Type II Band Alignment in ${\mathrm{Si}}_{1-\mathit{x}}{\mathrm{Ge}}_{\mathit{x}}/\mathrm{Si}\left(001\right)$ Quantum Wells: The Ubiquitous Type I Luminescence Results from Band Bending

We present experimental verification of type II band alignment in a coherently strained ${\mathrm{Si}}_{0.7}{\mathrm{Ge}}_{0.3}/\mathrm{Si}\left(001\right)$ quantum well by studying photoluminescence energy shifts under external strains. A recent determination of type I band alignment from a similar experiment is shown to result from band-bending effects due to high excitation. In high quality samples, the type II luminescence can be observed in the absence of external stress by using extremely low excitation. The type II luminescence differs from the well known type I spectrum in a dramatic but as yet unexplained change in the relative intensities of the phonon replicas.