Substrate dependence of adlayer optical response in reflectance anisotropy spectroscopy

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Abstract
Reflectance anisotropy spectroscopy (RAS) offers extreme surface sensitivity and can be used to study gas-solid and solid-liquidinterfaces. Although the technique has already made a considerable contribution to in situ monitoring of the epitaxialgrowth of semiconductors, its impact has thus far been limited by difficulties in interpreting experimental results. In particular, RAS spectra are related to both Δε 2 ′ and Δε 2 ″ , the real and imaginary parts of the surfacedielectricanisotropy, with weighting factors determined by the complex dielectric function of the substrate. Here RAS calculations are performed for a variety of substrates assuming a model overlayer dielectricanisotropy. Three distinct regimes of behavior are observed: RAS spectra can resemble either Δε 2 ′ , Δε 2 ″ or −Δε 2 ″ (or some combination of the three) depending on the substrate and the photon energy. It is therefore crucial to properly account for these substrate effects if RAS is to be used to determine the azimuthal orientation of adsorbates or to study the symmetries of surface states, for example. Similar considerations also apply to the related technique of surface differential reflectance. The Fresnel-based theoretical method used here and the substrate optical functions presented should serve as a useful database for future work as RAS is applied to new systems.