K2Te photocathode growth: A photoemission study

X-ray photoemission spectroscopy (XPS) has been used to investigate the gro wth process of K-Te thin layers with high quantum efficiency (QE) for photo emission in the UV range, prepared by sequential deposition of Te and K on a Mo substrate under ultrahigh vacuum conditions. Film formation occurs thr ough different steps, characterized by increasing QE up to a saturation val ue which correspond to K2Te stoichiometry. By quantitative, angle resolved XPS measurements, surface segregation of one monolayer of K has been detect ed. Films prepared starting from different amounts of Te exhibit the same K 2Te stoichiometry. However, the QE values are significantly different. Char ge transfer from K to Te has been detected during K deposition and monitore d by the shift of the Te 3d(5/2) photoemission line. At the end of the K ev aporation a shift of both Te 3d and K 2p photoemission lines has been obser ved, 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 compare d with that of Cs2Te, which is currently used as photocathode material in U V-laser driven photoinjectors. It has been shown that K2Te is more rugged t han Cs2Te. QE degradation is associated to the formation of TeO2 on the K2T e surface. (C) 2000 American Institute of Physics. [S0021-8979(00)05301-9].