Photocathode quantum efficiency of ultrathin Cs2Te 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.

Exploring the Cs-Te phase space via high-throughput density-functional theory calculations beyond the generalized-gradient approximation

“…64,68,70 D. The 2:1 composition of Cs-Te materials Among the variety of structures and compositions analyzed so far, the one with 2:1 relative Cs:Te content covers a promi-nent role in experimental photocathode research. 50,[71][72][73] Interestingly, this system is not only at the minimum of the computed convex hull (see Fig. 2c) but, from the analysis of the pDOS, it also exhibits the largest band gap (Fig.…”

Review of photocathodes for electron beam sources in particle accelerators

“…Furthermore, Cu atoms diffuse into Te and cause some island growth which leads to a lower QE compared to the Cs 2 Te cathode on a Mo substrate. 40 Besides the standard substrates (Cu and Mo), Nb 86 can be considered as a potential substrate for Cs 2 Te.…”

Ab Initio Quantum-Mechanical Predictions of Semiconducting Photocathode Materials

“…To start, these materials pose severe issues in terms of stability, lifetime, and reproducibility of their growth procedures [18,25]. Their high surface reactivity makes them subject to atmospheric contamination, which should be reduced to a minimum to prevent deterioration as well as to enhance sample durability and photoemission yield [6,26,27].…”

“…A number of challenges have to be tackled in order to gain sufficient insight to control and manipulate the composition of alkali antimonides and tellurides as well as their characteristics for photocathode applications (see Figure 1 ). To start, these materials pose severe issues in terms of stability, lifetime, and reproducibility of their growth procedures [ 18 , 25 ]. Their high surface reactivity makes them subject to atmospheric contamination, which should be reduced to a minimum to prevent deterioration as well as to enhance sample durability and photoemission yield [ 6 , 26 , 27 ].…”