Shichio Kawai

Control of electrical conductivity in laser deposited SrTiO3 thin films with Nb doping

Nb‐doped SrTiO3 (STO) thin films (Nb, 0<x<100 mol%) with a‐axis orientation have been prepared on nondoped STO (100) and LaAlO3 (100) single crystals using a pulsed laser deposition technique. Solid‐solution films are formed between STO and SrNbO3 without any impurity phases. Electrical conductivities of Nb‐doped STO films can be controlled from 10−7 to 104 S/cm by changing the concentration of the doped Nb. Carrier concentration of the Nb‐doped STO films also increases by increasing the Nb concentration.

Atomic layer and unit cell layer growth of (Ca,Sr)CuO2 thin film by laser molecular beam epitaxy

Thin films of (Ca,Sr)CuO2, the parent material of high Tc cuprate superconductors, have been formed by the laser ablation method under molecular beam epitaxial condition, and the growth mechanism has been investigated with reflection high‐energy electron diffraction (RHEED). Analyses of RHEED patterns and intensity oscillations show that this material grows with two‐dimensional layer growth. When all the metal elements are supplied simultaneously in NO2 atmosphere, the layer growth occurs with the unit‐cell layer of (Ca,Sr)CuO2. Furthermore, it has become evident that the growth unit can be separated into Ca (Sr) atomic layer and CuOx atomic layer by monitoring the RHEED intensity oscillation. The successive supply of each metal element leads to one atomic layer growth of this material.

Low‐temperature formation of multilayered Bi(Pb)‐Sr‐Ca‐Cu‐O thin films by successive deposition using laser ablation

Crystallized as‐grown Bi(Pb)‐Sr‐Ca‐Cu‐O thin films are formed by the laser ablation method at a substrate temperature as low as 480 °C under N2O gas flow. Moreover, the as‐grown thin film, which has a crystal structure consisting of four or five CuO2 layers between adjacent Bi2O2 layers, can be formed by the combination of N2O gas flow and the successive deposition method with excimer laser pulses.

Electrical Conduction and Phase Transition of Copper Sulfides

Electrical conductivity measurements have been made on copper sulfide crystals of compositions ranging from Cu1.8S to Cu2.0S and indium-doped Cu1.8S in the temperature region between 20°C and 220°C. Conductivity of Cu2-xS varied from 0.07 ohm-1cm-1 to 2400 ohm-1cm-1 as the deviation from stoichiometry increased from x=0 to x=0.2. It was found that Cu1.8S (digenite) and Cu1.96S (djurleite) undergo phase transitions at 90°C and at 93°C respectively. The transition temperature of Cu2S (chalcocite) increased from 98°C to 108°C as conductivity decreased from 52 ohm-1cm-1 to 0.07 ohm-1cm-1. An analysis of mixed conduction at β phase of low conductivity copper sulfide has shown that ionic conductivity is independent of the composition or the number of impurity cations in this material, and exhibits a temperature dependence with activation energy of 0.24 eV. Finally it is emphasized, after the discussion of the relation between electrical properties and crystal structure of copper sulfides, that the composition and crystalline phase are well characterized by the electrical conductivity in these materials.

Surface Structure and Electronic Property of Reduced SrTiO3(100) Surface Observed by Scanning Tunneling Microscopy/Spectroscopy

We have obtained the scanning tunneling microscopy (STM) images and scanning tunneling spectroscopy (STS) data with atomic-scale resolution for a SrTiO3(100) surface annealed in ultrahigh vacuum (UHV) at 1200°C. A √5×√5-R26.6° surface superstructure indicating oxygen vacancy ordering has been observed. The STS data provide evidence for a localized surface state arising from oxygen vacancies at 1.35 eV below the Fermi level. STM images of the √5×√5 structure correspond to the surface orbital of the ordered Ti-oxygen vacancy complex.

Epitaxial growth of LiNbO3 thin films by excimer laser ablation method and their surface acoustic wave properties

We have formed epitaxial LiNbO3 thin films of high quality on sapphire substrates (001) using an excimer‐laser ablation technique and evaluated the surface acoustic wave (SAW) properties of these films. It is demonstrated that the LiNbO3 thin films on the sapphire are the candidates for a high frequency SAW filter because of their high velocities of propagation (5300–6600 m/s) and good temperature coefficients (−34 to −80 ppm/ °C).

Growth of high purity LaB6 single crystals by multi-float zone passage

Abstract High purity LaB 6 single crystals have been grown by multi-float zone passage. Crystal growth was done under a pressurized atmosphere to prevent vaporization and dissociation of LaB 6 . No seed crystals for the selection of a preferred growth axis were used. The existence of an easy axis of growth was not found. The impurity concentrations were analysed by emission spectrography and residual resistance ratio measurement. The results show that the purity of the crystals after three passes were improved by about one order of magnitude compared with the crystals after only one zone pass. The grown crystals consist of sub-grains whose crystal axes were slightly misorientated with respect to each other. Electrical resistivity measurements for the purest crystal reveal the characteristic feature of conduction property of LaB 6 , i.e., polar phonon scattering of electrons plays an important role in the scattering mechanism of LaB 6 in addition to impurity scattering and acoustic phonon scattering.

Synthesis of diamond by laser-induced chemical vapor deposition

Diamond has been obtained by ArF excimer laser‐induced chemical vapor deposition. The reaction was carried out by use of C2H2 diluted with H2 as a source gas and at the pressure range of 8–75 Torr. The products were characterized by scanning electron microscopy and reflection electron diffraction. Deposits prepared in the temperature range of 40–800 °C, which were measured by the thermocouple attached to the substrate, show several lines of diamond in the reflection electron diffraction photographs. The fact that the laser beam must be concentrated for the diamond formation to occur strongly suggests that the reaction proceeds through a multiple photon process.

Preparation of PbTiO3 thin films at low temperature by an excimer laser ablation technique

We have formed PbTiO3 thin films on (100) SrTiO3 substrates at a temperature of 350 °C using an ArF excimer laser ablation technique. Until now, PbTiO3 films have not been formed at temperatures lower than 500 °C using other thin‐film techniques. The important points in the present study are the laser excitation of the substrate during the film growth and the lattice matching between the film and the substrate. The film deposited on the SrTiO3 substrate shows preferential orientation of the c‐axis perpendicular to the substrate surface.

The surface properties of TiC(001) and TiC(111) surfaces

Abstract The surface properties of the TiC(001) and TiC(111) surfaces such as atomic structure, electronic structure and chemisorption were investigated using low energy electron diffraction, angle-resolved X-ray photoelectron spectroscopy, angle-resolved UV photoelectron spectroscopy and ionscattering spectroscopy. The TiC(001) surface has a 1 × 1 structure with a nearly stoichiometric chemical composition, and the chemisorption is very similar to that of platinum. In contrast, the topmost layer of the TiC(111) surface with a 1 × 1 structure consists of titanium atoms, and its chemisorption resembles that of titanium.