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Synthesis and Characterization of Silver Doped Zinc Oxide Thin Films for Optoelectronic Devices

Permanent Link: http://ufdc.ufl.edu/UFE0041602/00001

Material Information

Title: Synthesis and Characterization of Silver Doped Zinc Oxide Thin Films for Optoelectronic Devices
Physical Description: 1 online resource (102 p.)
Language: english
Creator: Lugo, Fernando
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: devices, doping, epitaxial, film, silver, thin, zno
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: SYNTHESIS AND CHARACTERIZATION OF SILVER DOPED ZINC OXIDE THIN FILMS FOR OPTOELECTRONIC DEVICES The synthesis and properties of Ag-doped ZnO thin films were examined. Epitaxial films of 0.6 at.% Ag doped ZnO grown at moderately low temperatures (300 masculine ordinalC to 500 masculine ordinalC) by pulsed laser deposition yielded p-type material as determined by room temperature Hall measurements. Carrier (hole) concentrations ranging on the order mid-1015 cm-3 to mid-1019 cm-3 were realized. Growth at higher temperatures yielded n-type material, suggesting that the Ag substitution yielding an acceptor state is metastable. Photoluminescence measurements showed strong near-band edge emission with little to no mid-gap emission. The stability of the Ag-doped films was examined as well. Persistent photoconductivity was observed. ZnO buffer layers drastically improved the surface morphology of films thicker than 1.0 micron. Photoluminescence studies showed that Ag inclusion resulted in smaller non-radiative relaxation rates over surface states, which lead to UV emission enhancement. Room temperature PL measurements also showed a suppression of ZnO visible luminescence suggesting that Ag does not occupy interstitial sites or an antisite. Low temperature and temperature dependent PL spectroscopy revealed strong and dominant emissions originating from free electron recombination to Ag-related acceptor states around 3.31eV. The Amasculine ordinalX emission at 3.352 eV was also observed at low temperatures. Enhancement of the PL intensity with increasing grain size was observed. The nature of the acceptor related emissions was confirmed. The acceptor energy was estimated to be 124 meV. Weak deep level emission at low temperatures indicated that in the p-type ZnO:Ag native donor and acceptor defects are suppressed suggesting the observed acceptor related PL emissions and hole concentration are from the Ag in ZnO instead of native defects. High temperature ZnO buffers and lattice matched MgCaO buffers helped improve the UV emission of the Ag doped films. The room temperature PL spectrum of Ag-doped ZnO was compared to that of undoped, P-doped, Ga-doped, and Ag-Ga- codoped ZnO. The Ag-doped ZnO films showed superior optical properties. Finally, the fabrication and properties of rectifying Ag-doped ZnO/Ga-doped ZnO thin film junctions were reported. A rectifying behavior was observed in the I-V characteristic, consistent with Ag-doped ZnO being p-type and forming a p-n junction. The turn on voltage of the device was 3.0 V under forward bias. The reverse bias breakdown voltage was approximately 5.5 V. The highest light emission output power measured was 5.2x10-8 mW. At excitation currents of 10 mA, the applied voltage was approximately 2.0 V. After each measurement the light intensity decreased and the junction became Ohmic. The instability appears to be related to surface conduction and perhaps hydrogen incorporation. Finally, deposition of layers in reversed order (Ag-doped ZnO on bottom, Ga-doped ZnO on top) did not result in rectifying I-V characteristics. The reason for this is unclear but may relate to the differing growth temperatures used for the two layers.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Fernando Lugo.
Thesis: Thesis (Ph.D.)--University of Florida, 2010.
Local: Adviser: Norton, David P.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0041602:00001

Permanent Link: http://ufdc.ufl.edu/UFE0041602/00001

Material Information

Title: Synthesis and Characterization of Silver Doped Zinc Oxide Thin Films for Optoelectronic Devices
Physical Description: 1 online resource (102 p.)
Language: english
Creator: Lugo, Fernando
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: devices, doping, epitaxial, film, silver, thin, zno
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: SYNTHESIS AND CHARACTERIZATION OF SILVER DOPED ZINC OXIDE THIN FILMS FOR OPTOELECTRONIC DEVICES The synthesis and properties of Ag-doped ZnO thin films were examined. Epitaxial films of 0.6 at.% Ag doped ZnO grown at moderately low temperatures (300 masculine ordinalC to 500 masculine ordinalC) by pulsed laser deposition yielded p-type material as determined by room temperature Hall measurements. Carrier (hole) concentrations ranging on the order mid-1015 cm-3 to mid-1019 cm-3 were realized. Growth at higher temperatures yielded n-type material, suggesting that the Ag substitution yielding an acceptor state is metastable. Photoluminescence measurements showed strong near-band edge emission with little to no mid-gap emission. The stability of the Ag-doped films was examined as well. Persistent photoconductivity was observed. ZnO buffer layers drastically improved the surface morphology of films thicker than 1.0 micron. Photoluminescence studies showed that Ag inclusion resulted in smaller non-radiative relaxation rates over surface states, which lead to UV emission enhancement. Room temperature PL measurements also showed a suppression of ZnO visible luminescence suggesting that Ag does not occupy interstitial sites or an antisite. Low temperature and temperature dependent PL spectroscopy revealed strong and dominant emissions originating from free electron recombination to Ag-related acceptor states around 3.31eV. The Amasculine ordinalX emission at 3.352 eV was also observed at low temperatures. Enhancement of the PL intensity with increasing grain size was observed. The nature of the acceptor related emissions was confirmed. The acceptor energy was estimated to be 124 meV. Weak deep level emission at low temperatures indicated that in the p-type ZnO:Ag native donor and acceptor defects are suppressed suggesting the observed acceptor related PL emissions and hole concentration are from the Ag in ZnO instead of native defects. High temperature ZnO buffers and lattice matched MgCaO buffers helped improve the UV emission of the Ag doped films. The room temperature PL spectrum of Ag-doped ZnO was compared to that of undoped, P-doped, Ga-doped, and Ag-Ga- codoped ZnO. The Ag-doped ZnO films showed superior optical properties. Finally, the fabrication and properties of rectifying Ag-doped ZnO/Ga-doped ZnO thin film junctions were reported. A rectifying behavior was observed in the I-V characteristic, consistent with Ag-doped ZnO being p-type and forming a p-n junction. The turn on voltage of the device was 3.0 V under forward bias. The reverse bias breakdown voltage was approximately 5.5 V. The highest light emission output power measured was 5.2x10-8 mW. At excitation currents of 10 mA, the applied voltage was approximately 2.0 V. After each measurement the light intensity decreased and the junction became Ohmic. The instability appears to be related to surface conduction and perhaps hydrogen incorporation. Finally, deposition of layers in reversed order (Ag-doped ZnO on bottom, Ga-doped ZnO on top) did not result in rectifying I-V characteristics. The reason for this is unclear but may relate to the differing growth temperatures used for the two layers.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Fernando Lugo.
Thesis: Thesis (Ph.D.)--University of Florida, 2010.
Local: Adviser: Norton, David P.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0041602:00001


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1 SYNTHESIS AND CHARACTERIZATION OF SILVER DOPED ZINC OXIDE THIN FILMS FOR OPTOELECTRONIC DEVICES By FERNANDO LUGO A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUI REMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2010

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2 2010 Fernando Lugo

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3 To my future wife and kids

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4 ACKNOWLEDGMENTS I would like to thank to my advisor, Dr David P. Norton, for his guidance patience, and unconditio nal support throughout my undergraduate and graduate school years. I thank every committee member, Dr Stephen J. Pearton, Dr. Fan Ren, and Dr. Franky So for their advice and support on my research. A special thanks to Dr. Brent Gila for countless hours o f labor and advice in setting up photoluminescence equipment, and Dr. Cammy R. Abernathy for trus ting me with her lab equipment I would like to express a more than special thanks to all the group members in Dr. Erie, Dr. Yuanjie Li, Dr. Seemant Rawal. Dr. Li Chia Tien, Dr. Hyun Sik Kim, Dr. Patrick Sadik, Dr. Daniel Leu, Dr. Charlee Cal lender, Ryan Pate Joe Cianfrone, Seon Hoo Kim, and Kyeong Won Kim. It has truly been a pleasure to meet them and share 6 years o f research with every one of them. I am especially grateful to my collaborato Yu Lin Wang. They were kind enough to give some of their time to help me fabricate LED devices. I would also like to thank Ritesh Das, Andrew Gerge r, Galileo Sarasqueta and Sergey Maslov for their unconditional help with device fabrication and characterizations. This project would have been impossible to finish without their help. I must ackn owledge the Board of Eductation the Naval Office of Resear ch and Scientific Development, the National Scienc e Foundation and the College of Engineering at The University of Florida for their financial support and fellowships. My deepest gratitude goes to my roommates and ultimate club teammates. They provided me and still continue to provide their unconditional friendships and support over many years of training and competition They gave me the joy and privilege to compete and represent the University of Florida at the highest stage in ultimate.

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5 Finally, I would like to express my deepest appreciation and love to my parents brother and sister for their infinite love and support Simply, there are no words to express my undying admiration.

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6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 8 LIST OF FIGURES ................................ ................................ ................................ .......... 9 ABSTRACT ................................ ................................ ................................ ................... 12 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 15 2 LITERATURE REVIEW ................................ ................................ .......................... 18 2.1 General Properties of ZnO ................................ ................................ ................ 18 2.2 Doping of ZnO ................................ ................................ ................................ .. 18 2.2.1 Undoped ZnO and Its Native D efects ................................ ...................... 19 2.2.2 N type Doping ................................ ................................ ......................... 20 2.2.3 P type Doping ................................ ................................ .......................... 20 Nitrogen Doping ................................ ................................ ......................... 21 Other group V Dopants ................................ ................................ .............. 22 Silver Doping ................................ ................................ .............................. 23 2.3 ZnO Band Gap Engineering and Devices ................................ ......................... 24 2.3.1 Bandgap Engineering ................................ ................................ .............. 24 2.3.2 ZnO Devices ................................ ................................ ............................ 25 ZnO homojunction devices ................................ ................................ ......... 25 ZnO heterostructure devices ................................ ................................ ...... 26 ZnO based thin film t ransistors ................................ ................................ .. 27 3 MATERIALS AND CHARACTERIZATION TECHNIQUES ................................ ..... 35 3.1 Thin Film Synthesis ................................ ................................ ........................... 35 3.1.1 Pulsed Laser Deposition (PLD) ................................ ............................... 35 3.1.2 Target and Substrate P reparation ................................ ........................... 36 3.2 Characterization Techniques ................................ ................................ ............ 36 3.2.1 Hall Effect Measurement ................................ ................................ ......... 36 3.2.2 Photoluminescence (PL) ................................ ................................ ......... 37 3.2.3 X Ray Diffraction (XRD) ................................ ................................ .......... 38 3.2.4 Atomic Force Microscopy (AFM) ................................ ............................. 39 4 SILVER DOPED ZnO FILMS GROWN VIA PULSED LASER DEPOSITION ......... 42 4.1 Introduction ................................ ................................ ................................ ....... 42

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7 4.2 Experimental Procedures ................................ ................................ .................. 45 4.3 Results and Discussions ................................ ................................ ................... 45 4.3.1 Role of Ag Doping in ZnO Crystal Quality and Surface Mo rphology ....... 45 4.3.2 P Type Conductivity, Transport and Optical Stability of Ag Doped ZnO Films ................................ ................................ ................................ .............. 47 4.4 Summary ................................ ................................ ................................ .......... 50 5 P HOTOLUMINESCENCE STUDY OF ZnO ................................ ............................ 62 5.1 Introduction ................................ ................................ ................................ ....... 62 5.2 Experimental ................................ ................................ ................................ ..... 63 5.3 Results and Discussions ................................ ................................ ................... 64 5.3.1 PL Enhancement Via Ag I nclusion ................................ .......................... 64 5.3.2 Low Temperatur e and Temperature D ependent PL ................................ 65 5.3.3 Buffer Layer and Dopant Effect on the Optical P roperties of ZnO ........... 67 5.4 Summary ................................ ................................ ................................ .......... 68 6 Z INC OXIDE DEVICES ................................ ................................ ........................... 76 6.1 Introduction ................................ ................................ ................................ ....... 76 6.2 Experimental ................................ ................................ ................................ ..... 76 6.2.1 Silver Doped ZnO / Gallium Doped ZnO Thin Film Homojunction ........... 77 6.2.2 Silver Doped ZnO Thin Film Transistor (TFT) ................................ ......... 78 6.3 Results and Discussion ................................ ................................ ..................... 78 6.3.1 Rectifying Thin Film Junction ................................ ................................ ... 78 6.3.2 Silver D oped ZnO TFT ................................ ................................ ............ 80 6.4 Summary ................................ ................................ ................................ .......... 80 7 CONCLUSIONS ................................ ................................ ................................ ..... 89 7.1 P type Silver Doped ZnO Films ................................ ................................ ........ 89 7.2 Silver Related Acceptor State and Optimized Ultraviolet in Silver Doped ZnO Thin Films ................................ ................................ ................................ .... 90 7.3 Rectifying pn Junction and TFT Devices ................................ ........................... 91 LIST OF REFERENCES ................................ ................................ ............................... 93 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 102

