1 SYNTHESIS AND CHARACTERIZATION OF SILVER DOPED ZINC OXIDE THIN FILMS FOR
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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 REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2010
Transcript of 1 SYNTHESIS AND CHARACTERIZATION OF SILVER DOPED ZINC OXIDE THIN FILMS FOR
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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 REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
2010
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© 2010 Fernando Lugo
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To my future wife and kids
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ACKNOWLEDGMENTS
I would like to thank to my advisor, Dr David P. Norton, for his guidance, patience,
and unconditional 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 of labor and advice in setting up photoluminescence equipment, and
Dr. Cammy R. Abernathy for trusting me with her lab equipment.
I would like to express a more than special thanks to all the group members in Dr.
Norton’s group, Dr. Mat Ivill, Dr. George Erie, Dr. Yuanjie Li, Dr. Seemant Rawal. Dr. Li-
Chia Tien, Dr. Hyun-Sik Kim, Dr. Patrick Sadik, Dr. Daniel Leu, Dr. Charlee Callender,
Ryan Pate, Joe Cianfrone, Seon-Hoo Kim, and Kyeong-Won Kim. It has truly been a
pleasure to meet them and share 6 years of research with every one of them. I am
especially grateful to my collaborators in Dr. Ren’s group including 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 Gerger, 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 acknowledge the Board of Eductation, the Naval Office of Research and
Scientific Development, the National Science 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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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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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 Defects ...................................................... 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 transistors .................................................................. 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 Preparation ........................................................... 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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4.2 Experimental Procedures .................................................................................. 45 4.3 Results and Discussions ................................................................................... 45
4.3.1 Role of Ag Doping in ZnO Crystal Quality and Surface Morphology ....... 45 4.3.2 P-Type Conductivity, Transport and Optical Stability of Ag-Doped ZnO
Films.............................................................................................................. 47 4.4 Summary .......................................................................................................... 50
5 PHOTOLUMINESCENCE 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 Inclusion .......................................................... 64 5.3.2 Low Temperature and Temperature Dependent PL ................................ 65
5.3.3 Buffer Layer and Dopant Effect on the Optical Properties of ZnO ........... 67 5.4 Summary .......................................................................................................... 68
6 ZINC 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 Doped 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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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 device ................ 82
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LIST OF FIGURES
Figure page 2-1 Wurtzite structure of ZnO.................................................................................... 30
2-2 Bandgap and lattice constant of various semiconductors ................................... 30
2-3 The PL spectrum, EL spectrum, and EL image of the ZnO light emitting device. ................................................................................................................ 31
2-4 ZnO p-n homojunction a) EL spectrum under forward current injection and b) room temperature 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 heterostructure LED at 300 K and 500 K. ................................................................................................................. 32
2-7 Room temperature spectral photoresponsivity of the n-ZnO/-p-SiC photodiode 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 (Id) vs source–drain voltage (Vd) at gate voltages (Vg) between 0 and 50 V for a ZnO TFT. The corresponding transfer characteristic, Id vs Vg at a fixed Vd equal to 20 V, for the same 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) system ................................................................ 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 ºC, and (b) films grown at 300 ºC in oxygen pressures ranging from 1-75 mTorr. ................................................................... 53
4-2 Effect of Ag doping on ZnO d-spacing for films grown at 500 ºC ........................ 54
4-3 AFM images for films grown at (a)(b) 300ºC, (c) 400ºC, and (d) 500ºC.............. 54
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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 V·d/I as a function of magnetic field for (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 300ºC, P(O2) = 75 mTorr ...... 58
4-9 Effects of (a) UV light exposure and dark storage, showing (b) exponential decay of conductivity over time in dark storage for films grown at 300ºC, P(O2) = 75 mTorr ................................................................................................ 59
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. ........................................................................ 60
4-11 Room temperature absorption spectra for Ag-doped ZnO .................................. 61
5-1 Room temperature 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 undoped ZnO measured at 10 K .................................................. 72
5-4 PL spectra of Ag-doped ZnO (s2) measured at 15 K ......................................... 72
5-5 Plot of V·d·I-1 as a function of magnetic 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 intensity-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) VHalld/I as a function of applied magnetic field and (b) room temperature photoluminescence for a Ag-doped ZnO was grown at 500 ºC 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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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 characteristic.................................................................................... 86
6-5 The emission output power from the ZnO:Ag/ZnO:Ga/sapphire junction as measured using a Si photodiode. ....................................................................... 87
6-6 Schematic of ZnO:Ag thin film transistor ........................................................... 87
6-7 ZnO TFT grown on Si - output and transfer characteristics ............................... 88
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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy
SYNTHESIS AND CHARACTERIZATION OF SILVER DOPED ZINC OXIDE THIN
FILMS FOR OPTOELECTRONIC DEVICES
By
Fernando Lugo
May 2010
Chair: David P. Norton Major: Materials 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 ºC
to 500 ºC) 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 m.
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
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dominant emissions originating from free electron recombination to Ag-related acceptor
states around 3.31eV. The AºX 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
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characteristics. The reason for this is unclear but may relate to the differing growth
temperatures used for the two layers.
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CHAPTER 1 INTRODUCTION
In recent years, the market of electronic devices that source, detect, and control
light has grown rapidly. Light emitting diodes (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 is considered an alternative 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 epitaxial 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 ZnO films compare to GaN [5].
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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 candidate 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.
