Optical Absorption in SiGe/Si Quantum Well Structures Created bySubband Transitions

The absorption in Si_1-xGe_x/Si multiple quantum-well structures is measured. Several separated well absorption peaks corresponding to both intersubband and intervalance band transitions in the samples are observed. In the normal incidence, two broadband peaks are attributed to intervalence band transitions HH₀-SO₀(2.5μm), HH₀-LH₀(～ 3μm), respectively. Using 45°incidence of unpolarized light, both the intervalance band transitions and intersubband transitions are observed. The intervalance band transitions (HH₀-LH₀) are Ge composition dependent, but the intersubband transitions, HH₀-HH₁(5.9μm) and HH₀-HH₂(4.3μm), are not sensitive to the Ge composition.