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8 LIST OF TABLES Table page 2 1 Calculated defect energy level for group I and V dopants ................................ .. 29 4 1 Hall Data of 0.6 at% Ag doped ZnO films grown at various Temperatures and oxygen partial pressures ................................ ................................ .................... 52 5 1 Growth conditions considered in PL Study ................................ ......................... 70 6 1 Properties of n type and p type materials used in the junction devi ce ................ 82

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9 LIST OF FIGURES Figure page 2 1 Wurtzite structure of ZnO ................................ ................................ .................... 30 2 2 Bandgap and latt ice constant of various semiconductors ................................ ... 30 2 3 The PL spectrum, EL spectrum, and EL image of th e ZnO light emitting device ................................ ................................ ................................ ................ 31 2 4 Zn O p n homojunction a) EL spectrum under forward current injection and b) room tem perature I V characteristic ................................ ................................ .. 31 2 5 EL spectrum of an n ZnO/p GaN heterostructure ................................ .............. 32 2 6 EL spectra of n ZnO/ p Al0.12Ga0.88N heterostruc ture LED at 300 K and 500 K ................................ ................................ ................................ ................. 32 2 7 Room temperature spectral photoresponsivity of the n ZnO/ p SiC photodi ode illuminated both from the ZnO and 6H SiC (inset) sides for various reverse biases ................................ ................................ ....................... 33 2 8 (a) is a set of transistor curves of drain current ( I d ) vs source drain voltage ( V d ) at gate voltage s ( V g ) between 0 and 50 V for a ZnO TFT. The corresponding transfer characteristic, I d vs V g at a fixed Vd equal to 20 V, for the sa me ZnO TFT is shown in (b) ................................ ................................ .... 33 2 9 Electrical characteristics of a two layer gate insulator ZnO TFT prepared with a high carrier concentration ZnO layer: (a) Output characteristics and (b ) transfer characteristics ................................ ................................ ....................... 34 3 1 Pulsed laser deposition (PLD) syste m ................................ ................................ 40 3 2 Hall Effect diagram ................................ ................................ ............................. 40 3 3 Common radiative transition mechanism ................................ ............................ 41 3 4 Block diagram of atomic force microscope ................................ ......................... 41 4 1 Powder XRD pattern for films grown in (a) 25 mTorr for deposition temperature range of 300 600 C, and (b) films grown at 300 C in oxy gen pressures ranging from 1 75 mTorr. ................................ ................................ ... 53 4 2 Effect of Ag doping on ZnO d spacing for films grown at 500 C ........................ 54 4 3 AFM images for f ilms grown at (a)(b) 300C, (c) 400C, and (d) 500C .............. 54

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10 4 4 SZO film average roughness as a function of Ag content grown on different substrates and buffer layers ................................ ................................ ............... 55 4 5 Optical microscope images of the surface of ZnO grown (a) without buffer layer and (b) with HTB layer ................................ ................................ ............... 55 4 6 Plot of Vd/I as a function of magnetic field f or (a) an n type and (b) a p type Ag doped ZnO film. ................................ ................................ ............................ 56 4 7 Resistivity (a) and carrier concentration (b) as a function of growth conditions .. 57 4 8 Resistivity as a function of time for films grown at 300C, P(O 2 ) = 75 mTorr ...... 58 4 9 Effects of (a) UV light exposure and dark storage, showing (b) exponential decay of conductivi ty over time in dark storage for films grown at 300C, P(O 2 ) = 75 mTorr ................................ ................................ ................................ 59 4 10 Room temperature photoluminescence for Ag doped ZnO films grown at various temperatures in 25 mTorr of oxygen show ing (a) large wavelength and (b) narrow wavelength plots. ................................ ................................ ........ 60 4 11 Room temperature absorption spectra for Ag doped ZnO ................................ .. 61 5 1 Room t emperature PL spectra of undoped ZnO and Ag doped ZnO .................. 71 5 2 Band Edge Intensity as a function of grain size ................................ .................. 71 5 3 PL spectra of un doped ZnO measured at 10 K ................................ .................. 72 5 4 PL spectra of Ag doped ZnO (s2) measured at 15 K ................................ ......... 72 5 5 Plot of VdI 1 as a function of magneti c field for p type Ag doped ZnO s2 ......... 73 5 6 Acceptor related peak positions as a function of increasing temperature ........... 73 5 7 PL intensit y grain size relationship for localized bound exciton in Ag doped ZnO ................................ ................................ ................................ .................... 74 5 8 Room temperature UV PL emission of Ag doped ZnO grown on different buffer layers ................................ ................................ ................................ ........ 74 5 9 Room temperature PL spectra for ZnO doped with different elements ............... 75 6 1 Plot of (a) V Hall d/I as a function of applied magnetic field and (b) room temperature p hotoluminescence for a Ag doped ZnO was grown at 500 C in an oxygen pressure of 25 mTorr. ................................ ................................ ........ 83 6 2 The ZnO:Ag/ZnO:Ga/sapphire junction (a) schematic of structure and (b) Test for Ni Au contacts ................................ ................................ ....................... 84

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11 6 3 Homojunction I V characteristics ................................ ................................ ........ 85 6 4 The ZnO:Ga/ZnO:Ag/sapphire junction (a) schematic of structure and (b) junction I V c haracteristic. ................................ ................................ ................... 86 6 5 The emission output power from the ZnO:Ag/ZnO:Ga/sapphire junction as measured using a Si photodiode. ................................ ................................ ....... 87 6 6 Sc hematic of ZnO:Ag thin film transistor ................................ ........................... 87 6 7 ZnO TFT grown on Si output and transfer characteristics ............................... 88

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12 Abstract of Dissertation Presented to the Graduate S chool of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy SYNTHESIS AND CHARACTERIZATION OF SILVER DOPED ZINC OXIDE THIN FILMS FOR OPTOELECTRONIC DEVICES By Fernando Lugo May 201 0 Chair: David P. Norton Major: Material s Science and Engineering The synthesis and properties of Ag doped ZnO thin films were examined. Epitaxial films of 0.6 at.% Ag doped ZnO grown at moderately low temperatures (300 C to 500 C) by pulsed laser dep osition yielded p type material as determined by room temperature Hall measurements. Carrier (h ole ) concentrations ranging on the order mid 10 15 cm 3 to mid 10 19 cm 3 were realized. Growth at higher temperatures yielded n type material, suggesting that t he Ag substitution yielding an acceptor state is metastable. Photoluminescence measurements showed strong near band edge emission with little to no mid gap emission. The stability of the Ag doped films was examined as well. Persistent p hotoconductivity w as observed. ZnO buffer layers drastically improved the surface morphology of films thicker than 1.0 m. Photoluminescence studies showed that Ag inclusion resulted in smaller non radiative relaxation rates over surface states, which lead to UV emission e nhancement. Room temperature PL measurements also showed a suppression of ZnO visible luminescence suggesting that Ag does not occupy interstitial sites or an antisite. Low temperature and temperature dependent PL spectroscopy revealed strong and

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13 dominant emissions originating from free electron recombination to Ag related acceptor states around 3.31eV. The A X emission at 3.352 eV was also observed at low temperatures. Enhancement of the PL intensity with increasing grain size was observed. T he nature of the acceptor related emission s was confirmed The acceptor energy was estimated to be 124 meV. W eak deep level emission at low temperature s indicated that in the p type ZnO:Ag native donor and acceptor defects are suppressed suggesting the observed accepto r related PL emissions and hole concentration are from the Ag in ZnO instead of native defects. High temperature ZnO buffers and lattice match ed MgCaO buffers helped improve the UV emission of the Ag doped fi lms. T he room temperature PL spectrum of Ag dope d ZnO was compared to that of undoped, P doped, Ga doped, and Ag Ga codoped ZnO. The Ag doped ZnO films showed superior optical properties. Finally, the fabrication and properties of rectifying Ag doped ZnO/Ga doped ZnO thin film junctions were reported. A rectifying behavior was observed in the I V characteristic, consistent with Ag doped ZnO being p type and forming a p n junction. The turn on voltage of the device was 3.0 V under forward bias. The reverse bias breakdown voltage was approximately 5.5 V. The highest light emission output power measured was 5.2x10 8 mW. At excitation currents of 10 mA, the applied voltage was approximately 2.0 V. After each measurement the light intensity decreased and the junction became Ohmic. The instability appears to b e related to surface conduction and perhaps hydrogen incorporation. Finally, deposition of layers in reversed order (Ag doped ZnO on bottom, Ga doped ZnO on top) did not result in rectifying I V

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14 characteristics. The reason for this is unclear but may relat e to the differing growth temperatures used for the two layers.

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15 CHAPTER 1 INTRODUCTION In recent years, the market of electronic devices that source, detect, and control light has grown rapidly. Light emitting d iodes (LEDs) and laser advancements have significantly contributed to this rapid growth. Nitride based devices, in particular, have entered the communication, display, traffic signal, and automotive industry. In the near future, white LEDs are expected to develop as a major market replacing incandescent and fluorescent lamps in general lighting applications. The GaN semiconductor system has dominated the solid state lighting field for approximately two decades. The need for short wavelength photonic devices high power, and high frequency electronic devices in addition to the high quality synthesis of GaN has established its dominance. ZnO, however, has gained substantial interest in part because of its advantages over GaN and thus i s considered an alternati ve material. Initially, ZnO was studied for its polycrystalline properties and applications to facial powders, varistors, piezoelectric transducers, and transparent conductive films. Lately, however, large area bulk ZnO growth has been achieved [1], and e pitaxial thin film growth optimized [2]. Hence, the motivation for renewed focus on ZnO photonics research. ZnO has several advantages over GaN: ZnO has an exciton binding energy of 60 meV, while that of GaN is only 26 meV. This large binding energy is of particular interest because its excitonic emission may be used to obtain lasing action above room temperature [3]. ZnO is available in large area bulk wafers while no bulk wafers are available for GaN. Single crystal growth by seed vapor phase (SVP) is the method used to fabricate commercially available 2 inch wafers [4]. High quality homo epitaxial ZnO growth is possible using these native substrates. Thus, concentrations of dislocations and point defects due to lattice mismatch are relatively low in Zn O films compare to GaN [5].

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16 Highly resistant to ion beam induced build up damage. ZnO retains its crystallinity even after heavy ion bombardment and exhibits no defect saturation in its crystal bulk [6]. Such radiation resistance makes ZnO a fitting candi date for space and harsh environment applications. Wet chemical etch processing is possible. Wet etching processing is extremely important in device fabrication because it provides lower costs and relative ease of processing. In addition, ZnO exhibits a direct wide bandgap of 3.37 eV at room temperature. Mg x Zn 1 x O and Cd x Zn 1 x O ternary alloys are used to fabricate multiple quantum wells (MQW) structures. The bandgap can be varied from 3.0 to 4.0 eV using such alloys without changing ZnO wurtzite structure [7, 8]. Such bandgap tuning makes possible the fabrication of heterojunction structures for lasers and LEDs. Both n type and p type ZnO are required for the development of homojunction LEDs and laser diodes. Although strong n type ZnO is easy to obtain, reliable, high conductivity p type ZnO still remains a major challenge. Substitution of N for O has been the focus of most efforts in obtaining p type ZnO. Also, P and As albeit their large size compare to O, have been widely used to obtain p type conduct ivity. Compensating defects such as Oxygen vacancies (V o ) and Zinc interstitials (Zn i ), and relatively large energies necessary to create unfilled states in the deep valence band however, still remain an issue in attaining robust p type [9]. Therefore, re ducing the background compensating defect density and fabrication of high quality films is fundamental in obtaining robust p type conductivity. Silver, a rather limited studied, group IB element, is expected to favorably substitute Zn yielding a shallow ac ceptor state [10]. Unlike other group I elements, namely Li, Na, or K, Ag is expected to easily incorporate on the Zn site rather than occupy interstitial sites [11, 12]. The focus of this study was to address the effects of Ag

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17 doping on ZnO thin film prop erties grown by pulsed laser deposition (PLD). In addition, it addresses the fabrication of rec tifying junctions, and p MOS devices.

