Takabumi Humoto

Bio: Takabumi Humoto is an academic researcher. The author has contributed to research in topics: Magnetoresistance & Hall effect. The author has an hindex of 1, co-authored 1 publications receiving 50 citations.

Quantum size effect in thin bismuth films

Abstract: The dependence of the resistivity ρ magnetoresistance Δρ/ρ and Hall coefficient RH of thin Bi films (thickness d: 700–2000 A) on thickness and temperature was studied. Thin Bi films were grown epitaxially on freshly cleaved mica substrates by thermal evaporation in ultra-high vacuum (1–5×10-8 Torr). These films had a mosaic structure of equally oriented crystallites (grain size: about 1 µm). The plane of the films was perpendicular to the trigonal axis of Bi. The oscillatory thickness-dependence (its period Δd: 400 A) of the galvanomagnetic coefficients and the decrease of oscillation amplitudes with increasing temperature were observed. The present authors also observed that the temperature at which the resistivity minimum occurred and the rate of the increase of carrier concentration with increasing temperature and also the oscillatory thickness-dependence, and the authors explained these oscillatory behaviours in terms of quantum size effect (QSE). In order to compare the experimental results with the QSE theory, the numerical calculation from this theory for thin Bi films was done. These thickness dependence agreed well with the QSE theory.

Large-Area Dry Transfer of Single-Crystalline Epitaxial Bismuth Thin Films

Abstract: We report the first direct dry transfer of a single-crystalline thin film grown by molecular beam epitaxy. A double cantilever beam fracture technique was used to transfer epitaxial bismuth thin films grown on silicon (111) to silicon strips coated with epoxy. The transferred bismuth films retained electrical, optical, and structural properties comparable to the as-grown epitaxial films. Additionally, we isolated the bismuth thin films on freestanding flexible cured-epoxy post-transfer. The adhesion energy at the bismuth/silicon interface was measured to be ∼1 J/m2, comparable to that of exfoliated and wet transferred graphene. This low adhesion energy and ease of transfer is unexpected for an epitaxially grown film and may enable the study of bismuth’s unique electronic and spintronic properties on arbitrary substrates. Moreover, this method suggests a route to integrate other group-V epitaxial films (i.e., phosphorus) with arbitrary substrates, as well as potentially to isolate bismuthene, the atomic th...

Resistivities of titanium nitride films prepared onto silicon by an ion beam assisted deposition method

Abstract: Cubic titanium nitride (TiN) films preferentially oriented to the (200) lattice plane were deposited onto (111) silicon wafers using an ion beam assisted deposition technique with an electron cyclotron resonance ion source for ionizing nitrogen gas and an electron beam evaporator for evaporating Ti metals. The resistivities of the TiN films were inversely proportional to the average size of the crystallites making up the TiN films and decreased with increasing substrate temperature and film thickness. TiN films thicker than 50 nm had resistivities around 30 µ Ω cm, slightly higher than the resistivities of TiN crystals.

Prediction of Anisotropic Single-Dirac-Cones in Bi${}_{1-x}$Sb${}_{x}$ Thin Films

Abstract: The electronic band structures of Bi${}_{1-x}$Sb${}_{x}$ thin films can be varied as a function of temperature, pressure, stoichiometry, film thickness and growth orientation. We here show how different anisotropic single-Dirac-cones can be constructed in a Bi${}_{1-x}$Sb${}_{x}$ thin film for different applications or research purposes. For predicting anisotropic single-Dirac-cones, we have developed an iterative-two-dimensional-two-band model to get a consistent inverse-effective-mass-tensor and band-gap, which can be used in a general two-dimensional system that has a non-parabolic dispersion relation as in a Bi${}_{1-x}$Sb${}_{x}$ thin film system.

Effect of size quantization on the Einstein relation in ultrathin films of non-parabolic semiconductors

Abstract: An attempt is made to investigate the effect of size quantization on the diffusivity-mobility ratio of the carriers in ultrathin films of semiconductors having Kane-type nonparabolic energy bands. It is shown, takingn-type InSb as an example, that the ratio oscillates both with increasing film thickness and with increasing carrier concentration under degenerate conditions and remains unaffected otherwise. The corresponding results for parabolic semiconductors are also obtained from the expressions derived.