinc was detected on the annealed sample. The data are summarized in Table 6-7. The corresponding depth profiles are s hown in Figure 6-17. Carbon, gold, boron, tungsten, platinum, oxygen, and zinc were monitored as a function of sputter depth. The profiles obtained from the as deposited and 600C annealed W2B/Au samples were the same. No evidence of layer diffusion was observed in the annealed sample profile. The profiles obtained from the as deposited and 600C annealed W2B/Pt/Au samples revealed thin platinum layers and the presence of oxygen in the W2B layer for both samples. Platinum was detected in the gold layer of the annealed sample but not in the profile from the as-deposited sample. The as-deposited c ontacts are always ohmic, perhaps because of the residual sputter damage that leaves the near-surface region more n-type. As this damage is annealed out, the contact reverts to rectifying behavior in the cases where dissociation of the ZnO (as measured by the absence of Zn outdiffusion to the surface) is prevented by the Pt diffusion barrier. At temperatures around 900 C, the contact again shows ohmic behavior as the ZnO begins to dissociate. Figure 6-18 shows secondary electron images of W2B5/Pt/Au contacts on ZnO, either as-deposited (a) or after 600 C annealing (b). The thermal stability of these contacts is much inferior to that of the W2B, with blistering apparent at 600C. Figure 6-19 shows the I-V characteristics from W2B5/Pt/Au contacts on ZnO, as a function of annealing temperature. At 600C, the contacts show rectifying characteristics with barrier height 0.66 eV, ideality factor 4.5 and saturation current density of 2.4 10-5 Acm-2.

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95 Figure 6-20 shows AES surface scans of W2B5/Pt/Au contacts on ZnO, either as deposited (top) or after 600C annealing (bottom), while Figure 6-21 shows the corresponding depth profiles. The profiles obtained from the as deposited and 600 C annealed W2B5/Pt/Au samples are quite different from those of the W2B. The profile obtained from the as-deposited sample does not show the distinct layer structure that was observed in the W2B sample. Also, inter-diffusion of the layers is significant after annealing. The interpretation of the data for the W2B contacts is less clear-cut because of the significant reaction of the metal with the ZnO in the as-deposited condition. Figure 6-22 shows optical microscope images of CrB2/Pt/Au contacts on ZnO, either as-deposited (a) or after 500,600 or 700C annealing. The morphology is essentially featureless to anneal temperatur es of 500C but shows increasing degrees of metal reaction with the ZnO surface at higher temperatures. These contacts exhibit better thermal stability than either Pt or Au alone. The I-V characteristics from CrB2/Pt/Au contacts on ZnO are shown in Figure 6-23, as a function of annealing temperature. The contacts are ohmic as-deposited, and only in the case of the samples with Pt interlayer rectification is observed for anneal temperatures above 500C. After 600C anneals, the contacts showed rectifying behavior. The Schottky barrier height was obtained by fitting the forward characteristics to the relation for thermionic emission over a barrier. The ideality factor was also >2, suggesting transport mechanisms other than thermionic emission, such as recombination and surface leakage. The barrier height was ~0.39 eV, with saturation current density of 0.91 Acm-2. The work function of CrB2 is not known, but the value of the expected barrier height can be estimated from the e ndpoints of pure Cr and pure B, which are ~0.4

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96 and 0.35eV, respectively. Therefore, the experimental value for CrB2 determined on our samples seems physically reasonable. We would expect that the contacts properties of the as-deposited metal would be dominated by residual sputter-damage and once this is annealed out (at ~600C in this case), the intrinsic contact properties are revealed. The as-deposited contacts are always ohmic, perhaps because of the residual sputter damage that leaves the near-surface region more n-type. As this damage is annealed out, the contact reverts to rectifying behavior in the cases where dissociation of the ZnO (as measured by the absence of Zn out diffusion to the surface) is prevented by the Pt diffusion barrier. Annealing the contacts at temperatures greater than 700 C appears to produce high resistance phases, and the IV characteristics show back-to-back diode behavior. Figure 6-24 shows AES surface scans of CrB2/Pt/Au contacts on ZnO, either as-deposited (top) or after 600C annealing (bottom). Carbon, oxygen, and gold were detected on all of the as-deposited ZnO samples. Sulfur was detected on the as-deposited sample and zinc was detected on the annealed sample. The data are summarized in Table 6-7. The corresponding depth profiles are show n in Figure 6-25. The profiles obtained from the as deposited and 600C annealed CrB2/Pt/Au samples revealed no presence of oxygen in the CrB2 layer for both samples. The profile obtained from the annealed sample showed significant diffusion of layers. Oxygen and chromium were detected through the first ~500 of the gold layer. Platinum was not detected at the surface but the concentration did increase throughout the gold layer and it also diffused into the ZnO. Zinc was detected at a concentration of ~5 atomic percent throughout the gold layer. The apparent gold layer thickness was 1,500 . A distinct platinum layer is not observed in

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97 the profile. The platinum concentration peaks at the Au:CrB2 interface and again at the CrB2:ZnO interface. Summary Several metals, including Pt, W, W2B, W2B5, and CrB2, were examined for Schottky contacts to n-type, bulk ZnO. Surface cleaning using UV ozone was also investigated. For Pt contacts, the UV ozone treatment produces a change from ohmic behavior to rectifying behavior with Sc hottky barrier height of ~0.7 eV. W, W2B, W2B5, and CrB2 metals deposited by sputtering on ZnO produce non-rectifying contacts in the as-deposited state, but these convert to rectifying upon annealing at 500-600 C when a Pt diffusion barrier between the metal and the Au overlayer is used to prevent dissociation of the ZnO. The absence of the Pt diffusion barrier leads to ohmic behavior under all annealing conditions for both types of tungsten and boride contacts. The Schottky barrier heights for these contacts are in the range ~0.4 – 0.5 eV, which is comparable to that expected from the electron affinity. This barrier height is too low for use as a gate contact on transparent thin film transistors, but the metallization scheme may be promising for ohmic contacts in applications requiring improved thermal stability relative to more conventional metals.

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98 Table 6-1. Schottky contacts to ZnO and their respective barrier height and ideality factor from published works. Description Metal B (eV) Ref (a) n – type n-ZnO epi on Si E-beam Au (Schottky) E-beam Al (Ohmic) 0.59 1.5 149 n-ZnO epi on Al2O3 E-beam Ag (Schottky) E-beam Al (Ohmic) 0.69 – 0.83 1.33 150 n-ZnO epi on GaN/ Al2O3 Evaporated Au (Schottky) In (Ohmic) 0.37 – 0.66 1.8 – 3.2 151 n-ZnO epi on Al2O3 or n-ZnO bulk Evaporated Pd and Ag (Schottky) 0.59 – 0.68 1.4 – 1.95 152 n-ZnO epix on Al2O3 E-beam Ag or Au (Schottky) 0.84 1.5 160 n-ZnO bulk E-beam Au (Schottky) E-beam Ti (Ohmic) 0.60 – 0.71 1.03 – 1.17 153, 154, 158 n-ZnO bulk Vacuum deposition Au or Ag (Schottky) In (Ohmic) 0.64 – 0.69 1.6 – 1.8 118 n-ZnO bulk Pt (Schottky) Ti/Au (Ohmic) 0.89 – 0.93 1.15 95 n-ZnO bulk Pt (Schottky) Ti/Au (Ohmic) 0.79 1.51 155 n-ZnO bulk Evaporated Pd (Schottky) InGa or Ti (Ohmic) 0.75 1.4 156 n-ZnO bulk Pt ----157 n-ZnO bulk Vacuum deposition Ag or Au (Schottky) In (Ohmic) 0.5 – 0.6 1.3 – 1.6 161 n-ZnO bulk Resistively evaporated Au (Schottky) InGa (Ohmic) ----162 n-ZnO bulk Resistively evaporated Au or Pd (Schottky) 0.59 – 0.67 --163 (b) p type p-ZnMgO on glass E-beam Ti/Au or Pt/Au (Schottky) E-beam Ti/Au (Ohmic) 0.55 – 0.56 1.9 111 p-ZnO on glass Evaporated Au (Schottky) Mn (Ohmic) --2.7 – 3.5 159

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99 Table 6-2. Metal work function and ideal ba rrier heights for ZnO (electron affinity: 4.1 eV). Element Work Function (eV) Ideal Barrier Height (eV) B 4.45 0.35 Cr 4.5 0.4 Pt 5.64 1.54 Ti 4.33 0.23 W 4.55 0.45 Zr 4.05 -0.05 Table 6-3. Ideality factor, saturation current density and barrier height for Pt contacts measured at temperatures between 303473K on n-type ZnO, both before and after annealing at 300C.The contact diameter was 50 m in all cases. Temp (K) n Js (Acm-2) Barrier height (eV) (a) Pre-anneal 303 1.704 1.52X 10 -4 0.613 323 2.474 7.14X10-4 0.573 373 2.625 5.45X10-3 0.520 423 3.307 2.90X10-2 0.477 473 3.430 6.01X10-2 0.458 (b) Post-anneal 303 4.293 0.229 0.423 323 4.451 0.297 0.417 373 4.760 0.489 0.404 423 4.515 0.407 0.427 473 4.780 0.541 0.401

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100 Table 6-4. Carbon atomic concentrations on the ZnO surface before and after ozone treatment. Table 6-5. Summary of XPS data for O-related species before and after ozone cleaning. Peak assignments (eV): 530.4 ZnO, 531.5 OH and 533.2 water Table 6-6. Summary of electrical characteristics for W-based contacts after 700C anneals. No ozone 30 min ozone Barrier Height (eV) 0.45 0.49 Ideality Factor 4.5 3.2 Saturation Current (A-cm-2) 8.43 10-2 2.11 10-2 Treatment C 1s No sputter 29.5 1 min sputter 5.3 No ozone 2 min sputter 2.6 No sputter 5.8 1 min sputter 1.1 30 min ozone 2 min sputter 0.1 Peak Energy (eV) Intensity Treatment O1 O2 O1 O2 Ratio (O1/O2) No sputter 529.9 531.5 22365 18518 1.21 1 min sputter 529.8 530.4 27531 18751 1.47 No ozone 2 min sputter 529.8 530.3 31326 15837 1.98 No sputter 530.0 531.1 17745 17450 1.02 1 min sputter 529.8 530.4 18745 15227 1.23 30 min ozone 2 min sputter 529.9 530.4 21456 12781 1.68

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101 Table 6-7. Concentration of elements detected on the surface (in Atom%†) for W2B and W2B5 contacts. Sample ID C(1) Au(4) O(1) S(1) Zn(1) [0.076][0.038][0.212][0.652][0.278] W2B/Au as dep 55 44 1 nd nd W2B/Au 600C 55 40 3 nd 3 W2B/Pt/Au as dep 54 40 4 1 nd W2B/Pt/Au 600C 55 44 1 nd nd W2B5/Pt/Au as dep 57 40 1 2 nd W2B5/Pt/Au 600C 60 31 5 nd 4 † all concentrations are normalized to 100%. Table 6-8. Concentration of elements detected on the surfaces (in Atom%†) for CrB2 contacts. Sample ID C(1) O(1) S(1) Cr(2) Zn(1) Au(4) [0.076][0.212][0.652][0.265][0.278] [0.038] As deposited 51 1 2 nd nd 45 600 C annealed 22 37 1 10 7 23 † AES does not detect hydrogen and helium and all concentrations are normalized to 100%. Nd = element not detected. AES detection limits range from 0.1 – 1.0 atomic percent.

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102 (a) (b) Figure 6-1. Plan view optical micrographs of contacts before (a) and after (b) 300C annealing. The Pt Schottky contacts are the inner circles, while the Ti/Al/Pt/Au Ohmic contacts are the outer rings.

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103 0.00.51.01.52.0 0.0 3.0x10-46.0x10-49.0x10-4 Current (A)Bias (V) As-deposited 300 C, 1 min 30 C 30 C 200 C 200 C Figure 6-2. Forward I-V characteristics for as -deposited or 300C annealed Pt contacts on n-type ZnO, for two different measurement temperatures. -2.0-1.5-1.0-0.50.0 -2.0x10-4-1.6x10-4-1.2x10-4-8.0x10-5-4.0x10-50.0 Current (A)Bias (V) As-deposited 300 C, 1 min 30 C 30 C 200 C 200 C Figure 6-3. Reverse I-V characteristics for as-deposited or 300C annealed Pt contacts on n-type ZnO, for two different measurement temperatures.

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104 280320360400440480 0.36 0.40 0.44 0.48 0.52 0.56 0.60 0.64 Barrier Height (eV) Measurement Temperature (K) As-deposited 300 C, 1 min anneal Figure 6-4. Pt barrier height on n-type ZnO as a function of measurement temperature for as-deposited and 300C contacts.

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105 (a) (b) Figure 6-5. AFM scans of ZnO surfaces over 1 m2 area, either before (a) or after (b) UV ozone cleaning.

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106 (a) (b) Figure 6-6. AFM scans from the samples of Figure 6-5, showing the RMS roughness values.

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107 10008006004002000 -60000 -40000 -20000 0 20000 40000 60000 No Ozone Zn C O O Zn Zn N(E)Energy (eV) (a) 10008006004002000 -100000 -50000 0 50000 100000 150000 C O Zn Zn Zn 30 min OzoneN(E)Energy (eV) (b) Figure 6-7. XPS survey spectra of ZnO before (a) and after (b) UV ozone cleaning.

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108 525530535540 0 5000 10000 15000 20000 25000 30000 No OzoneIntensity (arb. unit)Energy (eV)(a)525530535540 0 5000 10000 15000 20000 25000 30000 30 min OzoneIntensity (arb. unit)Energy (eV)(b) Figure 6-8. XPS spectra from the region of O-bonded transitions, before (a) and after (b) UV ozone cleaning.

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109 -0.10-0.050.000.050.10 -1.0 -0.5 0.0 0.5 1.0 No ozoneCurrent (mA)Bias (V) Figure 6-9. I-V characteristic from Pt/A u contacts on ZnO without any ozone cleaning prior to metal deposition.

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110 0.00.10.20.30.40.5 0.000 0.005 0.010 0.015 0.020 B = 0.696 eV = 1.49 Js = 6.17 10-6 A-cm-2 30 min ozoneCurrent (mA)Bias (V)(a)-10-8-6-4-20 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 30 min ozoneCurrent (mA)Bias (V)(b) Figure 6-10. Forward (a) and reverse (b) I-V characteristics from Pt/Au contacts on ozone cleaned ZnO.

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111 -0.010-0.0050.0000.0050.010 -1.0 -0.5 0.0 0.5 1.0 Current (mA)Bias (V) As-sputtered 30 min ozone no ozone(a)-0.4-0.20.00.20.4 -0.1 0.0 0.1 0.2 0.3 Current (mA)Bias (V) 700 C, 1 min anneal 30 min ozone No ozone(b) Figure 6-11. I-V characteristics from W/P t/Au contacts on ZnO both as-deposited (a) and after annealing at 700C (b).

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112 02004006008001000 0 5000 10000 15000 20000 25000 30000 O Zn W Pt Au C 30 min ozone, as-depositedIntensity (arb. unit)Time (s)(a)02004006008001000 0 5000 10000 15000 20000 25000 Zn Zn W Au Pt C O 30 min ozone, 700 C annealIntensity (arb. unit)Time (s)(b) Figure 6-12. AES depth profiles of W/Pt/Au contacts both as-deposited (a) and after 700 C annealing (b).

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113 Figure 6-13. Secondary electron images of W2B/Au (a, c) or W2B/Pt/Au (b,d) contacts on ZnO, either as-deposited (a, b) or after 600 C annealing (c, d). (a) W2B/Au as deposited (d) W2B/Pt/Au 600C annealed (c) W2B/Au 600C annealed (b) W2B/Pt/Au as deposited

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114 -3-2-10123 -0.04 -0.02 0.00 0.02 0.04 Current (A)Voltage (V)W2B/Au (500/1000) As Deposited 500C 600C 700C 900C(a) -5.0-2.50.02.55.0 -0.04 -0.02 0.00 0.02 0.04 Current (A)Voltage (V)W2B/Pt/Au (500/200/1000) As Deposited 500 C 600 C 700 C 900 C(b) Figure 6-14. I-V characteristics from W2B/Au (a) or W2B/Pt/Au (b) contacts on ZnO, as a function of annealing temperature.

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115 450500550600650700750 0.37 0.38 0.39 0.40 0.41 0.42 0.43 W2B/Pt/Au Barrier Height Barrier Height (eV)Anneal Temperature (C)(a) 450500550600650700750 3.5 4.0 4.5 5.0 5.5 6.0 W2B/Pt/Au Ideality Factor Ideality FactorAnneal Temperature (C)(b) 450500550600650700750 0.0 0.5 1.0 1.5 2.0 W2B/Pt/Au Saturation Current Density Saturation Current Density (Acm-2)Anneal Temperature (C)(c) Figure 6-15. Barrier height (a), apparent ideality factor (b) and saturation current density (c) from W2B/Pt/Au Schottky contacts on n-type ZnO, as a function of anneal temperature.

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116 Figure 6-16. AES surface scans of W2B/Au (a, c) or W2B/Pt/Au (b, d) contacts on ZnO, either as-deposited (a, b) or after 600 C annealing (c, d). 500 1000 1500 2000 -6000 -5000 -4000 -3000 -2000 -1000 0 1000 2000 3000 4000 p Kinetic Energy (eV) C O A u A u A u A u A u A u A u Atomic % C1 55.2 Au4 43.7 O1 1.1W2B as deposited Surface survey W2B/Au as deposited 500 1000 1500 2000 Kinetic Energy (eV) 0 2000 4000 -2000 -4000 -6000 500 1000 1500 2000 -6000 -5000 -4000 -3000 -2000 -1000 0 1000 2000 3000 4000 p Kinetic Energy (eV) C O A u A u A u A u A u A u A u S S Atomic % C1 54.4 Au4 39.9 O1 4.3 S1 1.5W2B/Pt/Au As deposited Surface survey W2B/Pt/Au as-deposited Kinetic Energy (eV) 500 1000 1500 2000 Kinetic Energy (eV) 0 2000 4000 -2000 -4000 -6000 Countspersecond 500 1000 1500 2000 -8000 -6000 -4000 -2000 0 2000 4000 p Kinetic Energy (eV) C O Z n A u A u A u Z n Atomic % C1 54.6 Au4 40.3 Zn1 2.6 O1 2.5 A u A u A u A uW2B/Au 600C Surface survey W2B/Au 600C 500 1000 1500 2000 Kinetic Energy (eV) 0 2000 4000 -2000 -4000 -6000 -8000 500 1000 1500 2000 -6000 -5000 -4000 -3000 -2000 -1000 0 1000 2000 3000 4000 p Kinetic Energy (eV) C O A u A u A u A u A u A u A u A u Atomic % C1 55.4 Au4 43.6 O1 1.0W2B/Pt/Au 600C anneal Surface survey W2B/Pt/Au 600C 500 1000 1500 2000 Kinetic Energy (eV) 0 2000 4000 -2000 -4000 -6000 Countspersecond O C C C C O O O Zn Au Au Au Au Au Au Au Au Au Zn A Au Au Au Au Au Au Au Au Au Au Countspersecond Countspersecond Au Au Au S Atomic % C1 55.2 Au4 43.7 O 1 1 1 Atomic % C1 54.4 Au4 39.9 O1 4.3 S11.5 Atomic % C1 54.6 Au4 40.3 Zn1 2.6 O1 2.5 Atomic % C1 55.4 Au4 43.6 O 11 .0 (a) (b) (c) (d)

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117 Figure 6-17. AES depth profiles of W2B/Au (a, c) or W2B/Pt/Au (b,d) contacts on ZnO, either as-deposited (a, b) or after 600 C annealing (c, d). 05001000150020002500 0 20 40 60 80 100 W2B/Au as deposited O Zn B W C Au Atomic Concentration (%)Sputter Depth ()05001000150020002500 0 20 40 60 80 100 W2B/Pt/Au as deposited Zn O B W Pt Au C Atomic Concentration (%)Sputter Depth ()05001000150020002500 0 20 40 60 80 100 W2B/Au 600C C O Zn B W Au Atomic Concentration (%)Sputter Depth ()050010001500200025003000 0 20 40 60 80 100 W2B/Pt/Au 600C Zn O B W Pt Au C Atomic Concentration (%)Sputter Depth ()(a) (b) (c) (d)

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118 Figure 6-18. Secondary electron images of W2B5/Pt/Au contacts on ZnO, either asdeposited (a) or after 600 C annealing (b). (a) W2B5/Pt/Au as-deposited (b) W2B5/Pt/Au 600C annealed

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119 -3-2-10123 -0.04 -0.02 0.00 0.02 0.04 Current (A)Voltage (V)W2B5/Pt/Au (500/200/1000) As Deposited 500 C 600 C 700 C Figure 6-19. I-V characteristics from W2B5/Pt/Au contacts on ZnO, as a function of annealing temperature.

