i

TRANSPARENT CONDUCTIVE ELECTRODES

FOR GaN-BASED LIGHT EMITTING DEVICE

by

AHMAD HADI BIN ALI

Thesis submitted in fulfillment of the requirements

for the degree of

Doctor of Philosophy

FEBRUARY 2016

ii

ACKNOWLEDGEMENTS

First and foremost I wish to give all the praise to Almighty Allah for

giving me the strength and time to complete this research work. With His

blessings may this work be beneficial for the sustainable of humanity.

I am deeply indebted to my supervisor, Prof. Dr. Zainuriah Hassan and my

co-supervisor, Dr. Ahmad Shuhaimi bin Abu Bakar for their help, guidance and

encouragement throughout this work, without which it would be not have been

completed. They have thought me their professionalism and the profound art of

research, which inevitably are reflected in this thesis. For all these, and for

innumerable friendly discussions we have had, I am very grateful.

I would also like to express my thanks to Institute of Nano-

Optoelectronics Research and Technology (INOR) staff and to all my friends for

their co-operation and assistance. I dedicated this thesis to the soul of my father

Haji Ali bin Abdullah, God have mercy on him. Also, my deepest appreciation to

my mother Hajah Zainon bte Jaafar, my wife Siti Nur Kamariah bte Rubani, my

sons Ahmad Hazim, Ahmad Amri and Ahmad Syahmi, and parent-in-law for their

praiseworthy support through the course of my study.

Finally, I would like to thank the Universiti Tun Hussein Onn Malaysia

and Kementerian Pendidikan Tinggi for financial support in my study.

iii

TABLE OF CONTENTS

PAGE

ACKNOWLEDGEMENTS

ii

TABLE OF CONTENTS

iii

LIST OF TABLES

vii

LIST OF FIGURES

x

LIST OF SYMBOLS

xviii

LIST OF ABBREVIATIONS

xx

ABSTRAK

xxii

ABSTRACT

xxiv

CHAPTER 1 : INTRODUCTION

1

1.1 Introduction 1

1.1.1 Problem statement of the transparent

conductive electrodes

1

1.2.2 Problem statement of the GaN-based light

emitting diode

3

1.2 Research Objectives 6

1.3 Research Originality 6

1.4 Research Scope 7

1.5 Thesis Outline

7

CHAPTER 2 : LITERATURE REVIEW

9

2.1 Introduction 9

2.2 Overview of contact technology 9

2.2.1 Metal-contact technology on GaN 10

2.2.2 Transparent conductive oxide 18

2.2.3 ITO-based transparent conductive electrode 23

2.3 Overview of GaN 29

2.3.1 General properties of GaN 31

2.3.2 Growth of GaN 32

2.3.3 Structural defects of GaN 35

2.4 Overview of InGaN light emitting diode 37

2.4.1 Comparison of hetero-epitaxial growth of

InGaN LED on different substrates

38

2.4.2 Epitaxial growth of InGaN LED on Si 39

iv

substrate

2.5 Application of TCE on InGaN light emitting device

structure

41

CHAPTER 3 : THEORY

43

3.1 Introduction 43

3.2 Crystallite sizes of GaN and TCE 43

3.3 Metal-semiconductor contact based on energy band

principle

44

3.3.1 Current flow through metal-semiconductor

junctions

46

3.3.1.1 Thermionic emission 48

3.3.1.2 Thermionic field emission 48

3.3.1.3 Field emission 49

3.3.2 Schottky barrier 49

3.3.3 Ohmic contact 51

3.4 Current spreading layer 53

3.5 Optical principle of TCE 55

3.5.1 Figure of merit 57

3.6 Principle of InGaN light emitting diode 58

3.6.1 Defects generation in InGaN LED 58

3.6.2 Photoluminescence principle of InGaN LED 59

3.7 Growth, deposition, and characterization principles of

InGaN LED and TCE

60

3.7.1 Principle of metal organic chemical vapor

deposition

60

3.7.2 Principle of sputtering 61

3.7.3 Principle of heat treatment 63

3.7.4 Principle of x-ray diffraction 64

3.7.5 Principle of transfer length method 67

3.7.6 Principle of Hall effect measurement system

68

CHAPTER 4 : METHODOLOGY

72

4.1 Introduction 72

4.2 Deposition of transparent conducting electrode 72

4.2.1 Substrate cleaning 73

4.2.2 Deposition of thin Ni and Ag metal layer by

thermal evaporator

75

4.2.3 Deposition of Ti and Al by DC sputtering 78

4.2.4 Deposition of ITO by RF sputtering 81

4.2.5 Post-deposition annealing 84

4.3 Deposition of transparent conductive electrodes on

InGaN LED structure

84

4.3.1 Growth of InGaN-based light emitting device

structure

86

4.3.2 n-AlN/n-GaN multilayer intermediate layer 87

v

4.3.3 n-Al0.06Ga0.94N/n-GaN strain-layer-

superlattices under-layer

88

4.4 Characterization tools and equipments 88

4.4.1 X-ray diffraction 88

4.4.2 Energy dispersive x-ray spectroscopy 91

4.4.3 Field emission scanning electron microscopy 92

4.4.4 Atomic force microscope 92

4.4.5 Transfer length method 94

4.4.6 Hall effect 95

4.4.7 UV-visible spectrophotometry 95

4.4.8 Photoluminescence

96

CHAPTER 5 : TRANSPARENT CONDUCTIVE

ELECTRODES ON p-GaN

97

5.1 Introduction 97

5.2 Thin Ni/Ag metal contact on p-GaN 97

5.3 Indium tin oxide single-layer contact on p-GaN 102

5.4 Metal-ITO TCE contact on p-GaN 106

5.4.1 Ni/ITO on p-GaN 107

5.4.2 Ag/ITO TCE contact layer on p-GaN 111

5.5 Ni/Ag/ITO TCE contact on p-GaN 115

5.5.1 Optimization of the electrical properties of

Ni/Ag/ITO TCE on p-GaN

116

5.5.2 Optimization of the optical properties of

Ni/Ag/ITO TCE on p-GaN

118

5.5.3 Figure of merit of the Ni/Ag/ITO TCE 120

5.5.4 Structural properties of Ni/Ag/ITO TCE on

p-GaN

123

5.5.5 Morphological properties of Ni/Ag/ITO TCE

on p-GaN

125

5.5.6 Electrical properties of Ni/Ag/ITO TCE on p-

GaN

128

5.5.7 Optoelectronic properties of Ni/Ag/ITO TCE

multilayer contact on p-GaN

131

5.6 Summary

132

CHAPTER 6 : TRANSPARENT CONDUCTIVE

ELECTRODES ON n-GaN

133

6.1 Introduction 133

6.2 Thin metal Ti/Al contact on n-GaN 133

6.3 Indium tin oxide contact on n-GaN 138

6.4 Metal-ITO TCE on n-GaN 142

6.4.1 Ti/ITO contact on n-GaN 143

6.4.2 Al/ITO contact on n-GaN 147

6.5 Ti/Al/ITO TCE on n-GaN 151

6.5.1 Optimization of electrical properties of 152

vi

Ti/Al/ITO TCE on n-GaN

6.5.2 Optimization of the optical properties of

Ti/Al/ITO TCE on n-GaN

155

6.5.3 Analysis of figure of merit 157

6.5.4 Structural properties of Ti/Al/ITO TCE on n-

GaN

159

6.5.5 Morphological properties of Ti/Al/ITO TCE

on n-GaN

161

6.5.6 Electrical properties of Ti/Al/ITO TCE on n-

GaN

163

6.5.7 Optoelectronic properties of Ti/Al/ITO TCE

on n-GaN

166

6.6 Summary

167

CHAPTER 7 : InGaN LIGHT EMITTING DIODE

STRUCTURE

168

7.1 Introduction 168

7.2 Characterization of the InGaN light emitting diode

structure

168

7.2.1 Structural properties of the InGaN light

emitting diode structure

169

7.2.2 Morphological properties of the InGaN light

emitting diode structure

177

7.2.3 Optical properties of the InGaN-based LED 180

7.3 Ni/Ag/ITO TCE layer deposited on p-GaN top layer of

the InGaN light emitting diode structure

183

7.4 Summary

187

CHAPTER 8: CONCLUSIONS AND FUTURE WORK

189

8.1 Conclusions 189

8.2 Future work

190

REFERENCES

192

PUBLICATIONS

202

CONFERENCES 203

vii

LIST OF TABLES

PAGE

Table 2.1 Mechanical, thermal, electrical and optical properties of

wurtzitic AlN, GaN and InN

32

Table 3.1 Condition of work function to achieve ohmic

characteristics.

49

Table 4.1 The parameter used for the metal contact deposition by

thermal evaporation.

77

Table 4.2 Sputtering parameter of Ti on n-GaN and glass.

80

Table 4.3 Sputtering parameter of Al on n-GaN, glass and Ti layer.

81

Table 4.4 Parameter of ITO deposition by RF sputtering at

different film thicknesses.

82

Table 5.1 Elemental composition in wt% of Ni/Ag metal contact

layer on p-GaN for the as-deposited and 600C post-

annealed sample.

98

Table 5.2 Electrical properties of the as-deposited and post-

annealed sample of Ni/Ag thin metal contact layer on p-

GaN.

100

Table 5.3 Elemental composition in wt% of ITO TCO layer on p-

GaN for the as-deposited and 600C post-annealed

sample.

102

Table 5.4 Electrical properties of the as-deposited and post-

annealed sample of ITO contact layer on p-GaN.

