Characterization of Cs2Te photoemissive film: formation, spectral responses and pollution

In the framework of the Tesla Test Facility (TTF) collaboration for the Injector II, we present a study on the formation and characterization of photoemissive films of Cs2Te grown on Mo substrata. The measured Quantum Efficiency (QE) has been analyzed in terms of the preparation procedures followed to obtain the photoemissive films. Using AES and XPS techniques we have monitored the evaporated materials (Te and Cs) growth into a film using a `'step-by-step'' analysis. We observed the formation of different compounds before the formation of a stoichiometric Cs2Te. The spectral response of the emissive layers has been measured in the wavelength range 254-514 nm, produced from a high-pressure mercury lamp and CW lasers. In view of the applications of this photoemitter, it is important to know the effect of gas pollution on a film prepared with a standard procedure. Since oxygen is one of the most reactive gases, oxygen contamination on Cs2Te films has been chosen as a criterion for the investigation of QE degradation. An exposure to 5 x 10(-5) mbars of oxygen has been found to be enough to reduce the QE by one order of magnitude. The XPS technique has been used to study the effects of the oxidation. We also investigated the `'rejuvenation'' effect (i.e., the recovery of QE after the degradation induced by the oxygen exposure) produced by the contemporary action of heating and illumination with 254 nm radiation from a mercury lamp. The results reported in this paper allowed us to produce reproducible and reliable photocathodes (QE = 13% at 254 nm) and to understand the cathode formation steps. Moreover, we reached an initial understanding of the pollution and rejuvenation mechanisms.