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We have successfully demonstrated the simultaneous phase-locking of two terahertz quantum cascade lasers (THz-QCLs). These phase-locked THz-QCLs are used as local oscillators of a hot electron bolometer mixer (HEBM). The HEBM was simultaneously pumped by two phase-locked THz-QCLs for the multifrequency heterodyne detection of emission line spectra of methanol [CH3OH] at 2 THz using a gas cell. The phase-locking of THz-QCLs with arbitrary frequencies using an optical comb and a photomixer to generate THz references was also demonstrated. An advantage of this technique is that it expands the RF bandwidth of an HEBM.

To expand the intermediate frequency (IF) band and improve the sensitivity of a hot-electron bolometer mixer (HEBM), we proposed and examined a new HEBM structure using a magnetic thin film. We found that it was possible to suppress the superconductivity of the 5-nm-thick niobium nitride (NbN) thin film by the addition of a 1.8-nm-thick nickel (Ni) thin film. It was also confirmed that the superconductivity disappeared in the Au (70 nm)/Ni (1.8 nm)/NbN (5 nm) trilayer for forming the electrodes of the HEBM. By using the magnetic thin film for the electrodes, we suggested that the superconductivity of the HEBM strip would be affected and that hot spots would be formed near the electrodes. This approach is effective for shortening the hot-electron drift length and will lead to the expansion of the IF bandwidth. We think that the new structure lowers the required local oscillator power and improves the HEBM sensitivity by suppressing the proximity effect under the electrode. The IF bandwidth of the fabricated Ni-HEBMs was evaluated at 1.9 THz. We confirmed that the IF bandwidth expands, and the evaluated bandwidths were in the range of 5.1–5.7 GHz at 4 K. Ni-HEBMs with 0.1-μm strip length were also fabricated and evaluated. The IF bandwidth was about 6.9 GHz at 4 K.

In order to design antennas for mid-infrared (MIR) hot electron bolometers (HEBs), the surface impedance of Au thin films at cryogenic temperatures was evaluated using Fourier transform infrared spectroscopy with a sample cooling system. We fabricated resonator arrays that were constructed by gold thin film strips. The length, width, and thickness of the resonators were set at 1300, 200, and 55 nm, respectively. As the temperature was lowered, the resonant frequency was shifted to the low-frequency side. By fitting the resonant frequency to simulated results, the corrected surface reactance was established for designing MIR nano-antennas. MIR HEBs with a twin-slot antenna were designed and fabricated using the corrected surface impedance. The design frequency was 61 THz. The HEBs were formed as a superconducting strip with Au antennas and electrodes. Here, the superconductivity under the Au electrodes was suppressed by a Ni thin film. For evaluating the mixer properties of the MIR HEBs, measurement setup without beam splitter was constructed. In this setup, local oscillator power was applied directly to the HEB, and we confirmed that the critical current of the HEB can be reduced to be almost zero by the LO irradiation. The HEB responses with clear polarization dependencies were also observed.

We have demonstrated the phase-locking of a THz quantum cascade laser (THz-QCL) toward the realization of an accurate and stable local oscillator for a high-resolution receiver. The beat note between the THz-QCL and a THz reference was obtained by heterodyne mixing in a superconducting hot electron bolometer mixer (HEBM) and used for stabilizing the phase of 3.1 THz radiation from the THz-QCL. The phase-locked 3.1 THz radiation was fully evaluated with a superlattice harmonic mixer operating in the THz band, and this revealed that the THz-QCL synchronized with the microwave reference with a fractional frequency instability of 3×10 -15 at an averaging time of 100 s, corresponding to a center frequency deviation within 1 mHz, and the imposed phase noises during the heterodyne mixing were negligibly small.

We have proposed using nanoantennas with superconducting detector to improve the response performance of infrared detectors. Dipole nanoantennas that have an NbN load resistance were fabricated to evaluate the antenna properties. By using electromagnetic simulation, the antenna length was set at 2400 nm for the operation around 7-μm wavelength. The antennas were evaluated by using Fourier transform infrared spectroscopy. In an evaluation of spectral transmission characteristics, clear absorption caused by antenna effects was observed at around 1400 cm -1 . The results of the simulation qualitatively agreed with the results of the experiment. The antenna effective area was also evaluated, and the area was estimated to be approximately 5.4 μm 2 at 1420 cm -1 . We have fabricated a prototype of midinfrared (MIR) photodetectors that were formed with 45 dipole antennas with an NbN strip. The antennas were placed within an area of 20 × 20 μm 2 , and they were connected by bias line in series. By the irradiation of MIR (λ = 4.9 μm) light, the responses with high polarization dependence values caused by antenna effect were observed.

