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 Si1-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 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.