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120 Figure 6-20. AES surface scans of W2B5/Pt/Au contacts on ZnO, either as-deposited (a) or after 600 C annealing (b). 500 1000 1500 2000 -6000 -5000 -4000 -3000 -2000 -1000 0 1000 2000 3000 4000 042091401.spe Kinetic Energy (eV) c / s C O S A u A u A u A u A u A u A u Atomic % C1 57.3 Au4 40.2 S1 1.6 O1 0.9W2B5/Pt/Au As deposited Surface survey 500 1000 1500 2000 -1200 -1000 -800 -600 -400 -200 0 200 400 600 800 042091411.spe Kinetic Energy (eV) c / s C O Z n A u A u A u A u A u A u Atomic % C1 60.2 Au4 31.1 O1 5.1 Zn1 3.5W2B5/Pt/Au 600C annealed Surface survey 500 1000 1500 2000 500 1000 1500 2000 Kinetic Energy (eV) Kinetic Energy (eV) (a) W2B5/Pt/Au as deposited (b) W2B5/Pt/Au 600C Atomic % C1 57.3 Au4 40.2 S1 1.6 O10.9 Atomic % C1 60.2 Au4 31.1 O1 5.1 Zn1 3.5 0 2000 4000 -2000 -4000 -6000 Counts p er secon d -400 400 800 -800 -1200 0 Counts p er secon d Au Au Au Au Au Au C O C O S Au Au Au Au Au

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121 050010001500200025003000 0 20 40 60 80 100 (a) W2B5/Pt/Au as deposited O Zn W B Pt Au C Atomic Concentration (%)Sputter Depth ()050010001500200025003000 0 20 40 60 80 100 (b) W2B5/Pt/Au 600C W B Pt O Zn Au C Atomic Concentration (%)Sputter Depth () Figure 6-21. AES depth profiles of W2B5/Pt/Au contacts on ZnO, either as-deposited (a) or after 600 C annealing (b).

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122 (a) (b) (c) (d) Figure 6-22. Optical microscopy photos of CrB2 /Pt/Au on ZnO either as-deposited (a) or after annealing at 500 (b), 600 (c) or 700C (d).

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123 -3-2-10123 -0.04 -0.02 0.00 0.02 0.04 Current (A)Voltage (V) As deposited 500 C 600 C 700 C Figure 6-23. I-V characteristics from CrB2Pt/Au contacts on ZnO, as a function of annealing temperature.

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124 Figure 6-24. AES surface scans of Cr2B/Pt/Au contacts on ZnO, either as-deposited (a) or after 600 C annealing (b). 500 1000 1500 2000 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 x 104 042176211.spe c / s C O S Z n A u A u A u C r C r C r Atomic % O1 37.3 Au4 23.3 C1 22.2 Cr2 9.6 Zn1 6.6 S1 1.2Annealed sample Surface survey 500 1000 1500 2000 500 1000 1500 2000 -5000 -4000 -3000 -2000 -1000 0 1000 2000 3000 042176101.spe C O A u A u A u A u A u A u A u A u S S A u Atomic % C1 51.4 Au4 45.3 S1 2.0 O1 1.3As recieved Surface survey 500 1000 1500 2000 Counts per second Counts per second 3000 2000 1000 0 -4000 -1000 -2000 -3000 -5000 10000 -10000 -2000 -4000 -6000 -8000 8000 6000 4000 2000 0 (a) As-received (b) 600 C annealed Atomic % C 51.4 Au 45.3 S 2.0 O 1.3 Au Au O C S Au Au S C O Au Au Au C Zn Atomic % O 37.3 Cr 9.6 Au 23.3 Zn 6.6 C 22.2 S 1.8

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125 0500100015002000 0 20 40 60 80 100 Zn O Cr B Pt Au Atomic Concentration (%)Sputter Depth ()(a)050010001500200025003000 0 20 40 60 80 100 Zn Pt B Cr O Au Atomic Concentration (%)Sputter Depth ()(b) Figure 6-25. AES depth profiles of Cr2B/Pt/Au contacts on ZnO, either as-deposited (a) or after 600 C annealing (b).

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126 CHAPTER 7 PN JUNCTION DIODE Introduction The more robust p-type ZnO material from recent reports [18, 111, 139], aided by theoretical work [170-172], are indicative that minority carrier devices such as lightemitting diodes, laser diodes and transparent p-n junctions are likely to achieved in the future. Alivov et al. [173, 174] have recently made progress in that direction by demonstrating electroluminescence from p-AlGaN/n-ZnO junctions. Previous works have shown that an effective and reproducible route to achieving ptype material in the ZnO system is to lower the n-type background by adding Mg to increase the bandgap and then to dope the ZnMgO with P at high concentrations, followed by annealing to obtain type conversi on to p-type [46, 111, 175]. The resulting material has been demonstrated to be p-type from the capacitance-voltage properties of metal/insulator/P-doped (Zn,Mg)O diode structures which exhibit a polarity consistent with the P-doped (Zn,Mg)O layer being p-type [143]. In addition, thin-film junctions comprising n-type ZnO and P-doped (Zn,Mg)O display asymmetric current-voltage (I-V) characteristics that are consistent with the formation of a p-n junction at the interface. Simple Schottky contacts formed on the ZnMgO have also confirmed the p-type conduction with I-V characteristics. In this section, the temperature dependence of I-V characteristics of Zn0.9Mg0.1O/ZnO p-n junctions grown by pulsed la ser deposition on bulk, single-crystal

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127 ZnO substrates is reported. The p-n junctions exhibit negative temperature coefficients for reverse breakdown voltage. Experimental The starting substrates were (0001) undoped reseach grade bulk, single-crystal ZnO crystals from Cermet, Inc. The samples were epiready, one-side-Zn-face-polished by the manufacturer. The room temperature electron concentration and mobility established by van der Pauw measurements were ~1017 cm-3 and 190 cm2/V.s, respectively. Pulsed laser deposition was used for film growth. Phosphorus-doped (Zn0.9Mg0.1)O targets were fabricated using high-purity ZnO (99.9995%) and MgO (99.998%), with P2O5 (99.998%) serving as the doping agent. Use of the (Zn,Mg)O alloy reduces the residual n-type conductivity due to shallow defect donor states. The addition of Mg moves the conduction band edge up in energy and potentially away from the intrinsic shallow donor state, thus increasing the activation energy of the defect donors. The ablation targets were fabricated with a phosphorus doping level of 2 at. %. A KrF excimer laser was used as the ablation source. A laser repetition rate of 1 Hz was used, with a target to substrate distance of 4 cm and a laser pulse energy density of 1-3 J/cm2. The ZnO growth chamber exhibits a base pressure of 10-6 Torr. Film growth was performed at 400 C in an oxygen pressure of 20 mTorr. Previous work has shown that as-deposited phosphorus doped ZnO films are heavily n-type due to a compensating donor defect .A moderate temperature anneal suppresses the n-type behavior considerably. The samples were annealed in situ at 600 C in a 100 Torr O2 ambient for 60 min. It is necessary to grown a 0.8 m thick buffer of undoped (n-type) ZnO prior to growing the ZnMgO in order to achieve acceptable p-n junction quality. Without this buffer, the junctions displayed very poor rectification. The buffer was followed by 1.4 m of ZnMgO:P0.02. The p-side of the

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128 structures was contacted with Pt/Au (200/ 800 ) deposited by e-beam evaporation and patterned by lift-off with contact diameters ranging from 50 m to 375 m. The bulk ZnO substrate was given a Ti/Al/Pt/Au (200/400/200/800 ) full-backside contact annealed at 200 C for 1 min in N2. A schematic of the final structure is shown in Figure 7-1. The I-V characteristics were measured at temperatures up to 200C on a heated probe station using an Agilent 4156 parameter analyzer. Results and Discussion I-V characteristics at 30 C from the p-n junction structures are shown in Figure 7-2. The devices show clear rectification a nd polarity consistent with the ZnMgO being p-type. Table 7-1 shows the extracted values of reverse breakdown voltage (VRB), saturation current density (JS), forward voltage drop (VF) and on-state resistance (RON). The forward voltage drop of p-n junction rectifiers can be written as m i FV n n n e kT V 2ln (7-1) where k the Boltzmann's constant, T the absolute temperature, e the electronic charge, nand n+ the electron concentrations in the two end regions of the p-n junction (the p+-n and n+-n regions) and Vm is the voltage drop across the buffer region. Thus it is important to have a high quality buffer in order to minimize the turn-on voltage. At low current densities (< 1 A cm-2), the heterojunctions showed thermally activated behavior, nkT eV kT E Jaexp exp (7-2) where Ea is the activation energy and n the ideality factor. The rectifiers showed Ea ~ 1.7 eV but unphysically large ideality factors, which is consistent with several transport

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129 mechanisms being present, including defect–assisted tunneling and conventional carrier recombination in the space-charge region via a deep level near midgap in the ZnMgO [143]. Figure 7-3 shows the temperature dependence of reverse I-V characteristics for the Pt/Au contacted junctions. The reverse breakdown voltage decreases with temperature. The variation of VRB with temperature is plotted in Figure 7-4. The data can be represented by a relation of the form: ) ( 10 0T T V VRB RB (7-3) where the breakdown voltage showed a slightly negative temperature coefficient of 0.1-0.2 VK–1. It is desirable to have a positive temperature coefficient for breakdown if high temperature applications are envi saged, although both SiC and GaN devices typically showed negative values in the early stages of their development due to the presence of defects and non-optimized growth and processing. There is still room for optimization in both the growth and processing conditions for the p-n junctions. Summary ZnO-based p-n junctions using the ZnMgO/ ZnO heterostructure system have been demonstrated. The use of an n-type ZnO buffer on the ZnO substrate is critical in achieving acceptable rectification in the junctions. This is an important step in realizing minority carrier devices in the ZnO system.

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130 Bulk ZnO ZnOMgO:P0.02 (~1.4 m) Pt/Au (200/800 ) Ti/Al/Pt/Au (200/400/200/800 ) Buffer n-ZnO (~0.8 m) Table 7-1. Characteristics of p-ZnMgO/n-ZnO junctions measured on 50 um diode. Pt/Au VRB (eV) -9.0 Js (Acm-2) 4.6 10-9 Vf 4.0 Ron (m cm-2) 14.5 Figure 7-1. Schematic of ZnMgO/ZnO p-n junction structure.

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131 -10.0-7.5-5.0-2.50.02.55.0 -0.04 -0.02 0.00 0.02 0.04 Pt/Au 50 m Diode Current (A)Bias (V) Figure 7-2. I-V characteristics at 30C of ZnMgO/ZnO p-n junctions using Pt/Au as the ohmic contact to p-type ZnMgO.

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132 -8-6-4-20 -0.04 -0.03 -0.02 -0.01 0.00 Current (A)Bias (V)Pt/Au 210 m diode Measurement Temperatures 30 C 50 C 100 C 150 C 200 C Figure 7-3. Reverse I-V characteristics of ZnMgO/ZnO p-n junctions using Pt/Au as the ohmic contact on the p-type ZnMgO, as a function of the measurement temperature.

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133 050100150200250 1 2 3 4 5 6 7 Pt/Au Reverse Breakdown Voltage (V)Measurement Temperature (C) Figure 7-4. Measurement temperature dependence of the reverse breakdown voltage in ZnMgO/ZnO p-n junctions using Pt/Au as the ohmic contact on p-type ZnMgO.

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134 CHAPTER 8 CONCLUSIONS In this work, several processes in ZnO device fabrication were investigated. The topics included plasma etching, study of hydroge n in ZnO, and contact metallization. In addition, ZnMgO/ZnO heterostructure p-n j unction diode with good rectifying behavior was demonstrated. The etch mechanism for ZnO in plasma chemistries of CH4/H2/Ar and Cl2/Ar have been studied. For both chemistries the etch rate increases with ion energy as predicted from an ion-assisted chemical sputtering pr ocess. The etch rate using methane-based chemistry was faster than chlorine-based chemistry. The difference in their etch rates can be explained by the volatility of their respective probable etch by-products. High fidelity pattern transfer with smooth surface morphology and practical etch rates was attained by optimizing the etch parameters of the methane-based chemistry such that the Znand O-atoms were removed at equal rates to maintain stoichiometry. Photoluminescence revealed degradation to the optical properties by reducing the peak intensity by a factor of three even at low rf chuck powers. Dry et ching using inductively plasma etching with reasonable etch rates and anisotropic pattern transfer was demonstrated. When fabricating actual devices, it is necessary to reduce ion energy and flux to minimize damages. Thermal stability and diffusion characteristics of hydrogen in ZnO were examined. Hydrogen was introduced by two methods: direct ion implantation and plasma exposure. Ion implanted hydrogen concentration peaked at about 1 m deep with doses of 1015

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135 to1016 cm-3. The hydrogen completely evolved out, as determined by the lower detection limits of SIMS analysis, by annealing up to 700 C. RBS/C indicated minimal crystal damages; however, the photoluminescence intensity severely degraded by the ion induced damages. The optical properties recovered slightly but not completely upon annealing up to 700 C, and point defect recombination centers continued to dominate. Hydrogen introduced by plasma exposure reached up to ~30 m deep. Large diffusion depth suggests that hydrogen diffuses as an interstitial, with little trapping by the lattice elements or by defects or impurities. Complete removal of hydrogen incorporated by plasma exposure occurred by annealing up to 500 C. From C-V analysis, the carrier concentration increased after plasma exposure and decrease to below the as-grown condition after annealing at 600 C, suggesting that even though hydrogen is a shallow donor, the n-type conductivity arises from multiple impurities sources. The thermal stability of hydrogen in ZnO incorporation by both ion implantation and plasma exposure is significantly lower than GaN, in which annealing temperature of greater than 900 C is needed to remove the hydrogen under similar conditions. One explanation is that the hydrogen clusters do not formed readily in ZnO as they do commonly in GaN. Ion implanted hydrogen is more thermally stable than hydrogen introduced by plasma exposure due to the hydrogen decorating the implant-induced defects. E-beam evaporated Ti/Al/Pt/Au metallization was considered for ohmic contacts to bulk and thin-film ZnO have been investigated. Specific contact resistance of 10-4 cm2 for the nominally undoped (n ~ 1017 cm-3) bulk ZnO was obtained after annealing at 250 C, and specific contact resistances were in the range 10-7-10-8 cm2 for the heavily doped thin film, even in the as-deposited states AES revealed Ti-O interfacial reactions

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136 and intermixing between Al and Pt layers. The low specific contact resistance in the high carrier concentration thin film sample may be explained by the tunneling mechanism. AES revealed Ti-O interfacial reactions and intermixing between Al and Pt layers. These contacts show significant changes in morphology even for low temperature (200 C) anneals, suggesting that more thermally stable contacts schemes should be investigated. Both Au and Au/Ni/Au are found to provide low specific contact resistance on lightly doped p-ZnMgO after annealing at 600C. In both cases, the as-deposited contacts are rectifying and the transition to ohmic behavior is associated with out-diffusion of Zn from the ZnMgO. A minimum specific contact resistance of 7.6 10-6 cm2 was obtained with Au/Ni/Au, which is about a factor of 3 lower than for pure Au contacts annealed at the same temperature. Several metals and UV ozone treatments were examined for Schottky contacts to ntype ZnO. Pt contacts on n-type PLD ZnO th in film show rectifying behavior with a barrier height at 25oC of 0.61eV.This barrier height is reduced significantly (to 0.42 eV) after annealing at 300C. The main effect of the UV ozone treatment is found to be desorption of carbon contaminants from the ZnO surface, producing transition from ohmic behavior to rectifying behavior in Pt contacts to bulk ZnO with barrier height of ~0.7 eV. Other metals, W, W2B, W2B5, and CrB2 were also studied. Without a Pt diffusion barrier layer, the contacts produced non-rectifying current-voltage behavior in all cases. These contacts also showed non-rectifying behavior as-deposited condition, possibly due to the increase n-type conductivity near the surface by the residual sputter damages. The contacts displayed rectifying beha vior with barrier heights of ~0.4 –0.5 eV after annealing at >500 C to remove residual sputter-induced damages. The barrier

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137 heights are comparable to that expected from the electron affinity and are too low for use as a gate contact on transparent thin film transistors. However, the metallization scheme may be promising for ohmic contacts in applications requiring improved thermal stability relative to more conventional metals.

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138 APPENDIX A OHMIC CONTACT TABLES The following table summarizes the details and results of ohmic contacts on thin film and bulk ZnO other groups have done, including ones that were published after the completion of work presented in this dissertation.

