Yoshiharu Kakehi

Room-temperature epitaxial growth of NiO(1 1 1) thin films by pulsed laser deposition

The room-temperature epitaxial growth of NiO(111) thin films was successfully achieved on α-Al 2 O 3 (0001) substrates using a pulsed laser deposition method. The epitaxial growth mechanism and the effect of oxygen pressure on the film quality were investigated using X-ray diffraction, X-ray pole figure, reflection high-energy electron diffraction and atomic force microscopy. The orientation relationships of the films with respect to the substrates were NiO[111]∥α-Al 2 O 3 [0001], NiO[101]∥α-Al 2 O 3 [1010], and NiO[211]∥α-Al 2 O 3 [1120]. The films consisted of a lot of domains and showed sixfold symmetry. These results can be explained by the higher-order epitaxy mechanism enabling the fourfold longer in-plane lattice parameters of NiO(111) to match the threefold longer parameters of α-Al 2 O 3 (001) with <4.5% misfit. The crystallinity of the epitaxial films was significantly improved by expanding the in-plane lattice parameter. This is due to the relaxation of the lattice misfit between the film and the substrate at the initial growth.

Influence of Oxygen Flow Ratio on Properties of Zn2SnO4 Thin Films Deposited by RF Magnetron Sputtering

Zinc stannate (Zn2SnO4) thin films were deposited by RF magnetron sputtering on silica substrates at various [O2/(Ar+O2)] flow ratios. The influences of the [O2/(Ar+O2)] flow ratio on the crystalline structure, and the optical and electrical properties have been investigated. No sharp X-ray diffraction (XRD) peaks were observed in as-deposited thin films. After postdeposition annealing in air at 750°C, the thin films showed a preferred orientation of (111). The thin films exhibited a high transmittance in the visible spectrum irrespective of the [O2/(Ar+O2)] flow ratio or postdeposition annealing. The optical band gap was estimated to be 4.1 eV by analyzing the optical spectra of thin films annealed at 750°C. The composition ratio of Zn/Sn for thin films deposited in an Ar/O2 mixture was 2.0 and their electrical resistivity was on the order of 105 Ωcm. In contrast, the composition ratio of Zn/Sn for a thin film deposited in pure Ar was 1.5 and an electrical resistivity of 4.1 ×10-2 Ωcm was observed.

Epitaxial growth of CuScO2 thin films on sapphire a-plane substrates by pulsed laser deposition

An epitaxial film of CuScO2, a transparent oxide semiconductor with a delafossite structure, was grown on an α−Al2O3(112¯0) substrate by a pulsed laser deposition method using a single-phase Cu2Sc2Oδ target. A two-dimensional x-ray reciprocal space mapping measurement revealed that the film was single phase with a rhombohedral crystal structure. The film showed six-fold rotational symmetry in the basal plane, indicating that the film had a twinned domain structure. The epitaxial growth of CuScO2[3R](0001) thin films on α−Al2O3(112¯0) substrates is caused by the uniaxial locked epitaxy mechanism along the ⟨1¯21¯0⟩ direction of the film, and the orientation relationships of the film with respect to the substrate were CuScO2[3R](0001)∕∕α−Al2O3(112¯0) and CuScO2[3R][1¯21¯0]∕∕α−Al2O3[88¯01]. The optical transmittance of the film was larger than 65% in the visible/near-infrared regions, while the energy gap for direct allowed transition was estimated as 3.7 eV. The resistivity of the film, 9.3×106Ωcm at room te...

Effects of Postannealing on Orientation and Crystallinity of P-Type Transparent Conducting CuScO2 Thin Films

Highly c-axis-oriented CuScO2 thin films were successfully fabricated from polycrystalline CuScO2 thin films by pulsed laser deposition and followed by postannealing treatment. The oxygen pressure effects during postannealing on the surface morphology, and crystallographic and optical properties of the films were investigated. The growth orientation, crystallinity along both out-of-plane and in-plane directions, and surface morphology of the films were significantly improved by postannealing at an optimal oxygen pressure. Using a film postannealed under optimal annealing conditions, the optical and electrical properties of the film were measured. The optical average transmittance of the film was greater than 70% in the visible/near-infrared regions, and the energy gap for calculated direct allowed transition was 3.7 eV. The Hall coefficient measured using the van der Pauw electrode configuration was 1.9×10+1 cm3 C-1, indicating p-type conduction. The resistivity, carrier concentration, and Hall mobility of the film at room temperature were 9.9×10+1 Ω cm, 3.2×10+17 cm-3, and 2.0×10-1 cm2 V-1 s-1, respectively.

