Review of Ga2O3-based optoelectronic devices

Journal Pre-proof Review of Ga2O3 based optoelectronic devices Daoyou Guo, Qixin Guo, Zhengwei Chen, Zhenping Wu, Peigang Li, Weihua Tang PII:

S2542-5293(19)30137-3

DOI:

https://doi.org/10.1016/j.mtphys.2019.100157

Reference:

MTPHYS 100157

To appear in:

Materials Today Physics

Received Date: 14 September 2019 Revised Date:

28 October 2019

Accepted Date: 28 October 2019

Please cite this article as: D. Guo, Q. Guo, Z. Chen, Z. Wu, P. Li, W. Tang, Review of Ga2O3 based optoelectronic devices, Materials Today Physics, https://doi.org/10.1016/j.mtphys.2019.100157. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.

1

Review of Ga2O3 based optoelectronic devices

2

Daoyou Guo,a,b,c Qixin Guo,b,*1 Zhengwei Chen,d Zhenping Wu,d Peigang Li,d and Weihua Tangd,*

3

a

4

Province, Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, China.

5

b

6

Sage 840-8502, Japan.

7

c

State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, China

8

d

State Key Laboratory of Information Photonics and Optical Communications & Information Functional

9

Materials and Devices, School of Science, Beijing University of Posts and Telecommunications, Beijing 100876,

10

Center for Optoelectronics Materials and Devices & Key Laboratory of Optical Field Manipulation of Zhejiang

Department of Electrical and Electronic Engineering, Synchrotron Light Application Center, Saga University,

China.

Corresponding authors. E-mail address: [email protected] (Qixin Guo) and [email protected] (Weihua Tang)

1

11

Abstract: Gallium oxide (Ga2O3), with a ultrawide-bandgap of ~ 4.9 eV, has attracted

12

recently much scientific and technological attention due to its extensive future

13

applications in power electronics (field effect transistors, Schottky barrier diodes),

14

optoelectronics [phosphors and electroluminescent (EL) devices, solar-blind

15

photodetectors], memory (spintronic devices, resistance random access memory

16

devices), sensing systems (gas sensors, nuclear radiation detectors), and so on. Ga2O3

17

has six different polymorphs, known as α, β, γ, δ, ε, and κ. These various polymorphs

18

each have unique physical properties and can be widely used in various fields of

19

devices. Among them, β-Ga2O3 has been the most widely studied and utilized due to

20

its excellent chemical and thermal stability. Herein, we provide a review on

21

Ga2O3-based optoelectronics, with a detailed introduction of the phosphors and EL

22

devices and a concise summary of solar-blind photodetectors. We classify the

23

currently reported phosphors and EL devices based on Ga2O3 undoped and doped

24

with various elements (Eu, Er, Tm, Mn, Nd, Tb, Cr), and sorted out the latest

25

progresses of Ga2O3-based solar-blind photodetectors in various forms of bulk single

26

crystal,

27

metal-semiconductor-metal structure, Schottky junctions, heterojunctions, and pn

28

junctions. Finally, conclusions and future perspectives for Ga2O3 based optoelectronic

29

devices are presented.

nanostructures,

thin

films

with

30

2

various

device

structure

of

31

TABLE OF CONTENTS

32

1. INTRODUCTION.................................................................................................... 4

33

2. BASIC PROPERTIES AND DEVICE APPLICATIONS OF Ga2O3 ................. 5

34

2.1 CRYSTAL STRUCTURE AND PHASE TRANSFORMATION.............................................................. 5

35

2.2 BASIC PHYSICAL PROPERTIES AND POTENTIAL APPLICATIONS OF DEVICES ....................... 9

36

3. PHOSPHORS AND ELECTROLUMINESCENT DEVICES .......................... 13

37

3.1 UNDOPED ............................................................................................................................................... 13

38

3.2 Eu DOPED ............................................................................................................................................... 15

39

3.3 Er DOPED ................................................................................................................................................ 21

40

3.4 Tm DOPED .............................................................................................................................................. 24

41

3.5 Mn DOPED .............................................................................................................................................. 25

42

3.6 Nd DOPED ............................................................................................................................................... 27

43

3.7 Tb DOPED ............................................................................................................................................... 29

44

3.8 Cr DOPED ................................................................................................................................................ 34

45

4. SOLAR-BLIND PHOTODETECTORS.............................................................. 36

46

5. SUMMARY AND FUTURE PERSPECTIVES .................................................. 40

47

ACKNOWLEDGMENTS ......................................................................................... 41

3

48

1. Introduction

49

Gallium oxide (Ga2O3), with a bandgap of 4.9 eV, is an emerging

50

ultrawide-bandgap semiconductor material, which attracted recently much scientific

51

and technological attention due to their extensive future applications in power

52

electronics (field effect transistors, Schottky barrier diodes), optoelectronics

53

(phosphors and EL devices, solar-blind photodetectors), memory (spintronic devices,

54

resistance random access memory devices), sensing systems (gas sensors, nuclear

55

radiation detectors), deep ultraviolet transparent conductive oxide electrode,

56

photocatalyst, and so on [1-5]. Research on Ga2O3 becomes a hot topic world wide,

57

and the number of publications on Ga2O3 exhibits the exponential growth as a

58

function of time on logarithmic scale, as shown in Fig. 1. It is expected this trend will

59

continue for the foreseeable future as research funding for power device development

60

increases.

61 62 63

FIG.1 Number of publications on Ga2O3 as a function of time. (Data from the Scopus) 4

64

2. Basic properties and device applications of Ga2O3

65

2.1 Crystal structure and phase transformation

66

Ga2O3 can form six different polymorphs, known as α, β, γ, δ, ε, and κ, as shown

67

in Table I [3,6-8]. Among them, β-phase is the most stable while κ-phase is transient

68

[8]. Polymorphs are different not only in their crystal space group but also in the

69

coordination number of Ga ions. Although as many as above mentioned six

70

polymorphs of Ga2O3, there are likely four polymorphs: α, β, γ, and (δ/ε/κ). The

71

ε-phase is likely mimicking the κ phase due to rotational grains formed on sapphire

72

[9], while the δ-phase is a mixture of the β- and ε-phases [8]. TABLE I. Lattice parameters of Ga2O3 polymorphs.

73

Polymorph

System

Space group

Lattice parameters

Ref.

α

Hexagonal

c R3

a=b=4.98 Å, c=13.43 Å, α=β=90°,

[10]

γ=120° β

Monoclinic

a=12.23 Å, b=3.04 Å, c=5.80 Å,

C2/m

[11]

α=γ=90°, β=103.8° γ

Cubic

m Fd3

a=b=c=8.24 Å, α=β=γ=90°

[12]

δ

Cubic

Ia3

a=b=c=9.52 Å, α=β=γ=90°

[13]

ε

Hexagonal

P63mc

a=b=2.90 Å, c=9.26 Å, α=β=90°, γ=120°

[8]

κ

Orthorhombic

Pna21

a=5.05 Å, b=8.70 Å, c=9.28 Å,

[9]

α=β=γ=90°

74

α-Ga2O3 is hexagonal, with space group R 3 c, belongs to the corundum

75

structure[Fig. 2(a)] [10,14]. There are a variety of rhombohedral corundum-structured

76

III-oxide materials, that is, α-M2O3 (M = Al, Ga, In, Cr, Fe, V, and Ti). Owing to the

77

same crystal structure, alloys and heterostructures of them are essentially expected,

78

leading to novel functions in the future. In this area, Fujita’s group from Kyoto

79

University have done a lot of original innovative works [15-29]. In spite of metastable

80

phase, α-Ga2O3 and α-In2O3 thin films were epitaxially grown on sapphire (α-Al2O3) 5

81

substrates by using mist chemical vapour deposition (mist-CVD) [15,17,21,22,24],

82

and this enabled the growth of α-(Al,Ga,In)2O3 semiconductor alloys with which they

83

can achieve the “band gap engineering” from 3.8 eV to 8.8 eV [18,20,24]. They also

84

expect to tailor the materials functions with alloying transition-metal oxides (M = Cr,

85

Fe, V, and Ti) due to their unique properties with the α-(Al,Ga,In)2O3 semiconductor

86

alloys, leading to “function engineering” [20]. For example, α-Cr2O3 has been studied

87

as a surface coating material on stainless steels as an oxidation-resistant thin film

88

because of the excellent thermal stability, and also exhibits a magnetoelectric effect

89

[30-31]. α-V2O3 shows temperature-induced insulator-metal transition [32]. α-Fe2O3

90

shows weak ferromagnetism and is promising for applications to a spintronic material

91

[33]. Magnetization hysteresis observed for α-(Ga,Fe)2O3 at higher than room

92

temperature is an example of the “function engineering” realizing fusion of

93

semiconductor and magnetic properties in a material [34]. Meanwhile, α-Ga2O3 also

94

can be obtained by heating GaOOH prepared by hydrothermal method in air [6,35],

95

halide vapour phase epitaxy (HVPE) [36], and laser molecular beam epitaxy [37]. We

96

can expect designing a variety of alloys and heterostructures with the

97

corundum-structured III-oxide alloys toward novel multifunctional devices in the

98

future.

99

β-Ga2O3 has monoclinic structure, belongs to the space group of C2/m

100

[11,38-40].The unit cell of β-Ga2O3 is shown in Fig. 2(b). It contains two

101

crystallographically in equivalent Ga positions, one with tetrahedral geometry Ga (I)

102

and one with octahedral geometry Ga (II). The oxygen ions are arranged in a

103

“distorted cubic” close-packed array. Oxygen atoms have three crystallographically

104

different positions and are denoted as O(I), O(II) and O(III), respectively [41-42].

105

Two oxygen atoms are coordinated trigonally and one is coordinated tetrahedrally.

106

The bulk crystals of β-Ga2O3 can be grown directly from the melt with a melting point

107

of 1793

108

compared with the vapor growth techniques used to manufacture bulk crystals of GaN

109

and SiC, or more exotic wide bandgap semiconductors like diamond. The main melt

. The cost of producing larger area, uniform substrates is potentially lower

6

110

growth methods reported to date have included Czochralski (CZ) [43-46], float zone

111

(FZ) [47-52], vertical Bridgman (VB)/vertical gradient freeze (VGF) [53-56], and

112

edge-defined film-fed growth (EFG) methods [57-58]. The estimated electric field

113

breakdown of β-Ga2O3 is roughly a factor of two larger than the theoretical limits of

114

SiC and GaN [4,59-60]. In terms of power switching device figures-of-merit, the large

115

EB for β-Ga2O3 leads to a Baliga figure of merit almost three times larger than for

116

GaN and SiC. β-Ga2O3 is an interesting material system combining an

117

ultrawide-bandgap (~ 4.9 eV) with a decent mobility (~ 100 cm2/Vs), a high

118

breakdown field (8 MV/cm) and good thermal/chemical stability, which is promising

119

for power electronics and solar blind photodetectors, as well as extreme environment

120

electronics [high temperature, high radiation, and high voltage (low power) switching]

121

[2,4,59-67]. Furthermore, similar to GaN, β-Ga2O3 has shown the potential to form a

122

two-dimensional electron gas (2DEG) at including β-(AlxGa1-x)2O3/Ga2O3 and

123

β-(InxGa1-x)2O3/Ga2O3 hetero-interfaces and thus offers a basis for the development of

124

high electron mobility transistors (HEMTs) [68-73].

125

γ-Ga2O3 belongs to the space group of Fd3m, and has defective cubic spinel-type

126

structure (AB2O4-type) [Fig. 2(c)], which is similar to that of γ and η-Al2O3

127

[12,74-79]. The ideal spinel structure of AB2O4-type is represented by a 2×2×2

128

array of an fcc packed oxygen sub-cell, with the A and B cations occupying the 8a (of

129

the 64 available) tetrahedrally and the 16d (of 32) octahedrally coordinated interstitial

130

sites. First principles calculations indicate that the cation vacancies are mainly located

131

on the octahedral site. γ-Ga2O3 thin film can be obtained by the mist-CVD [80],

132

molecular beam epitaxy [81], and pulsed laser deposition (PLD) [82-84] with the use

133

of sapphire and spinel substrates. γ-Ga2O3 is promising for a spintronic material from

134

the facts that Mn-doped γ-Ga2O3 epitaxial layers shows room temperature

135

ferromagnetism [85-86].

136

δ-Ga2O3 belongs to the space group of Ia3, which is known as a body-centered

137

cubic crystal and isomorphous with other bixbyite crystals such as In2O3, Mn2O3 and

138

Ti2O3 [8,13]. However, thin film growth of the δ-Ga2O3 phase has not been reported 7

139

so far.

140

ε-Ga2O3 is hexagonal, with the space group of P63mc [Fig. 2(d)]. ε phase is the

141

second-most stable phase and is compatible with the common hexagonal wide

142

bandgap semiconductors GaN and SiC [87]. ε-Ga2O3 has a crystal structure similar to

143

κ-Al2O3 [9,40,88-90]. ε-Ga2O3 thin films were grown by HVPE on nitride, β-Ga2O3,

144

magnesia, and yttria-stabilized zirconia substrates [91-92], by plasma-assisted

145

molecular beam epitaxy on sapphire [93], and by metal organic chemical vapour

146

deposition on sapphire [94-95]. It is predicted that ε-Ga2O3 has a large spontaneous

147

polarization and therefore could produce high-density 2DEG used for conducting

148

channels in heterostructure field effect transistors. Ferroelectric properties were

149

observed in ε-Ga2O3 due to the presence of uncompensated electrical [94].

150

κ-Ga2O3 is orthorhombic, with the space group of Pna21 and the lattice

151

parameter of a=5.046 Å, b=8.702 Å, c=9.283 Å, α=β=γ=90°, which is usually

152

misinterpreted as the disordered structure with its P63mc space group symmetry

153

referred to as ε-Ga2O3 in the current literature [9].

154

155

156

FIG. 2. Crystal structures of several polymorphs of Ga2O3 [67].

β-Ga2O3 is the most common and the only stable phase up to 1800

, while

157

other modifications are low temperature phases. The formation energy of the

158

polymorphs has the following order β<ε<α<δ<γ [13]. Although other phases than

159

β-phase cannot be grown as bulk crystals from the melt, fluxor gas phase, they could 8

160

be obtained as thin films or thick layers. β-Ga2O3 can be obtained by baking any other

161

polymorph of Ga2O3 in air at temperatures above 500-700

162

converted into other phases at higher pressures or temperatures [10]. For example, it

163

undergoes a transition to the hexagonal α-Ga2O3 phase at a pressure of 4.4 GPa at

164

1000

165

to the trigonal α-Ga2O3 occurs at around 19.2 GPa under cold compression [97]. They

166

heated powdered samples to 2000 K at 30 GPa, and found that α-Ga2O3 is the most

167

stable structure at the high pressure [97]. This high pressure phase can remain as a

168

metastable phase if quenched to room temperature [98-99].

[Fig. 3], which can be

[96]. Ma et al. found that the phase transition from the monoclinic β-Ga2O3

169 170

171

172 173

FIG. 3. Interconversion relation of Ga2O3 polymorphs [8,67].

2.2 Basic physical properties and potential applications of devices Table Π compares the important material properties of major semiconductors with those of Ga2O3, which has important applications in the following device fields:

9

TABLE Π. Material properties of major semiconductors and Ga2O3

174

175

Materials parameters

Si

GaAs

GaP

4H-SiC

ZnO

GaN

Ga2O3

Diamond

AlN

MgO

Bandgap, Eg(eV)

1.1

1.43

2.27

3.3

3.35

3.4

4.2~5.3

5.5

6.2

7.8

Electron mobility, µ (cm2/Vs)

1400

8500

350

1000

200

1200

300

2000

135

-

Breakdown field, Eb (MV/cm)

0.3

0.6

1

2.5

3.3

8

10

2

-

Dielectric constant, Ɛ

11.8

12.9

11.1

9.7

8.7

9

10

5.5

8.5

9.9

Thermal conductivity k, (W/cmK)

1.5

0.55

1.1

2.7

0.6

2.1

10

3.2

-

Baliga (ƐµEb3)

1

853

339

-

870

24661

20

-

I)

8

0.23[010] 0.13[100]

3443

Ga2O3 is a promising material for deep ultraviolet transparent conductive

176

oxide (TCO) film electrode. It has an ultra-large bandgap of about 4.9 eV, with

177

excellent chemical and thermal stability and high UV-visible transmittance. Ga2O3

178

thin film n-type carrier concentration can be tuned the from low 1017 cm-3 up to 1020

179

cm-3 by doping with, for instance, Si, Ge or Sn [100-101]. These above characteristics

180

of Ga2O3 can simultaneously meet the requirements of good conductivity and high

181

optical transmittance of transparent conductive electrode. Compared to commercial

182

TCO electrodes (such as ITO, FTO, AZO, etc.), the TCO based on Ga2O3 has an

183

obvious advantage of a high transmittance to ultraviolet C, which will increase the

184

utilization of ultraviolet light in the device.

185

II)

Ga2O3, with a direct bandgap of ~ 4.9 eV, is a natural material for solar-blind

186

photodetector. Due to the strong absorption of deep ultraviolet light by stratospheric

187

ozone, the solar irradiation between 200 and 280 nm which is called solar-blind

188

region does not exist at the surface of the earth. Consequently, the photodetectors

189

operating in this region, the so-named solar-blind photodetectors, could detect very

190

weak signals accurately under sun and artificial illuminations due to the “black

191

background”. The bandgap of Ga2O3 directly corresponds to the solar-blind 10

192

ultraviolet detection band with wavelength less than 280 nm. There is no need of

193

doping in Ga2O3 to tune its bandgap, compared to AlGaN, MgZnO, etc., thereby

194

avoiding alloy composition fluctuations and phase separation. Solar-blind

195

photodetectors based on Ga2O3 have a vast and ever growing number of military and

196

civil surveillance applications such as missile tracking, fire detection, ozone holes

197

monitoring, chemical/biological analysis, and so on.

198

III)

Ga2O3 is an attractive substrate for the fabrication of efficient high-brightness

199

vertically-structured LEDs, and is a novel alternative to currently used powder

200

phosphors [102-110]. β-Ga2O3 as substrate for homoepitaxy as well as for

201

heteroepitaxial deposition of GaN-based devices. High-brightness blue-LEDs with

202

vertical current injection are demonstrated. By the vertical injection, the current

203

crowding inherent to horizontal structures is avoided, the heat is dissipated more

204

efficiently, and also the forward operation voltage decreases, thus also the heat losses

205

are diminished. The light comes out through the substrate, taking advantage of its

206

high transparency. Ga2O3 doped with rare-earths or other elements exhibits strong

207

luminescence emissions at various wavelengths, as a new multicolor-emitting

208

phosphors host material, which is promising for white light applications, in particular

209

for those involving high-brightness LEDs or LDs as primary light sources.

210

IV)

Ga2O3-based transistors and diodes possess fundamental electronic properties

211

that make it an ideal candidate for high-power devices [2,4,59-60,63-64,111-113].

212

The β-Ga2O3 theoretically has a high theoretical breakdown field (8MV/cm) that is

213

approximately 2 higher than the breakdown field of GaN (3.3MV/cm) and SiC

214

(2.5MV/cm), and 20 higher than the breakdown field of Si (0.3MV/cm). This high

215

breakdown field allows Ga2O3-based devices to be biased at a high drain voltage

216

(Vbreak-down≫100 V) while maintaining a large dynamic range. Furthermore, the wide

217

bandgap of Ga2O3 allows device operation at elevated temperature (>300

218

degradation. In addition, Ga2O3 has a high saturation electron velocity (vsat = 2×107

219

cm/s), which is partially accountable for the high current density.