104

Table 5.5 Elemental composition in wt% of Ni/ITO TCE layer on

p-GaN for the as-deposited and 600C post-annealed

sample.

107

Table 5.6 Electrical properties of the as-deposited and 600C post-

annealed sample of Ni/ITO layer on p-GaN.

109

Table 5.7 Elemental composition in wt% of Ag/ITO TCE contact

layer on p-GaN for the as-deposited and 600C post-

annealed sample.

111

Table 5.8 Electrical properties of the as-deposited and 600C post-

annealed sample of Ag/ITO layer on p-GaN.

113

viii

Table 5.9 Elemental composition in wt% of the Ni/Ag/ITO TCE

contact layer on p-GaN.

125

Table 5.10 Surface morphological characteristics of the Ni/Ag/ITO

TCE layer on p-GaN scanned by AFM over an area of

1.0 1.0 m2.

126

Table 5.11 Electrical properties of the as-deposited and post-

annealed characterized by Hall effect system.

129

Table 6.1 Elemental composition in wt% of Ti/Al metal contact

layer on n-GaN for the as-deposited and 600C post-

annealed sample.

134

Table 6.2 Electrical properties of the as-deposited and post-

annealed sample of Ti/Al thin metal layer on n-GaN.

136

Table 6.3 Elemental composition in wt% of ITO layer on n-GaN

for the as-deposited and 600C post-annealed sample.

139

Table 6.4 Electrical properties of the as-deposited and post-

annealed sample of ITO layer on n-GaN analyzed by

Hall effect system.

141

Table 6.5 Elemental composition in wt% of Ti/ITO TCE layer on

n-GaN for the as-deposited and 600C post-annealed

samples.

144

Table 6.6 Electrical properties of the as-deposited and 600C post-

annealed sample of Ti/ITO layer on n-GaN.

145

Table 6.7 Elemental composition in wt% of Al/ITO TCE layer on

n-GaN for the as-deposited and 600C post-annealed

sample.

148

Table 6.8 Electrical properties of the as-deposited and 600C post-

annealed sample of Al/ITO layer on n-GaN.

150

Table 6.9 Elemental composition in wt% of the Ti/Al/ITO TCE

layer on n-GaN.

161

Table 6.10 Electrical properties of the as-deposited and 600C post-

annealed samples analyzed by Hall effect system.

164

Table 7.1 Measured XRC (0004), )4220( and )0110( parameter

and calculated threading dislocation densities (TDD).

174

Table 7.2 Electrical properties of the as-deposited and post- 186

ix

annealed sample of Ni/Ag/ITO TCE layer on p-type

layer of the InGaN LED structure.

x

LIST OF FIGURES

PAGE

Figure 3.1 Band diagram of metal contact to semiconductor for m >

s (a) before and (b) after contact.

45

Figure 3.2 Band diagram of metal contact to semiconductor for m <

s (a) before and (b) after contact.

46

Figure 3.3 Schematic diagram of current conduction mechanisms of

a forward-biased Schottky barrier for the thermionic

emission process. The doping level in the semiconductor

is relatively low and Fermi level is below the conduction

band.

47

Figure 3.4 Potential energy diagram and current flow mechanisms

for a forward-biased Schottky barrier for thermionic field

emission and direct tunneling.

47

Figure 3.5 Current injection to LED structure (a) without TCE (b)

with TCE current spreading layer.

55

Figure 3.6 Mechanism of lattice mismatch and thermal mismatch in

GaN grown on Si (111) substrate.

60

Figure 3.7 Growth process of GaN epitaxial layer by MOCVD.

62

Figure 3.8 Basic principle of thin films deposition by sputtering

process.

62

Figure 3.9 X-ray diffraction by crystal lattice based on Bragg’s law.

65

Figure 3.10 Determination of contact and sheet resistance using the

TLM measurements (a) TLM pattern (b) resistance

between the contact pads vs. distance between the

contacts.

68

Figure 3.11 Simple diagram of Hall effect measurement setup using

bar shaped semiconductor.

69

Figure 3.12 Resistivity measurement using van der Pauw

configuration of (a) RA and (b) RB.

71

Figure 4.1 Flow chart of transparent conductive electrodes

deposition on p-GaN, n-GaN and glass substrates.

74

Figure 4.2 GaN template cleaning using acetone-acetone-IPA 76

xi

method.

Figure 4.3 Deposition of Ni and Ag metal thin films on p-GaN and

glass using Edwards Auto 306 thermal evaporator. Small

picture shows the control panel of the thermal

evaporator.

77

Figure 4.4 Simple diagram of (a) Ni layer (b) Ag layer and (c)

Ni/Ag layer deposited on p-GaN template and glass

substrates deposited by auto thermal evaporator.

78

Figure 4.5 Simple diagram of Ti thin metal layer DC sputtered on n-

GaN templates and glass substrates.

79

Figure 4.6 Simple diagram of Al layer sputtered on (a) n-GaN

template and glass substrates and (b) Ti layer.

80

Figure 4.7 RF/DC magnetron sputtering system.

83

Figure 4.8 Simplified diagram of ITO layer on (a) Ni (b) Ag (c)

Ni/Ag (d) Ti (e) Al (f) Ti/Al (g) p-GaN, n-GaN and glass

substrates.

83

Figure 4.9 Flow chart of the deposition of transparent conductive

electrodes on InGaN LED structure.

85

Figure 4.10 Schematic layer structure of (a) Sample A with a n-

Al0.06Ga0.94N/n-GaN SLS cladding under-layer and (b)

Sample B with Al0.03Ga0.97N cladding under-layer.

87

Figure 4.11 XRD setup for characterizing crystal structure of a

sample.

89

Figure 4.12 (a) Metal mask used to deposit metal contact and TCE

(b) samples after contact deposition with TLM pattern.

94

Figure 4.13 Van der Pauw technique to measure the electrical

resistivity, Hall coefficient, carrier mobility and carrier

concentration (a) simple schematic diagram (b) Van der

Pauw setup for Hall effect measurement.

96

Figure 5.1 EDXS elemental characteristics of Ni/Ag metal contact

layer on p-GaN for the (a) as-deposited and (b) 600C

post-annealed sample.

98

Figure 5.2 Morphological characteristics of Ni/Ag metal contact

layer on p-GaN of the (a) as-deposited and (b) 600C

post-annealed sample scanned by FESEM.

99

Figure 5.3 Current-voltage characteristics of Ni/Ag bi-metal contact 100

xii

layer on p-GaN for the as-deposited and post-annealed

sample.

Figure 5.4 Optical transmittance of Ni/Ag thin metal contact layer

on glass scanned by UV-visible spectrophotometry.

101

Figure 5.5 EDXS elemental characteristics of ITO contact layer on

p-GaN for the (a) as-deposited and (b) 600C post-

annealed sample.

103

Figure 5.6 Surface morphological characteristics of ITO contact

layer on p-GaN of the (a) as-deposited and (b) 600C

post-annealed sample.

103

Figure 5.7 Current-voltage characteristics of ITO layer on p-GaN

for the as-deposited and post-annealed sample.

105

Figure 5.8 Optical transmittance characteristics of ITO contact layer

deposited on glass.

106

Figure 5.9 EDXS elemental characteristics of Ni/ITO contact layer

on p-GaN for the (a) as-deposited and (b) 600C post-

annealed sample.

108

Figure 5.10 Morphological characteristics of Ni/ITO contact layer on

p-GaN of (a) as-deposited and (b) 600C post-annealed

sample.

108

Figure 5.11 Current-voltage characteristics of Ni/ITO layer on p-

GaN.

110

Figure 5.12 Optical transmittance of Ni/ITO TCE contact layer on

glass.

111

Figure 5.13 EDXS elemental characteristics of Ag/ITO contact layer

on p-GaN for the (a) as-deposited (b) 600C post-

annealed sample.

112

Figure 5.14 Morphological characteristics of Ag/ITO contact layer on

p-GaN for the (a) as-deposited and (b) 600C post-

annealed sample.

113

Figure 5.15 Current-voltage characteristics of Ag/ITO TCE layer on

p-GaN.

114

Figure 5.16 Optical transmittance of Ag/ITO TCE contact layer on

glass for the as-deposited and post-annealed sample.

115

Figure 5.17 Electrical resistivity of Ni/Ag/ITO TCE on p-GaN at

different post-annealing temperature.

117

xiii

Figure 5.18 Electrical resistivity of Ni/Ag/ITO TCE on p-GaN at

different post-annealing period.

117

Figure 5.19 Electrical resistivity of Ni/Ag/ITO TCE on p-GaN at

different post-annealing N2 gas flow rate.

118

Figure 5.20 Optical transmittance characteristics of Ni/Ag/ITO TCE

on glass at different post-annealing temperature.

119

Figure 5.21 Optical transmittance characteristics of Ni/Ag/ITO TCE

on glass at different post-annealing period.

120

Figure 5.22 Optical transmittance characteristics of Ni/Ag/ITO TCE

on glass at different post-annealing N2 gas flow.

121

Figure 5.23 Figure of merit, FOM of the Ni/Ag/ITO TCE contact

layer as a function of (a) temperature (b) period (c) N2

gas flow rate.

123

Figure 5.24 Phase analysis XRD of the as deposited and post-

annealed Ni/Ag/ITO TCE contact multi-layer on p-GaN.

124

Figure 5.25 Elemental composition of the (a) as-deposited (b) 600C

post-annealed Ni/Ag/ITO TCE layer on p-GaN.

125

Figure 5.26 Surface morphological characteristics of the Ni/Ag/ITO

TCE contact multilayer scanned by AFM over 1.0 1.0

m2 for (a) as-deposited (b) post-annealed samples in 2-

dimension and 3-dimension.