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18 CHAPTER 2 LITERATURE REVIEW 2.1 General Properties of ZnO At ambient conditions, ZnO exhibits a hexagonal wurtzite structure with a tetrahedral coordination typical of sp 3 covalent bonding, but has considerable ionic character. Although the wurtzite structure is thermodynamically stable at ambient conditions, a zinc blende structure may be obtained w hen grown on cubic substrates, while rocksalt (NaCl) when grown at high pressures. The lattice parameter of wurtzite ZnO are a = 3.2495 and c = 5.2069 [13]. The unit cell is composed of two interpenetrating hexagonal closed packed (HCP) sublattices, w here each Zn atom is surrounded by four O atoms in a tetrahedral coordination, and vice versa. There is a deviation from the ideal wurtzite crystal due to lattice expansions and ionicity [3]. Lattice expansions are attributed to free charges, point defects and threading dislocations. Thus, undoped ZnO is typically non stoichiometric and shows n type conductivity. The ionic radii are 0.60 and 1.38 for Zn +2 and O 2 respectively. Figure 2 1 shows the hexagonal wurtzite structure of ZnO. The wurtzite ZnO conduction band is made of an s 7 symmetry. Its valence band is made of a p like state, which splits into three bands due to crystal field and spin orbit interactions [14]. ZnO exhibits a direct and large bandgap, which allows it to sustain large electric fields and higher breakdown voltages. In addit ion, lower noise generating, high temperature, high power devices can be fabricated. 2.2 Doping of ZnO ZnO, a II VI semiconductor, emits light in the near UV region of the spectrum. The semiconductor large exciton bindin g energy (60 meV), has enabled room temperature

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19 lasing and stimulated emission at temperature up to 550 K, establishing ZnO as an interesting photonic semiconducting oxide [5]. The synthesis of heavily doped n type ZnO is easily accomplished via group III cation doping. However, the control over dopants and defects that may lead to high quality and robust p type still remains a major challenge to the fabrication of practical devices. 2.2.1 Undoped ZnO and Its Native D efects Undoped ZnO normally exhibits n t ype conductivity. The role of native defects such as vacancies (V O and V Zn ), interstitials (Zn i and O i ), and antisites (Zn O and O Zn ) in undoped ZnO is not yet clearly understood. Several studies [15 17] claim that such native defects create shallow donor s tates. D.C. Look et al suggested that Zn i rather than V O are the main cause for n type conductivity, acting like shallow donors, in ZnO [18]. However, more recent theoretical and experimental studies [19 24] argue that Zn i are unstable and diffuse at room temperature, while V O are deep compensating defects not responsible for the n type material. These studies suggest that hydrogen and group III elements impurities are more likely to be responsible for the intrinsic conductivity in ZnO. Theoretical work by Van de Walle [25] showed that interstitial H is a shallow donor in ZnO. This was confirmed by experimental results [26] that showed a three orders of magnitude increase in conductivity in ZnO films when grown in H 2 by pulsed laser deposition (PLD). Second ary ion mass spectroscopy (SIMS) analysis revealed Ca H complexes, where Ca donates an electric charge to a neighboring O atom that traps a H atom, allowing it to act as a shallow donor. Another first principle study [27] demonstrated that H can substitute an O atom an d act as a shallow donor as well.

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20 Unlike in terstitial H, substitutional H is stable and has a high migration energy, thus, making it a strong candi date for H related donors in as grown ZnO [28]. In order to efficiently dope and fully exploit t he intrinsic properties of ZnO, the effects of each of these native defects must first be fully understood and controlled. 2.2.2 N type Doping ZnO is easily doped n type by cation substitution using group III elements or anion substitution using group VI I elements. The most frequently used dopants are Al, Ga, and In, which have resulted in high quality, and highly conductive films. Myong et al [29] and Ataev et al. [30] reported resistivities as low as 6.2x10 4 and 1.2x10 4 cm for ZnO films doped with Al and Ga, respectively. Films with carrier concentrations of up to 10 21 cm 3 have been realized, which have led to their use as n type layers for LEDs and transparent Ohmic contacts. 2.2.3 P type Doping It is well known that n type ZnO is easily fabricat ed, while p type still remains a major obstacle. This is common in wide bandgap semiconductor because of the low formation energies for compensating defects. Candidates for p type doping include group V anions on oxygen sites or group I or IB cations on Z inc sites. Most research efforts have focused on group V dopants, namely, N [31, 32], As [33, 34], P [35 37], and Sb [38]. First principle calculations [39, 40], predicted that group I elements are shallow acceptor, while group V elements, for the most par t, are deep acceptors. C.H. Park et al. calculated defect energy levels relative to the valance band maximum (VBM) for each cation and anion substitution. The results are summarized in table 2 1. The VBM is made of anion p orbitals with smal l mixing of cat ion p d orbitals. T herefore, group I doping results in smaller

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21 perturbation and shallower defects than group V doping. However, group I elements rather occupy interstitial sites [11, 40] mitigated by their small size. Both P and As have significantly large r bond length and therefore are more likely to form antisites to avoid lattice strains. Theoretically, N is favored among group V elements because it has a similar bond length to ZnO, low ionization energy, and unlikely antisite formation (N Zn ) [40]. Nitrogen Doping As mentioned above, N is the most promising candidate for p type ZnO and a good deal of effort has been focused in using it as a shallow acceptor dopant. Several studies [41 43] have been devoted to find the right source for Nitrogen doping during film growth. X. Li et al [41] reported p type conductivity, with carrier concentrations ranging from 10 15 to 10 18 cm 3 using NO as its dopant source. W. Xu et al. [43], using both NO and N 2 O, obtained similar carrier concentrations and resistivi ties as low as 3.02 cm. Z Z. Ye et al [44] grew p type films using NH 3 O 2 and obtained carrier concentrations of 3.2x10 17 cm 3 and 1.8 cm 2 V 1 s 1 mobilities. Oxygen poor growth conditions are required to incorporate dopants into O sites, 47]. The low solubility of N is well known. Only about 0.1 % of the dopant seems to be electrically active, while the remaining inactive N acts as scattering centers resulting in low carrier mobility [45]. In o rder to increase the N solubility, codoping methods have been engineered using reactive donor dopants such as Ga, Al, and In. N codoping lowers the acceptor level in the bandgap due to strong interaction between acceptors and donors codopants [3]. SIMS res ults showed an increase of N solubility by a factor of 400 when using Ga as

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22 the reactive donor codopants. Furthermore, M. Joseph et al. [48], using N Ga codoping via pulsed laser deposition (PLD), successfully grew p type films. For these films the hole co ncentration was 4x10 19 cm 3 with 2.0 cm resistivity. H owever, the carrier mobility did not improve. Sing et al. [49], showed a relation between V O oxygen partial pressure and carrier type. Results showed that at ratios of oxygen partial pressure to total pressure (4x10 4 Torr) below 40% fi lms were n type, while films grown at ratios above 50% showed p type conductivity. Further increase of pressure, above ratios of 60%, resulted in high resistivities and low mobilities due to crystal degradation by oxygen related defects. P type ZnO was als o achieved using Al [50] and In [51] as codpants however, high carrier concentrations and incredibly high (~150 cm 2 V 1 s 1 ) mobilities bring the validity of the results into question. In other studies [52 54], despite achieving high concentration of N inc orporation, codoping yielded only n type ZnO. Other group V Dopants Fewer studies [55 63] have considered other group V elements for substitutional doping. Based on theoreti cal calculations, both P and As are predicted to be deep acceptors and carrier t ype inversion rather difficult to achieve. Other compensating defects, such as antisites, AX centers and V Zn are expected to be stable due to larger bond lengths and lattice strain relaxation. Despite theoretical predictions, several groups [55, 61, 64] have reported p type ZnO. Aoki et al. [57], realized p type ZnO by diffusing P into ZnO substrates via laser irradiation of Zn 3 P 2 Kim et al. [55], achieved carrier type conversion via rapid thermal annealing (RTA) of n type ZnO:P films. Other groups [60 62] used As, while Xiu et al. [63] used Sb as p type dopant. Furthermore,

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23 Capacitance Voltage (CV) measurements showed that Zn 0.9 Mg 0.1 O can be made p type using P 2 O 5 as the phosphorus source, but p type ZnO was not achieved [59]. The lack of reproduci bility, carrier type changes and lattice constant relaxation over time has raised doubts about the validity of the reports of p type ZnO. In addition, the apparent p type conductivity may be the result of interface and near surface states [64] and/or inhom ogeneous samples [65]. Therefore, a better understanding of the physical properties of point defects may be useful. Silver Doping In comparison to the group V elements, studies on group IB dopants, namely Cu or Ag, in ZnO have been rather limited [66 68]. Early reports argued that Ag substitution in ZnO forms a deep acceptor state 0.23 eV below the bottom of the conduction band [10]. However, recent studies suggest this may not to be the case. One study reported an acceptor state binding energy for the Ag 3d 10 states of only 200 meV [69]. Another study of the behavior of Ag in bulk ZnO suggests that Ag acts as an amphoteric dopant, yielding an acceptor state for substitution on the Zn site, and a donor state for interstitial defects [70]. First principles c alculations have examined the dopant energy levels and defect formation energies for group IB elements in ZnO [66]. The calculations estimate the acceptor state ionization energies for substitutional Ag, Cu, and Au to be 0.4, 0.7, 0.5 eV, respectively. Alt hough these represent relatively high ionization energies, the formation energies for these substitutional defects (Cu Zn Ag Zn and Au Zn ) are predicted to be low; energies for interstitial defects are predicted to be high. These calculations suggest that s olubility and self compensation may be less of an issue for group IB elements as compared to the group V dopants.

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24 Within the group IB elements, Ag has the lowest predicted transition energy (0.4 eV) [66], reflecting a weaker p d orbital repulsion as compar ed to Cu or Au. This weak repulsion is rooted in the large size and low atomic d orbital energy of Ag. Interestingly, the O rich conditions that have been suggested for preventing oxygen vacancy (V O ) and/or Zn interstitial (Zn i ) defects are consistent with the required conditions for substituting Ag onto the Zn site. A few groups have experimentally examined the properties of Ag doped ZnO. H. S. Kang et al. have reported the formation of p type ZnO via Ag doping in thin films grown by pulsed laser depositio n [71]. The formation of p type material was limited to deposition temperatures of 200 250C. Studies on Ag implanted ZnO suggest that Ag substitution on the Zn site becomes unstable at temperatures greater than 600C [72]. This is consistent with the esti mated 0.08 mol% bulk solid solubility of Ag in ZnO [73]. 2.3 ZnO Band Gap Engineering and Devices As mentioned before, p type ZnO must be accomplished in order to fabricate practical devices and therefore the vast majority of research has been dedicated to its realization. However, another important step in realizing ZnO based optoelectronic devices is bandgap modulation and indeed it has been demonstrated by Mg [74 79] and Cd [80 86] alloying. 2.3.1 Bandgap Engineering The band gap energy of a tern ary alloy AxZn1 xO (where A = Mg or Cd) is given by the following equation [87]: Eg(x) = (1 x)E ZnO + xE AO b x(1 x) (1) Where b is the bowing parameter and E AO and E ZnO are the band gap energies of compounds AO and ZnO, Respectively.

