High quantum efficiency GaAs photocathodes activated with Cs, O₂, and Te

GaAs photocathodes are the primary choice for generating spin-polarized electron beam with high brightness, high polarization, and fast polarization reversal. However, it suffers from short lifetime due to the highly reactive nature of the emission surface, resulting in substantial operational difficulties. Activating GaAs with a more robust material, such as Cs2Te, shows comparable polarization to that of Cs–O activation and increases the lifetime due to the robustness of the Cs2Te layer. However, previously reported photocathodes based on Cs–Te activation on GaAs suffer from 10× lower quantum efficiency (QE) compared to that activated with conventional Cs–O activation. Herein, we report activation recipes for GaAs photocathodes using Cs, O2, and Te. For Cs–Te activation, the QE was 6.6% at 532 nm. For Cs–O–Te activation, the QE was 8.8% at 532 nm and 4.5% at 780 nm. The negative electron affinity of the activated GaAs was directly measured and confirmed by low energy electron microscopy. We also report the activation layer chemical states and stoichiometry using in situ micro-spot synchrotron radiation x-ray photoelectron spectroscopy.