MgxZn1-xO and CdxZn1-xO 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 conductivity. Compensating
defects such as Oxygen vacancies (Vo) and Zinc interstitials (Zni), 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, reducing 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 acceptor 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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doping on ZnO thin film properties grown by pulsed laser deposition (PLD). In addition,
it addresses the fabrication of rectifying junctions, and p-MOS devices.
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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 sp3 covalent bonding, but has considerable ionic
character. Although the wurtzite structure is thermodynamically stable at ambient
conditions, a zinc-blende structure may be obtained when grown on cubic substrates,
while rocksalt (NaCl) when grown at high pressures. The lattice parameter of wurtzite
ZnO are a = 3.2495 Å and c = 5.2069 Å [13]. The unit cell is composed of two
interpenetrating hexagonal-closed-packed (HCP) sublattices, where 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 Å and 1.38 Å 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-like state having Γ7 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 addition, 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 binding energy (60 meV), has enabled room temperature
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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 Defects
Undoped ZnO normally exhibits n-type conductivity. The role of native defects
such as vacancies (VO and VZn), interstitials (Zni and Oi), and antisites (ZnO and OZn) in
undoped ZnO is not yet clearly understood. Several studies [15-17] claim that such
native defects create shallow donor states. D.C. Look et al. suggested that Zni rather
than VO 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 Zni
are unstable and diffuse at room temperature, while VO 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 H2 by pulsed laser
deposition (PLD). Secondary 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 and act as a shallow donor as well.
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Unlike interstitial H, substitutional H is stable and has a high migration energy, thus,
making it a strong candidate for H-related donors in as-grown ZnO [28].
In order to efficiently dope and fully exploit the 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 VII 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
1021 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 fabricated, 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 Zinc 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 part, 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
small mixing of cation p-d orbitals. Therefore, group I doping results in smaller
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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 larger 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 (NZn)
[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 1015 to 1018 cm-3, using NO as its dopant source. W. Xu et al. [43], using both NO
and N2O, obtained similar carrier concentrations and resistivities as low as 3.02 -cm.
Z-Z. Ye et al. [44] grew p-type films using NH3-O2 and obtained carrier concentrations of
3.2x1017 cm-3 and 1.8 cm2V-1s-1 mobilities.
Oxygen-poor growth conditions are required to incorporate dopants into O sites,
which promotes “hole killer” defects and compensation [39, 45-47]. 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 order 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 results showed an increase of N-solubility by a factor of 400 when using Ga as
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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 concentration was 4x1019 cm-3 with 2.0 -cm resistivity. However, the carrier
mobility did not improve. Sing et al. [49], showed a relation between VO, oxygen partial
pressure and carrier type. Results showed that at ratios of oxygen partial pressure to
total pressure (4x10-4 Torr) below 40% films 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 also achieved using Al [50] and In [51] as
codpants, however, high carrier concentrations and incredibly high (~150 cm2V-1s-1)
mobilities bring the validity of the results into question. In other studies [52-54], despite
achieving high concentration of N incorporation, 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 theoretical calculations, both P and As are predicted to be deep
acceptors and carrier type inversion rather difficult to achieve. Other compensating
defects, such as antisites, AX centers and VZn, 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 Zn3P2. 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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Capacitance-Voltage (CV) measurements showed that Zn0.9Mg0.1O can be made p-type
using P2O5 as the phosphorus source, but p-type ZnO was not achieved [59].
The lack of reproducibility, 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 inhomogeneous 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 3d10 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 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 (CuZn, AgZn, and AuZn) 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 group IB
elements as compared to the group V dopants.
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Within the group IB elements, Ag has the lowest predicted 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 size and low atomic d-orbital energy of Ag. Interestingly,
the O-rich conditions that have been suggested for preventing oxygen vacancy (VO)
and/or Zn interstitial (Zni) 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 deposition temperatures of 200-250ºC. Studies on Ag-
implanted ZnO suggest that Ag substitution on the Zn site becomes unstable at
temperatures greater than 600ºC [72]. This is consistent with the estimated 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 ternary alloy AxZn1-xO (where A = Mg or Cd) is given
by the following equation [87]:
Eg(x) = (1-x)EZnO + xEAO – bx(1-x) (1)
Where b is the bowing parameter and EAO and EZnO are the band-gap energies of
compounds AO and ZnO, Respectively.
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 band-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 (Eb > 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
Eg(y)=3.29−4.40y+5.93y2. 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, high 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 film, 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-
26
luminescence (EL) was observed under reverse 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 Figure 2-3.Similar results were
obtained for ZnO films deposited on GaAs substrate [89]. Arsenic 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, research 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 Cu2O and ZnO substrate as p and n layers
respectively, measurements revealed EL in both forward and reverse 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 different
emission bands as the result of carrier diffusion from low to high resistive layers. Other
materials such as Si [92], GaN [93], AlGaN [94], SrCu2O2 [95], NiO [96], CdTe [97], and
SiC [98] have been used with n-type ZnO to create useful heterostructure devices.
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, Al0.12Ga0.88N was used in order achieve carrier injection
into ZnO [94] and UV (389 nm) emissions attributed to excitonic recombination in ZnO,
were observed (see Figure 2-6). Furthermore, Alivov et al. [99] fabricated laser diodes
(LD) by growing n-GaN/n-ZnO/p-GaN structures, thus achieving carrier confinement. I-V
characteristics 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-ZnRh2O4 [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 photoresponse 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
The combination of transparency in the visible, excellent transistor characteristics,
and low temperature processing makes ZnO thin-film transistors (TFTs) attractive for
flexible electronics on temperature sensitive substrates. Carcia et.al [103],
demonstrated the fabrication of transparent ZnO TFTs by RF magnetron sputtering near
28
room temperatures using n-type Si substrates and 100 nm thick ZnO. High field effect
mobilities of 1.2 cm2/Vs and Ion/Ioff ratio of 1.6x106 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 insulator (SiO2 + SiNx) was used to obtain an Ion/Ioff
ratio of 105 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 fabrication 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.
