GaN与GaAs NEA光电阴极稳定性的比较

Comparison of Stability of GaN and GaAs Photocathode

利用自行研制的光电阴极激活评估实验系统,对激活后的反射式GaN及GaAs光电阴极进行了稳定性测试,获得了Cs/O激活一段时间后阴极随时间变化的光谱响应,通过计算得到量子效率曲线。结果表明:激活结束后GaN灵敏度可以在较长时间内保持稳定,而后缓慢衰减。而GaAs光电阴极的光电流随时间近似呈指数衰减。结合阴极表面双偶极层结构以及表面化学成分,分析原因主要是:两种阴极表面进行Cs/O激活后形成的双偶极子的结构不同、衰减过程中双偶极层化学成分变化方式不同决定。GaN光电阴极激活后Cs以复杂氧化物存在,更加稳定,灵敏度的衰减主要是由未分解的氧引起,而GaAs灵敏度下降的原因主要是表面双偶极层中的Cs极易脱附,影响其稳定性。

The photocurrent and quantum yield curves some time after Cs and Cs/O activation process for GaN and GaAs photocathode were given by using self-developing experimental system. The differences between the stability of negative electron affinity （NEA）GaN and GaAs cathodes was studied by evaluating these quantum yield curves. The variations in the photocurrent and quantum yield of NEA GaN and GaAs after the activation are different. The delayed decay in the yield in GaN occurs because larger changes in the surface dipole of the initial Cs/O activation layer are required before these emitters exhibit conversion from NEA to PEA surfaces, decreasing emission properties. The over cesiated GaAs emitters maintain the initial starting dipole that establishes the NEA surface until the excess Cs stabilizing the Cs/O adlayer is removed. The experiments indicate a wide bandgap emitter like GaN can out perform as an NEA photocathode with a large quantum yield and is more long lived than a GaAs emitter under the conditions tested. The GaN doesn＇t require over cesiation to delay the decay of the quantum yield.