Photocathode quantum efficiency of ultrathin Cs_{2}Te layers on Nb substrates

Abstract

The quantum efficiencies (QE) of photocathodes consisting of bulk Nb substrates coated with thin films of Cs_{2}Te are reported. Using the standard recipe for Cs_{2}Te deposition developed for Mo substrates (220 Å Te thickness), a QE∼11%–13% at light wavelength of 248 nm is achieved for the Nb substrates, consistent with that found on Mo. Systematic reduction of the Te thickness for both Mo and Nb substrates reveals a surprisingly high residual QE∼6% for a Te layer as thin as 15 Å. A phenomenological model based on the Spicer three-step model along with a solution of the Fresnel equations for reflectance, R, leads to a reasonable fit of the thickness dependence of QE and suggests that layers thinner than 15 Å may still have a relatively high QE. Preliminary investigation suggests an increased operational lifetime as well. Such an ultrathin, semiconducting Cs_{2}Te layer may be expected to produce minimal Ohmic losses for rf frequencies ∼1 GHz. The result thus opens the door to the potential development of a Nb (or Nb_{3}Sn) superconducting photocathode with relatively high QE and minimal rf impedance to be used in a superconducting radiofrequency (SRF) photoinjector.