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139Table A-1. Ohmic contacts to ZnO. Ref Material Growth technique Metals Surface treatment and deposition Electrical properties 95 Bulk ZnO (0001) n ~ 2 1017 cm-3 e ~ 100 cm2/Vs MAHK Co. Ti/Au Electron beam evaporation Nonalloyed specific contact resistance ~ 5 10-5 cm2 Anneal Temp ( C) Specific contact resistance ( cm2) As-deposited 7 10-1 96 Bulk ZnO (0001) Hydrothermally grown In (50 nm) KrF excimer laser irradiation 300 7.3 10-1 Specific contact resistance ( cm2) Conventional wet clean 7.3 10-4 97 n-type ZnO:Al on (0001) Al2O3 n ~ 3 1018 cm-3 e ~ 60 cm2/Vs Rf sputtering Ti/Au (50/100) nm Conventional wet clean or BCl3 plasma exposure BCl3 plasma exposure 1.5 10-5 Ga Doses (cm-2) Specific contact resistance ( cm2) None 4.1 10-3 1 1017 cm-2 3.1 10-4 3 1017 cm-2 3.3 10-4 98, 99 n-type ZnO:Al on (0001) Al2O3 Pulsed laser deposition Pt direct write contacts Ga-FIB surface modification 3 1016 cm-2 3.7 10-4 Anneal Temp ( C) Specific contact resistance ( cm2) As-deposited 2 10-2 300 2 10-4 100, 101 n-type ZnO:Al on (0001) Al2O3 thickness ~ 1 m n ~ 2 1017 cm-3 e ~ 22.5 cm2/Vs Rf magnetron sputtering Ti/Au (30/50) nm HCl:H2O (2:1) Electron beam evaporation 500 1 10-3

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140Table A-1. Continued. Ref Material Growth technique Metals Surface treatment and deposition Electrical properties Surface treatment Specific contact resistance ( cm2) As-grown 7.3 10-3 Ar-plasma 5.0 10-4 102 n-type ZnO:Al on (0001) Al2O3 thickness ~ 0.8 m n ~ 7 1017 cm-3 Rf magnetron sputtering Ti/Au (30/50) nm H2 or Ar plasma exposure Electron beam evaporation H2-plasma 4.3 10-5 Anneal Temp ( C) Specific contact resistance ( cm2) As-deposited 2.1 10-4 300 6.7 10-6 500 2.2 10-6 103 n-type ZnO:Al on (0001) Al2O3 thickness ~ 1 m n ~ 2 1018 cm-3 e ~ 17.5 cm2/Vs Rf magnetron sputtering Re/Ti/Au (2/20/60) nm Electron beam evaporation 700 1.7 10-7 Anneal Temp ( C) Specific contact resistance ( cm2) As-deposited 7.3 10-6 200 1.2 10-6 300 9.0 10-7 400 4.5 10-6 104 n-type ZnO:Al on (0001) Al2O3 n ~ 1.7 1018 cm-3 e ~ 59.7 cm2/Vs Metal-organic chemical-vapor deposition Ti/Al (300/3000) Solvent ultrasonication Electron beam evaporation 500 3.6 10-4

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141Table A-1. Continued. Ref Material Growth technique Metals Surface treatment and deposition Electrical properties Anneal Temp ( C) Specific contact resistance ( cm2) As-deposited 1.25 10-5 300 2 10-6 105, 106 n-type ZnO:Al on (0001) Al2O3 n ~ 2 1018 cm-3 e ~ 60 cm2/Vs Rf magnetron sputtering Al/Pt (20/50) nm Solvent ultrasonication and BOE etch Electron beam evaporation 600 8 10-5 Anneal Temp ( C) Specific contact resistance ( cm2) As-deposited 2.1 10-5 107 n-type ZnO:Al on (0001) Al2O3 thickness ~ 1.5 m n ~ 3 1018 cm-3 e ~ 60 cm2/Vs Rf magnetron sputtering Ru (100 nm) HCl:H2O (2:1) Electron beam evaporation 700 3.2 10-5 108 n-type ZnO:Al on (0001) Al2O3 thickness ~ 1 m n ~ 2 1018 cm-3 e ~ 60 cm2/Vs Rf magnetron sputtering Al (60 nm) Solvent ultrasonication and BOE etch Electron beam evaporation Nonalloyed specific contact resistance ~ 8 10-4 cm2 114 n-type ZnO on R-Al2O3 thickness: 200500nm n~ 1.6 1017 cm-3 MOCVD Al/Au (100/100) nm Electron beam evaporation 2.5 10-5 cm2

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142Table A-1. Continued. Ref Material Growth technique Metals Surface treatment and deposition Electrical properties Anneal Temp ( C) Specific contact resistance ( cm2) As-deposited 3.2 10-4 300 5.4 10-6 115 n-type ZnO on R-Al2O3 thickness: 300nm n~ 1.6 1017 cm-3 MOCVD Ta/Au (30/20) nm Electron beam evaporation 500 3.3 10-5 116 n-type ZnO on (0001) Al2O3 n ~ 1.7 1018 cm-3 e ~ 59.7 cm2/Vs MOCVD Ti/Al (300/3000) Electron beam evaporation Specific contact resistance ~ 9 10-7 cm2 after 300 C anneal Anneal Temp ( C) Specific contact resistance ( cm2) As-deposited 7.67 10-3 400 6.06 10-3 109 p-type ZnO:P on (0001) Al2O3 p ~ 1.0 0.2 1018 cm-3 h~2.0 cm2/Vs Rf magnetron sputtering Ni/Au (30/80) nm Electron beam evaporation 600 1.72 10-4 Anneal Temp ( C) Specific contact resistance ( cm2) As-deposited non-ohmic 300 8.8 10-4 400 8.1 10-4 500 7.7 10-4 110 p-type ZnO:As on (0001) Al2O3 p ~ 5 1016 cm-3 h~0.49 cm2/Vs Hybrid beam deposition Pt/ITO (2/200) nm Electron beam evaporation 600 3.5 10-3

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143Table A-1. Continued. Ref Material Growth technique Metals Surface treatment and deposition Electrical properties 111 p-type Zn0.9Mg0.1O:P0.02 on glass Thickness ~ 600 nm p ~ 1 1016 cm-3 Pulsed laser deposition Ti/Au or Ni/Au (200/800) Electron-beam evaporation Both linear I-V Ti/Au 600 C anneal: 3 10-3 cm2 Ni/Au annealed up to 500 C:10-2 cm2 112 p-type Zn0.9Mg0.1O:P0.02 on glass Thickness ~ 600 nm p ~ 2.7 1016 cm-3 h~6 cm2/Vs resistivity ~ 38 cm Pulsed laser deposition Ni/Au (200/800) Electron-beam evaporation 300 C anneal: 4 10-5 cm2 measured at 473K 113 p-type ZnO:As on ZnO/6H n-SiC Thickness ~ 0.5 m p ~ 4 1017 cm-3 Pulsed laser deposition In/Au, Ti/Au, or Ni/Au Electron beam evaporation In/Au and Ti/Au: back to back Schottky barrier Ni/Au showed ohmic behavior with annealing at 600 C

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144 APPENDIX B SCHOTTKY CONTACT TABLES The detailed descriptions and results of Schottky contacts done by other groups are compiled in the following tables. The information on Schottky contacts to thin film ZnO and bulk ZnO are shown in Table B-1 and Tabl e B-2, respectively. The tables include works that were published after the completion of work presented in this dissertation.

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145Table B-1. Schottky contacts to thin film ZnO Ref Material Growth technique Metals Surface treatment and deposition Electrical properties Integrated Schottky diode configuration (with Si3N4 guard ring): B (293K) = 0.586 eV = 1.5 Soft breakdown at V Leakage current = -0.014 A to 1V reverse bias 149 n-type (0002) ZnO on Al / Si (100) DC reactive magnetron sputtering Schottky: Au Ohmic: Al Electron-beam evaporation Schottky diode: B (293K) = Not given = 1.9 Soft breakdown at V Leakage current = -0.056 A to 1V reverse bias I-V measurements: B (295K) = 0.83 eV = 1.33 I0 = 4.62 10-12 A Schottky contact area = 1.77 10-4 cm-3 I-V-T measurements (T=265 to 340K): B (295K) = 0.69 eV A*= 0.15 Acm-2K-2 BF = 0.89eV 150 n-type ZnO (11 20) on Al2O3 (01 12, Rpane) Thickness ~ 5000-6000 n ~ 1 1017 cm-3 MOCVD Schottky: Ag (200 nm) Ohmic: Al (200 nm) Electron-beam evaporation C-V Measurements: BF (100 kHz) = 0.83 eV BF (1MHz) = 0.92 eV

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146Table B-1. Continued. Ref Material Growth technique Metals Surface treatment and deposition Electrical properties Tg (C) Polarity n (cm-3) B (eV) 400 O 41018 ----400 Zn 81019 0.55 3.2 300 O 71019 0.37 3.5 151 n-type ZnO:N on Gapolar GaN / (0001) Al2O3 Thickness ~ 450 600 nm Plasma-assisted molecular-beam epitaxy Schottky: Au Ohmic: In (on GaN layer) Organic cleaning Evaporated in vacuum chamber 300 Zn 21020 0.66 1.8 Surface Prep a b c PLD (0001) B (meV) 630 680 600 1.7 1.4 1.95 Metal Ag Pd PLD (11-20) B (meV) 590 680 if B (meV) 760 790 152 n-type (0001) ZnO on sapphire or (11 20) ZnO on sapphire Pulsed laser deposition Pd, Ag (a) Acetone ultrasonic (b) Acetone ultrasonic, toluene ultrasonic, DMSO ultrasonic (c) N2O plasma Evaporated 1.4 1.4

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147Table B-1. Continued. Ref Material Growth technique Metals Surface treatment and deposition Electrical properties 160 n-type (11 20) ZnO on sapphire Thickness ~ 500 nm n ~ 1016 cm-3 Metalorganic chemical vapor deposition Ag-ZnOAl/Au (Al or Ag: 2000, Au: 500) O-plasma clean Electron beam evaporation Ag B = 0.84 eV = 1.5 Js = 2.410-8 (A/cm2) Ti/Au B (293K) = 0.56 eV = 1.88 Reverse bias saturation current = 8.9 10-7A 111 p-type Zn0.9Mg0.1O:P0.02 on glass Thickness ~ 600 nm p ~ 11016 cm-3 Pulsed laser deposition Ohmic: Ti/Au (200/800) Schottky: Ti/Au or Pt/Au (200/800) Electron-beam evaporation Pt/Au B (293K) = 0.55 eV = 1.92 Reverse bias saturation current = 1.2310-6A 159 p-type ZnO on glass substrate thickness ~ 0.5-1 m Rf sputtering Ohmic: Mn Schottky: Au (10-20 nm) Evaporated varied from 2.7 to 3.5 (max 7) due to severe recombination processes

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148Table B-2. Schottky contacts on bulk n-type ZnO Ref Material Growth technique Metals Surface treatment and deposition Electrical properties Method a b c B (meV) 740 700 600 2.0 1.75 1.4 Metal Ag Pd Au Ni B (meV) 560 730 560 620 if B (meV) 760 840 ----1.5 1.75 2.0 1.7 152 Bulk ZnO, (0001) and (000 1) Eagle-Picher, Inc. by seeded chemical vapor transport Pd, Ag, Au, Ni Surface preparation: (a) acetone ultrasonic (b) acetone ultrasonic bath, toluene ultrasonic, DMSO ultrasonic (c) HCl etch Evaporated 156 Bulk ZnO (000 1) SPC Goodwill by hydrothermal technique Ohmic: Eutectic InGa or Ti film (500 ) Schottky: Pd (thickness ~ 500 diameters 0.8, 0.5, 0.3 mm) Electron beam evaporation B = 0.75 eV = 1.4

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149Table B-2. Continued. Ref Material Growth technique Metals Surface treatment and feposition Electrical properties In situ plasma cleaned 000 1 0001 B (eV) 0.60 0.71 1.03 1.17 Js (Acm-2) 210-4 410-6 Leakage current density (Acm-2) 9110-9 110-4 Reverse breakdown (V) 7.0 8.5 Annealed (0001) 80 C 150 C B (eV) 0.82 0.79 1.12 1.09 Js (Acm-2) 9.05 4.43 Leakage current density (Acm-2) 210-3 2010-3 153, 154, 158 Bulk ZnO, (000 1) and (0001) n ~ 11017 cm-3 Eagle-Picher, Inc. by seeded chemical vapor transport Ohmic: Ti (400 nm) Schottky: Au (thickness ~ 150 nm, diameter ~ 100 m) UHV remote plasma 20%O2, 80% He Electron-beam evaporation Reverse breakdown (V) 6 7 157 Bulk ZnO (000 1) and (0001) Not given Pt Resistivelyheated evaporation HREEL: No significant electronic interactions between Pt and ZnO (000 1). Electron transfer from metal oxide to metal results in Schottky barrier formation between Pt and ZnO (0001)

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150Table B-2. Continued. Ref Material Growth technique Metals Surface treatment and deposition Electrical properties Metal Au Ag Clean a b c b c Ir 10-8 (A) 8.6 2.1 1.6 2.5 2 n 1.8 1.6 1.8 1.6 1.8 Js 10-7 (Acm-2) 8.4 80 48 100 60 Ea (eV) 0.35 0.4 0.29 0.3 0.35 118 Bulk ZnO (0001) n ~ 91016 cm-3 e ~ 190 cm2/Vs Eagle-Picher, Inc. by seeded chemical vapor transport Ohmic: In Schotkky: Ag (1 mm diameter), Au (0.750.75 mm2) Surface cleaning: Organic solvents HCl HNO3 Vacuum deposition VC (V) 0.65 0.64 0.65 0.69 0.68 95 Bulk ZnO (0001) n ~ 21017 cm-3 e ~ 100 cm2/Vs MAHK Co. Ohmic: Ti/Au Schottky: Pt (thickness ~ 50nm, diameter ~200 m) Surface cleaning: Organic solvents and boiling hydrogen peroxide B (293K) = 0.89 eV by I-V B (293K) = 0.93 eV by C-V = 1.15 Leakage current ~6.510-8 A at -5V 155 Bulk ZnO (000 1) n ~ 51015 cm-3 e ~ 100 cm2/Vs MAHK Co. Ohmic: Ti/Au Schottky: Pt (thickness ~ 50nm, diameter ~400 m) Surface cleaning: Organic solvents and boiling (NH4)2Sx B (293K) = 0.79 eV = 1.51 Leakage current 3.7510-10 A at -5V

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151Table B-2. Continued. Ref Material Growth technique Metals Surface treatment and deposition Electrical properties 161 Bulk ZnO Undoped, Crdoped, Mndoped Flux method Ohmic: In on (0001) surface Schottky: Ag or Au (0.8 or 1 mm) on (000 1) face Vacuum evaporation on Schottky electrode and ionplating on ohmic electrode Barrier height not changed by Cr or Mn dopants B (300K) = 0.5-0.6 eV = 1.3-1.6 162 Bulk ZnO (000 1) Vapor phase technique Ohmic: InGa on (0001) face Schottky: Au (0.7 mm diameter, 200 nm thick) (000 1) face acetone ultrasonic bath, toluene ultrasonic, DMSO ultrasonic Resistively evaporated Reverse current = 10-9A at 1 V bias Method (eV) Au Pd Photoresponse 0.645 0.040.59 0.04 I-V 0.66 0.03 0.60 0.04 Activation energy 0.65 0.04 0.59 0.04 163 Bulk ZnO n ~ 11016 to 21017 cm-3 e ~ 200 cm2/Vs Not given Au or Pd (1000 thick, 100m diameter) Resistively evaporated Capacitance 0.67 0.03 0.61 0.05

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163 BIOGRAPHICAL SKETCH Kelly Ip was born on January 19, 1977, in Hong Kong. She immigrated to the United States with her family in 1986 and has lived in Florida ever since. After graduating high school with an International Baccalaureate Diploma in June 1996, she attended the University of Florida and earned a bachelors degree with honors in chemical engineering in December 2000. During her undergraduate studies, she interned for Milliken Chemical Company in Spartanburg, SC for two semesters through the Cooperative Education Program. She also conducted undergraduate research in the semiconductor field under the supervision of Prof. Fan Ren for four semesters. In 2001, Kelly worked briefly as a process development engineer in Agere Systems (formerly Bell Laboratories, Lucent Technologi es Microelectronics Group) in Orlando, FL. With guidance and encouragement from Prof. Ren, she enrolled in the Department of Materials Science and Engineering at the University of Florida and joined Prof. Stephen Peartons group in January 2002. Kelly continued her studies in the doctorate program under Prof. Pearton. In the summer of 2003 and 2004, she interned as a MESA Institute Fellow at Sandia National Laboratory in Albuquerque, NM, where she worked for Dr. Randy Shul.


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Title: Process Development for ZnO-Based Devices
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PROCESS DEVELOPMENT FOR ZnO-BASED DEVICES


By

KELLY PUI SZE IP














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

Kelly Pui Sze Ip

































To my family and all the special people in my life















ACKNOWLEDGMENTS

I would like to thank my advisor, Prof Stephen J. Pearton, the most integral part of

my graduate studies, for all the opportunities, guidance, motivation and support. I would

also like to thank my committee members, Prof. Cammy R. Abernathy, Prof. David P.

Norton, Prof. Fan Ren and Prof. Rajiv Singh, for their time, expertise and evaluation.

I thank Prof. Ren for introducing me to the world of semiconductors by welcoming

me into his research group when I was an undergraduate student. I am grateful for his

advice and encouragement that have helped me grow professionally and personally.

I would like to thank members of the research groups of Prof. Pearton, Prof. Ren

and Prof. Abernathy for their assistance and friendship, especially Kyu-Pil Lee, Kwang

Baik, Ben Luo, Jihyun Kim, Rishabh Mehandru, Jeff LaRoche, Brent Gila, Andrea

Onstine, Jennifer Hite, Jerry Thaler and Rachael Frazier, and countless others who have

made graduate school enjoyable. I appreciate Prof. Norton and collaborators from his

group, Young-Woo Heo and Yuanjie Li, for developing and growing the ZnO films used

in this dissertation.

I appreciate the valuable experience from the internship opportunities at Sandia

National Laboratory's Compound Semiconductor Research Laboratory (CSRL) made

possible by Randy Shul, my technical advisor, Kent Schubert, my technical manager, and

Regan Stinnet, from the MESA Institute. It was a pleasure to work with such dedicated

researchers, especially Randy, Albert Baca, Mark Overberg, Carlos Sanchez and









Stephanie Jones. I would like to thank them and the members of CSRL for their training

and assistance.

Most importantly, I express my deepest gratitude to family members and friends in

my life who have helped to shape me into the person I am today, professionally and

personally.
















TABLE OF CONTENTS

page

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

LIST OF TABLES ............................ ........... ..... .. ...... ....... ....... ix

LIST OF FIGURES ......... ......................... ...... ........ ............ xi

ABSTRACT ........ .......................... .. ...... .......... .......... xvii

CHAPTER

1 IN TRODU CTION ................................................. ...... .................

2 B A C K G R O U N D ............................................................ ........ .......... .... ....

Properties of ZnO ....................... .......................... ................... .5
G ro w th ................................................................................. 6
B u lk ............................................................. . 6
T h in F ilm ................................................................................... 8
D onors and A cceptors .................................................. ............... 8
Processing Techniques.............................................. 9
Dry Plasma Etching ................................. .......................... .... .........
Ion Im plantation ............................................................. 13
Rapid Thermal Annealing ................. .................................14
Characterization Techniques .............................................. ............... 15
Atom ic Force M icroscopy ........................................................ ..... ............. 15
A uger Electron Spectroscopy .......................................................... ....... ... 15
X-ray Photoelectron Spectroscopy .........................................15
E electrical M easurem ents ............................................................................... 16
Photolum inescence ....................... ................... ......... ......... ............... 16
Rutherford Backscattering Spectrometry/Channeling ......................................17
Scanning Electron M icroscopy ................................................................... 17
Secondary Ion Mass Spectrometry ...........................................18
S ty lu s P ro filo m etry ........................................................................................ 18

3 INDUCTIVELY COUPLED ETCHING OF ZnO ............ ... ............... 27

Intro du action ............. ................. .................................................................... 2 7
Experim ental M ethods................................................. 27









R esu lts an d D iscu ssion ....................................................................... ..................2 8
S u m m a ry ................................................................................................................ 3 0

4 HYDROGEN INCORPORATION, DIFFUSIVITY AND EVOLUTION IN ZnO ..39

In tro d u ctio n ............................................................................................................ 3 9
E xperim mental M ethod ......................................................................... ...................39
R results and D discussion ............................. .................... .. ........ .. .............4 1
S u m m a ry ......................................................................................................4 5

5 OHMIC CONTACTS TO ZnO ...........................................................................54

N -ty p e Z n O .............. ..................................................................................54
In tro d u ctio n .................................................................................................... 5 4
Bulk ZnO ............................ ............... 56
Experimental methods .......................................... .... ...... 56
R results and discussion................................................. 57
S u m m ary ...............................................................5 8
T h in F ilm n -Z n O ............................................................................................ 5 9
Experimental methods ...................................................59
R results and discussion ............................................. ........................... 60
S u m m ary ................................................................................. 6 3
p-ZnM gO Thin Film .................................. ........................... .. ......... 63
Introduction .................... .................. ....................... ....... 63
E xperim mental M ethods...............................................................................64
R results and D iscu ssion ......................................... ................... .................65
Sum m ary ......... .. ...................................................................................... 67

6 SCHOTTKY CONTACTS TO ZnO ............................... ...............85

In tro d u ctio n .............. ..... .......... ........................................................................... 8 5
Thin Film ZnO .......................................... .. .... .... ........... ..... 86
E xperim ental M ethods............................................................................... 86
Results and Discussion .............. .... .................................. 87
Su m m ary .............. ................................................................................89
B u lk Z n O ............................. ................................ .................. ...............8 9
E xperim ental M ethods.................................................................................... 89
Results and Discussion .............. .... .................................. 90
Su m m ary .............. ................................................................................97

7 PN JUNCTION DIODE ..... ......... .. ........ ....... ........126

Introdu action .............. ................. ............................................................... 12 6
E xperim mental ......... ......... ....................................................... 127
R results and D discussion ......... ................... ......... .................................... 128
Summary ................... ..................................... 129

8 CON CLU SION S .................................... .. ......... ..............134









APPENDIX

A OHMIC CONTACT TABLES .......................................................................138

B SCHOTTKY CONTACT TABLES..................................................................144

L IST O F R E FE R E N C E S ......................................................................... ................... 152

BIOGRAPH ICAL SKETCH .............................................................. ............... 163














































viii
















LIST OF TABLES


Table page

2-1. Electrical properties of GaN and ZnO.................................................................... 19

2-2. Basic physical properties of ZnO ........................................ ........................ 19

2-3. Properties of bulk ZnO grown by pressurized melt growth technique....................20

5-1. Ohmic contacts ZnO and their respective specific contact resistance from
published w orks ................................................. ................. 68

5-2. Sheet resistance, transfer length and specific contact resistance of annealed
Au or Au/Ni/Au contacts on p-ZnM gO. ...................................... ............... 68