220 221

V)

) without

Ga2O3 can also serve as a reactive oxide layer, sensitive to a wide variety of

gases, is a promising choice for gas sensing applications at high temperatures and in 11

222

harsh environments [114-121]. The gas sensing capability of Ga2O3 stems from

223

surface reactions with gas molecules, thus resulting in chemiresistive change in the

224

conductivity. Ga2O3 based gas sensors have important applications in monitoring

225

exhaust gases of automobiles, flue gases of incinerators, pollutant gases of refinery

226

plants, and explosive gases from military applications.

227

VI)

Ga2O3 is a potential photocatalyst for application in wastewater treatments,

228

particularly for the defluorination of fluorinated substances [122-127]. Compared to

229

TiO2, Ga2O3 has a higher bandgap and a higher redox potential. The photogenerated

230

hole and excited electron on the surface of Ga2O3 possess higher oxidation and

231

reduction power. Therefore, Ga2O3 is expected to efficiently degrade organic

232

substances attached to the catalyst surface; the elimination efficiency is higher in

233

comparison with the TiO2.

234

VII)

Ga2O3 doped with the transition metal elements can realize ferromagnetism

235

at room temperature, presenting a promising candidate for use in spintronic devices

236

that are capable of working at room temperature [20,34,85,128-131]. Guo et al.

237

observed the room temperature ferromagnetism in Mn-doped β-Ga2O3 epitaxial thin

238

films

239

corundum-structured α-(Ga1-xFex)2O3 alloy thin films and spinel-structure Mn-doped

240

γ-Ga2O3 thin films [20,34,85].

241

VIII)

[129].

Room

temperature

ferromagnetism

also

has

be

found

in

As an oxide semiconductor material, Ga2O3 is considered as one of ideal

242

candidates for resistance random access memory (RRAM) devices because of its

243

intrinsic high resistance characteristic and extraordinarily sensitive conductivity to the

244

oxygen [132-142]. Guo et al. observed the unipolar resistive switching behavior and

245

abnormal-bipolar resistive switching behavior respectively in the sandwich structure

246

of Pt/amorphous Ga2O3-x/Pt, which are stable and repeatable [132-133]. Ga2O3

247

RRAM devices based on the resistance switching is the potential next generation for

248

nonvolatile memory due to their outstanding features such as simplicity in structure,

249

high operation speed, low power consumption, high scalability, excellent endurance,

250

and multistate memory.

251

There are several review articles on various topics in Ga2O3, but there is a lack of 12

252

an overview of optoelectronic devices [1,3-4,63,65-66,73,143-145]. The present

253

review summarizes the application of Ga2O3 in solar-blind photodetector, phosphors

254

and EL devices.

255

3. Phosphors and electroluminescent devices

256

Ga2O3 has gained attention as new multicolor-emitting phosphors host material

257

for emissive display applications such as thin-film electroluminescent (TFEL)

258

displays, field emission displays, plasma display panels and fluorescent lamps

259

[105-106,146-160]. Conventional sulphide based phosphors have many disadvantages

260

such as the lack of primary colour emissions and chemical instability, especially in

261

regard to moisture. In contrast, β-Ga2O3 is very stable both chemically and physically.

262

Because of high electric strength of Ga2O3, it is possible to apply higher electric fields

263

to the Ga2O3 EL devices. Ga2O3 activated with transition metals, rare-earths or other

264

elements

265

High-luminance multicolor emissions can be realized by TFEL devices with a Mn, Cr,

266

Co, Sn or rare-earth activated Ga2O3 phosphor. High-luminance multicolor emissions

267

can be realized by TFEL devices with a Mn, Cr, Co, Sn or rare-earth activated Ga2O3

268

phosphor. Red and green emission colors can be obtained using Ga2O3 phosphors

269

activated with Eu3+, Cr, Co, Sm and Mn, Tb, Er, Ho, respectively (Table Ш). Yellow

270

and blue emission colors can be also obtained using Ga2O3 phosphors activated with

271

Tm, Ce, Sn and Dy, Eu2+, respectively (Table Ш).

exhibits

strong

luminescence

emissions

at

various

wavelengths.

272

Table Ш. Multicolor emissions with transition metal or rare-earth activated Ga2O3

273

phosphor Emission colors

Doping elements

Red

Eu3+, Cr, Co, Sm

Yellow

Tm, Ce, Sn

Green

Mn, Tb, Er, Ho

Blue

Dy, Eu2+

13

274

3.1 Undoped

275

In 2002, Hao et al. have prepared polycrystalline thin films of undoped Ga2O3 by

276

spray pyrolysis and observed abroad blue-green emission cathodoluminescence (CL)

277

spectrum [161]. The emission band can be separated into three Gaussian bands: Two

278

major emissions were centred at 497 nm (2.49 eV) and 526 nm (2.36 eV), while one

279

minor emission was centred at 424 nm (2.92 eV). The CL intensity was found

280

generally to increase with the increase of annealing temperature when these samples

281

were annealed in forming gas for 1 h, while emission peak position did not change.

282

Meanwhile, the intensity of CL emission band was affected strongly by the annealing

283

ambient, though the line position and line width did not change. The annealing

284

ambient changed from an oxidizing one (oxygen and air), through to an inert one (N2

285

or Ar) and on to a reducing one (forming gas), corresponding to a change from

286

oxygen-rich to oxygen-deficient Ga2O3 films. Forming gas created more oxygen

287

vacancies, and the CL intensity increased. On the other hand, the oxygen ambient

288

decreased these vacancies and the CL intensity decreased. The dependence of the CL

289

intensity on the annealing atmosphere and temperature was consistent with a model

290

that involved oxygen vacancies in the recombination [162]. Those three emission

291

could be attributed to: (a) recombination of electrons in shallow traps with holes in

292

deep traps, (b) recombination of electrons in deep traps with holes in shallow traps, (c)

293

recombination of electrons in deep traps with photo-excited or cathodo-excited holes

294

in the valence band, (d) recombination of photo-excited or cathodo-excited electrons

295

with holes in deep traps, as shown in Fig. 4 [161].

296

Three different PL bands corresponding to UV, blue and green emission were

297

observed in unintentionally doped Ga2O3 early, which were also attributed to the

298

recombination of an electron on a donor formed by oxygen vacancies with a hole on

299

an acceptor made up of either gallium vacancies or gallium oxygen vacancy pairs (VO,

300

VGa) by some researchers [163-177]. Because the thin films of Ga2O3 were prepared at

301

much lower temperature than was used to produce powders and single crystals, it is 14

302

possible that these films were oxygen deficient. Therefore, there could exist an

303

appreciable amount of singly ionized oxygen vacancies, gallium vacancies or

304

gallium-oxygen vacancy pairs (VO, VGa) in sprayed Ga2O3 thin films. As a result, the

305

emission could result from a rate-determining transfer of an electron from a donor

306

(oxygen vacancy) to a hole trapped at an acceptor site (gallium vacancy or a gallium

307

oxygen vacancy pair).

308

309

FIG. 4. Schematic diagram of the energy band of blue-light emission from undoped

310

Ga2O3. The carriers migrate through the solid lines and recombine via (a), (b), (c) and

311

(d) paths [161].

312

3.2 Eu doped

313

For Eu doped Ga2O3, there are two different luminescence spectrum dependent

314

on the Eu valence state (divalent or trivalent) [103,106,159,178-181]. Eu3+-doped

315

Ga2O3 is a promising red phosphor due to the 5D0 →7Fj transition band, while

316

Eu2+-doped Ga2O3 exhibits blue emission which can be attributed to the transition

317

band of 4f→5d [112,137]. Eu2+ is unstable in an oxidizing atmosphere, and it can be

318

oxidized to Eu3+ easily via Eu2+→ Eu3+. Annealing the phosphors at relative high 15

319

temperatures for hours in a reducing atmosphere is required for both crystallization of

320

compounds and reduction of Eu3+ to Eu2+ [137].

321

Layek et al. synthesized Eu2+/Eu3+-doped Ga2O3 colloidal nanocrystals (NCs)

322

with a metastable cubic crystal structure (γ phase) [178]. The PL spectra exhibit red

323

emission at 613 nm for Eu3+ doped and blue emission at 402 nm for Eu2+ doped,

324

which can be attributed to the transition band of 5D0 →7Fj and 4f→5d respectively. At

325

the high synthesis temperature, the PL intensities of both Eu2+ and Eu3+ increase with

326

increasing doping concentration, and the increase in intensity of Eu2+ is much more

327

pronounced. The overall scheme of the charge and energy transfer processes

328

responsible for the PL properties of Eu-doped Ga2O3 NCs is shown in Fig. 5. The

329

excitation above the Ga2O3 band edge energy leads to an efficient trapping of the

330

photo-excited electrons and holes in the donor and acceptor states, respectively,

331

resulting in a broad donor and acceptor pairs (DAP) emission of Ga2O3 NCs (thick

332

blue-green arrow). The nature of the donors are extensively in β-Ga2O3 single crystals.

333

These donor states are predominantly associated with one of the three different

334

oxygen vacancy sites (VO) and that electron transfer takes place along the b

335

crystallographic axis through a hopping or tunneling mechanism, depending on

336

temperature. The donor and acceptor defect states play a key role in the sensitization

337

of the Eu emission by nonradiative transfer of the DAP exciton energy to the dopant

338

ions (dashed lines in Fig. 5). The energy transfer is much more efficient for Eu3+

339

(thicker dashed line). Given the low absorptivity of f-f transitions, the energy transfer

340

is likely to be the Dexter rather than the Förster type. On the other hand, Eu2+ can be

341

readily excited directly (purple arrow), allowing for a control of the NC emission

342

color by selective excitation. The Eu2+ and Eu3+ centers behave independently, with

343

insignificant energy transfer between them, which could be associated with different

344

spatial distribution of the two species. While Eu2+ appears to be stabilized as an

345

internal dopant, Eu3+ is partitioned between the interior and surface of the NCs, with a

346

larger fraction on the NC surfaces. For high synthesis temperatures, Eu3+ in the

347

reaction mixture is readily reduced to Eu2+. The obtained NCs are also larger and on 16

348

average contain fewer native defects, a significant fraction of which could be located

349

in the interior of the NCs. These internal defects (i.e., oxygen vacancies with a single

350

positive charge) play a key role in the stabilization of Eu2+, as described above,

351

leading to a strong emission upon direct excitation of the Eu2+ dopant centers. A

352

decrease in the reaction temperature results in a lower degree of reduction of Eu3+ and

353

a higher concentration of native defects in the vicinity of NC surfaces, owing to an

354

increase in the surface-to-volume ratio. The lack of internally incorporated Eu2+ and

355

more effective sensitization of Eu3+ by surface-based NC defects lead to exclusive

356

Eu3+ emission. It is plausible that some non-oxidized Eu2+ ions remain on the surfaces

357

of NCs, where they are quenched by nonradiative relaxations. These manipulations of

358

the relative fractions of Eu2+ and Eu3+ centers and native defect concentration by

359

reaction conditions, such as temperature, time, and starting dopant precursor

360

concentration, enable the tuning of the PL emission in colloidal Ga2O3 NCs from blue

361

to red. The coexistence of Eu2+ with Eu3+ has not been reported for bulk (β-phase)

362

Ga2O3 prepared by solid state reaction or other high-temperature preparation methods,

363

such as PLD, spray pyrolysis, sputtering, and son on, as follows.

364

365

FIG. 5. Schematic representation of the charge and energy transfer processes involved

366

in the dual emission of Eu2+/Eu3+-doped Ga2O3 NCs. Charge transfer is indicated with

367

solid lines and energy transfer with dashed lines [178].

368

Chen et al. reported that Eu3+ doped Ga2O3 films with different Eu contents can 17

369

be obtained by changing the Eu composition in the targets on sapphire substrates by

370

PLD at substrate temperature as low as 500

371

nm were clearly observed in the Eu doped Ga2O3 films and the intensity increases

372

remarkably with the increase of Eu doping content, which can be attributed to the

373

transitions from 5D0 to 7F2 levels in Eu. While the intensity of the XRD peaks of

374

Ga2O3 decreases with increasing Eu contents, they speculate that the emission

375

intensity of the films was mainly determined by the amount of formation of Eu2O3. In

376

order to understand the nonradiative and radiative transition process, overcome

377

intensity quenching and increase the effective number of optically active Eu ion in the

378

host, the temperature dependence luminescence behavior of Eu doped Ga2O3 thin film

379

were investigated [183]. The emission intensity of Eu3+ decreased solely with elevated

380

temperature by using 325 nm light, while it had a maximum value at a certain

381

temperature under 488 nm light. Both of the experimental data were well fitted by the

382

luminescence dynamic equation models, suggesting that the variation of the emission

383

intensity may be attributed to the thermal activated distribution of electrons among 7Fj

384

and thermal quenching effect. They also have investigated substrate temperature

385

dependence of luminescence spectra in Eu doped Ga2O3 film grown by PLD at

386

various substrate temperature and found that the intensity of red emission line

387

observed at 613 nm increases with increasing substrate temperatures up to 400

388

and then decreases from 400 to 600

389

lattice and substituted in the lattice or localized at the grain boundaries at substrate

390

temperature as low as 400

391

have investigated structural properties of Eu doped Ga2O3 films grown by PLD at

392

different substrate temperature. X-ray rocking curve and Raman spectroscopy

393

measurements prove that the films grown at the substrate temperature above 400

394

are of monoclinic β-Ga2O3 structure and the crystalline quality of the films depends on

395

the substrate temperature. X-ray absorption near edge structure measurements indicate

396

that the valence of Eu ions in the Eu doped Ga2O3 films varies from mixture of

397

bivalent and trivalent to only trivalent with increasing substrate temperature.

398

Extended X-ray absorption fine structure analysis reveal that the Eu atoms doped in

[182]. Intense red emissions at 613

[183]. Are Eu ions incorporated on a Ga2O3

by PLD. Therefore, in another work [184], Chen et al.

18

399

Ga2O3 matrix are incorporated on Ga sites in Ga2O3 matrix even for the films with

400

amorphous structure grown at low substrate temperature.

401

However, since the substrate of sapphire is not conductive, it is not possible to

402

fabricate optical devices operating between the films and the substrates. GaAs is a

403

desirable conductive substrate due to its availability in large wafer size and its large

404

role in the electro optic industry. Moreover, as a gallium-based substrate, it is believed

405

that high quality Ga2O3 films could be epitaxial grown on GaAs. They have fabricated

406

Eu doped Ga2O3 thin films on GaAs substrate by using PLD at a substrate

407

temperatures of 500

408

controlled by adjusting the Eu contents in the targets. XPS spectra indicated that the

409

incorporation of trivalent Eu can substitute Ga3+ to form Eu2O3. Red LEDs of

410

Eu-related luminescence at about 611 nm in Eu:Ga2O3/GaAs heterojunctions was

411

observed, which originates from the 5D0→7F2 transition of Eu3+. Bright red light can

412

be observed with the naked eye at 6.0 V, providing an outlook on the future of Ga2O3

413

as a candidate for low driven voltage and small-scale displays such as mobile device

414

screens or micro-LED displays [185]. Under forward bias, the electrons will first

415

transit to the defect-related energy levels and then recombine with holes. The indirect

416

recombination of carriers in the Ga2O3 host could transfer energy to the Eu ions. The

417

work of Chen et al. also suggested that PLD is a promising method for obtaining high

418

quality Eu doped Ga2O3 films at low growth temperature, which paves the way for the

419

fabrication of optoelectronic devices based on Ga2O3 films.

[184]. The contents of Eu in Ga2O3 films can also be

420

As early as in 2000, Kitai’s group reported a high brightness of 840 cd/m2

421

located at 615 nm in on-filtered Ga2O3:Eu EL phosphor at 370V/650 Hz, which

422

exhibits the long-term stability and can be maintained for a single-insulating device

423

for over 2500 hours [149,152]. Hao et al. prepared the thin film of 1.0 at% Eu3+ doped

424

Ga2O3 by spray pyrolysis and observed a red CL spectrum [161]. Further, they

425

reported

426

of x = 0.643 and y = 0.356 with a dominant wavelength of 605 nm and a color purity

427

of 99%, and suggested that the spray pyrolysis process which avoids the use of a high

sprayed Ga2O3:Eu thin film can produce excellent chromaticity coordinates

19

428

vacuum apparatus may be more attractive from the point of view of large-scale and

429

large-area production of TFEL devices [157]. Wellenius et al. Produced Eu doped

430

gallium oxide thin film EL devices with bright, red emission 611 nm and relatively

431

low threshold voltages of 60 V using PLD, as shown in Fig. 6 [105-106]. The use of

432

transparent conducting electrodes of amorphous InGaZnO on transparent aluminum

433

titanium oxide/indium tin oxide/7059 Corning glass substrates resulted in a device

434

that is transparent throughout the visible spectrum. The origin of the red emission is

435

the 5D0 to 7F2 transition and is consistent with PL and CL results. At 100 V, with 1

436

kHz excitation, the luminance and chromaticity coordinates were 221 cd/m2 and x=

437

0.629, y= 0.355 respectively. The turn on voltage of the device is about 45 V ac, and

438

the device appears to be robust, operating at elevated voltages without degradation.

439

Brightness increased substantially for higher drive frequencies, with optimal emission

440

near 2 kHz for this device geometry [106].

441

442

FIG. 6. Electroluminescence spectrum illustrating the characteristic red emission of

443

Eu3+ ions. A device schematic and a picture of a working device are the inset [106]. 20

444

Marwoto et al. have grew Eu doped Ga2O3:Eu thin films using direct current

445

magnetron sputtering, and found that the broad red PL emission peak between 593

446

and 602 nm will be blue shift with the Eu concentration of 2% while it will be red

447

shift when it was 5% [156]. Kim et al. synthesized nanostructured and bulk

448

β-Ga2O3:Eu3+ phosphors through sol-gel and solid-state reaction techniques

449

respectively, and found both the luminescent intensity and color purity of

450

nanostructured β-Ga2O3:Eu3+ are enhanced in comparison with bulk one due to the

451

surface hindrance of resonant energy transfer [158]. Kang et al. synthesized Eu3+

452

doped β-Ga2O3 (3 mol%) hollow nanostructures by hydrothermal and calcination

453

processes, which showed strong red emission corresponding to the 5D4→7Fj transition

454

[186]. Theses hollow nanostructures also show the low blue emission intensity at

455

300-500 nm, which can be attributed to the recombination of an electron on a donor

456

with a hole on an acceptor formed by a gallium vacancy or gallium-oxygen vacancy

457

pair in the β-Ga2O3 crystal structures. López et al. also prepared Eu3+ doped Ga2O3

458

nanowires by implantation and rapid thermal annealing treatments, and observed the

459

most intense red emission located at 610 nm corresponding to the 5D0→7F2 transition

460

[187]. Lorenzi et al. presented the synthesis of Eu-doped γ-Ga2O3 hybrid

461

nanoparticles with an in situ organic capping resulting from a non-aqueous

462

solution-based benzyl alcohol synthesis route, and demonstrates the role of the

463

intrinsic and organic related activation of Eu3+ PL [76].

