Evolution of surface potential barrier for negative-electron-affinity GaAs photocathodes

Abstract

The evolution of surface potential barrier for reflection-mode GaAs photocathodes in an ultrahigh vacuum system has been investigated by using spectral response and angle-dependent x-ray photoelectron spectroscopy (ADXPS) measurements at room temperature. The escape probabilities of electrons emitted into vacuum are obtained as a function of the incident electron energy, surface barrier height, and thickness. Based on the new escape probability expressions, we obtain the surface barrier parameters of the reflection-mode negative-electron-affinity (NEA) cathodes from the fit of the spectral response curves by using quantum-efficiency equations. These parameters reveal the evolution of the NEA cathode surface during the degradation process. In addition, the surface layer structure of both the freshly activated and degraded cathodes is calculated from the ADXPS spectra. The calculated results are in fair agreement with the fitted barrier parameters.