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Ultra-wide bandgap semiconductor of β-Ga2O3 and its research progress of deep ultraviolet transparent electrode and solar-blind photodetector

Guo Dao-You Li Pei-Gang Chen Zheng-Wei Wu Zhen-Ping Tang Wei-Hua

Ultra-wide bandgap semiconductor of β-Ga2O3 and its research progress of deep ultraviolet transparent electrode and solar-blind photodetector

Guo Dao-You, Li Pei-Gang, Chen Zheng-Wei, Wu Zhen-Ping, Tang Wei-Hua
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  • Gallium oxide (Ga2O3), with a bandgap of about 4.9 eV, is a new type of ultra-wide bandgap semiconductor material. The Ga2O3 can crystallize into five different phases, i.e. α, β, γ, δ, and ε-phase. Among them, the monoclinic β-Ga2O3 (space group: C2/m) with the lattice parameters of a = 12.23 Å, b = 3.04 Å, c = 5.80 Å, and β = 103.7° has been recognized as the most stable phase. The β-Ga2O3 can be grown in bulk form from edge-defined film-fed growth with a low-cost method. With a high theoretical breakdown electrical field (8 MV/cm) and large Baliga’s figure of merit, the β-Ga2O3 is a potential candidate material for next-generation high-power electronics (including diode and field effect transistor) and extreme environment electronics [high temperature, high radiation, and high voltage (low power) switching]. Due to a high transmittance to the deep ultraviolet-visible light with a wavelength longer than 253 nm, the β-Ga2O3 is a natural material for solar-blind ultraviolet detection and deep-ultraviolet transparent conductive electrode. In this paper, the crystal structure, physical properties and device applications of Ga2O3 material are introduced. And the latest research progress of β-Ga2O3 in deep ultraviolet transparent conductive electrode and solar-blind ultraviolet photodetector are reviewed. Although Sn doped Ga2O3 thin film has a conductivity of up to 32.3 S/cm and a transmittance greater than 88%, there is still a long way to go for commercial transparent conductive electrode. At the same time, the development history of β-Ga2O3 solar-blind ultraviolet photodetectors based on material type (nanometer, single crystal and thin film) is described in chronological order. The photodetector based on quasi-two-dimensional β-Ga2O3 flakes shows the highest responsivity (1.8 × 105 A/W). The photodetector based on ZnO/Ga2O3 core/shell micron-wire has a best comprehensive performance, which exhibits a responsivity of 1.3 × 103 A/W and a response time ranging from 20 ${\text{μ}}{\rm{s}}$ to 254 nm light at –6 V. We look forward to applying the β-Ga2O3 based solar-blind ultraviolet photodetectors to military (such as: missile early warning and tracking, ultraviolet communication, harbor fog navigation, and so on) and civilian fields (such as ozone hole monitoring, disinfection and sterilization ultraviolet intensity monitoring, high voltage corona detection, forest fire ultraviolet monitoring, and so on).
      Corresponding author: Tang Wei-Hua, whtang@bupt.edu.cn
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  • 图 1  Ga2O3几个同分异构体的晶体结构

    Figure 1.  Crystal structures of several isomers of Ga2O3

    图 2  Ga2O3各同分异构体的相互转换关系[4]

    Figure 2.  Interconversion relation of Ga2O3 isomers[4]

    图 3  β-Ga2O3的晶体结构及晶格常数[2123]

    Figure 3.  Crystal structure and lattice constant of β-Ga2O3[2123]

    图 4  β-Ga2O3材料具有的物理性质及其对应的器件应用

    Figure 4.  Physical properties and device applications of β-Ga2O3 material

    图 5  (a)在不同温度下制备获得的Sn掺杂β-Ga2O3薄膜的透过率[75]; (b) Sn掺杂β-Ga2O3薄膜的导电率随沉积温度的变化关系[24]

    Figure 5.  (a) The transmittance of Sn-doped-Ga2O3 thin films prepared at different temperatures[75]; (b) the relationship between the conductivity of Sn doped -Ga2O3 thin films and the deposition temperature[24]

    图 6  Sn掺杂Ga2O3薄膜的透过率和带隙(a)[81]及电阻率(b)[82]随Sn掺杂浓度的变化关系

    Figure 6.  The relationship of the transmittance (a)[81], the band gap (a)[81], the resistivity (b)[82] with Sn different doping concentration in Sn-doped Ga2O3 thin films

    图 7  ITO与Ga2O3:ITO薄膜性能对比 (a)光输出功率–电流–电压特征曲线; (b)近紫外LED的电致发光光谱[85]

    Figure 7.  (a) Current versus light output power and forward voltage (L-I-V) characteristic curves and (b) typical electroluminescence spectra measured for near-ultraviolet LEDs with Ga2O3:ITO and ITO transparent conducting electrodes; the inset shows top-view SEM image of near-ultraviolet[85]

    图 8  Au-Ga2O3纳米线-Au光电探测器 (a)黑暗情况下的I–V特性曲线及其器件结构SEM图(插图); (b)–8 V偏压下对254 nm光的I–t响应特性曲线[91]

    Figure 8.  Au-Ga2O3 nanowire-Au photodetector: (a) I-V characteristic curve of the detector in dark. The inset of is a typical SEM image of the device, the scale bar: 200 nm; (b) real-time photoresponse of the detector to 254 nm light[91]

    图 9  β-Ga2O3纳米桥光电探测器的日盲光电性质 (a) 器件的结构示意图; (b) 50 V偏压下对254 nm光的I–t响应特性; (c) 黑暗及对365和254 nm光响应的I–V特性曲线; (d) 不同波长的光谱响应特性[88]

    Figure 9.  Solar blind photoelectric properties of photodetector based on the bridged β-Ga2O3 nanowires: (a) Schematic diagram of the devices; (b) time-dependent photoresponse of the bridged β-Ga2O3 nanowires measured in dry air under UVC (~2 mW cm–2 at 254 nm) illumination with a period of 60 s at a bias voltage of 50 V; (c) I-V characteristics of the bridged β-Ga2O3 nanowires in dark (squares), under 365 nm light (triangles), and under 254 nm light (circles). The I-V curve measured under 254 nm light is plotted on a linear scale in the inset; (d) spectral response of the bridged β-Ga2O3 nanowires revealing that the device is blind to solar light. The dashed line indicates the lowest wavelength of the solar spectrum on Earth[88]

    图 10  (a) Ga2O3纳米线光电探测器在不同偏压下的光谱响应[92]; (b)在Cr/Au电极上生长获得的Ga2O3纳米线光电探测器结构[93]; (c)不同温度下生长的Ga2O3纳米线对255 nm光的I–t响应曲线[93]; (d)不同偏压下的光谱响应[93]

    Figure 10.  (a) Room-temperature spectral responses of the Ga2O3 nanowires photodetector measured with different applied biases[92]; (b) Ga2O3 nanowire photodetector with Cr/Au as electrodes[93]; (c) transit responses measured from the three fabricated photodetectors grown at different temperatures[93]; (d) room-temperature spectral responses of the photodetector under different bias[93]

    图 11  (a)单条Ga2O3纳米带光电探测器的SEM图[94]; (b)不同带宽Ga2O3纳米带的光谱响应, 插图为探测器结构[94]; (c) In掺杂的Ga2O3单条纳米带光电探测器的光谱响应[95]; (d)纯Ga2O3和In:Ga2O3单条纳米带黑暗情况及在250 nm光照下的I–V曲线[95]

    Figure 11.  (a) SEM image of a Ga2O3 individual-nanobelt device[94]; (b) spectral response of the devices (nanobelts with different widths of 800 nm and 1.6 mm) measured at a bias of 15 V. The schematic configuration of a photoconductive measurement is inserted in the top-right corner[94]; (c) spectral response of an individual In-doped Ga2O3 nanobelt photodetector. The inset is a typical SEM image of an individual In-doped Ga2O3 nanobelt device[95]; (d) logarithmic plot of I-V curves of the individual Ga2O3 and In-doped Ga2O3 nanobelt photodetector under illumination with the 250 nm wavelength light and in dark conditions[95]

    图 12  (a) Ga2O3纳米花的SEM图; (b) Ga2O3纳米花对254 nm光的I–t响应曲线[97]

    Figure 12.  (a) SEM image of Ga2O3 nanoflowers; (b) I-t response curve of Ga2O3 nanoflowers to 254 nm light[97]

    图 13  ZnO/Ga2O3核/壳结构的日盲紫外探测器 (a)器件示意图; (b)黑暗和254 nm光照下的I–V特征曲线; (c)–6 V偏压下的光谱响应[100]; (d)0 V偏压下的光谱响应; (e)光电流衰减[101]. Au/Ga2O3纳米线Schottky型垂直结构的光电探测器 (f)器件示意图; (g)光谱响应; (h)光电流衰减[102]

    Figure 13.  Solar-blind ultraviolet photodetector based on Single ZnO-Ga2O3 core-shell microwire ZnO/Ga2O3 core-shell: (a) Device schematic diagram; (b)I-V characteristic curve in dark and under 254 nm light; (c) spectral response of the device at −6 V bias[100]; (d) the photoresponse spectrum of the device at 0 V; (e) the time response under the excitation of 266 nm pulse laser at 0 V[101]. Au/Ga2O3 nanowire Schottky vertical structure photodetector: (f) device schematic diagram; (g) spectral responses of the device at zero bias and under reverse bias of 10 V. Inset shows the responsivity of photodetectors at the wavelength of 254 nm as a function of reverse bias; (h) decay edge of the current response at reverse bias of 10 V[102].

    图 14  基于β-Ga2O3薄片的日盲紫外探测器 (a)机械剥离获得β-Ga2O3微米薄片及器件制作流程示意图; (b)器件的光学照片; (c)不同波长光照下的器件的I–t响应曲线; (d) 光谱响应曲线[103]; (e) β-Ga2O3微米薄片的反应离子刻蚀减薄[104]; (f) Ni/Au电极与β-Ga2O3薄片构成的MSM结构肖特基结日盲紫外探测器在不用波长下的I–V曲线; (g)能带结构示意图[105]; (h), (i)石墨烯电极与β-Ga2O3薄片构成的MSM结构日盲紫外探测器的SEM图[106]

    Figure 14.  Solar-blind ultraviolet photodetector based on β-Ga2O3 flake: (a) Schematic of the entire exfoliated β-Ga2O3 flake based photodetector fabrication process; (b) optical image of the fabricated photodetector; (c) time-dependent photoresponse of the fabricated photodetector under various illumination conditions (254, 365, 532 and 650 nm light exposure); (d) responsivity as a function of wavelength[103]; (e) the reactive ion etching assisted thinning of a β-Ga2O3 flake[104]; (f) the I-V curve; (g) energy band structure diagram of the schottky junction MSM structure solar-blind ultraviolet photodetector based on Ni/Au electrodes and β-Ga2O3 flake under different wavelengths[105]; (h), (i) the SEM image of the MSM structure solar-blind ultraviolet photodetector based on graphene electrode and β-Ga2O3 flake[106]

    图 15  垂直结构肖特基型β-Ga2O3单晶日盲紫外探测器 (a)制作过程[109]; (b)光谱响应[109]; (c)实物图[89]; (d)瞬态光响应[89]

    Figure 15.  Vertical solar-blind deep-ultraviolet schottky photodetectors based onβ-Ga2O3 substrates: (a) Fabrication process for photodetector[109]; (b) spectral responser[109]; (c) photograph of the flame detector. The dashed circles are on the edges of the transparent electrodes[89]; (d) transient response of the detector[89]

    图 16  (a) β-Ga2O3单晶与Au电极在不同温度下退火后的I–V曲线[110]; (b)未退火和400℃下退火后Au/β-Ga2O3单晶肖特基型光电探测器的光谱响应[110]; (c)在β-Ga2O3单晶上采用溶胶凝胶法制备高绝缘β-Ga2O3薄膜并与Au电极构成的光电探测器[111]; (d)有无高绝缘β-Ga2O3薄膜层的光谱响应对比图[111]

    Figure 16.  (a) Dark I-V characteristics of the Au-Ga2O3 Schottky photodiode annealed at various temperatures. The inset shows the device configuration[110]; (b) spectral response of the Au-Ga2O3 Schottky photodiode before and after annealing at 400℃. The inset shows the reverse I-V characteristics of the photodiode annealed at 400℃ taken in dark and under illumination with 240 nm light[110]; (c) schematic structure of a photodiode composed of a Au Schottky contact and a β-Ga2O3 single-crystal substrate with a sol-gel prepared cap layer.[111]; (d) spectral response of Ga2O3 photodiodes with and without a cap layer at reverse and forward biases of 3 V. The inset shows the incident light intensity dependence of the photocurrent at forward and reverse biases of 3 V under illumination with 250 nm light[111]

    图 17  石墨烯/β-Ga2O3单晶日盲紫外探测器[112] (a)器件结构示意图; (b)黑暗及365 nm光照下的I–V曲线; (c)光谱响应; (d)能带结构示意图

    Figure 17.  Solar-blind ultraviolet photodetectors based on graphene/β-Ga2O3 single crystal heterojunction[112]: (a) Schematic diagram of device structure; (b) I-V characteristics of the photodetectors in dark and under 365 nm light irradiation; (c) normalized spectral selectivity; (b) energy band diagram at forward bias voltage

    图 18  (a) Ga2O3薄膜的面内XRD图; (b) Ga2O3薄膜在黑暗及不同光照下的I–V曲线[90]

    Figure 18.  (a) In-plane XRD measurement results for the Ga2O3 film; (b) I-V characteristics of the Ga2O3 film photodetector in the dark, under black light irradiation, and under low-pressure mercury lamp irradiation[90]

    图 19  (a) Ga2O3/GaN光电探测器结构; (b) Ga2O3/GaN光电探测器在不同偏压下的光谱响应[117]; (c) Ga2O3/AlGaN/GaN光电探测器结构; (d) Ga2O3/AlGaN/GaN光电探测器在不同偏压下的光谱响应[118]; (e) Ga2O3/InGaN/GaN光电探测器结构; (f) Ga2O3/InGaN/GaN光电探测器在不同偏压下的光谱响应[119]; (g)有无Au纳米颗粒与Ga2O3界面形成的能带结构示意图; (h) Au纳米颗粒/Ga2O3光电探测器在不同偏压下的光谱响应[120]

    Figure 19.  Schematic diagram (a) and spectral responses under different bias (b) of Ga2O3/GaN photodetector[117]; Schematic diagram (c) and spectral responses under different bias (d) of Ga2O3/AlGaN/GaN photodetector[118]; Schematic diagram (e) and spectral responses under different bias (f) of Ga2O3/InGaN/GaN photodetector[119]; Energy band diagram of area near the surface of β-Ga2O3 and Au in the dark (g), spectral responses under different bias of Ga2O3/GaN-based metal-semiconductor-metal photodetectors covered with Au nanoparticles (h)[120]

    图 20  (a) Ga2O3/SiC光电探测器结构; (b) Ga2O3/SiC光电探测器在2 V反偏压下的光谱响应[121]

    Figure 20.  Schematic diagram (a) and spectral responses under 2 V reverse bias (b) of SiC/Ga2O3 photodetector[121]

    图 21  (a) Ga2O3薄膜MSM结构日盲紫外探测器的结构示意图[123]; (b) MSM结构中Ga2O3薄膜厚度对探测器光暗比的影响[124]; (c), (d) MSM结构阵列探测器[125]; (e)氧气氛退火处理构成的肖特基结与未退火欧姆接触MSM结构探测器的I–t曲线[126]. 不同元素掺杂Ga2O3薄膜MSM结构探测器的I–t曲线 (f) Mg掺杂[128]; (g) Mn掺杂[127]; (h) Zn掺杂[129]; (i) Sn掺杂[130]

