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摘要: 利用GaN光电阴极多信息量测试评估系统，对反射式梯度掺杂和均匀掺杂GaN光电阴极样品进行了激活及衰减后的量子效率测试，并测试衰减速率。在同样的衰减时间内，和均匀掺杂样品相比，梯度掺杂样品的衰减比例较小，衰减速率较慢，其原因在于梯度掺杂结构可在其发射层内部产生系列内建电场，致使其能带连续向下弯曲，导致其表面真空能级比均匀掺杂样品下降得更低，发射层表面形成的负电子亲和势更明显，造成发射层内的光生电子更易逸出，阴极量子效率的衰减变慢，从而使其稳定性强于均匀掺杂结构。Abstract: The GaN photocathode multi-information measurement and evaluation system is used to test the quantum efficiency of the reflective gradient-doped and uniformly doped GaN photocathode samples after activation and attenuation, and the attenuation rate test is performed. The gradient-doped sample has a smaller attenuation ratio and a slower decay rate than the uniform-doped sample within the same decay time because the gradient-doped structure can generate a series of built-in electric fields inside the emissive layer. As a result, the energy band can be continuously bent downwards, resulting in a lower surface vacuum level than that of the uniformly doped sample, and the negative electron affinity formed on the surface of the emission layer is more pronounced, resulting in easier escape of photogenerated electrons in the emission layer. The decay of the cathode quantum efficiency becomes slower, making it more stable than uniform-doped structures.
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Key words:
- GaN /
- photocathode /
- gradient-doping /
- built-in electric field /
- stability
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图 1 反射式均匀掺杂样品A和梯度掺杂样品B结构图
Figure 1. Structure of reflection-mode uniform-doping sample A and gradient-doping sample B
图 2 样品A和B在激活结束后的量子效率测试曲线
Figure 2. Quantum efficiency curves of sample A and sample B after activation
图 3 样品A和B的量子效率测试曲线
Figure 3. Quantum efficiency curves of GaN photocathodes in variable conditions
图 5 反射式GaN光电阴极的表面势垒示意图
Figure 5. Surface potential barrier of reflection-mode GaN photocathode
图 6 均匀掺杂样品A和梯度掺杂样品B能带结构示意图(Ec为导带能级，Ev为价带能级，EF为费米能级，E0为真空能级，Eg为GaN的禁带宽度)
Figure 6. Energy band structure of uniform-doping sample A and gradient-doping sample B(EC is the conduction band minimum, EV is the valence band maximum, EF is the Fermi level, E0 is the vacuum level, Eg is the band gap)
表 1 均匀掺杂样品A的衰减测试结果
Table 1. Quantum efficiency attenuation test results of sample A
波长/nm 240 260 280 300 320 340 激活后QE/% 51 36 27 20 16 14 12 h后QE/% 42 25 16 9.8 6.2 4.9 衰减比例/% 17.6 30.6 40.7 46 61.2 65 注：衰减比例为(激活后QE-12 h后QE)/激活后QE。 
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表 2 梯度掺杂样品B的衰减测试结果
Table 2. Quantum efficiency attenation test results of sample B
波长/nm 240 260 280 300 320 340 激活后QE/% 57 44 36 28 24 21 12 h后QE/% 48 32 24 18 13 10 衰减比例/% 15.8 27.3 33.3 35.7 45.8 52.4 注：衰减比例为(激活后QE-12 h后QE)/激活后QE。
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表 3 样品A和样品B的衰减速率测试结果
Table 3. Attenuation rate test results of decadence rate for uniform-doping sample A and gradient-doping sample B
衰减速率(a.u/h) 测试时间/h 2 4 6 8 10 12 梯度掺杂样品B 32 19.1 12.7 10.4 4.7 4.6 均匀掺杂样品A 40 30 19.1 17.6 14.3 8.3 注：衰减速率为(测试值1-测试值2)×100/测试值1。
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