Quasi-direct optical transitions in Ge nanocrystals embedded in GeO$_2$ matrix

GeO$_2$ films with germanium nanocrystals (NCs) were deposited from supersaturated GeO vapor with subsequent dissociation on Ge:GeO$_2$. The films were studied using photoluminescence (PL), Raman scattering, IR-spectroscopy techniques. Ge NCs in initial film have sizes about 6–8 nm and have no visible PL signal. The broad green-red PL peak was detected in Ge:GeO$_2$ films after thermal annealings. According to effective mass approach, maximum of PL signal from such relatively big Ge NCs should be in IR region. The experimentally observed PL signal is presumably originated due to quasi-direct $L_1-L_3'$ optical transitions “folded” in germanium NCs.