127

Figure 5.27 FESEM surface morphological of the Ni/Ag/ITO TCE

contact layer deposited on p-GaN for the (a) as-deposited

and (b) 600C post-annealed sample.

128

Figure 5.28 Current-voltage characteristics of Ni/Ag/ITO TCE

contact layer on p-GaN.

130

Figure 5.29 TLM graph plotted for the Ni/Ag/ITO TCE contact

multilayer.

131

Figure 6.1 Elemental characteristics of Ti/Al contact layer on n-

GaN scanned by EDXS for the (a) as-deposited and (b)

post-annealed sample.

134

Figure 6.2 Surface morphological characteristics of Ti/Al contact

layer on n-GaN of the (a) as-deposited and (b) 600C

post-annealed sample scanned by FESEM.

135

Figure 6.3 Current-voltage characteristics of Ti/Al bi-metal contact 137

xiv

layer on n-GaN for the as-deposited and post-annealed

sample.

Figure 6.4 Optical transmittance characteristics of Ti/Al thin metal

contact layer on glass for the as-deposited and post-

annealed sample.

138

Figure 6.5 EDXS elemental characteristics of ITO contact layer on

n-GaN for the (a) as-deposited (b) 600C post-annealed

sample.

139

Figure 6.6 Morphological characteristics of ITO contact layer on n-

GaN of (a) as-deposited and (b) 600C post-annealed

sample.

140

Figure 6.7 Current-voltage characteristics of ITO TCO layer on n-

GaN for the as-deposited and 600C post-annealed

sample.

142

Figure 6.8 Optical transmittance of ITO on glass for the as-

deposited and 600C post-annealed sample.

143

Figure 6.9 EDXS elemental characteristics of Ti/ITO contact layer

on n-GaN for the (a) as-deposited and (b) 600C post-

annealed sample.

144

Figure 6.10 Surface morphological characteristics of Ti/ITO contact

layer on n-GaN of (a) as-deposited and (b) 600C post-

annealed sample.

145

Figure 6.11 Current-voltage characteristics of Ti/ITO TCE layer on

n-GaN of the as-deposited and 600C post-annealed

sample.

146

Figure 6.12 Optical transmittance of Ti/ITO TCE layer on glass for

the as-deposited and 600C post-annealed sample.

147

Figure 6.13 EDXS elemental characteristics of Al/ITO contact layer

on n-GaN for the (a) as-deposited and (b) 600C post-

annealed sample.

148

Figure 6.14 Surface morphological characteristics of Al/ITO contact

layer on n-GaN of (a) as-deposited and (b) 600C post-

annealed sample.

149

Figure 6.15 Current-voltage characteristics of Al/ITO TCE layer on

n-GaN for the as-deposited and 600C post-annealed

sample.

150

xv

Figure 6.16 Optical transmittance characteristics of Al/ITO TCE

layer on n-GaN for the as-deposited and 600C post-

annealed sample.

151

Figure 6.17 Resistivity of Ti/Al/ITO TCE on n-GaN at different post-

annealing temperature.

153

Figure 6.18 Resistivity of Ti/Al/ITO TCE on n-GaN at different post-

annealing period.

154

Figure 6.19 Resistivity of Ti/Al/ITO TCE on n-GaN at different post-

annealing N2 gas flow.

154

Figure 6.20 Optical transmittance of Ti/Al/ITO TCE on glass at

different post-annealing temperature.

155

Figure 6.21 Optical transmittance of Ti/Al/ITO TCE on glass at

different post-annealing period.

156

Figure 6.22 Optical transmittance of Ti/Al/ITO TCE on glass at

different post-annealing N2 gas flow.

157

Figure 6.23 Figure of merit of the Ti/Al/ITO TCE layer on n-GaN at

variable post-annealing (a) temperature (b) period and (c)

N2 gas flow rate.

159

Figure 6.24 Phase analysis XRD of the as-deposited and 600C post-

annealed Ti/Al/ITO TCE on n-GaN.

160

Figure 6.25 EDXS properties of the (a) as-deposited (b) 600C post-

annealed Ti/Al/ITO TCE layer on n-GaN.

161

Figure 6.26 2-Dimensional and 3-Dimensional AFM morphological

for the (a) as-deposited (b) 600C post-annealed sample.

162

Figure 6.27 Surface morphological characteristics of the (a) as-

deposited (b) 600C post-annealed Ti/Al/ITO TCE layer

on n-GaN.

164

Figure 6.28 Current-voltage characteristics of Ti/Al/ITO TCE on n-

GaN for the as-deposited and post-annealed sample.

165

Figure 6.29 Specific contact resistance determined by transfer length

method (TLM) of the 600C post-annealed sample.

165

Figure 7.1 )4220( Phi scan analysis of the (a) Sample A and (b)

Sample B scanned over 360.

170

Figure 7.2 XRC omega-scan of Sample A and Sample B at (0004) 172

xvi

symmetric plane.

Figure 7.3 XRC omega-scan of Sample A and Sample B at )4220(

asymmetric plane.

173

Figure 7.4 XRC omega-scan of Sample A and Sample B at )0110(

asymmetric plane.

173

Figure 7.5 Triple axis XRC -2 spectrum of In0.11Ga0.89N/

In0.02Ga0.98N MQW LED structure of Sample A and

Sample B with n-Al0.06Ga0.94N/n-GaN SLS and n-

Al0.03Ga0.97N under-layer, respectively.

175

Figure 7.6 RSM recorded around 5110 diffraction plane for (a)

Sample A and (b) Sample B.

177

Figure 7.7 FESEM surface morphology of InGaN-based LED

projected 60 from the incident electron beam with (a)

Sample A and (b) Sample B.

178

Figure 7.8 Cross-sectional FESEM of GaN-based epitaxial layer of

(a) Sample A and (b) Sample B.

179

Figure 7.9 2D AFM surface morphology of the InGaN LED

structure (a) Sample A and (b) Sample B.

179

Figure 7.10 AFM 3D images on 5 5 m2 for (a) Sample A and (b)

Sample B of the InGaN MQW LED.

180

Figure 7.11 InGaN-based MQW emission profile via PL surface

mapping of (a) Sample A and (b) Sample B.

181

Figure 7.12 Intensity profile of InGaN-based MQW measured by PL

surface mapping for (a) Sample A and (b) Sample B.

182

Figure 7.13 PL characteristics of In0.11Ga0.89N/ In0.02Ga0.98N MQW

LED structure.

183

Figure 7.14 Phase analysis XRD of the Ni/Ag/ITO TCE layer on

InGaN LED structure.

184

Figure 7.15 2D and 3D surface morphological of the (a) as-deposited

and (b) 600C post-annealed Ni/Ag/ITO TCE on InGaN

LED scanned by AFM over an area of 5 5 m2.

185

Figure 7.16 Ohmic characteristics of the InGaN LED with

Ni/Ag/ITO TCE on p-GaN.

186

Figure 7.17 I-V characteristics of the InGaN-based LED with 187

xvii

Ni/Ag/ITO TCE on p-Gan and Al contact on backside of

n-Si (111) substrate.

xviii

LIST OF SYMBOLS

Absorption coefficient

Eg Band gap energy

B Barrier height

Bn Barrier height of n-type semiconductor

Bp Barrier height of p-type semiconductor

k Boltzmann constant 2

sb

Burgers vector

nb Carrier bulk density

b Carrier bulk resistivity

ND Carrier concentration of doped semiconductor

Carrier mobility

Rc Contact resistance

Wc Contact width

SCry Crystallite size

Angle

e Edge and mixed dislocation density

E Electric field vector

Electron affinity

s Electron affinity of semiconductor

q Elementary charge (1.602 10-19 C)

