  The quantum efficiencies (QE) of photocathodes consisting of bulk Nb
substrates coated with thin films of Cs2Te are reported. Using the standard
recipe for Cs2Te deposition developed for Mo substrates (220 {\AA} 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 {\AA}. A phenomenological model based on the
Spicer 3-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 {\AA} may still have a relatively high QE.
Preliminary investigation suggests an increased operational lifetime as well.
Such an ultra-thin, semiconducting Cs2Te 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 Nb3Sn) superconducting photocathode
with relatively high QE and minimal RF impedance to be used in a
superconducting radiofrequency (SRF) photoinjector.