6-1. Schottky contacts to ZnO and their respective barrier height and ideality
factor from published w orks. .............................................................................. 98

6-2. Metal work function and ideal barrier heights for ZnO (electron affinity:
4 .1 eV ) ................................................................................ 99

6-3. Ideality factor, saturation current density and barrier height for Pt contacts
measured at temperatures between 303-473K on n-type ZnO, both before and
after annealing at 3000C.The contact diameter was 50 [tm in all cases .................99

6-4. Carbon atomic concentrations on the ZnO surface before and after ozone
treatment ................... ..... ........ ... .................................. 100

6-5. Summary of XPS data for O-related species before and after ozone
cleaning ........................................................................... 100

6-6. Summary of electrical characteristics for W-based contacts after 700C
a n n e a ls ...................................... ......... ..................... ................ 1 0 0

6-7. Concentration of elements detected on the surface (in Atom%t) for W2B
and W 2B 5 contacts ............................................ ........................ 101

Table 6-8. Concentration of elements detected on the surfaces (in Atom%t) for CrB2
contacts ................ .... ......... ........................................101

Table 7-1. Characteristics of p-ZnMgO/n-ZnO junctions measured on diodes with
diam eter 50 tm ......................................................................130










A-1. Ohmic contacts to ZnO........................................................... .. .....139

B-1. Schottky contacts to thin film ZnO...........................................................145

B-2. Schottky contacts on bulk n-type ZnO ...................................................148





















































x















LIST OF FIGURES

Figure page

2-1. Crystal structure ofw urtzite ZnO .................................................. .....................20

2-2. Photographs of ZnO substrates grown by pressurized melt growth
technique: (a) 2-inch diameter wafer, (b) boules and wafers of various diameters
and (c) 1 cm 2 pieces. ........................... ....... ............ ...... ...... ...... 21

2-3. Pulsed laser deposition system .............................................................................. 22

2-4. ICP reactor. ..................................................................23

2-5. Electric and magnetic fields inside the reactor ........................................................ 23

2-6. Chemical etching process. (a) Generation of reactive species. (b)
Diffusion of reactive neutrals to surface. (c) Adsorption of reactive neutrals to
surface. (d) Chemical reaction with surface. (e) Desorption of volatile
byproducts. (f) Diffusion of byproducts into bulk gas. .........................................24

2-7. Physical etching process. (a) Generation of reactive species. (b)
Acceleration of ions to the surface. (c) Ions bombard surface. (d) Surface atoms
are ejected from the surface. ............................................ ............................ 24

2-8. Combination of chemical and physical etching process. (a) Generation of
reactive species. (b) Diffusion of reactive neutrals to surface. (b) Ion
bombardment to surface. (c) Adsorption of reactive neutrals to surface. (d)
Chemical reaction with surface. (e) Desorption of volatile byproducts. (f)
Diffusion of byproducts into bulk gas ............................................................25

2-9. Ion im plantation system ........................................ .............................................25

2-10. Principle of A FM ........................................... ............... .... ....... 26

2-11. The Auger process: (a) isolated atom, (b) inner core level electron
dislodged, leaving behind a vacancy, (c) an outer level electron fills the vacancy
and releases excess energy and (d) the excess energy ejects an Auger electron......26

3-1. Etch rates of ZnO as a function of rf chuck power in ICP CH 4/H2/Ar or
C12/Ar discharges. The dc self-bias on the cathode is also shown ........................32









3-2. Etch rate of ZnO in CH4/H2/Ar or C12/Ar plasmas as a function of the
average ion kinetic energy (plasma potential of 25 V minus the measured dc bias
voltage)......... ........................................................ ..... .... .. 32

3-3. PL spectra at 300K from ZnO before and after CH4/H2/Ar etching at
different rf chuck powers, shown on both linear (a) and log (b) scales .................33

3-4. Room temperature, deep-level PL emission from ZnO etched in ICP
CH4/H2/Ar discharges at different rf chuck powers. The data are shown on both
energy (a) and wavelength (b) scales. ........................................... ............... 34

3-5. AFM scans of ZnO before and after ICP CH4/H2/Ar etching at different rf
chuck powers. The z-scale is 150nm/div. .................................... .................35

3-6. RMS roughness of ZnO surfaces etched in ICP CH4/H2Ar discharges at
different rf chuck pow ers. ............................................................. .....................36

3-8. SEM micrographs of features etched into ZnO using a CH4/H2/Ar plasma.
The photoresist mask has been removed ............... ...... ...................38

4-1. SIMS profiles of 2H implanted into ZnO (100 keV, 1015 cm-2) before and
after annealing at different temperatures (5 min anneals)................... ............47

4-2. RBS spectra of bulk, single-crystal ZnO before and after 100 keV H+
implantation to a dose of 1016 cm-2 .......................................................................47

4-3. PL spectra at 300K of ZnO implanted with 2H+ ions (100 keV, 1015 cm-2)
as a function of post-implanted annealing temperature (5 min anneals)..................48

4-4. SIMS profiles of 2H in ZnO exposed to deuterium plasmas for 0.5 h at
different tem peratures. ....................................... ...........................48

4-6. SIMS profiles of 2H in ZnO exposed to deuterium plasma for 0.5 h at
2000C and then annealed at 4000C or 5000C for 5 mins ........................................49

4-7. Percentage of retained 2H implanted into ZnO (100 keV, 1015 cm-2) as a
function of annealing temperature (5 min anneals). The inset shows the data on a
log scale ...... ................ ...... ............ ...................................... ............................50

4-8. Donor concentration profiles in ZnO before and after plasma exposure
and after subsequent annealing. ........................................ .......................... 50

4-9. PL spectra from 2H plasma exposed ZnO. ..................................... ...............51

4-10. Detailed band edge and deep level emission PL spectra from 2H plasma
exposed Z nO ...................................................... ................. 52









4-11. 300K PL spectra from 2H implanted ZnO, as a function of subsequent
anneal tem perature. ..................................... ...... ........ .... ...... ...... 53

5-1. Schematic diagram (a) and secondary electron image (b) of circular TLM
pattern on ZnO substrate. ............................................... .............................. 69

5-2. Specific contact resistance as a function of anneal temperature for
Ti/Al/Pt/Au contacts on n-ZnO. Solvent chemical cleaning or H2 plasma
exposure of the surface prior to metallization was compared with the case of
depositing the metal on the as-received surface................... ................... ...............70

5-3. Secondary electron image (a) and AES depth profile (b) of as-deposited
T i/A l/P t/A u contact on Z nO .......................................................... .....................7 1

5-5. AES surface scans of Ti/Al/Pt/Au contacts on ZnO after annealing at
2500C (a), 3500C (b), 4500C (c) or 6000C (d) ............................................ ............ 73

5-6. AES depth profiles of Ti/Al/Pt/Au contacts on ZnO after annealing at
2500C (a), 3500C (b), 4500C (c) or 6000C (d) ............................................ ............ 74

5-7. Carrier mobility and resistivity of epi-ZnO as a function of post-growth
anneal tem perature. ..................................... ...... ........ .... ...... ...... 75

5-8. Schematic diagram (a), SEM (b) and microsope image (c) of the linear
TLM ohmic contact pads on ZnO mesa. ........................................ ............... 76

5-9. Carrier concentration of epi-ZnO and specific contact resistance of as-
deposited ohmic contacts measured at 300C versus post-growth anneal
temperature ................... ...... ........................ ........ ................ 77

5-10. Specific contact resistance as a function of measurement temperature for
samples with various carrier concentrations. The solid symbols represent
measurements prior to ohmic contact anneal. The corresponding open symbols
denote measurements after 2000C, 1 min anneal in N2 ambient............................77

5-11. Specific contact resistance versus measurement temperature of as-
deposited ohmic contact measured at 300C, and after annealing at 2000C, 1 min
m measured at 300C and 200 C ........................................... ........................... 78

5-12. In(pcT) versus 1000/T for samples with various carrier concentrations.
The solid symbols represent measurements prior to ohmic contact anneal. The
corresponding open symbols denote measurements after 2000C, 1 min anneal in
N 2 am b ien t............................................................................................ 7 8

5-13. AES surface scans (a) and surface scans (b) of Ti/Al/Pt/Au ohmic
contacts to ZnO after annealing at 200 C. ........................................ ...................79









5-14. I-V characteristics from Au/Ni/Au (a) or Au (b) contacts on p-type
ZnMgO before and after annealing at 600C................... ......................................80

5-15. AES depth profiles from Au/Ni/Au contacts on p-type ZnMgO before (a)
and after (b) annealing at 600 C. ........................................ ......................... 81

5-16. AES depth profiles from Au contacts on p-type ZnMgO before (a) and
after (b) annealing at 600 C ............................................ ............................ 82

5-17. AES surface scans from as-deposited Au (a) and after annealing at
6000C (b) or as-deposited Au/Ni/Au (c) and after annealing at 6000C (d)..............83

5-18. Cross-section TEM micrograph of Au/Ni/Au contact after annealing at
6 0 0 0C ..............................................................................8 4

6-1. Plan view optical micrographs of contacts before (a) and after (b) 3000C
annealing. The Pt Schottky contacts are the inner circles, while the Ti/Al/Pt/Au
Ohmic contacts are the outer rings. .............................................. ............... 102

6-2. Forward I-V characteristics for as-deposited or 3000C annealed Pt contacts
on n-type ZnO, for two different measurement temperatures ............. ...............103

6-3. Reverse I-V characteristics for as-deposited or 3000C annealed Pt contacts
on n-type ZnO, for two different measurement temperatures.............................103

6-4. Pt barrier height on n-type ZnO as a function of measurement temperature
for as-deposited and 3000C contacts. ......................................... ...............104

6-5. AFM scans of ZnO surfaces over 1 tm2 area, either before (a) or after (b)
U V ozone cleaning. ........................... ...................... .............. .. .............. 105

6-6. AFM scans from the samples of Figure 6-5, showing the RMS roughness
v a lu e s .................................. .......................................................... ............... 1 0 6

6-7. XPS survey spectra of ZnO before (a) and after (b) UV ozone cleaning. ..............107

6-8. XPS spectra from the region of O-bonded transitions, before (a) and after
(b) U V ozone cleaning. ............................................... ............................... 108

6-9. I-V characteristic from Pt/Au contacts on ZnO without any ozone
cleaning prior to m etal deposition ............. .................................... ............... 109

6-10. Forward (a) and reverse (b) I-V characteristics from Pt/Au contacts on
ozone cleaned ZnO ............................................................ ................... 110

6-11. I-V characteristics from W/Pt/Au contacts on ZnO both as-deposited (a)
and after annealing at 7000C (b). .................................... ............. ................... 111









6-12. AES depth profiles of W/Pt/Au contacts both as-deposited (a) and after
7000C annealing (b). ............................................. .... .... ... ........ .. .. 12

6-13. Secondary electron images of W2B/Au (a, c) or W2B/Pt/Au (b,d)
contacts on ZnO, either as-deposited (a, b) or after 600 C annealing (c, d). .......113

6-14. I-V characteristics from W2B/Au (a) or W2B/Pt/Au (b) contacts on ZnO,
as a function of annealing temperature. ............ .......... .................... 114

6-15. Barrier height (a), apparent ideality factor (b) and saturation current
density (c) from W2B/Pt/Au Schottky contacts on n-type ZnO, as a function of
anneal temperature. ............. ................... ....... ...............115

6-16. AES surface scans ofW2B/Au (a, c) or W2B/Pt/Au (b, d) contacts on
ZnO, either as-deposited (a, b) or after 6000C annealing (c, d). ............................116

6-17. AES depth profiles ofW2B/Au (a, c) or W2B/Pt/Au (b,d) contacts on
ZnO, either as-deposited (a, b) or after 6000C annealing (c, d). ............................117

6-18. Secondary electron images of W2B5/Pt/Au contacts on ZnO, either as-
deposited (a) or after 6000C annealing (b). .................. ................................. 118

6-19. I-V characteristics from W2B5/Pt/Au contacts on ZnO, as a function of
annealing temperature. .......... ..... ........................... 119

6-20. AES surface scans of W2B5/Pt/Au contacts on ZnO, either as-deposited
(a) or after 6000C annealing (b). ........................................ ........................ 120

6-21. AES depth profiles ofW2B5/Pt/Au contacts on ZnO, either as-deposited
(a) or after 6000C annealing (b). ........................................ ........................ 121

6-22. Optical microscopy photos of CrB2 /Pt/Au on ZnO either as-deposited
(a) or after annealing at 500 (b), 600 (c) or 7000C (d) ........................................122

6-23. I-V characteristics from CrB 2Pt/Au contacts on ZnO, as a function of
annealing tem perature. ................................................ ............................... 123

6-24. AES surface scans of Cr2B/Pt/Au contacts on ZnO, either as-deposited
(a) or after 6000C annealing (b). ........................................ ........................ 124

6-25. AES depth profiles of Cr2B/Pt/Au contacts on ZnO, either as-deposited
(a) or after 6000C annealing (b). ........................................ ........................ 125

7-1. Schematic of ZnMgO/ZnO p-n junction structure............................................130

7-2. I-V characteristics at 300C of ZnMgO/ZnO p-n junctions using Pt/Au as
the ohmic contact to p-type ZnM gO. ........................................ ............... 131









7-3. Reverse I-V characteristics of ZnMgO/ZnO p-n junctions using Pt/Au as
the ohmic contact on the p-type ZnMgO, as a function of the measurement
tem perature................ .............. ... .......... ......... .. .......... 132

7-4. Measurement temperature dependence of the reverse breakdown voltage
in ZnMgO/ZnO p-n junctions using Pt/Au as the ohmic contact on p-type
Z nM gO .............................................................................133















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

PROCESS DEVELOPMENT FOR ZnO-BASED DEVICES

By

Kelly Pui Sze Ip

August 2005

Chair: Stephen J. Pearton
Major Department: Materials Science and Engineering

Our study focused on process development for ZnO-based devices. Our intent was

to achieve practical plasma etching processes, to understand the role of hydrogen in ZnO

and to optimize ohmic and Schottky contacts to ZnO. We also demonstrated p-n diode

using ZnO/ZnMgO heterostructure.

Two different plasma chemistries, C12/Ar and CH4/H2/Ar, for etching ZnO were

examined. Methane-based chemistry is able to achieve practical etch rates and high

fidelity anisotropic pattern transfer. Hydrogen introduced into ZnO by ion implantation

and plasma exposure was investigated. Hydrogen incorporation depths of >25 |tm were

obtained in bulk, single-crystal ZnO during exposure to 2H plasmas for 30 min at 3000C

and completely removed by subsequent annealing at 500-6000C.

Ohmic contacts to both n-type and p-type ZnO were studied. Ti/Al/Pt/Au

metallization was considered for ohmic contacts to thin film and bulk n-type ZnO,

whereas Au/Ni/Au and Ni/Au were used on the p-type thin film. Schottky contacts to n-









type PLD thin film and bulk ZnO were investigated. The Pt contacts on thin film ZnO

show rectifying behavior with a barrier height at 250C of 0.61eV. This barrier height is

reduced significantly (to 0.42 eV) after annealing at 3000C. Several metals, including Pt

W, W2B, W2B5 and CrB2, were considered for contacts to n-type, bulk ZnO. For Pt

contacts, the UV ozone treatment produces a change from ohmic behavior to rectifying

behavior with Schottky barrier height of -0.7 eV. The W, W2B, W2B5, and CrB2 metals

deposited by sputtering on ZnO produce non-rectifying contacts in the as-deposited state,

but these convert to rectifying upon annealing at 500-6000C when a Pt diffusion barrier

between the metal and the Au overlayer is used to prevent dissociation of the ZnO.

We also reported the electrical characteristics of Zno.9Mgo.O1/ZnO p-n junctions

grown by pulsed laser deposition on bulk, single-crystal ZnO substrates. Acceptable

rectification in the junctions required growth of an n-type ZnO buffer on the ZnO

substrate before growth of the p-type, phosphorus-doped Zno.9Mgo.10. Without this

buffer, the junctions showed very high leakage current.


xviii














CHAPTER 1
INTRODUCTION

Since the invention of the first semiconductor transistor in 1947 by the scientists

of Bell Labs, the semiconductor industry has grown incessantly; fabricating faster,

smaller, more powerful devices while manufacturing in larger volume at lower costs.

Although the first semiconductor transistor was made from germanium (Ge), silicon (Si)

became the semiconductor of choice because Ge has low melting point that limits high

temperature processes and because of the lack of a naturally occurring germanium oxide

to prevent the surface from electrical leakage. Due to the maturity of its fabrication

technology, silicon continues to dominate commercial markets in discrete devices and

integrated circuits for computing, power switching, data storage and communication.

For high-speed and optoelectronic devices such as high-speed integrated circuits

and laser diodes, gallium arsenide (GaAs) is the material of choice. It exhibits superior

electron transport properties and special optical properties. GaAs has higher carrier

mobility and higher effective carrier velocity than Si, allowing faster devices. GaAs is a

direct bandgap semiconductor, whereas Si is indirect; making GaAs better suited for

optoelectronic devices. However, physical properties required for high-power,

high-temperature electronics and UV/blue light emitter applications are beyond the limits

of Si and GaAs. It is essential to investigate alternative materials and their growth and

processing techniques in order to achieve these devices [1,2]. Wide bandgap

semiconductors exhibit inherent properties such as larger bandgap, higher electron









mobility and higher breakdown field strength. Therefore, they are suitable for

high-power, high-temperature electronic devices and short-wavelength optoelectronics.

Zinc oxide is a direct, wide bandgap semiconductor material with many promising

properties for blue/UV optoelectronics, transparent electronics, spintronic devices and

sensor applications [3-9]. ZnO has been commonly used in its polycrystalline form for

over a hundred years in a wide range of applications: facial powders, ointments,

sunscreens, catalysts, lubricant additives, paint pigmentation, piezoelectric transducers,

varistors, and transparent conducting electrodes [10-14]. Its research interest has waxed

and waned as new prospective applications revive interest in the material, but the

applications have been limited by the technology available at the time.

The first use of ZnO for its semiconductor properties was for detectors in build-

your-own radio sets in the 1920s. A thin copper wire, known as a "cat's whisker," is

placed in contact with sensitive spots on a ZnO crystal. The metal/semiconductor

junction allows current to flow only in one direction, converting the incoming radio

waves from alternating current to direct current in the radio circuit. In 1957, the New

Jersey Zinc Company published the book entitled "Zinc Oxide Rediscovered" to promote

the material's "frontier" properties (semiconductor, luminescent, catalytic, ferrite,

photoconductive, and photochemical) and applications [15]. Research focused mainly on

growth, characterization and applications that do not require single crystals such as

varistors, surface acoustic wave devices and transparent conductive films.

Recent improvements in the growth of high quality, single crystalline ZnO in both

bulk and epitaxial forms has renewed interest in this material. Originally, research efforts

in ZnO growth were intended for gallium nitride (GaN) epitaxy. GaN is another wide,









direct bandgap semiconductor that has been the focus of intensive research for

high-power, high-frequency electronics that can operate at elevated temperatures and

UV/blue optoelectronics. The lack of a native substrate has led to a search for suitable

choices of substrate in other materials, including sapphire, silicon carbide (SiC) and ZnO

[16].

The work of Look and colleagues [18] played a major role in reviving interest in

ZnO research. The group began studying ZnO as a substrate for GaN epitaxy in the

1990s and realized the properties and potential of ZnO itself. They also organized the

First Zinc Oxide Workshop in 1999 that brought together researchers from all over the

world to disseminate their findings and to exchange ideas. Furthermore, Look and

colleagues published the first convincing results of carefully characterized p-type ZnO

homoepitaxial film grown by molecular beam epitaxy (MBE), a critical step in achieving

p-n junctions for light-emitting devices. Subsequent ZnO Workshops, in 2002 and 2004,

further encouraged research on ZnO [17-19].

As a wide bandgap semiconductor (Eg = 3.37 eV), ZnO is a candidate material for

blue/UV optoelectronics, including light-emitting diodes, lasers, and detectors. These

shorter wavelengths enable higher storage density in high-density optical storage

systems, since storage density is inversely proportional to the wavelength squared. Also,

ZnO blue or UV light-emitters could be employed in white solid-state lighting by using

them to excite phosphors. In addition, ZnO is transparent to visible light and can be

made into transparent transistors for active optical circuitry for color displays. Other

applications include communications, biological detectors, and gas sensors [3].






