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 coupling between Gamma and X-z electron states is allowed (forbidden) if the number of monolayers in AlAs slabs is even (odd). We use a simpler structure, namely, a single GaAs/AlAs/GaAs type-II double quantum well with thickness gradient to show experimental evidence of the AlAs-monolayer dependence of the Gamma -X coupling. A careful determination of layer thicknesses is obtained from electron microscopy and optical spectroscopy using additional quantum wells inserted in the structure. The results concerning the Gamma -X coupling are obtained from the study of the ratio of photoluminescence intensities of the zero-phonon line and the phonon replica and from their time decay. The variation of the Gamma -X coupling with AlAs thickness cannot be explained simply by the variation of the overlap of Gamma and X-z envelope functions. It clearly shows the monolayer dependence of the Gamma -X mixing potential. We develop a simple model to obtain the Gamma -X coupling in the case of nonabrupt interfaces. The amplitude of variation of the radiative recombination time due to the Gamma -X mixing is well reproduced within this model.