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)ₙ-(AlAs)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 Al0.5{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.