    Figure 21.  (a) Schematic diagram of the β-Ga2O3 thin film MSM structure photodetector[123; (b) the effect of Ga2O3 film thickness on light-dark ratio of the MSM structure photodetector[124]; (c), (d) MSM structure arrays photodetector[125]; (e)I-t curves of the β-Ga2O3 thin films MSM structure photodetector with unannealed (Ohmic-type up) and annealed treatment in O2 atmosphere (Schottky-type, down), respectively[126]. I-t curves of the MSM structure photodetector based on β-Ga2O3 thin films doped with different element: (f) Mg doped[128]; (g) Mn doped[127]; (h) Zn doped[129]; (i) Sn doped[130]

    图 22  石墨烯/Ga2O3/石墨烯垂直结构日盲紫外探测器的结构示意图[138](a)及其不同偏压下对254 nm紫外光的响应度(b)[138]; 纯Ga2O3及表面附着有Au纳米颗粒Ga2O3薄膜的紫外可见吸收(c)[139]和不同光照下的I–V曲线(d)[139]; 引入Al2O3薄层生长获得的Ga2O3薄膜/纳米线SEM图(e)[140]和不同光照下的I–V曲线(f)[140]

    Figure 22.  Schematic diagram (a) [138] and photoresponses to 254 nm ultraviolet light under different bias (b) [138] of graphene/Ga2O3/graphene vertical structure photodetector; UV-vis absorbance spectrum (c) [139] and I-V cures under the different wavelength light illumination (d) [139] of the bare Ga2O3 thin film and Au nanoparticles/Ga2O3 composite thin film; SEM image (e) and I-V cures under the different wavelength light illumination (f) [140] of Ga2O3 thin film/nanowire grown induced by Al2O3 thin layer[140]

    图 23  Ga2O3/NSTO异质结自供电探测器的结构示意图(a)[142] 、黑暗及254 nm不同光强下的I–V曲线(b)[142]和异质结界面处光生载流子输运的能带结构示意图(c)[142]; Ga2O3/P-Si PN结探测器的结构示意图(d)[143]; Ga2O3/Ga:ZnO异质结探测器的整流特性及结构示意图(e)[145]和光谱响应(f)[145]; Ga2O3/GaN PN结探测器的结构示意图(g)[146]和黑暗及不同波长光照下的I–V曲线(h)[146]; Sn:Ga2O3/GaN PN结探测器的光谱响应(i)[144]和不同波长光照下的I–t曲线(j)[147]; Ga2O3/SiC/P-Si PIN结(k)[148]和石墨烯/Ga2O3/SiC探测器的结构示意图(l)[149]

    Figure 23.  Schematic diagram (a) [142], I-V cures in dark and under 254 nm with different light intensity illumination (b) [142], and schematic energy band diagrams (c) [142] of the β-Ga2O3/NSTO heterojunction self-powered photodetector; Schematic diagram of Ga2O3/P-Si PN junction detector (d) [143]; Rectifier features (e), schematic diagram (e) and spectral response (f) of the Ga2O3/Ga:ZnO heterojunction photodetector[145]; Schematic diagram (g) [145], I-V cures in dark and under the different wavelength light illumination (h) [146]; Spectral response (i) and I-t cures under the different wavelength light illumination (j) of the Sn:Ga2O3/GaN PN junction photodetector[145]; Schematic diagram of Ga2O3/SiC/P-Si PIN junction photodetector (k) [148]and graphene/Ga2O3/SiC photodetector (l)[149]

    图 24  a-GaOx非晶薄膜和β-Ga2O3薄膜日盲紫外探测器[159] (a) MSM结构示意图; (b)光谱响应; (c)能带结构示意图

    Figure 24.  Solar-blind ultraviolet photodetector based on a-GaOx amorphous film and β-Ga2O3 film[159]: (a) MSM structure diagram; (b) spectral response; (c) energy band structure diagram

    图 25  MSM结构日盲紫外探测器 (a) MSM结构示意图[160]; (b) Ga2O3单晶和薄膜的光谱响应对比[160]; (c) MSM结构[162]; (d) Ga2O3薄膜不同气氛退火的光谱响应对比[161]; (e)不同氧压下生长的Ga2O3薄膜的光谱响应对比[162]; (f)不同In掺杂的Ga2O3薄膜的光谱响应对比图[163]

    Figure 25.  MSM structure solar-blind ultraviolet photodetector: (a) Schematic diagram of MSM structure[160]; (b) spectral response comparison of Ga2O3 single crystal and thin film[160]; (c) MSM structure[162]; (d) spectral response comparison of Ga2O3 thin films annealed in different atmospheres[161]; (e) spectral response comparison of Ga2O3 thin films grown under different oxygen pressures[162]; (f) spectral response comparison of Ga2O3 thin films doped with different concentrations of In elements[163]

    图 26  a-Ga2O3非晶薄膜日盲紫外探测器[169] (a)以石英为衬底的器件结构示意图; (b)光谱响应; (c)光衰减测试; (d)以柔性为衬底的器件结构示意图

    Figure 26.  Solar-blind ultraviolet photodetector based on a-Ga2O3 amorphous film[169]: (a) Schematic diagram of device structure with quartz substrate; (b) spectral response; (c) the decay of photoresponse; (d) schematic diagram of device structure with flexible substrate

    图 27  a-GaOx非晶薄膜日盲紫外探测器[171] (a)以玻璃为衬底的器件结构示意图; (b)黑暗和253 nm光照下的I–V曲线; 以聚酰亚胺为衬底的器件结构示意图(c)及黑暗和253 nm光照下的I–V曲线(d)

    Figure 27.  Solar-blind ultraviolet photodetector based on a-Ga2O3 amorphous film[171]: Schematic diagram of device structure with glass substrate (a) and I-V cures in dark and under the illumination of 253 nm light (b); Schematic diagram of device structure with polyimide substrate (c) and I-V cures in dark and under the illumination of 253 nm light (d)

    图 28  α-Ga2O3/ZnO异质结日盲紫外探测器[172] (a)光谱响应; (b)增益随偏压的变化; (c)瞬态光响应特性; (d)能带结构及器件结构示意图

    Figure 28.  Solar-blind ultraviolet photodetector based on α-Ga2O3/ZnO heterojunction[172] : (a) Spectral response; (b) variation of gain with bias; (c) transient photoresponse characteristics; (d) schematic diagram of energy band structure and device structure

    图 29  以N2O为反应气体获得的β-Ga2O3薄膜日盲紫外探测器 (a)生长原理示意图[176]; (b)黑暗和255 nm光照下的I–V曲线及MSM结构示意图[176]; (c)光谱响应及不同偏压下的光响应度[176]; (d)石墨烯/β-Ga2O3/GaN器件结构示意图[177]; (e)光谱响应[177]; (f)能带结构示意图[177]

    Figure 29.  Solar-blind ultraviolet photodetector based on β-Ga2O3 thin film grown using N2O as the reaction gas: (a) Schematic diagram of growth principle[176]; (b) I-V cures in dark and under 255 nm light illumination, and schematic diagram of MSM structure[176]; (c) spectral response and photoresponsivity under different bias[176]; (d) schematic diagram of graphene/β-Ga2O3/GaN devices[177]; (e) spectral response[177]; (f) energy band structure diagram[177]

    表 1  β-Ga2O3与主流半导体材料的基本物性比较[25]

    Table 1.  Comparison of basic physical properties of β-Ga2O3 with mainstream semiconductor materials[25]

    材料 Si GaAs GaP 4H-SiC ZnO GaN ß-Ga2O3 Diamond AlN MgO
    带隙Eg/eV 1.1 1.43 2.27 3.3 3.35 3.4 4.2—4.9 5.5 6.2 7.8
    迁移率${\text{μ}}$/cm2·Vs–1 1400 8500 350 1000 200 1200 300 2000 135
    击穿电场强度Eb/MV·cm–1 0.3 0.6 1.0 2.5 3.3 8 10 2
    相对介电常数ε 11.8 12.9 11.1 9.7 8.7 9 10 5.5 8.5 9.9
    导热率/W·cm–1·K–1 1.5 0.55 1.1 2.7 0.6 2.1 0.23[010] 0.13[100] 10 3.2
    巴利加优值/$\varepsilon {\text{μ}} {E_{\rm{b}}}^3$ 1 15 340 870 3444 24664
    DownLoad: CSV

    表 2  Ga2O3基透明导电电极薄膜的各参数指标汇总

    Table 2.  Parameters and indicators of Ga2O3-based transparent conductive electrode films

    薄膜类型 电导率/S·cm–1 面电阻/Ω·sq–1 载流子浓度/cm–3 迁移率/cm2·V–1·s–1 透过率/% 参考文献
    Ga2O3薄膜 7.6 - - - 85 [80]
    Sn:Ga2O3薄膜 1 - 1.4 × 1019 0.44 80 [78]
    Sn:Ga2O3薄膜 8.2 - - < 0.44 80 [24]
    Sn:Ga2O3薄膜 8.3 - - 12.03 85 [81]
    Sn:Ga2O3薄膜 32.3 - 2.4 × 1020 0.74 88 [82]
    Sn:Ga2O3单晶 23.4 - 2.3 × 1018 64.7 85 [79]
    (Ga, In)2O3薄膜 1.72 × 103 - 5 × 1020 - > 95 [83]
    Ga2O3/ITO薄膜 - 164 - - > 94 [84]
    Ga2O3/ITO薄膜 - 49 - - 93.8 [85]
    Ag/Ga2O3薄膜 - 42 - - 91 [86]
    Ga2O3/Cu/ITO - 50 - - 86 [87]
    DownLoad: CSV

    表 3  几种无线通信的比较

    Table 3.  Comparison of several wireless communications

    通信类别 非视距通信 抗干扰、防窃听 相对运动信号接收 传播距离调控 受环境气候时间影响
    无线电通信 易被干扰和窃听 很差 受环境影响
    激光通信 抗干扰、防窃听 较差 受环境影响
    红外通信 较易干扰、防窃听 较差 受环境时间影响
    紫外通信 抗干扰、防窃听 很好 很小、全天候
    DownLoad: CSV

    表 4  Ga2O3基日盲紫外探测器的各参数指标汇总

    Table 4.  Summary of parameters and indicators of Ga2O3 based solar-blind ultraviolet photodetector.

    光电探测器类型 光响应度/A·W–1 量子效率/% 暗电流/A 光暗比 响应时间/s 参考文献
    Ga2O3纳米线 - - 10–12 ≈ 2 × 103 2.2 × 10–1 [91]
    Ga2O3纳米线 - - < 10–12 3 × 104 < 2 × 10–2 [88]
    Ga2O3纳米线 8.0 × 10–4 0.39 2.4 × 10–10 ≈ 102 - [92]
    Ga2O3纳米线 3.4 × 10–3 1.37 - ≈ 102 - [93]
    ZnO/Ga2O3核壳微米线 1.3 × 103(–6 V) - 10–10 ≈ 106 2 × 10–5 [100]
    ZnO/Ga2O3核壳微米线 9.7 × 10–3(0 V) - 10–10 ≈ 7 × 102 10–4 [101]
    Ga2O3纳米线 6 × 10–4 - 10–11 ≈ 102 6.4 × 10–5 [102]
    Ga2O3纳米线 3.77 × 102 2.0 × 105 10–11 103 0.21 [107]
    石墨烯/Ga2O3纳米线 1.85 × 10-1 - 10–5 - 8 × 10–3 [108]
    Ga2O3纳米片 3.3 1.6 × 103 10–9 10 3 × 10–2 [96]
    Ga2O3纳米花(γ) - - 10–9 2.2 × 102 10–1 [97]
    Ga2O3纳米带 3.37 × 101 1.67 × 104 10–13 4.0 × 102 8.6 × 101 [94]
    Ga2O3纳米带 8.51 × 102 4.2 × 103 10–13 ≈ 103 < 3 × 10–1 [98]
    Ga2O3纳米带 1.93 × 101 9.4 × 103 10–10 ≈ 104 < 2 × 10–2 [99]
    In:Ga2O3纳米带 5.47 × 102 2.72 × 105 10–13 9.1 × 102 1 [95]
    Ga2O3微米带 1.8 × 105(–30 V) 8.8 × 105 10–6 2.57 0.67 [103]
    Ga2O3微米带 - - 10–4 - 1.4 [104]
    Ga2O3微米带 1.68 - 10–13 1.9 × 103 0.53 [105]
    石墨烯/Ga2O3微米带 2.98 × 101 - 10–13 ≈ 104 - [106]
    Ga2O3单晶 2.6—8.7 - 10–10 ≈ 103 - [109]
    Ga2O3单晶 3.7 × 10–2 1.8 × 101 10–10 1.5 × 104 9 × 10–3 [89]
    Ga2O3单晶 103 - 10–10 ≈ 106 - [110]
    Ga2O3单晶 4.3 2.1 × 101 10–11 105 - [111]
    石墨烯/Ga2O3单晶 3.93 × 101 1.96 × 104 10–6 103 2.2 × 102 [112]
    Ga2O3单晶 5 × 10–2 - 10–5 102 2.4 × 10–1 [160]
    Ga2O3单晶 3 × 10–3 - 10–8 101 1.4 × 10–1 [113]
    Ga2O3薄膜 8 × 10–5 - - - - [116]
    Ga2O3薄膜 3.7 × 10–2 1.8 × 101 10–9 - - [90]
    Ga2O3薄膜 4.53 × 10–1 > 102 10–10 105 - [117]
    Ga2O3薄膜 ≈ 101 - 10–10 103 - [118]
    Ga2O3薄膜 ≈ 101 - 10–7 103 - [119]
    Ga2O3薄膜 ≈ 102 - 10–10 102 - [120]
    Ga2O3薄膜 - - 10–11 105 - [122]
    Ga2O3薄膜 7.6 × 10–1 - 10–10 6 5 × 10–2 [152]
    Ga2O3薄膜 1.7 × 101 8.2 × 103 10–9 8.5 × 106 - [153]
    Ga2O3薄膜 - - 10–11 102 8 × 10–1 [154]
    Ga2O3薄膜 9.03 × 10–1 - 10–11 105 - [155]
    Ga2O3薄膜 2.59 × 102 7.9 × 104 10–10 104 4 × 10–1 [156]
    Ga2O3薄膜 - - 10–7 15 - [157]
    Ga2O3薄膜/晶体 1.8 8.7 × 102 10–6 36.9 - [158]
    a-GaOx非晶薄膜 7.0 × 101 - 10–10 1.2 × 105 2 × 10–2 [159]
    Ga2O3薄膜 4.2 - 10–11 1.6 × 104 4 × 10–2 [159]
    Ga2O3薄膜 9 × 10–3 - 10–5 101 1.8 × 10–1 [160]
    Al:Ga2O3薄膜 1.5 7.8 × 102 - - - [164]
    Si:Ga2O3薄膜 6 × 101 3 × 104 - 9 - [166]
    Si:Ga2O3薄膜 3.6 × 101 1.75 × 104 - 9 - [167]
    Zn:Ga2O3薄膜 2.1 × 102 - 10–11 5 × 104 1.4 [168]
    Ga2O3非晶薄膜 1.9 × 10–1 - 10–12 106 1.9 × 10–5 [169]
    Ga2O3非晶薄膜 4.5 × 101 - 10–10 104 2.97 × 10–6 [171]
    Ga2O3薄膜 1.5 - 10–9 103 - [175]
    Ga2O3薄膜 0.29 1.34 10–8 1.6 × 103 0.1 [173]
    Ga2O3薄膜 0.11 - 10–9 3.5 × 103 0.45 [174]
    Ga2O3薄膜 0.14 - 10–11 1.4 × 106 0.2 [174]
    Ga2O3薄膜 1.5 - 10–8 103 - [173]
    Ga2O3薄膜 2.6 × 101 - 10–8 104 0.18 [176]
    石墨烯/Ga2O3薄膜 1.28 × 101 - 10–8 - 2 × 10–3 [177]
    Ga2O3薄膜 9.6 × 101 4.76 × 104 10–6 - - [180]
    Ga2O3薄膜 5.86 × 10–5 - 10–9 1.8 × 101 0.1 [181]
    Ga2O3薄膜 1.5 × 102 7 × 104 10–11 105 1.3 [165]
    Ga2O3薄膜 1 × 10–1 - 10–8 - - [178]
    Ga2O3薄膜 - - 10–8 6 8.6 × 10–1 [123]
    Ga2O3薄膜 - - 10–9 1.3 × 101 6.2 × 10–1 [126]
    Ga2O3/Ga/Ga2O3薄膜 2.854 - 10–11 8 × 105 - [170]
    Mn:Ga2O3薄膜 7 × 10–2 3.6 × 101 10–9 6.7 × 101 2.8 × 10–1 [127]
    α-Ga2O3薄膜 1.5 × 10–2 7.39 10–9 3 × 101 - [137]
    α-Sn:Ga2O3薄膜 9.6 × 10–2 - 10–9 1.4 × 102 1.08 [132]
    α-Sn:Ga2O3薄膜 - - 10–7 4 8.73 [131]
    ε-Sn:Ga2O3薄膜 6.05 × 10–3 3.02 10–9 46.46 - [133]
    β-Sn:Ga2O3薄膜 3.61 × 10–2 - 10–8 19 1.37 [166]
    Zn:Ga2O3薄膜 - - 10–9 2 1.23 [134]
    Er:Ga2O3薄膜 - - 10–9 2.5 1.6 × 10–1 [76]
    Au NPs/Ga2O3薄膜 102 - 10–6 > 2 × 102 - [139]
    Ga2O3/p-Si异质结 3.7 × 102 1.8 × 105 10–8 9.4 × 102 1.8 [143]
    Ga2O3/ZnO异质结 3.5 × 10–1 1.7 × 102 10–10 1.5 × 101 6.2 × 10–1 [144]
    Ga2O3/NSTO异质结 4.3 × 101 2.1 × 104 10–6 2 × 101 7 × 10–2 [142]
    Ga2O3/Ga:ZnO异质结 7.6 × 10–4 - 10–9 2.6 × 102 2.7 × 10–1 [145]
    p-Si/i-SiC/n-Ga2O3 - - 10–8 5.4 × 103 - [148]
    石墨烯/Ga2O3/SiC 1.8 × 10–1 - 10–5 6.3 × 101 1.7 [149]
    石墨烯/Ga2O3/石墨烯 9.66 - 10–9 8.3 × 101 0.96 [138]
    Ga2O3/SiC/Al2O3 - - 10–9 7.7 - [141]
    Ga2O3/Al2O3 1.4 - 10–7 9.04 1.26 [140]
    Ga2O3/SiC异质结 7 × 10–2 - 10–10 - 9 × 10–3 [121]
    Ga2O3/GaN异质结 5.4 × 10–2 - 10–6 1.5 × 102 8 × 10–2 [146]
    Sn:Ga2O3/GaN异质结 3.05 - 10–11 104 1.8 × 10–2 [147]
    α-Ga2O3/ZnO异质结 1.1 × 104(–40 V) - 10–12 - 2.4 × 10–4 [172]
    Ga2O3/金刚石异质结 2 × 10–4 - 10–9 3.7 × 101 - [179]
    DownLoad: CSV
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  • Received Date:  15 October 2018
  • Accepted Date:  30 January 2019
  • Available Online:  06 June 2019
  • Published Online:  01 April 2019

