Photocathode quantum efficiency of ultrathin ${\mathrm{Cs}}_2\mathrm{Te}$ layers on Nb substrates

The quantum efficiencies (QE) of photocathodes consisting of bulk Nb substrates coated with thin films of ${\mathrm{Cs}}_2\mathrm{Te}$ are reported. Using the standard recipe for ${\mathrm{Cs}}_2\mathrm{Te}$ deposition developed for Mo substrates (220 Å Te thickness), a $\mathrm{QE}\sim 11\%–13\%$ 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 $\mathrm{QE}\sim 6\%$ 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 ${\mathrm{Cs}}_2\mathrm{Te}$ layer may be expected to produce minimal Ohmic losses for rf frequencies $\sim 1\mathrm{GHz}$. The result thus opens the door to the potential development of a Nb (or ${\mathrm{Nb}}_3\mathrm{Sn}$) superconducting photocathode with relatively high QE and minimal rf impedance to be used in a superconducting radiofrequency (SRF) photoinjector.

Figure 1

QE evolution of ${\mathrm{Cs}}_2\mathrm{Te}$ photocathode on Nb substrate during Cs deposition. Te210 means the cesiation was performed after 210 Å Te film was deposited, while Te25 and Te15 were done on 25 and 15 Å, respectively. Te0 is a cesiated Nb surface with no Te deposition.

Figure 2

Dependence of the quantum efficiency (QE) of ${\mathrm{Cs}}_2\mathrm{Te}$ films on Nb (blue circles) and Mo (red squares) substrates vs estimated ${\mathrm{Cs}}_2\mathrm{Te}$ thickness. ${\mathrm{Cs}}_2\mathrm{Te}$ thickness is estimated to be 1.65 times the Te thickness. Data points correspond to the average values while error bars represent the maximum and minimum measured QE for multiple samples of any given thickness. Solid curves are the phenomenological model fits described in the text using $n$ and $k$, the real and imaginary parts of the index of refraction, respectively, for the film and substrate.

Figure 3

Dependence of the transmission, $1\text{−}R$, vs film thickness determined from the Fresnel equations and optical constants of Nb, Mo and ${\mathrm{Cs}}_2\mathrm{Te}$ shown in the inset.