Role of localized interface states at type-II heterojunctions

At a heterojunction the constituents of which share neither common anion nor common cation, the interface can be considered as a very thin interlayer of a third material. Depending on the band alignments this built-in interface layer may be a thin quantum well for both electrons and holes. In this case it can trap carriers at the interface. We show why this effect is mainly significant at type-II heterojunctions. The effect is thus twofold: modification of the apparent band offset (related to the effective band gap), and enhancement of the overlap integrals between conduction and valence states, which leads to high transition probabilities. Here we first review a lot of heterojunctions likely to present this phenomenon. Then we show that this can be observed for ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{\mathit{y}\mathrm{-}\mathit{x}}$ ${\mathrm{In}}_{1\mathrm{-}\mathit{y}}$As/InP systems, and we detail the case of ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{In}}_{1\mathrm{-}\mathit{x}}$As/InP. We provide a semiempirical tight-binding calculation which includes strain effects at the interface. The resulting local density of states clearly demonstrates whether electrons or holes are trapped at the heterojunction.