Tadaoki Kusaka

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.

The Contact Resistance of the YBa2Cu3O7-δ-Metal Film System

The electrical properties and X-ray photoelectron spectroscopy (XPS) of Y1Ba2Cu3O7-δ (YBCO) contacts formed by Ag, Au, Sn, Zn, In and Al metal films have been studied. The Cu, In, Zn and Sn film contacts show high resistivities with semiconductive temperature dependence. The Au, Ag, and Pt film contacts have low resistivities with metallic temperature dependence. The XPS analyses show that Sn and Al react with the oxygen on the YBCO surface. However, little change in copper and oxygen states can be observed in Ag/YBCO and Au/YBCO systems. It is assumed that the oxygen-deficient layer at the interfaces of Sn/YBCO and Al/YBCO systems makes the contact resistance high, while the semiconductive layer is insignificant at the Ag/YBCO and Au/YBCO interfaces, and their resistance is low. Analyses indicate that Ag/YBCO and Au/YBCO systems provide very effective ohmic contacts.

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.

In situ formation of ohmic contact electrodes of Cu and Ag onto the fractured surface of (Bi, Pb)-Sr-Ca-Cu-O ceramics

Ohmic contacts on (Bi, Pb)-Sr-Ca-Cu-O (BSCCO) superconductor ceramics have been studied. When the electrodes were formed on the as-sintered surface of a BSCCO pellet, nonohmic characteristics were observed. Analysis by X-ray photoelectron spectroscopy suggests that the surface layer of the BSCCO pellet is degraded. When the fractured section of a BSCCO pellet was used, both Cu and Ag electrodes were found to be ohmic without thermal annealing.

Hybrid magneto-temperature sensor using contacts on a YBa2Cu3O7 - δ superconductor

Abstract A new type of hybrid magneto-temperature sensor has been fabricated on the surface of YBa 2 Cu 3 O 7 - δ (YBCO) ceramics by creating two types of electrical contacts. The type 1 contact, made by the ultrasonic bonding method, showed nonlinearity below the superconducting critical temperature ( T c ) in its current-voltage (I – V) curve, which was sensitive to the magnetic field. This was applicable as a strict magneto sensor. The type 2 contact was made by the spot-welding method and showed non-ohmic contact in its I – V curve, and was not affected by the magnetic field at all. This was applicable as a strict temperature sensor. Integration of these two types of electrical contacts on YBCO ceramics make an excellent and simple hybrid magneto-temperature sensor device. The features of this sensor are: simple structure and simple fabrication process: very small size; high sensitivity to a low magnetic field; and simple integration of magneto sensor and temperature sensors.

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.