4


Significant efforts in the last few years have been aimed at controlling conductivity

and improving crystal quality. However, to fully realize ZnO devices, material and

process development issues must be overcome. Our motivation was to develop practical

plasma etching processes, to understand the role of hydrogen in ZnO and to optimize

ohmic and Schottky contacts to ZnO.














CHAPTER 2
BACKGROUND

Properties of ZnO

ZnO has many attractive characteristics for electronic and optoelectronic devices.

Its electrical properties are compared to those of GaN in Table 2-1 [3,20,21]. It has direct

bandgap energy of 3.37 eV, which makes it transparent in visible light and operates in the

UV to blue wavelengths. The exciton binding energy is -60 meV for ZnO, as compared

to GaN -25 meV; the higher exciton binding energy enhances the luminescence

efficiency of light emission. The room-temperature electron Hall mobility in

single-crystal ZnO is -200 cm2 V-1, slightly lower than that of GaN, but ZnO has higher

saturation velocity. ZnO exhibits better radiation resistance than GaN for possible

devices used in space and nuclear applications [22]. ZnO can be grown on inexpensive

substrate, such as glass, at relatively low temperatures. Nanostructures, such as

nanowires and nanorods, have been demonstrated [23, 24]. These structures are ideal for

detection applications due to its large surface area-to-volume ratio. Recent works shows

ferromagnetism in ZnO by doping with transition metal, e.g. Mn, with practical Curie

temperatures for spintronic devices [25]. One attractive feature of ZnO is the ability to

bandgap tuning via divalent substitution on the cation site to form heterostructures.

Bandgap energy of -3.0 eV can be achieved by doping with Cd2+ [26], while Mg2

increases the bandgap energy to -4.0eV [27].

ZnO has a hexagonal wurtzite crystal structure, 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. Bonding between the Zn atoms and O

atoms is highly ionic, due to the large difference in their electronegative values (1.65 for

Zn and 3.44 for O). Alternating Zn and O layers form the crystal structure (Figure 2-1)

[21].

Basic physical properties of ZnO are shown in Table 2-2 [3, 20, 21]. Some of the

values compiled in the table remain uncertain; the disparity originates from the

inhomogeneity of the materials. Robust, reproducible p-type ZnO remains elusive, thus

the hole mobility and effective masses are still in debate. Crystal defects, such as

dislocations, may contribute to variation in thermal conductivity, as observed in GaN.

Growth

Bulk

Bulk, single-crystal ZnO substrates up to 2-inches in diameter are commercially

available. ZnO inherently has n-type conductivity. Good quality, bulk, single-crystal

ZnO substrates are primarily grown by one of the three methods: seeded vapor phase

technique [19], melt growth technique [29-31] or hydrothermal technique [32-37].

Hydrothermal solution growth has been limited to research laboratories until recently

while both seeded sublimation growth and melt growth have produced 2-inch diameter

wafers that are commercially available (Figure 2-2) [32]. The 1 x 1 cm2 substrates we

were purchased from Eagle-Picher Technologies, LLC (Joplin, MO) and Cermet, Inc.

(Atlanta, GA).

Seeded sublimation growth, also known as seeded vapor growth or chemical vapor

transport, is the technique used by Eagle-Picher Technologies, LLC (Joplin, MO, ZnO

division is now ZN Technologies in Brea, CA). High-purity ZnO powder is formed by

Zn vapor and 02 and is heated to 11000C inside a nearly closed horizontal tube. The H2









carrier gas is used to transport the vapor to a cooler region of the tube where the reaction

Zn (g) + H20 (g) ZnO (s) + H2 (g) is predicted to occur on a single-crystal seed. This

process is capable of producing 2-inch diameter, 1-cm thick crystal in about 150 to 175 h

[19].

Melt growth involves heating polycrystalline ZnO powder into a molten state and

allowing to crystallize into a single-crystal state. ZnO grown by the melt growth method

tends to have a large amount of defects (ZnOl-x) due to decomposition of ZnO at high

temperatures. In a pressurize melt growth method that utilizes a modified Bridgman

configuration (developed by Cermet, Inc., Atlanta, GA), an overpressure of oxygen

prevents reduction of the lattice. The growth apparatus consists of a water-cooled

crucible, such that a portion of the ZnO charge is solidified along the crucible wall (in

order to maintain same composition as the melt and to avoid contamination from the

crucible). The heat source uses radio frequency to induce eddy current to melt the charge

into the molten phase [29-31].

The hydrothermal growth technique involves heating precursors of nutrient and

mineralizer aqueous solution in a platinum-lined capsule inside an autoclave. The

nutrient consists of high-purity sintered ZnO polycrystal powder, and the mineralizer

solution is composed of LiOH and KOH [34] or Li2CO3, KOH and [35]. Single crystal

seed provides a template for growth as ZnO is precipitated from the solution. Typical

growth temperature and pressure are about 300 to 4000C and 80 to 100 MPa. The growth

rate of the hydrothermal growth technique is relatively slow (about 0.2 mm per day). The

size of the crystal grown by this technique is usually limited by the need for a Pt-liner to









prevent impurity contamination. Large area substrates (50 x 50 x 15 mm3 in size) have

been reported recently [34].

Thin Film

ZnO thin films are grown by various techniques, including molecular beam epitaxy

(MBE), chemical vapor deposition (CVD), pulsed laser ablation, atomic layer deposition

(PLD), reactive sputtering and spray pyrolysis [36-46]. Both n-type and p-type materials

have been reported. ZnO thin films can be grown on a wide range of substrates: sapphire,

Si, GaN, and even inexpensive materials such as glass. ZnO can be grown at relatively

low temperatures as compared to other wide bandgap semiconductors. Inexpensive

substrate and low-temperature growth make devices feasible to manufacture and lower

the cost of the final product.

The ZnO films we used were developed and grown using pulsed laser deposition by

Prof. Norton and his research group (Dr. Y.-W. Heo and Y. Li) [45, 46]. The PLD

system is shown in Figure 2-3. The main advantages of PLD growth are the ability to

achieve epitaxial growth at low temperatures and the ease of controlling and altering the

film composition by modifying the solid targets and background gases. Inside a vacuum

chamber, a high intensity laser is used to vaporize material from a solid target, forming a

highly directional plume. The solid target sources are made by pressing and sintering

high purity-material powder. Background gases can be introduced to form specific

reactions inside the plume. The material is transferred and deposited onto a heated

substrate several centimeters away.

Donors and Acceptors

As-grown, nominally undoped ZnO exhibits n-type conduction. This behavior can

be attributed to several possible sources: the presence of hydrogen [47, 48], Zn









interstitials or O vacancies [49, 50]. These intrinsic defects act as shallow donors and

reside approximately 0.01 to 0.05 eV below the conduction band. In addition, n-type

conduction can be controlled by excess Zn atoms or by adding Al, Ga, or In dopants. A

major obstacle is p-type doping in ZnO. High quality p-type ZnO has been elusive due to

asymmetrical doping limits that are common in wide bandgap semiconductors, which are

either n-type or p-type, but not both [49-51]. ZnTe, for instance, can be doped p-type

with relative ease, but n-type is difficult to achieve [52]. Shallow acceptors for practical

p-type conduction are difficult to achieve since dopants tend to form deep acceptor levels

instead of shallow acceptor levels and do not contribute significantly to hole conduction.

Several mechanisms offer insight to the origin of asymmetric doping limits: low

solubility of dopant in host material, strong lattice relaxations that force the dopant

energy level deep within the bandgap and the presence of native defects or dopant atoms

in interstitial sites that compensate for substitutional impurity levels by forming deep

level traps. Various p-type dopants (including Cu, Li, Ag, N, P, Sb, As) and co-doping of

group III (Ga, Al, In) with N have been investigated [53]. Many groups have reported

p-type conduction; however, the materials often exhibit ambiguous conduction and are

not consistently reproducible. As research continues in refining growth methods and

optimizing growth conditions, the material will become more robust over time.

Processing Techniques

Dry Plasma Etching

Etching refers to the crucial integrated circuits (IC) fabrication process of

transferring a pattern by removing specified areas. Wet chemical etching was widely

used in manufacturing until the 1960s. Even though this technique is inexpensive, the

feature size is limited to about 3 microns. The isotropic etching results in sloped









sidewalls and undercutting of the mask material. As feature dimension decreases to

microns and submicrons and device density per chip increases, anisotropic etching is

necessary. Dry etching techniques using gases as primary etch medium were developed

to meet this need. In addition to anisotropic pattern transfer, dry etching provides better

uniformity across the wafer, higher reproducibility, smoother surface morphology, and

better control capability than wet chemical etching. Three general types of dry etching

include plasma etching, ion beam milling, and reactive ion etch (RIE) [54, 55].

Inductively coupled plasma (ICP) etching was used in our study and is discussed in

detail.

ICP etching is a dry etching technique in which high-density plasmas are formed

in a dielectric vessel encircled by inductive coils (Figures 2-2 and 2-3). When an rf

power is applied to the coil, commonly referred to as the ICP source power, the

time-varying current flowing through the coil creates a magnetic flux along the axis of

the cylindrical vessel. This magnetic flux induces an electric field inside the vacuum

vessel. The electrons are accelerated and collide with the neutral operating gas, causing

the gas molecules to be ionized, excited or fragmented, forming high-density plasma.

The electrons in circular path are confined and only have a small chance of being lost to

the chamber walls; thus the dc self-bias remains low. The plasma generated as described

above consists of two kinds of active species: neutrals and ions. The material to be

etched sits on top of a small electrode that acts as parallel plate capacitor along with the

chamber as the second electrode. When an rf power, also known as electrode power or

chuck power, is applied to the sample stage, the electrons in the plasma accelerates back

and forth in the plasma from the changes in the sinusoidal field. Since electrons have









much lighter mass compared to the other species in the plasma, they respond more

rapidly to the frequency change than the other species. As the electrons impinge on

chamber surfaces, the chamber becomes slightly negative relative to the plasma. The

surface area of the chamber is larger than the sample stage, thus the negative charge is

concentrated on the sample stage. This bias attracts the ions toward the sample,

bombarding the surface to remove material. In an ICP system, the plasma density and the

ion energy and are effectively decoupled in order to achieve uniform density and energy

distributions and maintain low ion and electron energy low. This enables ICP etching to

reduce plasma damage while achieving fast etch rates.

The plasma generated as described above consists of two kinds of active species:

neutrals and ions. Neutrals are chemically reactive and etch the material by chemical

reactions, while ions are usually less reactive and are responsible for removing material

by physically bombarding the sample surface. The kinetic energy of the ions is

controlled by electrode bias. The electron density and ion density are equal on average,

but the density of neutrals, known as the plasma density, is typically higher. Anisotropic

profiles are obtained by superimposing an rf bias on the sample to independently control

ion energy and by using low pressure conditions to minimize ion scattering and lateral

etching.

The plasma is neutral but is positive relative to the electrode. It appears to glow

due the ion excitation from the electron movements. The recombination of charges at the

boundary surfaces surrounding the plasma creates a charge depletion layer, also known as

a sheath, dark space or dark region, resulting in diffusion of carriers to the boundaries.

The diffusion of electrons is faster than ions initially; thus an excess of positive ions is









left in the plasma and assumes plasma potential (Vp) with respect to the grounded walls.

The plasma and substrate potentials generate drift current to enhance the ion motions and

hinder the electron motions until steady-state condition is achieved. The difference in

electron and ion mobility also generates a sheath near the powered electrode. The dark

region, a small region in the plasma immediately above the sample, keeps the electrons

away due to the negatively charged electrode. The powered electrode reaches a self-bias

negative voltage (Vdc) with respect to the ground. Even though the voltage drop controls

the ion bombardment energy across the plasma sheath, it is difficult to measure;

therefore, it is common to monitor the Vdc. Note that the dc bias is not a basic parameter

and is characteristic to a particular piece of equipment.

Etching is accomplished by interaction of the plasma with the substrate. There

are three basic etching mechanisms: chemical etch process, physical etch process, and a

combination of both chemical and physical etching process (Figures 2-5, 2-6 and 2-7).

The chemical etch process is the chemical reaction that etches the substrate when active

species (neutrals) from the gas phase are absorbed on the surface material and react with

it to form a volatile product. The chemical etch rate is limited by the chemical reaction

rate or diffusion rate that depends on the volatility of the etch products since undesorbed

products coat the surface and prevent or hinder further reactions. Chemical etching is a

purely chemical process therefore etches isotropically, or equally in all directions.

Physical process, also known as sputtering, occurs when ions impinge normal to the

substrate surface. If the ions have sufficiently high energy, atoms, molecules or ions are

ejected from the substrate surface to achieve a vertical etch profile. The etch rate of

sputtering is slow, and the surface is often damaged from the ion bombardment. A









combination of both chemical and physical etching process, also known as energy-driven,

ion-enhanced mechanism, takes advantage of the effect of ion bombardment in the

presence of reactive neutral species. The energetic ions damage the surface and leave the

surface more reactive toward incident neutrals, leading to removal rates that exceed the

sum of separate sputtering and chemical etching. This process produces very fast etch

rates and anisotropic profile; therefore, it is desirable in high fidelity pattern transfer.

Ion Implantation

Ion implantation (Figure 2-8) is a physical process that introduces dopants by

means of high-voltage bombardment to achieve desired electrical properties in defined

areas with minimal lateral diffusion. Inside a vacuum chamber, a filament is heated to a

sufficiently high temperature where electrons are created from the filament surface. The

negatively charged electrons are attracted to an oppositely charged anode in the chamber.

As the electrons travel from the filament to the anode, they collide and create positively

charged ions from the dopant source molecules. The ions are separated in a mass

analyzer, a magnetic field that allows the passage of the desired species of positive ions

with specific characteristic arc radius based upon ion mass. The selected ions are

accelerated in an acceleration tube and then focused into a small diameter or several

parallel beams. The beam is scanned onto the wafer surface, and the ions physically

bombard the wafer. The ions enter the surface and come to rest below the surface as they

lose their energy through nuclear interactions and coulombic interactions, resulting in

Gaussian distribution concentration profile [56].

During implantation, the collisions with high-energy ions cause crystal damage to

the wafer, leading to poor electrical characteristics. In most cases, the carrier lifetime and

mobility decrease drastically. Also, only a small fraction of the implanted ions are









located in substitutional sites and contribute to carrier concentration. Annealing is

needed to repair the crystal damage and to activate the dopants. To determine the depth

and damage profile, Rutherford Backscattering and Channeling (RBS/C) analytical

technique is employed. Annealing process and RBS/C will be further discussed in the

subsequent sections.

Rapid Thermal Annealing

Annealing is a thermal process used for repairing the ion implantation damage,

diffusing dopants and alloying metal contacts. After ion implantation, annealing is

employed to repair the crystal damages caused by the high-energy ion bombardment that

degrade carrier lifetime and mobility. Since the majority of the implanted dopants reside

in the interstitial sites, the as-implanted materials have poor electrical properties.

Annealing provides thermal energy for the dopants to migrate to the substitutional sites

and contribute to the carrier concentration [57, 58].

Traditionally, tube furnaces were used for annealing after ion implantation.

However, furnace annealing causes the implanted atoms to diffuse laterally and requires

relatively long anneal time. Rapid thermal annealing was developed in order to

overcome these drawbacks.

Rapid thermal annealing (RTA) utilizes radiation heating from arc lamps or

tungsten-halogen lamps to heat the wafer in an inert atmosphere such as N2 or Ar. It can

attain higher temperature at a shorter time period than a conventional tube furnace, and

the overall anneal time is relatively short, usually taking seconds as compared to several

minutes to hours in a conventional tube furnace. RTA allows uniform heating and

cooling that reduces thermal gradients that can lead to warping and stress-induced

defects, enabling more dense design and fewer failures due to dislocations.









Characterization Techniques

Atomic Force Microscopy

Atomic force microscopy (AFM) (Figure 2-7) employs a microscopic tip on a

cantilever that deflects a laser beam depending on surface morphology and properties

through an interaction between the tip and the surface. The signal is measured with a

photodetector, amplified and converted into an image display on a cathode ray tube.

Depending on the type of surface, AFM can be performed in contact mode and tapping

mode.

Auger Electron Spectroscopy

Auger electron spectroscopy (AES) determines the elemental composition of the

few outermost atomic layers of materials. A focused beam of electrons with energies

from 3 keV to 30 keV bombards the surface of a specimen. The core-level electrons are

ejected from approximately 1 [tm within the sample, resulting in a vacancy in the core-

level. As the atom relaxes, an outer-level electron fills the core vacancy and releases

excess energy, which in turn, ejects an outer electron, known as an Auger electron. This

process is illustrated in Figure 2-8. The kinetic energy of the Auger electrons is

characteristic of each element, with the exception of hydrogen and helium. Therefore, by

measuring the energies of the Auger electrons, the near-surface composition of a

specimen can be identified. In addition, AES can provide compositional depth profile

from relative intensities of the elements present if the system is equipped with an ion gun

to sputter away material [59].

X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS), also known as electron spectroscopy for

chemical analysis (ESCA), provides similar information as AES. Instead of impinging









the sample surface with an electron beam, XPS utilizes a monoengergetic x-ray beam to

cause electrons to be ejected, usually two to 20 atomic layers deep. The variation of the

kinetic energies of the ejected electrons identifies the elements present and chemical

states of the elements [59].

Electrical Measurements

Current-voltage (I-V) measurements were taken to characterize the electrical

properties of the contacts. These measurements are performed on a semiconductor

parameter analyzer connected to a micromanipulator probe station. For vertical diodes,

the input voltage will be applied to a highly conductive copper disk on which the samples

were mounted on the backside with silver paste.

Photoluminescence

Photoluminescence (PL) is an analytical technique that provides information about

the optical properties of a substrate. A light source, such as He-Cd, Ar and Kr lasers,

with energy larger than the bandgap energy of the semiconductor being studied, generates

electron-hole pairs within the semiconductor. The excess carriers can recombine via

radiative and non-radiative recombination. Photoluminescence, the light emitted from

radiative recombination, is detected. The wavelength associated with the different

recombination mechanism is measured.

The luminescence from excitons, electrons and holes bound to each other, is

observed only at low temperatures in highly pure materials. As the temperature

increases, the exciton breaks up into free carriers from the thermal energy. Increase in

doping also causes the dissociation of excitons under local electric fields. Under these

conditions, the electrons and holes recombine via the band-to-band process. Since some

of the electrons may not lie at the bottom of the conduction band, their recombination and









holes will produce a high-energy tail in the luminescence spectrum. On the other hand,

the band-to-band recombination will yield a sharp cutoff at the wavelength corresponding

to the band gap of the material [59].

Rutherford Backscattering Spectrometry/Channeling

Depth profile of implanted ions and damages can be obtained by the Rutherford

Backscattering Spectrometry/Channeling (RBS/C) technique, which measures the energy

distribution of the backscattered ions from the implanted sample surface at a specific

angle. The energy of the backscattered ion is determined by the mass of the atomic

nucleus and the depth at which the elastic collisons take place.

A beam of high-energy ions impacts the surface of the specimen. The angle of the

analyzing ions affects the penetration depth. If the ions are injected parallel to the crystal

axis of the specimen, they penetrate considerably deeper than if injected randomly, due to

the lower stopping power from channeling. Deeper penetration results in higher

backscattered ions yield. The displacement of an atom, either as host or impurities, from

the crystal lattice also increases the backscattering yield. Therefore, the distribution of

displaced atoms that are caused by the radiation damage from ion implantation can be

measured by increasing the backscattered ion yield [59].

Scanning Electron Microscopy

Scanning electron microscopy (SEM) generates images from electrons instead of

light. A beam of electron is produced and accelerated from an electron gun. The electron

beam passes through a series of condenser and objective lenses, which focus the electron

beam. A scanning coil moves the beam across the specimen surface. The electron beam

interacts with the specimen, and electrons from the surface interaction volume, such as

backscattered, secondary, characteristic x-ray continuous x-ray, and Auger, are emitted.









The signals are collected, amplified and converted to a cathode ray tube image.