Energy difference between the Fermi energy and the conduction band

Ec Energy of the conduction band

Ev Energy of the valence band

Fm Fermi level of metal

Fs Fermi level of semiconductor

TC Figure of merit

v Frequency

s FWHM of XRC curve

VH Hall voltage or transverse voltage

nidl Ideality factor

n Index of refraction

a In-plane lattice parameters

I0 Intensity of the incident light

IT Intensity of the transmitted light

aGaN Lattice constant of GaN at a-axis

aSi Lattice constant of Si at a-axis

a Lattice mismatch

B Magnetic field vector

T Optical transmittance

c Out-of-plane lattice parameters

v Particle velocity

E Photon energy

h Planck constant

Rp Resistance between two contact pads

L Sample thickness

xix

K Scherrer constant

s Screw and mixed dislocation density

nsh Sheet density

Rs Sheet resistance

Rss Sheet resistance of semiconductor

d Spacing between crystal planes

Rsp Specific contact resistance

Rq Surface roughness

T Temperature

t Thickness

LT Transfer length

Wavelength

m Work function of metal

s Work function of semiconductor

xx

LIST OF ABBREVIATIONS

AlGaN Aluminum gallium nitride

AlN Aluminum nitride

arb. unit Arbitrary unit

AFM Atomic force microscope

CdO Cadmium oxide

CTE Coefficient of thermal expansion

CTLM Circular transmission line model

cps Count-per-second

EHP Electron-hole pair

EDXS Energy dispersive x-ray spectroscopy

EQE External quantum efficiency

FE Field emission

FESEM Field-emission scanning electron microscopy

FWHM Full-width at half-maximum

GaN Gallium nitride

HRXRD High resolution x-ray diffraction

InAlGaN Indium aluminum gallium nitride

InGaN Indium gallium nitride

InN Indium nitride

In2O3 Indium oxide

ITO Indium tin oxide

IR Infra-red

IDB Inversion domain boundaries

IPA Isopropyl alcohol

LED Light emitting diode

LT Low temperature

MOCVD Metal organic chemical vapor deposition

MOVPE Metal organic vapor phase epitaxy

MBE Molecular beam epitaxy

ML Multi layer

MQW Multi-quantum well

PAXRD Phase analysis x-ray diffraction

RTA Rapid thermal annealing

rlu Reciprocal lattice unit

RSM Reciprocal space mapping

RMS Root-mean-square

rpm Rotation-per-minute

SiC Silicon carbide

SF Stacking fault

sccm Standard cubic centimeter per minute

SLS Strain-layer superlattice

TE Thermionic emission

TFE Thermionic field emission

TD Threading dislocation

SnO2 Tin dioxide

TiO2 Titanium dioxide

xxi

TCE Transparent conductive electrode

TCO Transparent conductive oxide

TA Triple axis

UV Ultra-violet

UV-Vis Ultraviolet-visible

XRD X-ray diffraction

XRC X-ray rocking curve

ZnO Zinc oxide

xxii

ELEKTROD PENGALIR LUTSINAR UNTUK PERANTI PEMANCAR

CAHAYA BERASASKAN GaN

ABSTRAK

Sesentuh elektrik yang berkeupayaan sebagai pengalir arus elektrik yang

tinggi dan lutsinar optik yang baik dalam panjang gelombang cahaya nampak

adalah sangat penting untuk kecekapan peranti optoelektronik. Kajian ini

difokuskan ke atas elektrod pengalir lutsinar (TCE) yang dimendapkan ke atas

templat berasaskan GaN untuk aplikasi struktur diod pemancar cahaya (LED)

InGaN. Templat GaN jenis-p dan jenis-n digunakan sebagai templat pemendapan

untuk tujuan pengoptimuman TCE. Indium tin oksida (ITO) digunakan sebagai

sesentuh lapisan atas kerana ITO menawarkan kerintangan arus elektrik yang

rendah (~10-4 – 10-3 -cm), lutsinar optik dalam panjang gelombang cahaya

nampak yang tinggi (> 80 %), mempunyai kepekatan pembawa yang tinggi (~1021

cm-3) dan kelincahan pembawa yang baik. Untuk memperbaiki kerintangan arus

elektrik lapisan ITO, lapisan-bawah logam tipis dimendapkan di antara lapisan

ITO di atas dan templat GaN. Bagi templat p-GaN, lapisan-bawah Ni dan Ag

dimendapkan di bawah lapisan ITO, manakala bagi templat n-GaN, lapisan-

bawah logam tipis Ti dan Al digunakan. Sampel-sampel TCE dikenakan proses

sepuh lindap untuk memperbaiki ciri-ciri struktur TCE yang seterusnya akan

memperbaiki ciri-ciri elektrik dan optik TCE. Daripada proses pengoptimuman,

keadaan sepuh lindap yang terbaik untuk TCE adalah pada suhu 600C di bawah

aliran gas N2 2 L/min dengan tempoh sepuh lindap selama 15 min. Kerintangan

elektrik dan kebolehpancaran optik pada 470 nm bagi lapisan TCE Ni/Ag/ITO (5

nm / 5 nm / 80 nm) di atas p-GaN setelah melalui sepuh lindap selepas

pemendapan telah diukur sebagai 3.65 10-5 -cm dan 97 %, masing-masing.

xxiii

Bagi lapisan TCE Ti/Al/ITO (5 nm / 5 nm / 80 nm) yang dimendapkan di atas n-

GaN, kerintangan elektrik dan kebolehpancaran optik pada 470 nm telah diukur

sebagai 8.61 10-5 -cm and 95 %, masing-masing. Daripada pengiraan, angka

merit (FOM) bagi TCE Ni/Ag/ITO and Ti/Al/ITO tersepuh lindap ialah 9.51 10-

2 -1 and 5.91 10-2 -1, masing-masing, yang mana adalah lebih baik

berbanding sampel TCE sedia ada. Di samping pengoptimuman TCE, perigi

kuantum berbilang (MQW) LED InGaN yang ditumbuhkan dengan pemendapan

wap kimia logam organik (MOCVD) di atas substrat Si (111) dicirikan

berdasarkan sifat-sifat struktur dan optik. Didapati bahawa kepadatan kehelan

bebenang (TDD) berkurang dengan sisipan n-Al0.06Ga0.94N/n-GaN superkekisi

lapisan terikan (SLS) di atas tindanan lapisan tengah n-AlN/n-GaN dan

menghasilkan lapisan GaN di atas Si yang bebas daripada retak. Daripada

keputusan fotoluminesen, tenaga foton yang dipancarkan daripada LED dengan

lapisan-bawah SLS ialah 2.97 eV yang bersepadanan dengan panjang gelombang

417 nm. Pada arus 20 mA, voltan nyalaan bagi LED InGaN dengan TCE

Ni/Ag/ITO di atas lapisan p-GaN dan lapisan sesentuh Al di bawah substrat n-Si

ialah 7.4 V. Voltan pincang hadapan yang tinggi adalah disebabkan oleh

kerintangan daripada substrat Si dan lapisan penimbal berasaskan AlN di bawah

kawasan aktif.

xxiv

TRANSPARENT CONDUCTIVE ELECTRODES FOR GaN-BASED

LIGHT EMITTING DEVICE

ABSTRACT

Electrical contacts which possess high electrical current conductivity and

good optical transparency in visible wavelength are very important for the

efficiency of optoelectronic devices. This study focuses on transparent conductive

electrode (TCE) deposited on GaN-based templates for the application on InGaN

light emitting diode (LED) structure. P-type and n-type GaN templates were used

as depositing templates for optimization purposes of the TCE. Indium tin oxide

(ITO) is used as a top contact layer since ITO offers low electrical current

resistivity (~10-4 – 10-3 -cm), high optical transparency in visible wavelength (>

80 %), has high carrier concentration (~1021 cm-3) and good carrier mobility. In

order to improve the electrical current resistivity of the ITO layer, thin metal

under-layer was deposited between the top ITO layer and the GaN templates. For

the p-GaN templates, Ni and Ag thin metal under-layer were deposited under the

ITO top layer, whereas for the n-GaN templates, Ti and Al thin metal under-layer

were used. The TCE samples were subjected to post-annealing process in order to

improve the structural characteristics of the TCE which consequently will

improve the electrical and optical characteristics of the TCE. From the

optimization process, the best post-annealing condition for the TCE is at

temperature of 600C under N2 gas flow of 2 L/min with annealing period of 15

min. The electrical resistivity and optical transmittance at 470 nm of the

Ni/Ag/ITO (5 nm / 5 nm / 80 nm) TCE layer on p-GaN after post-deposition

annealing were measured as 3.65 10-5 -cm and 97 %, respectively. For the

Ti/Al/ITO (5 nm / 5 nm / 80 nm) TCE layer deposited on n-GaN, the electrical

xxv

resistivity and optical transmittance at 470 nm were measured as 8.61 10-5 -

cm and 95 %, respectively. From calculation, the figure of merit (FOM) of the

post-annealed Ni/Ag/ITO and Ti/Al/ITO TCE is 9.51 10-2 -1 and 5.91 10-2

-1, respectively, which is better than the as-deposited TCE samples. Besides the

TCE optimization, multi quantum-well (MQW) InGaN LED grown by metal

organic chemical vapor deposition (MOCVD) on Si (111) substrate was

characterized based on its structural and optical properties. It is found that the

threading dislocation densities (TDD) is reduced with the insertion of n-

Al0.06Ga0.94N/n-GaN strain-layer superlattices (SLS) on stack of n-AlN/n-GaN

intermediate layer and producing crack-free GaN epitaxial layers on Si. From

photoluminescence results, the emitted photon energy from the LED with SLS

under-layer is 2.97 eV corresponding to wavelength of 417 nm. At 20 mA current,

the turn-on voltage of the InGaN LED with Ni/Ag/ITO TCE on top of the p-GaN

layer and Al contact layer at the bottom of the n-Si substrate is 7.4 V. The high

forward voltage is mainly due to the resistance from the Si substrate and AlN-

based buffer layer under the active region.

1

CHAPTER 1

INTRODUCTION

1.1 Introduction

Transparent conductive electrodes (TCE) offer significant impact to the

optoelectronics device technology such as GaN-based light emitting and light

absorbing devices due to its unique properties of good optical transparency and high

electrical conductivity. The TCE can be deposited on optoelectronic device structure

as a single layer or multilayer depending on the device applications. The versatile

characteristics of this TCE including high optical transparency, low electrical

resistance and high thermal conductivity make it suitable as a current spreading layer

and light input/output layer for optoelectronics devices [1]. In general sense, the

TCE can be applied to automotive and architectural windows that act as a ultra-violet

(UV) and infra-red (IR) blocking layer.

1.1.1 Problem statement of the transparent conductive electrodes

Over decades, researchers have studied on the contact material and its

properties for the applications to the optoelectronic devices. Non-oxides or metal-

based contact electrodes with single layer structure such as platinum (Pt), titanium

(Ti), argentum (Ag), aluminum (Al), nickel (Ni) and Aurum (Au), as well as

multilayer thin films such as nickel/aurum (Ni/Au), nickel/argentum (Ni/Ag) and

Ti/Al has been deposited on various types of substrates such as silicon (Si) and

gallium nitride (GaN) [2-4]. These metal-based contacts provide good electrical

current conductivity due to the low electrical resistivity of the bulk metal. However

these metal-based electrodes pose low optical transmittance or semitransparent

2

characteristics due to the opaque properties of the metal as well as light reflectance

at the interfaces.