To improve the response performance of superconducting infrared detectors, we propose using a photonic antenna with a microdetector in conjunction with a nanostructure. In this paper, we report on the evaluated properties of a slot antenna and microstrip lines for the design of a midinfrared detector with a phased-array nanoslot antenna. We also explain the fabrication process for the detector. The antenna and microstrip line were designed for operation at a wave center near 5 μm. By measuring spectral reflectance characteristics, clear absorption by the antenna and the microstrip resonator was observed. To evaluate the validity of our process, we fabricated a midinfrared detector, which has structures composed of slot antennas and microstrip lines. We confirmed that the superconductivity of the superconducting detector was generally maintained in good condition after the fabrication process of midinfrared detectors.

This study discusses the electromagnetic modeling of fatigue cracks in plant environment to improve accuracy of depth sizing with eddy current testing. For the purpose, the factors that influence eddy current signals of fatigue cracks are evaluated. Here, martensitic layer and oxide fillings in fatigue cracks are focused on as influencing factors, and oxides are systematically filled in fatigue cracks by thermal treatment. The measured EC signals change depending on the presence and type of filling oxides. Numerical simulations are conducted to discuss the electromagnetic modeling of cracks with oxide fillings. The types of oxides in the cracks are identified by Raman spectroscopy, and the martensitic layers in vicinity of crack faces are observed by magnetic force microscopy. These results support for the electromagnetic model of environmental fatigue cracks developed in this study.

We fabricated Bi2Sr2CaCu2Ox (Bi-2212) SQUIDs with two stacks of intrinsic Josephson junctions by an acid-treatment process. The stacks were fabricated from a single crystal of Bi-2212. The in-plane loop of a dc-SQUID was surrounded by an acid-treated product, which was produced by immersing the Bi-2212 single crystal in dilute hydrochloric acid (pH=1.65). The critical aspect of this process is that the surrounding acid-treated product (BiOCl) is transparent. The SQUID exhibited a large hysteresis with a ratio of the critical current to the return current of 0.4 at 77 K. Applying a magnetic field modulated the critical current of a dc-SQUID for fields of up to about 10 G. In addition, we fabricated dc-SQUIDs attached to a shunt resistor, which exhibited nonhysteretic current-voltage characteristics. Appling a magnetic field modulated the voltage; the maximum voltage modulation was about 30 μV at 77 K.

To improve the response performance of superconducting infrared detectors, we have developed a fabrication process for nano-antennas. A nano-antenna consists of a dipole antenna, and a superconducting thin film strip placed in the antenna's center. By measuring the transition temperature of the superconducting strips, we confirmed that their superconductivity maintained a good condition after the nano-antenna fabrication process. We also evaluated nano-antenna characteristics using Fourier transform infrared spectroscopy. The evaluated antenna length and width were respectively set at around 2400 nm and 400 nm, and the antennas were placed at intervals of several micrometers around the area of 1 mm2 . In an evaluation of spectral transmission characteristics, clear absorption caused by antenna effects was observed at around 1400 cm-1. High polarization dependencies were also observed.

This paper describes the first experimental results for a waveguide-type all-NbN superconductor-insulator-superconductor (SIS) heterodyne mixer on an MgO substrate designed to operate over the gap frequency of Nb. The mixer consists of an NbN/MgO/NbN junction, which has a length of one wavelength at 880GHz as a tuning circuit, an NbN/MgO/NbN microstrip as a λ/4 impedance transformer, and an RF choke filter. The mixer chip was designed using a high-frequency-structure simulator. Its return-loss and embedding-impedance characteristics were examined using a 180-times-scaled mixer model. By optimizing the cutting and polishing processes for the MgO substrate, we were able to fabricate the mixer chip with an accuracy of less than 5 μm. We succeeded in mounting the chip on a mixer block and in estimating the receiver noise temperature. The uncorrected minimum double-sideband receiver noise temperature was 740 K at 824 GHz. A comparison of the receiver noise temperature in a quasi-optical SIS mixer fabricated on the same wafer as the waveguide mixer showed that input noise was the major contributor to receiver noise in the waveguide mixer.

The results of comparing the 1/f noise of NbN/MgO or AlN/NbN superconducting tunnel junctions that were epitaxially grown on a MgO (100) single-crystal substrate indicated that the 1/f noise level in a NbN/MgO/NbN junction was practically independent of the critical current density JC (evaluated from the quasiparticle current rise in the gap voltage). However, the 1/f noise level in a NbN/AlN/NbN junction increased suddenly with a decrease in JC. The reason is that since the crystal structure differs for NbN and AlN in the thick film state, as the AlN barrier thickness increases, the effect of the base crystal (NbN) weakens, and NbN/AlN epitaxial growth collapses. © 2005 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 88(12): 19–25, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.20063

The thermal response of a superconducting NbN thin-film meander line was studied by irradiating with a 20 ps pulse laser. A 10 nm-thick NbN thin film was prepared by dc magnetron sputtering and then processed to fabricate a 3 μm-wide, 125.5 mm-long meander line. The device was placed in a 4 K refrigerator, and the bias temperature was kept below the critical temperature T c . The end of an optical fiber was fixed at the front of a meander line, which was then directly irradiated by using the 20 ps pulse laser. The output voltage was observed with a digital oscilloscope and a low-noise amplifier. The output signals of the thermal response were clearly observed.

