Adsorption Kinetics of Cs on GaAs

Cesium adsorption on vacuum-cleaved p-GaAs (110) was monitored at 10−10 Torr concurrently by photoemission and x-ray photoelectron spectroscopy (ESCA). Cesium was supplied at R = 6.9×1012 atoms/cm2 sec by a calibrated valved room-temperature Knudsen effusion cell. ESCA results showed that the first monolayer of Cs contains N = 6.2×1014 atoms/cm2, that it adsorbs irreversibly, and that fractional coverage is θ = 1−exp(− Rt/N). Maximum photoyield (hence minimum work function) corresponds to θ = 1. The second layer of Cs decreases photoyield, but is desorbed with a time constant of less than 1 min.