464

3.3 Er doped

465

Er has been widely studied in different hosts for luminescence purposes, mainly

466

due to its green (4S3/2-4I15/2) and infrared (IR) (4I13/2–4I15/2) luminescence lines. The

467

former of green emission located at around 550 nm play an important role in

468

full-color display technology and in the development of white LEDs, while the latter

469

centred at around 1.54 µm has a great importance in the telecommunications industry

470

because of its coincidence with the wavelength at which the standard silica-based

471

optical fibres present the lowest losses. 21

472

Chen et al. epitaxially grew Er doped Ga2O3 films with various Er contents

473

Ga2O3 films on the sapphire substrate by PLD [188]. Temperature insensitive pure

474

green emissions at 550 nm are clearly observed for the Er doped Ga2O3 films. No

475

peak shift at 550 nm is found with temperatures ranging from 77 to 450 K, and the

476

intensity of the green emission increases with the increase of the Er doping content

477

and the decrease of temperature. The strongest peak at 550 nm is caused by the 4S3/2

478

to 4I15/2 transition, and a shoulder at longer wavelength is ascribed to Stark splitting

479

because the spine orbit splitting of the energy level. Other PL peaks observed at 524,

480

655, 850, and 975 nm can be assigned to the transitions from 2H11/2 → 4I15/2, 4F9/2 →

481

4

482

energy due to the recombination of electrons in the defect state with the

483

photogenerated holes can transfer to the excited states of Er ions. The normalized

484

intensity of the Er doped Ga2O3 films has a smaller variation with temperature

485

compared to Er doped GaN films, indicating that Ga2O3 is a good host material for Er

486

and potentially for other rare earth elements. As mentioned earlier, sapphire substrate

487

based devices is not possible to operate the photoelectric performance between the

488

films and the substrates since sapphire is not conductive. Moreover, Ga2O3:Er/Si

489

heterojunctions are especially attractive due to the well-known advantage of the Si

490

substrate and their prominent application in Si-based optoelectronic integrated circuits.

491

Thus, Er doped Ga2O3 thin films were deposited on Si substrate and their EL

492

properties were investigated [189]. Bright green emission (548 nm) can be observed

493

by naked eye from Ga2O3:Er/Si LEDs. The driven voltage of that LEDs is 6.2 V

494

which is lower than that of ZnO:Er/Si or GaN:Er/Si devices. The EL intensity

495

increases with the forward bias, and no peak shift in the EL spectrum is observed with

496

the injection current ranging from 3 to 60 mA. Moreover, the EL spectra has two

497

narrow, strong green emission bands centered at 524 and 548 nm, which are

498

consistent with the PL results. Since the wide bandgap of Ga2O3 contain more

499

defect-related level which will enhance the effects of recombination between

500

electrons in the defect-related level and the holes in the valence band, resulting in the

501

improvement of the energy transfer to Er ions [189-191].

I15/2, 4I9/2 → 4I15/2, and 4I11/2 → 4I15/2, respectively, as shown in Fig. 7. The related

22

502 503

FIG.7 Energy diagram of Ga2O3 and Er and the proposed mechanisms for laser

504

excitation less than 488 nm [188].

505

Wu et al. obtained highly oriented (201) Er3+-doped β-Ga2O3 thin films with

506

different Er doping concentrations on (0001) sapphire substrates using rf magnetron

507

sputtering, and systematically studied the crystal structure, optical absorption,

508

near-infrared luminescence, and ultraviolet photoresponse properties [192]. A

509

remarkable structural modification is induced by the Er doping from the XRD patterns.

510

The (402) diffraction peak positions of the Er doped thin films

511

lower 2θ compared to the undoped Ga2O3 thin film. The shift in the 2θ values

512

indicates an increase of the lattice constants, which can be attributed to the ionic radii

513

difference (30%) between Er3+and Ga3+ (Er3+: 0.89 Å , Ga3+:0.62 Å for octahedral

514

coordination and 0.47 Å for tetrahedral coordination, respectively). Meanwhile, the 23

gradually shift to

515

band gap of Er:Ga2O3 thin films gradually increases with the increase of Er content.

516

The evolution of lattice and energy band gap with increasing doping level confirms

517

the chemical substitution of Er3+ ions into the Ga2O3 crystal lattice. The down-shifting

518

near-infrared luminescence located at 1538 nm was observed under ultraviolet

519

excitation, which is attributed to the Er transition 4I13/2→4I15/2. The emission intensity

520

can be remarkably enhanced with increasing Er3+doping concentration. The most

521

probable site for the Er3+ ions substitution is at the distorted octahedral sites of the

522

Ga2O3 lattice with an inversion center, which implies the selection rules forbidding all

523

4f-4f electric dipole (ED) transitions. When the dopant concentration increases, the

524

elongation of the d spacing of (201) plane promotes the structure asymmetry of the

525

Ga2O3 host, approaching lower symmetry around Er3+ ions. The lower symmetry

526

means that the more uneven crystal-field components can mix opposite-parity into 4f

527

levels and subsequently increase the ED transition probabilities of the dopant ions,

528

resulting in the enhancement of NIR PL emission. They also had demonstrated the

529

existence of efficient energy transfer (ET) from the electron-hole pairs created in the

530

Ga2O3 host to the Er3+ ions.

531

Nogales et al. prepared Er doped β-Ga2O3 nanowires and microwires by a

532

vapour-solid process from an initial mixture of Ga2O3 and Er2O3 powders [193]. The

533

presence of rather intense Er intraionic blue, green and red emission lines, even at

534

room temperature, is observed in the CL spectra. Mapping of the main 555 nm

535

emission intensity shows a non-homogeneous distribution of Er ions in the

536

microstructures. They also observed strong near IR emission and weak green emission

537

from the Er doped β-Ga2O3 powder pellet, which is mainly excited through band to

538

band excitation followed by an energy transfer process and assigned to Er3+ in

539

octahedral Ga sites within the β-Ga2O3 [194]. Vincent et al. obtained single crystals of

540

conducting Er doped β-Ga2O3 through floating zone method, and determined some

541

crystal field levels of the 4I15/2 and 4I13/2 multiplets through the absorption and

542

emission spectra obtained at 1.5 µm in the 0.5% Er doped β-Ga2O3 sample [195].

543

Biljan et al. reported on facile solution combustion synthesis of Er doped 24

544

β-Ga2O3with urea as fuel [196]. Main characteristic of luminescence spectrum of

545

β-Ga2O3:Er is a strong Er3+ NIR emission at 1.54 µm due to the 4I13/2→4I15/2 transition

546

under the 1064 nm excitation. The intensity of this technologically important

547

transition can be significantly (around 60 times) increased by the Yb3+ codoping. It

548

also exhibits intense characteristic Er3+ luminescence in the visible region.

549

3.4 Tm-doped

550

Tm-activated phosphors have complicated energy level schemes due to the strong

551

deviation from Russell Saunders coupling in the (4f) configuration [197-206]. As a

552

consequence, the relaxation of the excited states of Tm3+ ions may take place via a

553

large number of relaxation paths and result in UV, visible, and IR emissions. Hao et al.

554

reported the Tm-doped Ga2O3 film exhibits a spectrum very similar to that for the

555

undoped film except for a minor peak around 813 nm relating to the transition from

556

1

557

which corresponds to the transitions between electronic energy levels of Tm3+. Instead,

558

a broad blue-green emission occurred over the range from 350 to 650 nm, which may

559

still be due to transitions involving Tm3+ levels or that it comes from the Ga2O3 host.

560

Furthermore, the luminance under the same conditions was 10 and 19 cd/m2 for the

561

Ga2O3 and Ga2O3:Tm films, respectively. Consequently, although the undoped film

562

exhibits a spectrum very similar to that for the Tm-doped film, the Tm dopant

563

enhances the CL emission intensity. This emission band can be separated into three

564

Gaussian bands: Two major emissions were centred at 497 and 526 nm, while one

565

minor emission was centred at 424 nm.

G4 →3H5 of the Tm3+ ion [161-162]. There is no obvious blue-emitting peak present

566

Guo et al. have deposited Tm doped Ga2O3 films on sapphire substrates by PLD

567

with changing Tm compositions in the targets [207]. Energy dispersive spectroscopy

568

results reveal that films with different Tm compositions can be tailored by changing

569

the Tm composition in the targets. XRD and Raman spectra analysis indicate that all

570

films have the monoclinic structure. PL measurements demonstrate that the emission 25

571

peaks at 460, 650 and 800 nm are observed from the Tm3+ 4f intrashell transitions

572

from 1G4 excited states to the 3H6, 3F4, and 3H5 states, respectively [Fig. 8].

573 574

575

FIG. 8. Energy levels and luminescent transitions in Tm doped Ga2O3 [207].

3.5 Mn-doped

576

In 1997, Minami et al. found Mn-activated Ga2O3 phosphors are very promising

577

as the thin-film emitting layer for TFEL devices [208]. A green emission with a

578

luminance of 232 cd/m2 was obtained for a Ga2O3:Mn TFEL device driven at 1 kHz.

579

When CaO is added to the host material Ga2O3 as the multicomponent oxide phosphor,

580

the EL emission color from CaO-Ga2O3:Mn TFEL devices changed from green to

581

orange as the CaO content was increased from 0 to 100 mol%. And a yellow emission

582

with a luminance of 1725 and 261 cd/m2 was obtained in a TFEL device using a

583

CaGa2O4:Mn thin film prepared with a CaO content of 50 mol%, when driven at 1

584

kHz and 60 Hz, respectively. However, the Ga2O3:Mn and CaO-Ga2O3:Mn phosphor

585

thin-film emitting layers were prepared by conventional planar rf magnetron

586

sputtering here, leading to a relative expensive cost of practical TFEL devices due to

587

the vacuum preparation process. In order to resolve this problem of cost, they

588

demonstrated the use of oxide phosphors as well as a chemical deposition technique 26

589

which eliminates the need for vacuum processes. They developed using a Ga2O3:Mn

590

thin film prepared by a sol-gel process using a relatively inexpensive and easy to

591

handle material gallium acetylacetonate as gallium source, and obtained the

592

high-luminance EL devices [Fig. 9] [153,209-210]. The sol-gel process, which

593

eliminates the need for vacuum processes, enabled the inexpensive preparation of

594

Ga2O3:Mn thin films on large-area thick ceramic sheet insulators. TFEL devices with

595

a Ga2O3:Mn thin-film emitting layer prepared by the sol-gel process at a deposition

596

temperature of 900

597

luminances of 1271 and 401 cd/m2 when driven at 1 kHz and 60 Hz, respectively.

598

Based on the above results, they concluded that a thin-film preparation method using a

599

sol-gel process is very promising for the fabrication of less expensive oxide phosphor

600

TFEL displays and flat panel TFEL lamps. In 2000, they have developed

601

multicolor-emitting TFEL devices using Mn and Cr co-doped Ga2O3 phosphors with

602

variations of both Mn and Cr from 0 to a content of 20 at.% [211]. They found that the

603

emission from Ga2O3:Mn,Cr TFEL devices were more strongly dependent on the Cr

604

content than on the Mn content doped into the phosphor-emitting layers. A change

605

from green to red in emission color and a high luminance above 100 cd/m2 were

606

obtained in the TFEL devices driven at 1 kHz, when the co-doped Cr content was

607

varied from 0 to 20 at.% under a constant Mn content of 0.3 at.% doping. In addition,

608

the emission color of a Ga2O3:Mn,Cr TFEL device co-doped with a Mn content of 0.3

609

at.% and a Cr content of 3 at.% changed from green to red with an increase of the

610

applied voltage. They pointed out that high-luminance multicolor emissions can be

611

realized by TFEL devices with a Mn, Cr, Co,Sn or rare-earth activated Ga2O3

612

phosphor [146,212-213].

and a post annealing temperature of 1000

27

exhibited

613

614

FIG. 9. Cross-sectional structure of a thick ceramic-insulating-layer-type

615

TFEL device [153].

616

Kim et al. fabricated the device with an inverted half-stack structure of

617

ITO/Ga2O3:Mn/lead zirconate titanate (PZT)/Au, and studied the alternating current

618

EL characteristics of Mn doped Ga2O3 thin film [151]. The devices exhibited a

619

broad-band PL emission peaked at around 507 nm in the green range, which is

620

accounted for by the 3d-3d intrashell transition from the 4T1 excited state level to the

621

6

622

EL emission were measured to be x=0.197 and y=0.623.

623

3.6 Nd doped

A1 ground state in the divalent Mn ion. The CIE chromaticity color coordinates of the

624

Nd doped Ga2O3 with NIR emission at 1100 nm corresponding to 4F3/2-4I11/2

625

transition is promise for high power laser media with a high stimulated emission

626

cross-section [214-220]. Wu et al. have investigated structural and NIR luminescence

627

of Nd-doped β-Ga2O3 thin films (Nd:Ga2O3) with different Nd doping concentrations

628

[215]. XPS results confirmed that Nd atoms have been effectively incorporated into

629

the oxide matrix and participate in the chemical bonding, exhibiting a trivalent Nd

630

(Nd3+). With an increase of Nd3+ content, the crystal lattice of the films expands,

631

while the energy band gap shrinks. Compared to undoped Ga2O3 film, the pronounced

632

NIR PL emission consists of three bands, corresponding to the infra-4f transitions of 28

633

Nd3+ ions from 4F3/2 level to 4I9/2 (905 nm), 4I11/2 (1067 nm), and 4I13/2 (1339 nm)

634

levels, respectively. And the intensity of NIR PL emission will be remarkably

635

enhanced with the increase of Nd3+ doping concentration. An enhancement factor of

636

main 4F3/2→4I11/2 transition band can reach up to 2.3. Similar to the discussion above

637

in the Er doped Ga2O3 films by Wu et al., the improvement of emission intensity with

638

the increasing doping concentration can be attributed to the lower symmetry around

639

Nd3+ ions. Notably, the NIR emission bands from Nd:Ga2O3 thin films present an

640

obvious blue-shift, the main luminescent peak (4F3/2→4I11/2) shifts from 1077 nm to

641

1065 nm. They attributed this phenomenon to the variation in the crystal field around

642

Nd3+ ions caused by lattice distortion. As discussed above, the lattice distortion is

643

gradually enhanced as the Nd concentration increases. The increase of the octahedral

644

distortion leads to the enhancement in the Stark splitting of the 4F3/2 multiplet. And the

645

enhanced Stark emitting levels are splitted from the 4F3/2 state, causing the blue-shift.

646

In above process, Ga2O3 acts as an effective light harvester to absorb UV photons and

647

subsequently transfer energy to Nd3+ ion, thereby resulting in the typical NIR

648

luminescence.The excitation energy of incident light is lower than the band gap of

649

Nd:Ga2O3. Owing to the existence of oxygen vacancy defects in the films, hence, the

650

ET process between the Ga2O3 host and Nd ion is as follows. Through ground state

651

absorption (GSA) process, the electrons are excited from the valence band to the

652

donor band (oxygen vacancy) by the light source.The released energy due to the

653

recombination of electrons in the defect state with the photogenerated holes can

654

transfer to the excited states of the Nd ions; thereby, NIR emissions of Nd take place.

655

Zhang et al. have deposited Nb-doped β-Ga2O3 thin films on the Si substrate

656

respectively by radio frequency magnetron technique, and investigated the effects of

657

Nb doping on the structural and optical properties of Ga2O3:Nb thin films [221-223].

658

They only observed the room temperature PL spectrum in the wavelength range of

659

350 ~ 650 nm, and did not obtained the PL spectrum in the NIR region [223].

29

660

661

FIG. 10. Energy level diagram of Ga2O3 and Nd3+, as well as the proposed

662

mechanisms less than 325 nm laser excitation [215].

663

3.7 Tb doped

664

Zhao et al. prepared one-dimensional Tb3+-doped β-Ga2O3 nanofibers by a

665

simple and cost-effective electrospinning process, and investigated the effects of Tb3+

666

doping on the PL of these nanofibers [224]. The diameter of the nanofibers annealed

667

at 900

668

to several tens to hundred micrometers. The β-Ga2O3:Tb3+ nanofiber shows the green

669

emission with the strongest peak at 550 nm under ultraviolet excitation, corresponding

670

to 5D4→7F5 transition of Tb3+ ions. The intensity of the green emission increases and

671

then decreases with the increase of the Tb3+ doping concentration, and reaches its

672

maximum at 3 mol% Tb3+ ions. The decrease of the fluorescence intensity with a

673

higher Tb3+ concentration can be attributed to the concentration quenching effect

674

because the addition of the activator over a sufficient quantity increased the distance

675

between the Tb3+ ions. Kang et al. synthesized Tb3+-doped β-Ga2O3 hollow

676

nanostructures with various Tb3+ concentrations by hydrothermal and calcination

677

processes [225]. They also observed the green emission spectra due to the 5D4→7Fj

ranged from 100 to 300 nm, and the lengths of the nanofibers reached up

30

678

transition and obtained the highest green emission intensity at 7 mol% of Tb3+ ions.

679

Tokida et al. synthesized Ga2O3:Tb3+ green phosphor by metal-organic

680

deposition (MOD) and reported the PL properties of this phosphor [155]. The

681

Tb3+-related green emission intensity shows an increase with the increase of the molar

682

ratio M = Tb/Ga and a saturation at M ~ 0.005-0.03, followed by the concentration

683

quenching at M > 0.03. The green emission intensity exhibits a gradual decrease with

684

increasing temperature above ~ 300 K, yielding quenching energies of 0.25 and 0.63

685

eV. They obtained the detailed energy-level scheme of Tb3+ ions in β-Ga2O3 from the

686

PL and PL excitation spectra of this phosphor, as shown in Fig. 11(a). A series of the

687

sharp PL peaks at λem ~ 490-623 nm correspond to the 4f8 (5Dj)→4f8 (7Fj) transitions in

688

Tb3+. Because of their parity- and spin-forbidden nature, the Tb3+-related emission

689

peaks are usually observed to be very weak. While their decay times are also very

690

long, in the milliseconds region. The Tb3+-related emission or absorption peaks are

691

observed at the three different wavelength regions: 5D4↔7F6 (490 nm) in the blue

692

region; 5D4↔7F5 (543 nm), 7F4 (588 nm) in the green region; and 5D4↔7F3 (623 nm),

693

7

694

F2 (655 nm), 7F1 (672 nm), 7F0 (685 nm) in the red region. The most intensive

emission peak is at the 5D4→7F5 transitions in the green region (543 nm), whereas the

695

5

D4↔7Fj (j= 0-2) red emission peaks are observed to be very weak. They put the PL

696

peak and PLE shoulder at ~ 490 nm attributed to the 5D4↔7F6 transitions. And no

697

large Stokes shift in the 5D4↔7F6 transitions is expected to occur. Very weak

698

absorptions at ~ 410-470 nm are due to the 7Fj→5D3 transitions with j=2-5. The

699

lowest 4f8 excited state in Tb3+ is surely 5D4, whereas the second lowest excited state

700

should be 5D3 or 5L10. As shown in Fig. 11(a), they assumed that the higher-lying

701

energy state labeled by 5Dj (j

702

5

3) contains a set of each nearly same energy state [e.g.,

D3 = (5D3, 5L10), 5D2 = (5D2, 5L6-9, 5G2-6), 5D1 = (5D1, 5H7), and 5D0 = (5D0, 5H5-6)].

