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We deposited SmBCO thin films on BaZrO3 (BZO)-buffered MgO(100) substrates by an off-axis magnetron sputtering method. SmBCO films were deposited at a fixed condition on stoichiometric and Zr-rich BZO-buffered MgO substrates. We found that the crystal orientation of SmBCO films changed from a-axis to c-axis orientation by using Zr-rich BZO. We speculate that the reaction between excess Zr oxides on the buffered substrates and deposited species at the initial stage of the SmBCO deposition is important in inducing these phenomena.

We have studied the fabrication process for c-axis-oriented Pr1.4Ba1.6Cu3Oy (P4) thin films as non-superconducting layers in HTS electronic devices. The P4 films were deposited by a pulsed laser deposition method. We found that gas control is a key factor for successful fabrication. Deposition in 200 mTorr oxygen, which was a suitable condition for deposition of c-axis-oriented YBCO thin films, resulted in c-axis-oriented P4 films containing BaPrO3 impurities. It seemed that oxygen stabilized Pr4+ and promoted the formation of BaPrO3. Pinholes were observed in the films. Energy loss of the ablated species by collision with excess atoms in the deposition chamber possibly brought about insufficient migration of the deposited atoms on the substrate and imperfect connection of the P4 grains. It was found that deposition in 40 mTorr oxygen yields P4 films without impurity phases or pinholes. Feasibility of the P4 film as an insulating layer in ramp-edge Josephson junctions and a buffer layer for Hg-based superconducting films is preliminarily studied.

The preparation process for epitaxial multilayer films consisting of Y 0.9 La 0.2 Ba 1.9 Cu 3 O y (La-YBCO) and SrSnO 3 (SSO) for high- T c superconductor (HTS) electronic devices was developed. Four-layer films of SSO/La-YBCO/SSO/La-YBCO were deposited on BaZrO 3 -buffered MgO(1 0 0) substrates by off-axis magnetron sputtering. During deposition, the substrates were directly heated from backside by thermal radiation of a heater. No adhesive paste, such as Ag, for fixing the substrate was used in order to avoid contamination. Deposition conditions, especially the heater temperature profiles and atmosphere during elevating the temperature, were carefully controlled. The multilayer films with the average surface roughness R a around 2.0 nm were prepared with high reproducibility. Continuous production of the high-quality films throughout one year proved excellent stability of the preparation process.

We have developed a stable preparation process for high-quality thin films of YBa2Cu3Oy (123)-type high-Tc superconductors (HTS) for the ground plane of single-flux-quantum (SFQ) devices. The films were deposited on MgO (100) substrates by an off-axis DC magnetron sputtering method. During deposition, the substrates were directly heated by thermal radiation from a heater. No adhesive paste was used for fixing the substrate. Possibility of chip contamination due to the paste could be completely eliminated. The insertion of a BaZrO3(BZO)-buffer layer was effective to obtain flat epitaxial 123 films with complete c -axis orientation. The flat ground plane could play a role as a suitable foundation for multilayer films of SFQ devices. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have developed a multilayer structure with a smooth surface and fabrication processes for HTS single-flux-quantum (SFQ) circuits. The multilayer structure with surface roughness Ra less than 2 nm, composed of a La0.2–Y0.9Ba1.9Cu3Ox ground plane and base electrode layer, and SrSnO3 insulating layers, was deposited by off-axis magnetron sputtering. We have fabricated interface-engineered junctions based on the multilayer structure using an La0.2–Yb0.9Ba1.9Cu3Ox counter-electrode layer prepared by pulsed laser deposition. The fabricated junctions exhibited excellent Josephson characteristics with a 1-σ spread in Ic as low as 8% for 1000 junctions. The sheet inductance values below 50 K were 0.8–1.0 pH per square. Operation of several elementary SFQ circuits including a toggle flip-flop, a confluence buffer, a set–reset flip-flop and other SFQ elements has been successfully demonstrated at 30–60 K.

Multilayer structures consisting of (Hg0.9Re0.1)Ba2CaCu2Oy [(Hg,Re)-1212] superconducting layers and CeO2 insulating layers have been successfully fabricated. A CeO2 film could be epitaxially grown on a (Hg,Re)-1212 film at a relatively low temperature of 500°C without deterioration of crystallinity and superconductivity for (Hg,Re)-1212. Moreover, another (Hg,Re)-1212 film was grown on CeO2/(Hg,Re)-1212 bilayer films to form trilayer films. The upper and lower (Hg,Re)-1212 layers for the trilayer films exhibited critical temperatures (Tc's) of 112–115 and 112–120 K, respectively. Ramp-edge-type interface-engineered Josephson junctions were fabricated using the (Hg,Re)-1212 multilayer structures. The junctions showed resistively shunted junction like current–voltage characteristics above 77–100 K, and a magnetic-field modulation of the critical current (Ic) of approximately 60% at 100 K.

