Hyogo Prefectural Institute of Technology

Unintentionally doped and Ge-doped α-Ga2O3 films were grown on m-plane sapphire substrates via mist-chemical vapor deposition at a substrate temperature of 600 °C and a growth rate of 3.3 μm/h. Hall-effect measurements at room temperature indicated that the unintentionally doped film exhibited a low carrier concentration of 3 × 1016 cm−3, and the highest electron mobility reached 99 cm2 V−1 s−1 for the Ge-doped α-Ga2O3 film with a carrier concentration of 5.8 × 1017 cm−3. Temperature-dependent Hall-effect measurements clarified the transport mechanism: scattering caused by dislocations and ionized impurities dominate at low temperatures in films with carrier concentrations of 1.7 × 1017 and 5.8 × 1017 cm−3, respectively, whereas polar optical phonons are the main origin of electron scattering around room temperature. The average densities of threading screw and edge dislocations in Ge-doped films prepared herein are 6.0 × 108 and 1.1 × 109 cm−2, respectively, which are lower than those for α-Ga2O3 films grown on sapphire substrates to date. The low carrier concentration and high electron mobility achieved by reducing the dislocation density suggest the promising application potential of α-Ga2O3 in power electronics and high-frequency devices.

Bacteria reduce toxic selenium oxyanions, such as selenite,toelemental selenium (Se0), forming selenium nanoparticles(SeNPs) either intracellularly or extracellularly. However, the mechanismthrough which extracellular SeNPs (Ex-SeNPs) are exported remainsunclear. In this study, we characterized Ex-SeNPs biosynthesized by Escherichia coli during the aerobic reduction ofselenite. The SeNPs appeared within 2 h of exposure, remained extracellular,and displayed a consistent spherical morphology (∼100 nm).Purified Ex-SeNPs consisted of an Se0 core enveloped bya membrane-like layer containing lipids, proteins, carbohydrates,peptidoglycan, and lipopolysaccharides. Fluorescence microscopy andgas chromatography–mass spectrometry indicated that the encapsulatedmembrane originates from the E. coli cell membrane. Notably, mutants deficient in the outer membraneproteins OmpC or TolA failed to excrete SeNPs, resulting in intracellularaccumulation despite efficient Se0 synthesis. Our findingssuggest that E. coli forms SeNPs intracellularlyand exports them via an envelope-dependent process, during which theparticles may become encapsulated in membrane-like structures. Thesefindings help clarify the mechanism underlying a membrane-dependentpathway for SeNP detoxification and export that had been suggestedbut not directly demonstrated.

Hexagonal boron nitride (h-BN) is a promising material for designing future electronic devices because of its superior dielectric properties. In this study, we fabricated bulk h-BN (sp2-bonded BN nano-network structure) on Si substrates using magnetically confined arc discharge plasma under various conditions. The effects of process gas conditions (Ar/N2 and N2) and impurity [tungsten (W)] incorporation were discussed. Regardless of the gas conditions, the presence of W atoms was found to significantly modulate the optical energy gap, which is supported by first-principles calculations. We investigated time-dependent dielectric breakdown (TDDB) mechanisms under constant voltage stress (CVS) and constant current stress (CCS). The time evolutions of the leakage current and the applied gate voltage during the TDDB measurements were analyzed to clarify the carrier-trapping and defect-generation mechanisms toward the final catastrophic dielectric breakdown. The field acceleration factors in the CVS-TDDB lifetime prediction fell within the general trend of SiO2-based films and were found to be a weak function of the gas condition and W concentration (CW), whereas the carrier-trapping and defect-generation dynamics during electrical stress depend on the gas conditions and CW. Based on the obtained results, we propose a prediction model for bulk h-BN degradation dynamics during CVS. We found that carrier trapping into preexisting sites and the probability of defect generation were enhanced by the bombardment of ions with higher energy during the bulk h-BN formation and a larger number of incorporated W atoms. These findings provide fundamental guidelines for the reliability assessment of bulk h-BN films for various applications.

With the recent breakthrough in microfabrication technology, microdevices in analytical chemistry have made remarkable progress and are now being put to practical use as micro chemical analysis systems such as micro-total analysis systems (μTAS) and Lab on a Chip. In microfluidic channels, which are an elemental function of these systems, it is necessary to make the channel walls partially hydrophilic or hydrophobic for solution control, but it has been difficult to obtain a continuous and spatially stable contact angle surface. In this study, hydrogen-containing silicon carbide films were deposited on silicon under various conditions using a sputtering deposition system, a newly developed (Shinko Seiki) PIG (Penning Ionization Gauge) plasma CVD method, in order to control the contact angle. This equipment can be applied to hot cathode Penning Ionization Gauge ion source, which can easily obtain high-density plasma for plasma CVD to produce thin films. The surface structures of these films were analyzed by photoelectron spectroscopy using synchrotron radiation to control the composition of the hydrogen-containing silicon carbide oxide films. As a result, hydrophilicity of the surface was obtained with increasing oxygen flow rate in the deposition. This result is important in that it can be used for channel walls of microdevices to obtain stable contact angle surfaces and to control fluid flow.

Recently, Micro Total Analysis System (µTAS) have been attracting attention. It is difficult to agitate high-viscosity liquids in microfluidic channels due to laminar flow. To facilitate stirring, we evaluated the characteristics of a micro hot stirrer using a SAW as a driving source. In the experiment, a high-viscosity liquid glycerin solution (90 wt%) was heated to 97°C in 180 seconds. In addition, the rotational speed of the liquid tank was increased approximately sixfold by heating from 40°C to 90°C. Using this micro hot stirrer, it was found that even high-viscosity liquids such as glycerin aqueous solution can be sufficiently heated to reduce viscosity and stirred. In the future, we plan to incorporate this micro stirrer into a micro continuous fluid actuator to polymerase chain reactions.

Rutile-type wide and ultrawide band-gap oxide semiconductors are emerging materials for high-power electronics and deep ultraviolet optoelectronics applications. A rutile-type GeO2-SnO2 alloy (r-GexSn1–xO2) recently found is one of such materials. Herein, we report low-temperature electron transport properties of r-GexSn1−xO2 thin films with x = 0.28 and 0.41. Based on resistivity and magnetoresistance measurements, along with the theory of quantum interference, it is suggested that Efros–Shklovskii variable-range hopping, i.e., hopping over the states within the Coulomb gap, is dominant at lower temperatures (T ≤ 10 and 15 K) in both r-Ge0.41Sn0.59O2 and r-Ge0.28Sn0.72O2. The negative and positive magnetoresistances observed at low temperatures are attributable to the quantum interference and field-induced spin alignment, respectively. The magnetoresistance measurements at higher temperatures suggest that both Mott variable–range hopping and thermally activated band conduction occur at T < 100 K and that almost pure thermally activated band conduction takes place at T ≥ 150 K.