Variation of quantum-yield curves for GaAs photocathodes under illumination

Abstract

The variation of the quantum-yield curves of reflection-mode and transmission-mode GaAs photocathodes under illumination has been studied using an instrument for measuring quantum yields at room temperature. The two kinds of photocathodes have an identical doping concentration and active-layer thickness, 1×1019cm−3 and 1.6μm, respectively. These photocathodes were illuminated by white light with an intensity of 100lx, and the yield curves were measured simultaneously every other hour. The measured results show that the shape of the yield curves for the reflection-mode photocathodes changes rapidly with increasing illumination time, whereas the shape of the curves for the transmission-mode photocathodes is almost constant, we find that the reason is the average path lengths that photoelectrons have to travel are different for the two kinds of photocathodes. In the yields of the transmission-mode photocathodes, all types of photoemission except Γ may be neglected, whereas all of the Γ, L, and hot-electron photoemission must be included in the yields of the reflection-mode photocathodes. Based on the two-minima diffusion model and the fit of the escape probability, we have revised the quantum-efficiency equation for the reflection-mode photocathodes and used this revised equation to explain the variation of their yield curves.