Ultra-wide bandgap semiconductor of β-Ga2O3 and its research progress of deep ultraviolet transparent electrode and solar-blind photodetector

    Corresponding author: Tang Wei-Hua, whtang@bupt.edu.cn
  • 1. Center for Optoelectronics Materials and Devices, Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, China
  • 2. Laboratory of Information Functional Materials and Devices, School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, China
  • 3. State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing 100876, China

Abstract: Gallium oxide (Ga2O3), with a bandgap of about 4.9 eV, is a new type of ultra-wide bandgap semiconductor material. The Ga2O3 can crystallize into five different phases, i.e. α, β, γ, δ, and ε-phase. Among them, the monoclinic β-Ga2O3 (space group: C2/m) with the lattice parameters of a = 12.23 Å, b = 3.04 Å, c = 5.80 Å, and β = 103.7° has been recognized as the most stable phase. The β-Ga2O3 can be grown in bulk form from edge-defined film-fed growth with a low-cost method. With a high theoretical breakdown electrical field (8 MV/cm) and large Baliga’s figure of merit, the β-Ga2O3 is a potential candidate material for next-generation high-power electronics (including diode and field effect transistor) and extreme environment electronics [high temperature, high radiation, and high voltage (low power) switching]. Due to a high transmittance to the deep ultraviolet-visible light with a wavelength longer than 253 nm, the β-Ga2O3 is a natural material for solar-blind ultraviolet detection and deep-ultraviolet transparent conductive electrode. In this paper, the crystal structure, physical properties and device applications of Ga2O3 material are introduced. And the latest research progress of β-Ga2O3 in deep ultraviolet transparent conductive electrode and solar-blind ultraviolet photodetector are reviewed. Although Sn doped Ga2O3 thin film has a conductivity of up to 32.3 S/cm and a transmittance greater than 88%, there is still a long way to go for commercial transparent conductive electrode. At the same time, the development history of β-Ga2O3 solar-blind ultraviolet photodetectors based on material type (nanometer, single crystal and thin film) is described in chronological order. The photodetector based on quasi-two-dimensional β-Ga2O3 flakes shows the highest responsivity (1.8 × 105 A/W). The photodetector based on ZnO/Ga2O3 core/shell micron-wire has a best comprehensive performance, which exhibits a responsivity of 1.3 × 103 A/W and a response time ranging from 20 ${\text{μ}}{\rm{s}}$ to 254 nm light at –6 V. We look forward to applying the β-Ga2O3 based solar-blind ultraviolet photodetectors to military (such as: missile early warning and tracking, ultraviolet communication, harbor fog navigation, and so on) and civilian fields (such as ozone hole monitoring, disinfection and sterilization ultraviolet intensity monitoring, high voltage corona detection, forest fire ultraviolet monitoring, and so on).

    • 硅由于具有良好的热性能与机械性能、在自然界中储量丰富、价格低廉、可以制备大尺寸高纯度的晶圆片等优势, 自20世纪50年代开始, 就作为第一代半导体的代表在微电子领域占据着不可替代的重要地位. 随着集成度的提高, 器件进一步微型化, 硅的缺陷也逐渐暴露出来, 硅的禁带宽度窄、击穿电场较低, 很难达到在高频、高功率器件和光电子方面应用的要求. 20世纪90年代, 以磷化铟、砷化镓、硅锗为代表的第二代半导体材料引起了科研人员的关注, 第二代半导体材料比硅具有更高的电子迁移率、更大的禁带宽度、更特殊的光电性能、更适用于高速高频高温大功率的电子器件. 近年来出现了碳化硅、氮化镓、氮化铝、硒化锌、氧化锌、氧化镓等禁带宽度Eg大于2.3 eV的第三代半导体材料, 相比前两代半导体材料, 这类材料的带隙大、击穿电场强度高、饱和电子漂移速度快、热导率大、介电常数小、抗辐射能力强, 具有良好的化学稳定性, 非常适合用来研制抗辐射、高频、大功率与高密度集成的半导体器件[1]. 氧化镓(Ga2O3)的禁带宽度为4.2—5.3 eV (不同晶体结构, 光学各向异性表现为不同的带隙), 是一种直接带隙的Ⅲ–VI族宽带隙半导体材料, 具有优良的化学和热稳定性, 是一种颇为看好的新型第三代半导体材料. 近年来, 特别是高质量2英寸Ga2O3单晶的成功获得以来, Ga2O3材料受到各国科研人员的高度关注, Ga2O3及相关材料专题国际研讨会也已举办过两次, 分别于2015年11月在日本京都大学及2017年9月在意大利帕尔玛大学成功举办. 国内, 南京大学郑有炓院士在2017年全国第二届宽禁带半导体学术会议上指出Ga2O3是宽禁带半导体未来的四大发展方向之一, 同时Ga2O3材料已被写入国家重点研发计划战略性先进电子材料及国家自然科学基金工程与材料科学部的申请指南中, 已然成为研究热点. 2019年初, 西安电子科技大学郝跃院士指出Ga2O3将致力于为人类提供更高效的生活.

    2.   Ga2O3材料的基本性质及器件应用前景
    • Ga2O3有五种同分异构体, 分别为α, β, γ, ε和δ[2-4]. α-Ga2O3属于三方晶系(Trigonal), 空间群为R-3c, 晶格常数为a = b = 4.98 Å, c = 13.43 Å, α = β = 90°, γ = 120°. 在A2O3结构中(其中A为金属), 以刚玉结构α相最为常见, 如: α-Al2O3, α-In2O3, α-Fe2O3, α-Cr2O3等, 而这些化合物往往具有丰富的物理性质, 可以结合α-Ga2O3和刚玉结构α-A2O3各自的优点[5-10], 制备特殊功能的连续固溶体化合物[5,11,12]. γ-Ga2O3属于立方晶系(Cubic), 空间群为Fd-3m, 晶格常数为a = b = c = 8.24 Å, α = β = γ = 90°, 属于有缺陷的尖晶石结构[13-17]. ε-Ga2O3属于六角晶系(Hexagonal), 空间群为P63mc, 晶格常数为a = b = 2.90 Å, c = 9.26 Å, α = β = 90°, γ = 120°[18,19], 晶体结构如图1所示. 在这些同分异构体中, β-Ga2O3最稳定, 其他相均为亚稳相, 这些亚稳相在一定的温度下都能转变为稳定的β相Ga2O3[4]. 例如: α-Ga2O3在温度600℃以上(干燥条件下)就能转变为β-Ga2O3, 而γ, ε和δ相则分别在650℃(干燥), 870℃(干燥)和300℃(湿)的温度条件下转变成β相[4], 如图2所示. 当然它们的逆过程同样也是可以实现的, 一般都是通过施加高压, 如β-Ga2O3在4 GPa的外力作用下可以转变为α-Ga2O3, 而α-Ga2O3在37 GPa的外力作用下则可以转变为ε-Ga2O3[20]. 常温常压下β-Ga2O3最稳定, 目前制备出的Ga2O3薄膜也以β相居多, 后面结构和性质的介绍也都以β-Ga2O3为主展开.

      Figure 1.  Crystal structures of several isomers of Ga2O3

      Figure 2.  Interconversion relation of Ga2O3 isomers[4]

      β-Ga2O3属于单斜晶系(Monoclinic), 空间群为C2/m, 晶格常数a = 12.23 Å, b = 3.04 Å, c = 5.80 Å, α = γ = 90°, β = 103.8°[21-23], 图3给出了β-Ga2O3单胞结构.β-Ga2O3的晶体结构为阴离子密堆积结构, Ga有两种不同的位置, 分别被O原子包围构成正四面体和正八面体, O则有三种不同的位置[24].

      Figure 3.  Crystal structure and lattice constant of β-Ga2O3[2123]

    • 表1列出了β-Ga2O3与主流半导体材料的基本物理性质[25], β-Ga2O3的摩尔质量为187.44 g/mol, 密度为5.88 g/cm3, 熔点为1740℃, 激子束缚能为30—40 meV, 介电常数ε为10. β-Ga2O3的带隙相对较宽, 具有光学各向异性的特点, 沿着不同的晶面带隙会有所差别, 变化范围为4.2—4.9 eV, 对应波长295—253 nm, 对紫外和可见光区都具有很高的透过率. 对于光学带隙较宽的半导体材料, 理想化学配比的β-Ga2O3材料理论上应是高绝缘体. 但在β-Ga2O3的生长制备过程中, 往往会无意间引入氧空位、Ga空位或Ga间隙原子等点缺陷, 获得偏离理想化学配比的氧化镓. 氧空位往往会在距离导带底大约0.04 eV的位置形成浅施主能级, 使得非故意掺杂的β-Ga2O3呈现出n型半导体性质, 氧空位的引入及浓度与制备方法和过程密切相关[26,27]. β-Ga2O3不仅具有光学各向异性, 而且具有电学各向异性, 在β-Ga2O3晶体结构中八面体Ga3+沿着b轴排列, 而八面体Ga3+组成的长链被认为载流子的传输通道, 因此沿着b轴的迁移率比较大[24].

      材料 Si GaAs GaP 4H-SiC ZnO GaN ß-Ga2O3 Diamond AlN MgO
      带隙Eg/eV 1.1 1.43 2.27 3.3 3.35 3.4 4.2—4.9 5.5 6.2 7.8
      迁移率${\text{μ}}$/cm2·Vs–1 1400 8500 350 1000 200 1200 300 2000 135
      击穿电场强度Eb/MV·cm–1 0.3 0.6 1.0 2.5 3.3 8 10 2
      相对介电常数ε 11.8 12.9 11.1 9.7 8.7 9 10 5.5 8.5 9.9
      导热率/W·cm–1·K–1 1.5 0.55 1.1 2.7 0.6 2.1 0.23[010] 0.13[100] 10 3.2
      巴利加优值/$\varepsilon {\text{μ}} {E_{\rm{b}}}^3$ 1 15 340 870 3444 24664

      Table 1.  Comparison of basic physical properties of β-Ga2O3 with mainstream semiconductor materials[25]

    • 由于β-Ga2O3材料具有优异的物理性能, 用相对较低的成本制备高质量的单晶对于工业化生产具有重要的意义. 相对于SiC和GaN来说, β-Ga2O3最突出的特征是可以采用高温熔体技术直接生长大尺寸高质量的单晶, 且具有晶体生长速度快、可实时观察、可采用缩颈工艺降低缺陷密度等优势[28-30]. 另外, 该晶体不需要使用像SiC及GaN那样的高温高压生长环境、系统集成和自动化控制(包括电路、气路等)复杂的晶体生长设备, 有助于降低设备成本. 目前, 几种熔融生长的技术已经成功地制备出了大尺寸的单晶, 主要有火焰法、光学浮区法、竖直布里奇曼/竖直梯度凝固、导模法、柴可拉斯基法等[28-30]. 当然还有一些其他生长晶体的方法, 比如助熔剂法, 气相沉积法等, 但是这些方法不太适用于大尺寸的单晶生长. 导模法已经成熟地应用于Al2O3单晶的生长, 由于Al2O3与Ga2O3类似, 很容易将生长Al2O3单晶的技术转移到Ga2O3单晶生长上. 目前利用该方法, 日本科学家已经制备出2英寸可商业化的单晶基片, 能够稳定制备4英寸的基片, 也制备出了6英寸的展示基片[29]. 国内最早报道Ga2O3单晶生长的单位是中国科学院上海光机所, 2006年他们采用浮区法成功制备出1英寸的Ga2O3单晶[30]. 近些年来, Ga2O3材料受到关注, 国内关于Ga2O3晶体生长的研究工作也越来越多, 2016年, 山东大学报道了采用导模法制备出1英寸的Ga2O3单晶[28,29]. 2017年, 媒体报道了同济大学与中科院上海硅酸盐研究所合作, 利用导模法成功制备出2英寸的Ga2O3单晶[28-30]. 2017年, 中国科学院安徽光学精密机械研究所采用提拉法, 制备出直径30 mm的Ga2O3单晶[28-30]. 天津中国电科46所利用导模法可以生长出(100), (010), (001), $\left( {\bar 201} \right)$面大于2英寸的β-Ga2O3单晶[28-30].

    • 相比于其他主流半导体材料, β-Ga2O3表现出诸多独特的物理性质, 决定了其在器件方面具有重要的应用前景, 如图4所示.

      Figure 4.  Physical properties and device applications of β-Ga2O3 material

      1) β-Ga2O3是一种有前景的深紫外透明导电氧化物薄膜电极材料.β-Ga2O3拥有约4.9 eV的超大带隙, 具有优异的化学和热稳定性以及高的紫外可见光透过率, 同时通过掺杂容易获得良好的N型导电(通过Sn, Si等施主杂质掺杂, 其载流子浓度可控制在1015—1019 cm–3的大范围内)[31], 可以同时满足透明导电电极所需的良好电导率和高光学透过率的要求. 相对于目前广泛应用的透明导电氧化物电极(如ITO, FTO, AZO等), 基于β-Ga2O3的透明导电电极具有一个明显的优势, 即具有高的紫外光透光率, 这将会增加器件对紫外光的利用.

