Preparation and operation of hydrogen cleaned GaAs(100) negative electron affinity photocathodes

We report on the preparation and operating conditions of negative electron affinity GaAs, cesium-oxygen activated photocathodes, cleaned using atomic hydrogen generated by a thermal cracking source. Atomic hydrogen is effecti ve in removing surface contaminants and in producing a smooth reconstructed surface free of oxides and carbon compounds at a relatively low GaAs tempe rature. A clean (2x4) reconstructed GaAs(100) reflection high-energy electr on diffraction (RHEED) pattern is obtained after atomic hydrogen cleaning. From the RHEED patterns, the thickness of the cesium layer that causes maxi mum photoemission is estimated to be similar to 0.5 monolayer. High quantum efficiency photocathodes (similar to 12%) are produced after activation to negative electron affinity. Atomic hydrogen cleaning is effective not only in the initial cleaning but also in reviving the photocathode after its pe rformance is degraded by operation. The GaAs is kept at 500 degrees C durin g the cleaning process, a temperature that preserves the arsenic stabilized surface necessary for negative electron affinity activation. By reducing t he accelerating voltage of the photoemitted electrons and the photocurrent intensity, and by keeping a low level-of continuous cesiation, the photocat hode lifetime can be significantly enhanced. (C) 1999 American Vacuum Socie ty. [S0734-2101(99)00903-3].