Many-Body and Correlation Effects in Surface and Interface Spectra of Optically Absorbing Materials

We consider the optical properties of Si surfaces and interfaces where all bonds are nominally saturated. Here, contributions arise mainly from many-body and correlation effects such as the surface-local-field effect and components related to derivatives of the substrate dielectric function with respect to critical point energy Eg and broadening parameter Γ. Differences Δ〈e〉 between pairs of ellipsometrically measured reference-quality pseudodielectric function spectra 〈e〉 can be modeled with the above two effects together with that of residual overlayers, including microscopic roughness. We show that derivative components follow from first-order time-dependent perturbation theory if we retain excited-state correlations caused by the finite penetration depth of light in optically absorbing materials. These correlations lead to a substantially different picture of optical absorption than ordinarily assumed.