Origin of U-shaped background density of interface states at nonlattice matched semiconductor interfaces

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Abstract
Recent experiments on the behavior of the density of interface states (DOIS) in the semiconductor band gap for Si/SiO2 as well as for III–V compound semiconductor/dielectric systems reveal the presence of a U-shaped background with specific structure riding on top. Among several suggestions for the origin of such states, the idea that strain/ disorder present at the interface may play a significant role has also been put forward. However, the nature of the states contributing to the background, and especially their energy distribution, has remained unexplained. In this paper we show that a thin disordered layer on the semiconductor side near the interface can produce such a U-shaped background in the DOIS arising from the Anderson localized states. Employing a simple model of a disordered system we provide the first derivation of their energy distribution valid over essentially the whole band tail region away from valence and conduction band edges. We have re-examined the existing data on Si/SiO2 interfaces in light of the above proposition and the new theoretical results. The exponential decay rates of the DOIS from the valence and conduction band edges, calculated on the basis of expected and realistic disorder, are found to provide a quantitative and consistent understanding of the behavior of the U-shaped curve throughout the gap region. It is thus suggested that lattice matched semiconductor-dielectric systems should be sought for applications requiring very low levels of interface state density.

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