OXIDE EFFECTS ON PHOTOEMISSION FROM HIGH CURRENT GaAs PHOTOCATHODES*

During four 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 90K, i. e. , above the COz 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 Gaz% 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 * Work supported by the Department of Energy under contract DE-AC03-76SF00515. (Presented at the Interdisciplinary Surface Science Conference, Liverpool, England, April 6-9, 1981 and submitted to Vacuum. )