      2) β-Ga2O3材料非常适合用于制作日盲紫外探测器. 由于臭氧层的吸收, 日盲波段(200—280 nm)的深紫外光在大气层中几乎是不存在的, 工作在该波段的日盲紫外探测器具有虚警率低的特点, 在航空和军事等方面具有重要的应用. 虽然对现有宽禁带半导体进行掺杂可实现对带隙的调控, 使其工作在日盲波段, 但要想获得高质量的化合物薄膜却非常艰难. 例如, 生长AlGaN薄膜往往需要极高温且难以外延成膜[32], 而ZnMgO在单晶纤维锌矿的结构下很难保持超过4.5 eV的带隙[33]. 带隙为4.9 eV的β-Ga2O3对应的吸收波长为253 nm, 是日盲型光电探测器的天然理想材料.

      3) β-Ga2O3在高温、高频、大功率电子器件领域有着广泛的应用前景, 如场效应晶体管等. β-Ga2O3的带隙(约4.9 eV)是Si的4倍多, 也比SiC的3.3 eV及GaN的3.4 eV大很多(表1). 通常情况下, 带隙越大, 击穿电场强度也会越大. 而具有较大击穿电场强度的材料在功率元器件中的性能越好, 根据一些已知的半导体带隙及相应的击穿电场强度对其进行拟合, 可以推测β-Ga2O3具有较大的击穿电场强度, 可达到8 MV/cm, 为商业化半导体Si的20倍以上, 也比常见的宽禁带半导体SiC和GaN高出1倍以上. β-Ga2O3材料除了具有耐高压的特性之外, 还具有一个非常重要的特性就是低功耗. 衡量低损失性的指标为“巴利加优值(Baliga’s figure of merit)”, 其公式为$\varepsilon {\text{μ}}{E_{\rm{b}}}^3$ (其中ε为介电常数, ${\text{μ}}$为迁移率, Eb为电场强度), 计算可得β-Ga2O3的巴加利优值为3444, 是常见SiC (巴加利优值为340)的10倍, 是GaN (巴加利优值为870)的4倍, 具体数值如表1所列. 基于β-Ga2O3耐高压和低损耗的特点, 其在场效应晶体管等高温高频大功率电子器件中具有重要的潜在应用. 近年来, 科学家们在β-Ga2O3场效应晶体管应用方面也做了大量工作, 最具代表性的课题组就是日本信息通信研究机构的Higashiwaki研究小组[25,31,34,35].

      4) β-Ga2O3不仅可用于功率元器件, 而且还可用于LED芯片、各类传感器元件及摄像元件等. 我们知道蓝色等短波长LED芯片是组成白色LED的重要基础部件, 基于GaN的LED芯片基板最被科研人员看好, 已经广泛用于蓝色、紫色及紫外等短波长LED. 目前, GaN的LED芯片都是在Al2O3衬底上制备获得, 而Al2O3具有高绝缘性, 需要采用横向配置阳极和阴极的横向结构.β-Ga2O3基板与Al2O3一样具有高的紫外—可见光透过率(高于80%), LED芯片发出的光能高效率地提取到外部. 但相比于Al2O3基板, β-Ga2O3晶体通过掺杂可以实现高的导电性, 可以在LED芯片表面和背面分别形成阳极和阴极构成垂直结构. 垂直结构相对于基于Al2O3基板的横向结构, 不仅可以使驱动电流均匀地分布, 而且可以大大降低元器件的电阻和热阻, 降低LED芯片的发热量, 可应用于需要大驱动电流的高功率LED. 基于β-Ga2O3基板垂直结构的LED单位面积光输出功率可达到Al2O3基板横向结构产品的10倍以上. 虽然SiC基板也可用于垂直结构的LED衬底, 但其生长成本相对较高, 而β-Ga2O3单晶则有望以更低成本来作为LED基板. 虽然使用β-Ga2O3基板的GaN基LED芯片目前还处在研发之中, 但已经获得了一定成果[36-42]. 比如, 日本信息通信研究机构的研究小组[36]在N型的β-Ga2O3单晶基板上, 采用金属有机化学气相沉积法(MOCVD)按序沉积了N型GaN层、多重量子阱构造的InGaN/GaN活性层以及p型GaN层, 并在β-Ga2O3基板侧和其另一侧分别镀上Ti/Au和Ag电极, 该元器件的大小为300 ${\text{μ}}{\rm{m}}$见方, 在200 mA的驱动电流下工作电压仅为3.3 V, 而该尺寸横向结构的产品在200 mA的驱动电流下工作电压则需要4.7 V.

      5) β-Ga2O3可以制作氧气和其他一些还原性气体的探测器. β-Ga2O3在高温条件下(800—1000℃)对氧气等还原气体较为敏感, 在较低温度(550—700℃)条件下对H2, CO和烷烃类还原性气体敏感, 其电阻率随着氧气、还原性气体浓度的改变而改变, 是一种良好的高温半导体气敏材料[43-55]. 气敏特性起源于气体与β-Ga2O3表面或体内的相互作用, 引起了β-Ga2O3材料电阻的显著变化. 相对于其他气敏材料, β-Ga2O3具有高稳定性、对湿度的低敏感性、快速反应性、自我清洁功能、不易老化等诸多优点, 可以制备火灾报警器(O2气敏传感器)和多种气体的探测器.

      6) Ga2O3作为宽禁带半导体材料, 通过过渡金属元素掺杂可实现室温铁磁性, 是一种稀磁半导体母体材料. 目前基于Ga2O3的磁半导体材料研究还相对较少. 2006年, Hayashi等[56]通过脉冲激光沉积(PLD)方法在蓝宝石衬底上制备了Mn掺杂Ga2O3薄膜并报道了其具有室温铁磁性, 他们得到的是具有尖晶石结构的γ-Ga2O3. 2008年, Pei等[57]利用第一性原理理论上计算了Mn掺杂β-Ga2O3的铁磁性, 并讨论了其中磁的相互作用. 2009年, Kaneko等[5]利用超声雾化气相沉积法在c面(0001)α-Al2O3基底上外延生长了α-(Ga1–xFex)2O3 (x = 0.24)薄膜, 并在110 K下研究了其铁磁性. 2013年, Kaneko等[12]制备了α-(Ga1–xFex)2O3(x = 0.24, 0.44, 0.58, 1.00)异质外延薄膜, 并研究了α-(Ga0.42Fe0.58)2O3的室温铁磁性能. 近年来, 我们课题组[58-60]提出了一种通过Ga2O3和过渡金属(Mn, Fe, Cr)薄层循环沉积经高温层间相互扩散实现掺杂并调控薄膜微结构的技术, 分别获得高浓度均匀掺杂的$\left( {\overline 2 01} \right)$晶面取向的β相(GaMn)2O3外延薄膜、Ga2O3/(Ga1–xFex)2O3多层薄膜和Cr掺杂的Ga2O3纳米蠕虫薄膜, 三种结构薄膜都观察到了室温铁磁性.

      7) 氧化镓作为一种氧化物半导体材料, 在低温真空条件下沉积薄膜会引入大量氧空位, 是一种理想的阻变材料. 目前, 基于氧化镓的阻变存储器也有一些报道[61-67], 我们课题组分别首次报道了基于氧化镓的单极型阻变行为[68]及反常双极型阻变行为[69,70], 也观察到了负微分电阻效应[71]. 虽然对其阻变的微观机理还存在很大争议, 但大家存在一个共识, 即氧空位/氧离子在电场作用下的迁移对阻变起着重要的作用. 例如, Gao等[61]研究了不同的上电极对氧化镓双极型阻变行为的影响, 指出氧空位在电极附近的移动是其双极型阻变行为的起源. Yang等[62]则认为双极型阻变行为来源于氧离子的移动所引起的氧化镓薄膜与下电极处有效厚度的形成和破灭. Aoki等[63]直接观察到了氧离子在氧化镓薄膜和电极界面处的聚集和驱散改变镓离子的价态, 通过观测到的实验现象指出氧离子在薄膜内部是整体移动的, 是一个体效应, 氧离子在薄膜与金属电极界面的聚集和驱散改变了电子在界面处的输运方式是其双极型阻变行为的起源.

      8) Ga2O3通过稀土元素掺杂, 可以制备成高亮度、多色彩的光致/电致发光薄膜器件[72-74], 同时基于Ga2O3的荧光粉具有化学稳定性好的优势, 可以用于荧光灯、场发射显示器、等离子平板显示器等, 我们课题组[75-77]分别通过Er, Nd, Pr掺杂Ga2O3研究了其红外的发光性能.

      当然Ga2O3的应用不仅限于此, 还有众多的领域待我们去研究开发. 下面就Ga2O3在深紫外透明导电电极和日盲紫外探测器中的研究进展做详细介绍.

    3.   Ga2O3在深紫外透明导电电极、日盲紫外探测中应用的研究进展
    • 透明导电薄膜具有在可见光区域透明及电阻率低等优异的光电性能, 被广泛应用在平面液晶显示器、节能视窗、太阳能电池透明电极等光电器件中. 传统透明导电薄膜(如ITO, FTO, AZO等)带隙较小, 全都小于4 eV, 在深紫外光区域(< 300 nm)不透明, 限制了在紫外光电器件的发展, 人们迫切需要带隙较大的深紫外透明导电薄膜材料. β-Ga2O3拥有超宽带隙, 具有高的紫外可见光透过率, 波长大于253 nm的紫外和可见光都能透过, 通过掺杂容易获得良好的N型导电, 同时具有优异的化学和热稳定性, 是一种有前景的深紫外透明导电氧化物薄膜电极材料.

      2000年, 日本科学家Orita等[78]采用PLD技术研究了Sn掺杂Ga2O3薄膜的生长, 本底真空为2 × 10–6 Pa, 靶材为掺杂浓度为1 mol% SnO2的β-Ga2O3陶瓷, 靶间距为30 mm, 氧压的调节范围为6 × 10–5—1.3 × 10–2 Pa, 生长温度为700—880℃, 衬底为SiO2或Al2O3单晶, 到达靶材表面的激光能量密度为5 J/cm2, 激光频率为10 Hz. 文中指出要想获得具有良好导电性和高透过率的薄膜, 需要适当减少氧压增加衬底温度, 最佳薄膜的电导率达到1 S/cm, 载流子浓度为1.4 × 1019 cm–3, 迁移率为0.44 cm2/Vs, 在可见光和近红外波段平均透过率高于80%, 如图5(a)所示. 2002年, Orita等[24]又报道了在低温下制备Sn掺杂的β-Ga2O3深紫外透明导电电极, 同样采用PLD方法, 此处的生长条件为本底真空为7 × 10–7 Pa, 靶材为掺杂浓度为5 mol% SnO2的β-Ga2O3和纯的β-Ga2O3陶瓷靶, 氧压为5 × 10–6 Pa, 靶间距为25 mm, 衬底温度为300—600 ℃, 衬底为Al2O3单晶, 到达靶材表面的激光能量密度为3.5 J/cm2, 激光频率为1 Hz. 实验结果表明, 当衬底温度为350 ℃以下时生长制备的薄膜为非晶, 而在380—435 ℃时获得沿着$\left( {\overline 2 01} \right)$晶面族择优生长的β-Ga2O3, 而当温度再高时将发生相变, 生长的薄膜为ε-Ga2O3, 随即导电性也明显下降, 如图5(b)所示. 最佳薄膜的电导率达到8.2 S/cm, 在可见光区和近红外波段平均透过率超过80%, 该薄膜是在衬底温度为380℃激光频率为1 Hz的条件下生长制备的.

      Figure 5.  (a) The transmittance of Sn-doped-Ga2O3 thin films prepared at different temperatures[75]; (b) the relationship between the conductivity of Sn doped -Ga2O3 thin films and the deposition temperature[24]

      2007年, Suzuki等[79]通过浮区法生长了Sn掺杂的β-Ga2O3单晶, 生长温度为1500 ℃, 最佳单晶的电阻率为4.27 × 10–2 Ω·cm, 载流子浓度为2.26 × 1018 cm–3, 迁移率为64.7 cm2/Vs, 在可见光和紫外光区的透过率高于85%.

      2012年, Ou等[80]采用PLD方法研究了不同衬底生长温度对Ga2O3薄膜晶体结构、表面形貌、导电性及透过率等性能的影响. 结果表明, 随着衬底温度的增加, 光学带隙、晶粒尺寸、表面粗糙度等逐渐增加, 制备深紫外透明导电电极的最佳生长温度是1000 ℃, 该薄膜电导率可达到7.6 S/cm, 在400—1000 nm波长范围内平均透过率高于85%.

      2015年, 山东大学Du等[81] 报道了采用MOCVD实现β-Ga2O3薄膜的同质外延生长, 研究了不同浓度Sn掺杂(1%—12%原子比)对薄膜结构和拉曼、光学、电学性质的影响. 通过Sn掺杂, β-Ga2O3薄膜的电阻下降8个数量级, 在Sn掺杂浓度为10 %时获得了应用于透明导电电极的最佳薄膜, 其电阻率为1.20 × 10–1 Ω·cm, 迁移率为12.03 cm2/Vs, 在可见光和紫外光区的透过率超过85%, 该薄膜是在衬底温度为700 ℃, 氧压为20 Torr (1 Torr ≈ 133.322 Pa)条件下生长的. 同时, Sn掺杂可以实现带隙在4.16—4.69 eV范围内的调控, 如图6(a)所示. 同年, Mi等[82]同样采用MOCVD法在(100)面MgAl2O4衬底上生长不同浓度Sn掺杂的β-Ga2O3薄膜, 实验结果表明, 在Sn掺杂浓度较低时薄膜表现为沿着(400)择优生长, 随着Sn掺杂浓度的增加, 薄膜的结晶质量降低, 表现为微晶, Sn掺杂使β-Ga2O3薄膜的电阻降低7个数量级, 在掺杂浓度为10%时电阻率最小(见图6(b)), 约为3.1 × 10–2 Ω·cm, 载流子浓度为2.4 × 1020 cm–3, 迁移率约为0.74 cm2/Vs, 在可见光波段的平均透过率为88%.

      Figure 6.  The relationship of the transmittance (a)[81], the band gap (a)[81], the resistivity (b)[82] with Sn different doping concentration in Sn-doped Ga2O3 thin films

      除掺杂Sn之外, Ga2O3还与其他材料形成多组分或多层结构用于透明导电电极, 例如, 1997年, Minami等[83]采用直流磁控溅射制备了Ga/In固溶体(Ga, In)2O3薄膜, 其最小电阻率为5.8 × 10–4 Ω·cm, 最大载流子浓度可达5 × 1020 cm–3, 可见光的透过率高达95 %以上; 2014年, Kim等[84]利用射频磁控溅射生长了Ga2O3/ITO薄膜, 并研究了不同气氛中(H2, N2和空气)不同温度下退火对该薄膜电学和光学性能的影响, 实验结果表明H2气氛中700℃下退火制备的薄膜表现出最佳的性能, 其面电阻为164 Ω/sq, 光学透过率高达94%, 跟P型GaN衬底形成Ohmic接触; 2014年, Kim等[85]生长并对比了ITO和Ga2O3:ITO薄膜的性能, ITO薄膜的面电阻为52 Ω/sq, 对波长为405 nm紫外光的透过率为84.4 %, 而Ga2O3:ITO薄膜的面电阻为49 Ω/sq, 对405 nm的透过率增加至93.8 %, 将两者用于近紫外LED的透明电极时, 以Ga2O3:ITO薄膜为上电极的器件的出射光强较ITO薄膜为上电极的器件高55%, 如图7所示. 2014年, Woo等[86]报道了Ag/Ga2O3双层薄膜在空气和N2中550℃下退火1 min后会由退火前的非Ohimc接触转变到Ohimc接触, 其接触电阻分别为3.06 × 10–2—7.34 × 10–2 Ω/cm2, 面电阻小于42 Ω/sq, 波长为380 nm处的透过率约为91%, 该材料可作为近紫外LED的P型透明导电电极; 2014年, Zhuang等[87]采用磁控溅射在柔性衬底PET上室温下生长了Ga2O3/Cu/ITO透明导电薄膜, 并研究了Ga2O3和Cu层的厚度对该薄膜光学及电学性能的影响, 在Ga2O3薄膜厚度为15 nm、Cu层厚度为3.7 nm时, 该薄膜获得最大的透过率, 为86%, 对应的面电阻为50 Ω/sq.