Meanwhile, oxides-based TCE or also known as transparent conductive

oxide (TCO) such as cadmium oxide (CdO), zinc oxide (ZnO), titanium dioxide

(TiO2), indium oxide (In2O3), tin oxide (SnO2) and indium tin oxide (ITO) offer

higher optical transmittance of more than 80% as compared to the metal-based TCE

(<80%) [5, 6]. The electrical resistivity is relatively higher than the metal-based

contacts with one order of magnitude, but still considered as low resistivity (~10-4-

cm) especially for light emitting device applications. An approach to increase the

electrical resistivity is by inserting metal under-layer or sandwiched metal layer.

Some group of researchers have conducted study on the multilayer TCO-metal such

as ZnO/Cu/ZnO, TiO2/Ag/TiO2, ITO/Ag/ITO, ITO/Ni/ITO in order to increase the

electrical conductivity of the TCO [7-10]. However the additional metal layer

degrades the optical transmittance characteristics. Therefore the metal layers must be

made very thin enough of less than 10 nm thickness in order to increase the light

transmittance, but this result in degradation of the electrical properties. Optimizing

the TCO-metal multilayer structures such as its thickness, TCO-metal material,

deposition conditions as well as pre and post deposition heat treatment can greatly

improve the optical and electrical properties of the TCE.

TCE can be deposited on substrates by many techniques such as electron

beam deposition and sputtering. The quality of the TCE depends on the target

material and conditions during deposition processes. However, most of the deposited

TCE such as ITO structures are amorphous in nature [11]. The amorphous nature of

the ITO reduces the electrical conductivity, increasing electrical resistivity and

reducing optical transmittance of the thin films. In addition, the substrate material

3

used also affects the TCE properties. Substrates with high carrier concentrations

such as n-type GaN (>1018 cm-3) helps in lowering the electrical resistivity of the

TCE as compared to the p-type GaN (<1018 cm-3) with lower carrier concentration

[12].

Since most of the room temperature deposited TCE produces amorphous thin

films, performing post deposition annealing of the TCE thin films can improve the

crystalline quality of the films consequently enhancing the electrical current

conductivity and increasing light transmittance characteristics. Some parameters

such as post-annealing temperature, duration and gas flow need to be carefully

optimized in order to get high quality TCE thin films.

1.1.2 Problem statement of the GaN-based light emitting diode

GaN-based materials have been extensively investigated over the past two

decades since found for its practical use in light emitting diodes (LED) technology.

GaN with its binary cousins, aluminum nitride (AlN) and indium nitride (InN), as

well its ternary, aluminum gallium nitride (AlGaN) and indium gallium nitride

(InGaN), along with their quarternary indium aluminum gallium nitride (InAlGaN),

is considered as one of the most important semiconductors after Si. This is due to

their unique structural, electrical and optical characteristics such as direct and large

bandgap (Eg ~ 0.7 eV-6.0 eV for InNGaNAlN), high carrier mobility, high

breakdown field, high thermal conductivity, chemical inertness and good mechanical

stability [13].

GaN-based LED offer an ultimate light sources in lighting technology since it

covers spectrum from UV to IR. Since found for practical use by Nakamura in the

early 1990s, many researchers have put numerous efforts to realizing high quality

4

and high efficiency GaN-based LED. Many approaches were used to hetero epitaxial

grow the LED such as by using metal organic chemical vapor deposition (MOCVD)

and molecular beam epitaxy (MBE). The hetero structure LED is epitaxially grown

on silicon carbide (SiC), sapphire (Al2O3) or silicon (Si) substrate.

One of the major challenges in growing the hetero structure GaN-based

epitaxial layer on third party substrates are the crystal defects such as lattice and

thermal expansion defects [14]. Highly crystal defects may deteriorates device

performance, reduce device lifetimes and alters the optical properties. The majority

defects within the nitrides epitaxial structure are threading dislocations generated

due to the lattice mismatch and differences in thermal expansion coefficient between

the epitaxial layer and the substrate. Threading dislocations can be classified into

edge dislocation, screw dislocation and mixed dislocation. The epitaxial stress

occurs due to the parameter mismatches between the substrates and the epitaxial

layer that can generate cracks through the epitaxial layer. Besides the lattice

mismatches, differences in coefficient of thermal expansion, CTE of the substrates

and the epitaxial layer will results in thermal stress. This stress can be a serious

problem as it causes some cracks through the epitaxial layer during cooling.

The most common substrates used for heteroepitaxial growth of III-nitrides

are sapphire, 6H-SiC and Si. Lattice mismatched between sapphire and GaN

epitaxial layer is higher than the SiC. In addition, the sapphire is thermally and

electrically insulating. SiC in the other hand have high thermal conductivity and very

good electrical conductivity. However the sapphire and SiC were limited in size to

no more than 4 inches as well as demanding high manufacturing cost. Si substrate on

the other hand offer larger wafer sizes with diameter up to 12 inches, dramatic cost

reduction as compared to the other substrates, have good thermal management as

5

compared to sapphire and easy integration with well-established Si-based

manufacturing technologies.

There are several significant challenges in growing GaN-based

heteroepitaxial structure on Si substrate with InGaN quantum wells active region.

Two major difficulties impeding the growth of high quality InGaN-based LED are

the large lattice mismatch (-17%) and thermal coefficient mismatch (116%) between

the GaN-based epitaxial layer and Si substrate [15]. These lattice and thermal

mismatch can generate structural defects including cracking, threading dislocation

(TD) and cloudy surface morphology through the entire device. TD generated from

Si substrate up to the InGaN-based multi-quantum well (MQW) active region can act

as a non-radiative recombination center by disrupting the light emission process,

resulting in the production of heat rather than light thus reducing the optical emission

efficiency of the LED [16].

Several approaches were proposed to overcome the inherent heterostructure

challenges of Si as a substrate for the GaN-based epi-layer. The lattice mismatched

and thermal stress in GaN-based epi-layer on Si can be controlled with the use of

buffer and intermediate layer. Many research groups have been investigated on these

buffer and intermediate layer for GaN growth on Si such as AlN buffer layer, AlGaN

buffer layer, AlGaN/GaN superlattice, AlN/GaN strain superlattices (SLS) buffer

interlayer on AlGaN/AlN nucleation layer, AlN/AlGaN low temperature (LT) buffer

layer [17], which will be discussed in detail in the next chapter. The lower lattice

parameter of AlN forces the GaN-based epi-layer to be grown under compression

stress, accordingly counteracting the thermal tensile stress acting from the Si

substrate.

6

Besides optimizing high quality structure of the GaN-based LED, good

quality ohmic contact with high optical transparency also plays an important role for

producing high external quantum efficiency, EQE. TCE with good optical

characteristics and high electrical conductivity can be used as contacts on top of the

p-GaN and n-GaN layer to spread the electrical current uniformly. In addition, with

high optical transparency, visible light generated from the active region can be

transmitted out through the TCE efficiently.

1.2 Research Objectives

The objectives of this research are to

(i) Study the improvement of the electrical and optical properties of

transparent conductive electrodes based on ITO and Ni/Ag metal thin

films sputter deposited on p-type GaN.

(ii) Study the improvement of the electrical and optical properties of

transparent conductive electrodes based on ITO and Ti/Al metal thin

films sputter deposited on n-type GaN.

(iii) Investigate the InGaN LED structure properties grown on Si (111)

substrate based on its structural, optical and electrical properties.

1.3 Research Originality

The main originality in this study lies on the combination of Ni and Ag thin

metal layer with the ITO layer deposited on p-GaN, and the combination of the Ti

and Al thin metal layer with the ITO layer deposited on n-GaN for transparent

conductive electrodes (TCE) with specific structural, morphological, electrical and

optical properties, deposited as a TCE contact layer on p-GaN, n-GaN and InGaN

7

light emitting device structure that was grown on Si (111) substrate by MOCVD,

that to the extent of our knowledge, have not been reported by other research groups.

The combination of ITO TCO with under-layer metal thin films will enhance the

electrical properties of the TCE as a current spreading layer. Moreover, detail study

on the effect of post-annealing on the TCE layer is expected to improve the TCE

properties with excellent ohmic behavior and excellent optical characteristics,

consequently increases the LED efficiency.

1.4 Research Scope

This study will focus on the deposition and optimization of ITO-based

transparent conductive electrodes with the insertion of Ni and Ag thin metal films

under the ITO on p-GaN; and Ti and Al thin metal films under the ITO on n-GaN.

Furthermore, this study will characterize the InGaN LED structure with the insertion

of strain layer superlattices under-layer and finally deposition of transparent

conductive contacts on p-GaN layer of the InGaN LED and Al contact layer on Si

(111) substrate.

1.5 Thesis Outline

The content of this dissertation is organized as follows:

Chapter 2 encompasses the literature overview of the metal contact and

transparent conductive electrodes technology on p-GaN, n-GaN and semiconductor

optoelectronic devices especially GaN-based light emitting device. The overview on

GaN-based light emitting device development especially related to the InGaN LED

structure development is also included.

8

Chapter 3 is related to the basic principles of the TCE including metal-based

and TCO-based transparent contact, principles of InGaN-based light emitting device

structure with multi-quantum-well (MQW) active region and basic concept of some

of the characterization techniques.

Chapter 4 explains the methods for sample preparations, depositions, growth

and characterizations. This includes the cleaning procedure, TCE deposition process,

post-deposition annealing, InGaN LED growth process followed by the

characterization of the TCE and InGaN LED structure based on its structural,

morphological, electrical and optical characteristics.

