Masaaki Isai

Oxygen sensing properties at high temperatures of β-Ga2O3 thin films deposited by the chemical solution deposition method

In this article, we report the fabrication of an oxygen sensor based on gallium oxide thin films prepared by the chemical solution deposition method. The oxygen sensing properties of the device (response and recovery time, sensitivity, stability) were investigated at high temperature (1000 °C). Attention was paid to the influence of the annealing conditions on the oxygen sensing properties of the Ga2O3 thin film sensor. Surface morphology of thin film samples, studied by using atomic force microscopy measurements, shows that there are grain size differences due to various annealing conditions (temperature and time period). The results show that the annealing conditions affect not only the grain size and surface structure of Ga2O3 thin films but also the oxygen sensing properties of the material. It was shown that the response time of the Ga2O3 sensors decreased when the annealing temperature and annealing time were increased. The sensitivity of the sensor devices it is also affected when the annealing par...

Crystal growth investigation of the magnetically highly sensitive InSb evaporated films

The crystal growth of the magnetically highly sensitive InSb films, prepared by a simple evaporation on mica substrates with a steep one‐dimensional temperature gradient, is investigated in detail. An optimum evaporation condition is presented. It is shown that the crystalline InSb with thickness less than 0.1 μm consists of single crystal domains oriented mainly along [110] or [111] direction, but that the [111] direction becomes completely dominant for the films thicker than 0.2 μm. The crystal morphology of the film is discussed in relation to the high magnetoresistance value obtained.

Influence of thickness on the galvanomagnetic properties of thin InSb films for highly sensitive magnetoresistance elements

Thin InSb films were prepared on mica substrates by evaporation of InSb to obtain highly sensitive magnetoresistance (MR) elements with high element resistance. The MR effect was investigated for elements with a thickness range of 0.15–0.5 μm. It was observed that only the 0.15‐μm‐thick InSb film consists of a p‐type, [111]‐oriented single crystal, while the films thicker than 0.2 μm show n‐type conduction. At room temperature, in a magnetic field of 1 T, the n‐type films with thicknesses of 0.2, 0.3, and 0.5 μm and length‐to‐width ratios of 0.12, 0.09, and 0.05, were found to have increasing mobilities of 2.2, 4.4, and 5.0 m2/V s, decreasing element resistances of 115, 44, and 16 Ω, and increasing MR(ΔR/R0) values of 394, 652, and 800%. Measurements of the temperature dependence of μ show that 0.15–0.3‐μm‐thick films have a small temperature coefficient of MR values at room temperature because μ peaks near that temperature. The 0.15–0.3‐μm‐thick InSb films are therefore most useful for low‐cost MR elemen...

Sensing characteristics and crystalline structure of InSb films for magnetic sensors

InSb evaporated films for magnetic sensors are prepared by controlling the evaporation rate and the substrate temperature gradient in order to obtain elements of high sensitivity. There are In needles in the films which are precipitated parallel to one another during the evaporation process. The In needles have diameters of 0.5∼2 μm and lengths of 50∼200 μm. The films have thicknesses of 1.0∼3.5 μm. The results of the sensing characteristics and crystalline structure of InSb films are presented. The latter was analyzed by an electron microscope. A magnetoresistance value (ΔR/R0) of 1400% is obtained at a length‐to‐width ratio (l/W) of 0.1 and an applied magnetic flux density (B) of 1 Wb/m2 (=T), at room temperature.

Magnetically sensitive InSb films prepared on SiO layers using hot‐wire recrystallization

Magnetically sensitive InSb films have been prepared by hot‐wire recrystallization. A room‐temperature magnetoresistance (MR) value of ΔR/R0=155%/1 T was obtained at 1 T with length to width ratio (L/W)=0.4. Samples were prepared by sequential deposition of In, InSb, and SiO onto unheated mica substrates. They were placed mica side toward the nichrome wire heater. The crystal properties of these films were investigated with a transmission electron microscope (TEM). The TEM investigations showed that single‐crystal InSb films were grown from the SiO layer. This preparation technique is useful because effective MR elements with large area (20×30 mm) can be prepared without a complicated process and huge cost.

