Kazunori Komori

Raman and electron microscopic studies of Si1−xGex alloy nanowires grown by chemical vapor deposition

Si1−xGex alloy nanowires (SiGeNWs) were grown by Au-catalyzed chemical vapor deposition and studied by Raman spectroscopy, transmission electron microscopy (TEM), and energy-dispersive x-ray spectroscopy (EDS) in TEM (TEM-EDS). The relationship between the growth parameters and the structure of the SiGeNWs was clarified by systematically changing the growth conditions over a wide range. Raman and TEM-EDS results demonstrated that the SiGeNWs consist of a lower Ge composition core and a higher Ge composition shell epitaxially grown on the surface of the core. The effects of oxidation on the structure of the SiGeNWs were studied. It was found that oxidation leads to segregation of the Ge atoms at the interface between the SiGeNWs and SiO2, which in turn results in a large inhomogeneity in Ge composition. Oxidation at a very low rate in a diluted oxygen gas atmosphere is required to avoid the formation of Ge particles and minimize the inhomogeneity.

Control of Y2O3-stabilized ZrO2 thin film orientation by modified bias sputtering

Abstract An attempt was made to control the crystal orientation of yttria-stabilized zirconia (YSZ) thin films using a modified bias sputtering technique. Two specially devised electrodes installed in a sputtering system were found to play an important role in obtaining films with a high degree of preferred orientation. An argon ion flux extracted from the discharge space formed by these electrodes appeared to impinge on the growing film to form films with this strong preferred orientation. The pole figure obtained by texture analysis revealed the clear evidence for alignment of the in-plane crystal axes of YSZ buffer layers for the growth of YBa 2 Cu 3 O y onto polycrystalline metal substrates. An apparent in-plane texturing was also confirmed in subsequently laser-deposited YBa 2 Cu 3 O y films. As a result, the current-carrying capacity of YBa 2 Cu 3 O y films was markedly improved.

rf Field dependence of surface resistance for a-b plane-textured YBa2Cu3O7-δ films deposited on copper substrate

Low rf loss at high rf field levels should be realized in order to apply high-Tc films to accelerator cavities. It is well known that c-axis perpendicular to the surface is essential to reduce Rs and its field dependence However, the effect of a–b plane texturing on Rs is not so clear because lack of experiments, especially for films deposited on metallic substrates. We developed a deposition technique that enables a–b plane texturing as well as c-axis orientation of YBCO films on a copper substrate. We prepared four samples with c-axis normal to the surface: two of them were a–b plane well textured films and the other two were a–b plane weakly textured films. The a–b plane well-textured films in low rf field exhibited a Rs higher than that of weakly textured films below 80 K. The rf field dependence measurements showed that the increase of Rs with increasing rf field for the a–b plane well-textured films was slower as compared with that of the weakly textured films.

Precise measurement of the microwave surface impedance of a YBa2Cu3O7−δ film on copper substrate

In measurement of the microwave surface resistance, Rs(T), of a high-Tc film with a host-cavity method, in which the cavity material is usually copper, the systematic error in Rs at low temperature can be significantly reduced through calibration with the data obtained by a niobium host cavity. Using a cavity excited in the TE011 mode at 13.6 GHz, the procedure is illustrated for a c-axis oriented YBa2Cu3O7−δ film fabricated on 36 mm diameter copper disk with yttria-stabilized-zirconia and chromium buffer layers. The temperature dependence in Rs(T) was consistent with that of the penetration depth; both quantities behavior could be fit well by a modified two-fluid model, in which the fraction of the pairing normal carriers obeyed (T/Tc)2 rather than (T/Tc)4 with some 20% of the charge carriers remaining normal.

