The authors report on a detailed photoluminescence study of a series of GaAs/AlAs type II quantum well structures. In these structures the thickness of the GaAs layers was kept constant at 25 AA and the thickness of the AlAs layers was varied between 28 and 114 AA. As the AlAs layer thickness was increased they observed a large change in the relative strengths of the zero-phonon and phonon-assisted type II exciton recombinations. They have interpreted this phenomenon as being due to a change in the nature of the lowest lying AlAs electron state from Xxy (k perpendicular to (001)) to Xz (k parallel to (001)) as the AlAs thickness was decreased, where (001) is the growth direction. This changeover is due to the competing effects of electron confinement and strain produced by the small but finite lattice mismatch between GaAs and AlAs. They have estimated the strain splitting of the different X states to be 23 meV. Calculations using this figure along with an envelope function treatment of the effects of electron confinement predict that the lowest confined Xz and Xxy states should cross at an AlAs thickness of the order of 60 AA. For the samples with the thickest AlAs layers (94 and 114 AA) they were able to resolve two zero-phonon lines which they attribute to the different X states, the measured splittings are in good agreement with the calculations.
