Photocathode Quantum Efficiency of Ultra-Thin Cs2Te Layers On Nb Substrates

TL;DR

This study investigates the quantum efficiency of ultra-thin Cs2Te layers on Nb substrates, demonstrating high QE with layers as thin as 15 Å and suggesting potential for superconducting RF photoinjectors.

Contribution

It introduces a phenomenological model for QE dependence on ultra-thin Cs2Te layers on Nb, showing high efficiency at minimal thickness and potential for superconducting applications.

Findings

Achieved 11-13% QE at 248 nm for Nb substrates.

Residual QE of ~6% with 15 Å Te layers.

Model fits thickness dependence and suggests high QE for layers thinner than 15 Å.

Abstract

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.