703

The PLE peaks or shoulders observed at ~ 310-380 nm is due to the 7F6→5Dj

704

transitions with j=0-3. The

705

transitions of 7F6→5D3 and/or 7F6→5L10, and so on. The PLE structure observed in the

706

250-300 nm region is due to the spin-forbidden 4f8→4f75d transitions in Tb3+, which

7

F6→5D3 (j=3) transitions mean the inner-f-shell

31

707

can be resolved into three independent peaks at ~ 260, 275, and 290 nm [7F6(4f8)→9T2

708

(4f75d) (triplet) transitions]. Due to the ultra-wide band gap of β-Ga2O3 (~ 4.9eV), an

709

electron energy transfer from the conduction band (CB) in the β-Ga2O3 host to the 9T2

710

(4f75d) states in Tb3+ results in the enhanced PLE strengths at ~ 260-290 nm. The

711

Tb3+-related absorption is observed at λ< 250 nm, which is caused by both the spin-

712

and parity-allowed 7F6(4f8)→9T2 (4f75d) transitions in Tb3+. And the 7T2 (4f75d) triplet

713

states are shown to be energetically located within the CB of β-Ga2O3.

714

32

715

FIG. 11. Electronic energy-level scheme for Tb3+singly doped [155] and (Tb3+,

716

Eu3+)-codoped [226] β-Ga2O3.

717

In view of excitation energy transfer and cooperative optical phenomena

718

commonly occur in codoped phosphors, which are an important concept to be used for

719

the improvement of phosphor performances. Sawada et al. investigated the codoping

720

effects of Tb3+ and Eu3+ in β-Ga2O3 crystallites by PL analysis, PLE spectroscopy, and

721

PL decay characteristics [226-227]. The XRD analysis indicated that only β-phase

722

Ga2O3 can be synthesized by the MOD and subsequent calcination at Tc

723

Above ~ 900

724

emission intensities. The Tb3+ and Eu3+ emission intensities in the Ga2O3:Tb3+, Eu3+

725

phosphors by excitation at λex = 266 nm showed remarkable concentration quenching

726

at

727

singly doped samples showed concentration quenching at

728

concentration quenching of the Tb3+ emission in the codoped sample was to be due to

729

an interaction between Tb3+ and Eu3+ in the β-Ga2O3 host. The quenching energies

730

determined here were 80 meV (45 meV) and 0.30 eV (0.20 eV) for the Tb3+ (Eu3+)

731

emission, regardless of the singly doped or codoped sample. Enhanced Eu3+ emission

732

intensity was observed under resonant excitation at λex = 488 nm (Tb3+; 7F6→5D4) in

733

the (Tb3+, Eu3+)-codoped β-Ga2O3 sample. They found PL and PLE measurements

734

showed that the (Tb3+, Eu3+)-codoped samples exhibit clearly different luminescence

735

properties when excited at wavelengths below or above λex ~ 350 nm, which

736

corresponds to the boundary in wavelength between the 4f6→charge transfer state and

737

4f6→4f6 transitions to occur in Eu3+. At λex> 350 nm, efficient energy transfer

738

occurred from Tb3+ to Eu3+, resulting in the decreased Tb3+ and enhanced Eu3+

739

emissions. At λex< 350 nm, not only the Tb3+ but also the Eu3+ emissions decreased

740

with increasing Tb3+/Eu3+ concentration. Furthermore, the PL intensity degradation in

741

the seconds time scale was observed by exciting light at λex< 350 nm. Such unusual

742

PL phenomena at λex< 350 nm seem to be due to an interaction between Tb3+ and Eu3+

743

with producing nonradiative relaxation channels in the Tb3+/Eu3+ emission pathway.

700

.

, calcination treatment greatly reduced the Tb3+ green and Eu3+ red

0.03 mol% (Tb3+) and

0.1 mol% (Eu3+), whereas those in the Tb3+/Eu3+

33

0.1 mol%. The

744

The energy-level scheme for the (Tb3+, Eu3+)-codoped β-Ga2O3 sample is shown in

745

Fig. 11(b). Various 4fN states for Tb3+ (N=8) and Eu3+ (N=6) are shown by the

746

horizontal lines. The 4f8→4f7 5d and 4f8→CTS transitions in Fig. 11(b) occur at

747

wavelengths ≥320 nm (Tb3+) and ≥350 nm (Eu3+). The electrons excited at the high

748

photon energy (λex=266 nm) will experience more nonradiative relaxation events than

749

those excited at the low photon energy (λex=488 nm). Such nonradiative relaxation

750

events would occur more dramatically if samples were more highly doped, known as

751

the concentration quenching. They further investigated the resonance energy transfer

752

process between the Tb3+ and Eu3+ ions in β-Ga2O3:Tb3+, Eu3+ phosphor by analyzing

753

the time-resolved PL spectra of Eu3+ ions exhibiting PL rising behavior [228]. The

754

intensity rising of the PL curve was observed in the time (t) range of t< 0.3 ms by the

755

excitation at 355 nm light. The time-resolved PL curves are well reproduced by

756

theoretical calculations based on a rate equation model including direct excitation

757

(pumping laser) and indirect excitation (energy transfer) processes of Eu3+. The

758

indirect excitation fraction of Eu3+ ions decreases with increasing the Eu

759

concentration, while the energy transfer rate and efficiency increases with increasing

760

the concentration due to the decrease in the average distance between the donor (Tb3+)

761

and acceptor (Eu3+) ions. Furthermore, the transfer efficiency estimated from Eu3+ PL

762

dynamics is confirmed to be larger than that from Tb3+ PL dynamics. This larger

763

efficiency reflects Eu3+ ions purely interacting with Tb3+ ions.

764

The effects of Tb3+ and Eu3+ codoped Ga2O3 also have been investigated by

765

others. Cabello et al. synthesized (Tb3+, Eu3+) codoped Ga2O3 films by photochemical

766

metallorganic deposition [229]. XRD analysis suggested that the films are amorphous

767

in nature. Under UV excitation, the Tb3+ singly doped film shows the characteristic

768

Tb3+ emission peaks attributed to 5D4→7Fj (j=6, 5, 4, 3) transitions of Tb ion. For the

769

(Tb3+, Eu3+)-codoped Ga2O3 film, the Tb3+ and Eu3+ emission peaks were observed

770

with decreased Tb3+ and Eu3+ emission intensities. Their results suggested an

771

interaction between Tb3+ and Eu3+ with producing nonradiative relaxation channels in

772

both the Tb3+ and Eu3+ emission pathways. They also reported these characteristic 34

773

Tb3+ emissions decrease and deteriorate in the transition metal co-doped films

774

(Ga2O3-x/Tb/M, where M=Mn or Cr) [230]. Wawrzynczyk et al. studied (Tb3+,

775

Eu3+)-codoped cubic γ-Ga2O3 nanoparticles prepared by thermal decomposition

776

reaction [231]. The luminescence from the Tb3+ or Eu3+ singly doped sample under

777

UV excitation showed a broad blue emission band and a characteristic series of sharp

778

emission lines coming from the lanthanide ions. After codoping with 1% of Tb3+ and

779

Eu3+, the nanoparticles showed only the broad blue emission band but not lanthanide

780

luminescence. The lanthanide luminescence quenching could result from the energy

781

exchange between neighbor Tb3+ and Eu3+ ions, followed by the nonradiative

782

relaxation. Patil et al. synthesized pure and Tb3+/Eu3+ codoped β-Ga2O3 nanoparticles

783

[159]. Strong blue emission is observed from un-doped gallium oxide nanoparticles,

784

while nanoparticles doped with Eu3+ and Tb3+ give strong red and green emissions,

785

respectively. When doped with Eu3+ and Tb3+ together, gallium oxide nanoparticles

786

emit white light. The CIE co-ordinate of the emitted light was found to be (0.33, 0.33),

787

which is well within the white light region. Sinha et al. also obtained the bright white

788

emission with the CIE co-ordinate values of 0.32 and 0.36 in Tb3+/Eu3+ codoped

789

Ga2O3 nanoparticles [160].

790

3.8 Cr doped

791

As early as in 1987, Vivien et al. have grown Cr3+ doped β-Ga2O3 single crystals

792

with various doping levels by the floating zone technique [232]. The broad band

793

4

794

with a 210 µs life time at the room temperature, while the 4T2 level is depopulated so

795

that fluorescence originates only from the 2E level with a 2.3 ms life time at 77 K. The

796

temperature dependence of the lifetime indicates that the 4T2 level lies approximately

797

600 cm-1 above the 2E level, which are similar to those of alexandrite: Cr3+. Walsh et

798

al. also observed a broadband luminescence in Cr3+ doped β-Ga2O3 crystal originated

799

mainly on the 4T2 level, and examined this luminescence over a wide range of

800

temperature to determine the radiative efficiency of the material and the relative

T2→4A2 fluorescence emission extending between 650 and 950 nm is dominated

35

801

positions of the 2E and 4T2 emitting levels [233]. PL and PLE spectroscopies studies

802

were widely performed on Cr3+ doped β-Ga2O3 crystal, and showed sharp red line

803

emissions superimposed on a broad luminescence band [234-237]. The intense and

804

broad PL peak at ~ 400 nm in nominally pure β-Ga2O3was observed to be abruptly

805

suppressed by Cr doping [235]. The luminescence properties of Cr3+ doped

806

nanoparticles, nanowires and thin films have also been studied using PL and PLE

807

spectroscopies [238-242]. Nogales et al. characterized the red emission obtained in Cr

808

doped β-Ga2O3 nano- and microwires, which is attributed to electronic transitions of

809

trivalent Cr ions in this host [238]. Temperature evolution of the luminescence time

810

decay is well fitted to a three level model where thermal population between the two

811

upper ones occurs. These three levels are the 4A2, 2E, and 4T2 of Cr3+, and a value of

812

60 meV is obtained for the 4T2-2E energy difference in this oxide. A rough estimation

813

of the Huang-Rhys factor for the 4T2-4A2 electronic transition in this host yields a

814

value of S ~ 5. Fujihara et al. have experimentally demonstrated that, under UV

815

excitation, the green emission from β-Ga2O3 thin film is quenched by the inclusion of

816

Cr3+ ions, and the material exhibits the red emission instead [243]. They infer that the

817

energy of the photo-excitation is more efficiently transferred to the Cr3+ activators

818

than to the defect centers which are responsible for the green emission.

819

Tokida et al. prepared Ga2O3:Cr3+ films by MOD and analyzed PL properties in

820

detail, such as the temperature-dependent PL intensity, peak energy, and spectral

821

width [244]. An activation energy of ~ 0.9 eV for the Cr3+ ions in β-Ga2O3 is

822

determined from a plot of PL intensity vs calcination temperature. The red-line

823

emission doublet R1 and R2 at ~ 1.8 eV and the broad emission band with a peak at

824

~ 1.7 eV are ascribed to the intra-d-shell electronic transitions of Cr3+ ions in the

825

β-Ga2O3 host. The zero-phonon line (ZPL) energies of the Cr3+ states, 2E, 4T2, 2T2, 4T1,

826

and 4T1, were determined to be 1.7884, 1.7979, 2.3, 2.40, and 3.35 eV at 300 K,

827

respectively. The high-energy luminescence tail of the broad 4T2→4A2 emission band

828

can be explained by the hot-carrier effect of the photo-excited electrons in the 4T2

829

state. The relative intensities of the R-line emission doublet can also be explained 36

830

(R1) very well by the population and depopulation of the electron numbers in the E

831

 (R2) states. Their proposed configurational-coordinate model shows a good and 2A

832

agreement with the experimental data, as schematically shown in Fig. 12. They also

833

found that the PL emission intensities in Eu3+-activated β-Ga2O3 and Tb3Ga5O12 were

834

strongly influenced by the Cr3+ red line emissions, where the Cr3+ ions were

835

unintentionally introduced in the synthesized phosphors [245-246].

836

837

838

839

FIG. 12. Schematic energy diagram of Cr3+ ions in β-Ga2O3 [244].

4. Solar-blind Photodetectors

840

37

841

Solar-blind photodetectors based on ultrawide-bandgap semiconductor, with the

842

high photosensitivity and low false alarm rate, have caused great concern recently due

843

to their wide potential applications in military fields (such as: ultraviolet

844

communication, missile early warning and tracking, rocket tail flame detection,

845

high-energy physics and so on) and civilian fields (such as: sterilization ultraviolet

846

intensity detection, biological medicine, fire warning, high voltage corona detection,

847

ozone hole monitoring and so on). To date, various ultra-wide band-gap

848

semiconductors such as AlGaN, ZnMgO, diamond, Ga2O3, etc., have been employed

849

to fabricate solar-blind photodetectors [247]. Although AlGaN with high Al

850

composition has been achieved solar-blind photodetection, the crystal quality of the

851

AlGaN epitaxial layer deteriorates rapidly with increasing Al composition [248].

852

Meanwhile, it is difficult to grow a single wurtzite ZnMgO with high Mg composition

853

due to phase segregation between ZnO wurtzite and MgO rock salt [249]. Diamond

854

has a band gap of 5.5 eV, and the sensitivity range is restricted to wavelengths below

855

225 nm. It is not possible to be used to detect entire solar-blind region due to their

856

mismatched bandgaps [250]. Among them, β-Ga2O3, with a direct bandgap of ~ 4.9

857

eV (corresponding to the solar-blind region), low cost, strong radiation hardness, and

858

high thermal and chemical stability, is a natural material for solar-blind photodetector.

859

Compared with ZnMgO and AlGaN, β-Ga2O3 is unnecessary to tune the band gap to

860

avoid phase separation and alloy composition fluctuations. In the last year, Ga2O3

861

based solar-blind photodetectors have been reviewed from various perspectives

862

[65-67,143,251-252]. Guo et al. comprehensively review the latest progresses of

863

solar-blind photodetectors based on Ga2O3 materials in various forms of bulk single

864

crystal, nanostructures, thin films (crystalline and amorphous) with various device

865

structure of metal-semiconductor-metal structure, Schottky junctions, heterojunctions,

866

and pn junctions [67]. Table ΙV summarizes the parameters of solar blind

867

photodetector

868

quasi-two-dimensional β-Ga2O3 microbelts mechanically exfoliated from bulk

869

β-Ga2O3 single crystal shows the highest photoresponsivity (1.8×105 A/W) [253],

870

followed by α-Ga2O3/ZnO heterojunction thin film (1.1×104 A/W) [254] and

based

on

Ga2O3.

The

38

photodetector

composed

of

871

ZnO/β-Ga2O3 core-shell heterojunction microwire (1.3×103 A/W) [255]. The

872

photodetector based on Au/β-Ga2O3 Schottky junction exhibits a photoresponsivity of

873

103 A/W [256]. Devices formed heterojunction or Schottky junction tend to have a

874

high gain. The photodetector composed of quasi-two-dimensional β-Ga2O3 microbelts

875

also has the highest external quantum efficiency(up to 8.8×105 %) [253], followed by

876

In-doped Ga2O3 nanobelts device (2.72×105 %) [257]. Compared with single crystal

877

and thin film devices, the photodetector based on Ga2O3 nanostructures tends to have

878

a higher dark current, which can be as low as the order of pA. The ratio of light

879

current to dark current (Idark/ILigh ) of Ga2O3 photodetector can reach 106 for all forms

880

of bulk single crystal, nanostructures, and thin films.The devices based on

881

heterojunction, Schottky junction or amorphous thin film often have faster response

882

speed, such as: graphene/Ga2O3 single crystal heterojunction (< 2.24 µs) [258],

883

ZnO/Ga2O3 core-shell heterojunction microwires (20 µs),255 Au/Ga2O3 nanowires

884

Schottky junction (64 µs) [259], amorphous Ga2O3 thin film (19.1 µs) [260]. The

885

photodetector based on ZnO/Ga2O3 core/shell micron-wire has a best comprehensive

886

performance, which exhibits a photoresponsivity of 1.3×103 A/W and a response time

887

of 20 µs to 254 nm light at -6 V [255].

888

t

Table ΙV Parameter list of Ga2O3 based solar-blind ultraviolet photodetector Photodetector type

Photoresponsivity, R (A/W)

External quantum efficiency, EQE ( % )

Idark (A)

Idark/ILight

Response time, t(s)

Ref.

Ga2O3 nanowire

-

-

≈ 10-12

≈ 2×103

2.2×10-1

261

-2

262

≈

-

-

Ga2O3 nanowire

8.0×10-4

3.9×10 -1

≈ 10-10

≈ 102

-

Ga2O3 nanowire

3.4×10-3

1.37

-

≈ 102

-

Ga2O3 nanowire

6×10

2

Ga2O3 nanowire

3.77×10

Ga2O3 nanowire

-1

7.1×10

1

2.0×10

5

2.5×10

2

≈ 10

-11

≈ 10

≈ 10

-11

3

≈ 10

1.08×10-2 (0V)

5.27×10-3

Ga2O3 nanorod array PEC

2.1×10-4 (0V)

Ga2O3 nanosheet

3.3

1.85×10

Ga2O3 nanorod array

Ga2O3 nanosheet (γ) Ga2O3 nano flower (γ) Ga2O3 nanobelt

-

3.37×10

264

6.4×10

-5

259

2.1×10

-1

265

1.9×10

-1

266

-3

8×10

≈ 10-13

9.14

3.8×10-1

268

-

-

-

5.6×10-2

269

1.6×103

≈ 10-9

10

3×10-2

270

-2

271

1

-5

2

263

-

-

-

10

< 2×10

2

2.6×10

-

Graphene/Ga2O3 nanowire

3×10

4

Ga2O3 nanowire

-4

10

-12

1.67×10

39

4

≈ 10

-9

1.64×10

≈ 10

-9

2.2×10

2

4.0×10

2

≈ 10

-13

4

6×10 10

-1

8.6×10

267

272 1

273

Ga2O3 nanobelt

8.51×102

4.2×103

≈ 10-13

≈ 103

< 3×10-1

274

Ga2O3 nanobelt

1.93×101

9.4×103

≈ 10-10

≈ 104

< 2×10-2

275

In:Ga2O3 nanobelt

5.47×102

2.72×105

≈ 10-13

9.1×102

1

257

Ga2O3 microbelt

5

Ga2O3 microbelt Ga2O3 microbelt

1.8×10 (-30V) -

8.8×10

5

-

1.68

2.57

≈ 10

-4

-

≈ 10

-13

-

≈ 10

-13

1

≈ 10

-6

6.7×10

-1

253

1.4

1.9×10 ≈ 10

3

276

5.3×10

-1

277

4

-

278

Graphene/Ga2O3 microbelt

2.98×10

Ga2O3 single crystal

2.6-8.7

-

≈ 10-10

≈ 103

-

279

Ga2O3 single crystal

3.7×10-2

1.8×101

≈ 10-10

1.5×104

9×10-3

280

Ga2O3 single crystal

103

-

≈ 10-10

≈ 106

-

256

Ga2O3 single crystal Graphene /Ga2O3 single crystal Graphene /Ga2O3 single crystal

