The effect of lattice parameter mismatch in NEA GaAs photocathodes grown on GaP/InxGa1−xP substrates

Abstract

Transmission-mode negative electron affinity (NEA) GaAs photocathodes were investigated. In order to assess quantitatively the effect of lattice-parameter mismatch between the cathode and substrate, several 1-μm-thick GaAs photocathode layers were vapor deposited onto GaP/InxGa1−xP composite substrates with x ranging from O to about 0.5; the corresponding lattice mismatch varied from 0.2 Å to approximately zero. Photoluminescence and ir free-carrier absorption measurements of the Zn doping concentration in the 1-μm-thick GaAs layers revealed that the doping concentration was about 3–6 times greater than that obtained on 15-μm-thick GaAs layers grown under identical Zn partial pressure conditions. Photoemission measurements showed that the transmission-mode quantum efficiency at a wavelength of 0.7 μm increased from about 0.045 electron/photon for GaAs on GaP to 0.13 electron/photon for GaAs on In0.50Ga0.50P as the lattice-parameter mismatch decreased to approximately zero. Correspondingly, transmission-mode white-light sensitivity values increased from 195 to 390 μA/lm. The photoemission spectral response curves were analyzed to determine values for back interface recombination velocity s and electron diffusion length L. In general, several combinations of these parameters gave a good fit to the data. However, for samples near the lattice match point, s was restricted to relatively low values and, for a given value of s, the corresponding value of L was significantly higher than that for any sample with an appreciable amount of lattice mismatch.