Analysis of quantum efficiency improvement in spin-polarized photocathode

GaAs/GaAsP strain-compensated superlattices (SLs) were developed for spin-polarized photocathode applications. High crystal quality was maintained with SL thicknesses up to 720 nm (90-pairs); however, the quantum efficiency (QE) did not increase linearly with the SL thickness but became saturated starting from an SL thickness of 192 nm (24-pairs). Time-resolved photoluminescence measurements revealed that the carrier lifetime in the GaAs/GaAsP strain-compensated SL was as short as 20.5 ps at room temperature, which causes the elimination of photoexcited electrons before emission. A simulation based on a diffusion model was implemented to quantitatively evaluate the effect of the carrier lifetime on the QE. The simulation results were in good agreement with the experimental results and demonstrate that a carrier lifetime of over 120 ps is required for a two-fold improvement of the QE.