We report here a general theory describing photoelectron transportation dynamics in GaAs semiconductor photocathodes. Gradient doping is incorporated in the model through the inclusion of directional carrier drift. The time-evolution of electron concentration in the active layer upon the injection of an excitation pulse is solved both numerically and analytically. The predictions of the model are compared with experiments via carrier-induced transient reflectivity change, which is measured for gradient-doped and uniform-doped photocathodes using femtosecond pump-probe reflectometry. Excellent agreement is found between the experiments and the theory, leading to the characterization of key device parameters such as diffusion constant and electron decay rates. Comparisons are also made between uniform doping and gradient doping for their characteristics in photoelectron transportation. Doping gradient is found to be able to accelerate electron accumulation on the device surface. These results offer new insights into the dynamics of III-V photocathodes and potentially open a new avenue toward experimental characterization of device parameters.
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