Multiband tight-binding approach to tunneling in semiconductor heterostructures: Application to Gamma X transfer in GaAs.

We study tunneling in semiconductor heterostructures where the constituent materials can have a direct or indirect bandgap. In order to have a good description of the lowest conduction band, we have used the nearest–neighbour sp3s∗ tight–binding model put forward by P. Vogl et al.. A recursive Green– function method yields transmission coefficients from which an expression for the current density may be written down. The method is applied to GaAs/AlAs heterostructures. Electrons may traverse the AlAs barriers via different tunneling states ψΓ and ψX (ΓX mixing). With an applied bias V > ∼0.5 V electrons may enter the GaAs collector contact in both the Γ and the X valley (ΓX transfer). We have studied a number of GaAs/AlAs structures. For very narrow barriers there is little ΓX transfer, but AlAs barriers wider than about 25 Å act as “ΓX filters”, i.e., most transmitted electrons have been transfered to the X valley.