      Figure 7.  (a) Current versus light output power and forward voltage (L-I-V) characteristic curves and (b) typical electroluminescence spectra measured for near-ultraviolet LEDs with Ga2O3:ITO and ITO transparent conducting electrodes; the inset shows top-view SEM image of near-ultraviolet[85]

      表2总结归纳了基于Ga2O3材料的透明导电电极的各项参数指标. 从表中可知, Minami等[83]获得的Ga/In固溶体(Ga, In)2O3薄膜具有最大的电导率(为1.72 × 103 S/cm), 但其对光透过截止波长增大至380 nm, 不再具有深紫外光透过的优势, 而掺Sn的Sn:Ga2O3薄膜电导率可以达到32.3 S/cm, 通过金属Ag层的复合可以获得42 Ω/sq的面电阻; 通过Sn, In等施主杂质掺杂, 载流子浓度可控制在1018—1020 cm–3的范围内; Sn掺杂的Sn:Ga2O3单晶具有最大的迁移率, 为64.7 cm2/Vs; 所有电极的透过率均在80%以上, 最高可大于95 %.

      薄膜类型 电导率/S·cm–1 面电阻/Ω·sq–1 载流子浓度/cm–3 迁移率/cm2·V–1·s–1 透过率/% 参考文献
      Ga2O3薄膜 7.6 - - - 85 [80]
      Sn:Ga2O3薄膜 1 - 1.4 × 1019 0.44 80 [78]
      Sn:Ga2O3薄膜 8.2 - - < 0.44 80 [24]
      Sn:Ga2O3薄膜 8.3 - - 12.03 85 [81]
      Sn:Ga2O3薄膜 32.3 - 2.4 × 1020 0.74 88 [82]
      Sn:Ga2O3单晶 23.4 - 2.3 × 1018 64.7 85 [79]
      (Ga, In)2O3薄膜 1.72 × 103 - 5 × 1020 - > 95 [83]
      Ga2O3/ITO薄膜 - 164 - - > 94 [84]
      Ga2O3/ITO薄膜 - 49 - - 93.8 [85]
      Ag/Ga2O3薄膜 - 42 - - 91 [86]
      Ga2O3/Cu/ITO - 50 - - 86 [87]

      Table 2.  Parameters and indicators of Ga2O3-based transparent conductive electrode films

    • 由于臭氧层的吸收, 在地球表面几乎不存在波长介于200—280 nm的深紫外光, 该波段的光称为日盲紫外光, 针对该波段的信号探测被称为日盲紫外探测. 由于不受太阳光背景的影响, 日盲紫外光信号探测灵敏度极高, 工作在此波段的通信准确率也极高, 在军事及航天航空等方面有广泛的应用, 加之红外对抗技术日趋成熟, 红外制导导弹的命中精度已受到严重威胁, 紫外通信特别是日盲紫外通信俨然已经成为各国军事竞赛的重点目标. 相对于其他几种无线通信, 日盲紫外通信除了上述提到的准确率极高外, 还有以下几个方面的优势: 1)非视距通信. 紫外通信依靠大气层的漫反射进行光信息的传输, 信号发射台和信号接收台无需在视距范围内, 是一种非视距通信; 2)安全性高. 在紫外通信中, 紫外光信号发射出来后, 要在大气层中经历无数个漫反射才能达到接收台, 如此以来, 即使敌方探测到信号也很难定位发射台的位置, 无法找出并摧毁它, 因此日盲紫外通信的安全性极高; 3)传播距离可控, 抗干扰、防窃听能力强. 大气层利用漫反射传输紫外光信号, 同时大气层对紫外光的吸收也是极强的, 利用这一特性可以控制紫外通信的距离. 例如, 若想控制紫外通信的范围在10 km范围之内, 只要调节发射台的紫外光信号强度, 使其光信号覆盖在10 km范围之内; 若为了减少被敌方探测到的概率, 想控制紫外通信的范围在1 km范围之内, 则适当降低发射台的紫外光信号强度, 使其光信号覆盖在1 km范围之内即可. 因此, 紫外通信的传播距离是可以调控的, 抗干扰、防窃听能力较强; 4)受环境影响很小, 可全天候工作. 由于日盲紫外通信工作在200—280 nm的日盲紫外区, 该波段在地球表面几乎不存在, 因此不管是白天还是黑夜, 日盲紫外通信可全天候工作, 不受可见光和红外光等其他波段光的影响. 表3给出了日盲紫外通信和其他几种无线通信的比较.

      通信类别 非视距通信 抗干扰、防窃听 相对运动信号接收 传播距离调控 受环境气候时间影响
      无线电通信 易被干扰和窃听 很差 受环境影响
      激光通信 抗干扰、防窃听 较差 受环境影响
      红外通信 较易干扰、防窃听 较差 受环境时间影响
      紫外通信 抗干扰、防窃听 很好 很小、全天候

      Table 3.  Comparison of several wireless communications

      当然, 除了在日盲紫外通信中的应用之外, 日盲紫外光电探测器还有其他方面的应用, 如国防预警与跟踪、生命科学、高压线电晕、臭氧层检测、气体探测与分析、火焰传感等. 然而目前市场上的紫外探测器都为真空紫外探测器件, 与之相比, 基于半导体材料的固态紫外探测器件具有体重小、功耗低、量子效率高、便于集成等特点近年来, 已经成为科研人员的研究热点. 日盲紫外探测器核心材料的禁带宽度往往要大于4.4 eV, 目前研究比较多的材料集中在AlGaN, ZnMgO和金刚石上[88-90]. 但AlGaN薄膜需要极高温生长并难以外延成膜, ZnMgO在单晶纤维锌矿的结构下很难保持超过4.5 eV的带隙, 金刚石具有固定的5.5 eV的带隙, 对应波长225 nm, 只占据日盲紫外波长的一小段. 而Ga2O3的禁带宽度约为4.9 eV, 对应波长253 nm, 且易于与Al2O3和In2O3形成连续固溶体实现日盲区的完全覆盖[90], 是一种天然的日盲紫外探测器材料. 以下将从Ga2O3的不同型态(纳米材料、单晶、薄膜)以论文发表的时间顺序来叙述Ga2O3基日盲探测器的最新研究进展.

    • 2006年, 中国科学院物理研究所Feng等[91]采用蒸发法在表面有10 nm Au的Si衬底上以N2气为载体在980 ℃下生长1 h获得β-Ga2O3纳米线. 器件的制备过程如下: 先对Si衬底进行热氧化处理, 生长一层约500 nm厚的SiO2绝缘层, 再在其上沉积厚度约为50 nm的Au电极, 最后将单根纳米线放于两个Au电极之上, 构成Au-Ga2O3-Au光电探测器结构, 如图8(a)插图所示. 该器件的暗电流非常小, 为pA级别, 在254 nm紫外光的光照下, 电导率有3个数量级的上升, 响应速度较快, 上升沿和下降沿的时间分别为0.22 和0.09 s, 如图8所示.

      Figure 8.  Au-Ga2O3 nanowire-Au photodetector: (a) I-V characteristic curve of the detector in dark. The inset of is a typical SEM image of the device, the scale bar: 200 nm; (b) real-time photoresponse of the detector to 254 nm light[91]

      2010年, Li等[88]采用一步化学气相沉积法制备了β-Ga2O3纳米线桥式结构, 具体过程如下: 在石英衬底上溅射一对间距为100 ${\text{μ}}{\rm{m}}$, 厚度为2 nm的Au电极, 以Ar和O2混合气为载体, Ga2O3粉末和石墨为原材料, 在不同的衬底温度下生长并制备桥式结构的β-Ga2O3纳米线探测器. 该探测器光暗比达3 × 104, 在254 nm光下的衰减时间τ < 20 ms, 探测器的截止波长约280 nm, 具有日盲区响应的特征, 如图9所示.

      Figure 9.  Solar blind photoelectric properties of photodetector based on the bridged β-Ga2O3 nanowires: (a) Schematic diagram of the devices; (b) time-dependent photoresponse of the bridged β-Ga2O3 nanowires measured in dry air under UVC (~2 mW cm–2 at 254 nm) illumination with a period of 60 s at a bias voltage of 50 V; (c) I-V characteristics of the bridged β-Ga2O3 nanowires in dark (squares), under 365 nm light (triangles), and under 254 nm light (circles). The I-V curve measured under 254 nm light is plotted on a linear scale in the inset; (d) spectral response of the bridged β-Ga2O3 nanowires revealing that the device is blind to solar light. The dashed line indicates the lowest wavelength of the solar spectrum on Earth[88]

      2010年, Weng等[92]通过气液固的方法直接氧化GaN薄膜得到多晶Ga2O3纳米线, 过程如下: 先采用MOCVD的方法在蓝宝石衬底上沉积一层较厚的GaN薄膜, 用稀盐酸去除表面的自然氧化层, 采用电子束蒸发法蒸镀一层3 nm厚的Au, 在500℃处理使其形成Au纳米颗粒, 最后将其升温至1100 ℃, 在O2气氛中生长2 h获得直径约为100 nm, 长度约为10 ${\text{μ}}{\rm{m}}$的Ga2O3纳米线. 该探测器采用叉指电极结构, 纳米线与电极形成良好的欧姆接触, 在10 V偏压下暗电流为2.44 × 10–10 A, 截止波长约为255 nm, 响应度为8.0 × 10–4 A/W, 如图10(a)所示. 2013年, Wu等[93]在指宽100 ${\text{μ}}{\rm{m}}$, 指长530 ${\text{μ}}{\rm{m}}$, 间距为5 ${\text{μ}}{\rm{m}}$的Cr/Au电极上采用气相沉积法不同温度下生长Ga2O3纳米线(见图10(b)), 随着衬底温度的增加, 纳米线的平均直径、长度和密度均增加, 并比较了不同温度下生长的纳米线对255 nm光的I–t响应(见图10(c)). 衬底温度为950 ℃的样品, 在5 V偏压下对255 nm波长的光响应为3.43 × 10–3 A/W, 如图10(d)所示.

      Figure 10.  (a) Room-temperature spectral responses of the Ga2O3 nanowires photodetector measured with different applied biases[92]; (b) Ga2O3 nanowire photodetector with Cr/Au as electrodes[93]; (c) transit responses measured from the three fabricated photodetectors grown at different temperatures[93]; (d) room-temperature spectral responses of the photodetector under different bias[93]

      2011年, Li等[94]以Au溶胶为催化剂、金属Ga为原材料、O2气氛为载体采用热蒸发的方法合成Ga2O3纳米带, 通过微加工制备了单条Ga2O3纳米带光电导探测器, 如图11(a)所示. 该器件对波长为250 nm的深紫外光具有很强的光电灵敏性, 光暗比大于4个数量级, 光响应时间小于0.3 s. 器件的参数如光电流、响应时间、量子效率等与光的强度、探测器所处的环境、纳米带的尺寸密切相关. 2012年, Tian等[95]以同样的方法合成了In掺杂的Ga2O3纳米带, 并对其材料进行了详细表征, In:Ga2O3纳米带光电探测器展现出高的灵敏度((9.99 × 104) %), 高的光响应度(5.47 × 10 2 A/W), 高的量子效率((2.72 × 105) %), 快的响应时间(上升沿和下降沿分别为1和0.6 s), 比纯的Ga2O3纳米带光电探测器具有更好的光电性能, 如图11(c)图11(d)所示.

      Figure 11.  (a) SEM image of a Ga2O3 individual-nanobelt device[94]; (b) spectral response of the devices (nanobelts with different widths of 800 nm and 1.6 mm) measured at a bias of 15 V. The schematic configuration of a photoconductive measurement is inserted in the top-right corner[94]; (c) spectral response of an individual In-doped Ga2O3 nanobelt photodetector. The inset is a typical SEM image of an individual In-doped Ga2O3 nanobelt device[95]; (d) logarithmic plot of I-V curves of the individual Ga2O3 and In-doped Ga2O3 nanobelt photodetector under illumination with the 250 nm wavelength light and in dark conditions[95]

      2014年, Feng等[96]采用热氧化GaSe纳米片的方法制备了二维Ga2O3纳米片, 纳米片为多晶, 厚度小于10 nm. 基于Ga2O3纳米片的光电探测器对254 nm的深紫外光具有快速的响应, 响应度为3.3 A/W, 外量子效率为1600%.

      2014年, Teng等[97]采用水热法制备了亚稳相的γ-Ga2O3纳米花, γ亚稳相的获得跟溶液中的PH值密切相关, 以γ-Ga2O3纳米花为材料的探测器展现出光暗比大、响应速度快等优异的日盲光电性能, 如图12所示.

      Figure 12.  (a) SEM image of Ga2O3 nanoflowers; (b) I-t response curve of Ga2O3 nanoflowers to 254 nm light[97]

      2014年, Zou等[98]以GaN粉末为原材料合成了沿(100)晶面生长的多层β-Ga2O3纳米带, 制备的探测器具有极低的暗电流(小于探测极限10–14A), 较高的光电流(> 21 nA), 快速的响应速度(< 0.3 s), 高的响应度(≈ 851 A/W), 高的外量子效率(≈ 4.2 × 103), 并能在433 K的高温下稳定工作.

      2015年, Zhong等[99]采用CVD方法制备了单晶的β-Ga2O3纳米带, 基于该纳米带的探测器对254 nm光的响应速度小于20 ms, 响应度19.31 A/W, 外量子效率为9427 %.

      2015年, Zhao等[100]采用CVD生长方法一步制备了以ZnO为核层、Ga2O3为壳层的ZnO/Ga2O3核/壳结构微米线, 核层ZnO和壳层Ga2O3都为高质量的单晶, 两种材料的界面非常陡峭, 并没有发现明显界面缺陷和位错. 分别在ZnO上镀In及在Ga2O3上镀Ti/Au电极, 制备获得ZnO/Ga2O3核/壳结构的日盲紫外探测器件, 如图13(a)所示. 器件的响应峰值在254 nm, 响应截止边为266 nm, 对日盲紫外光具有高灵敏度、高探测度、高量子效率和高速的响应. 在–6 V的电压驱动下, 器件的明暗电流比可以达到106以上(见图13(b)), 对254 nm的光响应度可达到1.3 × 103 A/W (见图13(c)), 探测率为9.91 × 1014 cm·Hz1/2/W, 同时响应时间小于20 ${\text{μ}}{\rm{s}}$, 该器件具有明显的雪崩增益, 其增益高达104, 主要性能高于目前商业Si雪崩二极管. Zhao等[101]于2017年报道了ZnO/Ga2O3核/壳结构日盲紫外探测器的自供电特性, 在0 V偏压下该器件对251 nm日盲区深紫外光的光响应度为9.7 mA/W (见图13(d)), 光响应上升时间和衰减时间分别为100和900 ${\text{μ}}{\rm{s}}$ (见图13(e)), 相比于之前报告的自供电日盲紫外探测器表现出更为优异的性能. 2016年, Chen等[102]以金属Ga为原材料采用简单的热氧化法生长Ga2O3纳米线阵列, 并在其上沉积一层约20 nm的Au, 制备获得了Au/Ga2O3纳米线肖特基型垂直结构的光电探测器, 如图13(f)所示. 该器件的光响应度截止波长约为270 nm, 在258 nm左右取得最大光响应值, 在偏压为–10 V时对应的光响度为0.6 mA/W (见图13(g)), 同时该器件具有较低的暗电流(偏压在–30 V时的暗电流为10 pA), 快速的响应时间(光衰减时间约为64 ${\text{μ}}{\rm{s}}$, 见图13(h)), 并具有自供电的特性.