29
Table 2-1. Calculated defect energy level for group I and V dopants
Defect Defect energy level (eV)*
Group I
LiZn NaZn KZn
0.09 0.17 0.32
NO 0.40 Group V PO 0.93
AsO 1.15 *Relative to valance band maximum (VBM)
30
Figure 2-1. Wurtzite structure of ZnO
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.50
1
2
3
4
5
6
7
8
Bandgap E
nerg
y (
eV
)
Lattice Parameter (Å)
MgO
CdO
ZnOGaN
AlN
InN
6H-SiC
Si
Ge
ZnTe
CdSe
InAs
MgSe
MgS
ZnSe
ZnS
CdS
InP
GaP
GaAs
AlAs
Figure 2-2. Bandgap and lattice constant of various semiconductors
31
Figure 2-3. The PL spectrum, EL spectrum, and EL image of the ZnO light emitting device. Reprinted with permission 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) room temperature 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 Physics Letters 460 (2008) 548.
32
Figure 2-5. EL spectrum of 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 heterostructure 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.
33
Figure 2-7. Room temperature 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, Ü. Özgür, S. Doğan, D. Johnstone, V. Avrutin, N. Onojima, C. Liu, J. Xie, Q. Fan, and H. Morkoç, Appl. Phys. Lett. 86, (2005) 241108.
Figure 2-8. (a) is a set of transistor curves of drain current (Id) vs source–drain voltage (Vd) at gate voltages (Vg) between 0 and 50 V for a ZnO TFT. The corresponding transfer characteristic, Id vs Vg 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.
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 permission from Figure 9 of S.Masuda, K.Kitamura, Y.Okumura, S.Miyatake,H.Tabata,and T. Kawai, J. Appl. Phys. 93, (2003) 1624.
35
CHAPTER 3 MATERIALS AND CHARACTERIZATION TECHNIQUES
The synthesis and characterization of Ag-doped ZnO thin films 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 (PO2), 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 were used to investigate the microstructure of the
films. The surface morphology of the films was characterized 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 ºC to 1000 ºC 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 opposite 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, structural
and chemical composition of the ablation target and background gas, chamber
pressure, and substrate temperature and distance to ablation target.
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/cm2 at the target surface. The
background gas used during all growths was ultra-high purity Oxygen.
3.1.2 Target and Substrate Preparation
Ablation targets were fabricated using ultra high purity ZnO and Ag2O powders.
The targets were sintered at 1000 ºC 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 some 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, and 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
(VH). 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:
𝑅𝐻 =𝑟(𝑝−𝑏2)
𝑞(𝑝+𝑏𝑛 )2 =𝑟𝐻
𝑞𝑝 (for p-type) or 𝑅𝐻 = −
𝑟𝐻
𝑞𝑛 (for n-type) (2)
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 VH
𝑅𝐻 =𝑡𝑉𝐻
𝐵𝐼=
1
𝑞𝑝 (for p-type) or 𝑅𝐻 = −
1
𝑞𝑛 (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 using the
following equations.
Mobility: 𝜇𝐻 =𝑅𝐻
𝜌; (4)
Resistivity: 𝜌 =𝑅𝑤𝑡
𝐿=
𝑉/𝐼
𝐿/𝑤𝑡 (5)
Where R is the resistance, w is the 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
Hall 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,
and 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
38
electron-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 valence band, allowing for bandgap determination. Information about
localized defects 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; therefore,
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 Diffraction (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 satisfy Bragg’s Law.
Bragg’s Law: 𝑛𝜆 = 2𝑑 sin 𝜃 (6)
where n is the order of diffraction, λ is the x-ray wavelength, d is the interplanar
spacing of crystal planes, and θ is the incident angle of x-ray.
In this study, the crystal structure of the films was characterized using 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
39
crystal quality and symmetry was obtained, from omega and phi scans, using the Philips
X’pert high resolution diffractometer.
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 that 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 resonance
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.
40
Figure 3-1. Pulsed laser deposition (PLD) system
Figure 3-2. Hall Effect diagram
41
Figure 3-3. Common radiative transition mechanism
Figure 3-4. Block diagram of atomic force microscope
42
CHAPTER 4 SILVER DOPED ZNO FILMS GROWN VIA PULSED LASER DEPOSITION
4.1 Introduction
ZnO is a wide bandgap semiconductor that is attracting significant attention for
thin-film electronics [113-115], nanoelectronics [116-118], photonics [119,120],
piezoelectrics [121,122] and sensor applications [3,123-125]. The crystal structure of
ZnO is wurtzite, with lattice parameters a = 3.25 Å and c = 5.12 Å [126]. 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 emission in the ultraviolet at
temperatures up to 550 K, establishing ZnO as an interesting photonic semiconducting
oxide [127]. An important issue in developing ZnO-based electronics is the formation of
robust p-type material. This is obviously relevant for the fabrication of pn junctions for
minority carrier injection [128-130]. 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 [132,133]. 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 challenging due to its propensity to create compensating
donor defects and the relatively large energy necessary to create unfilled states in the
deep valence band [9].
Candidate p-type dopants include the group V anions on the oxygen site or group
I or IB elements 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 [31,134],
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
NZn 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.
