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 an indirect band gap. In order to have a good description of the lowest conduction band, we have used the nearest-neighbor 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 psi(GAMMA) and psi(X) (GAMMAX mixing). With an applied bias V greater than or similar to 0. 5 V electrons may enter the GaAs collector contact in both the GAMMA and the X valleys (GAMMAx transfer). We have studied a number of GaAs/AlAs structures. For very narrow barriers there is little GAMMAX transfer, but AlAs barriers wider than about 25 angstrom act as ''GAMMAX filters,'' i.e., most transmitted electrons have been transferred to the X valley.