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25 Figure 2 2 shows the bandgap and lattice constant of various semiconductors. Band gap modulation can be achieved by alloying ZnO with MgO (Eg ~7.7 eV) in order to increases the band gap ZnO. On the other hand, alloying with CdO (Eg ~ 2.3 eV) results in a decrease of the ba nd gap. Ohtomo et al. [74] found a linear relation between Mg content and band gap up to about 3.9 eV. This band gap value corresponds to a Mg content of 0.33, above which segregation of MgO is observed. The properties of ZnO ZnMgO quantum well structures were also studied and quantization behavior and increased exciton binding energy (E b > 96 meV) were observed [77]. Similarly, Makino et al. [86] found that the band gap of ZnO can be decreased from 3.28 to 2.99 eV with Cd content of 7% and estimated a non linear band gap to content relation E g ( y ) y +5.93 y 2 XRD studies also showed that the unit cell volume increased, as both lattice parameter ( a and c ), increased with Cd content. 2.3.2 ZnO Devices To reiterate, the realization of reproducible, hi gh quality p type ZnO has been the bottleneck of ZnO device processing. Despite this difficulty, ZnO based homojunction devices have been fabricated [88 90]. However, most efforts have been focused on the making of p n heterostructure devices using ZnO as the n type layer [90 102]. ZnO homojunction devices Recently Liang et al. [88] deposited undoped ZnO films on heavily P doped n+ Si substrate via metal organic chemical vapor deposition (MOCVD). Upon diffusion of P, from the Si substrate into the ZnO fil m, a ZnO:P/ ZnO junction was formed. Current voltage (I V) characteristics showed good rectifying behavior with a turn on voltage of 4.2 V under forward bias and a reverse breakdown voltage of about 6 V. No electro

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26 luminescence (EL) was observed under reve rse bias; however, the blue white light under forward bias was clearly seen even with naked eyes in the dark. EL and PL measurements from the ZnO based device are shown in F igure 2 3 Similar results were obtained for ZnO films deposited on GaAs substrate [ 89]. A rsenic diffused from GaAs substrate was used to dope ZnO p type, while ZnO:Al was used as the n type layer. EL measurements revealed an emission peak centered at ~ 2.5 eV and a weaker shoulder at ~ 3.0 eV. Later, Sun et al. [90] reported EL emissions centered at 3.2 and 2.4 eV under forward biased for films doped with N and Ga as p type and n type dopants, respectively. EL spectrum of the device under a direct forward bias current of 40 mA at room temperature and IV characteristics is shown in Figure 2 4. The UV emission reported was comparable with the visible emission in the EL spectrum, which is a significant step forward in the performance of ZnO homojunction LEDs. ZnO heterostructure devices In the absence of reliable p type ZnO resea rch groups, in an effort to exploit ZnO many advantages, have spent a great deal of attention making heterostructure devices. When Sun et al [90] used Cu 2 O and ZnO substrate as p and n layers respectively, measurements revealed EL in both forward and rever se biases. Later, Tsurkan et al. [91] used p type ZnTe on n ZnO substrates varying carrier concentrations for each layer. Although, strong EL emissions were observed for all carrier concentrations under forward bias, the EL spectrum was dominated by differ ent emission bands as the result of carrier diffusion from low to high resistive layers. Other materials such as Si [92], GaN [93], AlGaN [94], SrCu 2 O 2 [95], NiO [96], CdTe [97], and SiC [98] have been used with n type ZnO to create useful heterostructure devices.

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27 The structural relationship between the materials forming the junction is an important factor in realizing high quality heterostructure devices, because the lattice mismatch between them causes defects that act as nonradiative centers and lowers the device efficiency. For this reason using p GaN (1.8% mismatch with ZnO) and n ZnO is promising. Alivov et al. [93] fabricated p GaN/n ZnO junctions. The observed EL emission, under forward bias, was likely to emerge from the p GaN since band alignment favors carrier injection from n ZnO into p GaN. Figure 2 5 shows the EL spectrum of the n ZnO/p GaN heterostructure and the broad band emission centered at 430 nm is seen. Subsequently, Al 0.12 Ga 0.88 N was used in order achieve carrier injection into ZnO [9 4] and UV (389 nm) emissions attributed to excitonic recombination in ZnO, were observed (see Figure 2 6) Furthermore, Alivov et al. [99] fabricated la ser diodes (LD) by growing n GaN/n ZnO/p GaN structures, thus achieving carrier confinement. I V charact eristics revealed low leakage current and a reverse breakdown voltage of 12 V, while strong EL emissions were also reported. Photodiodes using n ZnO layers have been realized as well [98,100 102]. P type materials employed in their fabrication include p ZnRh 2 O 4 [102], p Si [100], p NiO [96] and p 6H SiC [98,101]. High quality diodes that exhibit good photosensitivity to UV radiation and a photorespons e as high as 0.045 A/W at 7.5 V reversed bias have been reported [98,101] using p SiC. Results are shown in Figure 2 7. ZnO Based Thin Film Transistors T he combination of transparency in the visible, excellent transistor c haracteristics, and low temperature processing makes ZnO thin film transistors (TFTs) attractive for flexible electronics on temperature s ensitive substrates. Carcia et.al [1 03 ], demonstrated the fabrication of transparent ZnO TFTs by RF magnetron sputtering near

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28 room temperatures using n type Si substrate s and 100 nm thick ZnO. High fi e l d effect mobilities of 1.2 cm 2 /Vs and I on / I off ratio o f 1.6x10 6 with drain current greater than 10 5 A was achieved. The charge accumulation transistor curves are shown in Figure 2 8. Masuda et al. [104 ], succeeded at fabricating ZnO TFTs by pulsed laser deposition on glass substrates. A double layer insulato r (SiO 2 + SiN x ) was used to obtain an I on /I off ratio of 10 5 and an optical visible transmittance of more than 80%. The transistor curves for the double insulator TFTs are shown in Figure 2 9. Many other studies [ 105 112 ] have reported successful fabricatio n of ZnO TFTs grown at both low and room temperature in a variety of substrates such as amorphous glasses, plastics or metal foil. More reliable and efficient ZnO TFTs are expected to be fabricated in the near future, making invisible electronics possible.

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29 Table 2 1 Calculated defect energy level for group I and V dopants Defect Defect energy level (eV)* Group I Li Zn Na Zn K Zn 0.09 0.17 0.32 N O 0.40 Group V P O 0.93 As O 1.15 *Relative to valance band maximum (VBM)

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30 Figure 2 1 W urtzite structure of ZnO Figure 2 2 Bandgap and lattice constant of various semiconductors

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31 Figure 2 3 The PL spectrum, EL spectrum, and EL image of the ZnO light emitting device Reprinted with perm ission from Figure 4 of H.W. Liang, Q.J. Feng, J.C. Sun, J.Z. Zhao, J.M. Bian, L.Z. Hu, H.Q. Zhang, Y.M. Luo and G.T. Du, Semicond. Sci. Technol. 23, (2008) 025014 Figure 2 4 ZnO p n homojunction a) EL spectrum under forward current injection and b) r oom tem perature I V characteristic Reprinted with permission from Figure 4 of J.C. Sun, H.W. Liang, J.Z. Zhao, J.M. Bian, Q.J. Feng, L.Z. Hu, H.Q. Zhang, X.P. Liang, Y.M. Luo, G.T. Du Chemical Ph ysics Letters 460 (2008) 548.

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32 Figure 2 5. EL spectrum o f an n ZnO/p GaN heterostructure Reprinted with permission from Figure 4 of Y. I. Alivov, J. E. Van Nostrand, D. C. Look, M. V. Chukichev, and B. M. Ataev, Appl. Phys. Lett. 83 (2003) 2943. Figure 2 6 EL spectra of n ZnO/ p Al0.12Ga0.88N heterostruct ure LED at 300 K and 500 K Reprinted with permission from Figure 4 of Y. I. Alivov, E. V. Kalinina, A. E. Cherenkov, D. C. Look, B. M. Ataev, A. K. Omaev, M. V. Chukichev, and D. M. Bagnall, Appl. Phys. Lett. 83 (2003) 4719.

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33 Figure 2 7. Room temperatur e spectral photoresponsivity of the n ZnO/ p SiC photodiode illuminated both from the ZnO and 6H SiC (inset) sides for various reverse biases Reprinted with permission from Figure 3 of Y. I. Alivov, and H. Morko, Appl. Phys. Lett. 86 (2005) 241108. Figure 2 8. ( a ) is a set of transistor curves of drain current ( I d ) vs source drain voltage ( V d ) at gate v oltages ( V g ) between 0 and 50 V for a ZnO TFT The corresponding transfer characteristic, I d vs V g at a fixed Vd equal to 20 V, for the same ZnO TFT is shown in ( b ) Reprinted with permission from Figure 3 of P. F. Carcia, R. S. McLean, M. H. Reilly, and G Nunes, Appl. Phys. Lett. 82 (2003) 1117.

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34 Figure 2 9. Electrical characteristics of a two layer gate insulator ZnO TFT prepared with a high carrier concentration ZnO layer: ( a ) Output characteristics and ( b ) transfer characteristics Reprinted with per mission from Figure 9 of S.Masuda, K.Kitamura, Y.Okumura, S.Miyatake,H.Tabata,and T. Kawai, J. Appl. Phys. 93, (2003) 1624

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35 CHAPTER 3 MATERIALS AND CHARAC TERIZATION TECHNIQUE S The synthesis and characterization of Ag doped ZnO thin film s will be discussed in this chapter. Pulsed laser deposition (PLD) was employed to grow films with thickness ranging from 250 nm to 1.0 m. In order to find optimal doping conditions, oxygen partial pressures (PO 2 ), temperature, and doping levels were varied systematically. To better understand the doping effects on electrical properties Hall Effect measurements were performed, while the optical properties were studied by photoluminescence (PL) measurements. Scanning Electron Microscopy (SEM), powder and high resolution X ray diffraction we re used to investigate the microstructure of the films The surface morphology of the films was chara cterized by Atomic Force Microscopy (AFM). 3.1 Thin Film Synthesis 3.1.1 Pulsed Laser Deposition (PLD) Pulsed laser deposition (PLD) is a common thin film growth technique used in research studies because it allows for growths ranging from 25 C to 1000 C in nearly any desirable background gas. It consists of high power energy pulses that evaporate material from a target surface producing a plasma or plume of atoms, ions, and molecules. The ablated material then condenses on a substrate positioned opposi te to the ablation target, forming a thin film with the same composition as the target. Figure 3 1 shows a schematic of the PLD system (growth chamber and laser) used in this research. The quality of the films strongly depends on the laser wavelength, stru ctural and chemical composition of the ablation target and background gas, chamber pressure, and substrate temperature and distance to ablation target.

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36 The base pressure of the chamber used for these film growths was approximately 10 8 Torr. A KrF excimer laser with a 248nm wavelength used was the ablation source with an energy density of about 1.0 J/cm 2 at the target surface. The background gas used during all growths was ultra high purity Oxygen. 3.1.2 Target and Substrate P reparation Ablation targ ets were fabricated using ultra high purity ZnO and Ag 2 O powders. The targets were sintered at 1000 C in air. The concentrations of Ag in the ZnO targets were 0.75 at%, 0.1 and 0.5wt%. These concentrations of Ag were somewhat arbitrary, made based on som e rough assumptions regarding activation energy of the acceptor and desired hole concentration. Single crystal c plane sapphire and ZnO substrates were employed in this study. Substrates were cleaned using supersonic baths in trichloroethylene, acetone, an d methanol subsequently. The target to substrate distance was ranged from 3.5 to 6.0 cm. 3.2 Characterization Techniques 3.2.1 Hall Effect Measurement The Hall effect is widely used in semiconductor research because it permits one to determine the carrier density, mobility and majority carrier type. When a magnetic field is applied perpendicular to the current flow, charge carrier separation occurs resulting in an electrical field perpendicular to both the applied magnetic field and current direction. The potential drop across this electrical field is called the Hall voltage (V H ). Figure 3 2shows a schematic of the Hall effect. The Hall coefficient can also be extracted from Hall effect measurement and can be written as: = ( 2 ) ( + ) 2 = (for p type) or = (for n type) (2)

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37 Where b and r are scattering factors, n and p are electron and hole density, respectively. Also, it can be determined experimentally using the V H = = 1 (for p type) or = 1 (for n type) (3) Where B is the applied magnetic field, and t and I are the thickness and applied current on the sample. Other Hall parameters can be expressed usi ng the following equations. Mobility: = ; (4) Resistivity: = = / / (5) Where R is the resistance, w is t he sample width, L is the sample length, and V is the voltage across the sample. It should be noted that ZnO is a compensated semiconductor and in this study, in order to reliably delineate the carrier type and density for highly compensated samples, the H all measurements were performed at various magnetic field values and over a large magnetic field range. Furthermore, the measured transverse voltage is the sum of the Hall voltage, an offset voltage (due to non ideal contact geometry), magnetoresistance, a nd noise. Only the Hall voltage should be linearly dependent on applied magnetic field, making the sign of the Hall coefficient easily confirmed from the slope of the measured voltage as a function of applied magnetic field. 3.2.2 Photoluminescence (PL) Photoluminescence (PL) is a nondestructive technique used to characterize the bandgap, impurity and defect levels in semiconductors. The light source, having greater energy than the bandgap of the semiconductor, is directed to the sample and creates

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38 elect ron hole pairs by promoting carriers to allowable excited states. Upon returning to equilibrium states, the excess energy is released in the form of photons (radiative process) or phonons (nonradiative process). The energy of the emitted photon is related to the energy difference between the excited and equilibrium state. For direct bandgap semiconductors the most common radiative transition is that from the conduction to the vale nce band, allowing for bandgap determination. Information about localized def ects and impurity levels can be obtained from the emitted photoluminescence energy and amount, respectively. There are five common radiative transitions and are shown in Figure 3 3. In ZnO, most transitions are only detectable at very low temperatures; th erefore, both room and low (15 K) temperature PL were measured. Moreover, time resolved PL measurements were performed to better understand the nature of the defects present in the Ag doped ZnO films. 3.2.3 X Ray D iffraction ( XRD ) X ray diffraction (XRD) is a technique used to characterize the crystal structure, grain size, and preferred orientation in crystalline samples. A beam of X rays strike a sample interacting with crystal planes creating a diffraction pattern for constructive interactions that sati = 2 sin (6) where n is the order of diffraction, is the x ray wavelength, d is the interplanar ray. In this study, the crystal structure of the films was characterized usi ng high resolution X ray diffraction. The preferred orientation, presence of secondary phases, and lattice parameter changes were analyzed using the Philips APD3720 system. Information on

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39 crystal quality and symmetry was obtained, from omega and phi scans, using t he Philips 3.2.4 Atomic Force Microscopy (AFM) Atomic force microscopy (AFM) is a tool used to characterize the surface topology and morphology of thin films. It consists of a cantilever with a sharp tip at its end t hat is used to scan the sample surface (Figure 3 4) In this research, tapping mode AFM (Digital Instruments Dimension 3100) was employed to map out the topology of the ZnO film surface. In tapping mode, the cantilever oscillates near its resonanc e frequency. The oscillation amplitude decreases or increases due to interaction of forces acting on the cantilever as the AFM tip comes close the sample surface; thus, imaging the force of the oscillating contacts of the tip with the sample surface.