This growth condition promotes the formation of “hole-killer” defects yielding
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
group V dopants on the Zn site [135,136]. While several groups have been successful
in achieving p-type conductivity via group V doping [31, 33-39,134,137], low carrier
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 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
[69]. However, recent studies suggest this may not to be the case. One study reported
an acceptor state binding energy for the Ag 3d10 states of only 200 meV [70]. 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
44
defects [71]. 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 (CuZn, AgZn, and AuZn) 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 group IB
elements as compared to the group V dopants.
Within the group IB elements, Ag has the lowest predicted 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 size and low atomic d-orbital energy of Ag. Interestingly,
the O-rich conditions that have been suggested for preventing oxygen vacancy (VO)
and/or Zn interstitial (Zni) 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 deposition temperatures of 200-250ºC. Studies on Ag-
implanted ZnO suggest that Ag substitution on the Zn site becomes unstable at
temperatures greater than 600ºC [72]. This is consistent with the estimated 0.08 mol%
bulk solid solubility of Ag in ZnO [73]. In 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.
45
4.2 Experimental Procedures
The ZnO:Ag ablation targets were fabricated using ultra high purity ZnO and Ag2O
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 regarding 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 occasions, 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/cm2. The film thickness varied
from 300 nm to 1.1m 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 and 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 data for
46
films grown in 25 mTorr oxygen for the deposition temperatures ranging from 300-
600ºC. The results showed that the films are c-axis oriented and ZnO is the primary
phase. The diffraction does show the emergence of a secondary impurity phase for
growth at 400 oC and 500 oC. The small, broad peak could be associated with either
Ag2O 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
Ag2O bonding energies are present in the films with the impurity peak in the diffraction
pattern. Above 300 oC Ag2O is not stable and losses oxygen. Under these conditions,
the formation of Ag metal is expected for any Ag segregation, however, Ag quantities
are very small and no continuous secondary phase is observed. Figure 4-1b shows the
diffraction data for films grown at 300 oC for oxygen pressures ranging from 1-75 mTorr.
For growth at this low temperature, the impurity peak is not observed in the diffraction
data for the oxygen pressures considered. Also note that the ZnO (110) orientation was
not observed for growth at 300 ºC. No significant shift in d-spacing from that for
undoped ZnO was detected due to doping (Figure 4-2).
The surface morphology of the films was examined using atomic force
microscopy (AFM). Figure 4-3 shows AFM images for films grown at 300ºC, 400oC and
500ºC 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 [138,139]. In these experiments, it was speculated that the formation of Ag2O
in oxide growth provides a source of atomic oxygen through the continuous formation
and dissociation of Ag2O. The grain size measured for undoped ZnO was approximately
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 400oC and 500oC was 390 and
560 nm, respectively. The AFM results shows that as growth temperature is increased,
roughness also increased. At 300 ºC the average roughness was 5.36 nm while at 400
ºC and 500 ºC 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 100 nm. Such high
roughness makes it difficult to obtain reliable transport and optical properties. Growing
on mismatched substrates like sapphire introduces strain and a high density of interface
defects that may result in inhomogeneous carrier concentration measurements that may
yield the wrong carrier type [65]. It is worth noting that the surface of the thick SZO films
showed dark spots which may block any light coming out of the films during optical
characterization. Therefore, in an attempt to obtain smooth, 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 roughness 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 temperature
Hall measurements. As explained in chapter 3, in order to reliably delineate the carrier
48
type and 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 VHalld/I as a function of applied magnetic field, where VHall is
the Hall voltage, d is the film thickness, and I is the measurement current. It shows a
plot of V·d·I-1 as a function of magnetic field for two samples, a Ag-doped ZnO film
grown at 600 ºC that is n-type (as determined by the full Van der Pauw four-point
calculation), and another film grown at 300 ºC that is p-type. For the sample grown at
600 ºC, 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. From
the slope of the curve, the extracted hole carrier density is 5.2x1016 cm-3.
Figure 4-7 shows the results for resistivity and carrier concentration of 0.6 at%
SZO films for some of the growth conditions considered. For most deposition conditions,
the films were n-type (Table 4-1). For growth temperatures in the range of 300-500ºC,
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 ºC in oxygen
pressure of 10 and 25 mTorr. For p-type material, these conditions were optimal. For
these films, the hole carrier concentration was in the mid-1019 cm-3. The mobility for
films grown at 400 ºC and 500 ºC was 10.7 and 2.9 cm2/Vs, respectively. Note that for
growth at 300 ºC, P(O2)=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
oC, the carrier concentration and resistivity were independent of growth pressure.
Presumably, 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.
49
ZnO:Ag films grown at 300 oC, P(O2) = 75 mTorr were stored in the dark and Hall
measurements were performed to study the stability of the transport properties over
time. Before storing in the dark, the samples were exposed to indoor room light for
approximately 24 hours. Figure 4-8 shows resistivity vs. time for a film grown at 300 oC
in 75 mTorr of O2. Note that this was a different film from that considered in figure 4-6b,
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-term 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 the
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
254 nm) after being stored in the dark for some time. A persistent photoconductivity was
observed as shown in Figure 4-9a. The drop in resistivity was independent of the
wavelength used. The relaxation time for this set of films was about 24 hours. Figure 4-
9b 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, τ, was extracted to 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
50
films showed well-defined band edge emission around 377 nm. This emission line is
due to free exciton 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 - 500ºC. 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
better optical properties [68]. Note that there is little visible emission often seen in ZnO
films [140] due to recombination involving mid-gap states suggesting lower
concentration of compensating defects in the films.
