Coupled plasmon and phonon excitations in the space-charge layer on GaAs(110) surfaces

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Abstract
High-resolution electron-energy-loss spectroscopy (HREELS) is used to study the surface plasmon and phonon in the space-charge layer on doped GaAs(110) cleaved surfaces. Three losses are observed, with energies of about 27, 36, and 42 meV. The peak at 36 meV is attributed to an unscreened phonon in the depletion layer. The other two losses are due to the coupled modes of the plasmon and phonon propagating near the interface of the depletion layer and the bulk. The surface Fermi-level pinning is induced either by residual-gas interaction with the surface or by deliberate hydrogen adsorption; this in turn modifies the depletion-layer thickness. This modification results in a quite pronounced change of the relative intensities and noticeable energy shifts of these losses. A model energy-loss spectrum calculated in the framework of local-response theory is used to deduce the properties of the space-charge layer. Our model is the first to use a self-consistent free-carrier profile together with the Lindhard description of the local dielectric response.