MULTIBAND TIGHT-BINDING APPROACH TO TUNNELING IN SEMICONDUCTOR HETEROSTRUCTURES - APPLICATION TO GAMMA-X TRANSFER IN GAAS

We study tunneling in semiconductor heterostructures where the constit uent 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 th e nearest-neighbor sp3s tight-binding model put forward by P. Vogl et al. A recursive Green-function method yields transmission coefficient s from which an expression for the current density may be written down . The method is applied to GaAs/AlAs heterostructures. Electrons may t raverse the AlAs barriers via different tunneling states psi(GAMMA) an d psi(X) (GAMMAX mixing). With an applied bias V greater than or simil ar to 0. 5 V electrons may enter the GaAs collector contact in both th e GAMMA and the X valleys (GAMMAx transfer). We have studied a number of GaAs/AlAs structures. For very narrow barriers there is little GAMM AX transfer, but AlAs barriers wider than about 25 angstrom act as ''G AMMAX filters,'' i.e., most transmitted electrons have been transferre d to the X valley.