4.4 Summary
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 ºC to 500 ºC) by pulsed laser deposition yielded p-type material as
determined by room temperature Hall measurements. Hole concentrations on the order
mid-1015 to mid-1019 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
ºC 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 substitution for Zn (AgZn) 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 photoconductivity was also observed. High temperature ZnO buffer
layers drastically improved the surface morphology of films thicker than 1.0 m.
51
roughness below 10 nm were observed. Finally, in 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-
doped ZnO films are reported in the following chapters.
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(O2) (mTorr) G
row
th T
em
pe
ratu
re (
ºC)
1 10 25 50 75
300 C
16.7 -cm 4.0x10
16cm
-3
16.7 cm2V
-1s
-1
n-type
4.10 -cm 3.0x10
17cm
-3
5.10 cm2V
-1s
-1
n-type
0.47 -cm 1.3x10
19cm
-3
1.05 cm2V
-1s
-1
n-type
0.133 -cm 2.9x10
19cm
-3
1.63 cm2V
-1s
-1
n-type
11.96 -cm 5.2x10
16cm
-3
10.1 cm2V
-1s
-1
p-type
400 C
0.36 -cm 2.7x10
18cm
-3
6.53 cm2V
-1s
-1
n-type
119 -cm 8.6x10
15cm
-3
5.99 cm2V
-1s
-1
p-type
0.0194 -cm 2.9x10
19cm
-3
10.7 cm2V
-1s
-1
p-type
0.388 -cm 3.5x10
18cm
-3
4.68 cm2V
-1s
-1
Mixed-type
1.82 -cm 4.0x10
18cm
-3
0.85 cm2V
-1s
-1
n-type
500 C
0.041 -cm 1.7x10
19cm
-3
9.28 cm2V
-1s
-1
n-type
3.52 -cm 5.5x10
17cm
-3
3.2 cm2V
-1s
-1
p-type
0.0017 -cm 5.9x10
19cm
-3
2.9 cm2V
-1s
-1
p-type
0.088 -cm 5.0x10
18cm
-3
14.1 cm2V
-1s
-1
n-type
1.25 -cm 1.2x10
18cm
-3
4.34 cm2V
-1s
-1
n-type
600 C
0.053 -cm 5.2x10
18cm
-3
22.8 cm2V
-1s
-1
n-type
0.102 -cm 3.2x10
18cm
-3
15.5 cm2V
-1s
-1
n-type
0.142 -cm 4.3x10
18cm
-3
10.2 cm2V
-1s
-1
n-type
0.0692 -cm 4.1x10
18cm
-3
22.3 cm2V
-1s
-1
n-type
0.0273 -cm 9.7x10
18cm
-3
23.6 cm2V
-1s
-1
n-type
53
Figure 4-1. Powder XRD pattern for films grown in (a) 25 mTorr for deposition temperature range of 300-600 ºC, and (b) films grown at 300 ºC in oxygen pressures ranging from 1-75 mTorr.
30 40 50 60 70 80
Ag2O
(200)
Sapphire
a)
ZnO
(101)
ZnO
(002)
ZnO
(110)
ZnO
(004)
600 oC
500 oC
300 oC
400 oC
Undoped
Inte
nsity (
co
un
ts)
2 (odeg)
30 40 50 60 70 80
b)
Undoped
ZnO
(002) Sapphire
ZnO
(004)
75 mTorr
50mTorr
25 mTorr
1.0 mTorrInte
nsity (
co
un
ts)
2 (odeg)
54
0.0 0.1 0.2 0.3 0.4 0.5 0.6
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
Shift
in d
-spacin
g (
%)
Ag content (at%)
Figure 4-2. Effect of Ag doping on ZnO d-spacing for films grown at 500 ºC
Figure 4-3. AFM images for films grown at (a)(b) 300ºC, (c) 400ºC, and (d) 500ºC
55
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
10
20
30
40
50
60
70
80
90
100
Avera
ge R
oug
hness R
AV
E(n
m)
Ag content (at%)
HTB on Sapphire
No Buffer
HTB on ZnO
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 layer
56
Figure 4-6. Plot of V·d/I as a function of magnetic field for (a) an n-type and (b) a p-type Ag-doped ZnO film.
-10000 -5000 0 5000 10000-0.0010
-0.0005
0.0000
0.0005
0.0010
0.0015
0.0020a)
ZnO:Ag 600 oC 1.0 mTorr
n = 5.17x1018
cm-3
= 22.78 cm2/Vs
= 0.05296 -cm
Vd
/I (
Vcm
/A)
Field (G)
-10000 -5000 0 5000 10000-0.65
-0.60
-0.55
-0.50
-0.45
-0.40
-0.35
b)ZnO:Ag 300 oC 75 mTorr
p = 5.23x1016
cm-3
= 10.09 cm2/Vs
= 11.97 -cm
Vd
/I (
Vcm
/A)
Field (G)
57
Figure 4-7. Resistivity (a) and carrier concentration (b) as a function of growth
conditions
0 10 20 30 40 50 60 70 8010
-3
10-2
10-1
100
101
102
103
a)
Re
sis
tivity (
-cm
)
PO2 (mTorr)
300oC
400oC
500oC
600oC
0 10 20 30 40 50 60 70 80
1015
1016
1017
1018
1019
1020
b)
p-type
p-type
Ca
rrie
r C
on
ce
ntr
atio
n (
cm
-3)
PO2 (mTorr)
300oC
400oC
500oC
600oC
58
Figure 4-8. Resistivity as a function of time for films grown at 300 ºC, P(O2) = 75 mTorr
0 20 40 60 80 100 120 140 160 180 200
1000
1500
2000
2500
3000
3500
4000
p-type
n-type
Re
sis
tivity (
-cm
)
time (days)
59
Figure 4-9. Effects of (a) UV light exposure and dark storage, showing (b) exponential
decay of conductivity over time in dark storage for films grown at 300ºC, P(O2) = 75 mTorr
1600 1700 1800 1900 2000 2100 2200 2300 2400
0.0
2.0x10-5
4.0x10-5
6.0x10-5
8.0x10-5
1.0x10-4
1.2x10-4
1.4x10-4
b)
experimental curve
fitted exponential curve
CD = 0.02284 -cm
-1
= 227.85 min
= CDe
(-t/)
Co
nd
uctivity (
-cm
-1)
time (min)
1200 1500 1800 2100 2400
0
1x105
2x105
3x105 a)
UV light off
UV light on
= 254 nm
= 304 nm
= 365 nm
Re
sis
tivity (
-cm
)
time (min)
60
Figure 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.
