Hajime Asahi

Bio: Hajime Asahi is an academic researcher from University of Tokyo. The author has contributed to research in topics: Grain boundary & Bismuth. The author has an hindex of 3, co-authored 3 publications receiving 89 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.

Far-infrared microbolometer detectors

Abstract: The bismuth microbolometer is a simple, easily made detector suitable for use throughout the far-infrared, which has been integrated with a variety of planar antennas. The general thermal properties of these devices and some of the constraints on bolometer materials are discussed. The fabrication and performance of several different types of microbolometers and microthermocouples are described.

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.

Air-bridge microbolometer for far-infrared detection

Abstract: A new microbolometer for far-infrared detection has been fabricated that allows an increase in sensitivity of a factor of 4 over the best previously reported bolometer. By suspending the detector in the air above its substrate a reduction in the thermal conductance out of the device by a factor of 5 has been achieved. At a modulation frequency of 100 kHz this microbolometer has an electrical noise equivalent power of 2.8×10^−11 W(Hz)^−1/2. A thermal model is also presented that accurately fits the response of the detector.