PHOTOINDUCED INTERSUBBAND ABSORPTION IN SI SIGE QUANTUM-WELLS/

We have investigated photo-induced intersubband absorption in the vale nce band of Si/SiGe quantum wells. Carriers are optically generated in the quantum wells using an argon ion laser. The resulting infrared ab sorption is probed with a step-scan Fourier transform infrared spectro meter. The photo-induced infrared absorption in SiGe quantum wells is dominated by two contributions: the free carrier absorption, which is similar to bulk absorption in a uniformly doped SiGe layer, and the va lence subband absorption in the quantum wells. Both p- and s-polarized intersubband absorptions are measured. We have observed that the phot o-induced intersubband absorption in doped samples is shifted to lower energy as compared to direct intersubband absorption. This absorption process is attributed to carriers away from the Brillouin zone center . We show that the photo-induced technique is appropriate to study val ence band mixing effects and their influence on intersubband absorptio n.