      Figure 13.  Solar-blind ultraviolet photodetector based on Single ZnO-Ga2O3 core-shell microwire ZnO/Ga2O3 core-shell: (a) Device schematic diagram; (b)I-V characteristic curve in dark and under 254 nm light; (c) spectral response of the device at −6 V bias[100]; (d) the photoresponse spectrum of the device at 0 V; (e) the time response under the excitation of 266 nm pulse laser at 0 V[101]. Au/Ga2O3 nanowire Schottky vertical structure photodetector: (f) device schematic diagram; (g) spectral responses of the device at zero bias and under reverse bias of 10 V. Inset shows the responsivity of photodetectors at the wavelength of 254 nm as a function of reverse bias; (h) decay edge of the current response at reverse bias of 10 V[102].

      2016年, Oh等[103]通过机械剥离方式从Ga2O3晶体剥离获得准二维β-Ga2O3小薄片(厚度约为400 nm), 并将其转移到SiO2(300 nm)/p+-Si衬底上蒸镀Cr/Au (30 nm/70 nm)电极, 获得背栅场效应光晶体管, 如图14(a)所示. 在–30 V栅电压作用下, 该器件对254 nm的光响应度高达1.8 × 105 A/W, 在0 V情况下, 光响应度也能达到9.17 × 104 A/W. 考虑到机械剥离下来的β-Ga2O3薄片厚度较厚且厚度不可控, Kwon等[104]采用反应离子刻蚀技术对其进行减薄(见图14(e)), 调控β-Ga2O3薄片的厚度, 并在Al2O3衬底上蒸镀Ti/Au电极研究其日盲紫外光电特性. 同时, Oh等[105]采用Ni/Au电极与β-Ga2O3薄片构成肖特基, 制作了MSM结构的日盲紫外探测器, 该器件的暗电流为2.8 × 10–13A, 光暗比为1.92 × 103, 光响应度能达到1.68 A/W, 如图14(f)图14(g)所示. 最近, Oh等[106]改用紫外光透明的石墨烯替代传统金属做电极, 制成石墨烯/β-Ga2O3薄片/石墨烯MSM结构(图14(h)图14(i)), 在10 V偏压下, 对254 nm的光响应度为29.8 A/W, 光暗比为104, 对254和365 nm的光响应度抑制比R254 nm/R365 nm为9.5 × 103, 探测率为1.45 × 1012 J, 相比于采用Ni/Au电极, 光电性能大幅提升.

      Figure 14.  Solar-blind ultraviolet photodetector based on β-Ga2O3 flake: (a) Schematic of the entire exfoliated β-Ga2O3 flake based photodetector fabrication process; (b) optical image of the fabricated photodetector; (c) time-dependent photoresponse of the fabricated photodetector under various illumination conditions (254, 365, 532 and 650 nm light exposure); (d) responsivity as a function of wavelength[103]; (e) the reactive ion etching assisted thinning of a β-Ga2O3 flake[104]; (f) the I-V curve; (g) energy band structure diagram of the schottky junction MSM structure solar-blind ultraviolet photodetector based on Ni/Au electrodes and β-Ga2O3 flake under different wavelengths[105]; (h), (i) the SEM image of the MSM structure solar-blind ultraviolet photodetector based on graphene electrode and β-Ga2O3 flake[106]

      2016年, Du等[107]采用CVD方法生长β-Ga2O3纳米线并制备了纳米线日盲紫外探测器, 该器件的光响应截止波长为270 nm, 并在231 nm波长处获得峰值, 对231和290 nm的响应抑制比(I231 nm/I290 nm)大于3个量级, 展现出较好的日盲光电特性.

      2018年, 中国科学院苏州纳米所He等[108]在1000℃氧气气氛下通过热氧化法, 将Si衬底上的GaN纳米线阵列氧化获得Ga2O3纳米线阵列, 并通过转移单层石墨烯作为上电极, 构成垂直结构的日盲紫外探测器, 该探测器在–5 V偏压下对258 nm的光响应度为0.185 A/W, 对258和365 nm的光响应度抑制比R258 nm/R365 nm为3 × 104, 对254 nm的光响应度为8 ms.

    • 2008年, Oshima等[109]采用简单的热氧化β-Ga2O3单晶和真空蒸镀电极的方法制备获得了垂直结构的Ga2O3单晶肖特基型日盲紫外探测器, 制备过程如图15(a)所示. 为了减小Ga2O3单晶表面的氧空位, 降低载流子浓度, 先将Ga2O3单晶在1100℃下氧气氛中退火6 h, 退火后表面变得光滑并在表面形成一层高阻层, 然后将衬底背面用低压等离子体处理, 减小接触电阻, 采用Au/Ni和Au/Ti电极分别与衬底的表面和背面形成肖特基和欧姆接触. 该器件在 ± 3 V时的整流比为106, 在负向偏压下具有深紫外光电响应, 在200—260 nm的光响应度为2.6—8.7 A/W, 如图15(b)所示. 2009年, Oshima等[89]采用透明导电电极(聚苯乙烯磺酸, POEDT-PSS)作为上电极, 与半绝缘的β-Ga2O3高阻层形成肖特基接触(见图15(c)), 构成的器件对250和300 nm的光响应度比值约为1.5 × 104, 展现出显著的日盲光电特性, 在250 nm的量子效率约为18%, 具有快的光响应速度, 约为9 ms, 如图15(d)所示.

      Figure 15.  Vertical solar-blind deep-ultraviolet schottky photodetectors based onβ-Ga2O3 substrates: (a) Fabrication process for photodetector[109]; (b) spectral responser[109]; (c) photograph of the flame detector. The dashed circles are on the edges of the transparent electrodes[89]; (d) transient response of the detector[89]

      2009年, Suzuki等[110]在单晶β-Ga2O3衬底上沉积Au电极构成Au/β-Ga2O3肖特基接触, 研究了在不同温度下退火处理该结构的电学及光学性质(见图16(a)), 在400℃退火后器件的光响应相对于未退火样品提高了两个数量级, 该器件最大的光响应度为103 A/W (见图16(b)), 对240和350 nm的光响应度比值大于6个数量级. 2011年, Suzuki等[111]采用溶胶–凝胶方法在β-Ga2O3单晶衬底上生长一层β-Ga2O3高阻层并在其上镀Au电极形成了肖特基接触(见图16(c)), 覆盖有β-Ga2O3高阻层的器件具有更大的开启电压, 该器件的光响应度为4.3 A/W (见图16(d)), 对250 和350 nm的光响应度比值大于5个数量级.

      Figure 16.  (a) Dark I-V characteristics of the Au-Ga2O3 Schottky photodiode annealed at various temperatures. The inset shows the device configuration[110]; (b) spectral response of the Au-Ga2O3 Schottky photodiode before and after annealing at 400℃. The inset shows the reverse I-V characteristics of the photodiode annealed at 400℃ taken in dark and under illumination with 240 nm light[110]; (c) schematic structure of a photodiode composed of a Au Schottky contact and a β-Ga2O3 single-crystal substrate with a sol-gel prepared cap layer.[111]; (d) spectral response of Ga2O3 photodiodes with and without a cap layer at reverse and forward biases of 3 V. The inset shows the incident light intensity dependence of the photocurrent at forward and reverse biases of 3 V under illumination with 250 nm light[111]

      2016年, Kong等[112]在单晶β-Ga2O3衬底上转移石墨烯构成异质结结构, 该结构具有典型的整流及日盲紫外光电特性, 对254 nm光响应度为39.3 A/W, 量子效率为1.94 × 104 %, 如图17所示.

      Figure 17.  Solar-blind ultraviolet photodetectors based on graphene/β-Ga2O3 single crystal heterojunction[112]: (a) Schematic diagram of device structure; (b) I-V characteristics of the photodetectors in dark and under 365 nm light irradiation; (c) normalized spectral selectivity; (b) energy band diagram at forward bias voltage

      2017年, 山东大学Mu等[113]采用优化的导模法生长高质量的β-Ga2O3单晶, 通过一步机械剥离的方法获得β-Ga2O3晶体片, 并蒸镀Ti/Au电极制作MSM结构日盲紫外探测器, 在40 V偏压下该探测器对250 nm光响应度为3 mA/W, 光的衰减速度为0.14 s.

      2018年, 大连理工大学Yang等[114]采用真空热蒸发法在商业化的单晶β-Ga2O3衬底两头蒸镀Cu和Ti/Au电极分别构成肖特基和欧姆接触, 制备了垂直结构的肖特基结日盲紫外探测器. 该器件在 ± 2 V偏压的整流比为5 × 107, 探测器的截止波长为256 nm, 光响应度峰值波长为241 nm, 日盲/紫外光和日盲/可见光的抑制比分别为200和1000, 同时该器件在0 V偏压下日盲波段具有明显的响应, 表现为自供电特性.

    • 2006年, Ji等[115]采用热喷雾的方法制备Ga2O3薄膜, 该薄膜对波长大于275 nm的光的透过率大于80%, 在衬底温度为800℃时的带隙为5.16 eV, 该薄膜对太阳光不敏感, 而对254 nm的光有明显的电阻变化, 表现出对日盲紫外光的光敏特性.

      2007年, Kokubun等[116]采用溶胶凝胶方法在c面蓝宝石衬底上制备Ga2O3薄膜, 研究了不同温度下获得的β-Ga2O3薄膜光电探测器的光谱响应, 结果表明β-Ga2O3薄膜在深紫外波段有比较高的光电响应特性, 同时随着制备温度的升高, 探测器的截止波长逐渐变短, 这是因为当温度较高时蓝宝石衬底中的Al掺入到β-Ga2O3薄膜中, 禁带宽度会增大.

      2007年, Oshima等[90]采用等离子辅助分子束外延技术在c面蓝宝石衬底上生长沿$\left( {\overline 2 01} \right)$择优的β-Ga2O3薄膜, 尽管经过优化, 该薄膜还是会含有α-Ga2O3, 如图18(a)所示. 将该薄膜制成MSM器件, 在10 V时其暗电流只有1.2 nA, 对应6 GΩ的高阻, 对低压汞灯具有明显的光响应(见图18(a)), 对254 nm紫外光的响应度为0.037 A/W, 量子效率为18%.

      Figure 18.  (a) In-plane XRD measurement results for the Ga2O3 film; (b) I-V characteristics of the Ga2O3 film photodetector in the dark, under black light irradiation, and under low-pressure mercury lamp irradiation[90]

      2011年, Weng等[117]利用热氧化GaN外延薄膜方法获得了β-Ga2O3薄膜, 并镀上叉指电极制备光电探测器, 如图19(a)图19(b)所示. 在5 V偏压下, 暗电流只有1.39 × 10–10A, 在260 nm的紫外光照射下光电流增加至2.03 × 10–5 A, 光响度为0.453 A/W, 量子效率大于100 %. 深紫外光与可见光的响应度比值大于4个数量级. 2013年, Huang等[118]采用类似的方法制备了β-Ga2O3/AlGaN/GaN三段的光电探测器(见图19(c)图19(d)), 用于探测不同波段的紫外光[UV-A (315—400 nm), UV-B (280—315 nm), and UV-C (100—280 nm)]: 在偏压为1 V时, UV-C与UV-B以及UV-B与UV-A的光响应度比值分别为14.4和2157.9; 在偏压为10 V时, UV-A与可见光的光响应度比值为247.9, 展现出多波段紫外光探测的特性. 同时该组还研究了含β-Ga2O3包覆层的InGaN/GaN多量子阱的光电探测器(见图19(e)图19(f))[119], 覆盖有Au纳米颗粒的Ga2O3/GaN的MSM结构光电探测器(见图19(g)图19(h))[120].

      Figure 19.  Schematic diagram (a) and spectral responses under different bias (b) of Ga2O3/GaN photodetector[117]; Schematic diagram (c) and spectral responses under different bias (d) of Ga2O3/AlGaN/GaN photodetector[118]; Schematic diagram (e) and spectral responses under different bias (f) of Ga2O3/InGaN/GaN photodetector[119]; Energy band diagram of area near the surface of β-Ga2O3 and Au in the dark (g), spectral responses under different bias of Ga2O3/GaN-based metal-semiconductor-metal photodetectors covered with Au nanoparticles (h)[120]

      2013年, Nakagomi等[121]在P型6H-SiC衬底上生长了约200 nm厚的β-Ga2O3薄膜构成β-Ga2O3/SiC异质结(见图20), 该器件表现出良好的整流特性. 在负向电压下, 光电流随着紫外光强度的增加而增加, 在波长为210—260 nm范围内具有相对较高的光响应度(接近0.07 A/W), 并具有快速的光响应速度(ms量级).

      Figure 20.  Schematic diagram (a) and spectral responses under 2 V reverse bias (b) of SiC/Ga2O3 photodetector[121]

      2013年, Ravadgar等[122]采用MOCVD在蓝宝石衬底上生长β-Ga2O3薄膜并分别在700℃, 800℃和900℃空气中退火30 min, 实验结果表明光电探测器的光暗比由退火前的1个数量级上升至退火后的大于4个数量级. 在700℃下退火的样品, 器件暗电流降低至70 pA, 光暗比大于5个数量级.

      从2013年开始, 我们课题组在β-Ga2O3薄膜日盲紫外探测器方面做了大量工作. 采用激光分子束外延(L-MBE)技术在(0001)α-Al2O3衬底上不同条件下(不同氧压不同衬底温度)生长Ga2O3薄膜, 并在氧压为5 × 10–3 Pa、衬底温度为750℃时获得了单一取向且结晶性良好的β-Ga2O3外延薄膜, 制备了基于β-Ga2O3薄膜的MSM结构日盲紫外探测器(见图21(a)所示), Au/Ti电极与β-Ga2O3薄膜形成欧姆接触, 该探测器对254 nm的光极为敏感, 而对365 nm的光几乎不响应[123]. MSM结构中β-Ga2O3薄膜的厚度对探测器光电性能的影响明显, 当薄膜较薄时不足以吸收全部的日盲紫外光, 而当薄膜较厚时又将会影响光生载流子的有效分离. 实验结果表明, 随着β-Ga2O3薄膜厚度的增加, 探测器的性能呈现先增加后减小的趋势, 并在厚度约为303 nm时取得最佳值[124], 如图21(b)所示. 探测器阵列既可通过单元的串联或并联提高探测灵敏度, 又可用作成像, 应用前景非常广阔, 我们课题组首次制作了基于β-Ga2O3薄膜的32 × 32, 16 × 16, 8 × 8, 4 × 4日盲紫外探测器阵列(见图21(c)图21(d)所示), 单个探测单元在–10 V的偏压下对250 nm深紫外光响应度为0.89 A/W[125]. 另外, 考虑到肖特基结可调控载流子的输运, 增加探测器的光电性能, 而β-Ga2O3薄膜中较多的表面缺陷容易使金属电极Ti与β-Ga2O3薄膜形成欧姆接触; 另一方面, β-Ga2O3薄膜中存在的大量氧空位等缺陷降低了探测器的响应速度. 为此, 我们通过在氧气氛中原位退火处理的办法, 有效地减少了β-Ga2O3薄膜中的氧空位, 使金属电极Ti与β-Ga2O3薄膜的接触由欧姆型向肖特基型转变. 经过退火处理后制备的器件表现出更高的光暗比(由退火前的2.7增加至12.9), 更快的响应速度(衰减时间由退火前的τd1/τd2 = 2.16 s/24.55 s减小至0.83 s/8.14 s)(见图21(e)所示), 这主要归结于金-半界面处耗尽层宽度的变化, 调控了载流子的输运方式, 我们通过能带图对其机理进行了详细的解释[126]. 同时, 还利用掺杂元素Mn3+/Mn2+价态转变, 有效地抑制了β-Ga2O3薄膜内部的滋生载流子, 减小了氧空位, 降低了暗电流, 提高了探测器的光暗比及响应速度[127](见图21(g)所示). 虽然经改善后的探测器性能有所提高, 但受限于MSM结构, 探测器的各性能指标都很低, Mn:Ga2O3薄膜探测器的光响应度仅为7 × 10–2 A/W, 量子效率为36%, 光暗比为67, 响应时间为0.28 s. 我们课题组[128-137]还研究了不同元素掺杂(如Mg, Sn, Zn, Er)、不同晶相(如α, β, ε)对Ga2O3薄膜日盲紫外探测器的影响, 器件性能总结在表4中. 同时, 考虑到MSM结构对光的利用率不高, Ai等[138]采用对深紫外光具有高透过率的石墨烯作为上下电极, 构筑石墨烯/β-Ga2O3/石墨烯垂直结构的日盲紫外探测器, 在10 V偏压下, 该器件对254 nm的光响应度达9.66 A/W, 如图22(a)图22(b)所示. 表面等离子激元对光具有共振吸收的特性, An等[139]在β-Ga2O3薄膜表面生长Au纳米颗粒, 附着有Au颗粒的薄膜除了对小于250 nm的深紫外光有强烈吸收之外, 还对510 nm附近的可见光具有宽的吸收峰(见图22(c)所示), 可实现多波段的光探测, 同时Au纳米颗粒的引入还能降低探测器的暗电流, 提高光电性能. Cui等[140]和Huang等[141]还通过引入SiC, Al2O3层构成多层膜并制备日盲紫外探测器, Al2O3层的引入会获得Ga2O3纳米线/薄膜复合结构[140].