Preparation of in-plane textured large-area YBCO films on (100) MgO with CeO2/YSZ buffer layers

Epitaxial YBa2Cu3Oy (YBCO) thin films with smooth surfaces have been successfully grown on large area (100) MgO (30×30 mm2) substrates buffered by CeO2/YSZ (Y2O3-stabilized ZrO2) double layers. The use of the double buffer layers and the employment of a new technique for controlling substrate temperature greatly enhance not only the YBCO film quality, but also the reproducibility of deposition. An excellent in-plane epitaxy was achieved with no evidence of misoriented grains in the YBCO/CeO2/YSZ/MgO structure. Good microwave properties were observed for YBCO films grown on buffered MgO compared with those on bare MgO.

Dependence on the microwave field of the surface resistance for YBa2Cu3O7−δ films fabricated on copper substrates

The microwave field dependence of the surface resistance for YBa2Cu3O7−δ films was studied at field levels up to 400 A/m. The c-axis normal YBa2Cu3O7−δ films were deposited onto copper disks 36 mm in diameter by a laser ablation method. The surface resistance increased linearly below 50 K as the microwave magnetic field increased, which is explained better by the critical-state model than by the coupled-grain model. The critical current density decreased almost linearly with increasing temperature from 2.8×105 A/cm2 at 20 K to 4×104 A/cm2 at 77 K.

Double-Crystalline Silicon Channel Thin Film Transistors Fabricated Using Continuous-Wave Green Laser for Large Organic Light-Emitting Diode Displays

We developed double-crystalline silicon channel thin film transistors (TFTs) whose active region consisted of a polysilicon layer and a microcrystalline silicon layer. The polysilicon layer was formed by continuous-wave green laser annealing with scalability to large substrates. The microcrystalline silicon layer formed on the polysilicon layer provides a sufficient channel etching margin for the fabrication of TFTs on large substrates without degradation of TFT mobility. The kink-free output characteristics were realized by a band-gap engineering method using a microcrystalline silicon layer. The TFT characteristics desirable for organic light-emitting diode display applications, namely, high mobility, high reliability, and kink-free output characteristics, have been successfully demonstrated.

Microwave Properties and Critical Current Density of YBCO Thin Films Deposited on CeO2-Buffered Sapphire Substrates Using Sputtering Targets of Different Compositions

The effect of the composition of a YBCO film on the microwave properties deposited on sapphire substrates with a CeO2 buffer layer was investigated. CeO2 and YBCO films were deposited by RF magnetron sputtering with targets of different compositions. Y-rich crack-free YBCO films with a thickness of 0.5 µm, deposited onto a 25-nm-thick CeO2 film, showed good crystallinity and a low microwave surface resistance. Microwave surface resistances of 0.09 mΩ at 20 K and 0.7 mΩ at 77 K were obtained at a frequency of 12 GHz, and a critical current density of 0.1 MA/cm2 at 77 K was estimated for the 0.5-µm-thick YBa1.5Cu2.5Oy film.

Microwave properties of YBCO films deposited on in-plane textured CeO2/YSZ buffer layers

Abstract Yttria-stabilized zirconia (YSZ) and CeO 2 buffer layers were used on MgO substrates to control the in-plane epitaxy of YBCO films. Substrate temperature control method in RF magnetron sputtering deposition was improved and the deposition was performed in good reproducibility. XRD results revealed an excellent in-plane epitaxy with no evidence of misoriented grains in a YBCO/CeO 2 /YSZ/MgO structure.

Grain orientation of the YBa2Cu3Oy superconducting thin films prepared by the low pressure plasma flash evaporation technique and its dependence on the starting composition

Abstract We have prepared superconducting thin films on MgO(100) substrates by a deposition technique using low pressure plasma flash evaporation. The r.f. plasma was sustained under 20 Torr by a 13.56 MHz r.f. generator operated at 8 kW. The films show various grain orientations depending on the conditions of deposition. With some modifications in the starting material composition, superconducting properties of the best film achieved were T c (resistivity ϱ = 0) = 90 K and J c ( at 77 K and zero field ) = 1.7 × 10 5 A cm 2 .