4.3

2.1×10

3.93×10

1

-2

1.03×10 (0V) -2

1

1.96×10

4

≈ 10

≈ 10 ≈ 10

5

-11

-6

-10

-5

10

5

10

3

10

2

< 2.24×10

10

2

-1

283

-

281

2.2×10

2

282 -6

258

-

≈ 10

3 × 10-3

-

≈ 10-8

101

1.4×10-1

284

Ga2O3 single crystal

4

-

≈ 10-10

≈ 102

3×10-1

285

Ga2O3 single crystal

1.28×10-3

-

≈ 10-10

1.5×101

8×10 -2

286

-

-

-

287

Ga2O3 single crystal

5×10

Ga2O3 single crystal

Ga2O3 thin film Ga2O3 thin film Ga2O3 thin film

8×10

-5

3.7×10

4.53×10

Ga2O3 thin film

≈ 10

-1

Ga2O3 thin film

≈ 101

Ga2O3 thin film

-

-2

-1

-1

1.8×10

1

-

10

-

290 291

103

-

-

≈ 10

-11

5

-

≈ 10

-10

-

Ga2O3 thin film

289

≈ 10-7

Ga2O3 thin film

2.59×10

-

3

-

8.2×103

Ga2O3 thin film

10

≈ 10

1.7×101

2

288

-

Ga2O3 thin film

9.03×10

5

-10

7.6×10

Ga2O3 thin film

-

≈ 10

Ga2O3 thin film

-1

≈ 10

-10

>10

2

-9

2.4×10

-

10

170 -2

6

5×10

≈ 10-9

8.5×106

-

293

-

≈ 10-11

102

8×10-1

294

-

≈ 10

-11

10

5

-

295

≈ 10

-10

10

4

7.9×10

4

≈ 10

4

≈ 10

-7

-11

4×10

1.5×10

1

5.7×10

4

-1

-

296 297

-1

Ga2O3 thin film

8.4

4.1×10

Ga2O3 thin film

8.9×101

4.1×102

≈ 10-10

105

2.5×10-1

299

Ga2O3 thin film/crystal

1.8

8.7×102

≈ 10-6

3.69×101

-

300

Ga2O3 thin film

4.2

-

≈ 10-11

1.6×104

4×10-2

-

-5

Ga2O3 thin film Al: Ga2O3 thin film Si:Ga2O3 thin film

9×10

-3

1.5 6×10

≈ 10

7.8×10 1

3×10 1

2

4

1.8×10

-1

298

301 283

-

-

-

302

-

9

-

303

4

-

9

-

304

Si:Ga2O3 thin film

3.6×10

Si:Ga2O3 thin film

1.6×10-1

8.75×101

-

104

-

305

Zn:Ga2O3 thin film

2.1×102

-

≈ 10-11

5×104

1.4

306

a-GaOx amorphous thin film

7×101

-

≈ 10-10

1.2×105

2×10-2

-

-12

Ga2O3 amorphous thin film

1.9×10

-1

1.75×10

10

1

1×10

292

40

≈ 10

10

6

1.9×10

-5

301 260

Ga2O3 amorphous thin film

4.5×101

-

≈ 10-10

104

1.48×10-4

307

Ga2O3 thin film

1.2×10-4

-

≈ 10-10

102

-

308

Ga2O3 thin film

1.5

-

≈ 10-9

103

Ga2O3 thin film Ga2O3 thin film Ga2O3 thin film

2.9×10

-1

1.1×10

-1

1.4×10

-1

1.34 -

≈ 10

-8

≈ 10

-9

≈ 10

-

-11

-8

Ga2O3 thin film

1.5

-

≈ 10

Ga2O3 thin film

2.6×101

-

≈ 10-8

Graphene/Ga2O3 thin film

1.28×101

-

Ga2O3 thin film

9.6×101

4.76×104

-5

Ga2O3 thin film

5.86×10

Ga2O3 thin film

2

Ga2O3 thin film

1.5×10 10

-1

7×10

4

1.6×10 3.5×10

3

4.5×10

1.4×10

6

-1

2×10

310 -1

311 311

104

1.8×10-1

312

≈ 10-8

-

2×10-3

313

≈ 10-6

-

-

314

-9

10

3

10

309

1.8×10

-11

10

≈ 10

-8

-

-8

≈ 10

-

309 -1

-

≈ 10

-

3

1

5

-1

315

1.3

316

10

-

309

6

8.6×10

-1

247

Ga2O3 thin film

-

-

≈ 10

Ga2O3 thin film

-

-

≈ 10-9

1.3×101

6.2×10-1

317

Ga2O3 thin film

-

-

≈ 10-11

1.6×104

1.6×10-2

318

Ga2O3/Ga/Ga2O3 thin film

2.85

-

≈ 10-11

8×105

-

319

Mn:Ga2O3 thin film α-Ga2O3 thin film α-Sn:Ga2O3 thin film

7×10

-2

1.5×10

-2

9.6×10

-2

3.6×10

1

7.39 -

≈ 10

-9

6.7×10

≈ 10

-9

1

≈ 10

-9

1.4×10

-7

3×10

1

2.8×10

-1

320

-

37

1.08

321

4

8.73

322

2

α-Sn:Ga2O3 thin film

-

-

≈ 10

ε-Sn:Ga2O3 thin film

6.05×10-3

3.02

≈ 10-9

4.6×101

-

323

β-Sn:Ga2O3 thin film

3.61×10-2

-

≈ 10-8

19

1.37

303

-

≈ 10

-9

2

1. 23

≈ 10

-9

≈ 10

-6

≈ 10

-8

9.4×10

Zn:Ga2O3 thin film Er:Ga2O3 thin film Au NPs/Ga2O3 thin film

10

2

2

>2×10

1.6×10

192

2

-

325

2

1.8

326

Ga2O3/p-Si heterojunction

3.7×10

Ga2O3 /NSTO heterojunction

4.3×101

2.1×104

≈ 10-6

2×101

7×10-2

327

Ga2O3/Ga:ZnO heterojunction

7.6×10-4

-

≈ 10-9

2.6×102

2.7×10-1

328

Ga2O3/Cu2O pn junction PEC

4.2×10-4

-

-

-

1.01×101

35

5.4×10

3

-

329

1.7

p-Si/i-SiC/n-Ga2O3 Graphene/Ga2O3 /SiC Graphene/Ga2O3/graphene

1.8×10

1.8×10

5

2.5

324 -1

-1

9.66

-

≈ 10

-8

≈ 10

-5

6.3×10

1

≈ 10

-9

8.3×10

1

-9

7.7

-

332

330

9.6×10

-1

331

Ga2O3/SiC/Al2O3

-

-

≈ 10

Ga2O3 /Al2 O3

1.4

-

≈ 10-7

9.04

1.26

333

Ga2O3 /SiC heterojunction

7×10-2

-

≈ 10-10

-

9×10-3

334

Ga2O3 /SiC heterojunction

6.78×10-2

3.3×101

≈ 10-10

3.84×102

3.1×10-1

335

Ga2O3/GaN heterojunction Ga2O3/GaN heterojunction nanowire array Sn:Ga2O3/GaN heterojunction Ga2O3/ZnO heterojunction

5.4×10

-2

5.5×10

2

3.05 3.5×10

2.25×10

3

-1

1.7×10

41

2

≈ 10

-6

≈ 10

-7

≈ 10

-11

≈ 10

-10

1.5×10

2

5 10

4

1.5×10

1

8×10

-2

336

8×10

-2

337

1.8×10

-2

338

6.2×10

-1

339

α-Ga2O3/ZnO heterojunction

1.1×104 (-40V)

-

≈ 10-12

-

2.4×10- 4

254

ZnO/Ga2O3 core-shell nanowire arrays

1.38×102 (0V)

-

-

2.64×104

8.6×10-5

340

ZnO/Ga2O3 core-shell microwire

1.3×103 (-6V)

-

≈ 10-10

≈ 106

2×10-5

255

-

-10

ZnO/Ga2O3 core-shell microwire Ga2 O3/diamond heterojunction α-Ga2O3/Cu2O heterojunction

-3

9.7×10 (0V) 2×10

-4

4.2×10

≈ 10

-

-4

-

α-Ga2O3 solid/liquid heterojunction

1.44×10

-3

7×10

β-Ga2O3 solid/liquid heterojunction

3.81×10-3

Ga2O3/CuSCN core-shell microwire heterojunction

1.33×10-2 (5V)

-1

≈ 10

-9

≈ 10

-6

≈ 7×10

2

10

1

-

3.7×10 ≈2

-4

10

341 342

1

343 -1

344

-

≈6

1.7×10

1.86

-

≈ 2.9×101

1.6×10-1

344

-

10-12

4.14×104

3.5×10-2

345

889

42

890

5. SUMMARY AND FUTURE PERSPECTIVES

891

Ga2O3 is a new type of ultrawide-bandgap semiconductor material, which can

892

crystallize in six different phases (known as α, β, γ, δ, ε, and κ-phases). These various

893

polymorphs each have unique physical properties and can be widely used in various

894

fields of devices. Among them, the monoclinic β-Ga2O3 (space group: C2/m) has been

895

recognized as the most stable phase, which can be grown in bulk form from EFG with

896

a low-cost method. Also, β-Ga2O3 has been the most widely studied and utilized,

897

which can be attributed to two main reasons: 1) β-Ga2O3 has more commercial value

898

than other phases because its low cost and good thermal/chemical stability; 2)

899

β-Ga2O3 is easy to prepare while other phases are relatively difficult. With a large

900

bandgap of about 4.9 eV and a high theoretical breakdown electrical field (8 MV/cm),

901

β-Ga2O3 is a potential candidate material for next generation optoelectronics,

902

high-power electronics, and extreme environment electronics [high temperature, high

903

radiation, and high voltage (low power) switching]. In this review, we mainly

904

summarize the application of Ga2O3 in optoelectronic devices (including phosphors

905

and EL devices, solar-blind photodetectors). β-Ga2O3 emerges as a new

906

multicolor-emitting phosphor host material for emissive display applications on

907

account of its extremely stable chemically and thermally. Red and green emission

908

colors can be obtained using Ga2O3 phosphors activated with Eu3+, Cr, Co, Sm and

909

Mn, Tb, Er, Ho, respectively. Yellow and blue emission colors can be also obtained

910

using Ga2O3 phosphors activated with Tm, Ce, Sn and Dy, Eu2+, respectively. Because

911

of high electric strength of Ga2O3, it is possible to apply higher electric fields to the

912

Ga2O3 EL devices. High-luminance multicolor emissions can be realized by TFEL

913

devices with transition metal or rare-earth activated Ga2O3 phosphor. Meanwhile,

914

Ga2O3 is a natural choice for use in solar-blind photodetector due to the virtues of

915

direct bandgap of ~ 4.9 eV which is in the solar-blind region. The performance

916

parameters of β-Ga2O3 solar-blind photodetectors based on various material type of

917

nanostructures, bulk single crystal and thin film are summarized. The photodetector

918

based on quasi-two-dimensional β-Ga2O3 microbelts shows the highest responsivity 43

919

(1.8×105 A/W). The photodetector based on ZnO/Ga2O3 core/shell micron-wire has a

920

best comprehensive performance, which exhibits a responsivity of 1.3×103 A/W and a

921

response time of 20 µs to 254 nm light at -6 V. We look forward to the application of

922

β-Ga2O3 based solar-blind ultraviolet photodetectors in military (such as: missile early

923

warning and tracking, ultraviolet communication, harbor fog navigation, and so on)

924

and civilian fields (such as: ozone hole monitoring, disinfection and sterilization

925

ultraviolet intensity monitoring, high voltage corona detection, forest fire ultraviolet

926

monitoring, and so on) as soon as possible.

927

44

928

ACKNOWLEDGMENTS

929

This work was supported by the National Natural Science Foundation of China (No.

930

61704153, 51572241, 61774019, 51572033), Zhejiang Public Service Technology

931

Research Program/Analytical Test (LGC19F040001), Beijing Municipal Commission

932

of Science and Technology (SX2018-04), Visiting Scholar Foundation of State Key

933

Lab of Silicon Materials (SKL2019-08), and Fundamental Research Funds of

934

Zhejiang Sci-Tech University (2019Q061).

45

936

[1] S. J. Pearton, J. C. Yang, P. H. Cary, F. Ren, J. Kim, M. J. Tadjer, M. A. Mastro, Appl. Phy. Rev. 5 (2018) 011301.

937

[2] M. Higashiwaki, G. H. Jessen, Appl. Phys. Lett. 112 (2018) 060401.

938

[3] Z. Galazka, Semicond. Sci. Tech. 33 (2018) 113001.

939

[4] M. Higashiwaki, H. Murakami, Y. Kumagai, A. Kuramata, Jpn. J. Appl. Phys. 55 (2016) 1202A1.

935

940

942

[5] M. A. Mastro, A. Kuramata, J. Calkins, J. Kim, F. Ren, S. J. Peartong, ECS J. Solid State Sc. 6 (2017) P356.

943

[6] R. Roy, V. G. Hill, E. F. Osborn, J. Am. Chem. Soc. 74 (1952) 719.

944

[7] S. Penner, C. Zhuo, R. Thalinger, M. Grünbacher, C. Hejny, S. Vanicek, M. Noisternig, Monatsh. Chem. 147 (2015) 289.

941

945 946 947

[8] H. Y. Playford, A. C. Hannon, E. R. Barney, R. I. Walton, Chem. Eur. J. 19 (2013) 2803.

949

[9] I. Cora, F. Mezzadri, F. Boschi, M. Bosi, M. Čaplovičová, G. Calestani, I. Dódony, B. Pécz, R. Fornari, CrystEngComm 19 (2017) 1509.

950

[10] M. Marezio, J. P. Remeika, J. Chem. Phys. 46 (1967) 1862.

951

[11] V. M. Bermudez, Chem. Phys. 323 (2006) 193.

952

[12] H. Y. Playford, A. C. Hannon, M. G. Tucker, D. M. Dawson, S. E. Ashbrook, R. J. Kastiban, J. Sloan, R. I. Walton, J. Chem. Phys. C 118 (2014) 16188.

948

953

955

[13] S. Yoshioka, H. Hayashi, A. Kuwabara, F. Oba, K. Matsunaga, I. Tanaka, J. Phys.: Condens. Mat. 19 (2007) 346211.

956

[14] L. P. Dong, J. G. Yu, Y. M. Zhang, R. X. Jia, Comp. Mater. Sci. 156 (2019) 273.

957

[15] D. Shinohara, S. Fujita, Jpn. J. Appl. Phys. 47 (2008) 7311.

958

[16] K. Kaneko, T. Nomura, S. Fujita, Phys. Status Solidi (c) 7 (2010) 2467.

959

[17] K. Akaiwa, S. Fujita, Jpn. J. Appl. Phys. 51 (2012) 070203.

960

[18] H. Ito, K. Kaneko, S. Fujita, Jpn. J. Appl. Phys. 51 (2012) 100207.

961

[19] K. Kaneko, H. Kawanowa, H. Ito, S. Fujita, Jpn. J. Appl. Phys. 51 (2012) 020201.

954

962

46

963

[20] K. Kaneko, I. Kakeya, S. Komori, S. Fujita, J. Appl. Phys. 113 (2013) 233901.

964

[21] S. D. Lee, K. Akaiwa, S. Fujita, Phys. Status Solidi (c) 10 (2013) 1592.

965

[22] N. Suzuki, K. Kaneko, S. Fujita, J. Cryst. Growth 364 (2013) 30.

966

[23] S. Fujita, K. Kaneko, J. Cryst. Growth 401 (2014) 588.

967

[24] N. Suzuki, K. Kaneko, S. Fujita, J. Cryst. Growth 401 (2014) 670.

968

[25] K. Kaneko, S. Fujita, T. Hitora, Jpn. J. Appl. Phys. 57 (2018) 02CB18.

969

[26] R. Jinno, T. Uchida, K. Kaneko, S. Fujita, Phys. Status Solidi (b) 255 (2018) 1700326.

970 971 972

[27] S. Kan, S. Takemoto, K. Kaneko, I. Takahashi, M. Sugimoto, T. Shinohe, S. Fujita, Appl. Phys. Lett. 113 (2018) 212104.

974

[28] T. Uchida, R. Jinno, S. Takemoto, K. Kaneko, S. Fujita, Jpn. J. Appl. Phys. 57 (2018) 040314.

975

[29] T. Uchida, K. Kaneko, S. Fujita, MRS Adv. 3 (2018) 171.

976 977

[30] K. Toyoki, Y. Shiratsuchi, A. Kobane, S. Harimoto, S. Onoue, H. Nomura, R. Nakatani, J. Appl. Phys. 117 (2015) 17D902.

978

[31] C. S. Cheng, H. Gomi, H. Sakata, Phys. Status Solidi (a) 155 (1996) 417.

979

[32] D. B. McWhan, J. P. Remeika, Phys. Rev. B 2 (1970) 3734.

980

[33] T. Fujii, Solid State Ionics 172 (2004) 289.

981

[34] K. Kaneko, T. Nomura, I. Kakeya, S. Fujita, Appl. Phys. Express 2 (2009) 075501.

973

982 983 984 985

[35] C. R. He, D. Y. Guo, K. Chen, S. L. Wang, J. Q. Shen, N. Zhao, A. P. Liu, Y. Y. Zheng, P. G. Li, Z. P. Wu, C. R. Li, F. M. Wu, W. H. Tang, ACS Appl. Nano Mater. 2 (2019) 4095.

986

[36] Y. Oshima, E. G. Víllora, K. Shimamura, Appl. Phys. Express 8 (2015) 055501.

987

[37] D. Y. Guo, X. L. Zhao, Y. S. Zhi, W. Cui, Y. Q. Huang, Y. H. An, P. G. Li, Z. P. Wu, W. H. Tang, Mater. Lett. 164 (2016) 364.

988 989 990

[38] M. Yamaga, E. G. Víllora, K. Shimamura, N. Ichinose, M. Honda, Phys. Rev. B 68 (2003) 155207. 47

991

[39] T. Zacherle, P. C. Schmidt, M. Martin, Phys Rev. B 87 (2013) 235206.

992

[40] M. Orita, H. Hiramatsu, H. Ohta, M. Hirano, H. Hosono, Thin Solid Films 411 (2002) 134.

993 994 995 996 997 998

[41] C. Janowitz, V. Scherer, M. Mohamed, A. Krapf, H. Dwelk, R. Manzke, Z. Galazka, R. Uecker, K. Irmscher, R. Fornari, M. Michling, D. Schmeißer, J. R. Weber, J. B. Varley, C. G. Van de Walle, New J. Phys. 13 (2011) 085014. [42] S. I. Stepanov, V. I. Nikolaev, V. E. Bougrov, A. E. Romanov, Rev. Adv. Mater. Sci. 44 (2016) 63.

1001

[43] Z. Galazka, R. Uecker, D. Klimm, K. Irmscher, M. Naumann, M. Pietsch, A. Kwasniewski, R. Bertram, S. Ganschow, M. Bickermann, ECS J. Solid State Sc. 6 (2016) Q3007.

1002

[44] Z. Galazka, K. Irmscher, R. Uecker, R. Bertram, M. Pietsch, A. Kwasniewski, M.

1003

Naumann, T. Schulz, R. Schewski, D. Klimm, M. Bickermann, J. Cryst. Growth 404 (2014) 184.

999 1000

1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015

[45] Z. Galazka, R. Uecker, K. Irmscher, M. Albrecht, D. Klimm, M. Pietsch, M. Brützam, R. Bertram, S. Ganschow, R. Fornari, Cryst. Res. Technol. 45 (2010) 1229. [46] K. Irmscher, Z. Galazka, M. Pietsch, R. Uecker, R. Fornari, J. Appl. Phys. 110 (2011) 063720. [47] M Saurat, A Revcolevschi, Revue Internationale Des Hautes Temperatures Et. Des. Refractaires 8 (1971) 291. [48] E. G. Víllora, K. Shimamura, Y. Yoshikawa, K. Aoki, N. Ichinose, J. Cryst. Growth 270 (2004) 420. [49] J. G. Zhang, B. Li, C. T. Xia, G. Q. Pei, Q. Deng, Z. H. Yang, W. S. Xu, H. S. Shi, F. Wu, Y. Q. Wu, J. Xu, J. Phys. Chem. Solids 67 (2006) 2448.

