$\Gamma -X$ electron level crossover in ZnSe/BeTe multiple quantum wells

We report on the photoluminescence (PL) properties of thin BeTe/ZnSe/BeTe type II quantum wells (QW’s), where the $\Gamma$ electron level in ZnSe is close to the resonance with the X electron level in BeTe. The $\Gamma -X$ levels crossover modifies the nature of the interband optical transitions which are indirect in real space but direct in $\mathbf{k}$ space for the thick enough QW’s and direct in real space but indirect in $\mathbf{k}$ space for the QW’s thinner than 2–3 monolayers. Approaching the crossover condition from the side of thicker QW’s leads to a thresholdlike quenching and spectral narrowing of the PL band. The latter effect can be explained taking into account the inhomogeneous broadening of the $\Gamma$ electron level.