Chapter 5 presents the results of the ITO-based TCE on p-GaN with Ni and

Ag metal thin layer under the ITO. The post-annealing effects on the TCE based on

the structural, morphological, electrical and optical properties are discussed.

Chapter 6 presents the results of the ITO-based TCE on n-GaN with Ti and

Al metal thin layer under the ITO. The post-annealing effects on the TCE based on

the structural, morphological, electrical and optical properties are discussed.

Chapter 7 presents the structural and optical characterization results of the

InGaN light emitting device grown on Si (111). The results on the implementation of

TCE on p-type layer of the InGaN LED structure are also briefly discussed.

Finally in Chapter 8, conclusion on TCE and InGaN LED characterization

results covered in this thesis with recommendations for further research work will be

given.

9

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

Transparent conductive electrodes including non-oxide (metal only conductor

contact) and oxide (oxide metal) either single layer or multilayer has been

investigated for its structural, electrical, optical and morphological properties for

semiconductor optoelectronic applications. Good ohmic contact and high optical

transmittance characteristics of the TCE on semiconductor devices such as GaN-

based light emitting devices can improve the power and optical efficiency of the

device. This chapter overview on the transparent conductive electrodes technology

from the non-oxide metal contacts (eg. Ni, Ag, Ti and Al), transparent conductive

oxide (eg. CdO, ZnO and ITO) and ITO-metal TCE (eg. Ni/ITO, Ag/ITO and

Ti/ITO). Further sections will overview on the technology development of the

InGaN light emitting structure.

2.2 Overview of contact technology

Improvements in metal-semiconductor contacts have become a critical factor

for better technology along with the advancing properties of the semiconductor

devices. In recent years, GaN itself has been proven to be excellent choice for light-

emitting devices especially for white light and high power device applications. The

successes of all GaN related devices for high brightness and high efficient LED

application depend largely on having excellent contact properties to these devices.

Contact to semiconductor basically consists of region of semiconductor surface just

below first metal layer, metal semiconductor interface and few layers of contacts

10

metallization above the semiconductor. Invariably the as-deposited contact does not

give the desired properties (either low resistance or high Schottky barrier). So the

contacts were undergone treatment such as plasma and heat treatment which results

in formation of different complex inter-metallic compounds by way of solid state

reaction among metal layers and semiconductor surfaces.

2.2.1 Metal-contact technology on GaN

GaN has received much attention due to its unique properties of wide and

direct energy band gap which makes it suitable for high efficiency light-emitting

device applications. Good ohmic metal-semiconductor contacts may enhance the

device efficiency. High quality metal-semiconductor ohmic contact with low

electrical resistance is vital to ensure the high efficiency electrical current flow

through the contact to the GaN-based devices. In addition, the development of

multilayer contact has been utilized to obtain low resistance ohmic contact

performance. Furthermore, thermal annealing can be used in order to get ohmic

characteristics of the metal contacts, besides improving the thermal stability and

electrical characteristics. Some research groups have deposited metal-based thin film

contact such as Ti/Al [18] and Ti/Al/Ni/Au [19, 20] on semiconductor substrates.

Chen and his group introducing Ni/Au on Ti/Al in order to preventing inter-

diffusion of Ti, Al, Au and also as an anti-oxidation of the contacting layer [21].

They performed two-step annealing process at 900C under N2 for 30s to lower

specific contact resistance, c to 9.65 10-7 -cm2. Other group of researcher

reported on the improvement of the specific contact resistance as low as 3 10-6 -

cm2 with multilayer metallization ohmic contact of Ti/Al/Ni/Au on AlGaN/GaN

after three steps rapid thermal annealing, RTA from 400C to 830C [19]. Greco et.

11

al. reported on Ti/Al ohmic contact on AlGaN/GaN/Si with electrical resistivity of

3.33 10-5 -cm and 1.23 10-5 -cm for the high defect density (12 109 cm-2)

and the low defect density (4 109 cm-2) in the epitaxial structure, respectively [18].

Metal contact on p-GaN

The high contact resistance of p-GaN is one of major problems in the

realization of long-lifetime and reliability operation of GaN-based devices. It is due

to the difficulty to grow heavily p-type doped in GaN (>1018 cm-3) and to get an

appropriate metal having work functions larger than that of p-GaN (~7.5 eV) [22].

The difficulty to form low ohmic contact on p-GaN is also due to the extremely large

Schottky barrier height formed at metal/p-GaN interfaces. The performance of

InGaN LED such as the operation voltage is strongly affected by contact resistance.

Power dissipation due to voltage drop at the p-GaN/metal contact interface generates

Joule heat, thus causes high junction temperature which could degrade the

performance of the device. It is thus crucial to develop high-quality ohmic contact on

p-GaN to enhance device performance.

However it is difficult to obtain ohmic contact on p-GaN with specific

contact resistance of less than 10-4 -cm2 due to the low activation energy of Mg

dopant and the tendency of GaN surface to preferentially lose N during processing

[23]. Various metallization method including surface preparation methods,

metallization layer, deposition techniques and annealing treatments have been

investigated to obtain low specific contact resistivity and low energy barrier between

the metal contacts and p-GaN [24-26]. Single layer, bi-layer, and multilayer

metallization contacts based on high work function metals such as Ni, Pt, Au and Pd

have been investigated for ohmic contact formation. Thus multilayer metallic films

12

with high work function such as Ni/Au [27] and Ni/Ag [28, 29] have been used as a

contact on p-GaN.

Cao has reported on the improvement of electrical and optical characteristics

with the fabrication of Ni/Au current spreading layer on p-GaN after annealing in air

at 550C for 5 min [27]. The high conductivity and transparency in visible regime of

the Ni/Au make it a good candidate as a current spreading layer for InGaN LED. The

low contact resistance was attributed to the formation of intermediate NiO (band gap

~ 4 eV) embedded with Au-rich and Ni-Ga-O islands which was believed to be low

barrier contact to p-GaN. Annealing process also helps in reducing the contact

resistance due to the hydrogen atoms bonded with Mg or N in p-GaN are removed

during the annealing process, which in conjunction increases the hole concentration

in p-GaN layer.

Qin studied on Ni/Au ohmic contact on top p-GaN layer of InGaN/GaN

MQW blue LED [30]. They found that the Ni layer play a role of reducing the

Schottky barrier while the Au layer play a role of spreading the injection current.

They also found that the three steps annealing can improves the contact properties

significantly. Since Ni has a relatively high metal work function as well as good

adhesion to nitrides, it was utilized as an intermediate film [31]. Hassan reported on

the Ni/Ag metal contacts on p-GaN instead of Au since Ag is cheaper than Au, has

low electrical resistivity (1.59 10-6 -cm) and good thermal conductivity (1

cal/cm-s-C). They achieved specific contact resistance of 9.9 10-2 -cm2 after

undergone thermal annealing at 700C for 15 min with subsequent cryogenic cooling

treatment for another 10 min [32]. Further research by Jang and Lee was reported on

the improved specific contact resistance of 5.2 10-5 -cm2 with the fabrication of

Ni/Ag/Ru/Ni/Au multilayer metal contacts on p-GaN after annealing at 500C for 1

13

min in O2 ambient. Ru was used in order to act as a diffusion barrier for Ag out-

diffusion to the surface, resulting in excellent thermal stability [33]. They also

reported on Ni/Ag metal contacts on p-GaN with achieved specific contact resistance

of 5.2 10-5 -cm2 after annealing at 500C for 1 min in O2 ambient. Out-diffused

of Ga atoms from GaN could dissolve in the Ag layer to form Ag-Ga solid solutions,

leaving Ga vacancies below the contact. Ga vacancies could increase the net hole

concentration and reduce the surface band bending, resulting in the ohmic contact

formation [34].

Cho used Pd/Ni/Au multilayer metallization contact on p-GaN and achieved

ohmic characteristics after thermal annealing at 500 for 1 min in N2 [35]. Pd was

used for its high work function and high reactivity, besides the Pd properties that act

as an acceptor in GaN, causing the near surface region to be highly doped. In

addition, the transformation of Ni to NiO during thermal annealing at the interface of

Ni and GaN produce a layer with high hole concentration which led to the reduction

of contact resistance. Hong-Xia reported on a novel Ni/Ag/Pt ohmic contact on p-

GaN [36]. The specific contact resistance improves to 26 10-6 -cm2 after thermal

annealing at 500C for 3 min in O2 ambient. Pt layer can improve the surface

morphology and thermal reliability, Ag plays a key role in achieving good ohmic

contact due to the out-diffusion of Ga into Ag forming vacancies which increase the

hole concentration while the surface contamination of p-GaN is reduced by Ni.

Chang reports on Ni/Ag/Au ohmic contact on p-GaN which achieved specific

contact resistance of 4.35 10-4 -cm2 after thermal annealing at 500C for 10 min

in O2 ambient [37]. Ni/Au and Ag are combined in order to form low resistance and

high reflective contact.

14

Chou et. al. show that the addition of 10 at.% of Al with Ag in Ni/Ag

contacts reduce the specific contact resistance to 10-2 -cm2 after thermal annealing

at 500C for 10 min in air [38]. The addition of a small amount of Al additives in

Ni/Ag can effectively prevent the formation of Ag agglomeration on the p-GaN

surface, consequently improves the properties of the annealed contacts. De-Sheng et.

al. shows an improvement of the Ni/Ag ohmic metal contact with specific contact

resistance of 2.5 10-4 -cm2 on p-GaN after annealing at 550C for 1 min in O2

[28]. Kim fabricated Ni-Co solid solution/Au on p-GaN with thermal annealing

process at 550C for 1 min in air [39]. The annealing process improves the ohmic

characteristics of the contact as well as the optical transmittance of ~70%. The Co

used might contribute to increase in the carrier concentration by extracting hydrogen

from p-GaN thus lowering the contact resistance.

