Polarization-dependent absorption in Ge/SiGe multiple quantum wells: Theory and experiment

Polarization resolved absorption spectra of a strain-compensated Ge multiple quantum well (MQW) structure with Ge-rich SiGe barriers have been calculated with an sp³d⁵s^ tight-binding model and measured for light propagating perpendicular to the growth direction. The MQW was grown by low-energy plasma-enhanced chemical vapor deposition and consists of 50 Ge quantum wells deposited onto a thick graded Si_1-xGeₓ buffer layer. The MQW was structurally characterized by high-resolution x-ray diffraction. The measured absorption spectra show clear quantum confined excitonic transitions related to the Ge Γ point band gap, and strong dependence on the incident light polarization, as expected from selection rules for type I direct gap quantum confined systems. A good agreement between theoretically predicted spectra and experimental data is found, demonstrating light and heavy hole polarization-dependent selection rules in Ge MQWs.