Takashi Tachiki

Characteristics of Vanadium Oxide Thin Films Prepared by Metal–Organic Decomposition for Bolometer Detectors

Highly axis-oriented V2O5 thin films were fabricated by metal–organic decomposition (MOD) on SiO2/Si substrates. V2O5 thin films were reduced to VOx thin films by heat treatment with a temperature of 530–580 °C, pressure of 1.2–3 Pa in O2 and heating time of 2–5 h. The VOx thin films exhibited an abrupt transition, with a resistivity change of up to 3 orders of magnitude at a transition temperature of about 55 °C. The hysteresis loop width in the transition was about 3 °C. The grain size of the VOx thin films with an abrupt transition was 200–300 nm. Furthermore, these films had a temperature coefficient of resistivity (TCR) of up to 2.2%/K at 300 K.

Fabrication and Evaluation of Thin-Film Spiral-Antenna-Coupled VOx Microbolometer by Metal–Organic Decomposition

A VOx microbolometer coupled with thin-film spiral antenna was fabricated on a fused-quartz substrate by metal–organic decomposition (MOD). The size of the bolometer was 1 ×52 µm2, and the antenna was designed for operating at 75–110 GHz. The DC sensitivity and responsivity of the bolometer were 540 W-1 and 124 V/W at the bias current of 0.5 mA under the irradiation of a 94 GHz electromagnetic wave, respectively. These values were over one order higher than those of the Bi microbolometer, which is conventionally utilized as a detector in terahertz and infrared regions. The antenna-coupled VOx microbolometer with a high responsivity was realized by MOD.

Microwave-induced current steps in intrinsic Josephson junctions patterned on Bi2Sr2CaCu2O8 single crystal

With a 10u200aμm×10u200aμm mesa patterned on Bi2Sr2CaCu2O8 single crystals, we measure the current–voltage (I–V) curves of a stack of intrinsic Josephson junctions. Current steps are observed at an equal voltage spacing of 4 mV when the sample is subjected to microwave radiation at around 7 GHz. With increase of the microwave power, more steps occur while the spacing between neighboring steps does not seem to change. The magnitude of each step depends on the microwave power in an oscillating way. Tuning the microwave frequency causes such steps to occur over separate frequency ranges, and each range is quite narrow. A temperature rise from 4.2 to 14.3 K completely quenches the step structures. Possible explanations for the step structures, based on resonances excited by microwave or geometric resonances in the junction cavity, are discussed.

Intrinsic Josephson junctions in oxygen-deficient YBa2Cu3O7−δ film deposited on a substrate step

Using c-axis-oriented oxygen-deficient YBa2Cu3O7−δ film deposited across a low-angle step on a SrTiO3 substrate, we successfully demonstrated intrinsic Josephson effects. In addition to several voltage jumps of large amplitudes (a few millivolts) and remarkable hysteresis on the dc current–voltage curves, we observed upturns on the current–voltage curves under microwave irradiation which appeared at increasingly high voltages with increasing microwave power. We proposed to explain this observation in terms of high-order microwave-induced Shapiro steps.

High DC sensitivity of VOx bolometer thin films on Si3N4/SiO2 membranes fabricated by metal–organic decomposition

Well axis-oriented V2O5 thin films were fabricated on Si3N4/SiO2/Si substrates by metal–organic decomposition (MOD). These films were reduced to VOx films by heat treatment, and the films had temperature coefficient of resistances (TCRs) of 3.2–3.7%/K at 300 K. Then VOx thin film on Si3N4/SiO2 membranes was realized by wet etching of the backside of the Si substrates using a 26 wt % KOH solution. The VOx thin film on Si3N4/SiO2 membrane had a high DC sensitivity of 6130 W−1, which was over 10 times higher than that of the film on the Si3N4/SiO2/Si substrate.

Terahertz-wave radiation emitted by intrinsic Josephson junctions

Radiation properties of terahertz (THz) electromagnetic waves emitted from stacks of 3 and 50 intrinsic Josephson junctions by flowing bias currents have been numerically investigated for high-frequency device applications. A novel multibranch structure in the resistive state was obtained for the current-voltage characteristics of the stacks. Each branch corresponded to an individual mode of a two-dimensional cavity resonance for Josephson plasma waves in the directions parallel and perpendicular to the c-axis of the stack. The electromagnetic field distribution in the stack exhibited standing-wave patterns not only for the in-phase mode between junctions but also for the other modes in different branches. Maximum radiation power was attained for the in-phase mode, and the corresponding bias point was measured for the 3- and 50-junction stacks. The peak power in the 50-junction stack was approximately 50 times larger than that in the 3-junction stack. The frequency spectra at maximum power exhibited a sha...