We developed a new fabrication process of stacked intrinsic Josephson junctions using superconducting Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi-2212) single crystals. In the proposed self-planarizing process, the Bi-2212 crystal around the junction window was modified to insulator by soaking into the solution of dilute (0.05–0.2%) hydrochloric acid for 5 s. Energy dispersive X-ray (EDX) spectroscopy revealed that the acid-treated Bi-2212 exhibited the decrease of Cu and Sr contents in the crystals. The current–voltage characteristic of the stacks showed distinct resistive branches with large hysteresis at 77 K. The number of intrinsic junctions in the stacks linearly decreased with decreasing the concentration of the solution. The well controllability of the number of junctions in the self-planarized stacks may be useful for electronic device applications.

Low-frequency noise properties of epitaxial NbN/AlN/NbN tunnel junctions have been investigated as a function of the bias voltage. The subharmonic gap structures (SGSs) were clearly observed in the differential resistance dV/dI-V curves. The SGSs were explained with Octavio, Tinkham, Blonder, Klapwijk (OTBK) theory based on the multiple-Andreev reflection (MAR) phenomena. To estimate the amplitude of the voltage noise power spectra S/sub V/(f) in the range 1-10/sup 5/ Hz, we extended Rogers and Buhrman empirical theory. The 1/f noise parameter /spl eta/ in this theory is given as /spl eta/=S/sub V/(f)I/sub b//sup 2/R/sub d//sup 2//fA, where A is the junction area, I/sub b/ is the bias current and R/sub d/ is the dynamic resistance. The /spl eta/ values (/spl eta//sub sg/) biased at the sub-gap voltage were larger than that (/spl eta//sub ag/) of the above-gap voltage. In addition, the /spl eta//sub sg/-V curves in the sub-gap region have dip and peak structures related to the SGSs in the dI/dV-V curves. In contrast, for the above-gap bias voltage, the /spl eta//sub ag/ values did not depend on the bias voltage. We concluded from these results that the excess 1/f noise in the sub-gap bias voltage was caused by the fluctuation of the MAR conductance.

To extend the application range of epitaxial NbN devices, we developed epitaxial TiN thin-film resistors. TiN has approximately the same lattice constant and crystalline structure as NbN and MgO. However, high quality TiN thin films show superconductivity at the liquid helium temperature. We attempted to remove this superconductivity using TiO impurity. The ratio of Ar and N/sub 2/ was fixed at 5:1 and the O/sub 2/ flow ratio varied from 0 to 1.6% in relation to the total gas flow. When O/sub 2/ ranged from 0.5 to 1.6%, superconductivity vanished and the resistivity obtained was about 50-145 /spl mu//spl Omega/cm at 4.2 K. X-ray analysis showed that the TiN films grew epitaxially on the MgO substrate as did the NbN films on the TiN films. Using the TiN resistor, a fully epitaxial NbN Josephson array oscillator was fabricated and tested. The oscillator was formed with 10 TiN-shunted NbN/MgO/NbN tunnel junctions and NbN microstrip resonators. At around 1.0THz, Shapiro steps induced by the Josephson oscillation were observed in the detector I-V characteristics.

We investigated subharmonic gap structures in the current-voltage (I-V) characteristics of NbN tunnel junctions by observing frequency response using a quasioptical mixer configuration. We chose a 10.5-/spl mu/m-long by 0.6-/spl mu/m-wide junction for the mixer so that the junction behaved as a distributed element at submillimeter-wave frequencies in wideband observations. In the presence of electromagnetic waves, subharmonic photon-assisted tunneling steps with a voltage width of hf/2e could clearly be observed below and above the half-gap voltage in the I-V curves of junctions at measured frequencies from 600 to 950 GHz, where h is Planck's constant, f is frequency, and e is electron charge. This evidence is consistent with a consequence of multiple Andreev reflection at low transmissions or multiparticle tunneling.

Interplay between microwave field and Andreev current in epitaxial NbN/AlN/NbN tunnel junctions have been investigated. The subharmonic-gap structures (d V /d I dip-structures) were clearly observed for almost all junctions without microwave field, and the voltage positions of d V /d I dips were well explained by Octavio, Tinkham, Blonder and Klapwijk (OTBK) theory based on multiple-Andreev reflection (MAR). In the presence of microwave field, the d V /d I – V curve only in a high- J c (5 kA/cm 2 ) junction has a rich subgap structure due to photon-assisted MAR. The observed microwave-induced d V /d I structure was qualitatively explained by the OTBK theory with an effective time-dependent voltage model.