      Figure 21.  (a) Schematic diagram of the β-Ga2O3 thin film MSM structure photodetector[123; (b) the effect of Ga2O3 film thickness on light-dark ratio of the MSM structure photodetector[124]; (c), (d) MSM structure arrays photodetector[125]; (e)I-t curves of the β-Ga2O3 thin films MSM structure photodetector with unannealed (Ohmic-type up) and annealed treatment in O2 atmosphere (Schottky-type, down), respectively[126]. I-t curves of the MSM structure photodetector based on β-Ga2O3 thin films doped with different element: (f) Mg doped[128]; (g) Mn doped[127]; (h) Zn doped[129]; (i) Sn doped[130]

      Figure 22.  Schematic diagram (a) [138] and photoresponses to 254 nm ultraviolet light under different bias (b) [138] of graphene/Ga2O3/graphene vertical structure photodetector; UV-vis absorbance spectrum (c) [139] and I-V cures under the different wavelength light illumination (d) [139] of the bare Ga2O3 thin film and Au nanoparticles/Ga2O3 composite thin film; SEM image (e) and I-V cures under the different wavelength light illumination (f) [140] of Ga2O3 thin film/nanowire grown induced by Al2O3 thin layer[140]

      光电探测器类型 光响应度/A·W–1 量子效率/% 暗电流/A 光暗比 响应时间/s 参考文献
      Ga2O3纳米线 - - 10–12 ≈ 2 × 103 2.2 × 10–1 [91]
      Ga2O3纳米线 - - < 10–12 3 × 104 < 2 × 10–2 [88]
      Ga2O3纳米线 8.0 × 10–4 0.39 2.4 × 10–10 ≈ 102 - [92]
      Ga2O3纳米线 3.4 × 10–3 1.37 - ≈ 102 - [93]
      ZnO/Ga2O3核壳微米线 1.3 × 103(–6 V) - 10–10 ≈ 106 2 × 10–5 [100]
      ZnO/Ga2O3核壳微米线 9.7 × 10–3(0 V) - 10–10 ≈ 7 × 102 10–4 [101]
      Ga2O3纳米线 6 × 10–4 - 10–11 ≈ 102 6.4 × 10–5 [102]
      Ga2O3纳米线 3.77 × 102 2.0 × 105 10–11 103 0.21 [107]
      石墨烯/Ga2O3纳米线 1.85 × 10-1 - 10–5 - 8 × 10–3 [108]
      Ga2O3纳米片 3.3 1.6 × 103 10–9 10 3 × 10–2 [96]
      Ga2O3纳米花(γ) - - 10–9 2.2 × 102 10–1 [97]
      Ga2O3纳米带 3.37 × 101 1.67 × 104 10–13 4.0 × 102 8.6 × 101 [94]
      Ga2O3纳米带 8.51 × 102 4.2 × 103 10–13 ≈ 103 < 3 × 10–1 [98]
      Ga2O3纳米带 1.93 × 101 9.4 × 103 10–10 ≈ 104 < 2 × 10–2 [99]
      In:Ga2O3纳米带 5.47 × 102 2.72 × 105 10–13 9.1 × 102 1 [95]
      Ga2O3微米带 1.8 × 105(–30 V) 8.8 × 105 10–6 2.57 0.67 [103]
      Ga2O3微米带 - - 10–4 - 1.4 [104]
      Ga2O3微米带 1.68 - 10–13 1.9 × 103 0.53 [105]
      石墨烯/Ga2O3微米带 2.98 × 101 - 10–13 ≈ 104 - [106]
      Ga2O3单晶 2.6—8.7 - 10–10 ≈ 103 - [109]
      Ga2O3单晶 3.7 × 10–2 1.8 × 101 10–10 1.5 × 104 9 × 10–3 [89]
      Ga2O3单晶 103 - 10–10 ≈ 106 - [110]
      Ga2O3单晶 4.3 2.1 × 101 10–11 105 - [111]
      石墨烯/Ga2O3单晶 3.93 × 101 1.96 × 104 10–6 103 2.2 × 102 [112]
      Ga2O3单晶 5 × 10–2 - 10–5 102 2.4 × 10–1 [160]
      Ga2O3单晶 3 × 10–3 - 10–8 101 1.4 × 10–1 [113]
      Ga2O3薄膜 8 × 10–5 - - - - [116]
      Ga2O3薄膜 3.7 × 10–2 1.8 × 101 10–9 - - [90]
      Ga2O3薄膜 4.53 × 10–1 > 102 10–10 105 - [117]
      Ga2O3薄膜 ≈ 101 - 10–10 103 - [118]
      Ga2O3薄膜 ≈ 101 - 10–7 103 - [119]
      Ga2O3薄膜 ≈ 102 - 10–10 102 - [120]
      Ga2O3薄膜 - - 10–11 105 - [122]
      Ga2O3薄膜 7.6 × 10–1 - 10–10 6 5 × 10–2 [152]
      Ga2O3薄膜 1.7 × 101 8.2 × 103 10–9 8.5 × 106 - [153]
      Ga2O3薄膜 - - 10–11 102 8 × 10–1 [154]
      Ga2O3薄膜 9.03 × 10–1 - 10–11 105 - [155]
      Ga2O3薄膜 2.59 × 102 7.9 × 104 10–10 104 4 × 10–1 [156]
      Ga2O3薄膜 - - 10–7 15 - [157]
      Ga2O3薄膜/晶体 1.8 8.7 × 102 10–6 36.9 - [158]
      a-GaOx非晶薄膜 7.0 × 101 - 10–10 1.2 × 105 2 × 10–2 [159]
      Ga2O3薄膜 4.2 - 10–11 1.6 × 104 4 × 10–2 [159]
      Ga2O3薄膜 9 × 10–3 - 10–5 101 1.8 × 10–1 [160]
      Al:Ga2O3薄膜 1.5 7.8 × 102 - - - [164]
      Si:Ga2O3薄膜 6 × 101 3 × 104 - 9 - [166]
      Si:Ga2O3薄膜 3.6 × 101 1.75 × 104 - 9 - [167]
      Zn:Ga2O3薄膜 2.1 × 102 - 10–11 5 × 104 1.4 [168]
      Ga2O3非晶薄膜 1.9 × 10–1 - 10–12 106 1.9 × 10–5 [169]
      Ga2O3非晶薄膜 4.5 × 101 - 10–10 104 2.97 × 10–6 [171]
      Ga2O3薄膜 1.5 - 10–9 103 - [175]
      Ga2O3薄膜 0.29 1.34 10–8 1.6 × 103 0.1 [173]
      Ga2O3薄膜 0.11 - 10–9 3.5 × 103 0.45 [174]
      Ga2O3薄膜 0.14 - 10–11 1.4 × 106 0.2 [174]
      Ga2O3薄膜 1.5 - 10–8 103 - [173]
      Ga2O3薄膜 2.6 × 101 - 10–8 104 0.18 [176]
      石墨烯/Ga2O3薄膜 1.28 × 101 - 10–8 - 2 × 10–3 [177]
      Ga2O3薄膜 9.6 × 101 4.76 × 104 10–6 - - [180]
      Ga2O3薄膜 5.86 × 10–5 - 10–9 1.8 × 101 0.1 [181]
      Ga2O3薄膜 1.5 × 102 7 × 104 10–11 105 1.3 [165]
      Ga2O3薄膜 1 × 10–1 - 10–8 - - [178]
      Ga2O3薄膜 - - 10–8 6 8.6 × 10–1 [123]
      Ga2O3薄膜 - - 10–9 1.3 × 101 6.2 × 10–1 [126]
      Ga2O3/Ga/Ga2O3薄膜 2.854 - 10–11 8 × 105 - [170]
      Mn:Ga2O3薄膜 7 × 10–2 3.6 × 101 10–9 6.7 × 101 2.8 × 10–1 [127]
      α-Ga2O3薄膜 1.5 × 10–2 7.39 10–9 3 × 101 - [137]
      α-Sn:Ga2O3薄膜 9.6 × 10–2 - 10–9 1.4 × 102 1.08 [132]
      α-Sn:Ga2O3薄膜 - - 10–7 4 8.73 [131]
      ε-Sn:Ga2O3薄膜 6.05 × 10–3 3.02 10–9 46.46 - [133]
      β-Sn:Ga2O3薄膜 3.61 × 10–2 - 10–8 19 1.37 [166]
      Zn:Ga2O3薄膜 - - 10–9 2 1.23 [134]
      Er:Ga2O3薄膜 - - 10–9 2.5 1.6 × 10–1 [76]
      Au NPs/Ga2O3薄膜 102 - 10–6 > 2 × 102 - [139]
      Ga2O3/p-Si异质结 3.7 × 102 1.8 × 105 10–8 9.4 × 102 1.8 [143]
      Ga2O3/ZnO异质结 3.5 × 10–1 1.7 × 102 10–10 1.5 × 101 6.2 × 10–1 [144]
      Ga2O3/NSTO异质结 4.3 × 101 2.1 × 104 10–6 2 × 101 7 × 10–2 [142]
      Ga2O3/Ga:ZnO异质结 7.6 × 10–4 - 10–9 2.6 × 102 2.7 × 10–1 [145]
      p-Si/i-SiC/n-Ga2O3 - - 10–8 5.4 × 103 - [148]
      石墨烯/Ga2O3/SiC 1.8 × 10–1 - 10–5 6.3 × 101 1.7 [149]
      石墨烯/Ga2O3/石墨烯 9.66 - 10–9 8.3 × 101 0.96 [138]
      Ga2O3/SiC/Al2O3 - - 10–9 7.7 - [141]
      Ga2O3/Al2O3 1.4 - 10–7 9.04 1.26 [140]
      Ga2O3/SiC异质结 7 × 10–2 - 10–10 - 9 × 10–3 [121]
      Ga2O3/GaN异质结 5.4 × 10–2 - 10–6 1.5 × 102 8 × 10–2 [146]
      Sn:Ga2O3/GaN异质结 3.05 - 10–11 104 1.8 × 10–2 [147]
      α-Ga2O3/ZnO异质结 1.1 × 104(–40 V) - 10–12 - 2.4 × 10–4 [172]
      Ga2O3/金刚石异质结 2 × 10–4 - 10–9 3.7 × 101 - [179]

      Table 4.  Summary of parameters and indicators of Ga2O3 based solar-blind ultraviolet photodetector.

      MSM结构光电探测器往往具有持续光电导现象, 器件响应速度较慢, 并需要在外加电源下工作, 通过构建异质结、PN结、肖特基结等引入结效应[142-151], 利用内建电场实现光生载流子的快速有效分离, 可在0 V下工作, 无需外加偏压, 具有自供电的特性. Guo等[142]在N型的NSTO衬底上生长β-Ga2O3薄膜构筑β-Ga2O3/NSTO异质结, 在254 nm光照下, 该结构I–V曲线由黑暗情况下的正向整流转变为负向整流, 如图23(a)图23(c)所示. 在0 V偏压下, 该器件的光暗比为20, 衰减时间为0.07 s, 在–10 V偏压下, 对254 nm深紫外光的响应度为43.31 A/W, 外量子效率为(2.1 × 104) %. 在商业化的P型Si衬底上构建β-Ga2O3/Si异质结(见图23(d)), 该结构具有显著的载流子倍增效应, 在–3 V偏压下, 器件对254 nm的光响应度达370 A/W, 对应的外量子效率为(1.8 × 105) %, 光暗比为940, 光响应度及量子效率较MSM结构增加了近4个数量级[143]. 同时, 我们还在ZnO, Ga掺杂的ZnO衬底上分别制备了β-Ga2O3/ZnO[144], β-Ga2O3/Ga:ZnO[145]异质结自供电紫外探测器(见图23(e)图23(f)). 以生长在蓝宝石基底上的P型GaN厚膜为衬底沉积N型β-Ga2O3薄膜构筑PN结[146], 该结构对254和365 nm紫外光都有响应, 在0 V偏压下对365 nm的光响应度为54.43 mA/W, 响应速度为0.08 s, 光暗比152, 探测率为1.23 × 1011 cm Hz1/2/W, 如图23(g)图23(h)所示. 考虑到Sn掺杂可以增加电子浓度, 提高N型β-Ga2O3费米能级的位置, 增加P型GaN厚膜与N型β-Ga2O3薄膜间的能级势垒差, 促进光生载流子的分离, Guo等[147]构建了GaN/Sn:Ga2O3 PN结及自供电紫外探测器, 在0 V偏压下对254 nm的光响应度为3.05 A/W, 紫外与可见光的抑制比为R254 nm/R400 nm = 5.9 × 103, 该器件拥有低的暗电流(1.8 × 10–11A), 高的光暗比(I/I = 104), 响应速度为 18 ms, 探测率为1.69 × 1013 cm·Hz1/2·W–1, 如图23(i)图23(j)所示. 同时An等[148]还在PN结中引入绝缘层构筑PIN结(见图23(k)), Qu等[149]以石墨烯作为异质结器件的上电极来提高探测器性能(见图23(l)).

      Figure 23.  Schematic diagram (a) [142], I-V cures in dark and under 254 nm with different light intensity illumination (b) [142], and schematic energy band diagrams (c) [142] of the β-Ga2O3/NSTO heterojunction self-powered photodetector; Schematic diagram of Ga2O3/P-Si PN junction detector (d) [143]; Rectifier features (e), schematic diagram (e) and spectral response (f) of the Ga2O3/Ga:ZnO heterojunction photodetector[145]; Schematic diagram (g) [145], I-V cures in dark and under the different wavelength light illumination (h) [146]; Spectral response (i) and I-t cures under the different wavelength light illumination (j) of the Sn:Ga2O3/GaN PN junction photodetector[145]; Schematic diagram of Ga2O3/SiC/P-Si PIN junction photodetector (k) [148]and graphene/Ga2O3/SiC photodetector (l)[149]

      2014年, Guo等[152]c面蓝宝石衬底上采用金属有机沉积方法研究了不同生长温度对Ga2O3薄膜结构、表面及光学性质的影响, 随着温度的增加Ga2O3薄膜的结晶性能增加, 晶粒尺寸增大, 表面粗糙度增加, 制备的MSM光电探测光响应度为0.76 A/W, 响应速度达50 ms.

      2015年, Hu等[153]a面蓝宝石衬底上采用MOCVD生长Ga2O3薄膜并镀上Au叉指电极, 构成Au/Ga2O3/Au结构, 该结构展现出载流子倍增的效果, 截止波长为260 nm, 对255 nm具有最大的光响应, 在偏压为20 V时的光响应度为17 A/W, 量子效率为8228 %.

      2015年, Sheng等[154]采用分子束外延(MBE)技术生长了β-Ga2O3薄膜并分别在不同气氛(O2, N2, 真空)不同温度下(800℃, 900℃, 1000℃和1100℃)退火, 研究它们的日盲光电特性.

