Tamm states and donors at InAs/AlSb interfaces

Localized interface states, viz., generalized Tamm states, can be induced by an interfacial InSb bond in the InAs/AlSb heterojunction system, as confirmed by electronic structure calculations. The calculated energies of the interfacial Tamm states, however, are too low to account for the observed carrier concentrations in InAs quantum wells. Native defects capable of accounting for the observed carrier concentrations are identified, and their electronic structures calculated: AlSb in an AlSb layer is responsible for the semi-insulating character of thin InAs quantum wells and the n-type character of wide wells, and AsAl at an AlAs-like interface is responsible for the high values of electron concentration in the wells. The decrease of electron concentrations with temperature can be attributed to partial freezing of electrons into the shallow levels associated with ionized donors.