$K2Te$ photocathode growth: A photoemission study Available to Purchase

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 $Te 3d5/2$ 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.