GaAs photocathode cleaning by atomic hydrogen from a plasma source

Abstract

A clean GaAs surface is required in order to fabricate negative-electron-affinity photocathodes. Surface preparation is routinely performed by chemical cleaning and heating in ultra-high vacuum. These processes could damage the surface and produce photocathodes with low quantum efficiencies. Here an alternative technique which overcomes these problems, namely chemical cleaning and heating in ultra-high vacuum, is used. A helical resonator discharge produces hydrogen plasma which is used in a down-flow configuration to clean the GaAs(100) surface of a bulk crystal and strained layer GaAs prior to activation to negative electron affinity. Atomic hydrogen reacts with the oxides and hydrocarbons on the surface, forming volatile compounds. The photocathodes are prepared by depositing caesium onto the clean semiconductor surface. The photoemission current increases with surface cleaning. The reflection high-energy electron-diffraction pattern after atomic hydrogen cleaning reveals a clean 2×4 reconstructed GaAs(100) surface compared with a halo before cleaning.