In situ characterization of thermal cleaned surface for preparing superior transmission-mode GaAs photocathodes.

Considering that it is impractical to utilize in situ surface diagnostic means to determine the surface cleanliness of transmission-mode GaAs photocathodes in the vacuum device manufacturing process, the thermal desorption technique with the aid of the quadrupole mass spectrometer during the thermal cleaning process is employed to in situ characterize the thermal cleaned surface. The desorption behaviors for various impurity gases during the thermal cleaning process are analyzed. The experimental results show that the amount of desorbed impurity gases varies due to the different heat treatment temperatures. Through the verification of Cs/O activation and quantum efficiency measurement, it is found that the desorption behaviors of the specific impurity gases including AsH and As are crucial to surface cleanliness of transmission-mode GaAs photocathodes, which relate to the final photoemission capability. This simple and reliable criterion provides an effective way to guide the thermal cleaning process of transmission-mode GaAs photocathodes, and the desorption behaviors assist in in situ evaluation of surface cleanliness.