Hara Toru | Nazarbayev University (original) (raw)

Papers by Hara Toru

Although sulfur has a high theoretical gravimetric capacity, 1672 mAh/g, its insulating nature re... more Although sulfur has a high theoretical gravimetric capacity, 1672 mAh/g, its insulating
nature requires a large amount of conducting additives: this tends to result in a low massloading
of active material (sulfur), and thereby, a lower capacity than expected. Therefore,
an optimal choice of conducting agents and of the method for sulfur/conductingagent
integration is critically important. In this paper, we report that the areal capacity
of 4.9 mAh/cm2 was achieved at sulfur mass loading of 4.1 mg/cm2 by casting sulfur/
polyacrylonitrile/ketjenblack (S/PAN/KB) cathode composite into carbon fiber paper.
This is the highest value among published/reported ones even though it does not contain
expensive nanosized carbon materials such as carbon nanotubes, graphene, or graphene
derivatives, and competitive enough with the conventional LiCoO2-based cathodes (e.g.,
LiCoO2, <20 mg/cm2 corresponding to <2.8 mAh/cm2). Furthermore, the combination
of sulfur/PAN-based composite and PAN-based carbon fiber paper enabled the sulfurbased
composite to be used even in carbonate-based electrolyte solution that many
lithium/sulfur battery researchers avoid the use of it because of severer irreversible active
material loss than in electrolyte solutions without carbonate-based solutions, and even at
the highest mass-loading ever reported (the more sulfur is loaded, the more decomposed
sulfides deposit at an anode surface).

We tried to relate the relaxation currents of Pt/62 nm-thick-(Ba 0.5 Sr 0.5 )TiO 3 /Pt capacitors... more We tried to relate the relaxation currents of Pt/62 nm-thick-(Ba 0.5 Sr 0.5 )TiO 3 /Pt capacitors to the results of ultraviolet photoemission spectroscopic measurements for the 62 nm-thick-(Ba 0.5 Sr 0.5 )TiO 3 /Pt specimens. The slowest relaxation (159-313 s at applied voltages of 1.5-3 V, and at a measuring temperature of 40 8C) and the relatively faster relaxations (4.92-5.43 and 0.25-0.46 s) were assigned as the electron-detrapping from the localized state at 0.80 eV below the quasi-Fermi level, from the localized state at 0.55 eV below the quasi-Fermi level, and from the localized state at 0.30 eV below the quasi-Fermi level, respectively. The decrease in the relaxation time caused by the increase in bias voltage is probably due to the decreasing depletion width. The decrease in depletion width is probably due to the detrapping of electrons from deep localized states in accordance with the downward bending of quasi-Fermi level in the depletion layer. The bending is produced by the decrease in relative dielectric constant in the depletion layer in accordance with the increasing of bias voltage. q

A strontium titanate (SrTiO 3 ) thin film showed high oxygen sensitivity on the ppb order at room... more A strontium titanate (SrTiO 3 ) thin film showed high oxygen sensitivity on the ppb order at room temperature. The sensitivity is enough to monitor oxygen contamination in semiconductor manufacturing processes, which is becoming increasingly important as semiconductor devices are scaled down to the nanometer generation. The SrTiO 3 film was prepared using atomic layer deposition (ALD), which has been established and found suitable for mass production.

We investigated the leakage current versus voltage (I-V) characteristics, the capacitance versus ... more We investigated the leakage current versus voltage (I-V) characteristics, the capacitance versus thickness of (Ba 0:5 Sr 0:5 )TiO 3 film (C-t) characteristics, and the relaxation currents of sputtered (Ba 0:5 Sr 0:5 )TiO 3 films with the thickness of 40-166 nm. The I-V characteristics can be explained by the partially depleted model especially when the thickness of (Ba 0:5 Sr 0:5 )TiO 3 film exceeds 62 nm. The C-t characteristics indicate that the relative dielectric constant in the internal layer (out of the depletion layer) does not change by applied voltages. This can be explained by assuming that the electric field is concentrated at the partially depleted layer, and that the relative dielectric constant in the depletion layer decreases in accordance with the increasing of applied voltage. The relaxation currents may be explained by assuming that the relative dielectric constant in the depletion layer decreases in accordance with the increasing of applied voltage.

This work is intended to draw attention to the origin of the electronic structures near surfaces ... more This work is intended to draw attention to the origin of the electronic structures near surfaces of perovskite type oxides. Deep states were observed by ultraviolet photoelectron spectroscopic measurements. The film thickness dependent electronic structures near surfaces of (Ba 0.5 Sr 0.5 )TiO 3 thin films were observed. As for the 117-308 nm thick (Ba 0.5 Sr 0.5 )TiO 3 films, deep states were lying at 0.20, 0.55, and 0.85 eV below the quasi-fermi level, respectively. However, as for the 40 nm thick (Ba 0.5 Sr 0.5 )TiO 3 film, the states were overlapped. The A-site doping affected electronic structures near surfaces of SrTiO 3 single crystals. No evolution of deep states in non-doped SrTiO 3 single crystal was observed. However, the evolution of deep states in La-doped SrTiO 3 single crystal was observed.