350 400 450 500 550
0.05
0.10
0.15
0.20
0.25
a)
300 oC
400 oC
500 oC
Inte
nsity (
a.u
)
Wavelength (nm)
340 350 360 370 380 390 400 410 420 430
0.00
0.05
0.10
0.15
0.20
0.25b)
500 oC
400 oC
300 oC
377 nm
Inte
nsity (
a.u
)
Wavelength (nm)
61
200 250 300 350 400 450 500 550 600-50
0
50
100
150
200
250
300
350
4006.0 5.0 4.0 3.0
(hx
10
10 (
eV
/cm
)2
(nm)
Eg=3.246 eV
Figure 4-11. Room temperature absorption spectra for Ag-doped ZnO
62
CHAPTER 5 PHOTOLUMINESCENCE STUDY OF ZINC OXIDE
5.1 Introduction
Light emitting diodes are expected to develop as a major market; therefore, it is
important to investigate the optical properties of different ZnO films. Not only does it
help to analyze the structure and other properties of ZnO, but also it contributes to the
optimization of the growth process of ZnO. In order to make high-performance
optoelectronic devices on ZnO, it is necessary to investigate 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 involving an electron from the conduction 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 and 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 temperature 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 substantial 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 donor, leaving
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 optical 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 buffer 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-doped ZnO was
realized using Ag 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. The 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 paint. The substrate to target distance was 3.5 cm. Some of the Ag-
doped ZnO samples were grown on a high temperature ZnO buffer layers. The growth
temperature and P(O2) was 800 ºC and 1mTorr respectively and were 50 nm thick. ZnO
lattice-matched MgCaO buffers 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. Table 5-1 lists the growth condition, doping content, and
64
heat treatment used for all samples analyzed. The resistivity, Hall mobility and carrier
concentration were measured using a four-point van der Pauw method. Contacts for the
Hall measurements were soldered 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 Results and Discussions
5.3.1 PL Enhancement via Ag Inclusion
The room temperature PL spectra of undoped ZnO (s1) and Ag-doped ZnO (s2)
are shown in Figure 5-1. The undoped sample shows two characteristic emission
bands. The near band edge emission centered at 377 nm which originates from free
exciton recombination [141] and a dominant broad deep level emission centered at
505nm, which originates from structural defects and impurities [142]. 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 have better optical properties [68]. It has been suggested that if
Ag occupies a Zn site (AgZn), 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 [143] and thus better exciton diffusion
through the crystal and higher 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 abs. units to 8.4 abs. units as grain size increased from 168 nm to 330 nm
65
respectively. Further grain growth to 560 nm resulted in higher PL intensity (11.7 abs.
units). The suppression of the broad deep band suggests that Ag does not occupy an
interstitial site or an antisite [144].
5.3.2 Low Temperature and Temperature Dependent PL
Hwang et. al. [145], resolved both donor and acceptor bound exciton transitions in
in high quality undoped ZnO. The PL spectra measured at 10 K is shown in Figure 5-3.
The undoped sample shows two main peaks. A strong PL emission centered at 3.363
eV which originates from localized exciton recombination bound to a donor state (DºX)
[31, 144-148]. The next PL peak is slightly weaker and it is centered at 3.352 eV and
has been previously identified as the acceptor-bound exciton transition (AºX) [149].
Meyer et al. [146] showed that 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, Aº) are expected around 3.31 eV
[144,145,150] but are rarely seen in undoped samples. Similarly, the Ag-doped sample
shows a series of main peaks followed by small shoulders shown in Figure 5-4. The first
peak centered at 3.353 eV corresponds to AºX emissions, preceded to its right by a
small shoulder around 3.36 eV corresponding to DºX. A strong peak centered at 3.31
eV dominates the spectrum. As mentioned above this emission originates from free
electrons to neutral acceptor state transitions, in this case, believed to be Ag related. It
should be noted that the PL intensity from DºX (3.36 eV) is relatively small compared to
the AºX and (e,Aº) peaks suggesting that the Fermi level should be near the acceptor
level. This is consistent with the p-type conductivity of the sample measured by Hall
(see Figure 5-5). These results are also consistent with those found in P-doped ZnO by
66
Li et al [150]. 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
peaks 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,Aº) peak position to
blue-shift at low temperatures from 15 K to 110 K, followed by a red-shift at higher
temperatures. This is a typical behavior seen in PL peaks assigned to radiative
recombination of free-neutral acceptor [144,150]. 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
𝐸𝑒𝐴 = 𝐸𝑔 − 𝐸𝐴 +𝐾𝐵𝑇
2 (7)
where Eg and EA 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
EA = Eg(3.437) - EeA(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 sizes, s2 and s3, were compared at low
temperatures. The low temperature PL spectra showed that both peaks assigned AºX
and (e,Aº) drastically increased their intensities with grain size. As shown in Figure 5-7,
the peak AºX intensity increased from 81 abs. units (s2) to 136 abs. units (s3), a 68.7%
increase in emission intensity with increasing grain size. The (e,Aº) peak intensity
increased 12.7 % from 213 abs. units (s2) to 240 abs. units (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
67
to improve with grain growth and lowering of surface states, hence the drastic increase
in intensity from the AºX peak (3.35 eV). In the case of free electrons, fewer surface
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 Properties of 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- doped 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 the 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. Improvement in roughness and
less structural defects was also observed when the lattice-matched MgCaO buffer was
employed. In addition, the MgCaO buffer may induce band bending in ZnO at the
interface changing the electronic states of the defects responsible for visible emission
and thus enhancing the UV emission [148,149] 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
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 AºX 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
emissions 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
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 properties.