Depending on the specimen and the equipment setup, the contrast in the final image

provides information on the specimen composition, topography and morphology. The

main advantages of using electrons for image formation are high magnification, high

resolution and large depth of fields [59].

Secondary Ion Mass Spectrometry

Secondary ion mass spectrometry (SIMS) is a highly sensitive chemical

characterization technique. Primary ions, such as Cs+, 02 0- and Ar, bombard the

specimen in an ultra high vacuum environment, sputtering away secondary ions from the

specimen surface. A small fraction of the ejected atoms are ionized either positively or

negatively, and they are called secondary electrons. The composition of the surface is

determined by the secondary electrons that are individually detected and tabulated using a

mass spectrometer, as a function of their mass-to-charge ratio.

There are two modes of SIMS, static or dynamic. In the static mode, a low

primary-ion flux <1014cm-2 is used, leaving the specimen surface relatively undisturbed.

The majority of secondary ions originate in the top one or two monolayers of the

samples. The dynamic mode monitors the selected secondary ion intensities as a function

of the sputtering time, resulting in a concentration versus depth profile. The depth

resolution of this technique ranges from 5 to 20 nm [59].

Stylus Profilometry

Stylus profilometry is used to measure the topographical features of a specimen

surface, such as roughness, step height, width and spacing. A probe, or stylus, contacts

the surface of the specimen and follows height variation as it scans across the surface.

The height variations are converted into electrical signals, providing a cross-sectional









topographical profile of the specimen. In this work, the etch rate was calculated by the

depth, as measured by the profilometer, over a specified period of time.

Table 2-1. Electrical properties of GaN and ZnO.
Property GaN ZnO
Direct bandgap energy (eV) 3.4 3.4
Electron mobility (cm2/Vs) 220 200
Hole mobility (cm2/Vs) 10 5.50
Electron effective mass 0.27 mo 0.24 mo
Hole effective mass 0.80 mo 0.59 mo
Exciton binding energy (meV) 28 60


Table 2-2. Basic physical properties of ZnO.
Property


Lattice parameters at 300 K (nm)

Density (g cm-3)
Stable phase at 300 K
Melting point (C)
Thermal conductivity

Linear thermal expansion coefficient

Static dielectric constant
Refractive index
Energy bandgap (eV)

Intrinsic carrier concentration (cm-3)

Exciton binding energy (meV)
Electron effective mass
Electron Hall mobility, n-type at 300 K (cm2V-ls-1)
Hole effective mass
Hole Hall mobility, p-type at 300 K (cm2V-1s-1)


Value
ao: 0.32495
co: 0.52069
5.606
Wurtzite
1975
0.6, 1-1.2
ao: 6.5 x 10-6
co: 3.0 x 10.6
8.656
2.008, 2.029
Direct, 3.37
<106
Max n-type doping: n
Max p-type doping: p
60
0.24
200
0.59
5 50


1020
1017








Table 2-3. Properties of bulk ZnO grown by pressurized melt growth technique.
Temperature (K) Resistivity (f cm) Density (cm-3) Mobility (cm2 V' s')
296 9.430 x 10-2 5.045 x 1017 131
78 5.770 x 10-1 3.640 1016 298









Zn


@00


Figure 2-1. Crystal structure ofwurtzite ZnO.



















(a)








(b)








(c)
Figure 2-2. Photographs of ZnO substrates grown by pressurized melt growth technique:
(a) 2-inch diameter wafer, (b) boules and wafers of various diameters and (c)
1 cm2 pieces.











RF PLASMA
RHEED SCREEN








Z n ....................
.............. ...... : IO N
ION
Mg ik GAUGE


EFFUSION CELLS


Figure 2-3. Pulsed laser deposition system.


SUBSTRATE
HEATER



















2 MHz
Power
supply


electrode


Figure 2-4. ICP reactor.


/



Magneti Field
\


\ Rf Current
< ---- ----------------------
--l----------------------- --

Induced E Field


Figure 2-5. Electric and magnetic fields inside the reactor.


10










) _,Q Q:


(b) Q
(c)- Q


4Q (e)


* Electron
SReactive neutral
SIon
SSubstrate atom


Figure 2-6. Chemical etching process. (a) Generation of reactive species. (b) Diffusion
of reactive neutrals to surface. (c) Adsorption of reactive neutrals to surface.
(d) Chemical reaction with surface. (e) Desorption of volatile byproducts. (f)
Diffusion of byproducts into bulk gas.


Q:


*
Z (d)


Sample Negatively biased


Figure 2-7. Physical etching process. (a) Generation of reactive species. (b)
Acceleration of ions to the surface. (c) Ions bombard surface. (d) Surface
atoms are ejected from the surface.


a) _. Q


(b)~,~









(a)


(bl)
(cL _


(b2)
S(d (e)
(d)i


Figure 2-8. Combination of chemical and physical etching process. (a) Generation of
reactive species. (b) Diffusion of reactive neutrals to surface. (b) Ion
bombardment to surface. (c) Adsorption of reactive neutrals to surface. (d)
Chemical reaction with surface. (e) Desorption of volatile byproducts. (f)
Diffusion of byproducts into bulk gas.


Mass Analyser


Ion
Source


Figure 2-9. Ion implantation system.


















Laser beam


Specimen surface
Figure 2-10. Principle of AFM.


Figure 2-11. The Auger process: (a) isolated atom, (b) inner core level electron
dislodged, leaving behind a vacancy, (c) an outer level electron fills the
vacancy and releases excess energy and (d) the excess energy ejects an Auger
electron.


Electron
O Vacancy

O Auger Electron














CHAPTER 3
INDUCTIVELY COUPLED ETCHING OF ZNO

Introduction

Etching process is a crucial step in device fabrication to form features and patterns.

Numerous wet etchants have been reported for ZnO, including NH4C1, HNO3/HCl and

HF [6,60-63]; however, little are known about its dry etching characteristics and the

associated mechanisms and effects on the optical properties of the material. Some initial

results have appeared on plasma etching of sputter-deposited thin films [62-65], while

plasma-induced damage from high ion density Ar or H2 discharges was found to increase

the conductivity of the near surface of similar samples and lead to improved n-type ohmic

contact resistivities [66].

In this section, the etching characteristics of high-quality, bulk single-crystal ZnO

in inductively-coupled plasmas (ICP) of either CH4/H2/Ar or C12/Ar, which are two

common chemistries for II-VI and III-V compound semiconductors [67], were studied.

The etching mechanism was investigated by varying the ion impact energy and by

examining the effects on both the luminescence efficiency and near-surface stoichiometry

of the ZnO.

Experimental Methods

The bulk, wurtzite (0001) ZnO crystals from Eagle-Picher Technologies were

nominally undoped (n 8 x 1016 cm-3, mobility 190 cm2/V-s at 300K from Hall

measurements). Photoresist masked and unmasked samples were exposed to CH4/H2/Ar

(3/8/5 sccm) or C12/Ar (10/5 sccm) discharges in a Unaxis 790 ICP reactor









(St. Petersburg, FL). The 2 MHz power applied to the ICP source was held constant at

500W, and the rf (13.56MHz) chuck power was varied from 50 300W. The source

power controls the ion density while the rf chuck power affects the ion energy. Over the

rf power range investigated, the dc self-bias on the sample electrode varied from -75V to

-344 V for C12/Ar and -91 V to -294 V for CH4/H2/Ar. Etch rates were obtained from

stylus profilometry measurements while PL spectra were obtained at 300K using He-Cd

laser excitation. AES surface scans were used to determine the near-surface

stoichiometry, and SEM was used to examine the anisotropy.

Results and Discussion

Figure 3-1 shows the ZnO etch rates in both chemistries as a function of rf power.

Figure 3-2 displays the ZnO etch rate in both types of plasma chemistry as a function of

the substrate bias, Vb. The x-axis is plotted as the square root of the average ion energy,

which is the plasma potential, -25V for this particular equipment, minus the dc self-bias.

A commonly accepted model for an etching process occurring by ion-enhanced

sputtering in a collision-cascade process predicts the etch rate will be proportional to

E0-ETH0.5, where E is the ion energy and ETH is the threshold energy [68]. Therefore, a

plot of etch rate versus E.5 should be a straight line with an x-intercept equal to ETH. In

the case of CH4/H2/Ar, which produced the faster etch rates, the value of ETH is -96 eV.

The C12/Ar data would indicate negative activation energy, but this is an artifact of the

complexity of the ion energy distribution in that chemistry, as reported in detail

previously [69-72]. The fact that CH4/H2/Ar exhibits an ion-assisted etch mechanism is

consistent with the moderate vapor pressure for the expected group II etch product,

namely (CH3)2Zn, with a vapor pressure of 301 mTorr at 20C [64, 73] and the high bond

strength of ZnO. To form the etch product, the Zn-O bonds must first be broken by ion









bombardment. The ZnCl2 etch product has a lower vapor pressure (1 mTorr at 428 C)

[65], which explains the slower etch rates for this chemistry. Given the higher etch rates

for CH4/H2/Ar, the remainder of the study is focused on this plasma chemistry.

Figure 3-3 shows 300K PL spectra from the samples etched in CH4/H2/Ar as a

function of rf chuck powers. The overall PL intensity is decreased from both the band

edge (3.2 eV) and the deep-level emission bands (2.3 2.6 eV). There is a decrease of

approximately a factor of 4 even for the low bias condition, which shows that ZnO is

susceptible to ion-induced damage during plasma etching. In etching real device

structures, it would be necessary to minimize both ion energy and ion flux toward the end

of the process in order to minimize lattice damage.

Figure 3-4 shows a close-up of the deep level emission spectra. The intensity of

these transitions is also decreased by the etch process. An increase in band edge intensity

and suppression of the deep-level emission reported for H2 plasma exposure of ZnO [64,

66] were not observed, suggesting that the Ar ion bombardment component dominates

during CH4/H2/Ar etching at room temperature. At this stage, it is not clear if this is a

result of hydrogen incorporation, or simply a decrease in overall intensity from the

introduction of non-radiative centers.

Figure 3-5 shows AFM surface scans taken before and after etching with

CH4/H2/Ar at different rf chuck powers. The surface morphology depends on the

incident ion energy and most likely results from differences in the removal rates of Zn

and O etch products. Figure 3-6 shows the measured root-mean-square (RMS) roughness

measured over 5x5 |tm2 areas, as a function of the rf chuck power. The roughness goes

through a minimum at 200 W rf power, corresponding to an ion energy of -251 eV. At









this incident ion energy, the surface roughness is approximately the same as for the

unetched control material. Under these conditions the etch rate is -2000 A-min1, which

is a practical value for most device processing applications. The near-surface

stoichiometry of the ZnO was unaffected by CH4/H2/Ar etching, as determined by AES.

A smooth surface morphology is a good indicator that both Zn and O are removed

equally during etching. Figure 3-7 shows AES surface scans before and after etching for

3 mins at the 200W rf chuck power (-167eV average ion energy) condition.

Approximately 60 A of ZnO was removed by Ar ion sputtering in the AES analysis

chamber prior to analysis to remove adventitious carbon and other atmospheric

contaminants. The Zn-to-O ratio is identical within experimental error and demonstrates

that the CH4/H2/Ar plasma chemistry is capable of equi-rate removal of the Zn and O

etch products during ICP etching.

Given this result and the fact that the etching occurs through an ion-assisted

mechanism, smooth, anisotropic pattern transfer is expected. Figure 3-8 shows SEM

micrographs of features etched in ZnO using a photoresist mask and CH4/H2/Ar

discharge. The vertical sidewalls are an indication that the etch products are volatile only

with additional ion-assistance. In addition, the etched field shows only a slight degree of

roughening, consistent with the fact that the surface retains its stoichiometry. The

sidewall striations on the bottom figure are due to initial photoresist sidewall roughness,

but one can observe excellent high fidelity pattern transfer into the ZnO.

Summary

The etch mechanism for ZnO in plasma chemistries of CH4/H2/Ar and C12/Ar have

been investigated. For both chemistries the etch rate increases with ion energy as

predicted from an ion-assisted chemical sputtering process. Smooth, anisotropic pattern






31


transfer in ZnO can be achieved with ICP CH4/H2/Ar discharges. Under optimum

conditions, the etched surface is smooth and stoichiometric under these conditions and

the etching proceeds by an ion-driven mechanism with a threshold ion energy of 96eV.

ICP C12/Ar discharges also produce practical etch rates for ZnO but are slower than with

CH4/H2/Ar due to low volatility of the ZnCl2 etch product. The luminescence intensity

from the ZnO is significantly degraded by the dry etching by creating non-radiative

defects.












5000


4000


3000


2000


000 -


400

350

300

250

200

150

100

50


0 I I I I I I I0-0
0 50 100 150 200 250 300 350
RF Power (W)

Figure 3-1. Etch rates of ZnO as a function of rf chuck power in ICP CH4/H2/Ar or
C12/Ar discharges. The dc self-bias on the cathode is also shown.


3000

2500

" 2000

2 1500

| 1000


12 14 16 18


Square Root (25+Vb)


Figure 3-2. Etch rate of ZnO in CH4/H2/Ar or C2/Ar plasmas as a function of the
average ion kinetic energy (plasma potential of 25 V minus the measured dc
bias voltage).


-A- Cl,/Ar dc bias
CH4/H,/Ar dc bias









A C1,/Ar etch rate
SCH /H,/Ar etch rate


- U Cl/Ar Chemistry
CH/H2/Ar Chemisty


























2.0 2.5 3.0 3.5
Energy (eV)


- 0.1




a 0.01
0
c^


2.0 2.5 3.0 3.5


Energy (eV)


Figure 3-3. PL spectra at 300K from ZnO before and after CH4/H2/Ar etching at different
rf chuck powers, shown on both linear (a) and log (b) scales.











0.07

0.06
Control-
0.05

0.04

0.03
00W
0.02 50W rf

0.01
200W rf
0.00
2.0 2.5
Energy (eV)

(a)

0.07

0.06

0.05
Control
0.04

S0.03 300W rf
S /50W rf
S0.02-

0.01
200W rf
0.00
450 500 550 600 650
Wavelength (nm)

(b)

Figure 3-4. Room temperature, deep-level PL emission from ZnO etched in ICP
CH4/H2/Ar discharges at different rf chuck powers. The data are shown on
both energy (a) and wavelength (b) scales.





























1OOW rf


2I
4
~IM


200W rf


300W rf


UN


Figure 3-5. AFM scans of ZnO before and after ICP CH4/H2/Ar etching at different rf
chuck powers. The z-scale is 150nm/div.


Control


50W rf


3 "
S -


2














ZnO
CH4/H2/Ar
500W ICP Power


Control


0 50 100 150 200 250 300

rf Power (W)


Figure 3-6. RMS roughness of ZnO surfaces etched in ICP Ch4/H2Ar discharges at
different rf chuck powers.


.........._















;~ww-w w-MY-N4


50 450 850 1250 1650
Kinetic Energy (eV)


2050


450


850 1250
Kinetic Energy (eV)


1650


Figure 3-7. AES surface scan before (a) and after (b) CH4/H2/Ar etching. The spectra
was taken at a depth of -60 A by first sputtering briefly with an Ar+ beam.


(a) ZnO control


C


Zn


(b) ZnO etched


7rw1


C


Zn


2050











































Figure 3-8. SEM micrographs of features etched into ZnO using a CH4/H2/Ar plasma.
The photoresist mask has been removed.














CHAPTER 4
HYDROGEN INCORPORATION, DIFFUSIVITY AND EVOLUTION IN ZNO

Introduction

There is particular interest in the properties of hydrogen in ZnO, because of the

predictions from density functional theory and total energy calculations that it should be a

shallow donor [47, 74-77]. The generally observed n-type conductivity, therefore, may at

least in fact be explained by the presence of residual hydrogen from the growth ambient,

rather than to native defects such as Zn interstitials or O vacancies. Some experimental

support for the predicted observations of its muonium counterpart [48, 78] and from

electron paramagnetic resonance of single-crystal samples [79]. There have been many

other studies on the effects of hydrogen on the electrical and optical properties of ZnO

[80-88], but no detailed studies have been performed on the thermal stability and

diffusion behavior of hydrogen introduced by ion implantation and by plasma exposure.

In this section, the retention of hydrogen by ion implantation and plasma exposure

in single-crystal, bulk ZnO as a function of annealing temperature is presented. The

effects of the implantation on both the crystal quality and optical properties of the

material were also examined. Changes in the electrical and optical properties of the

plasma-exposed ZnO are also discussed.

Experimental Method

Bulk, wurtzite (0001) ZnO crystals from Eagle-Picher Technologies of grade I

quality were employed for all experiments. The samples were nominally undoped with









as-received n-type carrier concentrations of 1017 cm-3 and a room temperature mobility

of 190 cm2/V-s.

To investigate the thermal stability of implanted hydrogen, either 2H+ or 1H+ ions

were implanted into the Zn faces of the samples. In the latter case, implantation was

performed with a 1.7 MV tandem accelerator (NEC, 55DH-4, Middleton, WI) at 25C

with 100 keV H+ ions using a beam flux of 1.3x 1013 cm-2s-1 to a dose of either 1015 or

1016 cm-2. During implantation, samples were tilted by -7 relative to the incident ion

beam to minimize channeling. After implantation, these samples were characterized by

RBS/C using the same accelerator with 1.8 MeV 4He+ ions incident along the [0001]

direction and backscattered at -168 relative to the incident beam direction. The RBS/C

spectra were accumulated for long enough that the random yield at the depth of the bulk

defect peak corresponded to -4000 counts. The 2H+ implantation was also performed at

energy of 100keV to a dose of 1015 cm-2. Annealing was performed for 5 mins at

500-700C under flowing N2 rapid thermal annealing furnace with the samples in a face-

to-face configuration. These samples were examined by photoluminescence at 300K

using a He-Cd laser and by SIMS. The latter was performed using a Cs+ ion beam with

14.5 keV energy and 24 incident angle.

To study the thermal stability and diffusion behavior of hydrogen introduced by

plasma exposure, the samples were exposed to 2H plasmas at temperatures of 100-300C

in a Plasma Therm 720 series reactor operating at 900 mTorr with 50 W of 13.56 MHz

power. Some of these samples were subsequently annealed at temperatures up to 6000C

under flowing N2 ambients for 5 mins. SIMS measurements were used to obtain the

deuterium profiles as a function of plasma exposure or annealing temperature. The









electrical properties of some of the samples were examined by electrochemical

capacitance-voltage measurements using a 0.2 M NaOH/0.1 M EDTA electrolyte as the

rectifying contact. Finally, optical properties were measured using photoluminescence

spectroscopy at variable temperatures, with a He-Cd laser as the excitation source.

Results and Discussion

Figure 4-1 shows the SIMS profiles of implanted 2H as a function of subsequent

annealing temperature. The effects of the annealing is an evolution of 2H out of the ZnO

crystal, with the remaining deuterium atoms of each temperature decorating the residual

implant damage. The peak in the as-implanted profile occurred at 0.96 [am, in good

agreement with the projected range from Transfer-of-Ion-in-Matter (TRIM) simulations.

The thermal stability of the implanted 2H is considerably lower in ZnO than in GaN [89],

where temperatures of 900C are needed to remove deuterium to below the detection

limit (-3 x 1015 cm-3) of SIMS and this suggests that slow-diffusing H2 molecules or

larger clusters do not form during the anneal. Since conventional out-diffusion profiles

were not observed, it was impossible to estimate a diffusion coefficient for the 2H in

ZnO. The results reported here are consistent with an implant-damaged trap-controlled

release of 2H from the ZnO lattice for temperatures >500C.

RBS/C showed that implantation of 1H, even at much higher doses (1016 cm-2), did

not affect the backscattering yield near the ZnO surface (Figure 4-2). However, there

was a slight but detectable increase in scattering peak deeper in the sample, in the region

where the nuclear energy loss profile of 100keV H+ is a maximum. The RBS/C yield at

this depth was -6.5% of the random level before H implantation and -7.8% after

implantation to the dose of 1016 cm2.