An estimation of the impurities causing n‐ to p‐type inversion of conduction in thin InSb films on mica substrates

The 0.1–0.9‐μm‐thick thin InSb films were prepared on mica substrates by evaporation of InSb to obtain highly sensitive magnetoresistance elements. It was observed that films thinner than 0.15 μm show p‐type conduction while films thicker than 0.2 μm show n‐type conduction in the extrinsic temperature range. It was also observed that the film, thinned from 0.3 to 0.1 μm by etching from the surface side, showed p‐type conduction. The cause of these phenomena was investigated using a secondary ion mass spectroscopy in‐depth analysis. It was observed that K, Al, Si and O atoms, constituents of mica substrates, diffused into InSb films from mica substrates. In these atoms, it was estimated that K, Al, and Si atoms could act as acceptors in an InSb crystal contributing to the inversion of conduction type.

Prevention against oxidation of Mn evaporant during reactive evaporation process

Manganese oxide films for lithium secondary batteries were prepared using a reactive evaporation method. The Mn metal in the crucible suffers severe oxidation during the reactive evaporation process, during which its deposition rate deteriorates with increasing deposition run. So it is difficult to maintain the stoichiometry of films from run to run. To prevent deteriorations, a quartz ampoule has been installed at the bottom of the Mn crucible, which isolated successfully the Mn evaporant from incoming oxygen atoms. It improved the reproducibility of film composition because of stabilizing of the deposition rate. The performance of quartz ampoule was compared with that of Mo separator and stainless steel (SUS) cell.

A possible mechanism for the recrystallization of InSb film on mica substrates

The recrystallization of InSb films while growing through the melting–solidification process was investigated in detail. A great fluctuation of the film temperature was seen in some cases of the growing film, especially in the early stage of the evaporation process. This fluctuation of the film temperature gives rise to the melting–solidification process. In this work, two distinct types of temperature fluctuations were observed. It was found that the shape of the temperature fluctuation curves can be reasonably explained by the variation of deposition rate in such a way that a higher deposition rate at the early stage of evaporation yields higher temperature fluctuations. The deposition rate was also found to depend on the preheating time of source, hence the optimum deposition rate was easily obtained by varying this time. The optimum substrate temperature and preheating time for high quality InSb films were 260 °C and 180 s, respectively. The present method greatly improves the reproducibility of prepa...

Inversion of conduction type in thin InSb films prepared on glass substrates

Thin InSb films with average thicknesses of 0.15 and 0.4 μm were prepared on glass substrates by evaporation of In and Sb to obtain homogeneous and In droplets‐free films. The substrate‐temperature dependence of some properties of the film was investigated between 350 and 470 °C. Highly (111)‐oriented 0.15an 450 °C, respectively. It was observed that 0.4‐μm‐thick films show p‐type conduction in the extrinsic temperature range. The cause of these phenomena was investigated using a secondary ion mass spectroscopy in‐depth analysis. As a representative in the constituents of glass substrates, the diffusion of Na, K, Mg, Ca, Ba, B, Al, Si, Pb, and O atoms into the InSb films was investigated more precisely. Among these atoms, it was estimated that K, Mg, and Si atoms could act as acceptors in an InSb crystal contributing to the inversion of conduction type.

In situ monitoring of the recrystallization process of InSb films by using a TV camera

Thin InSb films for highly sensitive magnetoresistance elements were prepared with high reproducibility by using a TV camera monitoring system. The recrystallization process was observed by viewing with real‐time video recording. This monitoring system can detect the dendritic growth through the melting‐solidification process as well as reevaporation during growth. This system was evaluated by investigating the correlations between morphology changes and parameters of the process, and also by the properties of the resulting films.