70
Table 5-1.Growth conditions considered in PL Study Growth
Condition No.
Dopant at% Substrate Buffer Growth
Temperature Growth
Pressure Anneal
S1 Undoped - Al2O3 - 500 ºC 10 mTorr -
S2 Ag 0.6 Al2O3 - 500 ºC 10 mTorr -
S3 Ag 0.6 Al2O3 - 500 ºC 25 mTorr -
S4 Ag 0.6 Al2O3 ZnO 500 ºC 10 mTorr -
S5 Ag 0.6 GaN MgCaO 500 ºC 10mTorr
S6 Ga 0.22 Al2O3 ZnO 650 ºC 1 mTorr -
S7 P 0.5 Al2O3 - 700 ºC 150 mTorr RTA
950 ºC
S8 Ag, P 0.6, 0.5 Al2O3 - 500 ºC 10 mTorr -
71
350 400 450 500 550 600 650
0
2
4
6
8
10
Undoped ZnO s1
Ag-doped ZnO s2
PL
in
ten
sity (
a.u
)
Wavelength (nm)
383 nm
377 nm
505 nm
visible luminescence
suppression
300 K
Figure 5-1. Room temperature PL spectra of undoped ZnO and Ag doped ZnO
100 200 300 400 500 600
0
2
4
6
8
10
12
PL
In
ten
sity (
a.u
)
Grain Size (nm)
Undoped ZnO s1
Ag-doped ZnO s2
Ag-doped ZnO s3
Figure 5-2. Band Edge Intensity as a function of grain size
72
Figure 5-3. PL spectra of undoped ZnO measured at 10 K Reprinted with 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
3.1 3.2 3.3 3.4 3.5
0
100
200
300
Ag-doped ZnO - s2
(e, Ao)
3.31 eV
DAP-1LO
3.17 eV
DAP
3.25 eV
AoX
3.353 eV
PL
In
ten
sity (
a.u
)
Energy (eV)
DoX
3.36 eV
15 K
73
Figure 5-5. Plot of V·d·I-1 as a function of magnetic field for p-type Ag-doped ZnO s2
20 40 60 80 100 120 140 160 180 2003.18
3.19
3.20
3.21
3.22
3.23
3.24
3.25
3.26
3.27
3.28
3.29
3.30
3.31
3.32
Red-shift
Pe
ak P
ositio
n (
eV
)
Temperature (K)
Blue-shift
(e,Ao) Peak
DAP Peak
Figure 5-6. Acceptor related peak positions as a function of increasing temperature
-12000 -8000 -4000 0 4000 8000-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Vd
/I (
Vcm
/A)
Field (G)
p = 5.19x1019 cm-3
Ag-doped ZnO - s2
74
3.1 3.2 3.3 3.4 3.5
0
50
100
150
200
250
AoX
3.35eV
PL
In
ten
sity (
a.u
)
Energy (eV)
s3 D: 560 nm
s2 D: 333 nm
(e,Ao)
3.31eV
Ag-doped ZnO
15 K
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
75
350 400 450 500 550 600 650 700
0
2
4
6
8
10
PL
In
ten
sity (
a.u
)
Wavelength (nm)
Ag-doped ZnO
Ga-doped ZnO
P-doped ZnO
Undoped ZnO
Ag,P co-doped ZnO
300 K
Figure 5-9. Room temperature PL spectra for ZnO doped with different elements
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 rectifying junctions [128, 129]. Studies have indicated that Ag can act
as an amphoteric dopant, yielding an acceptor state for substitution on the Zn site, and
a donor state 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, first-principles calculations 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 ionization energy is relatively high, the formation
energy for this substitutional defect (AgZn) 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 thin 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 5x1019 cm-3,
as well as, excellent UV photoluminescence 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
conditions 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
77
devices considered in this chapter, including junctions, the p-type SZO films and layers
were grown at 500 ºC in an oxygen pressures of 10 and 25 mTorr.
6.2.1 Silver-Doped ZnO / Gallium-Doped ZnO Thin Film Homojunction
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/cm2. Ablation targets were fabricated
using ultra high purity ZnO and Ag2O powders. The targets were sintered at 1000 ºC 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 trichloroethylene, 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 with 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 ºC in an oxygen pressure of 1 mTorr. 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. The resistivity, Hall mobility and carrier
concentration were measured using a four-point van der Pauw 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.