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40 Figure 3 1 Pulsed laser deposition (PLD) system Figure 3 2 Hall Effect diagram

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41 Figure 3 3 Common radiative transition mechanism Figure 3 4 Block diagram of atomic force microscope

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42 CHAPTER 4 SILVER DOPED ZNO FIL MS GROWN VIA PULSED LASER DEPOSITION 4.1 Introduction ZnO is a wide bandgap semiconductor that is attracting significant attention for thin film electronics [1 13 115], nanoelectronics [116 118], photonics [119,120], piezoelectrics [121,122] and sensor applications [3,123 12 5 ] The crystal structure of ZnO is wurtzite, with lattice parameters a = 3.25 and c = 5.12 [1 2 6 ]. ZnO has a direct bandgap of 3.2 eV and a relatively large exciton binding energy of 60 meV. This has enabled room temperature lasing and stimulated emissi on in the ultraviolet at temperatures up to 550 K, establishing ZnO as an interesting p hotonic semiconducting oxide [12 7 ]. An important issue in developing ZnO based electronics is the formation of robust p type material. This is obviously relevant for th e fabrication of pn junctions fo r minority carrier injection [128 13 0 ]. It is also important for p channel thin film transistors [20] as well as in spin doped ZnO where the ferromagnetic ordering appears linked to carrier density and carrier type [ 13 2 13 3 ] Undoped ZnO is normally n type due to native defects that create shallow donor states [15 ]. The synthesis of heavily doped n type ZnO is easily accomplished via group III cation doping. In contrast, achieving p type conductivity in ZnO is quite challeng ing due to its propensity to create compensating donor defects and the relatively large energy necessary to create unfilled sta tes in the deep valence band [9 ]. Candidate p type dopants include the group V anions on the oxygen site or group I or IB elemen ts on the Zn site. Previous studies [11,12 ] indicate that group I elements, namely Li, Na or K, do not easily incorporate on the Zn site, but rather occupy interstitial sites. Most research efforts have focused on the group V dopants, including N [ 31,13 4 ],

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43 As [33,34], P [35 37] and Sb [38 ]. N is the most favored dopant candidate for p type doping in ZnO based on similar bond length, low ionization energy and lack of antisite N Zn defects [39 ]. Experimentally, the solubility of N in ZnO appears to be low. Furthermore, the tendency to form N N complexes that are not acceptor defects further reduces the effectiveness of this dopant. In practice, only a small fraction of the N incorporated into ZnO is observed to be electrically active. The remaining inactive N may act as scattering centers, possibly resulting in low carrier mobility [ 45 ]. Moreover, O poor growth conditions are required to incorporate group V dopants onto the O sites. efects yieldi ng compensation [39, 45 47 ]. Calculations for As, P, or Sb substitution on the O lattice site predict high energies of formation and high acceptor ionization energies. Both computational and experimental evidence suggest that the acceptor states observed for As, P, or Sb doped ZnO may be the result of complexes involving Zn site vacancies and gr oup V dopants on the Zn site [135,13 6 ]. While several groups have been successful in achieving p type con ductivity via group V doping [31, 33 39 134,13 7 ], low carr ier mobilities and carrier concentration remain important issues. In comparison to the group V elements, studies on group IB dopants, namely Cu or Ag, in ZnO have been rather l imited [66 68 ]. Early reports argued that Ag substitution in ZnO forms a deep a cceptor state 0.23 eV below the b ottom of the conduction band [69 ]. However, recent studies suggest this may not to be the case. One study reported an acceptor state binding energy for the Ag 3d 10 states of only 200 meV [70 ]. Another study of the behavio r of Ag in bulk ZnO suggests that Ag acts as an amphoteric dopant, yielding an acceptor state for substitution on the Zn site, and a donor state for interstitial

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44 defects [ 7 1 ]. First principles calculations have examined the dopant energy levels and defect formation energies for group IB elements in ZnO [66 ]. The calculations estimate the acceptor state ionization energies for substitutional Ag, Cu, and Au to be 0.4, 0.7, 0.5 eV, respectively. Although these represent relatively high ionization energies, the formation energies for these substitutional defects (Cu Zn Ag Zn and Au Zn ) are predicted to be low; energies for interstitial defects are predicted to be high. These calculations suggest that solubility and self compensation may be less of an issue for gr oup IB elements as compared to the group V dopants. Within the group IB elements, Ag has the lowest predicte d transition energy (0.4 eV) [66 ], reflecting a weaker p d orbital repulsion as compared to Cu or Au. This weak repulsion is rooted in the large siz e and low atomic d orbital energy of Ag. Interestingly, the O rich conditions that have been suggested for preventing oxygen vacancy (V O ) and/or Zn interstitial (Zn i ) defects are consistent with the required conditions for substituting Ag onto the Zn site. A few groups have experimentally examined the properties of Ag doped ZnO. H. S. Kang et al. have reported the formation of p type ZnO via Ag doping in thin films grown by pulsed laser deposition [71 ]. The formation of p type material was limited to depo sition temperatures of 200 250C. Studies on Ag implanted ZnO suggest that Ag substitution on the Zn site becomes unstable at tem peratures greater than 600C [72 ]. This is consistent with the estimated 0.08 mol% bulk s olid solubility of Ag in ZnO [73 ]. I n this chapter, the synthesis and properties of Ag doped ZnO films grown by pulsed laser deposition is examined, focusing on the formation of p type material, as well as delineating the stability of the transport properties.

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45 4.2 Experimental Procedures Th e ZnO:Ag a blation targets were fabricated using ultra high purity ZnO and Ag 2 O powders. The concentration of Ag in the ZnO targets was 0.6 at%, and 0.3 at%. As mentioned before, the Ag content was somewhat arbitrary, made based on some rough assumptions re garding activation energy of the acceptor and desired hole concentration. The ZnO:Ag films were grown on c plane (0001) sapphire substrate at temperatures ranging from 300 C to 600 C in oxygen partial pressure ranging from 1.0 to 75 mTorr. In specific oc casions, undoped ZnO buffer layers were grown in 1.0 mTorr of oxygen at 400 C or 800 C. A KrF excimer laser was used as the ablation source at a frequency of 1.0 Hz with an energy density of 1.5 J/cm 2 The film thickness varied from 300 nm to 1.1 m for the ZnO:Ag thin films while the ZnO buffer(when employed) thickness was kept fixed at 50 nm. High resolution x ray diffraction (Phillips, XRD3720) was used to characterize the crystal quality of the films. Atomic force microscopy (AFM Dimension 3100 ) was used to observe the surface morphology. The resistivity, Hall mobility and carrier concentration were measured using a four point van der Pauw method with a commercial LakeShore Hall measuring system Measurements were taken in the dark, room light a nd various UV light wavelengths, in order to analyze the behavior of photocarriers in the doped films. The room temperature optical properties were analyzed using photoluminescence. For this, a He Cd laser operating at 325 nm was used for excitation. 4 .3 Results and Discussions 4.3 .1 Role of Ag Doping in ZnO Crystal Quality and Surface Morphology The crystalline structure and orientation of the deposited ZnO:Ag films was examined using x ray diffraction. Figure 4 1a shows a log plot of the diffraction d ata for

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46 films grown in 25 mTorr oxygen for the deposition temperatures ranging from 300 600C. The results showed t hat the films are c axis oriented and ZnO is the primary phase. The diffraction does show the emergence of a secondary impurity phase for gro wth at 400 o C and 500 o C. The small, broad peak could be associated with either Ag 2 O or Ag metal. In order to delineate the bonding state of the Ag, X ray photoelectron spectroscopy ( XPS ) was performed on these films. The results show that both Ag and Ag 2 O bonding energies are present in the films with the impurity peak in the diffraction pattern Above 300 o C Ag 2 O is not stable and losses o xygen Under these conditions, the formation of Ag metal is expected for any Ag segregation, however, Ag quantities are very small and no contin uous secondary phase is observed Figure 4 1b shows the diffraction data for films grown at 300 o C for oxygen pressures ranging from 1 75 mTorr. For growth at this low temperature, the impurity peak is not observed in the diff raction data for the oxygen pressures considered. Also note that the ZnO (110) orientation was not observed for growth at 300 C. N o significant shift in d spacing from that for undoped ZnO was detected due to doping (Figure 4 2) The surface morphology o f the films was examined using atomic for ce microscopy (AFM). Figure 4 3 shows AFM images for films grown a t 300C 400 o C and 500C in an oxygen partial pressure of 25 mTorr. The grain size increased with increasing temperature and Ag inclusion. Note that an enhancement in grain size via the inclusion of Ag in the deposition flux has been reported for various oxide thin film materials [138,13 9 ]. In these experiments, it was speculated that the formation of Ag 2 O in oxide growth provides a source of atomic ox ygen through the continuous formation and dissociation of Ag 2 O. The grain size measured for undoped ZnO was approximately

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47 150 nm. For films grown under the same condition but doped with Ag, the grain size increased to 275 nm. The grain size of films grown at 400 o C and 500 o C was 390 and 560 nm, respectively. The AFM results shows that as growth temperature is increased, roughness also increased. At 300 C the average roughness was 5.36 nm while at 400 C and 500 C the roughness increased to 17.62 nm and 28. 24 nm respectively. As for thicker films the roughness increased drastically As mentioned above, the highest roughness reported for films with thickness less than 500 nm was 28.24 nm, while films with thickness above 1.0 m showed roughness as high as 10 0 nm. Such high roughness makes it difficult to obtain reliable transport a nd optical properties. Growing o n mismatch ed substrates like sapphire introduces strain and a high density of interface defects that may result in inhomogeneous carrier concentratio n measurements that may yield the wrong carrier type [65]. It is worth noting that the surface of the thick SZO films showe d dark spots which may block any light coming out of the films during optical characterization Therefore, in an attempt to obtain sm ooth, clear and low defect films, 50 nm thick ZnO buffer layers were grown at 400 C and 800 C in 1.0 mTorr of oxygen prior to the growth of the 1.0 m thick SZO film. The results showed that high temperature buffers (HTB) grown at 800 C, improve the ro ughness of the thicker SZO films to 25nm and when the HTB is grown on ZnO substrates the SZO films roughness drops below 10 nm (Figure 4 4 ). In addition, SZO films grown with a high temperature buffer layer resulted in clear surfaces as shown in Figure 4 5 4.3 .2 P Type Conductivity, Transport and Optical Stability of Ag Doped ZnO Films The transport properties of the films were determined using room temperat ur e Hall measurements. As explained in chapter 3, in order to reliably delineate the carrier

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48 type an d density for these highly compensated samples, the Hall measurements were performed at various magnetic field values and over a large magnetic field range. Figure 4 6 shows a plot of V H all d/I as a function of applied magnetic field, where V H all is the Ha ll voltage, d is the film thickness, and I is the measurement current. It shows a plot of VdI 1 as a function of magnetic field for two samples, a Ag doped ZnO film grown at 600 C that is n type (as determined by the full Van der Pauw four point calcula tion), and another film grown at 300 C that is p type. For the sample grown at 600 C, the slope is clearly negative, indicating n type. In Figure 4 6b, there is an obvious positive slope to the data as field is increased, indicating p type behavior. F rom the slope of the curve, the extracted hole carrier density is 5.2x10 16 cm 3 Figure 4 7 shows the results for resistivity and carrier concentration of 0.6 at% SZO films for some of th e growth conditions considered. For most deposition conditions, the films were n type (Table 4 1) For growth temperatures in the range of 300 500C, results showed an initial drop in film resistivity, followed by a rise as growth pressure was increased. P type ZnO was realized for films grown at 400 500 C in oxyg en pre ssure of 10 and 25 mTorr. For p type material, these conditions were optimal. For these films, the hole carrier concentration was in the mid 10 19 cm 3 The mobility for films grown at 400 C and 500 C was 10.7 and 2.9 cm 2 /Vs, respectively. Note that for g rowth at 300 C, P(O 2 )=75 mTorr, and 400 C and 500 C in 10mTorr of oxygen, low carrier concentration p type ZnO:Ag was also realized. At a growth temperature of 600 o C, the carrier concentration and resistivity were independent of growth pressure. Presum ably, the Ag was driven out of the Zn site yielding only n type films. All films grown with Ag content other than 0.6 at% were n type.

