Pre-Emission Study of Photoelectron Dynamics in a GaAs/AlGaAs Photocathode

Understanding the carrier dynamics in III-V semiconductor photocathodes is of great importance to the development of many novel optoelectronic systems such as low-light imagers and coherent x-ray sources. Past experimental effort to study ultrafast response of GaAs photocathodes has relied exclusively on post-photoemission measurement, i.e. via characterization of the temporal properties of emitted electron bunches. Such an indirect approach is not only difficult to achieve experimentally but also liable to sample surface conditions when assessing key carrier dynamics such as photoelectron diffusion and decay. In this report, we present a pre-photoemission investigation of photoelectron transport and population decay in a GaAs/AlGaAs photocathode based on pump-probe transient reflectometry. By measuring the ultrafast behaviors of reflectivity after the excitation of a femtosecond optical pulse, we are able to evaluate the transient evolution of photoelectron population near the device surface, which provides a direct picture of electron diffusion and decay. A carrier diffusion model shows excellent agreement with experiment. Experiment-theory comparisons also suggests a diffusion coefficient much greater than some of the post-photoemission predictions. In addition, the free-electron population decay near the surface is found to be of bi-exponential nature. Possible physical mechanisms underlying these decay lifetimes are discussed.