Crossover from positive to negative magnetoresistance by the rise of electron temperature for Si:Sb in the variable-range hopping regime

Abstract Magnetoresistance (MR) measurements in Si:Sb with the Sb concentration just below the critical concentration for metal–insulator transition have been carried out by stepping into the non-ohmic region of the electric current density. We have found that the MR sensitively depends on the electric current density. For the insulating sample positive MR is observed in the low current density region. A crossover from positive to negative MR is found as the current density increases. It is considered that these phenomena originate from the rise of the electron temperature and are related to the transport in the upper-Hubbard band.

INFRARED RADIATION DETECTOR WITH YBA2CU3OX THIN FILM

We investigated an infrared (IR) radiation detector using a c-axis oriented YBa2Cu3OX thin film with a microbridge structure. The photoresponse was measured under IR radiation and was found to depend on the bias current. A bolometric response was observed under relatively low bias current, which was inversely proportional to the square root of the modulation frequency of IR irradiation. The frequency dependence can be explained by a heat diffusion process that is similar to that of a microbolometer. With increasing bias current, a nonbolometric response was observed just above the superconducting transition temperature (Tc). This was caused by the enhancement of additional flux creep induced by modulated IR light, which was equivalent to an increase in resistance. The maximum responsivity and noise equivalent power (NEP) were 1.7×104 V/W and 2.1×10-14 W/Hz1/2, respectively.

Electrical Properties of Cr–N Films Deposited by Sputtering: Application to Cryogenic Temperature Sensors

The electrical properties of chrome nitride (Cr–N) films have been investigated for application to cryogenic temperature sensors. The films were deposited on silica substrates by RF magnetron sputtering under various total pressures without substrate heating. The electrical resistivity at room temperature increased from 1.4×10-3–1.7×10-3 to 3.4×10-3–4.0×10-3 Ω cm as the total pressure increased. X-ray diffraction measurements revealed that the crystallinity was improved in the high-total-pressure region. The temperature dependence of the electrical resistivity for Cr–N films exhibited semiconducting behavior. The slope of the electrical resistivity versus temperature increased with increasing total pressure. When a magnetic field was applied parallel to the current direction, the change in magnetoresistance at 3.7 K for Cr–N films was less than 1% in the range of 0 to 10 T. The results indicated that temperature sensors made of Cr–N films are available in high magnetic fields.

Effect of postannealing on properties of ZnO-SnO2 thin film transistors

Herein, the influence of postannealing on the properties of ZnO-SnO2 (ZTO) thin-film transistors (TFTs) was investigated. Postannealing in ambient air induced recovery of the electrical properties of wet-etch-damaged TFTs and a decrease in the subthreshold swing. Also, the field effect mobility increased with increasing postannealing temperature. Further improvement of the electrical properties of ZTO TFTs was not obtained with a further increase in annealing time. After postannealing at 200 °C for 60 min in the air, the ZTO TFTs exhibited a field effect mobility of about 10 cm2/V s.Herein, the influence of postannealing on the properties of ZnO-SnO2 (ZTO) thin-film transistors (TFTs) was investigated. Postannealing in ambient air induced recovery of the electrical properties of wet-etch-damaged TFTs and a decrease in the subthreshold swing. Also, the field effect mobility increased with increasing postannealing temperature. Further improvement of the electrical properties of ZTO TFTs was not obtained with a further increase in annealing time. After postannealing at 200 °C for 60 min in the air, the ZTO TFTs exhibited a field effect mobility of about 10 cm2/V s.

Optical response of LaCaMnO thin film prepared by pulsed laser deposition

Abstract An epitaxial film of La0.7Ca0.3MnO3 has been successfully fabricated by pulsed laser deposition. The film shows a phase transition from paramagnetism to ferromagnetism at around 190 K with a steep resistance change. This resistance change can be used for a bolometric photosensor and results in detectivity D*, noise equivalent power (NEP) becomes D* = 2.7×108 cm √Hz W−1 and NEP/√B = 7.4×10−10 W √Hz, where B is the band width of an amplifier and is assumed to be 1 Hz.

IR Detector Prepared by Y-Ba-Cu-O Thin Film

An Infrared radiation detector (IR) has been investigated by using a superconducting YBa2Cu3Ox(YBC0) thin film. The YBCO thin films were prepared on both LaAlO3 and MgO substrates by an rf-sputtering equipment and were patterned to form a microbridge. The infrared light, which was generated by a semiconductor laser diode(λ =827nm), was incident upon the microbridge. The temperature dependence of the photoresponse was proportional to dR/dT, and showed the sharp maximum at superconducting transition edge under relatively low modulation frequency and low bias current. When the IR detector was prepared on LaAlO3 substrate, the noise equivalent power(NEP) and responsivity were 1 × 10-12 W/ \(\sqrt {Hz} \) and 3.7 × 105 V/W, respectively. As increasing the bias current, non-bolometric behavior was clearly observed in superconducting transition tail.