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49 ZnO:Ag films grown at 300 o C, P(O 2 ) = 75 mTorr were stored in the dark and H all measurements were perfor med to study the stability of the transport properties over time Before storing in the dark, the samples were exposed to indoor room light for ap proximately 24 hours. Figure 4 8 shows resistivity vs. time for a film grown at 300 o C in 75 mTorr of O 2 Note that this was a different film fr om that considered in figure 4 6 b, and exhibited a higher resistivity. Results demonstrate a large increase in resistivity from 694 to 3704 cm in the first week of dark storage. This spike in resistivity is mainly due to the relaxation of persistent photocarriers created by light exposure. After a week in the dark, the resistivity of the film gradually decreases. When considering the long t erm stability of the Ag doped ZnO films, one may need to consider the possible effects of oxygen absorption, hydroxide formation, or diffused hydrogen, the latter being important since the hydrogen diffusion rate is quite high. This may be the cause of th e subsequent decrease in resistivity and change in carrier type. The change in carrier type was observed just after 120 days of storage. Another set of films grown in the same conditions were exposed to three different UV light wavelengths (365, 304, and 2 54 nm) after being stored in the dark for some time. A persistent photoconductivity was observed as shown in Figure 4 9 a. The drop in resistivity was independent of the wavelength used. The relaxation time for this set of films was about 24 hours. Figure 4 9 b shows the conductivity curve as a function of time when films are placed in the dark after UV light exposure. An exponential decay curve can be fitted and the relaxation time constant was extracted t o be 227.9 min. Room temperature photoluminescence measurements were performed on the Ag doped ZnO films. The PL spectra are shown in Figure 4 10 The Ag doped ZnO

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50 films sh owed well defined band edge emission around 377 nm This emission line is due to free excit on recombination around 3.27 eV, which is very close to the bandgap (3.25 eV) found by room temperature absorption measurements shown in Figure 4 11. The photoluminescence intensity was highest for films grown at 400 500C. This enhancement may be due to a reduction in surface states, which are deleterious to UV emission. Other studies on the effect a monovalent dopant on the photoluminescence of ZnO showed that Ag enhances the efficiency of exciton recombination, so as to have b etter optical properties [68]. Note that there is little visible emis sion often seen in ZnO films [14 0] due to recombination involving mid gap states suggesting lower concentration of compensating defects in the films. 4.4 Summary The synthesis and proper ties of Ag doped ZnO thin films were examined. Epit axial ZnO films doped with 0.6 at% Ag content grown at moderately low temperatures (300 C to 500 C) by pulsed laser deposition yielded p type material as determined by room temperature Hall measurements Hole concentrations on the order mid 10 1 5 to mid 10 19 cm 3 range were realized. Growth at higher temperatures yielded n type material, suggesting that the Ag was driven out of the substitutional site above 500 C and that Ag substitution yielding an acce ptor state is metastable. Photoluminescence measurements showed strong near band edge emission with little mid gap emission as the result of Ag substitution for Zn (Ag Zn ) and reduction of surface states deleterious to UV photoluminescence emission The st ability of the Ag doped films was examined as well. Presumably hydrogen incorporation caused the films to turn n type after about 120 days. Persistent photoconductivity was also observed. High temperature ZnO buffer layers drastically imp roved the surface morphology of films thicker than 1.0 m.

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51 roughness below 10 nm were observed. Finally, i n developing electroluminescent junctions, the realization of robust UV photoluminescence in p type ZnO may prove advantageous. A detailed PL study and results for the rectifying junctions utilizing Ag dop ed ZnO films are reported in the following chapters.

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52 Table 4 1 Hall Data of 0.6 at% Ag doped ZnO films grown at various Temperatures and oxygen partial pressures Growth Pressure P(O 2 ) (mTorr) Growth Temper ature (C) 1 10 25 50 75 300 C 16.7 cm 4.0 x10 1 6 cm 3 16.7 cm 2 V 1 s 1 n type 4.10 cm 3.0 x10 1 7 cm 3 5.10 cm 2 V 1 s 1 n type 0 47 cm 1.3 x10 1 9 cm 3 1.05 cm 2 V 1 s 1 n type 0.133 cm 2.9 x10 1 9 cm 3 1.6 3 cm 2 V 1 s 1 n type 11.96 cm 5.2x10 16 cm 3 10.1 cm 2 V 1 s 1 p type 400 C 0.36 cm 2.7 x10 1 8 cm 3 6 .53 cm 2 V 1 s 1 n type 119 cm 8.6x10 15 cm 3 5.99 cm 2 V 1 s 1 p type 0.019 4 cm 2.9x10 19 cm 3 10.7 cm 2 V 1 s 1 p type 0.388 cm 3.5x10 18 cm 3 4.68 cm 2 V 1 s 1 Mixed type 1. 82 cm 4.0 x10 1 8 cm 3 0.85 cm 2 V 1 s 1 n type 500 C 0.041 cm 1.7 x10 1 9 cm 3 9.28 cm 2 V 1 s 1 n type 3.52 cm 5.5x10 17 cm 3 3.2 cm 2 V 1 s 1 p type 0.0017 cm 5.9x10 19 cm 3 2.9 cm 2 V 1 s 1 p type 0.088 cm 5.0 x10 1 8 cm 3 14.1 cm 2 V 1 s 1 n type 1. 25 cm 1 .2x10 1 8 cm 3 4. 3 4 cm 2 V 1 s 1 n type 600 C 0.053 cm 5 .2x10 1 8 cm 3 22.8 cm 2 V 1 s 1 n type 0.1 02 cm 3.2x10 1 8 cm 3 15.5 cm 2 V 1 s 1 n type 0. 1 42 cm 4.3 x10 1 8 cm 3 10.2 cm 2 V 1 s 1 n type 0.0692 cm 4.1 x10 1 8 cm 3 22. 3 cm 2 V 1 s 1 n type 0.0273 cm 9.7 x10 1 8 cm 3 23.6 cm 2 V 1 s 1 n type

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53 Figure 4 1 Pow der XRD pattern for films grown in (a) 25 mTorr for dep osition temperature range of 300 600 C and (b) films grown at 300 C in oxygen pressures ranging from 1 75 mTorr.

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54 Figure 4 2 Effect of Ag doping on ZnO d spacing for film s grown at 500 C Figure 4 3 AFM images for films grown at (a)(b) 300C, (c) 400C and (d) 500C

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55 Figure 4 4 SZO film average roughness as a function of Ag content grown on different substrates and buffer layers Figure 4 5 Optical microscope images of the surface of ZnO grown (a) without buffer layer and (b) with HTB laye r

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56 Figure 4 6 Plot of Vd/I as a function of magnetic field for (a) an n type and (b) a p type Ag doped ZnO film.

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57 Figure 4 7 Resistivity (a) and carrier concentration (b) as a function of growth conditions

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58 Figure 4 8 Resistivity as a function of time for films grown at 300 C, P(O 2 ) = 75 mTorr

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59 Figure 4 9 Effects of (a) UV light exposure and dark storage, showing (b) exponential decay of conductivi ty over time in dark storage for films grown at 300 C, P(O 2 ) = 75 mTorr

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60 Fig ure 4 10 Room temperature photoluminescence for Ag doped ZnO films grown at various temperatures in 25 mTorr of oxygen showing (a) large wavelength and (b) narrow wavelength plots.

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61 Figure 4 11. Room temperature absorpti on spectra for Ag doped ZnO

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62 CHAPTER 5 PHOTOLUMINESCENCE ST UDY OF ZINC OXIDE 5.1 Introduction L ight emitting diode s are expected to develop as a major market; therefore, it is important to investigate the optical properties of differe nt ZnO films. Not only does it help to analyze the structure and other properties of ZnO, but also it contributes to the optimizatio n of the growth process of ZnO In order to make high performance optoelectronic devices on ZnO, it is necessary to investig ate the transitions processes in ZnO. The main optical transitions seen in ZnO originate from free excitons, bound excitons, two electron satellites ( TES) and donor acceptor pairs. The related free exciton transitions invol ving an electron from the condu ction band and a hole from ZnO three valance bands are named as A (corresponding to the heavy hole), B (corresponding to the light hole), and C (corresponding to crystal field split band). These transitions dominate the near bandgap intrinsic absorption an d emission spectra [ 141 ] and occur at energies between 3.37 and 3.45 eV. Bound exciton emissions originate from discrete electronic energy levels caused by dopants or defects. Normally, neutral shallow donor bound exciton transitions dominate the low tempe rature PL spectrum due to the presence of donor sources (unintentional impurities or other shallow level defects) and they occur at energies between 3.34 and 3.37 eV. Acceptor bound exciton transitions are sometimes seen in ZnO samples that contain a subst antial amount of acceptors and can be seen at energies between 3.30 and 3.35 eV. TES transitions occur at this same range in high quality ZnO samples. This transition process is generated by the radiative recombination of an exciton bound to a neutral dono r, leaving

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63 the donor in the excited state (2s or 2p state) Emissions originated from donor acceptor recombination and respective phonon replicas are expected at energies between 3.05 and 3.26eV. There are a lot of experimental techniques for the study of the optic al transitions processes in ZnO. In this chapter the defects in ZnO are analyzed in detail using both room temperature and low temperature (15 K) photoluminescence spectroscopy. Furthermore, the effect of co doping, grain growth and different buff er layers on the optical properties of ZnO is discussed. 5.2 Experimental In this study, several films with different dopants were grown via pulsed laser deposition. Ag, P, and Ga were used individually to dope ZnO, while co dope d ZnO was realized using A g and P. The Ag content in the ZnO ablation target was 0.6 at%. The other ablation targets were fabricated using ZnO doped with 0.5 at% P, and ZnO doped with 0.22 at% Ga content. Co doped ZnO ablation target was fabricated using 0.5 at% P and 0.6 at% Ag. T he films were grown on single crystal c oriented sapphire and GaN substrates. Prior to growth, the substrates were cleaned in an ultrasonic bath using trichloroethylene, acetone, and methanol. The substrates were attached to the heater plate using silver p aint. The substrate to target distance was 3.5 cm. Some of the Ag doped ZnO samples were grown on a h igh temperature ZnO buffer layers The growth temperature and P ( O 2 ) was 800 C and 1mTorr respectively and were 50 nm thick. ZnO lattice match ed MgCaO buff ers grown on GaN substrate via molecular beam epitaxy (MBE) were also employed in this study. Note that buffer layers fabricated using MBE were grown by another research group, thus, this section discuss only the effects on photoluminescence properties. Ta ble 5 1 lists the growth condition, doping content, and

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64 heat treatment used for all samples analyzed. T he resistivity, Hall mobility and carrier concentration were measured using a four point van der Pauw method Contacts for the Hall measurements were sol dered indium. Photoluminescence was measured using a He Cd continuous wave laser operating at 325 nm. A CTI Cryogenic vacuum pump was used to lower the temperature of the samples during low temperature and temperature dependent PL measurements. 5 .3 Result s and Discussions 5 .3.1 PL Enhancement via Ag I nclusion The room temperature PL spectra of undoped ZnO (s1) and Ag doped ZnO (s2) are shown in Figure 5 1. The undoped sample shows t w o characteristic emission bands. The ne ar band edge emission centered at 3 77 nm which originates fro m free exciton recombination [14 1 ] and a dominant broad deep level emission centered at 50 5nm, which originates from structural defects and impurities [ 14 2 ]. The Ag doped sample shows the opposite behavior, an enhanced UV emission (383 nm) and a suppressed deep level emission. Other studies on the effect a monovalent dopant on the photoluminescence of ZnO showed that Ag enhances the efficiency of exciton recombination, so as to ha ve better optical properties [68 ]. It has been sugge sted that if Ag occupies a Zn site (Ag Zn ), photocarriers may escape more easily from Ag ions resulting in more excitons and higher excitonic recombination. As discussed in the previous chapter, Ag inclusion also causes grain growth which results in smaller non radiative relaxation rates over the surface states [ 14 3 ] and thus better exciton diffusion through the crystal and hi gher UV PL intensities. Figure 5 2 shows the UV emission PL intensity increasing as a function of increasing grain size. The intensity increased from 0.75 a bs u nits to 8.4 a bs u nits as grain size increased from 168 nm to 330 nm