For the optimum design of integrated receivers operating above the gap frequency of Nb, we have designed, fabricated and tested NbN-based quasi-optical superconductor/insulator/superconductor (SIS) mixers. The mixer chip incorporates a resonant half-wavelength epitaxial NbN/AlN/NbN junction, a twin-slot antenna and their coupling circuits. We adopted two kinds of coupling circuit between the antenna and the SIS junction: one is an in-phase feed with a length of 95 µm and the other is an anti-phase feed of 30 µm length. It was found that the anti-phase mixer reveals a 3 dB bandwidth of 43% of the centre frequency; the uncorrected double-sideband receiver noise temperature TRX = 691 K at 0.91 THz and TRX = 844 K at 0.80 THz, while 17% and TRX = 1250 K at 0.79 THz for the in-phase version. Possible reasons for this difference are discussed, which could be transmission loss and its robustness with respect to the variation of junction parameters. These experimental results suggest the NbN-based distributed mixer with the anti-phase feed is a better candidate for wide-band integrated receivers operating above 0.7 THz.

Phase‐locking of two longitudinal oscillating modes of a dual‐mode microchip laser that has 101 GHz longitudinal‐mode spacing has been successfully demonstrated by using electronic negative‐feedback control. Since the two longitudinal modes were oscillating simultaneously in the same laser cavity, the common‐mode fluctuation cancels, resulted in the small required control bandwidth of ∼4 kHz. This narrow‐bandwidth phase‐locking technique is useful for generating high‐spectral‐purity millimetre‐wave signals with a considerably simpler setup.

MgB 2 thin films were grown on sapphire (0 0 0 1) substrates. The thin films were deposited using the co-evaporation method, in which individual deposition rates were well controlled. The as-grown thin films demonstrated superconductivity without the use of any post-annealing process. The dependence of the characteristics of the thin film on the substrate temperature and evaporation rates was investigated. The thin films did not show superconductivity at a high substrate temperature and low deposition rate, but increasing the deposition rate greatly improved the quality of the film. It was found that a high substrate temperature and high deposition rates were essential to produce high-quality films. The critical temperature for the MgB 2 film was around 35 K. MgB 2 thin films made using the co-evaporation method can be expected to have excellent properties after further optimization.

To improve reproducibility of full-epitaxial NbN/MgO/NbN tunnel junctions, we fabricated junctions using an ion-beam sputtered MgO tunnel barrier. The tunnel junctions were fabricated on single-crystal MgO(100) substratesand X-ray analysis showed that all the layers of NbN/MgO/NbN forming the junction had been epitaxially grown. The junctions showed good tunneling characteristics, with large gap voltages and small subgap leakage currents. The junctions were fabricated in the range of R N A products of 25-26,000 Ωμm 2 . By using ion-beam sputtering, the stability of the MgO tunnel barrier's deposition rate could be increased and the controllability of the barrier thickness was improved.

We present the high-speed test results of a tandem-Banyan network switch (TBNS) component based on superconductor single-flux-quantum technology. The tested circuit is called a transmission checker (TMC) and is one component of the TBNS. The circuit was designed by using the CONNECT SFQ cell library developed for NEC standard fabrication technology. To perform the high-speed test, an on-chip test system composed of shift resisters and a high-speed clock generator were integrated together with the TMC circuit. The total number of Josephson junctions, including the on-chip test system, was 1545. We observed correct output waveforms at up to 40 GHz. The bias margin of the TMC circuit was ±6.1% up to 27 GHz and ±1.7% at 40 GHz.

We have designed and fabricated waveguide superconductor-insulator-superconductor (SIS) receivers at 870 GHz with NbN/AlN/NbN tunnel junctions on an MgO substrate. A waveguide probe was incorporated into the mixer chip for a waveguide-to-microstrip transition. As the tuning circuit for the SIS mixer, we designed a resonant distributed junction whose length is one wavelength and critical current density is 20 kA/cm/sup 2/. The mixing properties were calculated based on the quantum theory of mixing. The SSB mixer noise temperature should be less than 100 K and conversion gain larger than -8.0 dB in the frequency range from 840 to 920 GHz. We also observed the resonance steps of the fabricated SIS mixer. The tuning frequency was slightly higher than the designed frequency.

We have designed an all-NbN superconductor-insulator-superconductor mixer with a broadband tuning circuit for the terahertz band, which enables the use of junctions with a lower current density. It uses the reactance compensation efficiently produced by two half-wavelength distributed junctions connected by a half-wavelength microstripline. Based on the design and simulated noise performance, a tuning circuit using NbN junctions with a current density of 25 kA/cm/sup 2/ should be able to cover the 780-950 GHz band with an SSB receiver noise temperature below 4hfk/sub B/.

MgB/sub 2/ thin film growth on sapphire (0001) and MgO substrates is reported. The thin films were deposited by using the co-evaporation method, in which the deposition rates were well controlled separately. The as-grown thin films demonstrated superconductivity without the use of any post-annealing process. The critical temperature dependence of the substrate temperature and the evaporation rates were investigated, and it was found that a high substrate temperature and high deposition rates are needed to produce high-quality films. Below a substrate temperature of 250/spl deg/C, the films exhibited no x-ray diffraction peaks, but above it, the films tended to grow epitaxially to c-axis on sapphire (0001) substrate. The critical temperature of MgB/sub 2/ film was over 30 K, and MgB/sub 2/ thin films made by co-evaporation method are expected to have excellent properties after further optimization.

