Study on the spectral response curve shape variation of the reflection-mode GaN NEA photocathodes

摘要

The spectral response curves of the reflection-mode NEA GaN photocathodes are measured during the activation process and the degradation process by using the spectral response testing instrument, the results show the shapes of the spectral curves in the two procedures vary continuously. With the formation of the dipoles during the procedure of the activation of the photocathode, the effective electron affinity value on the surface decrease continuously, thus the spectrum response will increase continuously, especially the spectrum response increase more quickly in long wave-length band. When the activation process is over, because of the desorption of the Cs element, the double-dipole layer in the surface of the photocathode will be damaged, so the effective electron affinity value on the surface increases continuously, thus the spectrum response will decrease continuously, especially decrease more quickly in long wave-length band. This phenomenon cannot be explained with the regular quantum efficiency formula, the photon excited electron with high energy may result in this. With the energy increasing of the incident photon, the energy distribution of the electrons will shift toward the higher energy area, meanwhile the surface potential barrier influence the electron with low energy more severely to escape into the vacuum than those with high energy. These two reasons mentioned above will cause the shape variation of the spectral response curves.