Ag films have poor adhesion to the substrate and easily agglomerate at

elevated temperature in air. Son et. al. reported on low specific contact resistance of

8.2 10-6 -cm2 of the Ni/Ag/Ni multilayer metal contact on p-GaN after annealing

at 450C for 1 min in air [24]. They deposited thin Ni over-layer on top of the Ag

contact layer in order to prevent surface diffusion of Ag atoms during annealing,

leading to smoother surface morphology and low contact resistivity. The Mg doping

concentration is generally about 1020 cm-3, but only 0.1-1% of the Mg atoms are

activated, due to high activation energy (~170 meV) and the formation of Mg-H

complexes decreasing the number of active carrier which lead to carrier density to ~

1017 cm-3 [24]. Chuah et. al. reported the Ni/Ag metallization scheme on p-GaN

achieving specific contact resistance of 1.74000 10-1 -cm2 after thermal annealing

at 700C for 10 min under N2 flow [29]. They deduce that high Mg doping may lead

to the creation of a large number of deep level defects in p-GaN, leading to the

15

reduction of the depletion region in p-GaN near the interface and increases the

probability of thermionic filed emission. Further investigation by Chuah et. al. also

reported their study on different metallization of Ir/Ag [40]. They used Ir due to its

ability to diffuse through the oxide contamination at the metal/p-GaN interface layer.

They achieved lower specific contact resistivity after the same thermal annealing

process as compared to the reported Ni/Ag contacts. Song et. al. studied on the role

of diffusion barrier metals (Pt and Ti) on the Ni/Ag/Pt or Ti/Au deposited on p-GaN

for flip chip LED [41]. They found that the Pt diffusion barrier shows lower specific

contacts resistance of 3.8 10-3 -cm2 as compared to the Ti of 8.1 10-3 -cm2,

respectively, after thermal annealing at 380C for 1 min in air. Youngjun et. al.

investigated on the carrier transport mechanism of Ni/Ag/Pt contacts to Mg-doped p-

GaN [42]. They prove through experiment and calculation that the contact behavior

was found to strongly depend on the Mg doping concentration.

Further study was conducted by Huang et. al. on the effect of hydrogen

treatment on ohmic contact to p-GaN [26]. The interfacial oxide layer on the p-GaN

surface was found to be the main reason for causing the nonlinear I-V behavior for

the H2 untreated p-GaN films. Surface inversion of p-GaN layer was successfully

achieved by H2 treatment at high temperature of 1000C. This consequently increase

nitrogen vacancy density, pinned the surface Fermi level close to the conduction-

band edge, reduce the Schottky barrier height that allows the electrons to flow easily

over the barrier from the metal contact to p-GaN, lowering the metal contact

resistance and thus improve the I-V characteristics to the linear behavior. Other

group of researcher used graphene with the insertion of thin Ti/Al metal layer to

improve the contact resistance although loss ~20% of the optical transmittance [43].

Graphene is used as a contact layer since it is highly transparent, high electrical, high

16

thermal conductivities and high mechanical flexibility. After thermal annealing, the

I-V characteristics of the Gr/Ti/Au contact become ohmic as compared the nonlinear

of the as-deposited contact. Kim et. al. reported on a low specific resistance (6.1

10-5 -cm2) of Pd/Zn/Ag metal contacts after annealing at 500C for 1 min in air

[44]. The Pd interlayer is used to enhance the adhesion of the Zn on p-GaN. In

addition, Pd was expected to form ohmic contact on p-GaN since Pd has a large

work function. InGaN LED with Pd/Zn/Ag contacts exhibited a forward voltage of

3.01 V at 20 mA.

Metal contact on n-GaN

Ti- or Al-based metallization contacts such as Ti/Al [45], Ti/Au [46] and

Ti/Al/Ni/Au [21] had been widely used as contacts to n-GaN as it is widely

recognized as the ohmic contact yielding the lowest resistivity. Among many contact

metallization schemes on n-GaN, Ti/Al-based contacts are widely utilized. Ti

metallization schemes reduces contact resistance by forming a low work function

TiN alloys with the under GaN layer. In such metallization contacts, low specific

contact resistance ranging from ~ 10-5 – 10-8 -cm2 have been reported, which are

good enough for the operation of the optoelectronic devices.

Recent results have shown that the additional layers on the Ti/Al contact

layers such as Ni/Au, Pt/Au, Ti/Au can reduce the contact resistivity especially after

undergone heat treatment or post-annealing process. Al reduces the reaction between

Ti and GaN. Ti /Al contact on AlGaN/GaN annealed at low temperature have a

resistivity of 1.67-5.45 -cm [47]. Placidi et. al. has conducted a study on the effects

of cap layer on Ti/Al ohmic contact on n-GaN [48]. They have shown that the

specific contact resistance of the Ti/Al with top protected layer of SiO2 is lower than

17

the unprotected top contact layer. Kim et. al. deposited Ta-based ohmic contact with

Ti/Al upper-layer and Ni/Au over-layers on AlGaN/GaN [49]. After post-annealing

at 700C for 1 min, they achieved ohmic contact of Ta/Ti/Al/Ni/Au with specific

contact resistance of 7.5 10-7 -cm2 and root-mean-square surface roughness of

28.8 nm. Ti/Al (40/160 nm) contacts exhibited linear current-voltage characteristics

after post-annealing at 300C and 400C, and remained nearly linear after post-

annealing at 700C with barrier height, B of 0.35eV [45].

Further study by Dobos et. al. that reported on the structural effects of Ti/Au

ohmic contact on n-GaN before and after thermal annealing [46]. They showed that

the Au diffused through Ti layer and Au rich grains were formed under Ti layer. TiN

was also formed at the metal-GaN interface. The annealed contact shows very

smooth surface morphology. Jeon et. al. studied on Cr/Al ohmic contact on N-polar

n-GaN for vertical LED [50]. They found that the annealed specific contact

resistance of the Cr/Al contacts increases by one order of magnitude than the as-

deposited samples. Cr layer may serve as a diffusion barrier to the out-diffusion of

Ga atoms during annealing. Ga atoms are dominantly out-diffused into the metal

electrodes because of lower formation energy of Ga vacancy compared to that of N

vacancy in n-GaN. The acceptor-like defects could give rise to electrical

compensation in the N-polar n-GaN, resulting in a decrease in the electron

concentration and hence increase in the effective Schottky barrier heights.

Furthermore, Dobos et. al. investigated the effects of the Cr/Au contacts to the n-

GaN for its electrical, structural and morphological characteristics [51]. After

annealing in vacuum at temperature of 700C and 900C, the I-V characteristics of

the contact become linear. The Cr was selected as the first contact layer because it

has a work function (4.5 eV) which is close to the n-GaN, and because of its reactive

18

nature, it also acts as an oxide reducing agent. Further study was performed by

Dobos et. al. on Ti/Cr/Al contacts on n-GaN and post-annealing process at 400, 700

and 900C for 10 min in vacuum [52]. Annealing process at 700 and 900C yielded

inter-diffusion, lateral diffusion and alloying of the metal contacts and the GaN. The

diffusion of Cr into the epi-layer of GaN led to ohmic behavior and increase series

resistance at 700C.

Borysiewicz et. al. studied on thermal stability of the

Ti/Al/TiN/Ti/Al/TiN/Ti/Al/TiN ohmic contact to n-GaN [53]. The contact remains

ohmic and morphologically unaltered after aging annealing at 300, 400 and 500C in

ambient air for 100h but the specific contact resistance increases by one order of

magnitude as compared to the as-deposited samples. A thin Ti oxynitride layer was

found to form after the aging process that most probably leads to the observed rise in

the specific contact resistance. By employing the Ti/Al/Ti/Al/Ti/Au

(30/30/30/30/30/60 nm) multi-layered structure, better ohmic characteristics were

demonstrated, significantly reduced contact resistance and enhanced surface

morphology of the contact electrodes [54]. The current transport characteristics

facilitated at 39.35 mA at V = 1V, with specific contact resistancec of 4.52 10-6 -

cm2 and contact resistance of 0.52 mm [54].

2.2.2 Transparent conductive oxide

As discussed in section 2.2.1, different types of metal-based contacts were

deposited on p- and n-type GaN for the application on GaN-based devices. Types of

contacts on p-GaN and n-GaN need to be optimized carefully in order to improve the

device performance. For LED, low resistance ohmic contact either on p-GaN or n-

GaN are very important for optimum device performance. However, the metal-based

19

contacts are almost opaque or semi-transparent that less light from the device can be

emitted out even though with low input power. In order to achieve high optical

transparency, the metal contacts need to be made very thin (less than 20 nm) which

results in non-uniform surface morphology and consequently degrades the electrical

characteristics.

Contact or electrode with good optical transparent characteristics and low

electrical resistance are very essential for efficient LED. The low electrical contact

resistivity can acts as a good current spreading layer as well as allow less power

input to the device, whereas the high optical transparency contact allows the light

generated emitted out with very minimum optical loss. Doped oxide-based thin film

contact with much better transparency and good electrical characteristics has been

studied and continuing to be developed. These highly transparent and conductive

thin films contact, also known as transparent conductive oxide (TCO) were achieved

by introducing native defects such as oxygen vacancies, or by doping with higher

doping elements [55]. Many types of TCO such as CdO, ZnO, In2O3, SnO2 and ITO

have been studied extensively for their unique properties as well as for the optimum

device performance.

