Band-population interference phenomena in the electroreflectance of narrow-gap semiconductors under heavy surface accumulation

Band-population effects have been well established as being responsible for portions of the electroreflectance spectrum, in a metal-oxide-semiconductor configuration, of narrow-gap semiconductors. In this paper we show both experimentally and theoretically that these effects lead to unexpectedly strong interference phenomena and broadening under accumulation conditions at the surface. Specifically, as the gate bias is increased into accumulation the electroreflectance signal splits into two structures one of which shifts to the blue with increasing bias; the other is relatively insensitive to bias. The results can be directly correlated with the amount of surface band bending, the screening length, the bulk Fermi level, and band nonparabolicity. A simple oscillator model is used to describe the changes in the complex dielectric function. This model, when used with the Aspnes-Frova relation for spatially varying dielectrics, shows more clearly the origin of the observed phenomenon. This analysis facilitates the interpretation of electroreflectance data in degenerate semiconductors and allows determination of surface-related parameters.