AlAs-monolayer dependence of the Gamma-X coupling in GaAs-AlAs type-II heterostructures

For ideal type-II GaAs/AlAs superlattices it was predicted that the couplin g between Gamma and X-z electron states is allowed (forbidden) if the numbe r of monolayers in AlAs slabs is even (odd). We use a simpler structure, na mely, a single GaAs/AlAs/GaAs type-II double quantum well with thickness gr adient to show experimental evidence of the AlAs-monolayer dependence of th e Gamma -X coupling. A careful determination of layer thicknesses is obtain ed from electron microscopy and optical spectroscopy using additional quant um wells inserted in the structure. The results concerning the Gamma -X cou pling are obtained from the study of the ratio of photoluminescence intensi ties of the zero-phonon line and the phonon replica and from their time dec ay. The variation of the Gamma -X coupling with AlAs thickness cannot be ex plained simply by the variation of the overlap of Gamma and X-z envelope fu nctions. It clearly shows the monolayer dependence of the Gamma -X mixing p otential. We develop a simple model to obtain the Gamma -X coupling in the case of nonabrupt interfaces. The amplitude of variation of the radiative r ecombination time due to the Gamma -X mixing is well reproduced within this model.