To develop an efficient local oscillator (LO) integrated with a SIS mixer operating above the gap frequency of Nb, we have investigated the radiation power P of NbN-based flux-flow-type Josephson oscillators (FFO's). The designed and fabricated chip incorporates FFO's, SIS power detectors (DET's), and their coupling circuits. Both FFO's and DET's consist of epitaxial NbN/AlN/NbN junctions with high critical current density J/sub C/ (15 200 nW, enough for the optimum pumping of a SIS mixer with rf-resistance of 50 /spl Omega/, is coupled to DET's for 0.5-0.9 THz. The coupling bandwidth is larger than 20% of its central frequency. In the band, the radiation frequency is tuned by the control current through the FFO of 10-100 mA. The maximum coupling frequency of the present experiment is quantitatively agreed with the theoretical one. The peak power of 1.3 /spl mu/W is detected at 0.76 THz. The dissipated power in a FFO is smaller than 500 /spl mu/W, which is 10/sup -4/ times less than that of semiconductor sources. These results indicate the feasibility of NbN-based FFO's for a practical on-chip LO.

We measured the low frequency noise characteristics and subharmonic gap structures of epitaxial NbN/AlN/NbN tunnel junctions with different current densities. For all of the junctions, the voltage noise power spectrum S/sub v/(f) showed a frequency dependence that is well described by 1/f behavior. Subharmonic gap structures were clearly observed in the dV/dI-V curves at voltage 2/spl Delta//ne, and the values of n increased with the current density J/sub c/ in the junctions. We estimated the 1/f noise parameter /spl eta/ using the empirical theory of Rogers and Buhrman for the S/sub v/(f), and investigated the correlation between the /spl eta/ and the inverse junction quality 1/Q. We found that the /spl eta/-1/Q relationship for our epitaxial tunnel junctions gave a different behavior for nonepitaxial tunnel junctions. The tunnel barrier properties were investigated by applying a pulse voltage to the junctions at 4.2 K, and measuring the applied voltage dependence of the I-V characteristics and low frequency noise amplitude.

To improve the performance of superconducting analog devices at frequencies exceeding the Nb gap frequency, we developed a fabrication process to grow epitaxial NbN/MgO/NbN trilayers. To evaluate the RF performance of the trilayers, we fabricated Josephson junctions with an epitaxial NbN/MgO/NbN microstrip resonator. A Josephson junction was placed in the center of the resonator and it was used as both an oscillator and a detector. The width and length of the microstrip resonator were 10 and 1000 /spl mu/m, respectively. Current steps of up to 1.9 mV resulting from coupling with the resonator were observed in the I-V characteristics. We found that the height of the current steps depended on the loss of the resonator. By assuming that the sum of the dielectric loss and the radiation loss is much smaller than the conductor loss, we estimated the surface resistance of the epitaxial NbN thin films. The surface resistance of the epitaxial NbN films was estimated to be approximately 2.6-88 m/spl Omega/ at 0.1-1.1 THz.

We report on the fabrication of as-grown MgB/sub 2/ thin films and MgB/sub 2//AlN/NbN thin film heterostructures having as-grown MgB/sub 2/ thin films. The MgB/sub 2/ thin films deposited by using a carousel-type sputtering system at substrate temperatures T/sub s/ of 252/spl deg/C typically showed a critical temperature of 28 K. The results of XRD measurements of the thin films indicated a c-axis orientation. The surface morphology of the films fabricated at T/sub s/=252/spl deg/C was determined to be very flat and smooth from SEM images. The MgB/sub 2//AlN/NbN trilayer was continuously deposited on sapphire [001] substrates in a single vacuum run. The AlN-barrier and NbN-counter layers were deposited by using DC-reactive magnetron sputtering at ambient substrate temperatures. The junctions were fabricated by using conventional photolithography, reactive ion etching, and electron cyclotron resonance etching techniques. The junctions demonstrated excellent quasiparticle tunneling characteristics having ideal dependence of the normal resistance on the junction area and on the AlN-barrier thickness. Also, the DC magnetic field and temperature dependences of the supercurrent I/sub c/ were measured to investigate the Josephson tunneling behavior in the MgB/sub 2//AlN/NbN junctions. The junction I/sub c/-H curve showed the ideal Fraunhofer pattern.