      2015年, Yu等[155]采用PLD技术在c面蓝宝石衬底上, 不同温度下(400—1000℃)沉积了β-Ga2O3薄膜, 在800℃生长样品获得器件的暗电流为1.2 × 10–11 A, 光暗比约为105, 对250 nm的光响应度为0.903 A/W.

      2016年, 电子科技大学Liu等[156]采用MBE技术研究了同质缓冲层对β-Ga2O3薄膜探测器的影响, 他们发现同质缓冲层的引入可以提高器件性能, 器件的暗电流为0.04 nA, 光暗比为104, 光响应度为259 A/W, 外量子效率为7.9 × 104 %. 同时, Qian等[157]还研究了四端日盲紫外探测器, Liu等[158]在Si掺杂的Ga2O3单晶上生长高绝缘的β-Ga2O3薄膜层, 并蒸镀Ni/Au电极形成肖特基结, 研究其日盲紫外光电性能. 2017年, Qian等[159]采用磁控溅射在低温450℃生长获得高度非化学计量比的a-GaOx非晶薄膜, 并制作了超高光响应度、快响应速度的日盲紫外探测器, 该器件在10 V偏压下对250 nm的光响度可达70.26 A/W, 截止波长为265.5 nm, 对250和350 nm光响应度抑制比R250 nm/R350 nm大于105, 探测率为1.26 × 1014 J. 同时, 该组对比基于MBE生长的β-Ga2O3薄膜日盲探测器, 该器件在10 V偏压下对250 nm的光响度为4.21 A/W, R250 nm/R350 nm抑制比为104, 性能不如基于a-GaOx非晶薄膜的探测器, 如图24所示.

      Figure 24.  Solar-blind ultraviolet photodetector based on a-GaOx amorphous film and β-Ga2O3 film[159]: (a) MSM structure diagram; (b) spectral response; (c) energy band structure diagram

      2016年, 西安电子科技大学Feng等[160]采用L-MBE技术在蓝宝石衬底上生长β-Ga2O3薄膜制备了MSM结构日盲紫外探测器, 并与Ga2O3晶体材料进行对比, 发现基于体材料的Ga2O3探测器展现出更为优异的性能, 其光响应峰值波长较薄膜探测器要短, 在40 V偏压下, 晶体Ga2O3探测器对光响应峰值波长252 nm的光响应度为0.05 A/W, 是薄膜探测器的5.6倍(薄膜Ga2O3探测器对光响应峰值波长244 nm的光响应度为0.009 A/W). Feng等[161]和Huang等[162]还研究了生长氧压、后退火处理对Ga2O3薄膜日盲紫外探测器光电性能的影响, 实验结果表明, 减少Ga2O3薄膜内部的氧空位能有效地提高探测器性能. Zhang等[163]和Feng[164]等还通过Al, In掺杂有效调控探测器的光谱响应并提高光响应度, 如图25所示.2018年, 西安电子科技大学Xu等[165]采用湿化学气相法生长β-Ga2O3薄膜并制备MSM结构的日盲紫外探测器, 在20 V偏压下对254 nm的光响应度大于150 A/W, 暗电流为14 pA, 光暗比大于105, 量子效率超过(7 × 104)%.

      Figure 25.  MSM structure solar-blind ultraviolet photodetector: (a) Schematic diagram of MSM structure[160]; (b) spectral response comparison of Ga2O3 single crystal and thin film[160]; (c) MSM structure[162]; (d) spectral response comparison of Ga2O3 thin films annealed in different atmospheres[161]; (e) spectral response comparison of Ga2O3 thin films grown under different oxygen pressures[162]; (f) spectral response comparison of Ga2O3 thin films doped with different concentrations of In elements[163]

      2016年, Ahn等[166,167]生长了Si掺杂的β-Ga2O3薄膜并研究其日盲紫外光电性能. 2017年, Alema等[168]获得了Zn掺杂的β-Ga2O3外延薄膜并制备日盲紫外光电探测器.

      2017年, 中国科学院物理所Cui等[169]采用磁控溅射方法分别在石英和柔性衬底上常温下生长Ga2O3非晶薄膜, 研究了不同氧压对探测器性能的影响, 该器件具有快速的响应速度, 光响应衰减时间仅为19.1 ${\text{μ}}{\rm{s}}$, 光响应度为0.19 A/W, 光暗比大于104, 如图26所示. 2018年, Cui等[170]通过预埋金属Ga层并进行后退火处理制备Ga2O3/Ga/Ga2O3多层薄膜, 并制作成日盲探测器. 随着Ga层厚度的增加, 探测器的光电流及光响应度先增加后减小, 在Ga层厚度为20 nm时, 器件展现出最佳的性能, 在–10 V偏压下的暗电流为8.52 pA, 光暗比为8 × 105, 在–15 V偏压下其光响应度为2.85 A/W, 相比于纯Ga2O3薄膜, 光响应度增加了220倍.

      Figure 26.  Solar-blind ultraviolet photodetector based on a-Ga2O3 amorphous film[169]: (a) Schematic diagram of device structure with quartz substrate; (b) spectral response; (c) the decay of photoresponse; (d) schematic diagram of device structure with flexible substrate

      2017年, Lee等[171]采用原子层沉积法在玻璃和柔性衬底聚酰亚胺上较低温度下(< 250 ℃)沉积超薄a-GaOx非晶薄膜并制作日盲紫外探测器, 基于30 nm的a-GaOx非晶薄膜探测器对253 nm的光响应度为45.11 A/W, 光截止波长为300 nm, 10 V偏压下的暗电流为200 pA, 光暗比大于104, 光响应速度为2.97 ${\text{μ}}{\rm{s}}$, 如图27所示.

      Figure 27.  Solar-blind ultraviolet photodetector based on a-Ga2O3 amorphous film[171]: Schematic diagram of device structure with glass substrate (a) and I-V cures in dark and under the illumination of 253 nm light (b); Schematic diagram of device structure with polyimide substrate (c) and I-V cures in dark and under the illumination of 253 nm light (d)

      2017年, 南京大学Chen等[172]采用L-MBE技术在非极性的ZnO $\left( {11\overline 2 0} \right)$晶面上外延生长单晶α-Ga2O3薄膜, 并制备了基于Au/α-Ga2O3/ZnO异质结结构的高性能肖特基势垒雪崩二极管. 该器件具有自供电的特性, 0 V偏压下暗电流为pA级别, 对紫外/可见光的光响应度抑制比为103, 探测率为9.66 × 1012 cm·Hz1/2·W−1; 在–5 V偏压下, 该探测器为双波段响应器件, 光响应度峰值波长位于255 和365 nm处, 对应的光响应度分别为0.50和0.071 A/W; 在–40 V偏压下, 该器件表现为高的雪崩增益, 对254 nm的深紫外光的光响应度高达1.1 × 104 A/W, 总增益超过105, 如图28所示.

      Figure 28.  Solar-blind ultraviolet photodetector based on α-Ga2O3/ZnO heterojunction[172] : (a) Spectral response; (b) variation of gain with bias; (c) transient photoresponse characteristics; (d) schematic diagram of energy band structure and device structure

      2017年, Patil-Chaudhari等[173]通过高温氧化的方式将Si掺杂的GaAs晶圆片在1050℃下热处理, 使其表面形成β-Ga2O3薄膜制备日盲紫外探测器, 该器件对270 nm的光响应度为0.29 A/W, 光暗比为1.6 × 103, 外量子效率为1.34%.

      2017年, Rafique等[174]采用低压化学气相沉积法在c面蓝宝石衬底上生长β-Ga2O3薄膜并研究了热退火对其日盲紫外光电性能的影响, 实验结果表明, 在氧气氛中1000℃下退火1 h有效地减少了氧空位, 提高了光电性能, 对250 nm深紫外波长的光暗比和光响应度分别由退火前的3.5 × 103和0.11 A/W提高至退火后的1.44 × 106和0.14 A/W, 对250和405 nm的光响应度抑制比R250 nm/R405 nm由退火前的4.47 × 102提升至退火后的4.4 × 105.

      2017年, Pratiyush等[175]采用等离子体辅助分子束外延技术在c面蓝宝石衬底上外延生长β-Ga2O3薄膜并采用电子束蒸发技术蒸镀Ni/Au叉指电极构筑肖特基接触, 在4 V偏压下对236—240 nm的光响应度为1.5 A/W, 紫外/可见抑制比 > 105, 在20 V偏压下暗电流小于10 nA, 光暗比 > 103.

      2018年, 中山大学Zhang等[176]通过改进的MOCVD法采用弱氧化性的N2O替代传统O2作为反应气体生长β-Ga2O3薄膜并制作日盲紫外探测器, 该器件在10 V偏压下, 对255 nm的光响应度为26.1 A/W, 光暗比为104, 响应速度为0.18 s, 相比于在O2气氛生长的β-Ga2O3薄膜, 光电性能有大幅提升. 2018年, Lin等[177]在P-GaN衬底上生长β-Ga2O3薄膜并转移石墨烯作为上电极制作垂直结构的日盲紫外探测器, 在–6 V偏压下对254 nm的光响应度为12.8 A/W, 探测率为1.3 × 1013, 响应速度为2 ms, 如图29所示.

      Figure 29.  Solar-blind ultraviolet photodetector based on β-Ga2O3 thin film grown using N2O as the reaction gas: (a) Schematic diagram of growth principle[176]; (b) I-V cures in dark and under 255 nm light illumination, and schematic diagram of MSM structure[176]; (c) spectral response and photoresponsivity under different bias[176]; (d) schematic diagram of graphene/β-Ga2O3/GaN devices[177]; (e) spectral response[177]; (f) energy band structure diagram[177]

      2018年, Jaiswal等[178]采用微波辐射技术在GaN衬底上原位生长Ga2O3薄膜, 并蒸镀Ni/Au电极制作MSM结构的探测器, 该器件的光响应峰值波长为236 nm, 在22 V的光响应度为0.1 A/W, 对230和400 nm的光响应度抑制比R230 nm/R400 nm > 103.

      2018年, 郑州大学Chen等[179]采用等离子体增强化学气相沉积方法在金刚石衬底上生长β-Ga2O3薄膜, 构筑金刚石/β-Ga2O3异质结光电探测器, 该器件可工作在0 V偏压下, 具有自供电的特性, 截止波长为270 nm, 光暗比为37, 对峰值波长244 nm的光响应度为0.2 mA/W, 紫外/可见光响应度抑制比为R244 nm/R400 nm = 1.4 × 102.

      2018年, Arora等[180]采用磁控溅射方法在商业化的Si衬底上生长β-Ga2O3薄膜并通过引入Ga2O3种子层提高日盲光电特性, 该器件结构具有自供电的特性, 在0 V偏压下对254 nm的光暗比 > 103, 暗电流为1.43 pA, 探测器具有很好的稳定性和很高的重复性; 在5 V偏压下, 对250 nm的光响应度为96.13 A/W, 外量子效率为4.76 × 104.

      2018年, Shen等[181]采用廉价的溶胶凝胶法生长Ga2O3薄膜, 当退火温度超过700℃时将获得纯相的β-Ga2O3薄膜, 基于该薄膜的探测器光暗比为18.34, 响应速度为0.1 s.

      表4总结了基于Ga2O3材料(形式包括纳米、单晶、薄膜)日盲紫外探测器的各参数指标, 从表中可知, 从单晶体材料上机械剥离下来的准二维β-Ga2O3微米小薄片所构成的日盲紫外探测器具有最高光响应度(为1.8 × 105 A/W)[103], 其次是与ZnO构成异质结的α-Ga2O3/ZnO异质结(为1.1 × 104 A/W)[172] 及ZnO/β-Ga2O3核/壳结构微米线探测器 (为1.3 × 103 A/W)[100], 以及与Au电极构成Schottky结的Au/β-Ga2O3单晶光电探测器(为103 A/W)[99]; 形成异质结或Schottky结的器件往往具有很高的增益[99,100]; 准二维β-Ga2O3的微米小薄片具有最高的量子效率(达(8.8 × 105) %)[103], 其次是掺In的Ga2O3纳米带(为(2.72 × 105) %)[95]; 相比于单晶和薄膜材料, 纳米Ga2O3往往具有更低的暗电流, 可低至零点几个pA[94,95,98,105]; 基于纳米、单晶、薄膜类型的Ga2O3光电探测器的光暗比都可以达到106 [99,100,152,169]; 而对于光的响应时间, 基于非晶Ga2O3薄膜的探测器具有最快的响应时间(仅为2.97 ${\text{μ}}{\rm{s}}$)[171], 同时从表中总结可知异质结或肖特基结器件往往具有更快的响应速度, 如ZnO/Ga2O3核/壳结构微米线探测器的响应时间仅为20 ${\text{μ}}{\rm{s}}$[100], Au/Ga2O3纳米线肖特基型垂直结构光电探测器的响应速度也能达到64 ${\text{μ}}{\rm{s}}$[102], 而基于Ga2O3/SiC异质结探测器及其响应速度可以达到9 ms[121].

    4.   结论与展望
    • β-Ga2O3的禁带宽度为4.9 eV, 该带隙对应的波长为253 nm, 为日盲波段的核心区, 可以说β-Ga2O3是一种天然的日盲深紫外探测和紫外可见光透明材料. 本综述介绍了Ga2O3的晶体结构和基本物性, 并综述了β-Ga2O3在紫外透明导电电极和日盲紫外探测器应用中的研究进展. 在紫外透明电极方面, Sn掺杂的Ga2O3薄膜电导率可到32.3 S/cm, 透过率大于88%. 商业化的透明导电电极要求薄膜电导率大于104 S/cm, 对紫外可见光的透过率超过85%. 使Ga2O3薄膜的电导率达到商业化值并保持其带隙不变是关键, 通过四价元素掺杂(如Sn或Si等)并不断改善薄膜生长工艺, 必将能引来Ga2O3基紫外可见光透明电极的产业化. 同时, 本文从纳米、单晶、薄膜三种材料形态按时间顺序叙述了Ga2O3基日盲紫外探测器的发展历程. 从材料形态上来看, 基于单根纳米材料的探测器展现出最高的光响应度, ZnO/Ga2O3核/壳微米线的探测器具有最佳的综合性能, 其对254 nm深紫外光的光响应度为1.3 × 103 A/W, 响应时间为20 ${\text{μ}}{\rm{s}}$. 但单根纳米材料的面积极小, 对应的光响应度会较大, 但实际上光照是一大片的, 不仅仅只照射到纳米材料上. 同时, 基于纳米材料的探测器制作过程复杂, 不利于产业化. 单晶虽然也有很高的光响应度, 但单晶衬底价格昂贵. 薄膜是Ga2O3日盲紫外探测器产业化最有前景的材料形态, 通过MOCVD, LPCVD, 磁控溅射等方式可以在4英寸的蓝宝石衬底上生长β-Ga2O3外延薄膜, 蒸镀电极并制作成可商业化的探测器, 整个流程在工艺界已相对成熟, 目前MSM结构的Ga2O3日盲紫外探测器性能已经达到商业化参数. 另一方面, 基于肖特基结、异质结、PN结结构的Ga2O3基紫外探测器展现出自供电的特性, 在无需外加电源的情况下也能正常工作, 在特殊场合、极端条件下具有重要应用. 我们期盼Ga2O3基日盲紫外探测器能早日应用于导弹预警跟踪、紫外通讯、港口破雾导航等军用及臭氧空洞监测、消毒杀菌紫外线强度监测、高压电晕检测、森林防火紫外监测等民用领域. Ga2O3的P型掺杂一直是其难点, 也是Ga2O3在电子器件领域产业化的关键点, 目前几乎没有可靠的P型掺杂的报道, 这可能会是一个影响其应用的根本问题. 理论计算表明, 由于Ga2O3的迁移率低, 在Ga2O3中易发生空穴自陷, 这会降低有效的P型导电性. 同时, 所有的受体掺杂剂都会产生深的受主能级, 而不能产生P型导电性. 一种可能的解决方式是将N型Ga2O3与其他具有P型导电性的半导体材料(如碘化铜, 氧化铜和氧化镍)结合.

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