Journal of Applied Physics, Vol. 66, No. 7, pp. 3066-3073, October 1989.

(Received 24 March 1989; accepted for publication 21 June 1989)

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A self-consistent ensemble Monte Carlo particle model to simulate the dynamics of photogenerated carrier transport in GaAs surface space-charge fields is presented. Dependence of transport parameters, such as relaxation processes, carrier screening, and velocity overshoot, on pulse vidth, injection level, and excitation energy is investigated and related to the experimentally observable potential drop across the surface space-charge region. In the present study, the high-field conditions are produced by charged surface states, and are to be modified by the redistribution of injected carriers. The photocarrier-induced change in the electric field, which can be measured using electro-optic sampling techniques, is found to be due to the spatial separation of the electron-hole pairs. The simulation results show that at a high injection level a maximum potential drop would result due to complete carrier screening of the surface charge field. The rate of change of this potential drop also would increase with increasing level of injection. These results are in good qualitative agreement with experimental observations. The simulation predicts an enhanced degree of velocity overshoot at decreasing injection energies and, at below L-minima excitation, a second increase in the potential resulting from the redistribution of carriers in the bulk. In this way, it is indicated that the study of the onset of photoconductivity can be used to probe such transport properties in GaAs.

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