Historically, CdO is one of the first examples of a TCO reported in 1907

[56]. CdO thin films contact were achieved by DC magnetron reactive sputtering

[57, 58], thermal evaporation [59], spray pyrolysis [60], atmospheric-pressure

chemical vapor deposition [61] and electrochemical deposition technique [62]. CdO

thin films have attracted much attention because of their high Hall mobility values

and high electron concentration (1019- 1021 cm-3) [56, 61]. This n-type conductivity

CdO has a face centered cubic (FCC) crystal structure with lattice constant of 4.694

Å. It has wide direct band gap between 2.2 and 2.7 eV and narrow indirect band gap

20

of 2.1 eV [61, 62]. It has high transmittance in the visible spectral region of 80-95%

[63], high electrical conductivity and low electrical resistivity of ~1.4 10-4 -cm

makes it useful for various optoelectronics applications [63]. Although the CdO

TCO have the desired good electrical and optical characteristics, they face

tremendous obstacles in penetrating the market due to the high toxicity of Cd [55].

Besides CdO, ZnO is II-VI semiconductor and is one of the promising TCO

that offer good electrical conductivity and highly transmittance especially for wide

bandgap optoelectronics and microelectronics device applications. ZnO thin films

have been deposited on GaN by many techniques such as RF magnetron sputtering

[64, 65], atmospheric-pressure metal-organic chemical vapor deposition (AP-

MOCVD) [66], metal-organic chemical vapor deposition (MOCVD) [67] and pulsed

laser deposition (PLD) [68, 69]. ZnO is used with GaN to grow ultraviolet (UV)

light emitters [70]. ZnO is n-type semiconductor with direct band gap energy of 3.37

eV [71] and carrier concentration of ~1017 cm-3. It offers some advantages including

large 60 meV exciton binding energy at room temperature [72], low cost, non-

toxicity, etch ability with wet chemicals, high temperature stability and abundant

material resources [6]. The ZnO TCO have similar refractive indices with GaN in the

range of 2.1-2.5 which have the advantages of reducing internal light reflections

especially for the GaN-based light-emitting devices [73]. In addition, the ZnO has

relatively small lattice mismatched with GaN of less than 2% and small difference

between the in-plane linear thermal expansion coefficients (ZnO = 6.51 10-6 K-1

and GaN = 5.59 10-6 K-1) [66]. However the ZnO-based TCO degrade much faster

than other TCO such as ITO when exposed to damp and hot environment. The as-

grown ZnO naturally exhibits n-type characteristics due to the oxygen vacancies,

growing p-type ZnO are difficult due to the self-compensation mechanism involving

21

oxygen vacancies, zinc interstial and low solubility of dopant [74]. Since most of

ZnO is used with GaN to produce light emitters in UV region, doping the ZnO films

with group III-elements such as Ga, Al and In have been realized with very attractive

electrical and optical applications as a TCO for electrode applications. Ga-doped

ZnO (GZO) have been extensively studied due to several advantages such as has

high resistance to oxidation and less lattice deformation as compared to other

materials [68]. GZO has been reported to have the lowest resistivity of 8.12 10-5 -

cm and high transmittance ~90% in the visible spectrum ranges [75]. The dopant

concentration of the GZO is more often in the range of 1020 – 1022 cm-3 with Hall

mobility of 14-40 cm2/V-s [68, 76, 77]. Other research groups reported on the Al-

doped ZnO thin films forming AZO. The AZO reaches optical transmittance of 80-

90% at visible spectrum. The reported carrier concentration of the AZO thin films is

in the range of 1020-1021 cm-3 with the lowest resistivity of 8.54 10-5 -cm.

In2O3 is a wide band gap semiconductor showing direct band gap of about

3.7 eV can be used for various applications in optoelectronics such as light-emitting

and detector devices [78]. Various techniques have been used to prepare the

In2O3including pulsed electron beam deposition (PED) [79], thermal oxidation [78],

pulsed DC magnetron sputtering [80] and pulsed laser deposition (PLD) [81] . Many

types of substrates such as c-cut sapphire [79], glass [80], Si[82], GaAs and quartz

[81] were used for the deposition of the In2O3. Adurodija et. al. reported on low

resistivity of 3.5 10-4 -cm of the In2O3 thin films deposited on Si (100) with

thickness of 100 nm [82]. Tripathi et. al. shows average optical transmission of

~96% of the 200 nm thick In2O3 thin films deposited on quartz and glass substrate

[81]. They also successfully shows the optical band gap values of In2O3 thin films of

3.64 eV and 3.79 eV deposited on quartz and glass substrates, respectively.

22

SnO2 belongs to TCO family that combining the unique properties of low

electrical resistivity and high optical transmittance in the visible ranges. It is n-type

semiconductor with a wide band gap of 3.6 eV and composes of a tetragonal rutile

crystal structure [83]. Many methods used to deposit the SnO2 such as spray

pyrolysis [84], thermal evaporation [85], DC and RF magnetron sputtering [86, 87].

SnO2 is a low cost and multifunctional materials that show high electrical

conductivity, high exciton binding energy of ~ 130 meV, mechanically strong,

chemically stable and have good adhesion to most of the substrate [86-88]. SnO2 thin

films deposited by DC magnetron sputtering on glass shows electrical resistivity of

4.3 10-2 -cm with Hall mobility of 15 cm2/V-s and carrier concentration of 2

1019 -cm for ~150 nm thick thin films [87]. Amma et. al. reported on low electrical

resistivity of 2.92 10-3 -cm for 170 nm thick SnO2 thin films [84] whereas

Alhuthali et. al. reported on high optical transmittance of 66 nm thick SnO2 thin

films of ~94% at visible spectrum [86]. Study conducted by Yu et. al. shows very

low electrical resistivity of 6.5 10-5 -cm and optical transmittance of ~73% at

~470 nm with the multilayer structure of SnO2/Cu/ SnO2 [89]. Further investigation

by Yu et. al. by using multilayer SnO2/Ag/SnO2 shows improved optical

transmittance as high as 94.8% at ~470 nm and low electrical resistivity of 1.0 10-4

-cm [90].

Although CdO is the first TCO thin films realized more than a century ago

and have good electrical and optical transmittance characteristics, the CdO has a

relatively narrow band gap and high toxicity. Despites the advantageous properties

of ZnO especially for the application in short wavelength LED and LD, but the ZnO-

based TCO degrade much faster than other TCO such as ITO when exposed to damp

and hot environment. Even though the band gap energy of In2O3 is lower than the

23

SnO2, a compound of both of them exhibit excellent electrical and optical properties.

As compared to other TCO including CdO, ZnO, In2O3 and SnO2, ITO is the

predominant TCO used in optoelectronic devices due to the ease of its processing

[55].

2.2.3 ITO-based transparent conductive electrode

ITO is preferred as a contact layer on optoelectronic semiconductor devices

as it offers good electrical conductivity, highly transparent to visible light, high

reflection in infrared region, strong absorption in UV region, mechanically hard,

chemically inert transparent material and have good adhesion to GaN [91, 92]. The

ITO also serves as a good current spreading layer when connected to the external

potential for application in visible light emitting diode. The additional current

spreading layer on p-GaN and n-GaN is important to achieve uniform current

distribution and light emission. For top emitting InGaN LED structure, the top

transparent conductive layer which acts as a window layer is important for efficient

light extraction. Electrical current from the external potential is spread uniformly by

the TCO layer to the active region through the p-type semiconductor layer

consequently producing photon in term of visible light. This visible light is

transmitted out through the transparent TCO contact layer without losing much of its

intensity and optical spectrum to the contact layer.

ITO can be deposited on the substrates by many techniques including sol-gel

processes [93, 94], thermal evaporator [95, 96], electron beam evaporation [97, 98],

pulsed laser deposition (PLD) [99], RF/DC sputtering [100, 101], plasma enhanced

chemical vapor deposition (PECVD) [102], metal-organic chemical vapor deposition

24

(MOCVD) [103] and atmospheric pressure chemical vapor deposition (APCVD)

[104].

Many substrates have been used for ITO deposition depending on the

application such as Si [105], Si (100) [106], p-Si (100) [107] glass [108-110], quartz

glass [101, 111, 112], polyethylene terephtalate (PET) [106, 113], p-GaN [97, 114,

115] and n-GaN [116-118]. The ITO is widely used for applications including solar

cells [119], flat panel display [120], gas sensor [121, 122], organic light emitting

diodes (OLED) [123], GaN-based vertical light-emitting diodes (VLED) [124] and

InGaN/GaN LED [125, 126].

Sol-gel method (or wet chemical method) offers several advantages including

simplicity, low cost, easily controlled doping level, no need for vacuum in the

deposition chamber and feasible preparation of large area films [94]. But this method

involved wet chemical that need delicate processes. In addition, organic solvents

used in this deposition process like ethylene glycol and ethanol are often flammable

and harmful to the environment [93]. Thermal evaporation offer easy preparation for

deposition, low cost of target material and less radiation damage to the substrates

[105]. Electron beam evaporation has the advantages of low cost, high purity and

high deposition rate [91]. However the evaporation process produces lower quality

of thin films as compared to the sputtering methods due to the evaporation process

itself. Sputtering method in turn, including RF and DC magnetron sputtering

provides high deposition rate, large deposition areas, high purity of the target

materials, clean deposition environment, low vacuum environment, controllable thin

films stoichiometry, less damaged areas, good films adhesion, simplicity of the

growth process, relatively low sputtering energy particle generation and high plasma

density [127]. Although other thin films technique such as the chemical vapor