While the structural properties of the ZnO were minimally affected by the hydrogen

(or deuterium) implantation, the optical properties were severely degraded. Room

temperature cathodoluminescence showed that even for a dose of 1015 cm-2 1H ions, the

intensity of near-gap emission has reduced by more than 3 orders of magnitude as

compared to control values. This is due to the formation of effective non-radiative

recombination centers associated with ion-beam produced defects. Similar results were

obtained from PL measurements. Figure 4-3 shows the 300K spectra from the 2H+

implanted samples annealed at different temperatures. The band-edge luminescence is

still severely degraded even after 700C anneals where the 2H has been completely

evolved from the crystal. This indicates that point defect recombination centers are still

controlling the optical quality under these conditions. Kucheyev et al. [90] have found

that resistance of ZnO can be increased by about 7 orders of magnitude as a result of trap

introduction by ion irradiation.

Figure 4-4 shows SIMS profiles of 2H in plasma exposed ZnO, for different sample

temperatures during the plasma treatment. The profiles follow those expected for

diffusion from a constant or semi-infinite source, i.e.


C(x,t)=Co erfc (2-1)


where C(x,t) is the concentration at a distance x for diffusion time t, Co is the solid

solubility and D is the diffusivity of 2H in ZnO [91]. The incorporation depths of 2H are

very large compared to those in GaN or GaAs under similar conditions, where depths of

1-2 |tm are observed [89, 92]. The rapid diffusion of the hydrogen suggests that it

diffuses as an interstitial, with little trapping by the lattice elements or by defects or

impurities. The position of H in the lattice after immobilization has not yet been









determined experimentally, but from theory the lowest energy states for H is at a

bond-centered position forming an O-H bond, while for H2 the anti-bonding Zn site is

most stable [47].

Using a simple estimate of the diffusivity D, from D = X2/4t, and where Xis the

distance at which 2H concentration has fallen to 5 x 1015 cm-3 (Figure 4-4), we can

estimate the activation energy for diffusion from the data shown in Arrhenius form

(Figure 4-5). The extracted activation energy, Ea, is 0.17 + 12 eV for 2H in ZnO. Note

that the absolute diffusivities of 1H would be -40% larger because of the relationship for

diffusivities of isotopes, i.e.

D1H M2H 11/2 (2-2)
S~(2-2)


The small activation energy is consistent with the notion that the atomic hydrogen

diffuses in interstitial form.

Figure 4-6 shows SIMS profiles of a ZnO sample exposed to 2H plasma of 0.5 h at

2000C, then annealed for 5 mins under N2 at different temperatures. There is significant

loss of 2H even after a short anneal at 4000C, with virtually all of it evolved out of the

crystal by 5000C. This is also in sharp contrast to 2H in GaN, where much higher

temperatures (>8000C) are needed to evolve the deuterium out of the sample [89, 92]

To compare this data to the thermal stability of 2H incorporated by direct

implantation, Figure 4-7 shows the percentage of 2H remaining as a function of annealing

temperature for incorporation by either plasma exposure or implantation. The 2H

concentrations were obtained by integrating the area under the curves in the SIMS data.

The 2H is slightly more thermally stable in the latter case, most likely due to trapping at









residual damage in the ZnO carried by the nuclear stopping process. It is evident that the

thermal stability of implanted deuterium is not that high with 12% of the initial dose

retained after 500C anneals and -0.2% after 600C anneals. Lavrov et al. [93] have

identified two hydrogen-related defects in ZnO, by using local vibrational mode

spectroscopy. The H-I center consists of a hydrogen atom at the bond centered site,

while the H-II center contains two inequivalent hydrogen atoms bound primarily to two

oxygen atoms.

Figure 4-8 shows donor concentration profiles in the ZnO before and after plasma

exposure and following subsequent annealing. The 2H plasma treatment causes an

increase in donor concentration, consistent with past reports. In that case, the effect was

attributed to hydrogen passivation of compensating acceptor impurities present in the

as-grown ZnO epitaxial layers [81]. An alternative explanation is that the hydrogen

induces a donor state and thereby increases the free electron concentration [47].

Subsequent annealing reduces the carrier density to slightly below the initial value in the

as-received ZnO, which may indicate that it contained hydrogen as a result of the growth

process. It is important to note that the n-type conductivity probably arises from multiple

impurity sources [3, 22, 94] and cannot be unambiguously assign all of the changes to the

presence of hydrogen.

Figure 4-9 shows the PL spectrum from a plasma treated sample as a function of

measurement temperature. The sample shows strong band-edge luminescence and a

small deep-level band (-2.6 eV). Past reports have shown that the efficiency of band-

edge emission was increased by plasma hydrogenation of various types of ZnO [82];

however, the degree of improvement depended on the impurity and defect concentration









in the original samples [82, 84]. No significant difference in the intensity or shape of the

PL spectra as a result of plasma hydrogenation of our samples was observed in these

experiments.

More detail on the measurement temperature dependence of the band-edge and

deep-level emissions from the plasma deuterated ZnO (Figure 4-10). As expected and as

reported previously [84], the bandedge intensity increases significantly as the temperature

is lowered and the deep level emission is quenched. The overall intensity of the plasma

treated ZnO remains much higher than the material hydrogenated by direct implantation

of protons or deuterons. Figure 4-11 shows 300K PL spectra from ZnO after 2H

implantation at a dose of 1015 cm-3, followed by annealing at different temperatures. The

implantation step severely degrades the band edge intensity, and even annealing at 7000C

where all of the 2H has been evolved from the ZnO, the intensity remains about 2 orders

of magnitude lower than in the unimplanted material.

Summary

Hydrogen is found to exhibit a very rapid diffusion in ZnO when incorporated by

plasma exposure, with D of 8.7 x 10-10 cm2/Vs at 3000C. The low activation energy for

diffusion is indicative of interstitial motion. All of the plasma-incorporated hydrogen is

removed from the ZnO by annealing at > 5000C. When the hydrogen is incorporated by

direct implantation, the thermal stability is somewhat higher, due to trapping at residual

damage. Optical degradation is more severe when the hydrogen is incorporated by

implantation then by plasma exposure. The photoluminescence intensity does not

completely recover even after annealing to the point where all the hydrogen is essentially

removed. The free electron concentration increases after plasma hydrogenation,






46


consistent with the small ionization energy predicted for H in ZnO [47] and the

experimentally measured energy of 60 + 10 meV for muonium in ZnO [48]. The

electrical activity and rapid diffusivity of H or ZnO must be taken into account when

designing device fabrication processes such as deposition of dielectrics using SiH4 as a

precursor or dry etching involving use of CH4/H2/Ar plasmas since these could lead to

significant changes in near-surface conductivity.




























0.5 1.0 1.5 2.0
Depth (ptm)


SSIMS profiles of 2H implanted into ZnO (100 keV, 1015 cm-2) before and
after annealing at different temperatures (5 min anneals).


Depth (A)
10000 7500 5000 2500


0.6 0.8 1.0 1.2
Energy (MeV)


1.4 1.6


Figure 4-2. RBS spectra of bulk, single-crystal ZnO before and after 100 keV H
implantation to a dose of 1016 cm2.


Figure 4-1






600


)















Control















2.0 2.5 3.0 3.5
Energy (eV)


Figure 4-3. PL spectra at 300K of ZnO implanted with 2H+ ions (100 keV, 1015 cm2) as a
function of post-implanted annealing temperature (5 min anneals).



1E18 -
H plasma treatment


1E17



1E16



1E15
1000C 2000C 3000C


0 5 10 15 20 25 30
Depth (pm)


Figure 4-4. SIMS profiles of 2H in ZnO exposed to deuterium plasmas for 0.5 h at
different temperatures.












1E-9







1E-10


1E-11 I I 1 I I
1.5 2.0 2.5 3.0
1000/T (K)


Figure 4-5. Arrhenius plot of diffusivity for 2H in ZnO.



1E18
2H plasma 200C
No anneal Post treatment anneal

1E17













Depth (iLm)
Figure 4-6. SIMS profiles ofH in ZnO exposed to deuterium plasma for 0.5 h at 2005m


and then annealed at 400C or 500C for 5 minds.
10 15 20 25
Depth (gm)


Figure 4-6. SIMS profiles of 21H in ZnO exposed to deuterium plasma for 0.5 h at 200'C
and then annealed at 400'C or 500'C for 5 mins.


2H plasma exposure
in bulk ZnO






















60


20


0 100 200 300 400 500
Anneal Temperature (C)


600 700


Figure 4-7. Percentage of retained 2H implanted into ZnO (100 keV, 1015 cm-2) as a
function of annealing temperature (5 min anneals). The inset shows the data
on a log scale.


4x101





S3x101
04




S2x10





1xl01


Depth (pm)


Figure 4-8. Donor concentration profiles in ZnO before and after plasma exposure and
after subsequent annealing.


100

0E \
S10
2\

I -

0 200 400 600
Anneal Temperature (C)



100 keV 2H ZnO
5 min anneals \
-- --


-- 2H Plasma 2000C + 6000C Anneal
-- H Plasma 200C
As Grown *



S* *









o o *.O"OO0" *






51



101


50 K
10
'--- 200 K
-- 300 K
10-1



10-2



10-3
2.5 3.0 3.5

Energy (eV)


Figure 4-9. PL spectra from 2H plasma exposed ZnO.














0.01


2.4 2.5 2.6 2.7
Energy (eV)


1





0.1





0.01


3.15 3.20 3.25 3.30 3.35 3.40 3.45
Energy (eV)


Figure 4-10. Detailed band edge and deep level emission PL spectra from 2H plasma
exposed ZnO.










0.014
600 0C 7000C Anneal

0.012 6000C Anneal
S-- 5000C Anneal
--No Anneal
;: 0.010 -

S700 C
S 0.008 -


S 0.006
500 OC
0.004 -
No anneal
1.5 2.0 2.5 3.0 3.5

Energy (eV)


Figure 4-11. 300K PL spectra from 2H implanted ZnO, as a function of subsequent
anneal temperature.














CHAPTER 5
OHMIC CONTACTS TO ZnO

N-type ZnO

Introduction

Ohmic contacts are essential to connect electronic devices to allow the current to

flow in and out. For ohmic contact, the ideal contact resistance is minimal and negligibly

small, in which current and voltage relationship is linear and obeys ohm's law. In order

to attain devices with acceptable characteristics, high quality ohmic contacts with low

specific contact resistance are imperative.

The achievement of acceptable device characteristics relies heavily on developing

low specific contact resistance ohmic metallization schemes. Table 5-1 summarizes the

results of reported by other groups [95-116]. A more detailed table is located in

Appendix A. Various metallization schemes and results for ohmic contact to n-ZnO have

been reported. Specific contact resistance of 3 x 10-4 Qfcm2 was reported for Pt-Ga

contacts [98, 99]. Ti/Au ohmic contacts on Al-doped epitaxial layers resulted in specific

contact resistances of 2 x 10-4 Q cm2 after 300 C anneal [100, 101] and 4.3 x 10-5 Qfcm2

with plasma surface treatment prior to metal deposition [102]. Non-alloyed Al on

epitaxial n-type ZnO yielded specific contact resistance of 2.5 x 10-5 Qfcm2 [114]. The

specific contact resistance of Ta/Au on n-ZnO epitaxial layer was 4.3 x 10-6 Qfcm2 [115].

For Al/Pt ohmic contacts to Al-doped ZnO (n 8 x 1018 cm-3), the specific contact

resistances of nonalloyed ohmic contacts and after 300 C anneal were 1.2 x 10-5 Q-cm2









[106] and 2 x 10-6 Qfcm2 [105], respectively. Lowest specific contact resistance of 9.0 x

10-7 Qfcm2 was reported for Ti/Al on n+- ZnO with n 1.7 x 1018 cm-3 after 300 C

anneal [116].

The usual approaches involve surface cleaning to reduce barrier height or increase

the effective carrier concentration of the surface through preferential ion of oxygen.

Minimum contact resistance generally occurs for post-deposition annealing temperatures

of 200 C to 300 OC on doped samples treated so as to further increase the near-surface

carrier concentration.

In this section, the annealing temperature dependence of contact resistance and

morphology for Ti/Al/Pt/Au contacts on high quality, bulk ZnO substrates are reported.

Two different surface cleaning procedures were employed, although it was found that in

general the as-received surface produced the lowest specific contact resistances. In

addition, the effects carrier concentration on resistance of Ti/Al/Pt/Au contacts to n-type

ZnO thin film were also investigated

Previous reports have shown that Au, Ni, Pt and Ni/Au metallurgy can be used as

ohmic metallization on thin films of p-type ZnO or ZnMgO [109, 111]. The lowest

reported specific contact resistance of 1.7 x 10-4 Q*cm2 after annealing at 6000C in air

was achieved for Ni/Au on P-doped ZnO produced by sputtering [109]. More work is

needed to establish the contact formation mechanism as well as the intermixing and

morphology of the contact metallurgy during the anneal process. A study of Au and

Au/Ni/Au contacts to p-type ZnMgO grown by pulsed laser deposition is presented.









Bulk ZnO

Experimental methods

Bulk, wurtzite (0001) crystals obtained from Cermet, Inc. were used for all

experiments. The samples were nominally undoped (n 1017 cm-3) and the Zn-

terminated surface was used in all cases. A circular transmission line method (C-TLM)

pattern was created by lift-off of e-beam deposited Ti /Al / Pt /Au (200/400/200/800 A)

on the front surface of the samples. Three different types of surface preparation wee used

prior to the metal deposition, namely the as-received condition, sequential cleaning in

acetone, tricholorethylene, and methanol (3 minutes in each solvent), and finally, a one-

minute exposure to an inductively-coupled plasma of H2 (5 mTorr, 100 W rf chuck

power, 300 W surface power) in a PlasmaTherm 790 reactor (St. Petersburg, FL). The

latter treatment is predicted to increase the near-surface doping through introduction of

hydrogen donors. A schematic diagram and micrographs of a completed sample are

shown in Figure 5-1. The contact pad spacing varied from 5 to 45 |jm. Samples were

annealed at temperatures up to 6000C for 1 min under flowing N2. The specific contact

resistance, pc, was derived from the circular TLM based on measurements on Agilent

4156 Precision Semiconductor Parameter Analyzer (Palo Alto, CA), while the inter-

diffusion of the metal layers was examined by AES on a Physical Electronics 660

Scanning Auger Microprobe (Chanhassen, MN) with a 10 keV, 1 pA beam at 300 from

the sample normal. Depth profiling during AES analysis was achieved by sputtering with

a 3 keV Ar ion beam at a current of 2 pA rastered over a 3 mm2 area. The sputter rate for

the metals was in the range of 90 to 200 A / minute. Optical and secondary electron

images of the analysis areas were also obtained.









Results and discussion

The specific contact resistance, Pc, was obtained form the circular TLM

measurements with the relationships

S= s Rlnf I LT o0(Ro L LT Ko (R ILT) ( -
2r Ro[ Ro I, (RoIL) R, K,(R, / L,T)


Pc = RsL (5-2)

where RT is the total resistance, Rs is the sheet resistance, R1 is the outer radius of the

annular gap, Ro is the inner radius of the annular gap, Io, I1, Ko, and K1 are the modified

Bessel functions, LT is the transfer length, and Pc is the specific contact resistance [117].

The resulting data is shown as a function of annealing temperature (Figure 5-2). The

minimum pc values were obtained after 2500C anneals, with the as-deposited samples

exhibiting non-ohmic behavior. The pc values for anneals at 3500C and then slightly

decrease before again showing non-ohmic characteristics after higher temperature

(6000C) processing.

Figure 5-3 shows a secondary electron image of the as-deposited contact on ZnO,

along with the AES depth profile. The surface morphology is excellent and there is no

intermixing of the individual metal layers. The metal morphology started to roughen

after 3500C anneals as shown in the secondary electron images (Figure 5-4). As will be

seen later from the AES data, the Al diffuses out towards the surface and begins to

oxidize, while the Au diffuses in through the Pt layer. For Ti/Au contacts on ZnO, Kim

et al. [101] found formation of TiO2 (srilankite and rutile) phase and TiO at the

semiconductor-metal interface even in as-deposited samples. After annealing at 3000C,

the reaction of Ti and O was more complete while the Au began to form Ti-Au phases









such as TiAu2 and Ti3Au by 5000C. The surface morphology of the contacts in our work

does not degrade significantly above 3500C, even though the intermetallic reactions are

more prevalent.

AES surface scans from samples annealed at 2500C, 3500C, 4500C and 6000C are

shown in Figure 5-5. Adventitious carbon is present on all samples, but Al is not

detected on the surface until the 3500C anneal. The amount of Al on the surface

increases with increasing anneal temperature, and the presence of Al on the surface is

likely to be accompanied by the associated oxidation of the metal. After 6000C anneals,

even Pt is detected on the surface, confirming the reacted nature of the contact after this

treatment.

The associated AES elemental depth profiles are shown in Figure 5-6. The 250C

annealed sample shows some interfacial reaction of Ti with the ZnO to form Ti-O phases,

as reported previously [100, 101] and some interaction between Pt and Al, in-diffusion of

Au and more significant formation of Ti-O phases at the semiconductor-metal interface.

The dissociation of the ZnO and the further intermixing of the metal increases at higher

temperatures leading to the completely intermixing contract after annealing at 6000C.

Summary

A minimum specific contact resistance of 6 x 10-4 Qfcm2 was obtained for

Ti/Al/Pt/Au ohmic contacts on undoped (n 1017 cm-3) bulk ZnO substrates. The

contacts do not show ohmic behavior in the as-deposited state and reached their

minimum resistance after 2500C annealing. Higher processing temperatures led to a

degraded contact resistance and reaction of the metal layers with each other and of Ti

reaction with the ZnO. Solvent cleaning or H2 plasma exposure did not improve contact









resistance in our case relative to the as-received state. Future work should focus on the

doping dependence of specific contact resistance and on finding more thermally stable

contact metallurgies for ZnO.

Thin Film n-ZnO

Experimental methods

The phosphorus-doped ZnO epitaxial films in this study were grown by pulsed

laser deposition (PLD) on single crystal (0001) A1203 substrate, using a ZnO: Po.02 target

and a KrF excimer laser ablation source. The laser repetition rate and laser pulse energy

density were 1 Hz and 3 J-cm-2, respectively. The films were grown at 4000C in an

oxygen pressure of 20 mTorr. The samples were annealed in the PLD chamber at

temperatures ranging from 425 to 6000C in 02 ambient (100 mTorr) for 60 min. The

resulting film thickness ranged from 350 nm to 500 nm. Four-point van der Pauw Hall

measurements were performed. Figure 5-7 shows the carrier mobility and the resistivity

of the films after post-growth anneal at various temperatures. The conductivity of the

ZnO films was n-type in all cases. Increasing post-growth anneal temperature decreases

the carrier mobility and increases the resisitivity.

Transmission line method (TLM) patterns, consisting of 100 |tm2 contact pads and

gap spacings varying from 5 to 80 itm, were created by dry etching of mesa and lifting

off of e-beam evaporated metals. The samples were etched in an Unaxis 790 reactor in a

CH4/Ar/H2 plasma with gas flows of 3, 5, and 8 sccm, respectively, under 5 mTorr

pressure, 200 W rf power, and 500 W ICP power for 3 min. The samples were then

deposited with Ti/Al/Pt/Au (200/400/200/800 A) by e-beam evaporation. After metal

deposition, the samples were annealed at 2000C for 1 min in N2 ambient. Schematic









diagram and secondary electron image of the ohmic contact pads on a ZnO mesa are

shown in Figure 5-8.

The specific contact resistance, pc, was derived from the TLM-based measurements

on Agilent 4156C Precision Semiconductor Parameter. The measurement temperature

was regulated with a Wentworth Labs Tempchuck TC-100 (Brookfield, CT), ranging

from 300C to 2000C. The inter-diffusion of the metal layers was examined by AES on a

Physical Electronics 660 Scanning Auger Microprobe with a 10 keV, 1 ptA beam at 30

from the sample normal. Depth profiling during AES analysis was achieved by

sputtering with a 3 keV Ar ion beam at a current of 2 ptA rastered over a 3 mm2 area.

The sputter rate for the metals was in the range of 90 to 200 A / minute.

