Electronic states in GaAs-AlAs short-period superlattices: energy levels and symmetry

We have made a comprehensive photoluminescence (PL) study of the low lying conduction band states of (GaAs)_m/(AlAs)_n type-II short-period superlattices (SL) with m, n ≤ 4, in order to determine their energy and symmetry. The symmetry is found from the shift and splitting of the levels under uniaxial stress, and from the no-phonon oscillator strengths determined by time-resolved PL. Our samples are found to be true superlattices obeying the optical selection rules predicted by the space group symmetry, which determines whether a transition is indirect or pseudo-direct. However, selection rules depending on parity with respect to reflection in the growth plane are not obeyed. The results are compared with theoretical calculations from the literature. When strain due to lattice mismatch is taken into account, the ordering of the levels is found to agree with the most recent calculations, except for the case m = n = 1. In this SL the lowest conduction band state is found to derive from the bulk X valley, rather than from the L valley as predicted. We confirm that this discrepancy can be resolved if there is an ordered interchange of a certain fraction of the Ga and Al atoms, and our data support the theoretical prediction that such an ordered intermixed SL may in fact be more stable than either the perfect SL or the random alloy.