Fumitaka Ohashi

Complex changes in the framework of endohedrally Na-doped type II Si clathrates with respect to Na content

Crystal structures of endohedrally Na-doped type II silicon clathrates with variable Na content were refined by Rietveld analysis. Type II Si clathrates have two types of cages: small and large. The large cages were preferentially occupied by Na atoms. Upon occupation by Na in the large cages, the lattice constant of the clathrates decreased slightly. The attractive interaction of the Na atoms in the large cages with a framework that caused displacement of the Na atoms from the center might influence lattice shrinkage. Moreover, in the region where almost large cages contained Na atoms and the small cages were partially occupied by Na, the lattice constant increased with the Na content; however, the relationship between these features was complex. The lattice constant demonstrated a linear relationship with the size of the small cages; however, the enlargement of the small cages was not linear with respect to its Na occupancy, resulting in complex changes in the lattice constant with respect to the Na content. It was inferred that enlargement of the small cages by Na insertion may be dependent on the Na occupancy of neighboring small cages.

Effects of Substrate Temperature on Properties of Amorphous InxGa1-xN Films Deposited by Reactive Radio Frequency Sputtering

Amorphous indium gallium nitride (a-InxGa1-xN) films were deposited at various substrate temperatures TS by simultaneous reactive rf-magnetron sputtering using GaN and InN targets. The nitrogen composition ratio increased with an increase in TS, and became almost the same as the stoichiometry ratio at a TS above 100 °C. In the X-ray diffraction patterns of the films deposited at a TS below 200 °C, no perceivable peaks assigned to crystalline InxGa1-xN were found. In those deposited at a TS above 300 °C, however, small broad peaks assigned to crystalline InxGa1-xN were observed. The dark conductivity σd increased with an increase in TS. On the other hand, the photoconductivty σp increased with an increase in TS below 200 °C and a large change in σp was not obtained at a TS above 300 °C. The photosensitivity σp/σd showed its maximum (1175) at a TS of 200 °C.

A thin film of a type II Ge clathrate epitaxially grown on a Ge substrate

A thin film of a type II Ge clathrate, NaxGe136, 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 NaxGe136 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 direction normal to the substrate surface. It was also suggested that the lattice mismatch between NaxGe136 and the Ge substrate is relaxed by a buffer layer of α-Ge having a triple-period superlattice. The electrical resistivity of the NaxGe136 film was estimated from the I–V measurements to be in the order of 101–102 Ω m.

Group IV clathrates for photovoltaic applications

Recent studies on type-II Si/Ge clathrates for photovoltaic (PV) applications are reviewed. The band-gap energies (E g) experimentally estimated for Si and Ge clathrates are discussed on the basis of the comparison with theoretical calculation. For the Si and Ge clathrates, E g = 1.7–1.9 and 0.6–0.8 eV are acceptable values, respectively. Thin films of type-II Si clathrates have been successfully synthesized on Si wafers. A device was fabricated using a Si clathrate thin film, and its PV response was observed. For the Ge clathrate, a single-crystal-like thin film was identified to epitaxially grow on a Ge(111) wafer. The PV response and epitaxial growth of group IV clathrates found in recent studies strongly suggest that these new materials will be the next-generation platform for semiconductor technology.

Electron-Spin Resonance of Type II Si-Clathrate Thin Film for New Solar Cell Material

Silicon clathrate thin films, Na x Si136 (x > 5), were prepared on Si (111) substrates. The X-ray diffraction (XRD) pattern shows a mixture of the type II Si-clathrate crystalline and amorphous phases. The electron-spin resonance (ESR) spectrum for the Si-clathrate thin film consists of two lines with a set of \( g \) value and width, (\( g \), \( \upgamma \)(mT)) = (2.005, 1) and (2.002, 0.1). In comparison, the ESR spectra for Na x Si136 (x = 5.5, 11) polycrystalline powders have a fairly broad line with (\( g \), \( \upgamma \)(mT)) = (~2.05, 15) other than the above two lines. Such broad line may be assigned to a Na-Na pair and/or a Na-cluster in the Si-clathrate polycrystalline powders. These ESR results suggest that the dominant line with (\( g \), \( \upgamma \)(mT)) = (2.005, 1) observed for the Si-clathrate thin film is assigned to electron trapped at a Si-deficit with a dangling bond created in the amorphous phase.