We have measured the temperature dependence of the low-frequency noise properties across for mesa stacks of intrinsic Josephson junctions (IJJs) in single crystals of Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub y/ (BSCCO) in the absence of an external magnetic field. The measured curves for noise spectrum density S/sub V/(f) showed a 1/f frequency dependence at temperatures in the range from 4.3 K to 150 K except 36 K. We estimated the magnitude of the S/sub V/(f) by using Rogers and Buhrman's empirical theory and Machlup's formula. From the observed curves for S/sub V/(f), we estimated a 1/f noise level /spl eta/ for a BSCCO IJJ biased at a sub-gap voltage of approximately 10/sup -5/ /spl mu/m/sup 2/, and this value was almost completely independent of temperature in the range between 4.3 K and 40 K. We also found that the /spl eta/ values above T/sub c/ were smaller by a factor of approximately 10 than those below T/sub c/. The /spl eta/ value for the BSCCO IJJ is approximately two orders of magnitude greater than for an epitaxial NbN/MgO/NbN tunnel junction biased above-gap voltage. We consider that the observed variation in the noise level with temperature is not interpretable in terms of a magnetic origin and the 1/f noise in a BSCCO IJJ may be somehow associated with stress or defects in the BiO and/or SrO layers act as tunnel barriers.

We fabricated Josephson junctions having an epitaxial NbN/MgO/NbN microstrip resonator to evaluate the RF performance of the epitaxial NbN/MgO/NbN trilayers. All of the NbN and the MgO layers that formed the junctions and the resonators were confirmed to be epitaxial growth by X-ray analysis. The width of the microstrip resonator was 0.7 μm and the length was changed from 1000 to 99.3 μm. At the resonant frequency, current steps appeared in the I – V characteristics. The steps were observed up to 2.5 mV. By fitting the simulated results to the measured data, the surface resistances of the epitaxial NbN thin films were estimated to be about 35–88 m Ω in the 0.7–1.1 THz range.

We present the results of an investigation of the crystal structures and superconducting properties of as-grown MgB2 superconducting thin films deposited on Al2O3 substrates having different crystal planes (C- and R-planes). The MgB2 thin films were prepared by a multiple-target sputtering method without performing a post-annealing process. The MgB2 films deposited on the C-plane Al2O3 substrates were found to have a c-axis orientation, from the results of 2θ/θ x-ray diffraction (XRD) measurements. However, there were no MgB2 peaks for the films grown on the R-plane Al2O3 substrates. Even though the maximum Tc values of both films deposited on the C- and R-plane Al2O3 substrates were nearly the same, Tc,onset ~ 28 K and residual resistivity ratio ~ 1.2, it is very interesting to note that the resistivity of the films on the C-plane Al2O3, ρ ~ 70 μΩ cm at 40 K, was lower than that on the R-plane Al2O3, ρ ~ 300 μΩ cm at 40 K. These results indicate that the grain size of MgB2 in the films deposited on the C-plane Al2O3 substrates was larger than that on the R-plane Al2O3, indicating a good agreement with the XRD measurements. The surface morphology of the as-grown MgB2 thin films was investigated using scanning electron microscopy. The surfaces of the films on the C- and R-plane Al2O3 substrates were found to be very flat and smooth. This result indicates the high potential for fabricating Josephson tunnel junctions using as-grown MgB2 thin films. Presented at the IUMRS-ICEM conference, Xi'an, China, 10–14 June 2002.

We fabricated full-epitaxial-NbN Josephson array oscillators to operate over the Nb-gap frequency. The oscillator was formed with 40 shunted NbN/MgO/NbN tunnel junctions and NbN microstrip lines. An NbN/MgO/NbN tunnel junction with an open-stub was used as a detector and was directly connected to the oscillator through the narrow microstrip line. X-ray analysis confirmed that all NbN and MgO layers of the oscillator grew epitaxially. At around 680 GHz, Shapiro steps and photon-assisted tunneling steps induced by Josephson oscillation were clearly observed in the detector I – V characteristics.

To achieve the full potential of superconducting circuits based on Josephson junctions, it is necessary to reduce the junction size to the sub-micron regime. We developed an advanced fabrication process by using electron-beam lithography (EBL) and chemical–mechanical polishing (CMP). Trilayers of NbN/AlN/NbN with a critical current density ( J c ) over 10 kA/cm 2 were prepared by an RF sputtering method. The junction area was defined by EBL, and an electrical contact between the counter electrode and the wiring layer is made by planarization using CMP. The standard deviation ( σ ) of the critical current was 2.2%, 3.2%, 4.5%, and 7.0% for the junction sizes of 1.1, 0.8, 0.6, and 0.4 μm, respectively. This result indicates that integration of more than 10,000 junctions with the minimum size of 0.6 μm is possible.

As part of an effort to develop heterodyne rf receivers operating in the terahertz band, we have fabricated hot-electron bolometers (HEB) using NbN thin films, and investigated their intermediate frequency (IF) bandwidths. The NbN thin films we studied were grown on single-crystal MgO substrates by dc reactive sputtering. Our 2.8-nm-thick films exhibited superior superconducting properties, including transition temperatures TC of 9.7 K and 20 K resistivities ρ20 of 185 µΩ·cm. Using electron beam diffraction, high-resolution TEM images, and other techniques to study the crystallinity of these films, we determined that the NbN grows epitaxially on the MgO substrate as single-crystal material. The HEB elements we made consisted of a NbN microstrip, a Au electrode section, and an Al antenna section. In the course of studying the IF bandwidths of these devices in the millimeter-wave range, we found one device with an IF bandwidth of 2.5 GHz, the highest of any device we made. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 85(7): 77–83, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.1111

