Short-period GaAs-AlAs superlattices: Optical properties and electronic structure

We report the results of low-temperature photoluminescence and photoluminescence excitation studies of short-period, n=m, (GaAs${)}_{\mathrm{n}}$-(AlAs${)}_{\mathrm{m}}$ superlattices fabricated by molecular-beam epitaxy. Values of n ranged between 2 and 8. We find that the smallest energy gap does not approximate to that of an ${\mathrm{Al}}_{0.5}$ ${\mathrm{Ga}}_{0.5}$As alloy until n+m≤4 monolayers. The limits of a simple Kronig-Penney model of the electronic states are explored in relation to our observations. For the direct gap, good agreement is achieved between experiment and theory down to m=n≥4 monolayers. The description of the ‘‘pseudodirect’’ gap breaks down at n=m∼6 monolayers. Reasons for the failure of the simple description are discussed.