Oxide effects on photoemission from high current GaAs photocathodes

Abstract

During 4 years of on line operation of the SLAC polarized electron gun (PEGGY) and polarized LEED (PLEED) system, we have observed and characterized the failure modes of the GaAs (100) photocathodes (PC's) used in these systems. Several modes are observed. Gradual decreases in electron polarization and intensity are attributed to the physisorption of CO2 on the PC's during running at LN2 temperatures. Such PC's can be rejuvenated by warming to 90 K, i.e. above the CO2 desorption temperature. These PC's recover 90% of their original intensity. A second well-characterized failure mode results from overheating the PC during in situ heat cleaning prior to activation. In this mode, As is preferentially evaporated from the GaAs, leaving a Ga2O3 layer on the surface. This effect has been studied by AES sputter profiling which indicates that the substantial thickness of the oxide layer blocks photoemission. These PC's may only be recovered by chemically removing the oxide layer. A third mode which is not as well characterized appears for thin Ga oxide layers. Properties of these PC's include reduced emission and the presence of a cutoff bias level. Such PC's are also not recoverable in situ.