Free-carrier and intersubband infrared absorption in p-type ${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$/Si multiple quantum wells

Infrared absorption has been investigated in high-quality p-type ${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$/Si multiple quantum wells grown by UHV–chemical vapor deposition. Sample parameters have been chosen in order to obtain an intersubband absorption peak at λ≊10 μm. Direct transmission as well as transmission through multipass waveguides have been measured for various radiation polarizations, doping levels, and temperatures. Transmission of single uniformly doped SiGe epilayers has also been performed. Both intersubband and free-carrier absorptions have been quantitatively analyzed using a modified Drude model, while the valence states have been obtained from an envelope-function formalism. The relative contributions of both free carriers and intersubband transitions to the total absorbance have been accurately determined as a function of polarization and doping level. The absorption coefficients have been deduced for radiation electric field along the growth axis and parallel to the plane of the layers. Finally, the infrared absorption at normal incidence is discussed as well as the selection rules of the intersubband transitions.