Face-to-Face Annealing of Bi2Sr2CaCu2O8+δ/MgO Precursor Films Fabricated by Metal–Organic Decomposition

To improve the uniformity of Bi2Sr2CaCu2O8+δ (Bi-2212)/MgO thin film, a face-to-face annealing method was introduced for a Bi-2212 precursor film (i.e., the surface of a precursor film was covered with that of another precursor film) fabricated by metal–organic decomposition (MOD). Compared with the Bi-2212 film fabricated by conventional single-substrate annealing, the coexistence of a Bi-2201 phase due to the thermal decomposition of Bi-2212 was drastically suppressed. The dominant rotation angle of the Bi-2212 film was η=45° ([100] of Bi-2212 ∥ [110] of MgO) and the full width at half maximum (FWHM) of the diffraction peak indicating η=45° became narrower with the face-to-face annealing. Moreover, a smooth surface with a grain size of approximately 10 µm was also observed. By improving the uniformity of the Bi-2212 phase and in-plane orientation, the critical current density at 4.2 K of the film fabricated by the face-to-face annealing became one order of magnitude higher than that of the film fabricated by conventional single-substrate annealing.

Harmonic frequency mixing in Bi2Sr2CaCu2O8+X intrinsic Josephson junctions

In a c-axis junction stack (10u200aμm×10u200aμm in a-b plane) patterned on a Bi2Sr2CaCu2O8+X single crystal, we performed harmonic mixings between a 100 GHz signal and up to the 98th harmonic of a local oscillator at about 1 GHz. The dependence of PIF on V, where PIF is the mixing output at the intermediate frequency and V is the dc voltage across the junction stack, was strongly affected by the local oscillator power levels. For low values of local oscillator power, the optimum operation point where PIF became maximum was around zero bias, and typical multi-branch structure of PIF vs V curves were observed. For high values of local oscillator power, the optimum operation point moved to the high-voltage region. In both cases, interesting oscillations occurred on the PIF vs V curves. Our experimental results led us to believe that intrinsic Josephson junctions can be good candidates for high frequency applications and the harmonic mixing may be a useful probe to investigate plasma phenomena in the layered supercon...

Transport characteristics in c-axis La/sub 2-x/Sr/sub x/CuO/sub 4/ (LSCO) single crystals

c-axis micro-bridges of La/sub 2-x/Sr/sub x/CuO/sub 4/ (LSCO) single crystals were fabricated by the focused-ion-beam (FIB) etching method. Small rectangular LSCO pieces were fabricated by cutting and grinding single crystals of underdoped LSCO of x=0.09. The size of LSCO single crystals between electrodes was cut to (5-20)/spl times/40 /spl mu/m/sup 2/ in the ab-plane by using the FIB etching method. The junction resistance exhibited clear two-step structures corresponding to the superconducting transition of two different components of crystal structure. Superconductor-insulator-superconductor (SIS) like-branch structures on I-V curves of the LSCO stacks were observed for the first time. The branch structures show voltage jumps of several tens mV in the range of 2 K to 5 K with temperature dependence. When the temperature is changed from 2 K to 5 K, the critical current around zero bias regions splits into a few small voltage jumps with intervals of several mV in the range of 1 mV to 3 mV.

Optimization of SrTiO3 Precursor Films on MgO Substrates Prepared by the Metal?Organic Decomposition Method

The narrow window for the preparation conditions of SrTiO3 (STO) thin films used as a buffer layer for high-Tc superconducting films synthesized by the metal–organic decomposition (MOD) method is reported. The values of the full width at half maximum (FWHM) of STO 200 observed from the rocking curve in X-ray diffraction (XRD) analysis are markedly suppressed up to 1.8° in the optimum range of precursor temperature. The minimum value of the in-plane FWHM of STO 110 was 2.65°. Below the optimum precursor temperature range, it was found that STO precursor films contained carbonates as a residue of metal–organic materials by X-ray photoelectron spectroscopy (XPS). A random nucleation was observed in the STO precursor films by atomic force microscopy (AFM) above the optimum range of precursor temperature.