We have considered perovskite-type dielectric materials as promising candidates for use as highly... more We have considered perovskite-type dielectric materials as promising candidates for use as highly sensitive oxygen sensors because of the indispensable interaction between ionic polarization in perovskites and adsorbed oxygen on the perovskite surface. In this study, a gas calibration system equipped with a ZrO 2 -based oxygen pump (SIOC-200C, STLAB, Japan) was used. Among the materials studied, Cr:SrTiO 3 shows sufficient sensitivity to detect residual oxygen in helium ambient at ppb levels. In contrast to Cr:SrTiO 3 , Cr:BaTiO 3 shows no oxygen sensitivity. The merits of Cr:SrTiO 3 might be attributed to the coexistence of large and small polarons. Perovskite-based sensor is expected to provide a solution for real-time monitoring of residual oxygen during semiconductor manufacturing.

In this paper, we show some results on oxygen-concentration-dependent electrical resistances of S... more In this paper, we show some results on oxygen-concentration-dependent electrical resistances of SrTiO 3 -based thin films and their possible new application. It is widely known that SrTiO 3 -based ceramics can be used as alternatives to ZrO 2 -based high-temperature oxygen sensors, which are categorized as ionic conductors operated at high temperatures, such as 1,273 K. In contrast, our thin-film-based devices, which are operated at room temperature, could be studied from the viewpoint of the polaron theory. #

Semiconductive dielectric SrTiO 3 -based thin films are promising candidates for monitoring trace... more Semiconductive dielectric SrTiO 3 -based thin films are promising candidates for monitoring trace levels of oxygen during in situ monitoring of semiconductor manufacturing processes. In our preliminary studies, it was confirmed that SrTiO 3 -based thin films have a high oxygen sensitivity of ppb order at room temperature. In this paper, we will show that the prototype SrTiO 3 -based oxygen sensor detects trace levels of oxygen, even at ppb concentrations. It is expected that the SrTiO 3 -based sensor can be used for the real-time monitoring of residual oxygen in the future. #

In this study, the effect of oxygen (O 2 ) adsorption on polaron conduction in nanometer-scale Nb... more In this study, the effect of oxygen (O 2 ) adsorption on polaron conduction in nanometer-scale Nb 5þ -, Fe 3þ -, and Cr 3þ -doped SrTiO 3 thin films was investigated. The SrTiO 3 -based thin films were epitaxially grown onto SrTiO 3 (001) single-crystal substrates by pulsed laser deposition (PLD). It was found that polaron diameter decreases owing to O 2 adsorption; this can be understood by assuming that O 2 adsorbates induce local distortions of TiO 6 unit cells, at which conduction electrons are frequently trapped. Furthermore, the polaron diameter of Sr(Ti,Nb)O 3 and Sr(Ti,Fe)O 3 exhibited less dependence on O 2 concentration than that of Sr(Ti,Cr)O 3 ; this can be explained by the shielding of electron-phonon interaction by doped electrons in Sr(Ti,Nb)O 3 , and by the electronic structure of Sr(Ti,Fe)O 3 . #

Our previous studies showed that SrTiO 3 -based thin films can be used to detect trace amounts of... more Our previous studies showed that SrTiO 3 -based thin films can be used to detect trace amounts of oxygen. The sensitivity to oxygen of the films was attributed to the polaronic nature of SrTiO 3 . In this study, it was observed that the application of an electric field resulted in a decrease in electrical resistance (hereafter, the aging effect) possibly in the same way as a dc electrical degradation in ceramic capacitors, which is due to the demixing of the oxygen vacancies (the electrical migration of oxygen vacancies leading to their pileup at the interface between SrTiO 3 and electrodes). The sensitivity to oxygen of the films was maintained even after aging. #

We have previously reported that SrTiO 3 -based thin films can be used to detect trace amounts of... more We have previously reported that SrTiO 3 -based thin films can be used to detect trace amounts of oxygen (e.g., down to 0.001 ppt). However, the SrTiO 3 surfaces are highly active against oxygen adsorption. Therefore, the slow desorption response can affect the response speed of the sensors against the sudden change in oxygen concentration. We have reported that UV light irradiation is effective for accelerating oxygen desorption, and in this paper, we will show that the increase in the irradiation intensity of UV light can be an efficient way of improving the response speed. As an example, an apparent hysteresis in the sensor resistance versus oxygen concentration curve, which is shown after the UV irradiation at a low intensity, diminishes after the UV irradiation at a high intensity, even after short-lasting treatments.

