Characteristics of Photovoltage Spectrum on Surfaces of Gallium Nitride Photocathode Film Materials
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摘要: 在蓝宝石基底上外延生长了多层结构氮化镓光阴极薄膜材料并进行表面光电压测试；对比分析了掺杂类型、厚度和掺杂方式对氮化镓材料表面光电压的影响，确定了多层结构氮化镓材料表面光电压产生机理；借助亚带隙激光辅助，针对均匀掺杂和δ-掺杂氮化镓（GaN）光电阴极薄膜材料进行了表面光电压测试；实验数据表明，相较于均匀掺杂，δ-掺杂可以获得更好生长质量，但也提高了在能级(Ev+0.65)eV~(Ev+1.07)eV范围的缺陷态密度。Abstract: In this study, we epitaxially grew a multilayer structure of gallium nitride (GaN) photocathode film material on a sapphire substrate and conducted a surface photovoltage test. The effects of doping type, thickness, and doping method on the surface photovoltage of the gallium nitride material were compared and analyzed, and the mechanism of surface photovoltage generation of the multi-layered gallium nitride material was determined. A surface photovoltage test was performed on uniformly doped and delta-doped gallium nitride photocathode thin film materials using sub-band-gap laser. Experimental data shows that better growth quality was achieved using δ-doping than that achieved using uniform doping; however, δ-doping increased the density of defect states in the (Ev+0.65)–(Ev+1.07) eV energy levels.
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Key words:
- gallium nitride /
- photocathode /
- photovoltage spectrum on the surface
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图 2 表面光电压测试系统整体结构示意图
Figure 2. Overall structure diagram of surface photovoltage test structure
图 3 样品3~样品7能带结构简图
注：EV、EC和EF分别为导带、价带和费米能级；w1、w2和w3分别为三个空间电荷区宽度；VS、VD1和VD2为三个空间电荷区电势，箭头表示电场方向。
Figure 3. Simplified diagram of the band structure of samples 3 to 7
Note: EV, EC and EF are conduction band, valence band and Fermi level respectively; w1, w2 and w3 are the widths of three space charge regions respectively; VS, VD1 and VD2 are the three space charge region potentials, and the arrows indicate the direction of the electric field
图 4 样品1表面光电压曲线及微分曲线
Figure 4. Surface photovoltage curve and differential curve of sample 1
图 6 不同厚度样品表面光电压谱
Figure 6. Surface photovoltage spectra of samples with different thickness
图 7 不同掺杂方式P-GaN/N-GaN/Al2O3结构表面光电压谱
Figure 7. Surface photovoltage spectra of P-GaN/N-GaN/Al2O3 structures with different doping methods
图 8 不同掺杂方式P-GaN/N-GaN/Al2O3结构激光辅助表面光电压谱
Figure 8. Laser-assisted surface photovoltage spectroscopy of P-GaN/N-GaN/Al2O3 structure with different doping methods
表 1 在基底上外延生长的样品规格
Table 1. Specifications of samples grown epitaxially on the substrate
Samples Thickness of epitaxial layer/μm Doping type 1 2 Undoped 2 2 N-type 3 0.2 P-type 4 0.5 P-type 5 2 P-type 6 0.0032 P-type 7 0.5 P-type 
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