Results and discussion

Current and voltage information obtained from electrical measurements are curve

fitted with the corresponding equations to determine the specific contact resistance. For

linear TLM, the total resistance, Rs, and specific contact resistance, pc, are given by


RT =2Rc +RsL (5-3)



PC =- (5-4)
Rs



where Rc is the contact resistance, Rs is the sheet resistance, L is the distance between

two pads, W is the width of the pad. The carrier concentration in the ZnO films and the

specific contact resistance of as-deposited contacts on these films, measured at room

temperature as a function of post-growth anneal temperatures (Figure 5-9). The carrier









concentration ranged from 7.5 x 1015 cm-3 after 6000C post-growth anneal to

1.5 x 1020 cm-3 for the unannealed sample. The as-deposited ohmic contacts with the

lowest specific contact resistance (8.7 x 10-7 Qfcm2) were obtained for the sample with

the highest carrier concentration, as expected.

Figure 5-10 compares the specific contact resistance of the Ti/Al/Pt/Au contacts

both before and after annealing at 2000C, for measurement temperatures up to 480K. At

the lower doping levels, there is little variation in contact resistance versus measurement

temperature in the range we measured. In addition, in most cases, the 2000C anneal did

not improve contact resistance significantly. The lowest p, was achieved for the 200C

annealed contact on the n=2.4 x 1018 cm-3 ZnO subsequently measured at 2000C.

The data as a function of both carrier concentration in the ZnO, before and after

2000C annealing and for measurement temperatures of 30 or 2000C are compiled in

Figure 5-11. The lowest specific contact resistance in the as-deposited unannealedd)

samples was 8.7 x 10.7 Qfcm2 in the ZnO with the highest carrier concentration

1.5 x 1020 cm-3. The lowest specific contact resistances in the 2000C annealed samples

were 3.9 x 10-7 Qfcm2 and 2.2 x 10-8 Qfcm2 obtained in samples with carrier

concentrations of 6.0 x 1019 cm-3 measured at 300C and 2.4 x 1018 cm-3 measured at

2000C, respectively.

The low specific contact resistance in the highest carrier concentration sample

(1.5 x 1020 cm-3) may be explained by the tunneling process, which is given by


Pc exp [E OB 1 (5-5)
^- h JDw









where q is the electronic charge, 4Bn the barrier height, as the semiconductor permittivity,

m* the effective mass, h the Planck's constant, and ND the donor density. The strong

influence on doping is attributable to the ND-1/2 term. Another possible transport

mechanism is thermionic emission, in which specific contact resistance is given by


Pc q= exp U (5-6)
qAT kT)

where k is the Boltzmann's constant, A* the Richardson constant, and T the measurement

temperature. The In (pcT) dependence on (1/T) for the thermionic emission is illustrated

in Figure 5-12. For the sample with carrier concentration of 2.4 x 1018 cm-3, 4Bn

extracted from the In (pcT) versus 1/T plot was 0.21 eV before 2000C anneal and 0.29 eV

after this anneal. The energy required for thermionic emission is greater after annealing,

even though a decrease in specific contact resistance is observed.

Finally, Figure 5-13 shows AES depth profiles of the Ti/Al/Pt/Au contact after

annealing at 2000C. The initially sharp interfaces between the different metals are

degraded by reactions occurring, especially between the Ti and the ZnO to form Ti-O

phases and between the Pt and Al. The O appears to diffuse outward while the Pt diffuses

inward. The as-deposited contacts showed very sharp interfaces between the different

metals and between the Ti and the ZnO. Anneals at 6000C almost completely intermixed

the contact metallurgy. These results are similar to the same contact metallization on

bulk ZnO as discussed above.

Low thermal stability of both ohmic and Schottky contacts on ZnO appears to be a

significant problem in this materials system [118], and there is an apparent need to

investigate refractory metals with better thermal properties if applications such as high









temperature electronics or lasers operating at high current densities are to be realized.

The relatively low thermal stability of these metals on ZnO is in sharp contrast to the

stability of the same metal system on GaN [119, 120].

Summary

The lowest specific contact resistance of 8.7 x 10-7 Qfcm2 for nonalloyed ohmic

contact was achieved for ZnO with an n-type carrier concentration of 1.5 x 1020 cm3. In

the 2000C annealed samples, the minimum specific contact resistances of

3.9 x 10-7 Qfcm2 and 2.2 x 10-8 Qcm2 were obtained in samples with carrier

concentrations of 6.0 x 1019 cm-3 measured at 300C and 2.4 x 1018 cm-3 measured at

2000C, respectively. The low specific contact resistance in the high carrier concentration

sample may be explained by the tunneling mechanism. For the sample with carrier

concentration of 2.4 x 101 cm-3, 4Bn extracted from the In (pcT) versus 1/T plot based on

the thermionic emission mechanism are 0.21 eV before anneal and 0.29 eV after anneal.

AES revealed Ti-O interfacial reactions and intermixing between Al and Pt layers.

In summary, specific contact resistances in the range 10-7-10-8 Q-cm2 were obtained

for Ti/Al/Pt/Au contacts on heavily n-type ZnO thin films, even in the as-deposited state.

However, the contacts show significant changes in morphology even for low temperature

(2000C) anneals, and this suggests that more thermally stable contacts schemes should be

investigated.

p-ZnMgO Thin Film

Introduction

There is increasing interest in the ZnMgO/ZnO/ZnCdO system for solid-state

lighting applications [121-123]. The larger exciton binding energy relative to GaN,









commercial availability of large area ZnO substrates, ability to grow at lower epitaxial

growth temperatures and availability of wet etchants suggest that there may be

advantages relative to the nitrides in some of these applications. Most of the recent

interest has derived from advances in p-type doping [18, 113, 124-143] and

demonstrations of a variety of n-ZnO/p-AlGaN and n-ZnO/p-ZnO/ScMgAl04

electroluminescent devices have followed [111, 144-148]. Substantial improvement is

necessary to establish robust p-type doping, which often exhibits very low mobilities and

poor optical properties [124, 125]. Recent reports also show it may revert to n-type

conductivity over a few days at room temperature [147]. In addition to achieving stable

and high hole concentrations, work is also needed to develop low resistance p-ohmic

contacts. Previous reports have shown that Au, Ni, Pt and Ni/Au metallurgy can be used

as ohmic metallization on thin films of p-type ZnO or ZnMgO [111, 148]. The lowest

reported specific contact resistance of 1.7 x 10-4 Q.cm2 after annealing at 6000C in air

was achieved for Ni/Au on P-doped ZnO produced by sputtering [148].

More work is also needed to establish the contact formation mechanism as well as

the intermixing and morphology of the contact metallurgy during the anneal process. In

this section, we report on a study of Au and Au/Ni/Au contacts to p-type ZnMgO grown

by pulsed laser deposition. In both cases, the as-deposited contacts are rectifying and the

transition to ohmic behavior is associated with out-diffusion of Zn from the ZnMgO.

Experimental Methods

The phosphorus-doped (Zno.9Mgo.1)O epitaxial films were grown by pulsed laser

deposition (PLD) on c-plane sapphire substrates. The target was fabricated using

high-purity ZnO (99.9995%) and MgO (99.998%), mixed with P205 (99.998%) as the









doping agent. The phosphorus doping level in the target was 2 at.%. The addition of Mg

shifts the conduction band edge to higher energy, reducing the n-type background carrier

concentration. The phosphorus introduces an acceptor level at -0.25 eV from the valence

band [146]. The ZnO growth chamber base pressure was 4x 10-6 Torr. An undoped ZnO

buffer layer (-50 nm) was initially deposited at 4000C and 20 mTorr oxygen partial

pressure before the growth of P-doped (Zno.9Mgo.i)O films at a substrate temperature of

5000C under oxygen partial pressure of 150 mTorr. The total film thickness was 400 to

600 nm. Hall measurements showed a 250C hole concentration of 2.7 x 1016 cm3, a

mobility of 8 cm2 /Vs and a resistivity of 35 Q-cm. The films exhibited good crystallinity

with c-axis orientation.

Contact metallurgy of either Au(1000A) or Au/Ni/Au (200/200/800 A) was

deposited by electron-beam evaporation and patterned by resist lift-off to form a circular

transmission line pattern (C-TLM) with inner-outer ring spacings of 5 to 45 im. The

samples were annealed for 1 min at 6000C in N2 and the current-voltage (I-V)

characteristics recorded on an Agilent 4156 parameter analyzer. AES depth profiling was

performed on a Physical Electronics 660 Scanning Auger Microprobe. The electron

beam conditions were 10keV, 1 IA beam current at 300 from sample normal. For depth

profiling, the ion beam conditions were 3 keV Ar+, 2.0 ptA, 3 mm2 raster, with measured

sputter rate of 110 A/minute. Cross-section transmission electron microscopy (TEM)

was also performed on the annealed samples.

Results and Discussion

The as-deposited contacts showed rectifying behavior for both Au and Au/Ni/Au,

but the I-V characteristics became ohmic for annealing at 6000C (Figure 5-14). Previous









work reported ohmic behavior with specific contact resistance of- 8 x 10-3 Q.cm2 in

as-deposited Ni/Au contacts on p-type ZnO of higher initial hole concentration

(1018 cm-3) than used in this experiment [148]. The contact behavior observed here are

similar to Ni/Au contacts on p-GaN, which also has relatively low hole densities due to

the high ionization energy of the acceptor dopant. The electrical properties of our

contacts to p-ZnMgO before and after 6000C annealing are compiled in Table 5-2. The

specific contact resistance of 7.6x10-6 Q'cm2 is the lowest value reported for any contact

to p-ZnO or ZnMgO. Au alone also produces a low specific contact resistance, indicating

that Ni is not necessary to achieve good ohmic contacts, though it does help produce a

lower contact resistance. Lim et al. [148] reported a value of 1.7 x 10-4 Qcm2 for Ni/Au

on sputtered p-ZnO, where the minimum specific contact resistance was achieved for

6000C anneals in air. In their case, annealing was found to produce NiO and Au3Zn

phases that may play a role in the current conduction mechanism.

Figure 5-15 shows the AES depth profiles from the Au/Ni/Au contacts before and

after annealing. A part of the Ni layer has outdiffused through the Au overlayer even in

the as-deposited case and most likely becomes oxidized on the surface. After annealing

at 6000C, the flat profiles for the Ni and Au concentrations indicate that they have

completely reacted. By sharp contrast, the Au-only contacts show little change in the Au

profile (Figure 5-16) after annealing. In both cases however, there is diffusion of Zn

through the metallization after annealing,as shown by the AES surface scans in Figure

5-17. The formation of Zn vacancies in the ZnMgO would increase the near-surface hole

concentration and improve contact resistance. This has also been suggested by Lim et al.

[144] as a contributing factor in the conduction mechanism in their Ni/Au contacts on









p-ZnO. The fact that Au-only contacts produce low contact resistance suggests that Au

acts to enhance Zn vacancy formation, perhaps through formation of the Au3Zn phases.

When Ni is also present, it may react to form Ni-Zn phases as well as NiO, both of which

may play a role in achieving low contact resistance.

Figure 5-18 shows a cross-section TEM of the annealed Au/Ni/Au contact on

p-ZnMgO. The morphology of the contact becomes roughened, consistent with the

intermixing observed in the AES depth profiles. Different grains with dimensions of

order 100 nm are visible in the metal layer. It remains to be seen if this will be an issue in

terms of uniformity of contact properties and current density in device structures such as

light-emitting diodes, though generally, the contact dimensions on such devices are very

large (>100 .im) relative to the grain size.

Summary

Both Au and Au/Ni/Au are found to provide low specific contact resistance on

lightly doped p-ZnMgO after annealing at 6000C. In both cases, the as-deposited

contacts are rectifying and the transition to ohmic behavior is associated with out-

diffusion of Zn from the ZnMgO. A minimum specific contact resistance of

7.6 x 10-6 Q.cm2 was obtained with Au/Ni/Au, which is about a factor of 3 lower than for

pure Au contacts annealed at the same temperature.









Table 5-1. Ohmic contacts ZnO and their respective specific contact resistance from
published works.
Lowest p Re
Description Metal t p, Ref
(Qcm )


(a) n type
n-ZnO bulk
n-ZnO bulk
n-ZnO epi on A1203
n-ZnO epi on A1203
n-ZnO epi on A1203
n-ZnO epi on A1203
n-ZnO epi on A1203
n-ZnO epi on A1203
n-ZnO epi on A1203
n-ZnO epi on A1203
n-ZnO epi on A1203
n-ZnO epi on A1203
n-ZnO epi on A1203
n-ZnO epi on A1203
(b) p type
p-ZnO epi on A1203
p-ZnO epi on A1203
p-ZnMgO epi on glass

p-ZnMgO epi on glass
p-ZnO on SiC


E-beam Ti/Au
E-beam In
Ti/Au
FIB direct write Ga-Pt
E-beam Ti/Au
E-beam Ti/Au
E-beam Re/Ti/Au
E-beam Ti/Al
E-beam Al/Pt
E-beam Ru
E-beam Al
E-beam Al
E-beam Ta/Au
E-beam Ti/Al

E-beam Ti/Au
E-beam Pt/ITO
E-beam Ti/Au or
Ni/Au
E-beam Ni/Au
E-beam In/Au, Ti/Au,
or Ni/Au


Table 5-2. Sheet resistance, transfer length and specific contact resistance of annealed
Au or Au/Ni/Au contacts on p-ZnMgO.
Rs (QD/0) Lt (|tm) Re (Qcm-2)
Au (6000C annealed) 7.5 x 103 0.57 2.5 x 10-5
Au/Ni/Au (6000C annealed) 1.7 x 103 0.68 7.6 x 10-6


5x10-5
7x10-1
1.5x10-5
3.1x 104
2x10-4
4.3x10-5
1.7x10-7
9.0x10-7
2x10-6
2.1x 10-5
8x10-4
2.5x10-5
5.4x10-6
9x10.7

1.72x10-4
7.7x10-4
3x10-3

4x10-5


95
96
97
98,99
100, 101
102
103
104
105, 106
107
108
114
115
116

109
110
111

112
113








000
000


Cross-sectional view of circular TLM
Ro


Figure 5-1. Schematic diagram (a) and secondary electron image (b) of circular TLM
pattern on ZnO substrate.












1.0x10-2


--- As Received
8.0x10lO --- H2 Plasma
Solvent


6.0x103


4.0x103


2.0x103


250 300 350 400 450


Anneal Temperature (oC)


Figure 5-2. Specific contact resistance as a function of anneal temperature for
Ti/Al/Pt/Au contacts on n-ZnO. Solvent chemical cleaning or H2 plasma
exposure of the surface prior to metallization was compared with the case of
depositing the metal on the as-received surface.
























4/21/03 3.OkV 40.0 X 5
7 control wafer


100
Au Pt
80
I \Ti

6 60


= 40

o 20


0 1000 2000 3000 4000
Depth (A)
(b)

Figure 5-3. Secondary electron image (a) and AES depth profile (b) of as-deposited
Ti/Al/Pt/Au contact on ZnO.


(a)






























Figure 5-4. Secondary electron images of Ti/Al/Pt/Au contacts on ZnO after annealing at
2500C (a), 3500C (b), 4500C (c) or 6000C (d).


4/50 0.k500X 5.jm




































500 1000 1500 2000
Energy (eV)


1

-05




-05

-1

-1 5

-2


500 1000 1500 2000
Energy (eV)


o (d)




Al
C AI Au
C Zn A Pt Pt Pt

Atomic %
0 40.7
Al 3.2
C 10.8
Au 9.6
Pt 6.5
Zn 1.1


500 1000 1500
Energy (eV)


2000


Figure 5-5. AES surface scans of Ti/Al/Pt/Au contacts on ZnO after annealing at 2500C

(a), 3500C (b), 4500C (c) or 6000C (d).


Energy (eV)


05


0











(b) 3500C


0 1000 2000
Depth (A)


3000 4000


(c) 4500C


0 1000



(d) 6000C


2000
Depth (A)


3000 4000


0 1000


2000 3000
Depth (A)


4000


0 1000


2000 3000
Depth (A)


Figure 5-6. AES depth profiles of Ti/Al/Pt/Au contacts on ZnO after annealing at 2500C
(a), 3500C (b), 4500C (c) or 6000C (d).


4000


(a) 250C
















II I 1V


102
15
S101


10 -- Mobility 100 0
= --*- Resistivity

10-
; 5
10-2

010-3
I l I l I I 1 0 -3
0 100 200 300 400 500 600

Post-growth Anneal Temperature (oC)


Figure 5-7. Carrier mobility and resistivity of epi-ZnO as a function of post-growth
anneal temperature.







Metal Pads
/ \


Semiconductor film


Figure 5-8. Schematic diagram (a), SEM (b) and microsope image (c) of the linear TLM
ohmic contact pads on ZnO mesa.


"I


4;;,

















1020


1019


1018


1016


0A A




-- -Carrier Concentration
-A- p measured at 30 oC




A-





0 100 200 300 400 500 600

Post-growth Anneal Temperature (oC)


10-3



4
10-








10 P
ft
!X


Figure 5-9. Carrier concentration of epi-ZnO and specific contact resistance of as-
deposited ohmic contacts measured at 300C versus post-growth anneal
temperature.


10-4



10o5


106



10-
l -6







10
280
10-8
280


V V V


S10 3
3.-- -2 x -- cm-3
3.2 x 1017 cm'3


7.5 x 10 cm-3


1.5 x 1020 cm3


6.0 x 109 cm


doste 20 Anneal
s-deposited 200C Anneal


S 0-
A- -A-
-- -o-
A A
V V


2.4x 1018 cm-3


320 360 400 440 480


Measurement Temperature (K)


Figure 5-10. Specific contact resistance as a function of measurement temperature for
samples with various carrier concentrations. The solid symbols represent
measurements prior to ohmic contact anneal. The corresponding open
symbols denote measurements after 2000C, 1 min anneal in N2 ambient.


~ ~


/


As
















10 -
? 10-4


10


S06 As-deposited
S-*-2000C anneal,
0 measured at 30 C
107 -A 2000C anneal,
measured at 2000C


1015 10 1 0 10 1018 1 10 1020 1021

Carrier Concentration (cm 3)


Figure 5-11. Specific contact resistance versus measurement temperature of as-deposited
ohmic contact measured at 300C, and after annealing at 2000C, 1 min
measured at 300C and 2000C.


2.2 2.4 2.6 2.8
1000/T (1/K)


3.0 3.2 3.4


Figure 5-12. In(pcT) versus 1000/T for samples with various carrier concentrations. The
solid symbols represent measurements prior to ohmic contact anneal. The
corresponding open symbols denote measurements after 2000C, 1 min anneal
in N2 ambient.


0


-2
-2 -


-4


E -6


-8


-10


-12
2.0







4000

2000

0

-2000

-4000


-60001 ,
0 500


100


1000 1500
Kinetic Energy (eV)


(a)
2000 2500


1000 2000
Sputter Depth (A)


3000


(b)
Figure 5-13. AES surface scans (a) and surface scans (b) of Ti/Al/Pt/Au ohmic contacts
to ZnO after annealing at 2000C.












Au/Ni/Au/Zno.Mgo.O:Po.o


-5 -4 -3 -2 -1 0 1 2 3 4 5
Bias (V)


Au/Zno.9Mg.1O:Po.o


600 C Annealed




As-deposited
-/


-5 -4 -3 -2 -1 0 1 2 3 4 5
Bias (V)


Figure 5-14. I-V characteristics from Au/Ni/Au (a) or Au (b) contacts on p-type ZnMgO
before and after annealing at 6000C.


0.02



S 0.00



-0.02


0.04



0.02



S0.00



-0.02


-n 04






81




Au/Ni/Au/Zn .9Mg O::P o2 As-deposited


0 500 1000 1500 2000 2500
Sputter Depth (A)



Au/Ni/Au/Zn .9Mgo.O:Po.2 6000C Annealed


0 500 1000 1500
Sputter Depth (A)


2000 2500


(b)

Figure 5-15. AES depth profiles from Au/Ni/Au contacts on p-type ZnMgO before (a)
and after (b) annealing at 6000C.












100


a 80
80


60

40
| 40
o
-,e


0 500 1000 1500 2000 2500 3000 3500
Sputter Depth (A)


Au/Zn.9MgoO:Po02 6000C Annealed


O



C



0 500 1000 1500 2000 2500 3000 3500
Sputter Depth (A)


Figure 5-16. AES depth profiles from Au contacts on p-type ZnMgO before (a) and after
(b) annealing at 6000C.