78
6.2.2 Silver-Doped ZnO Thin Film Transistor (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 laser was operated at a repetition
rate of 1 Hz and an average energy density of 1 J/cm2. Ablation targets were fabricated
using ultra high purity ZnO and Ag2O powders. The targets were sintered at 1000 ºC in
air. The concentration of Ag in the ZnO target was 0.6 at%. A heavily doped p+ Si wafer
was used as substrate. The thermal oxide (SiO2, 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 was open
by photolithography and buffer oxide etch (BOE) etching (1:6, 4 minute etch) and the
metal used was Indium. The as grown Ag-doped ZnO film was grown 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 plot of VHalld/I as a function
of applied magnetic field and room temperature photoluminescence for a Ag-doped ZnO
grown at 500 ºC in an oxygen pressure of 25 mTorr. The pn junction fabrication started
with 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
fabricated as well as the test for Ni-Au contacts. The size of the devices was 180m in
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 the 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 [151,152]. Note that, without the metallization of contacts, the device was
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 this 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
Agilent 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 were
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 emission
or rectifying I-V characteristics. Unfortunately, the relatively unstable nature of light
emission under bias made it not possible to obtain a useful electroluminescence
80
spectrum. The instability appears to be related to surface conduction and perhaps
hydrogen incorporation as discussed elsewhere [153,154].
6.3.2 Silver Doped 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 the
film by changing its crystallinity from single to polycrystalline. The mobility of the
channel was 3.5 cm2/Vs and the Ion/Ioff ratio was 105. 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 (Al2O3) 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 5x1019 cm-3, as well 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 output power measured was
5.2x10-8 mW. At excitation currents of 10 mA, the applied voltage was approximately
81
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.
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 cm2/Vs and a Ion/Ioff ratio of 105. The subtheshold slope was determined
to be 4.4 V/decade.
82
Table 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(cm2/V-s) 27.10 2.87
Carrier Concentration (cm-3) 5.0x1019 5.9x1019
83
-10000 -5000 0 5000 10000
8.0x10-5
8.1x10-5
8.2x10-5
8.3x10-5
8.4x10-5
8.5x10-5
8.6x10-5
8.7x10-5
a)
p = 5.9x1019
cm-3
= 2.87 cm2/Vs
= 0.00198 -cm
ZnO:Ag 500 oC 25 mTorr
Vd
/I (
Vcm
/A)
Field (G)
350 400 450 500 550 600 650 700
0.010
0.015
0.020
0.025
0.030
b)
377 nm
Inte
nsity (
a.u
.)
Wavelength (nm)
Room Temperature PL
ZnO:Ag 500 oC 25 mTorr
Figure 6-1. Plot of (a) VHalld/I as a function of applied magnetic field and (b) room temperature photoluminescence for a Ag-doped ZnO was grown at 500 ºC in an oxygen pressure of 25 mTorr.
84
Figure 6-2. The ZnO:Ag/ZnO:Ga/sapphire junction (a) schematic of structure and (b) Test for Ni-Au contacts
85
Figure 6-3. Homojunction I-V characteristics
86
Figure 6-4. The ZnO:Ga/ZnO:Ag/sapphire junction (a) schematic of structure and (b) junction I-V characteristic.
87
0.000 0.005 0.010 0.015
0.0
1.0x10-8
2.0x10-8
3.0x10-8
4.0x10-8
5.0x10-8
6.0x10-8
3rd time
2nd time
1st time
Lig
ht In
ten
sity (
mW
)
Current (A)
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
88
Figure 6-7. ZnO TFT grown on Si - output and transfer characteristics
0 5 10 15 20
0.0
5.0x10-7
1.0x10-6
1.5x10-6
2.0x10-6
2.5x10-6
3.0x10-6
3.5x10-6
20V
12VDra
in C
urr
en
t (A
)
Drain Voltage(V)
0V4V8V
16V
VG
-30 -20 -10 0 10 20 30 40
10-8
10-7
10-6
10-5
10-4
10-3
10-2
Dra
in c
urr
ent(
A)
Gate Voltage (V)
Vd=15V
89
CHAPTER 7 CONCLUSIONS
The research presented in this dissertation focused on Ag-doped ZnO thin film
growth by pulsed laser 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
optical 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 ºC to 500 ºC) by pulsed laser deposition yielded p-type material as
determined by room temperature Hall measurements. Hole concentrations on the order
mid-1015 to mid-1019 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
ºC 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 substitution for Zn (AgZn) 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 photoconductivity was also observed. High temperature ZnO buffer
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 Optimized Ultraviolet in Silver-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 revealed strong
and dominant emissions originating from free electron recombination to Ag-related
acceptor states around 3.31eV. The AºX 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
emissions 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
91
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 properties.
7.3 Rectifying pn Junction and TFT Devices
Both p-type conductivity in Ag-doped ZnO with carrier concentrations as high as
5x1019 cm-3, as well as, excellent UV photoluminescence emission were achieved. 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 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.
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 mobility of the channel was 3.5 cm2/Vs
and the Ion/Ioff ratio was 105. 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 crystallinity issues has been suggested. The
92
conductivity is expected to remain p-type however a top gate TFT structure would be
required.
93
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BIOGRAPHICAL SKETCH
Fernando Lugo was born in 1983, in Barquisimeto, Venezuela. After graduating
from high school in 2001, he attended Miami Dade College and earned an Associate of
Arts degree in May 2003. He then began to pursue a Bachelor’s degree at The
University of Florida in Material Science and Engineering. He received his Bachelor’s
degree in the spring of 2006. He conducted his undergraduate research for bachelor’s
degree on the growth 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 the
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 University of Florida Ultimate Frisbee club team from 2007 to 2010.
