Surface-layer modulation of electron concentrations in InAs–AlSb quantum wells

A strong dependence of the electron concentration in not-intentionally doped InAs–AlSb quantum wells on the nature of the surface layer is demonstrated. We find that quantum wells capped with a GaSb layer result in much higher electron concentrations than wells capped with an InAs layer. The difference in concentration increases as the top barrier thickness of the well decreases. These changes are due to a shift in the pinning position of the Fermi level at the surface. For InAs surface layers, the Fermi-level pinning position also depends sensitively on the presence of residual Sb, which is found to move the pinning position further into the conduction band of InAs. The Fermi level is found to be pinned above the conduction band edge of bulk InAs by 80 meV for as-grown InAs surface layers, and 300 meV for etch-exposed InAs surface layers.