We have developed an eight-band finite-difference envelope function approximation model capable of reproducing in almost all situations the true D 2d or C 2v symmetry of [001] grown zinc-blende heterostructures. We have used our model to study the relative contributions of the bulk inversion asymmetry (BIA) and structural inversion asymmetry to the spin splitting in the conduction band of asymmetric AlSb/GaSb/InAs/AlSb quantum wells, and clarify apparently contradictory statements about the

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... tive magnitude of the two contributions. We show that, in the system under study, the inclusion of BIA effects changes considerably the angular dependence and the magnitude of the splitting. We also investigate how BIA changes the transmission properties of a resonant tunnelling structure.