Enhancement of near-infrared response for GaAs-based photocathode with laminated graded-bandgap structure: theory and experiment

Abstract

To enhance the quantum efficiency of GaAs-based photocathode in the near-infrared range, a laminated graded-bandgap photocathode structure consisting of a distributed Bragg reflection layer and a graded-bandgap emission layer is proposed. The theoretical optical properties and quantum efficiency of this proposed photocathode are simulated based on the finite-different time-domain method and the one-dimensional continuity equations, respectively. The absorptivity at 1064 nm can be significantly increased for the new-type photocathode because of the secondary absorption caused by the distributed Bragg reflection layer. With the improvement of absorptivity at 1064 nm, the quantum efficiency at specific wavelength is enhanced significantly. The contribution of the DBR structure and built-in electric field to the quantum efficiency is theoretical investigated respectively. According to theoretical structure design, the cathode samples grown by epitaxial technique are prepared to verify the theoretical prediction of quantum efficiency enhancement. After the treatment of surface cleaning and Cs/NF3 activation, the experimental results show that the minimum reflectivity at 1064 nm is realized, and the quantum efficiency of the sample with proposed structure is ten times than that of the sample without distributed Bragg reflection layer at 1064 nm. This research provides a guidance for the design of novel photocathode structures with enhanced response at special wavelength.