Photo-induced intersubband absorption in Si/Si1−xGex quantum wells

We have investigated photo-induced intersubband absorption in the valence band of Si/SiGe quantum wells. Carriers are optically generated in the quantum wells using interband optical pumping. Both p- and s-polarized intersubband absorptions of 3 nm 30% SiGe multi-quantum wells are measured using a waveguide geometry. At normal incidence, the infrared modulation is surprisingly quenched around 10 μm. We show that this result can be interpreted by taking into account the Fabry-Perot reflections in the multi-quantum well stack and the anomalous dispersion associated with intersubband transitions. The intersubband absorption is found to be dependent on the interband excitation wavelength. This indicates that the s-polarized photo-induced absorptions which involve bound-to-bound and bound-to-continuum transitions are probably assisted by potential fluctuations linked to interface roughness.