CeO2 thin films as insulating layers for high-temperature superconducting digital devices were studied. The dependence on substrate temperature and oxygen pressure of the surface morphology and crystallinity of CeO2 thin films prepared by pulsed laser deposition were investigated. CeO2 thin films with a flat and closely grained surface were obtained at a relatively low oxygen pressure of 3.6 Pa, whereas higher oxygen pressure led to CeO2 thin films with a rough surface and columnar grains. The recovery of oxygen content in superconducting layers was examined for multilayer structures with CeO2 thin films. Enough oxygen was supplied to the upper and lower superconducting layers when the multilayer was cooled slowly in 3 × 10−4 Pa oxygen pressure after deposition. Resistively shunted junction type I-V characteristics were confirmed for interface-engineered ramp-edge junctions in a multilayer structure including four superconducting layers with CeO2 thin films.

This paper describes an overview of our plan and recent progress on the HTS sampler development with the aim of demonstrating its potential performance of a bandwidth over 100 GHz using a compact cryocooler. Several changes from the NEC’s former work have been made in the device fabrication process, the circuit design and the system design. We employed the HTS circuit process with a lower ground-plane structure, and improved the circuit design and the related packaging technology, which enabled a higher bandwidth. We have focused on observing 50 GHz waveforms by the sampler as the mid-point goal, and then succeeded in observing a 45 GHz sinusoidal waveform in magnetically coupled current measurement and a 50 GHz waveform in voltage measurement with a high-frequency module. An on-chip sampler observing short-duration sampling pulse was also designed and the observation was successfully demonstrated.

We examined the deposition conditions for SrSnO 3 films with a smooth surface and good crystallinity on both ground plane (GP) and MgO substrate, and successfully found the optimum conditions. SrSnO 3 insulating layers were prepared by an rf magnetron sputtering method. SrSnO 3 films deposited at gas pressure of 80 mTorr (Ar + 50% O 2 ) and rf power of 50 W with optimized target-substrate configuration had a smooth surface with the average roughness R a less than 1 nm. The measured X-ray rocking curve for the (2 0 0) reflection peak showed a small FWHM value of 0.3°. X-ray fluorescence analysis (XRF) and inductively coupled plasma analysis (ICP) revealed that the Sn/Sr cationic ratio for the films were close to unity. Employing the optimized SrSnO 3 films, we fabricated interface-engineered ramp-edge junctions on a patterned GP. The fabricated junctions exhibited RSJ-type I – V characteristics with I c R n products of about 2.1 mV at 4.2 K.

We succeeded in observing high-frequency current waveforms up to 45 GHz using a high-temperature superconducting (HTS) sampler. In this experiment, we used a sampler circuit with a superconducting pickup coil, which magnetically detects current signals flowing through a micro-strip line on a printed board placed outside the cryochamber. This type of measurement enables non-contact current-waveform observation that seems useful for analyses of EMI, defects in LSI, etc. Computer simulation reveals that one of our latest versions of HTS sampler circuits having Josephson transmission lines with optimized biases as buffers has a potential of sampling high-frequency signals with a bandwidth above 100 GHz. To realize the circuit parameters required in the simulations, we developed an HTS circuit fabrication process employing a lower ground plane structure with SrSnO 3 insulating layers. We consider that improvement of the circuit fabrication process and optimization of the pickup coil lead to much higher signal frequency observable by the sampler.

We have examined feasibility of MgO thin films as an insulating layer in high- T c SFQ digital devices. When MgO thin films were prepared at 350 °C by RF magnetron sputtering on Y 0.9 La 0.2 Ba 1.9 Cu 3 O y (La–YBCO) thin films, they showed a rough surface and existence of (1 1 1) orientation. Employing a 10-nm-thick BaZrO 3 buffer layer on the La–YBCO films, we successfully fabricated MgO thin films with a flat surface. The prepared MgO thin film on the La–YBCO film had a dielectric constant of 11.5. La–YBCO thin films on the MgO thin films had (0 0 l ) orientation and in-plane alignment. It was also found that the MgO thin film had a lower etching rate than those for La–YBCO films and conventional insulating thin films such as SrSnO 3 and CeO 2 .