1017

[50] E. G. Víllora, Y. Morioka, T. Atou, T. Sugawara, M. Kikuchi, T. Fukuda, Phys. Status Solidi (c) 193 (2002) 187.

1018

[51] S. Ohira, N. Arai, Phys. Status Solidi (c) 5 (2008) 3116.

1019

[52] T. Oshima, N. Arai, N. Suzuki, S. Ohira, S. Fujita, Thin Solid Films 516 5768.

1016

1020

48

(2008)

1021 1022 1023 1024 1025

[53] V. I. Nikolaev, V. Maslov, S. I. Stepanov, A. I. Pechnikov, V. Krymov, I. P. Nikitina, L. I. Guzilova, V. E. Bougrov, A. E. Romanov, J. Cryst. Growth 457 (2017) 132. [54] M. Mohamed, K. Irmscher, C. Janowitz, Z. Galazka, R. Manzke, R. Fornari, Appl. Phys. Lett. 101 (2012) 132106.

1027

[55] J. G. Zhang, C. T. Xia, Q. Deng, W. S. Xu, H. S. Shi, F. Wu, J. Xu, J. Phys. Chem. Solids 67 (2006) 1656.

1028

[56] K. Suzuki, T. Okamoto, M. Takata, Ceram. Int. 30 (2004) 1679.

1029 1030

[57] A. Kuramata, K. Koshi, S. Watanabe, Y. Yamaoka, T. Masui, S. Yamakoshi, Jpn. J. Appl. Phys. 55 (2016) 1202A2.

1031

[58] H. H. Tippins, Phys. Rev. 140 (1965) A316.

1032

[59] M. Higashiwaki, K. Sasaki, A. Kuramata, T. Masui, S. Yamakoshi, Appl. Phys.

1026

1033

Lett. 100 (2012) 013504.

1035

[60] M. Higashiwaki, K. Sasaki, H. Murakami, Y. Kumagai, A. Koukitu, A. Kuramata, T. Masui, S. Yamakoshi, Semicond. Sci. Tech. 31 (2016) 034001.

1036

[61] S. J. Pearton, F. Ren, M. Tadjer, J. Kim, J. Appl. Phys. 124 (2018) 220901.

1037

[62] M. H. Wong, A. Takeyama, T. Makino, T. Ohshima, K. Sasaki, A. Kuramata, S. Yamakoshi, M. Higashiwaki, Appl. Phys. Lett. 112 (2018) 023503.

1034

1038 1039 1040

[63] M. Higashiwaki, K. Sasaki, A. Kuramata, T. Masui, S. Yamakoshi, Phys. Status Solidi (c) 211 (2014) 21.

1042

[64] K. Sasaki, M. Higashiwaki, A. Kuramata, T. Masui, S. Yamakoshi, J. Cryst. Growth 378 (2013) 591.

1043

[65] J. J. Xu, W. Zheng, F. Huang, J. Mater. Chem. C 7 (2019) 8753.

1044

[66] Y. Qin, S. B. Long, H. Dong, Q. M. He, G. Z. Jian, Y. Zhang, X. H. Hou, P. J. Tan, Z. F. Zhang, H. B. Lv, Q. Liu, M. Liu, Chin. Phys. B 28 (2019) 018501.

1041

1045

1047

[67] D. Y. Guo, P. G. Li, Z. W. Chen, Z. P. Wu, W. H. Tang, Acta Phys. Sin.-Ch. Ed. 68 (2019).

1048

[68] Y. W. Zhang, A. Neal, Z. B. Xia, C. D. Joishi, J. M. Johnson, Y. H. Zheng, S.

1046

1049 1050

Bajaj, M. Brenner, D. Dorsey, K. Chabak, Appl. Phys. Lett. 112 (2018) 173502. [69] T. S. Wang, W. Li, C. Y. Ni, A. Janotti, Phys. Rev. Appl. 10 (2018) 011003. 49

1052

[70] Z. Cheng, N. Tanen, C. Chang, J. Shi, J. McCandless, D. Muller, D. Jena, H. G. Xing, S. Graham, arXiv preprint arXiv:1905.00139 (2019).

1053

[71] Y. Zhang, A. Neal, Z. B. Xia, C. Joishi, Y. Zheng, S. Bajaj, M. Brenner, S. Mou,

1051

1054 1055 1056 1057 1058 1059

D. Dorsey, K. Chabak, arXiv preprint arXiv:1802.04426 (2018). [72] R. Wakabayashi, M. Hattori, K. Yoshimatsu, K. Horiba, H. Kumigashira, A. Ohtomo, Appl. Phys. Lett. 112 (2018) 232103. [73] F. H. Teherani, G. A. Khodaparast, Y. V. Xu, J. Wu, V. P. Dravid, D. Pavlidis, M. Razeghi, R. McClintock, D. J. Rogers, J. H. Park, Proc. SPIE 10533, Oxide-based Materials and Devices IX 105330 (2018).

1061

[74] M. Mitome, S. Kohiki, T. Nagai, K. Kurashima, K. Kimoto, Y. Bando, Cryst. Growth Des. 13 (2013) 3577.

1062

[75] S. S. Farvid, T. Wang, P. V. Radovanovic, J. Am. Chem. Soc. 133 (2011) 6711.

1063

[76] R. Lorenzi, A. Paleari, N. V. Golubev, E. S. Ignat'eva, V. N. Sigaev, M.s Niederberger, A. Lauria, J. Mater. Chem. C 3 (2015) 41.

1060

1064 1065 1066 1067 1068

[77] M. Takahashi, T. Nakatani, S. Iwamoto, T. Watanabe, M. Inoue, J. Phys.: Condens. Mat. 18 (2006) 5745. [78] M. Zinkevich, F. M. Morales, H. Nitsche, M. Ahrens, M. Rühle, F. Aldinger, Z. Metallkd. 95 (2004) 756.

1070

[79] Q. Liu, D. Y. Guo, K. Chen, Y. L. Su, S. L. Wang, P. G. Li, W. H. Tang, J. Alloy. Compd. 731 (2018) 1225.

1071

[80] T. Oshima, T. Nakazono, A. Mukai, A. Ohtomo, J. Cryst. Growth 359 (2012) 60.

1072

[81] T. Oshima, Y. Kato, M. Oda, T. Hitora, and M. Kasu, Appl. Phys. Express 10

1069

1073 1074 1075 1076 1077 1078 1079

(2017) 051104. [82] T. Oshima, K. Matsuyama, K. Yoshimatsu, A. Ohtomo, J. Cryst. Growth 421 (2015) 23. [83] H. Hayashi, R. Huang, F. Oba, T. Hirayama, I. Tanaka, J. Mater. Res. 26 (2011) 578. [84] H. Hayashi, R. Huang, F. Oba, T. Hirayama, I. Tanaka, J. Mater. Sci. 46 (2011) 4169.

50

1081

[85] H. Hayashi, R. Huang, H. Ikeno, F. Oba, S. Yoshioka, I. Tanaka, S. Sonoda, Appl. Phys. Lett. 89 (2006) 181903.

1082

[86] R. Huang, H. Hayashi, F. Oba, I. Tanaka, J. Appl. Phys. 101 (2007) 063526.

1083 1084

[87] X. C. Xia, Y. P. Chen, Q. J. Feng, H. W. Liang, P. C. Tao, M. X. Xu, G. T. Du, Appl. Phys. Lett. 108 (2016) 202103.

1085

[88] K. Matsuzaki, H. Hiramatsu, K. Nomura, H. Yanagi, T. Kamiya, M. Hirano, H.

1080

1086

Hosono, Thin Solid Films 496 (2006) 37.

1088

[89] B. Ollivier, R. Retoux, P. Lacorre, D. Massiot, G. Férey, J. Mater. Chem. 7 (1997) 1049.

1089

[90] X. Y. Bi, Z. P. Wu, Y. Q. Huang, W. H. Tang, AIP Adv. 8 (2018), 025008.

1090 1091

[91] Y. Oshima, E. G. Víllora, Y. Matsushita, S. Yamamoto, K. Shimamura, Jpn. J. Appl. Phys. 118 (2015) 085301.

1092

[92] H. Nishinaka, D. Tahara, M. Yoshimoto, Jpn. J. Appl. Phys. 55 (2016) 1202BC.

1093

[93] M. Kracht, A. Karg, J. Schörmann, M. Weinhold, D. Zink, F. Michel, M. Rohnke, M. Schowalter, B. Gerken, A. Rosenauer, Phys. Rev. Appl. 8 (2017) 054002.

1087

1094 1095 1096

[94] F. Mezzadri, G. Calestani, F. Boschi, D. Delmonte, M. Bosi, R. Fornari, Inorg. Chem. 55 (2016) 12079.

1099

[95] R. Fornari, M. Pavesi, V. Montedoro, D. Klimm, F. Mezzadri, I. Cora, B. Pécz, F. Boschi, A. Parisini, A. Baraldi, C. Ferrari, E. Gombia, M. Bosi, Acta Mater. 140 (2017) 411.

1100

[96] J. P. Remeika, M. Marezio, Appl. Phys. Lett. 8 (1966) 87.

1101

[97] Y. M. Ma, H. Y. Chen, K. F. Yang, M. Li, Q. L. Cui, J. Liu, G. T. Zou, Chin. Phys. Lett. 25 (2008) 1603.

1097 1098

1102

1104

[98] H. He, R. Orlando, M. A. Blanco, R. Pandey, E. Amzallag, I. Baraille, M. Rérat, Phys. Rev. B 74 (2006) 195123.

1105

[99] H. He, M. A. Blanco, R. Pandey, Appl. Phys. Lett. 88 (2006) 261904.

1106

[100] K. D. Leedy, K. D. Chabak, V. Vasilyev, D. C. Look, J. J. Boeckl, J. L. Brown, S. E. Tetlak, A. J. Green, N. A. Moser, A. Crespo, D. B. Thomson, R. C.

1103

1107 1108

Fitch, J. P. McCandless, G. H. Jessen, Appl. Phys. Lett. 111 (2017) 012103.

51

1110

[101] E. Chikoidze, H .J. von Bardeleben, K. Akaiwa, E. Shigematsu, K. Kaneko, S. Fujita, Y. Dumont, J. Appl. Phys. 120 (2016) 025109.

1111

[102]

1109

1112 1113 1114

F. H. Teherani, D. C. Look, D. J. Rogers, E. G. Víllora, S. Arjoca, K.

Shimamura, D. Inomata, K. Aoki, 8987 (2014) 89871U. [103] M. Peres, K. Lorenz, E. Alves, E. Nogales, B. Méndez, X. Biquard, B. Daudin, E. G. Víllora, K. Shimamura, J. Phys. D: Appl. Phys. 50 (2017) 325101.

1116

[104] P. Wellenius, E. R. Smith, S. M. LeBoeuf, H. O. Everitt, J. F. Muth, J. Appl. Phys. 107 (2010) 103111.

1117

[105]

1118 1119

[106] P. Wellenius, A. Suresh, J. V. Foreman, H. O. Everitt, J. F. Muth, Mater. Sci. Eng.: B 146 (2008) 252.

1120

[107]

1115

1121

P. Wellenius, A. Suresh, J. F. Muth, Appl. Phys. Lett. 92 (2008) 021111.

P. Gollakota, A. Dhawan, P. Wellenius, L. M. Lunardi, J. F. Muth, Y. N.

Saripalli, H. Y. Peng, H. O. Everitt, Appl. Phys. Lett. 88 (2006) 221906.

1122

[108]

1123

[109] S. E. Choi, Y. T. Oh, H. K. Ham, T. W. Kim, G. S. Heo, J. W. Park, B. H. Choi, D. C. Shin, Curr. Appl. Phys. 11 (2011) S255.

1124 1125 1126 1127

C. F. Lin, K. T. Chen, K. P. Huang, IEEE Electr. Device L. 31 (2010) 1431.

[110] K. Iizuka, Y. Morishima, A. Kuramata, Y. J. Shen, C. Y. Tsai, Y. Y. Su, G. Liu, T. C. Hsu, J. H. Yeh, Gallium Nitride Materials and Devices X (2015) (unpublished).

1129

[111] Z. Liu, P. G. Li, Y. S. Zhi, X. L. Wang, X. L. Chu, W. H. Tang, Chin. Phys. B 28 (2019) 017105.

1130

[112]

1128

1131 1132 1133

M. Higashiwaki, K. Sasaki, T. Kamimura, M. H. Wong, D. Krishnamurthy, A.

Kuramata, T. Masui, S. Yamakoshi, Appl. Phys. Lett. 103 (2013) 123511 . [113] J. C. Yang, F. Ren, M. Tadjer, S. J. Pearton, A. Kuramata, ECS J. Solid State Sc. 7 (2018) Q92.

1135

[114] M. Bartic, C. I. Baban, H. Suzuki, M. Ogita, M. Isai, J. Am. Ceram. Soc. 90 (2007) 2879.

1136

[115]

1137

[116] M. Bartic, M. Ogita, M. Isai, C. L. Baban, H. Suzuki, J. Appl. Phys. 102 (2007) 023709.

1134

1138

C. Baban, Y. Toyoda, M. Ogita, Thin Solid Films 484 (2005) 369.

52

1139

[117]

1140

[118] L. Mazeina, F. K. Perkins, V. M. Bermudez, S. P. Arnold, S. M. Prokes, Langmuir 26 (2010) 13722.

1141

M. Bartic, Phys. Status Solidi (a) 213 (2016) 457.

1143

[119] L. Mazeina, V. M. Bermudez, F. K. Perkins, S. P. Arnold, S. M. Prokes, Sensor. Actuat. B: Chem. 151 (2010) 114.

1144

[120]

1142

1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156

P. Feng, X. Y. Xue, Y. G. Liu, Q. Wan, T. H. Wang, Appl. Phys. Lett. 89

(2006) 112114. [121] A. Trinchi, S. Kaciulis, L. Pandolfi, M. K. Ghantasala, Y. X. Li, W. Wlodarski, S. Viticoli, E. Comini, G. Sberveglieri, Sensor. Actuat. B: Chem. 103 (2004) 129. [122] X. Wang, Q. Xu, M. Li, S. Shen, X. Wang, Y. Wang, Z. Feng, J. Shi, H. Han, C. Li, Angew. Chem. Int. Edit. 51 (2012) 13089. [123] Y. D. Hou, X. C. Wang, L. Wu, Z. X. Ding, X. Z. Fu, Environ. Sci. Tech. 40 (2006) 5799. [124] S. Q. Jin, X. Wang, X. L. Wang, M. G. Ju, S. Shen, W. Z. Liang, Y. Zhao, Z. C. Feng, H. Y. Playford, R. I. Walton, J. Phys. Chem. C 119 (2015) 18221. [125] Y. D. Hou, L. Wu, X. C. Wang, Z. X. Ding, Z. H. Li, X. Z. Fu, J. Catal. 250 (2007) 12.

1158

[126] X. F. Li, X. Z. Zhen, S. G. Meng, J. J. Xian, Y. Shao, X. Z. Fu, D. Z. Li, Environ. Sci. Tech. 47 (2013) 9911.

1159

[127]

B. X. Zhao, M. Lv, L. Zhou, J. Environ. Sci.-China 24 (2012) 774.

1160

[128]

G. Q. Pei, C. T. Xia, Y. J. Dong, B. Wu, T. Wang, J. Xu, Scripta Mater. 58

1157

1161 1162 1163 1164 1165 1166 1167

(2008) 943. [129] D. Y. Guo, Z. P. Wu, Y. H. An, X. J. Li, X. C. Guo, X. L. Chu, C. L. Sun, M. Lei, L. H. Li, L. X. Cao, P. G. Li, W. H. Tang, J. Mater. Chem. C 3 (2015) 1830. [130] D. Y. Guo, Z. P. Wu, P. G. Li, Q. J. Wang, M. Lei, L. H. Li, W. H. Tang, RSC Adv. 5 (2015) 12894. [131] D. Y. Guo, Y. H. An, W. Cui, Y. S. Zhi, X. L. Zhao, M. Lei, L. H. Li, P. G. Li, Z. P. Wu, W. H. Tang, Sci. Rep. 6 (2016) 25166.

53

1169

[132] D. Y. Guo, Z. P. Wu, Y. H. An, P. G. Li, P. C. Wang, X. L. Chu, X. C. Guo, Y. S. Zhi, M. Lei, L. H. Li, W. H. Tang, Appl. Phys. Lett. 106 (2015) 042105.

1170

[133]

1168

1171 1172 1173 1174 1175 1176 1177 1178 1179

D. Y. Guo, Z. P. Wu, L. J. Zhang, T. Yang, Q. R. Hu, M. Lei, P. G. Li, L. H.

Li, W. H. Tang, Appl. Phys. Lett. 107 (2015) 032104. [134] D. Y. Guo, Y. P. Qian, Y. L. Su, H. Z. Shi, P. G. Li, J. T. Wu, S. L. Wang, C. Cui, W. H. Tang, AIP Adv. 7 (2017) 065312. [135] X. Gao, Y. D. Xia, J. F. Ji, H. N. Xu, Y. Su, H. T. Li, C. J. Yang, H. X. Guo, J. Yin, Z. G. Liu, Appl. Phys. Lett. 97 (2010) 193501. [136] J. B. Yang, T. C. Chang, J. J. Huang, S. C. Chen, P. C. Yang, Y. T. Chen, H. C. Tseng, S. M. Sze, A. K. Chu, M. J. Tsai, Thin Solid Films 529 (2013) 200. [137] P. C. Wang, P. G. Li, Y. S. Zhi, D. Y. Guo, A. Q. Pan, J. M. Zhan, H. Liu, J. Q. Shen, W. H. Tang, Appl. Phys. Lett. 107 (2015) 262110.

1181

[138] Y. Aoki, C. Wiemann, V. Feyer, H. S. Kim, C. M. Schneider, H. Yoo, M. Martin, Nat. Commun. 5 (2014) 3473.

1182

[139]

C. W. Hsu, L. J. Chou, Nano Lett. 12 (2012) 4247.

1183

[140]

D. Y. Lee, T. Y. Tseng, J. Appl. Phys. 110 (2011) 114117.

1184

[141] J. J. Huang, T. C. Chang, J. B. Yang, S. C. Chen, P. C. Yang, Y. T. Chen, H. C. Tseng, S. M. Sze, A. K. Chu, M. J. Tsai, IEEE Electr. Device L. 33 (2012) 1387.

1180

1185

1187

[142] J. B. Yang, T. C. Chang, J. J. Huang, Y. T. Chen, P. C. Yang, H. C. Tseng, A. K. Chu, S. M. Sze, M. J. Tsai, Thin Solid Films 528 (2013) 26.

1188

[143]

X. H. Chen, F. F. Ren, S. L. Gu, J. D. Ye, Photonics Res. 7 (2019) 381.

1189

[144]

S. Kumar, R. Singh, Phys. Status Solidi -R 7 (2013) 781.

1190 1191

[145] H. Morkoc, F. Omnès, C. W. Litton, E. Monroy, E. Muñoz, J. L. Reverchon, Proc. of SPIE 6473 (2007) 64730E.

1192

[146]

T. Miyata, T. Nakatani, T. Minami, J. Lumin. 87 (2000) 1183.

1193

[147]

P. C. Stanish, P. V. Radovanovic, J. Phys. Chem. C 120 (2016) 19566.

1194

[148]

S. C. Vanithakumari, K. K. Nanda, Adv. Mater. 21 (2009) 3581.

1195

[149] T. Xiao, A. H. Kitai, G. Liu, A. Nakua, J. Barbier, Appl. Phys. Lett. 72 (1998) 3356.

