Synthesis and evaluation of optical properties of SiGe alloy clathrate films

Type II SiGe alloy clathrate thin films were synthesized using Si(111) substrates coated with 50 nm Ge films as starting materials. During annealing in an Ar atmosphere, Na and Ge atoms diffused into the Si substrate. Subsequent annealing under vacuum conditions transformed these precursors into type II clathrate structures. The film thickness and Ge composition were increased and decreased, respectively, as the Ar annealing duration was increased. Photoluminescence peaks were observed around 1.6 eV, which is lower than that of type II Si clathrates, indicating the narrowing of the bandgap energy due to inclusion of Ge into the clathrate framework.