The performance of a hot-electron bolometric (HEB) mixer based on NbN from 0.9 to 2.5 THz was investigated using a quasi-optical receiver configuration. An HEB mixer is an ultra-thin superconducting NbN strip located at the feed point of a thick normal conducting Au spiral antenna on a high-resistivity Si substrate. The active area of the mixer was 3 nm thick, 0.4 μm long and 4 μm wide. The quasi-optics consisted of an MgO hyperhemisphere with anti-reflection caps made of Kapton-JP polyimide film and an offset parabola to reduce input losses. The frequency dependence of the double-sideband receiver noise temperature was investigated at several frequencies by using a backward wave oscillator or an optically pumped far-infrared laser as the local oscillator. Results demonstrated low-noise and wide-band characteristics, below 1 K GHz−1 over the measured frequency range. At 917 GHz, the measured receiver noise temperature was 550 K across a 500 MHz intermediate-frequency bandwidth centred at 1.5 GHz, which is slightly better than that of other HEB mixers at around this frequency. This paper was presented at the 8th International Superconductive Electronics Conference, Osaka, Japan, 19–22 June 2001.

We report the fabrication and testing of hot electron bolometric mixers with an ultrathin NbN film as heterodyne receivers operating at terahertz frequencies. We found that the quality of the NbN strip is usually degraded by damage resulting from the fabrication process, and we developed a process for reducing such damage. We also fabricated HEB mixers with a new structure in order to study the IF bandwidth determined by the original quality of the NbN thin films. Investigations at 100 GHz revealed that the widest IF bandwidth of 2.0 GHz was obtained by a mixer based on a 2.8 nm-thick NbN film. An HEB mixer based on the 2.8 nm-thick NbN film was also fabricated and evaluated. The receiver noise temperature at 900 GHz was 780 K, the absorbed LO power was about 400 nW, and the conversion gain was -13 dB.

To improve the performance of Josephson oscillators and SIS mixers over 700 GHz, we have developed a fabrication process to grow epitaxial NbN/MgO/NbN trilayers by reactive dc-sputtering. Trilayers were fabricated on a single-crystal MgO substrate at ambient temperature. NbN and MgO films were deposited by reactive dc-sputtering with an Nb and Mg target, respectively. The MgO inter-layer thickness was changed up to 480 nm, but, T/sub C/ and 20-K resistivity of the NbN upper-layer showed no remarkable dependency and they were 15.7 K and about 60 /spl mu//spl Omega/cm, respectively. To evaluate the RF performance of the epitaxial NbN films, we fabricated Josephson junctions with a microstrip resonator constructed from an epitaxial NbN/MgO/NbN trilayer. The I-V characteristics of the junction exhibited resonance steps up to 2.5 mV, which suggests that the epitaxial NbN films have low loss up to 1.2 THz.

Low frequency noise characteristics of epitaxial NbN/AlN/NbN tunnel junctions have been investigated. For all of our junctions with different current densities, the voltage noise power spectrum S/sub v/(f) showed a frequency dependence of 1/f type. The magnitude of the S/sub v/(f) exhibited two distinct types of dependency on the current density and AlN barrier thickness. We consider that this may result from a difference in the crystal structures of the AlN barrier by consulting with the barrier thickness dependence of tunnel barrier heights. We also estimated the 1/f noise parameter /spl eta/ using the Rogers and Buhrman's empirical theory for the S/sub v/(f), and investigated the relationship between the /spl eta/ and the current density J/sub c/ and the tunnel barrier thickness d/sub AlN/. The tunnel barrier was characterized by investigating the /spl eta/-d/sub AlN/ relation. It was found that the /spl eta/-J/sub c/ and /spl eta/-d/sub AlN/ relation in our high current density junctions, i.e, in the epitaxial tunnel junctions, are different from nonepitaxial tunnel junctions.

We have designed, fabricated, and tested terahertz superconductor-insulator-superconductor (SIS) mixers having a self-compensated NbN/AlN/NbN tunnel junction and an epitaxially grown NbN/MgO/NbN microstripline. The junction as a distributed element was 0.8-/spl mu/m wide and 2.4-/spl mu/m long, and its current density was 50 kA/cm/sup 2/. The microstripline consisted of a 200-nm-thick NbN ground plane, a 180-nm-thick MgO insulator, and a 350-nm-thick NbN wiring layer. An investigation of the mixer in our receiver configuration showed flat noise characteristics from 870 to 960 GHz. The lowest receiver noise temperature of about 550 K was obtained at 909 GHz, including a 9-/spl mu/m-thick Mylar beam splitter loss and other optical losses. These characteristics suggest that SIS mixers with self-compensated NbN/AlN/NbN junctions and NbN/MgO/NbN microstriplines are appropriate for used in wideband and low-noise operations at terahertz frequencies.