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65 respectively. Further grain growth to 560 nm resulted in higher PL intensity (11.7 a bs u nits ). The suppression of the broad deep band suggests that Ag does not occupy an interstitial site or an antisite [1 4 4 ]. 5.3.2 Low Temperature and Temperature D ependent PL Hwang et. al. [ 14 5 ], resolved both donor and acceptor bound exciton transitions in in high quality undoped ZnO. The PL spectra measured at 10 K is shown in F igure 5 3 The undoped sample shows two main peaks. A strong PL emi ssion centered at 3.363 eV which originates from localized exciton recombination bound to a donor state (D X) [ 31, 144 14 8 ] The next PL peak is slightly w eaker and it is centered at 3. 352 e V and has been previously identified as the acceptor bound exciton transition (A X) [ 14 9 ]. Meyer et al. [ 14 6 ] showed th at PL emission originating from donor acceptor pairs (DAP) and DAP LO phonon replicas can be found centered at 3.24 eV and 3.17 eV respectively The nature of the acceptor state is unclear, however emission from free exciton to neutral acceptor transitions ( e, A ) are expected around 3.31 eV [ 14 4 ,14 5 ,15 0 ] but are rarely seen in undoped samples Similarl y, the Ag doped sample shows a s eries of main peaks followed by small shoulders shown in Figure 5 4 The first peak cente red at 3.353 eV corresponds to A X emissions, preceded to its right by a small shoulder around 3.36 eV corresponding to D X. A strong peak centered at 3.31 e V dominat es the spectrum As mentioned above this e mission originates from free electrons to neutral acceptor state transitions in this case, believed to be Ag related. It should be not ed that the PL intensity from D X (3.36 eV) is relatively small compared to the A X and (e,A) peak s suggesting that the Fermi level should be near the acceptor level. This is consistent with the p type conductivity of the sampl e measured by Hall (see Figure 5 5 ). These results are also consistent with those found in P doped ZnO by

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66 L i e t al [ 15 0 ]. The following shoulders centered at 3.25 eV and 3.17 eV correspond to DAP and DAP LO emissions, respectively. Temperature dependent PL was employed to further study the nature of the main peak s centered at 3.35 eV and 3.31 eV. Figure 5 6 is a plot of peak position as a function of temperature. Increasing the temperature causes the (e,A ) peak position to blue shift at low temperatures from 15 K to 110 K, followed by a red shift at higher temperature s This is a typical behavior seen in PL pe aks assigned to radi ative recombination of free neutral acceptor [ 1 4 4 ,1 5 0 ]. The acceptor energy of the Ag dopant was estimated from the free to neutral acceptor transition at 3.311 eV. The transition energy is given by = + 2 (7) where E g and E A are the band gap and acceptor energies, respectively. Since the thermal energy can be neglected at 15 K, the acceptor energy can be roughly estimated E A = E g (3.437) E eA (3.313) to obtain 124 meV. Finally, in order to delineate the excitonic nature of the peak centered at 3.352 Ag doped ZnO films with different grain size s, s2 and s3, were compared at low temp eratures The l ow temperature PL spectra showed that both peaks assigned A X and (e, A ) drastically increase d their intensities with grain size. As shown in Figure 5 7 the peak A X intensity increased from 81 a bs u nits (s2) to 136 a bs u nits (s3), a 68.7 % increase in emission intensity with increasing grain size. The (e, A ) peak intensity increased 12.7 % from 213 a bs u nits (s2) to 240 a bs u nits (s3), while other non excitonic peaks and shoulders remained relatively unchanged. The grain size of s2 and s3 is 333 nm and 560 nm respectively. Exciton diffusion through the crystal is expected

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67 to improve with grain growth and lowering of surface states, hence the drastic increase in intensity from the A X peak (3.35 eV). In the case of free electrons, fewer s urface states and lower relaxation rates are expected to aid in their recombination process. Such PL intensities and shallow acceptor energy suggest that Ag can yield a robust acceptor level. 5.3.3 Buffer Layer and Dopant Effect on the Optical P roperties o f ZnO In the previous sections, it was discussed how Ag inclusion improves the optical properties of ZnO as well as yielding an acceptor state clearly identifiable by PL measurements. In order to further improve the PL properties of ZnO, a high temperature buffer (HTB) and a lattice matched MgCaO buffer were employed individually prior to the growth of Ag dope d films. Figure 5 8 compares the room temperature PL results. The near band edge emission is centered at 378 nm for all samples. As mentioned above t he enhancement in the UV PL is readily seen when ZnO is doped with Ag. UV PL intensity increased further with the use of the HTB, while the maximum intensity is observed when the lattice match MgCaO is employed. The HTB reduces surface states by improving the surface roughness and clearing the surface from dark spots as shown in chapter 4 enhances UV PL emission of the films. Improve ment in roughness and less structural defects was also observed when the lattice matched MgCaO buffer was employed. In additi on, the MgCaO buffer may induce band bending in ZnO at the interface changing the electronic states of the defects responsible for visible emission and th u s enhancing the UV emission [14 8 14 9 ] further Figure 5 9 shows the room temperature PL spectra of ZnO doped with different elements. The Ag doped ZnO sample has a stronger UV emission than any other

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68 sample including Ga doped ZnO, which is currently used in most ZnO based heterojunctions. Codoping ZnO with Ag and P deteriorated the optical properties 5.4 Summary Silver doped ZnO films were grown via PLD and their optical properties were discussed. Results showed that Ag inclusion lead to grain growth and smaller non radiative relaxation rates over surface states, which lead to UV emission enhancement. Room temperature PL measurements also showed a suppression of ZnO visible luminescence suggesting that Ag does not occupy interstitial sites or an antisite. Low temperature and temperature dependent PL spectroscopy revealed strong and dominant emissions originating from free electron recombination to Ag related acceptor states around 3.31 eV. The A X emission at 3.352 eV was also observed at low temperatures. Enhancement of the PL intensity with increasing grain size and the peak position blue shits (low temperatures) followed by red shits (high temperatures) with increasing temperature, confirmed the nature of the emission. Donor acceptor pair emission and corresponding phonon replicas were also observed. The acceptor energy was estimated to be 124 meV. The very weak deep level emission (not shown), again in the low tem perature PL spectra indicates that in the p type ZnO:Ag native donor and acceptor defects are suppressed suggesting the observed acceptor related PL emissions and hole concentration are fro m the Ag in ZnO instead of native defects. High temperature ZnO buffers and lattice match MgCaO buffers helped improve the UV emission of the Ag doped films. A combination of band bending at the MgCaO ZnO interface and reduction of surface states may be re sponsible for such enhancement. No visible luminescence was observed. Finally, the room temperature PL

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69 spectrum of Ag doped ZnO was compared to that of undoped, P doped, Ga doped, and Ag Ga codoped ZnO. The Ag doped ZnO films showed better optical propert ies.

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70 T able 5 1 .Growth conditions considered in PL Study Growth Condition No. Dopant at% Substrate Buffer Growth Temperature Growth Pressure Anneal S1 Undoped Al 2 O 3 500 C 10 mTorr S2 Ag 0.6 Al 2 O 3 500 C 10 mTorr S3 Ag 0.6 Al 2 O 3 500 C 25 mTorr S4 Ag 0.6 Al 2 O 3 ZnO 500 C 10 mTorr S5 Ag 0.6 GaN MgCaO 500 C 10mTorr S6 Ga 0.22 Al 2 O 3 ZnO 650 C 1 mTorr S7 P 0.5 Al 2 O 3 7 00 C 150 mTorr RTA 950 C S8 Ag, P 0.6, 0. 5 Al 2 O 3 500 C 10 mTorr

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71 Figure 5 1 Room temperature PL spectra of undoped ZnO and Ag doped ZnO Figure 5 2 Band Edge Intensity as a function of grain size

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72 Figure 5 3 PL spectra of undoped ZnO measured at 10 K Reprinted w ith permission from Figure 1 of D K Hwang H.S. Kim J.H. Lim Appl. Phys.Lett. 86 (2005) 151917 Figure 5 4 PL spectra of Ag doped ZnO (s2) measured at 15 K

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73 Figure 5 5 Plot of VdI 1 as a function of magnetic field for p type Ag doped ZnO s2 Figure 5 6 Acceptor related peak positions as a function of increasing temperature

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74 Figure 5 7. PL intensity grain size relationship for localized bound exciton in Ag doped ZnO Figure 5 8. Room temperature UV PL emission of Ag doped ZnO grown on different buffer layers

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75 Figure 5 9. Room temperature PL spectra for ZnO doped with different elements

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76 CHAPTER 6 ZINC OXIDE DEVICES 6.1 Introduction An important issue in developing ZnO based electronics is the formation of p type material and rec tifying junctions [128 1 29 ]. Studies have indicated that Ag can act as an amphoteric dopant, yielding an acceptor state for su bstitution on the Zn site, and a donor st ate for interstitial defects [70 ]. In the previous chapters, it was shown that Ag is more likely to occupy a substitutional site yielding acceptor state for selected growth conditions. Moreover, f irst principles cal culations have been used to estimate the dopant energy levels and defect formation energies for Ag in ZnO [ 66 ]. The calculations estimate the acceptor state ionization energies for substitutional Ag to be 0.4 eV. Although this predicted value for ionizati on energy is relatively high, the formation energy for this substitutional defect (Ag Zn ) is predicted to be low; energies for interstitial defects are predicted to be high. H. S. Kang et al. recently reported the formation of p type ZnO via Ag doping in th in films grown by pulsed laser deposition [ 67 ]. This study has also examined the growth conditions and stability of p type Ag doped ZnO films Results showed p type conductivity with carrier concentrations as high as 5x10 19 cm 3 as well as, excellent UV p hotoluminescence emission. In this chapter the fabrication and properties of rectifying Ag doped ZnO/Ga doped ZnO thin film junctions is reported. 6.2 Experimental As mentioned above film growth experiments over a wide range of deposition condition s revealed a deposition parameter space where p type SZO thin films could be realized (see Table 4 1). The p type conductivity in the Ag doped ZnO layers was confirmed using Hall measurements taken at various magnetic fields. For the films and

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77 devices cons idered in this chapter, including junctions, the p type SZO films and layers were grown at 500 C in an oxygen pressures of 10 and 25 mTorr. 6.2 .1 Silver Doped ZnO / Gallium Doped ZnO Thin Film Homoj unction The Ag doped ZnO films were grown via pulsed laser deposition. A 254 nm KrF excimer laser was used as the ablation source. The laser was operated at a repetition rate of 1 Hz and an average energy density of 1 J/cm 2 Ablation targets were fabricated using ultra high purity ZnO and Ag 2 O powders. The t argets were sintered at 1000 C in air. The concentration of Ag in the ZnO target was 0.6 at%. Single crystal c plane sapphire substrates were employed for this study. Prior to growth, the substrates were cleaned in an ultrasonic bath using trichloroe thyle ne, acetone, and methanol. The substrates were attached to the heater platen using silver paint. The substrate to target distance was 3.5 cm. Ag doped ZnO layer thickness varied from 250 nm to 450 nm. Ga doped ZnO laser ablation targets were fabricated wit h high purity (99.9995 %) ZnO mixed with gallium oxide (99.998 %) as the doping agent. The gallium doping level in the target was 1 at. %. Ga doped ZnO, which served as the n type layer, was grown on c plane sapphire at 700 C in an oxygen pressure of 1 mT orr. Otherwise, the ablation conditions are as described above for the Ag doped ZnO film. The thickness for the n type Ga doped ZnO layer was about 350 nm. T he resistivity, Hall mobility and carrier concentration were measured using a four point van der Pa uw method Contacts for the Hall measurements were soldered indium. Photoluminescence was measured using a He Cd continuous wave laser operating at 325 nm. Finally, the I V characteristics of Ag doped ZnO/Ga doped ZnO junctions were analyzed

