Synthesis and optical properties of Ge clathrate films with and without Al doping

An attempt to prepare thin films of Al-doped type II Ge clathrates Na- x (Al y Ge1-y )(136) was made by using amorphous Ge films including 0-7 atomic percent Al as the starting materials. The preparations were conducted using our recently established method, which enables the deposition of Na on the starting material with simultaneous or subsequent annealing under a high vacuum. X-ray diffraction measurements confirmed the type II clathrate structure in the prepared film. Smaller lattice constants and an Al-Ge related Raman peak found in the Al-included samples suggested the partial substitution of Al for Ge in the clathrate structure. Optical transmission measurements indicated that the fundamental absorption edge is shifted to higher energies due to the inclusion of Al.