Influence of surface charge on free-carrier density profiles in GaAs films: Application to second-harmonic generation by free carriers

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Abstract
We calculate, self-consistently, free-carrier density profiles and wave functions in GaAs films with surface charge present, by virtue of charged surface states or an applied external electric field. We explore the regime from the case where the free carriers are distributed roughly uniformly over the film, to the case where the surface charge localizes them to the near vicinity of one or both surfaces. This is done by both a Fourier transform and a real-space procedure (finite-difference method); the latter is found highly efficient. We also calculate the integrated strength of second-harmonic current induced in the free carriers near the surface by far-infrared radiation with frequency well below the free-carrier plasma frequency. We find values of the dimensionless parameter which controls the strength of the second-harmonic current to be substantially smaller than those reported recently by Weber and Liebsch, and argue that screening provided by interband transitions is responsible for this difference.