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78 6.2 .2 Silver Doped ZnO T hin F ilm T ransistor (TFT) A bottom gate thin film transistor (TFT) using Ag doped ZnO as the active (p type) layer was fabricated. The film was grown by pulsed laser deposition. A 254 nm KrF excimer laser was used as the ablation source. The las er was operated at a repetition rate of 1 Hz and an average energy density of 1 J/cm 2 Ablation targets were fabricated using ultra high purity ZnO and Ag 2 O powders. The targets were sintered at 1000 C in air. The concentration of Ag in the ZnO target was 0.6 at%. A h eavily doped p + Si wafer was used as substrate. The thermal oxide (SiO 2 dry oxygen source) was 100 nm thick. The source and drain electrodes were patterned by shadow mask. Ni and Au metal contacts were used and the thickness was 20 nm and 80 nm, respectively. The width and length of the electrodes was 750 nm and 50 nm, respectively. The gate w as open by photolithography and buffer oxide etch ( BOE ) etching (1:6, 4 minute etch) and the metal used was In dium The as grown Ag doped ZnO film was gr own without etching to form the specific pattern of the channel. Figure 6 4 shows the schematic of the TFT. 6.3 Results and Discussion 6.3 .1 Rectifying Thin Film Junction Table 6 1 shows the transport properties of Ag doped and Ga doped ZnO films grown under corresponding conditions. Figure 6 1 shows a p lot of V H all d/I as a function of applied magnetic field and room temperature photolum inescence for a Ag doped ZnO grown at 500 C in an oxygen pressure of 25 mTorr. The pn junction fabrication started wi th device isolation and followed with p mesa definition using dilute phosphoric acid solution. Electron beam deposited Ni (20nm)/Au (80nm) and Ti (20nm)/Au(80nm) were used as the p and n Ohmic metallization. Figure 6 2 shows a schematic of the devices fab ricated as well as the test for Ni Au contacts. The size of the devices was 180 m in

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79 diameter for the active area. A rectifying behavior is observed in the I V curve as seen in Figure 6 3 consistent with the Ag doped ZnO layer being p type and forming a pn junction with the Ga doped ZnO layer. The forward bias turn on voltage for t he junction shown was 3.0 V. Reverse bias breakdown occurred at approximately 5.5 V. This rectifying behavior is similar to that seen for ZnO junctions incorporating group V dopants [1 5 1 15 2 ]. Note that, without the metallization of contacts, the device wa s optically transparent in the visible range. Also, note that the deposition of the layered structure in reverse order (Ag doped ZnO on bottom, Ga doped ZnO on top) did not result in rectifying I V characteristics as shown in Figure 6 4 The reason for thi s is unclear but may relate to the differing growth temperatures used for the two layers. The rectifying junction was then examined for light emission intensity. The current voltage (I V) characteristics were measured at 300 K using a probe station and Ag ilent 4145B parameter analyzer. The emission output power from the structures was measured using a Si photodiode. The results for multiple measurements on a typical rectifying junction are shown in Figure 6 5 Note that the non zero light emission with no excitation current is an artifact of the null offset for Si photodiode. The highest emission output power measured was 5.2x10 8 mW. The excitation current used did not exceed 20 mA. At 10mA, the applied voltage was approximately 2.0 V. Several devices we re fabricated that displayed these characteristics. Note that, after each measurement, the light intensity decreased The junction became ohmic after only a few measurements. Subsequent annealing of these junctions did not result in recovery of the light e mission or rectifying I V characteristics. Unfortunately, the relatively unstable nature of light emission under bias made it not possible to obtain a useful electroluminescence

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80 spectrum. The instability appears to be related to surface conduction and perh aps hydrogen incorpor a tion as discussed elsewhere [1 5 3,1 5 4 ]. 6.3.2 Silver D oped ZnO TFT Figure 6 5 shows the measured output and transfer characteristics of Ag doped ZnO TFT grown on Si. Note that the channel behavior is that of an n type TFT operating in charge accumulation mode. The reason for the n type behavior of Ag doped ZnO is unclear. It is believed that growing ZnO on Si may have altered the conductivity of th e film by changing its crystallinity from single to polycrystalline. The mobility of the c hannel was 3.5 cm 2 /Vs and the I on /I off ratio was 10 5 The determined subthreshold slope is 4.4 V/decade. This value suggests that the crystallinity of the channel is very poor. Fabrication of a TFT structure on sapphire (Al 2 O 3 ) is expected to yield better film crystallinity and not to affect the conductivity of the channel, possibly resulting in a p channel TFT. 6.4 Summary In previous chapters, results showed p type conductivity in Ag doped ZnO with carrier concentrations as high as 5x10 19 cm 3 as wel l as, excellent UV photoluminescence emission. In this chapter, the fabrication and properties of rectifying Ag doped ZnO/Ga doped ZnO thin film junctions were reported. A rectifying behavior was observed in the I V characteristic, consistent with Ag doped ZnO being p type and forming a p n junction. The turn on voltage of the device was 3.0 V under forward bias. The reverse bias breakdown voltage was approximately 5.5 V. Devices were optically transparent in the visible range. The highest light emission ou tput power measured was 5.2x10 8 mW. At excitation currents of 10 mA, the applied voltage was approximately

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81 2.0 V. After each measurement the light intensity decreased and the junction became Ohmic. The instability appears to be related to surface conducti on and perhaps hydrogen incorporation. Finally, deposition of layers in reversed order (Ag doped ZnO on bottom, Ga doped ZnO on top) did not result in rectifying I V characteristics. The reason for this is unclear but may relate to the differing growth tem peratures used for the two layers. Thin film transistor structures were also fabricated. Although no p channel behavior was observed, the measured output and transfer characteristics revealed a mobility of 3.5 cm 2 /Vs and a I on /I off ratio of 10 5 The subthe shold slope was determined to be 4.4 V/decade.

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82 T able 6 1 Properties of n type and p type materials used in the junction device Transport Properties Ga doped ZnO n type Ag doped ZnO p type Resistivity ( cm) 0.00127 0.00198 Mobility(cm 2 / V s) 27.10 2.87 Carrier Concentration (cm 3 ) 5.0x10 19 5.9x10 19

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83 Figure 6 1 Plot of (a) V H all d/I as a function of applied magnetic field and (b) room temperature photoluminescence for a Ag doped ZnO was grown at 500 C in an oxygen pressure of 25 mTorr.

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84 Figure 6 2 The ZnO :Ag/ZnO:Ga/sapphire junction (a) schematic of structure and (b ) Test for Ni Au contacts

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85 Figure 6 3 Homojunction I V characteristics

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86 Figure 6 4 The ZnO:Ga/ZnO:Ag/sapphire junction (a) schematic of structure and (b) junction I V characteristic.

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87 Figure 6 5 The emission output power from the ZnO:Ag/ZnO:Ga/sapphire junction as measured using a Si photodiode. Figure 6 6. Schematic of ZnO:Ag thin film transistor

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88 Figure 6 7 ZnO TFT grown on Si output and transfer characteristic s

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89 CHAPTER 7 CONCLUSIONS The research presented in this dissertation focused on Ag doped ZnO thin film grow th by pulsed lase r deposition for the understanding of Ag behavior as a p type dopant of ZnO and realization of light emitting diodes. Firstly, Ag doped ZnO films were investigated to understand the effect of Ag doping on ZnO structural, transport, and opt ical properties and to obtain robust p type ZnO films. P type ZnO films were achieved by doping with Ag without the need of post annealing steps Photoluminescence properties were examined and optimized Lastly, ZnO LEDs were fabricated by using Ag doped p type ZnO layer. Light emission was observed. 7.1 P type Silver Doped ZnO Films The synthesis and properties of Ag doped ZnO thin films were examined. Epitaxial ZnO films doped with 0.6 at% Ag content grown at moderately low temperatures (300 C to 500 C ) by pulsed laser deposition yielded p type material as determined by room temperature Hall measurements. Hole concentrations on the order mid 10 1 5 to mid 10 19 cm 3 range were realized. Growth at higher temperatures yielded n type material, suggesting that the Ag was driven out of the substitutional site above 500 C and that Ag substitution yielding an acceptor state is metastable. Photoluminescence measurements showed strong near band edge emission with little mid gap emission as the result of Ag substitu tion for Zn (Ag Zn ) and reduction of surface states deleterious to UV photoluminescence emission. The stability of the Ag doped films was examined as well. Presumably hydrogen incorporation caused the films to turn n type after about 120 days. Persistent p hotoconductivity was also observed. High temperature ZnO buffer

PAGE 90

90 layers drastically improved the surface morphology of films thicker than 1.0 m. roughness below 10 nm were observed for films grown on ZnO substrates. 7.2 Silver Related Acceptor State and Op timized Ultraviolet in Silv er Doped ZnO Thin Films Silver doped ZnO films were grown via PLD and their optical properties were discussed. Results showed that Ag inclusion lead to grain growth and smaller non radiative relaxation rates over surface states, which lead to UV emission enhancement. Room temperature PL measurements also showed a suppression of ZnO visible luminescence suggesting that Ag does not occupy interstitial sites or an antisite. Low temperature and temperature dependent PL spectroscopy r evealed strong and dominant emissions originating from free electron recombination to Ag related acceptor states around 3.31eV. The A X emission at 3.352 eV was also observed at low temperatures. Enhancement of the PL intensity with increasing grain size, and the peak position blue shits (low temperatures) followed by red shits (high temperatures) with increasing temperature, confirmed the nature of the emission. Donor acceptor pair emission and corresponding phonon replicas were also observed. The acceptor energy was estimated to be 124 meV. The very weak deep level emission (not shown), again in the low temperature PL spectra indicates that in the p type ZnO:Ag native donor and acceptor defects are suppressed suggesting the observed acceptor related PL emi ssions and hole concentration are from the Ag in ZnO instead of native defects. High temperature ZnO buffers and lattice match MgCaO buffers helped improve the UV emission of the Ag doped films. A combination of band bending at the MgCaO ZnO interface and reduction of surface states may be responsible for such enhancement. No visible luminescence was observed. Finally, the room temperature PL

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91 spectrum of Ag doped ZnO was compared to that of undoped, P doped, Ga doped, and Ag Ga codoped ZnO. The Ag doped Zn O films showed better optical properties. 7.3 Rectifying pn Junction and TFT Device s Both p type conductivity in Ag doped ZnO with carrier concentrations as high as 5x10 19 cm 3 as well as, excellent UV photoluminescence emission were achieved. The fabrica tion and properties of rectifying Ag doped ZnO/Ga doped ZnO thin film junctions were reported. A rectifying behavior was observed in the I V characteristic, consistent with Ag doped ZnO being p type and forming a p n junction. The turn on voltage of the de vice was 3.0 V under forward bias. The reverse bias breakdown voltage was approximately 5.5 V. Devices were optically transparent in the visible range. The highest light emission output power measured was 5.2x10 8 mW. At excitation currents of 10 mA, the a pplied voltage was approximately 2.0 V. After each measurement the light intensity decreased and the junction became Ohmic. The instability appears to be related to surface conduction and perhaps hydrogen incorporation. Finally, deposition of layers in rev ersed order (Ag doped ZnO on bottom, Ga doped ZnO on top) did not result in rectifying I V characteristics. The reason for this is unclear but may relate to the differing growth temperatures used for the two layers. An n type Ag doped ZnO TFT operating in charge accumulation mode was fabricated. The reason for the n type behavior of Ag doped ZnO is unclear. It is believed that growing ZnO on Si may have altered the conductivity of the film by changing its crystallinity from single to polycrystalline. The mo bility of the channel was 3.5 cm 2 /Vs and the I on /I off ratio was 10 5 The determined subthreshold slope is 4.4 V/decade. This value suggests that the crystallinity of the channel is very poor. Growing the Ag doped ZnO TFT structure on sapphire, to avoid cry stallinity issues has been suggested. The

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92 conductivi ty is expected to remain p type however a top gate TFT structure would be required.

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102 BIOGRAPHICAL SKETCH Fernando Lugo was born in 19 8 3 in Barquisimeto Venezuela After graduating from high school in 2001 he attended Miami Dade College and earned an Associate of Arts degr ee in May 2003 He then began to pursue a Bachelor he University of Florida in Material Science and Engineering. He received his Bachelor degree in the spring of 2006 He conducted his undergraduate research for bachelor degree on the grow th of ZnO materials for optoelectronic applications under the supervision of Dr. David P. Norton In summer 2006, he enrolled in graduate school at the University of Florida in the Department of Materials Science and Engineering to pursue a Ph.D under th e advisement of Dr. David P. Norton. His main research involved growth and characterization of ZnO thin films for light emitting diodes. He is a SEAGEP fellow and is the co author of more than 10 journal and conference papers. He was a member of the Univer sity of Florida Ultimate Frisbee club team from 2007 to 2010