Abstract
We report measurements of reflectance difference (RD) spectra for $c(4\ifmmode\times\else\texttimes\fi{}4)$ and $(2\ifmmode\times\else\texttimes\fi{}4)$ reconstructed (001) surfaces of GaAs. Surface dimers induce an inhomogeneous orthorhombic strain field that penetrates several monolayers into the crystal. The thickness $d$ of this strained region is smaller than the penetration depth $L$ of the probing light, and an inhomogeneous anisotropy formalism can be used. The dielectric function of GaAs at room temperature can be described by a discrete (Lorentzian) excitonic line shape. For the contribution of the region near the surface to the optical response, the excitonic response should be at least partly quenched because of the electric field present near the surface. Thus, a one-electron contribution (logarithmic line shape) may have to be used to fit the RD spectra. By using an inhomogeneous perturbation formalism and a logarithmic line shape, we have been able to isolate the strain component induced by the dimers around ${E}_{1}$ and ${E}_{1}+{\ensuremath{\Delta}}_{1}$ critical points. We also inferred that the RD spectra include a component related to surface roughness.
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