Intersubband Absorption in Modulation-Doped p-Type S i/S i_1-x G e _x Quantum Wells: A Systematic Study

A study of intersubband infrared (IR) absorption in modulation-doped p-type Si/SiGe quantum wells is presented for SiGe wells with widths between 26 Å and 36 Å and Ge contents in the range from 19% to 50%. Absorption lines between 725 cm^-1 and 1830 cm^-1 are observed. Depending on the barrier height (i.e., on the Ge content of the wells), the heavy-hole subbands excited by the IR 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 calculation are in excellent agreement with the measured absorption spectra.