Robust optical orientation of spins in Ge/SiGe quantum wells

Compared to other materials systems, group IV compounds have a greater potential in the field of spintronics because of the long spin coherence time and because Si and Ge can be isotopically purified. Despite such advantages, initialization of spins by angular momentum transfer from light is still one of the main challenges in Si-based systems, and effcient, optical initialization remains to be demonstrated. Our approach, accomplished via band-structure engineering, tackles such long-standing issue. By measuring circular polarization of luminescence above 80%, we demonstrate spin orientation for photo-excited carriers at type I Ge/SiGe quantum wells. Spin orientation is observed even after carrier relaxation from Gamma to L minima of the conduction band, paving the way towards joint implementations of spin and photonic functionalities onto mainstream Si electronics.