We investigate the surface band structure in Mg2Si thin films using the first principle band structure calculations. When the film is stacked along the  direction, surface band structures appear inside quantum confinement band structures originates from the bulk band structure. When the film is stacked along the  direction, the semiconductor gap retains while a direct band gap appears.
Magnesium induced crystallization (Mg-MIC), that is, the low temperature crystallization process of an amorphous silicon (a-Si) film activated by magnesium has first been investigated. The crystallization temperature is evaluated as low as 450 °C from Raman spectroscopy in contrast to 600–800 °C in the solid phase crystallization (SPC) process (only a-Si film is heated). The crystallization is found to occur via the formation of intermediate phase: the silicide (Mg2Si) as reported in the Ni-MIC studies. A filamentary structure expected from the previous studies is not observed.
We obtained Mg2Si1−xSnx films on the c-plane sapphire and the (100) CaF2 substrates using the radio frequency (RF) magnetron co-sputtering method under various sputtering area ratio of Mg chips to Mg2Si (Sn) target and a subsequent two-step annealing process up to 400 °C. The X-ray diffraction (XRD) and energy-dispersive X-ray spectrometry (EDS) analysis of the samples confirmed that the obtained films were ternary Mg2Si1−xSnx films with a composition of x ≈ 0.31. Optical microscopy and EDS mapping images of Mg2Si and Mg2Si1−xSnx (x = 0.31) films after annealing at 400 °C showed remarkable Mg desorption from Mg2Si1−xSnx films, but not from Mg2Si films. The Hall effect measurements revealed that all Mg2Si1−xSnx films annealed at 400 °C had a p-type conductivity. The first-principles calculations suggested that a combination of two different types of defects, Sn substitution at Si site (SnSi) and Mg vacancy (VMg), which acts as an acceptor, could be the origin of the p-type conductivity of Mg2Si1−xSnx films.
Direct transition energies of Mg2Si were obtained by photoreflectance (PR) spectra of a highly-oriented Mg2Si(111)//Si(111) film. In the PR spectra at 9 K, direct transition energies of E1 = 2.38 eV, E2 = 2.58 eV, E3 = 2.69 eV, and E4 = 2.82 eV were observed. In the temperature dependence of PR spectra, E1 and E2 shifted to lower energy at high temperatures, but there was no temperature dependence of transition energies in E3 and E4. These results showed that the temperature dependences of band structure in Mg2Si differ at direct transition points.
Si clathrate thin films were fabricated by using Si(111) substrates, Na lumps and NaH powder at various conditions in two steps annealing process. The duration of the first annealing to prepare precursor films affected the thickness and the surface morphology of the final products, i.e., Si clathrate films. The annealing duration of 18 h led to Si clathrate films of 1–2 µm in thickness. Attempts to control the rate of reduction of Na from the precursor films were carried out in the second step annealing under vacuum. The obtained results suggested that the rate of Na reduction affects the structure type (type I or II) of clathrate.
Guest free Type II Si–Ge alloy clathrate exhibits tunable band gap energy depending upon the composition ratio of Si and Ge. In an attempt to synthesize guest free type II Si–Ge alloy clathrates in various Si–Ge compositions, Na4(Si1−yGey)4 precursors were prepared with the variation of y from 0 to 1 and then annealed in a sealed glass tube together with ionic liquid. The obtained samples were characterized by powder XRD experiments and EDX measurements. The synthesis of Si–Ge clathrates were recognized as Si or Ge rich compositions. EDX data and Rietveld analyses of XRD data allowed us to verify the Si–Ge alloy clathrate with low Na content.
CaF2 nanostructures were synthesized from Ca-silicide powders by a diluted aqueous HF treatment. Commercially-available CaSi2 crystal powders and calcium silicide powders prepared by mechanical alloying were used as the source materials, and CaF2 nanosheet bundles and nanobunches of the CaF2 nanoparticles were obtained, respectively. The morphological property of the resulting CaF2 nanostructures was characterized by electron microscopy. It was found that the morphology of the resulting products depended on the starting materials. In addition, the growth mechanism of the CaF2 nanostructures was discussed from a topological synthesis point of view.
Fe3O4 thin films of approximately 30 nm in thickness were grown on Si substrate at 573 K by means of ion beam sputter deposition (IBSD) technique, using oxygen or argon ions to sputter Fe3O4 solid target. The effect of these irradiation atmospheres on the chemical composition and crystallinity of the thin films was investigated using XPS (X-ray Photoelectron Spectroscopy), RHEED (Reflection High-Energy Electron Diffraction), and XRD (X-Ray Diffraction). We revealed that the oxygen atmosphere improves the crystallinity of the film as compared with argon atmosphere, but also causes the formation of iron oxide phases other than Fe3O4. On the other hand, the obtained results for Fe3O4 thin films prepared in argon atmosphere had poor crystallinity. Furthermore, the results of RHEED suggested that the preferential growth orientations of the film are different depending on the irradiation atmospheres.
Fe/nitrogen-doped carbon (50 nm)/Fe3Si tri-layered films were fabricated on Si(111) substrates by physical vapor deposition with a mask method, and the magnetic and electrical properties were investigated. Spin-valve signals were observed in magnetoresistance curves measured in local structure at not only 5 K but also room temperature. It was demonstrated that spin transport in variable hopping is possible for N-doped carbon.
We investigated an electrical conduction mechanism of Si/FeSi2 composite films by measuring a frequency dependence of AC electrical conductivity. The results were analyzed based upon Jonscher’s power law. The hopping conduction obeying the Jonscher’s power law was observed for a-Si single films after annealing as well as Si/FeSi2 composite films annealed at 550 and 900 °C. From the analysis of XRD, optical absorption, TEM/EDS and AC conductivity, we conclude that the electrical conduction mechanisms in polycrystalline (poly)-Si/β-FeSi2 annealed at 550 °C and poly-Si/α- and β-FeSi2 annealed at 900 °C are due to electron hopping via the conduction band of β-FeSi2 and α-FeSi2 nanocrystals embedded in poly-Si thin films, respectively.
Lithium silicide Li12Si7 (orthorhombic) and Li7Si3 (trigonal), composed of Li ions and Si clusters were synthesized by heat treatment of Li and Si mixture. Their high-pressure properties were investigated by synchrotron X-ray diffraction (XRD) measurements using a diamond anvil cell (DAC). Compression was successfully made up to 16 GPa for Li12Si7 and 20 GPa for Li7Si3, but no phase transition was observed. The bulk modulus was obtained from the fitting by Murnaghan equation of state. The obtained bulk moduli were compared with those of other lithium silicides, Si and Li, and there were found to be correlation between the bulk modulus and the Li–Si composition ratio.