Intra-valence-band mixing in strain-compensated SiGe quantum wells

We explore the midinfrared absorption of strain-compensated $p\text{\ensuremath{-}}{\mathrm{Si}}_{0.2}{\mathrm{Ge}}_{0.8}∕\mathrm{Si}$ quantum wells for various well thicknesses and temperatures. Owing to the large band offset due to the large bi-axial strain contrast between the wells and barriers, the intersubband transitions energies from the ground state to the excited heavy hole (HH), light hole (LH), and split-off (SO) hole states up to $\ensuremath{\sim}0.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ are resolved. When HH2 is within $\ensuremath{\sim}30\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ of LH1 or SO1 a partial transfer of the HH1-HH2 oscillator strength to the HH1-LH1 or HH1-SO1 transitions is observed, which is otherwise forbidden for light polarized perpendicular to the plane of the wells. This is a clear sign of mixing between the HH and LH or SO states. A large temperature induced broadening of HH1-HH2 transition peak is observed for narrow wells indicating nonparabolic dispersion of the HH2 states due to the mixing with the LH/SO continuum. We found that the observations are in good agreement with the six-band $\mathbf{k}∙\mathbf{p}$ theory. A possible role of many-body effects in the temperature-induced negative peak shift is discussed.