A thin film of a type II Ge clathrate, Na x Ge 136 , was epitaxially grown on a (111) substrate of Ge with a diamond structure (α-Ge). A Zintl phase NaGe film was synthesized in advance by a reaction of the substrate surface with Na vapor under an Ar atmosphere, and was highly oriented such that the NaGe(100) planes were parallel to the Ge(111) surface. The NaGe film was transformed to the Na x Ge 136 film by heat treatment under dynamic vacuum. XRD measurements demonstrated that the prepared film consisting of twin crystals with a (111) twin plane was epitaxially grown with the <111> direction normal to the substrate surface. It was also suggested that the lattice mismatch between Na x Ge 136 and the Ge substrate is relaxed by a buffer layer of α-Ge having a triple-period superlattice. The electrical resistivity of the Na x Ge 136 film was estimated from the I-V measurements to be in the order of 10 1 -10 2 Ω m.
ExperimentalFrequently, type II clathrates are synthesized from Zintl alloys of alkali elements and Si or Ge. 1,3,12,16,20,21 Fig. 1(b) shows 5630
Type II Ge clathrate (Na x Ge 136 ) 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 Na x Ge 136 was developed by applying the electric field in an Ar environment at 275 °C. Rietveld refinement analysis of Na x Ge 136 films showed an almost guest free nature (Na contents x ∼ 0) using this new technique. In addition, absorbance spectra obtained from Na x Ge 136 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 Na x Ge 136 .
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