K2Te photocathode growth:: A photoemission study

X-ray photoemission spectroscopy (XPS) has been used to investigate the growth process of K-Te thin layers with high quantum efficiency (QE) for photoemission in the UV range, prepared by sequential deposition of Te and K on a Mo substrate under ultrahigh vacuum conditions. Film formation occurs through different steps, characterized by increasing QE up to a saturation value which correspond to K2Te stoichiometry. By quantitative, angle resolved XPS measurements, surface segregation of one monolayer of K has been detected. Films prepared starting from different amounts of Te exhibit the same K2Te stoichiometry. However, the QE values are significantly different. Charge transfer from K to Te has been detected during K deposition and monitored by the shift of the Te 3d(5/2) photoemission line. At the end of the K evaporation a shift of both Te 3d and K 2p photoemission lines has been observed, due to a band bending mechanism. The degradation by oxygen exposure of the quantum efficiency at 254 nm of K2Te has also been studied and compared with that of Cs2Te, which is currently used as photocathode material in UV-laser driven photoinjectors. It has been shown that K2Te is more rugged than Cs2Te. QE degradation is associated to the formation of TeO2 on the K2Te surface. (C) 2000 American Institute of Physics. [S0021-8979(00)05301-9].