Some fluoro organic lithium salts have attractive features as organic electrolyte such as solubility, conductivity, and oxidation potentials. The fluoro organic lithium salts with SO2 groups show higher conductivities than those with CO groups. Methide (CF3SO2)3CLi and imide (RfSO2)2NLi salts show higher conductivities than oxide salts RfSO2OLi with only one RfSO2 group. The lithium salts with long fluoroalkyl groups and two or more RfSO2 groups show high oxidation potentials and high aluminum corrosion potentials over 4.3 V. The battery performances of modified imide salts such as (C2F5SO2)2NLi, (C4F9SO2)(CF3SO2)NLi, and ((CF3)2CHOSO2)2NLi are satisfactory. The cycle characteristics of the imide salt cells are better than those of the LiPF6 cell. Especially, the ((CF3)2CHOSO2)2NLi shows the best cycle characteristics. In XPS analysis the anode surfaces of the imide cells are covered with imide salt components.

We report on tunneling properties and interface structures for high-quality NbN/AlN/NbN tunnel junctions fabricated on ambient temperature MgO substrates. Junction quality and electrical parameters were systematically investigated in a very wide range for current density. The junctions show a very good junction quality with a high gap voltage, large I/sub c/R/sub N/ product, and large R/sub sg//R/sub N/ ratio as the current density varied from 100 A/cm/sup 2/ to above 100 kA/cm/sup 2/. The average barrier heights of the NbN/AlN/NbN tunnel junctions are calculated from the barrier thickness dependence of the critical current density. We found that the current density has two distinct types of dependency on the AlN barrier thickness, corresponding to two average barrier heights in different regions for the current density.

At present, switching devices such as GCT (Gate Commutated Turn-off) thyristor and HVIGBT (High Voltage IGBT) are remarkably progressing and able to switch fast at high DC operation voltage above 2.5kV. However, a freewheeling diode which should realize the recovery performance similar to the turn-on performance of switching devices is not developed. In the high voltage diode, a main problem of the recovery performance is to prevent the voltage-oscillation. The voltage-oscillation brings miss-operation of driving circuit or voltage destruction of oneself. As the diode during the recovery operation can be regarded as a combination of one capacitance and two resistances, the oscillation mechanism was studied by replacing the diode to LCR circuit. As this results, it was clear that the diode during the recovery operation made surely the oscillation. On the basis of this analysis, the diode structure to mitigate oscillation was proposed.

The operation of an external-cavity semiconductor diode laser with selectable coherence and a tunable wavelength has been successfully demonstrated through the control of the intracavity polarisation state. The laser has an intracavity half-wave plate that is able to rotate the orientation of the polarisation axis with regard to diffraction grating, where the grating acts as a polarisation- and wavelength-selective element. As a result, it was possible to continuously sweep the correlation widths from 230 kHz to 2.0 MHz by simply rotating the half-wave plate.

The charge/discharge characteristics and mechanisms of low-crystallized carbons which have larger capacity than graphite have been investigated. Low-crystallized carbons have two principal types of charge curve versus Li metal. Hard carbons prepared at 1100 °C (H11) show charge curves with a low average potential, whereas soft carbons pyrolyzed at 700 °C (S7) show those with a high average potential. These results might depend on the lithium diffusion rate in their non-crystallized sites. The 18650-type Li-ion batteries using H11 have comparable capacity versus graphite, whereas the batteries using S7 have low capacity because of their low charge/discharge efficiency.

In our continuous study on organic lithium salts, we found that the imide salt (RfSO 2 )(R′fSO 2 )NLi shows high conductivity in organic solvents, and that the modification of the imide salts change the anodic oxidation potential and the dissolution potential of aluminum current collector. Lithium imide with long fluoroalkyl groups such as (C 4 F 9 SO 2 )(CF 3 SO 2 )NLi and the new ester-type imide salts, ((CF 3 ) 2 CHOSO 2 ) 2 NLi do not dissolve the aluminum collector up to 4.8 and 4.3 V, respectively. We also examined the polymer analogue of the ester-type imide salt such as (-CH 2 (CF 2 ) 4 CH 2 OSO 2 N(Li)SO 2 O-) 9–10 .

Hot electron bolometers (HEBs) are expected to be used as low-noise heterodyne mixers for applications in radio astronomy and remote sensing in the terahertz frequency region. We fabricated and tested HEB mixers for these applications; however, those we made with the conventional process easily broke due to thermal stress and electrical surges. To improve their durability, a new process involving fluoride radical etching was developed. In this process, HEB mixers were fabricated using a relatively thick NbN strip (several tens of nm). Finally, the NbN strip was etched that was less than 4-nm thick enough to operate as HEB mixers by using the fluoride radical etching. We confirmed that fluoride radical etching caused no damage to the etched NbN strip. The dc properties of the HEB mixers fabricated with the new process did not change for fourteen 4.2-300 K thermal cycles for 50 days. The receiver noise temperature of the HEB mixer was evaluated and it was about 615 K at 780 GHz.