Optical Absorption in SiGe/Si Quantum Well Structures Created by Subband Transitions

The absorption in Si_(1-x)Ge_(x)/Si multiple quantum-well structures is measured. Several separated well absorption peaks corresponding to both intersubband and intervalence band transitions in the samples are observed. In the normal incidence, two broadband peaks are attributed to intervalence band transitions HH0-SO0(2.5μm),HH_(0)-LH_(0)(~ 3μm), respectively. Using 45° incidence of unpolarized light, both the intervalence band and intersubband transitions are observed. The intervalence band transitions (HH_(0)-LH_(0) ) are Ge composition dependent, but the intersubband transitions, HH_(0)-HH_(1) (5.9μm) and HH_(0)-HH_(2)( 4.3 μm), are not sensitive to the Ge composition.