1186

1196

54

1198

[150] H. B. Xie, L. M. Chen, Y. N. Liu, K. L. Huang, Solid State Commun. 141 (2007) 12.

1199

[151]

1200 1201

[152] D. Stodilka, A. H. Kitai, Z. Huang, K. Cook, SID Symposium Digest of Technical Papers. Oxford, UK: Blackwell Publishing Ltd. 31 (2000) 11.

1202

[153]

1197

1203

J. H. Kim, K. H. Yoon, J. Korean Phys. Soc. 53 (2008) 818.

T. Minami, T. Shirai, T. Nakatani, T. Miyata, Jpn. J. Appl. Phys. 39 (2000)

L524.

1205

[154] J. F. Wager, J. C. Hitt, B. A. Baukol, J. P. Bender, D. A. Keszler, J. Lumin. 97 (2002) 68.

1206

[155]

Y. Tokida, S. Adachi, ECS J. Solid State Sc. 3 (2014) R100.

1207

[156]

P. Marwoto, S. Sugianto, E. Wibowo, J. Theor. Appl. Phys. 6 (2012) 17.

1208

[157] J. H. Hao, Z. D. Lou, I. Renaud, M. Cocivera, Thin Solid Films 467 (2004) 182 .

1204

1209 1210 1211

[158] J. S. Kim, H. E. Kim, H. L. Park, G. C. Kim, Solid State Commun. 132 (2004) 459.

1213

[159] P. Patil, J. Park, S. Y. Lee, J. K. Park, S. H. Cho, Appl. Sci. Conv. Tech. 23 (2014) 296.

1214

[160]

G. Sinha, A. Patra, Chem. Phys. Lett. 473 (2009) 151.

1215

[161]

J. H. Hao, M. Cocivera, J. Phys. D: Appl. Phys. 35 (2002) 433.

1216

[162]

J. H. Hao, G. G. He, X. W. Huang, R. Wu, J. Rare Earth. 24 (2006) 728.

1217

[163]

L. Binet, D. Gourier, Appl. Phys. Lett. 77 (2000) 1138.

1218

[164]

L. Binet, D. Gourier, J. Phys. Chem. Solids 59 (1998) 1241.

1219

[165]

T. Harwig and F. Kellendonk, J. Solid State Chem. 24 (1978) 255.

1220

[166] V. I. Vasiltsiv, Y. M. Zakharko, Y. I. Rym, Ukrainskii Fizicheskii Zhurnal 33 (1988) 1320.

1212

1221

1223

[167] C. H. Liang, G. W. Meng, G. Z. Wang, Y. W. Wang, L. D. Zhang, S. Y. Zhang, Appl. Phys. Lett. 78 (2001) 3202.

1224

[168]

1222

T. Wang, P. V. Radovanovic, J. Phys. Chem. C 115 (2011) 18473. 55

1226

[169] T. Wang, S. S. Farvid, M. Abulikemu, P. V. Radovanovic, J. Am. Chem. Soc. 132 (2010) 9250.

1227

[170]

1225

1228 1229 1230 1231 1232 1233 1234

P. Ravadgar, R. H. Horng, S. D. Yao, H. Y. Lee, B. R. Wu, S. L. Ou, L. W. Tu,

Opt. Express 21 (2013) 24599. [171] G. Sinha, D. Ganguli, S. Chaudhuri, J. Phys.: Condens. Mat. 18 (2006) 11167. [172] T. Yanagida, G. Okada, T. Kato, D. Nakauchi, S. Yanagida, Appl. Phys. Express 9 (2016) 042601. [173] S. Z. Luan, L. P. Dong, X. F. Ma, R. X. Jia, J. Alloy. Compd. 812 (2020) 152026.

1236

[174] A. Luchechko, V. Vasyltsiv, L. Kostyk, O. Tsvetkova, A. I. Popov, Nucl. Instrum. Meth. B 441 (2019) 12.

1237

[175]

J. L. Jiang, J. Zhang, J. P. Li, D. Y. Xu, Chem. Phys. Lett. 719 (2019) 8.

1238

[176]

L. Dong, R. Jia, B. Xin, B. Peng, Y. Zhang, Sci. Rep. 7 (2017) 40160.

1239

[177] L. P. Dong, R. X. Jia, B. Xin, Y. M. Zhang, J. Vac. Sci. Technol. A 34 (2016) 060602.

1235

1240 1241 1242 1243 1244 1245 1246 1247 1248 1249 1250 1251 1252 1253 1254

[178] A. Layek, B. Yildirim, V. Ghodsi, L. N. Hutfluss, M. Hegde, T. Wang, P. V. Radovanovic, Chem. Mater. 27 (2015) 6030. [179] G. C. Liu, X. C. Duan, H. B. Li, D. W. Liang, Mater. Chem. Phys. 110 (2008) 206. [180] Y. Quan, S. Q. Liu, K. L. Huang, D. Fang, X. Y. Zhang, H. W. Hou, T. Nonferr. Metal. Soc. 20 (2010) 1458. [181] N. F. Santos, J. Rodrigues, A. J. S. Fernandes, L. C. Alves, E. Alves, F. M. Costa, T. Monteiro, Appl. Surf. Sci. 258 (2012) 9157. [182] Z. W. Chen, K. Saito, T. Tanaka, M. Nishio, M. Arita, Q. X. Guo, J. Cryst. Growth 430 (2015) 28. [183] Z. W. Chen, X. Wang, F. B. Zhang, S. Noda, K. Saito, T. Tanaka, M.Nishio, Q. X. Guo, J. Lumin. 177 (2016) 48. [184] Z. W. Chen, K. Nishihagi, X. Wang, C. Y. Hu, M. Arita, K. Saito, T. Tanaka, Q. X. Guo, J. Lumin. 194 (2018) 374. 56

1256

[185] Z. W. Chen, D. Y. Guo, P. G. Li, Z. W. Chen, W. H. Tang, Q. X. Guo, Appl. Phys. Express 12 (2019) 061009.

1257

[186]

1255

1258 1259 1260 1261 1262 1263 1264

B. K. Kang, S. R. Mang, H. D. Lim, K. M. Song, Y. H. Song, D. H. Go, M. K.

Jung, K. Senthil, D. H. Yoon, Mater. Chem. Phys. 147 (2014) 178. [187] I. López, K. Lorenz, E. Nogales, B. Méndez, J. Piqueras, E. Alves, J. A. García, J. Mater. Sci. 49 (2014) 1279. [188] Z. W. Chen, X. Wang, S.Noda, K. Saito, T. Tanaka, M. Nishio, M. Arita, Q. X. Guo, Superlattice. Microst. 90 (2016) 207. [189] Z. W. Chen, X. Wang, F. B. Zhang, S. Noda, K. Saito, T. Tanaka, M. Nishio, M. Arita, and Q. X. Guo, Appl. Phys. Lett. 109 (2016) 022107.

1266

[190] Z. W. Chen, K. Nishihagi, X. Wang, K. Saito, T. Tanaka, M. Nishio, M. Arita, Q. X. Guo, Appl. Phys. Lett. 109 (2016) 102106.

1267

[191]

1268 1269

[192] Z. P. Wu, G. X. Bai, Y. Y. Qu, D. Y. Guo, L. H. Li, P. G. Li, J. H. Hao, W. H. Tang, Appl. Phys. Lett. 108 (2016) 211903.

1270

[193]

1271

[194] E. Nogales, J. A. García, B. Méndez, J. Piqueras, K. Lorenz, E. Alves, J. Phys. D: Appl. Phys. 41 (2008) 065406.

1265

1272

Z. W. Chen, K. Saito, T. Tanaka, Q. X. Guo, CrystEngComm 19 (2017) 4448.

E. Nogales, B. Mendez, J. Piqueras, Nanotechnology 19 (2008) 035713.

1274

[195] J. Vincent, O. Guillot-Noël, L. Binet, P. Aschehoug, Y. Le Du, F. Beaudoux, P. Goldner, J. Appl. Phys. 104 (2008) 033519.

1275

[196]

1276

[197] Y. B. Shin, H. T. Lim, Y. G. Choi, Y. S. Kim, J. Heo, J. Am. Ceram. Soc. 83 (2000) 787.

1273

1277 1278 1279

T. Biljan, A. Gajović, Z. Meić, J. Lumin. 128 (2008) 377.

[198] D. L. Yang, H. Gong, E. Y. B. Pun, X. Zhao, H. Lin, Opt. Express 18 (2010) 18997.

1281

[199] G. Y. Zhao, Y. Tian, S. K. Wang, H. Y. Fan, L. L. Hu, J. Phys. B: Condens. Mat. 407 (2012) 4622.

1282

[200]

Y. S. Han, J. H. Song, J. Heo, J. Appl. Phys. 94 (2003) 2817.

1283

[201]

J. Heo, Y. B. Shin, J. N. Jang, Appl. Opt. 34 (1995) 4284.

1280

57

1285

[202] D. M. Shi, Q. Y. Zhang, G. F. Yang, Y. H. Liu, Z. H. Jiang, Chin. Phys. Lett. 23 (2006) 478.

1286

[203]

1284

1287

D. M. Shi, Y. G. Zhao, Q. Qian, D. D. Chen, Q. Y. Zhang, J. Alloy. Compd.

499 (2010) 126.

1288

[204]

D. M. Shi, Y. G. Zhao, X. F. Wang, J. Liu, Phys. Procedia 48 (2013) 172.

1289

[205]

B. Zhou, L. L. Tao, W. Jin, Y. H. Tsang, IEEE Photonic. Tech. L. 24 (2012)

1290 1291 1292

1726. [206] D. C. Zhou, D. Y. Jin, Q. M. Ni, X. Y. Song, X. M. Bai, K. X. Han, J. Am. Ceram. Soc. (2019).

1294

[207] Q. X. Guo, K. Nishihagi, Z. W. Chen, K. Saito, T. Tanaka, Thin Solid Films 639 (2017) 123.

1295

[208]

1293

1296

T. Minami, H. Yamada, Y. Kubota, T. Miyata, Jpn. J. Appl. Phys. 36 (1997)

L1191.

1298

[209] T. Minami, T. Miyata, T. Shirai, T. Nakatani, MRS Online Proceedings Library Archive 621 (2000).

1299

[210]

T. Miyata, T. Nakatani, T. Minami, Superficies Y Vacío (1999) 70.

1300

[211]

T. Minami, T. Nakatani, T. Miyata, J. Vac. Sci. Technol. A 18 (2000) 1234 .

1301

[212]

T. Minami, T. Miyata, Y. Suzuki, Jpn. J. Appl. Phys. 42 (2003) L1018.

1302

[213]

T. Minami, H. Toda, T. Miyata, J. Vac. Sci. Technol. A 19 (2001) 1742.

1303

[214] R. Kumaran, T. Tiedje, S. E. Webster, S. Penson, W. Li, Opt. Lett. 35 (2010) 3793.

1297

1304

1306

[215] Z. P. Wu, G. X. Bai, Q. R. Hu, D. Y. Guo, C. L. Sun, L. Y. Ji, M. Lei, L. H. Li, P. G. Li, J. H. Hao, W. H. Tang, Appl. Phys. Lett. 106 (2015) 171910.

1307

[216]

1305

1308 1309 1310 1311 1312

L. R. P. Kassab, S. H. Tatumi, C. M. S. Mendes, L. C. Courrol, N. U. Wetter,

Opt. Express 6 (2000) 104. [217]

L. R. P. Kassab, R. D. Mansano, L. D. S. Zambom, V. D. D. Cacho, Braz. J.

Phys. 36 (2006) 451. [218] E. Antic-Fidancev, M. Lemaitre-Blaise, M. Latroche, P. Porcher, J. Coutures, J. P. Coutures, J. Alloy. Compd. 250 (1997) 342. 58

1314

[219] J. Zhang, X. T. Tao, C. M. Dong, Z. T. Jia, H. H. Yu, Y. Z. Zhang, Y. C. Zhi, M. H. Jiang, Laser Phys. Lett. 6 (2008) 355.

1315

[220]

1313

1316 1317 1318 1319 1320 1321 1322

A. A. Kaminskii, G. A. Bogomolova, D. N. Vylegzhanin, K. S. Bagdasarov,

A. M. Kevorkov, M. M. Gritsenko, Phys. Status Solidi (c) 38 (1976) 409. [221] H. Zhang, J. X. Deng, Z. W. Pan, Z. Y. Bai, L. Kong, J. Y. Wang, Vacuum 146 (2017) 93. [222] H. Zhang, J. X. Deng, L. Kong, Z. W. Pan, Z. Y. Bai, J. Y. Wang, Micro Nano Lett. 14 (2019) 701. [223] H. Zhang, J. X. Deng, P. Duan, R. D. Li, Z. W. Pan, Z. Y. Bai, L. Kong, J. Y. Wang, Vacuum 155 (2018) 465.

1324

[224] J. G. Zhao, W. Y. Zhang, E. Q. Xie, Z. L. Liu, J. W. Feng, Z. J. Liu, Mat. Sci. Eng.: B 176 (2011) 932.

1325

[225]

B. K. Kang, S. R. Mang, D. H. Yoon, Mater. Lett. 111 (2013) 67.

1326

[226]

K. Sawada, T. Nakamura, S. Adachi, J. Alloy. Compd. 678 (2016) 448.

1327

[227]

K. Sawada, T. Nakamura, S. Adachi, ECS J. Solid State Sc. 5 (2016) R67.

1328

[228]

K. Sawada, T. Nakamura, J. Lum. 215 (2019) 116616.

1329

[229]

G. Cabello, A. Araneda, L. Lillo, C. Caro, C. Venegas, M. Tejos, B. Chornik,

1323

1330

Solid State Sci. 27 (2014) 24.

1332

[230] G. Cabello, L. Lillo, C. Caro, M. A. Soto-Arriaza, B. Chornik, G. E. Buono-Core, Ceram. Int. 39 (2013) 2443.

1333

[231]

1334 1335

[232] D. Vivien, B. Viana, A. Revcolevschi, J. D. Barrie, B. Dunn, P. Nelson, O. M. Stafsudd, J. Lum. 39 (1987) 29.

1336

[233]

1331

1337 1338 1339 1340

D. Wawrzynczyk, M. Nyk, M. Samoc, Chem. Phys. 456 (2015) 73.

C. G. Walsh, J. F. Donegan, T. J. Glynn, G. P. Morgan, G. F. Imbusch, J. P.

Remeika, J. Lum. 40 (1988) 103. [234]

J. G. Zhang, B. Li, C. T. Xia, J. Xu, Q. Deng, X. D. Xu, F. Wu, W. S. Xu, H.

S. Shi, G. Q. Pei, Y. Q. Wu, Sci. China Ser. E. 50 (2007) 51. [235]

H. Wakai, Y. Sinya, A. Yamanaka, Phys. Status Solidi (c) 8 (2011) 537.

59

1341 1342 1343 1344 1345 1346 1347 1348

[236] T. C. Lovejoy, R. Chen, E. N. Yitamben, V. Shutthanadan, S. M. Heald, E. G. Villora, K. Shimamura, S. Zheng, S. T. Dunham, F. S. Ohuchi, M. A. Olmstead, J. Appl. Phys. 111 (2012) 123716. [237] Z. Galazka, S. Ganschow, A. Fiedler, R. Bertram, D. Klimm, K. Irmscher, R. Schewski, M. Pietsch, M. Albrecht, M. Bickermann, J. Cryst. Growth 486 (2018) 82. [238] E. Nogales, J. A. García, B. Méndez, J. Piqueras, J. Appl. Phys. 101 (2007) 033517.

1350

[239] I. López, E. Nogales, B. Méndez, J. Piqueras, A. Peche, J. Ramírez-Castellanos, J. M. González-Calbet, J. Phys. Chem. C 117 (2013) 3036.

1351

[240]

Y. H. Wang, K. Xu, D. R. Li, H. Zhao, Z. F. Hu, Opt. Mater. 36 (2014) 1798.

1352

[241]

M. Alonso-Orts, E. Nogales, J.  M. San Juan, M.  L. Nó, J. Piqueras, B.

1349

1353

Méndez, Phys. Rev. Appl. 9 (2018).

1355

[242] X. S. Wang, J. Q. Situ, X. Y. Ying, H. Chen, H. F. Pan, Y. Jin, Y. Z. Du, Acta Biomater. 22 (2015) 164.

1356

[243]

S. Fujihara, Y. Shibata, J. Lum. 121 (2006) 470.

1357

[244]

Y. Tokida, S. Adachi, J. Appl. Phys. 112 (2012) 063522.

1358

[245]

Y. Tokida, S. Adachi, Jpn. J. Appl. Phys. 52 (2013) 101102.

1359

[246]

K. Sawada, S. Adachi, ECS J. Solid State Sc. 3 (2014) R238.

1360

[247]

D. Y. Guo, Z. P. Wu, P. G. Li, Y. H. An, H. Liu, X. C. Guo, H. Yan, G. F.

1354

1361

Wang, C. L. Sun, L. H. Li, W. H. Tang, Opt. Mater. Express 4 (2014) 1067.

1362

[248]

1363 1364

[249] X. L. Du, Z. X. Mei, Z. L. Liu, Y. Guo, T. C. Zhang, Y. N. Hou, Z. Zhang, Q. K. Xue, A. Y. Kuznetsov, Adv. Mater. 21 (2009) 4625.

1365

[250]

Y. J. Lu, C. N. Lin, C. X. Shan, Adv. Opt. Mater. 6 (2018) 1800359.

1366

[251]

C. Xie, X. T. Lu, X. W. Tong, Z. X. Zhang, F. X. Liang, L. Liang, L. B. Luo,

1367 1368 1369

M. Razeghi, P. IEEE 90 (2002) 1006.

Y. C. Wu, Adv. Funct. Mater., 1806006 (2019). [252] C. Q. Zhou, Q. Ai, X. Chen, X. H. Gao, K. W. Liu, D. Z. Shen, Chin. Phys. B 28 (2019) 048503. 60

1371

[253] S. Oh, J. Kim, F. Ren, S. J. Pearton, J. Kim, J. Mater. Chem. C 4 (2016) 9245.

