Tsukasa Terada

Bio: Tsukasa Terada is an academic researcher from Osaka University. The author has contributed to research in topics: Thermoelectric effect & Thermal conductivity. The author has an hindex of 5, co-authored 11 publications receiving 91 citations.

Low thermal conductivity in single crystalline epitaxial germanane films

Abstract: We investigated thermal conductivity of epitaxial germanane films: stacked structure of hydrogenated germanenes. It was confirmed that single crystalline germanane films were epitaxially grown on Ge(111). The films exhibited low out-of-plane thermal conductivity of 1.1 ± 0.3 W m−1 K−1 which is lower than other layered materials composed of heavy atoms. This came from weak van der Waals interlayer interaction related to weak polarization in germanane composed of smaller atoms. This demonstrates that choice of small constituent atoms for weakening van der Waals interlayer interaction is a promising thermal conductivity reduction outline for developing ecofriendly high performance thermoelectric layered materials.

Phonon transport in the nano-system of Si and SiGe films with Ge nanodots and approach to ultralow thermal conductivity.

Abstract: Phonon transport in the nano-system has been studied using well-designed nanostructured materials to observe and control the interesting phonon behaviors like ballistic phonon transport. Recently, we observed drastic thermal conductivity reduction in the films containing well-controlled nanodots. Here, we investigate whether this comes from the interference effect in ballistic phonon transport by comparing the thermal properties of the Si or Si0.75Ge0.25 films containing Ge nanodots. The experimentally-obtained thermal resistance of the nanodot layer shows peculiar nanodot size dependence in the Si films and a constant value in the SiGe films. From the phonon simulation results, interestingly, it is clearly found that in the nanostructured Si film, phonons travel in a non-diffusive way (ballistic phonon transport). On the other hand, in the nanostructured SiGe film, although simple diffusive phonon transport occurs, extremely-low thermal conductivity (∼0.81 W m-1 K-1) close to that of amorphous Si0.7Ge0.3 (∼0.7 W m-1 K-1) is achieved due to the combination of the alloy phonon scattering and Ge nanodot scattering.

Low thermal conductivity of complex thermoelectric barium silicide film epitaxially grown on Si

Abstract: We achieved substantially low thermal conductivity by introducing some crystal disorder into complex material BaSi2 films on Si substrates for realization of a high-performance thermoelectric material. The BaSi2 films/Si exhibited a low thermal conductivity of 0.96 W m−1 K−1 without nanostructuring, which is less than about two-thirds value of bulk BaSi2 and is the lowest among ecofriendly silicide materials. This substantially low thermal conductivity was brought by both the use of complex material with intrinsically low phonon group velocity and the introduction of point defects as the crystal disorder. The first-principles calculations revealed that the point defect modulates the phonon dispersion relation lowering longitudinal acoustic phonon group velocity. However, the transverse acoustic phonon group velocity was increased at the same time, resulting in a negligible change in average acoustic phonon group velocity. This indicated that the crystal disorder effect related to point defects in this system is enhancement of phonon scattering, not lowering phonon group velocity. The BaSi2 films/Si with point defects exhibited a higher thermoelectric power factor (2.9 μW cm−1 K−2) than bulk BaSi2. These results highlight that complex material BaSi2 film/Si with point defects, having substantially low thermal conductivity, is a candidate as a thermoelectric power generator material in the sensor network.

Reactivity of Au with ultrathin Si layers: A photoemission study

Abstract: We present a photoemission study on reactivity of the Au–Si system. We studied gold films evaporated atop ultrathin silicon layers previously deposited on GaAs. Following analysis of both the Si 2p core level and the Au 5d valence band spectra related response, we show that the reaction mechanism between Au and Si is affected by structural imperfections of the silicon layer. This is in sharp opposition to some current models of the reactivity mechanism.

[The 4th Thin Film and Surface Physics Division Award Speech] Methodology of thermoelectric power factor enhancement by controlling nanowire interface

Abstract: The simultaneous realization of low thermal conductivity and high thermoelectric power factor in materials has long been the goal for the social use of high-performance thermoelectric modules. Nanostructuring approaches have drawn considerable attention because of the success in reducing thermal conductivity. On the contrary, enhancement of the thermoelectric power factor, namely, the simultaneous increase of the Seebeck coefficient and electrical conductivity, has been difficult. We propose a method for the power factor enhancement by introducing coherent homoepitaxial interfaces with controlled dopant concentration, which enables the quasiballistic transmission of high-energy carriers. The wavenumber of the high-energy carriers is nearly conserved through the interfaces, resulting in simultaneous realization of a high Seebeck coefficient and relatively high electrical mobility. Here, we experimentally demonstrate the dopant-controlled epitaxial interface effect for the thermoelectric power factor enhancement using our \"embedded-ZnO nanowire structure\" having high-quality nanowire interfaces. This presents the methodology for substantial power factor enhancement by interface carrier scattering.

High-Uniformity Threshold Switching HfO2-Based Selectors with Patterned Ag Nanodots.

Abstract: High-performance selector devices are essential for emerging nonvolatile memories to implement high-density memory storage and large-scale neuromorphic computing. Device uniformity is one of the key challenges which limit the practical applications of threshold switching selectors. Here, high-uniformity threshold switching HfO2-based selectors are fabricated by using e-beam lithography to pattern controllable Ag nanodots (NDs) with high order and uniform size in the cross-point region. The selectors exhibit excellent bidirectional threshold switching performance, including low leakage current ( 108), high endurance (>108 cycles), and fast switching speed (≈75 ns). The patterned Ag NDs in the selector help control the number of Ag atoms diffusing into HfO2 and confine the positions to form reproducible filaments. According to the statistical analysis, the Ag NDs selectors show much smaller cycle-to-cycle and device-to-device variations (C V < 10%) compared to control samples with nonpatterned Ag thin film. Furthermore, when integrating the Ag NDs selector with resistive switching memory in one-selector-one-resistor (1S1R) structure, the reduced selector variation helps significantly reduce the bit error rate in 1S1R crossbar array. The high-uniformity Ag NDs selectors offer great potential in the fabrication of large-scale 1S1R crossbar arrays for future memory and neuromorphic computing applications.

Gold-enhanced oxidation of MBE-grown silicon nanowires

Abstract: Thermal oxidation of MBE-grown silicon nanowires with a gold droplet on their tips was investigated. Two kinds of oxidation behavior were observed: (i) enhanced axial oxidation, if there was a direct contact between the gold droplet and the nanowire, and (ii) conventional oxidation for nanowires when there was no direct contact between the gold and silicon. For a dry atmosphere, such enhanced oxidation takes place at temperatures down to 500 °C. Under a wet atmosphere, remarkable oxidation was observed even for temperatures down to 250 °C.