We have developed an HTS multilayer structure including a superconducting ground plane and interface-engineered ramp-edge Josephson junctions for single flux quantum (SFQ) circuits, and tried to improve reproducibility in the multilayer and junction process. We found that one of the crucial factors to improve the reproducibility is use of a ground plane with small average surface roughness (R/sub a/) of typically less than 2 nm and a high T/sub c/ above 87 K. By employing La/sub 0.2/-Y/sub 0.9/Ba/sub 1.9/Cu/sub 3/O/sub x/ (La-YBCO) thin films deposited by dc off-axis magnetron sputtering, such high quality was maintained for 3-month successive use of a ceramic target. Use of such a high-quality ground plane reproducibly led to SrSnO/sub 3//La-YBCO/SrSnO/sub 3//La-YBCO //MgO multilayer structures with R/sub a//spl les/2 nm and 100-junction arrays with 1-/spl sigma/I/sub c/ spread of 6-17% at 4.2 K depending on the average I/sub c/ level. The run-to-run reproducibility of average junction I/sub c/ and sheet inductance of both the base- and counter-electrodes for three samples was /spl plusmn/12%, /spl plusmn/3.0%, and /spl plusmn/3.4%, respectively, which are substantially smaller than those previously reported.

(Hg0.9Re0.1)Ba2CaCu2Oy [(Hg, Re)-1212] grain boundary Josephson junctions have been fabricated on SrTiO3 (STO) [001]-tilt bicrystal substrates with thin CeO2 and Y0.9La0.2Ba1.9Cu3Oy (YLBCO) buffer layers. These buffer layers effectively suppressed the formation of undesirable precipitates, which existed along the grain boundaries in (Hg, Re)-1212 films directly fabricated on STO bicrystal substrates without a buffer layer. The junctions on both the buffer layers exhibited resistively shunted junction (RSJ)-type current–voltage (I–V) characteristics and more homogeneous current distribution than those for the junctions on the STO bicrystal substrate. For the junctions on the YLBCO buffer layers, RSJ-type I–V curves were observed at temperatures up to 112–114 K, being very close to the film Tc of 115–117 K, while a substantially lower junction Tc was observed for those on the CeO2 buffer layers. The junctions on YLBCO exhibited more systematic variations of junction parameters with the misorientation angle θ than those for the junctions on STO. These results and the reduction of the critical current density with θ apparently smaller than that for the junctions on STO are probably attributed to no inclusion of precipitates and superior crystallinity near the grain boundaries. The junctions on the YLBCO buffer layers with θ = 36.8° were found to have a magnitude of Ic suitable for applications to superconducting quantum interference devices (SQUIDs) at temperatures higher than 77 K. The dc SQUIDs fabricated using such junctions exhibited voltage–flux modulation at temperatures up to 110 K.

We have fabricated a multilayer structure for single flux quantum (SFQ) circuit application using a high-temperature superconductor (HTS). La 0.2 -Y 0.9 Ba 1.9 Cu 3 O x (La-YBCO) base electrode layers were prepared by a dc or rf magnetron sputtering method. The reproducibility of film quality for dc-sputtered La-YBCO films was better than that for rf-sputtered films, and the dc sputtered films exhibited the average surface roughness R a less than 1.0 nm and a T c zero value of 88 K. By using the dc-sputtered La-YBCO films, a multilayer structure of SrSnO 3 /La-YBCO/ SrSnO 3 /La-YBCO on MgO substrate with R a below 2.0 nm was obtained. Interface-modified ramp-edge junctions with La 0.2 -Yb 0.9 Ba 1.9 Cu 3 O x (La-YbBCO) counter electrodes have been fabricated by using this multilayer structure with dc-sputtered films. The fabricated junctions exhibited RSJ-type I - V characteristics with I c R n products of about 3 mV at 4.2 K. We also obtained a 1-σ I c spread of 8% for a 1000-junction series-array. The sheet inductance values at 4.2 K for the base and counter electrodes on La-YBCO ground planes were 0.8 pH and 0.7 pH per square, respectively. Operation of several types of elementary SFQ circuits has been successfully demonstrated hv using this multilayer structure.

Multilayer structures of MZrO3 (M=Ba, Sr) and Y0.9Ba1.9La0.2Cu3Oy (La–YBCO) were investigated with the aim of fabricating reliable ramp-edge Josephson junctions on a superconducting ground plane. It was found that the quality of the multilayer structures of MZrO3 and La–YBCO depends on the composition of MZrO3 thin films. When Ba/Zr=1, both BaZrO3/La–YBCO and La–YBCO/BaZrO3 bilayer films exhibited clear interfaces and smooth surfaces. However, when BaZrO3 films contained excess Ba, BaZrO3 thin films prepared on La–YBCO showed rough surfaces and poor adhesion. For SrZrO3 thin films, we obtained experimental results suggesting a strong correlation between Sr/Zr composition in the films and the quality of La–YBCO on the SrZrO3 films.

