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Growth of Ge clathrate on sapphire and optical properties

Type II Ge clathrate (NaxGe136) films containing a small amount of Na were successfully synthesized on a sapphire substrate by thermal annealing of precursor NaGe composite films. A new technique for Na removal from NaxGe136 was developed by applying the electric field in an Ar environment at 275 °C. Rietveld refinement analysis of NaxGe136 films showed an almost guest free nature (Na contents x ∼ 0) using this new technique. In addition, absorbance spectra obtained from NaxGe136 films showed a decrease in the absorbance intensity in the lower photon energy (<0.5 eV) with annealing. It suggests the decrease in free carrier absorption by the removal of Na atoms in NaxGe136.