Intersubband absorption in modulation doped p-type Si_1-xGe_x quantum wells: Theory and experiment

A study of intersubband infrared absorption in modulation doped p-type Si/SiGe quantum wells is presented for SiGe wells with thicknesses between 22 angstrom and 64 angstrom and Ge contents in the range from 23% to 58%. The peak positions of the absorption lines are observed between 500 cm^-1 and 2200 cm^-1. Depending on the barrier height (i.e., on the Ge content of the wells), the heavy-hole states excited by the infrared radiation are either localized in the wells or strongly mixed with barrier-bound states and therefore delocalized. The shape of the absorption line correspondingly changes from a narrow Lorentz line to a rather broad absorption band. Using the structural parameters determined by high-resolution triple-axis x-ray diffraction, the results of a self-consistent Luttinger-Kohn type envelope function approach with the explicit inclusion of the strain in the quantum wells are in excellent agreement with the measured spectra.