Semiconductive dielectric SrTiO 3 -based thin films are promising candidates for in situ monitori... more Semiconductive dielectric SrTiO 3 -based thin films are promising candidates for in situ monitoring of trace levels of oxygen in semiconductor or organic light-emitting diode (LED) display manufacturing processes. SrTiO 3 -based thin films, which are deposited by atomic-layer deposition (ALD), exhibit high sensitivity to oxygen at room temperature; however, the sensitivity can be affected by SrO-based surface segregation. In this study, we will show that annealing temperature and time are the key parameters for decreasing the coverage of oxygen-insensitive SrO-based surface segregation. #

The oxygen sensing properties of SrTiO 3 -based thin films have been investigated at room tempera... more The oxygen sensing properties of SrTiO 3 -based thin films have been investigated at room temperature. First, nondoped SrTiO 3 was investigated. Although such a material is highly sensitive, its electrical resistance was too high and not feasible for practical use. Donor (Nb 5þ ) could lower the resistance of SrTiO 3 ; however, the sensitivity was lost. UV-light irradiation or t 2 -type acceptor (Cr 3þ ) doping could lower the resistance and yet maintain the sensitivity. In contrast to t 2 -type acceptor doping, e-type acceptor (Fe 3þ ) doping could lower the resistance; however, the sensitivity was lost. In this paper, we discuss the phenomena from the viewpoint of polaron-controlled carrier conduction.

Semiconductive dielectric SrTiO 3 thin films are promising candidates for the in situ monitoring ... more Semiconductive dielectric SrTiO 3 thin films are promising candidates for the in situ monitoring of trace levels of oxygen in semiconductor manufacturing processes. Highly sensitive oxygen sensors were prepared by atomic-layer deposition (ALD) or pulsed-laser deposition (PLD) of SrTiO 3 ; however, there was a problem with ALD-SrTiO 3 that surface defects, such as SrO surface segregation with a minor contribution from SrO 2 , affected the detection below an oxygen concentration [P O2 /(P O2 + P He )] of 1.1 © 10 ¹12 , causing the reversion of ALD-SrTiO 3 to a slightly high-resistance state. The coverage of the segregated surface of SrO and SrO 2 on the ALD-SrTiO 3 was considerably higher than that observed on the PLD-SrTiO 3 surface, however, the reversion could be markedly suppressed by the additional thermal treatment, probably owing to gathering small SrO-based surface islands (building-up of bigger islands), resulting in the exposure of clean surfaces.

The defect-related relaxation behaviors and the leakage of fresh and/or dc-electrically degraded ... more The defect-related relaxation behaviors and the leakage of fresh and/or dc-electrically degraded specimens of Au/(Ba 0.5 Sr 0.5 )TiO 3 /Pt capacitors on the TiO 2 coated sapphire substrates were investigated. The relaxation behaviors of (Ba 0.5 Sr 0.5 )TiO 3 films are assumed to be the electron-detrapping in the depletion layer. The local electric field enhancement due to oxygen vacancies near the Pt/(Ba 0.5 Sr 0.5 )TiO 3 interface, which is estimated from the Poisson equation, was assumed to be sufficiently high as a cause of tunneling conduction, and assumed to be a cause of dc-electrical degradation.

The relative dielectric constant versus voltage ( ) characteristics and the current density versu... more The relative dielectric constant versus voltage ( ) characteristics and the current density versus electric field ( ) characteristics of (Ba 0 5 Sr 0 5 )TiO 3 films, which have intentionally inserted oxygen depleted layers near the bottom electrodes, were investigated as a model of dc-electrical degradation phenomena. Our investigation demonstrated that the intentionally inserted oxygen depleted layer is the cause of the tunneling conduction.

The phenomena of dc electrical degradation of (Ba 0 5 Sr 0 5 )TiO 3 thin films was studied. From ... more The phenomena of dc electrical degradation of (Ba 0 5 Sr 0 5 )TiO 3 thin films was studied. From our experimental and analytical results of current versus voltage ( -) characteristics, it was shown that the degraded devices exhibited analogous leakage behaviors with the devices which have thin intercalated (Ba 0 5 Sr 0 5 )TiO 3 layers with intentionally introduced oxygen vacancies between cathodes and thick (Ba 0 5 Sr 0 5 )TiO 3 layers without intentionally introduced oxygen vacancies. This could be explained by assuming that oxygen vacancies accumulate at the interfaces between the cathodes and the (Ba 0 5 Sr 0 5 )TiO 3 films after fatigue.

Polycrystalline Dy-doped SrTiO3 thin films deposited by pulsed laser deposition (PLD) on CeO2-buf... more Polycrystalline Dy-doped SrTiO3 thin films deposited by pulsed laser deposition (PLD) on CeO2-buffered yttria-stabilized zirconia (YSZ) single crystal substrates showed oxygen sensing characteristics at room temperature. The oxygen gas sensing characteristics depended on the amount of Dy-doping, and 1.5 mol% Dy-doping was most effective. The oxygen gas sensing characteristics were closely related to leakage current characteristics of Dy-doped SrTiO3 thin film deposited on ZnIn2O4/YSZ/ Si(001) substrates. The suppression of leakage current by Dy-doping suggests that Ti 4+ is substituted by Dy 3+ , and the Dy 3+ acts as an acceptor. Further doping of Dy 3+ brought about the increase of the leakage current, and it lowered the oxygen gas sensitivity.