Bandstructure analysis of strain compensated Si/SiGe quantum cascade structures

Si/SixGe1−x quantum well samples with various barrier and well widths as well as Ge concentrations (0.70 < x < 0.85) have been prepared by low temperature molecular beam epitaxy on relaxed buffer layers. Composition and thickness of the strain symmetrized structures are determined by transmission electron microscopy, X-ray diffraction and X-ray reflectivity. The unique set of structural parameters, determined with high accuracy in combination with polarization resolved intersubband absorption and photoluminescence spectra deduced from the same set of samples allows for a detailed comparison with k·p calculations. The observed shift of the band gap luminescence to lower energy with increasing Ge concentration in the well is found to agree very well with accepted values for the valence band offset of 0.72-0.74 eV between Si and Ge pseudomorphically strained to Si0.5Ge0.5.