Na_2KSb光电阴极与GaAs光电阴极比较研究

Study on the Difference between GaAs Cathode and Na_2KSb Cathode

测量了三代像增强器和超二代像增强器的阴极光谱反射率,结果表明在500～800 nm波长范围内,GaAs阴极的平均光谱反射率仅为6.5%,而Na2KSb阴极的平均光谱反射率却高达22%。采用减反技术之后,Na2KSb阴极的平均光谱反射率降为10%,与GaAs阴极相比还有一定的差距,因此还有进一步改进和提高的空间。测量了三代像增强器和超二代像增强器光电阴极的荧光谱,结果表明在785 nm波长激光的激发条件下,GaAs阴极的荧光谱峰值波长与Na2KSb阴极的荧光谱峰值波长基本相同,但荧光谱半峰宽和峰值强度却区别很大。说明GaAs阴极和Na2KSb阴极在吸收785nm波长光子后,所激发的电子跃迁的能级基本相同,但跃迁电子的数量区别却很大。GaAs阴极荧光谱的半峰宽较窄,说明GaAs阴极的晶格较Na2KSb阴极的晶格更完整。当Na2KSb阴极的膜层厚度从180 nm变为195 nm之后,由于跃迁电子距真空界面的距离增大,因此导致短波的量子效率减小。尽管膜层厚度加厚,长波光子的吸收更充分,但因受到电子扩散长度的限制,长波量子效率仅仅略有增加。这说明Na2KSb阴极的电子扩散长度远远小于GaAs阴极的电子扩散长度。GaAs阴极表面吸附Cs-O层之后,表面电子亲和势会降低,而多碱阴极表面吸附Cs-Sb层之后,不仅表面电子亲和势会降低,而且跃迁电子的能级会提高,跃迁电子的数量也会增加,这说明多碱阴极在进行表面Cs激活之后,阴极膜层内部的能带结构发生了变化。所以要提高多碱阴极的灵敏度,除了要控制好表面的Cs激活工艺之外,还需要控制好Na2KSb基础层的结构。只有一个结构良好的Na2KSb基础层,在Cs激活之后,能带结构的变化才会有利于跃迁电子能级的提高。

This paper described and analyzed the measured results of the cathode spectral reflectance in the third generation image intensifier and super gen. II image intensifier. The results showed that in the wavelength range of 500-800 nm, the average spectral reflectance of GaAs cathode is only 6.5%, while the average spectral reflectance of NaEKSb cathode is as high as up to 22%. By applying the anti-reflection technology, the average spectral reflectance of NaEKSb cathode is reduced to 10%, and still has very big difference comparing with GaAs cathode. Therefore there is a further space for improving and enhancing. Besides the luminescence spectrum of photocathode in the third generation image intensifier and super gen. II image intensifier were also measured in this paper. The results showed that in the laser excitation condition of 785 nm wavelength, peak wavelength of fluorescent spectrum for GaAs cathode and Na2KSb cathode are basically the same, but there is big difference at the half peak width and peak intensity of fluorescence spectrum. This says that the energy level of transition electron are basically the same for GaAs and Na2KSb cathodes after the absorption the photon of 785 nm wavelength, but the numbers are different. The half peak width of fluorescence spectrum in GaAs cathode is narrow. This showed that GaAs has more complete lattice than Na2KSb lattice. When the film thickness of Na2KSb cathode gets thicker, as the distance for transition electron to vacuum interface has been increased, the quantum efficiency in short wavelength would be decreased. Although the film thickness is getting thicker, light absorption is more sufficient, but due to the limited electron diffusion length, long wavelength quantum efficiency would be increased only slightly. After cathode surface was adsorbed with Cs-Sb layer, not only the surface electron affinity can be reduced, but the energy level and number of the transition electron would be increased. This shows that when cathode surface was activated with Cs, the energy band of cathode films would change. So to improve the cathode sensitivity, in addition to the control surface Cs activation process, also it is needed to control Na2KSb base layer structure. Only a good Na2KSb base layer was obtained after Cs activation, energy band structure changes can help to elevate the energy level of transition electron.