1372

[254]

1370

1373 1374 1375 1376 1377 1378 1379 1380 1381 1382 1383 1384 1385 1386 1387 1388 1389 1390 1391 1392 1393 1394 1395 1396 1397 1398 1399

X. H. Chen, Y. Xu, D. Zhou, S. Yang, F. F. Ren, H. Lu, K. Tang, S. L. Gu, R.

Zhang, Y. D. Zheng, J. D. Ye, ACS Appl. Mater. Inter. 9 (2017) 36997. [255] B. Zhao, F. Wang, H. Chen, Y. Wang, M. Jiang, X. Fang, D. Zhao, Nano Lett. 15 (2015) 3988. [256] R. Suzuki, S. Nakagomi, Y. Kokubun, N. Arai, S. Ohira, Appl. Phys. Lett. 94 (22), 222102 (2009). [257] W. Tian, C. Y. Zhi, T. Y. Zhai, S. M. Chen, X. Wang, M. Y. Liao, D. Golberg, Y. Bando, J. Mater. Chem. 22 (34), 17984 (2012). [258] M. Z. Chen, J. G. Ma, P. Li, H. Y. Xu, Y. C. Liu, Opt. Express 27 (6), 8717 (2019). [259] X. Chen, K. Liu, Z. Zhang, C. Wang, B. Li, H. Zhao, D. Zhao, D. Shen, ACS Appl. Mater. Inter. 8 (2016) 4185. [260] S. J. Cui, Z. X. Mei, Y. H. Zhang, H. L. Liang, X. L. Du, Adv. Opt. Mater. 5 (2017) 1700454. [261] P. Feng, J. Y. Zhang, Q. H. Li, T. H. Wang, Appl. Phys. Lett. 88 (2006) 153107. [262] Y. B. Li, T. Tokizono, M. Y. Liao, M. Zhong, Y. Koide, I. Yamada, J. J. Delaunay, Adv. Funct. Mater. 20 (2010) 3972. [263] W. Y. Weng, T. J. Hsueh, S. J. Chang, G. J. Huang, S. P. Chang, IEEE Photonic. Tech. L. 22 (2010) 709. [264] Y. L. Wu, S. J. Chang, W. Y. Weng, C. H. Liu, T. Y. Tsai, C. L. Hsu, K. C. Chen, IEEE Sens. J. 13 (2013) 2368. [265] J. Y. Du, J. Xing, C. Ge, H. Liu, P. Y. Liu, H. Y. Hao, J. J. Dong, Z. Y. Zheng, H. Gao, J. Phys. D: Appl. Phys. 49 (2016) 425105. [266] S. L. Wang, H. L. Sun, Z. Wang, X. H. Zeng, G. Ungar, D. Y. Guo, J. Q. Shen, P. G. Li, A. P. Liu, C. R. Li, W. H. Tang, J. Alloy. Compd. 787 (2019) 133. [267] T. He, Y. K. Zhao, X. D. Zhang, W. K. Lin, K. Fu, C. Sun, F. F. Shi, X. Y. Ding, G. H. Yu, K. Zhang, Nanophotonics 7 (2018) 1557. 61

1401

[268] S. L. Wang, K. Chen, H. L. Zhao, C. R. He, C. Wu, D. Y. Guo, N. Zhao, G. Ungar, J. Q. Shen, X. L. Chu, RSC Adv. 9 (2019) 6064.

1402

[269]

1400

1403 1404 1405 1406 1407 1408 1409 1410 1411 1412 1413 1414 1415 1416 1417

J. H. Zhang, S. J. Jiao, D. B. Wang, S. M. Ni, S. Y. Gao, J. Z. Wang, J. Mater.

Chem. C 7 (2019) 6867. [270] W. Feng, X. N. Wang, J. Zhang, L. F. Wang, W. Zheng, P. A. Hu, W. W. Cao, B. Yang, J. Mater. Chem. C 2 (2014) 3254. [271] Y. Q. Wang, L. X. Song, Y. Teng, F. Wang, W. P. Wang, M. M. Ruan, Z. Yang, Z. Y. Xu, J. Mater. Chem. C 7 (2019) 1477. [272] Y. Teng, X. S. Le, A. Ponchel, Z. K. Yang, J. Xia, Adv. Mater. 26 (2014) 6238. [273] L. Li, E. Auer, M. Liao, X. Fang, T. Zhai, U. K. Gautam, A. Lugstein, Y. Koide, Y. Bando, D. Golberg, Nanoscale 3 (2011) 1120. [274] R. Zou, Z. Zhang, Q. Liu, J. Hu, L. Sang, M. Liao, W. Zhang, Small 10 (2014) 1848. [275] M. Z. Zhong, Z. M. Wei, X. Q. Meng, F. M. Wu, J. B. Li, J. Alloy. Compd. 619 (2015) 572. [276] Y. Kwon, G. Lee, S. Oh, J. Kim, S. J. Pearton, F. Ren, Appl. Phys. Lett. 110 (2017) 131901.

1419

[277] S. Oh, M. A. Mastro, M. J. Tadjer, J. Kim, ECS J. Solid State Sc. 6 (2017) Q79.

1420

[278]

S. Oh, C. K. Kim, J. Kim, ACS Photonics 5 (2017) 1123.

1421

[279]

T. Oshima, T. Okuno, N. Arai, N. Suzuki, S. Ohira, S. Fujita, Appl. Phys.

1418

1422

Express 1 (2008) 011202.

1424

[280] T. Oshima, T. Okuno, N. Arai, N. Suzuki, H. Hino, S. Fujita, Jpn. J. Appl. Phys. 48 (2009) 011605.

1425

[281]

1426

[282] W. Y. Kong, G. A. Wu, K. Y. Wang, T. F. Zhang, Y. F. Zou, D. D. Wang, L. B. Luo, Adv. Mater. 28 (2016) 10725.

1423

1427 1428 1429

R. Suzuki, S. Nakagomi, Y. Kokubun, Appl. Phys. Lett. 98 (2011) 131114.

[283] Q. Feng, L. Huang, G. Q. Han, F. G. Li, X. Li, L. W. Fang, X. Y. Xing, J. C. Zhang, W. X. Mu, Z. T. Jia, IEEE T. Electron Dev. 63 (2016) 3578. 62

1431

[284] W. X. Mu, Z. T. Jia, Y. R.Yin, Q. Q. Hu, J. Zhang, Q. Feng, Y. Hao, X. T. Tao, CrystEngComm 19 (2017) 5122.

1432

[285]

1430

1433 1434

A. S. Pratiyush, U. U. Muazzam, S. Kumar, P. Vijayakumar, S.

Ganesamoorthy, N. Subramanian, R. Muralidharan, D. N. Nath, IEEE Photonic. Tech. L. 31 (2019) 923.

1436

[286] L. P. Dong, J. G. Yu, R. X. Jia, J. C. Hu, Y. M. Zhang, J. W. Sun, Opt. Mater. Express 9 (2019) 1191.

1437

[287]

1435

1438

Y. Kokubun, K. Miura, F. Endo, S. Nakagomi, Appl. Phys. Lett. 90 (2007)

031912.

1439

[288]

1440

[289] W. Y. Weng, T. J. Hsueh, S. J. Chang, G. J. Huang, H. T. Hsueh, IEEE Sens. J. 11 (2010) 999.

1441 1442 1443 1444 1445 1446 1447

T. Oshima, T. Okuno, S. Fujita, Jpn. J. Appl. Phys. 46 (2007) 7217.

[290] Z. D. Huang, W. Y. Weng, S. J. Chang, C. J. Chiu, T. J. Hsueh, S. L. Wu, IEEE Sens. J. 13 (2013) 3462. [291] Z. D. Huang, W. Y. Weng, S. J. Chang, Y. F. Hua, C. J. Chiu, T. J. Hsueh, S. L. Wu, IEEE Sens. J. 13 (2012) 1187. [292] P. Guo, J. Xiong, X. H. Zhao, T. Sheng, C. Yue, B. W. Tao, X. Z. Liu, J. Mater. Sci.: Mater. El. 25 (2014) 3629.

1449

[293] G. C. Hu, C. X. Shan, N. Zhang, M. M. Jiang, S. P. Wang, D. Z. Shen, Express 23 (2015) 13554.

1450

[294]

T. Sheng, X. Z. Liu, L. X. Qian, B. Xu, Y. Y. Zhang, Rare Metals 1 (2015).

1451

[295]

F. P. Yu, S. L. Ou, D. S. Wuu, Opt. Mater. Express 5 (2015) 1240.

1452

[296] X. Z. Liu, P. Guo, T. Sheng, L. X. Qian, W. L. Zhang, Y. R. Li, Opt. Mater. 51 (2016) 203.

1448

1453 1454 1455 1456 1457 1458 1459

Opt.

[297] L. X. Qian, X. Z. Liu, T. Sheng, W. L. Zhang, Y. R. Li, P. T. Lai, AIP Adv. 6 (2016) 045009. [298] Y. Qin, H. D. Sun, S. B. Long, G. S. Tompa, T. Salagaj, H. Dong, Q. M. He, G. Z. Jian, Q. Liu, H. B. Lv, M. Liu, IEEE Electr. Device L. 1 (2019). [299] Y. K. Peng, Y. Zhang, Z. W. Chen, D. Y. Guo, X. Zhang, P. G. Li, Z. P. Wu, W. H. Tang, IEEE Photonic. Tech. L. 30 (2018) 993. 63

1460

[300]

1461

[301] L. X. Qian, Z. H. Wu, Y. Y. Zhang, P. T. Lai, X. Z. Liu, Y. R. Li, ACS Photonics 4 (2017) 2203.

1462

X. Z. Liu, C. Yue, C. T. Xia, W. L. Zhang, Chin. Phys. B 25 (2016) 017201.

1464

[302] Q. Feng, X. Li, G. Q. Han, L. Huang, F. G. Li, W. H. Tang, J. C. Zhang, Y. Hao, Opt. Mater. Express 7 (2017) 1240.

1465

[303]

1463

1466 1467 1468

S. Ahn, Y. H. Lin, F. Ren, S. Oh, Y. Jung, G. Yang, J. Kim, M. A. Mastro, J.

K. Hite, C. R. E. Jr, J. Vac. Sci. Technol. B. 34 (2016) 041213. [304] S. Ahn, F. Ren, S. Oh, Y. Jung, J. Kim, M. A. Mastro, J. K. Hite, C. R. E. Jr, S. J. Pearton, J. Vac. Sci. Technol. B. 34 (2016) 041207.

1470

[305] F. Alema, B. Hertog, P. Mukhopadhyay, Y. Zhang, A. Mauze, A. Osinsky, W. V. Schoenfeld, J. S. Speck, T. Vogt, APL Mater. 7 (2019) 022527.

1471

[306]

1469

1472 1473 1474 1475 1476 1477 1478 1479

F. Alema, B. Hertog, O. Ledyaev, D. Volovik, G. Thoma, R. Miller, A.

Osinsky, P. Mukhopadhyay, S. Bakhshi, H. Ali, W. V. Schoenfeld, Phys. Status Solidi (a) 214 (2017) 1600688. [307] S. H. Lee, S. B. Kim, Y. J. Moon, S. M. Kim, H. J. Jung, M. S. Seo, K. M. Lee, S. K. Kim, S. W. Lee, ACS Photonics 4 (2017) 2937. [308] S. J. Jiao, H. L. Lu, X. H. Wang, Y. Y. Nie, D. B.Wang, S. Y. Gao, J. Z. Wang, ECS J. Solid State Sc. 8 (2019) Q3086. [309] A. S. Pratiyush, S. Krishnamoorthy, S. V. Solanke, Z. B. Xia, R. Muralidharan, S. Rajan, D. N. Nath, Appl. Phys. Lett. 110 (2017) 221107.

1481

[310] D. P. Chaudhari, M. Ombaba, J. Y. Oh, H. Mao, K. H. Montgomery, A. Lange, S. Mahajan, J. M. Woodall, M. S. Islam, IEEE Sens. J. 9 (2017) 1.

1482

[311]

1483

[312] D. Zhang, W. Zheng, R. C. Lin, T. T. Li, Z. J. Zhang, F. Huang, J. Alloy. Compd. 735 (2018) 150.

1480

1484

S. Rafique, L. Han, H. P. Zhao, Phys. Status Solidi (a) 214 (2017) 1700063.

1486

[313] R. Lin, W. Zheng, D. Zhang, Z. Zhang, Q. Liao, L. Yang, F. Huang, ACS Appl. Mater. Inter. 10 (2018) 22419.

1487

[314]

1488

[315] H. Shen, Y. N. Yin, K. Tian, K. Baskaran, L. B. Duan, X. R. Zhao, A. Tiwari, J. Alloy. Compd. 766 (2018) 601.

1485

1489

K. Arora, N. Goel, M. Kumar, M. Kumar, ACS Photonics 5 (2018) 2391.

64

1491

[316] Y. Xu, Z. Y. An, L. X. Zhang, Q. Feng, J. C. Zhang, C. F. Zhang, Y. Hao, Opt. Mater. Express 8 (2018) 2941.

1492

[317]

1490

1493 1494 1495 1496 1497 1498 1499 1500 1501 1502 1503 1504 1505 1506 1507 1508 1509 1510 1511 1512 1513 1514 1515 1516 1517 1518 1519

D. Y. Guo, Z. P. Wu, Y. H. An, X. C. Guo, X. L. Chu, C. L. Sun, L. H. Li, P.

G. Li, W. H. Tang, Appl. Phys. Lett. 105 (2014) 023507. [318] X. Wang, Z. W. Chen, D. Y. Guo, X. Zhang, Z. P. Wu, P. G. Li, W. H. Tang, Opt. Mater. Express 8 (2018) 2918. [319] S. J. Cui, Z. X. Mei, Y. N. Hou, Q. S. Chen, H. L. Liang, Y. H. Zhang, W. X. Huo, X. L. Du, Chin. Phys. B 27 (2018) 067301. [320] D. Y. Guo, P. G .Li, Z. P .Wu, W. Cui, X. L .Zhao, M. Lei, L. H .Li, W. H. Tang, Sci. Rep. 6 (2016) 24190. [321] X. L. Zhao, Z. P. Wu, D. Y. Guo, W. Cui, P. G. Li, Y. H. An, L. H. Li, W. H Tang, Semicond. Sci. Tech. 31 (2016) 065010. [322] X. L .Zhao, Z. P .Wu, W. Cui, Y. S. Zhi, D. Y. Guo, L. H. Li, W. H. Tang, ACS Appl. Mater. Inter. 9 (2017) 983. [323] X. L. Zhao, Y. S. Zhi, W. Cui, D. Y. Guo, Z. P. Wu, P. G. Li, L. H. Li, W. H. Tang, Opt. Mater. 62 (2016) 651. [324] X. L. Zhao, Z. P. Wu, Y. S. Zhi, Y. H. An, W. Cui, L. H. Li, W. H. Tang, Phys. D: Appl. Phys. 50 (2017) 085102.

J.

[325] Y. H. An, X. L. Chu, Y. Q. Huang, Y. S. Zhi, D. Y. Guo, P. G. Li, Z. P. Wu, W. H. Tang, Prog. Nat. Sci. 26 (2016) 65. [326] X. C. Guo, N. H. Hao, D. Y. Guo, Z. P. Wu, Y. H. An, X. L. Chu, L. H. Li, P. G. Li, M. Lei, W. H. Tang, J. Alloy. Compd. 660 (2016) 136. [327] D. Y. Guo, H. Liu, P. G. Li, Z. P. Wu, S. L. Wang, C. Cui, C. R. Li, W. H. Tang, ACS Appl. Mater. Inter. 9 (2017) 1619. [328] Z. P. Wu, L. Jiao, X. L. Wang, D. Y. Guo, W. H. Li, L. H. Li, F. Huang, W. H. Tang, J. Mater. Chem. C 5 (2017) 8688. [329] Y. H. An, Y. S. Zhi, Z. P. Wu, W. Cui, X. L. Zhao, D. Y. Guo, P. G. Li, W. H. Tang, Appl. Phys. A 122 (2016). [330] Y. Y. Qu, Z. P. Wu, M. L. Ai, D. Y. Guo, Y. H. An, H. J. Yang, L. H. Li, W. H. Tang, J. Alloy. Compd. 680 (2016) 247. 65

1521

[331] M. L. Ai, D. Y. Guo, Y. Y. Qu, W. Cui, Z. P. Wu, P. G. Li, L. H. Li, W. H. Tang, J. Alloy. Compd. 692 (2017) 634.

1522

[332]

1520

1523 1524 1525 1526 1527 1528 1529 1530 1531 1532 1533 1534 1535 1536 1537 1538 1539 1540

Y. Q. Huang, H. Wu, Y. S. Zhi, Y. L. Huang, D. Y. Guo, Z. P. Wu, P. G. Li, Z.

W. Chen, W. H. Tang, Appl. Phys. A 124 (2018). [333] W. Cui, D. Y. Guo, X. L. Zhao, Z. P. Wu, P. G. Li, L. H. Li, C. Cui, W. H. Tang, RSC Adv. 6 (2016) 100683. [334] S. Nakagomi, T. Momo, S. Takahashi, Y. Kokubun, Appl. Phys. Lett. 103 (2013) 072105. [335] J. G. Yu, Z. Z. Nie, L. P. Dong, L. Yuan, D. J. Li, Y. Huang, L. C. Zhang, Y. M. Zhang, R. X. Jia, J. Alloy. Compd. 798 (2019) 458. [336] P. G. Li, H. Z. Shi, K. Chen, D. Y. Guo, W. Cui, Y. S. Zhi, S. L. Wang, Z. P. Wu, Z. W. Chen, W. H. Tang, J. Mater. Chem. C 5 (2017) 10562. [337] T. He, X. D. Zhang, X. Y. Ding, C. Sun, Y. K. Zhao, Q. Yu, J. Q. Ning, R. X. Wang, G. H. Yu, S. L. Lu, K. Zhang, X. P. Zhang, B. S. Zhang, Adv. Opt. Mater., 1801563 (2019). [338] D. Y. Guo, Y. L. Su, H. Z. Shi, P. G. Li, N. Zhao, J. H. Ye, S. L. Wang, A. P. Liu, Z. W. Chen, C. R. Li, W. H. Tang, ACS Nano 12 (2018) 12827. [339] D. Y. Guo, H. Z. Shi, Y. P. Qian, M. Lv, P. G. Li, Y. L. Su, Q. Liu, K. Chen, S. L. Wang, C. Cui, C. R. Li, W. H. Tang, Semicond. Sci. Tech. 32 (2017) 03LT01. [340] D. T. You, C. X. Xu, J. Zhao, W. Zhang, F. F. Qin, J. P. Chen, Z. L. Shi, J. Mater. Chem. C 7 (2019) 3056.

1542

[341] B. Zhao, F. Wang, H. Y. Chen, L. X. Zheng, L. X. Su, D. X. Zhao, X. S. Fang, Adv. Funct. Mater. 27 (2017) 1700264.

1543

[342]

1541

1544 1545 1546 1547 1548 1549 1550

Y. C. Chen, Y. J. Lu, C. N. Lin, Y. Z. Tian, C. J. Gao, L. Dong, C. X. Shan,

J. Mater. Chem. C 6 (2018) 5727. [343] C. R. He, D. Y. Guo, K. Chen, S. L. Wang, J. Q. Shen, N. Zhao, A. P. Liu, Y. Y. Zheng, P. G. Li, Z. P. Wu, C. R. Li, F. M. Wu, W. H. Tang, ACS Appl. Nano Mater. 2 (2019) 4095. [344] K. Chen, S. L. Wang, C. R. He, H. W. Zhu, H. L. Zhao, D. Y. Guo, Z. W. Chen, J. Q. Shen, P. G. Li, A. P. Liu, C. R. Li, F. M. Wu, W. H. Tang, ACS Appl. Nano Mater. 2 (2019) 6169. 66

1551 1552

[345] S. Li, D. Y. Guo, P. G. Li, X. Wang, Y. H. Wang, Z. Y. Yan, Z. Liu, Y. S. Zhi, Y. Q. Huang, Z. P. Wu, W. H .Tang, ACS Appl. Mater. Inter. 11 (2019) 35105.

1553

67

Highlights 1. Ga2O3 emerges as a new multicolor-emitting phosphor host material for emissive display applications. 2. Ga2O3 also is a natural choice for use in solar-blind photodetector due to the appropriate bandgap of ~ 4.9 eV. 3. We summarized the application of Ga2O3 in optoelectronic devices and presented the future perspectives.

Conflict of Interest Statement The authors declare no conflict of interest. Daoyou Guo, Qixin Guo, Zhengwei Chen, Zhenping Wu, Peigang Li, Weihua Tang