Abstract
Second-order nonlinear mixing is evaluated as a probe of the depletion electric field in the near-surface region of GaAs(001). A phenomenological model is presented whereby the nonlinear susceptibility is expanded in the depletion electric field. For GaAs, three terms contribute to the observed nonlinear mixing: the dipole-allowed bulk contribution and first- and second-order contributions in the depletion electric field. All three contributions can be isolated by a combination of rotational anisotropy and photomodulation studies. The second-harmonic signal from oxidized GaAs(001) surfaces was measured as a function of azimuthal sample orientation, photomodulation, and dopant density for fundamental photon energies in the region 1.17--1.51 eV. The first-order depletion-electric-field contribution dominates the second-order contribution, and can be comparable to the intrinsic bulk contribution for high surface fields. The results rule out significant contributions arising from either bulk electric quadrupole or surface dipole effects.
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