Preparation of LuBa 2 Cu 3 O y (Lu-123) thin films by a pulsed laser deposition (PLD) technique was studied. Thin films with c -axis orientation were successfully obtained on SrTiO 3 (1 0 0) substrates at a relatively low temperatures of 600–660 °C. These films exhibited a T c (zero) of 82 K. We fabricated interface-modified ramp-edge Josephson junctions using the Lu-123 and La-doped Y-123 for counter and base electrodes, respectively. The Lu-123 electrode was deposited at 635 °C. The obtained junctions exhibited RCSJ-type current–voltage characteristics. The Lu-123 electrode without a ground plane showed moderately low sheet inductance of 3 pH/□.

Approximately 160-nm-thick (Hg0.9Re0.1)Ba2CaCu2Oy ((Hg,Re)-1212) thin films were fabricated on SrTiO3 (STO) vicinal substrates with the tilt angle of 5–18.4°. By employing Y0.9La0.2Ba1.9Cu3Oy or Pr1.4Ba1.6Cu3Oy thin films as buffer layers on the STO vicinal substrates, single-phase (Hg,Re)-1212 films with the c-axis normal to STO (001) and a Tc of 115–117 K were successfully grown for all the tilt angles. It was confirmed that the crystallinity of the vicinal films was comparable to that for conventional films on STO (001) substrates. The c-axis resistivity estimated from their anisotropic transport properties clearly exhibited semiconductor-like behavior below 180 K, which is similar to the case for optimally-doped Bi2Sr2CaCu2Oy (Bi-2212), though the resistivity anisotropy of approximately 1000 is about one order of magnitude smaller than that of Bi-2212. Moreover, multiple branch structures characteristic of intrinsic Josephson junctions were observed in the current-voltage curves in the direction across their a-b planes.

We examined the fabrication conditions to obtain high-quality ramp-edge Josephson junctions on a liquid-phase-epitaxy YBa2Cu3Oy (LPE-YBCO) ground plane, in particular, focusing on the fabrication of a suitable insulating layer on the ground plane and the post-annealing conditions to load oxygen to the ground plane. A (LaAlO3)0.3–(SrAl0.5Ta0.5O3)0.7 (LSAT) insulating film on the ground planes exhibited a conductance ranging from 10−4 to 10−8 S after deposition of an upper superconducting film, suggesting existence of some leak paths through the LSAT insulating layer. By introducing approximately 30 nm thick SrTiO3 (STO) buffer layers on both side of the LSAT insulating layer. We reproducibly obtained a conductance lower than 10−8 S. The dielectric constant of the STO/LSAT/STO layer was 32, which was slightly larger than that of the single LSAT layer. It was found that a very slow cooling rate of 1.0 °C/h in oxygen was needed to fully oxidize the ground plane through the STO/LSAT/STO insulating layers, while the oxidation time could be effectively reduced by introducing via holes in the insulating layer at an interval of 200 μm. Ramp-edge junctions on LPE-YBCO ground planes with STO/LSAT/STO insulating layers exhibited a 1σ-spread in Ic of 8% for 100-junction series-arrays and a sheet inductance of 0.7 pH/□ at 4.2 K.

As an insulating layer between a superconducting ground plane and a base electrode of Josephson junctions for high- T c devices, (LaAlO 3 ) 0.3 –(SrAl 0.5 Ta 0.5 O 3 ) 0.7 (LSAT) thin films and BaZrO 3 (BZO) thin films were studied. LSAT films deposited on LPE–YBCO and La–YBCO films exhibited columnar grain growth. Insertion of thin SrTiO 3 (STO) buffer layer between LSAT and YBCO was found effective to suppress this columnar grain growth. Although BZO films on LSAT and STO substrates had smooth surfaces with the average roughness R a ∼0.2 nm, the film deposited on La–YBCO showed a rough surface and poor adhesion. By employing a thin STO or LSAT buffer layer between BZO and La–YBCO, the film adhesion and surface quality were much improved. The relative permittivity values of STO/LSAT/STO and STO/BZO/STO were 30 and 20, respectively, indicating a high potential of STO-buffered BZO for use as insulating layers in high- T c devices.

(Hg 0.9 Re 0.1 )Ba 2 CaCu 2 O y ((Hg,Re)-1212) thin films were fabricated on buffered SrTiO 3 (STO) substrates. BaZrO 3 , (LaAlO 3 ) 0.3 –(SrAl 0.5 Ta 0.5 O 3 ) 0.7 , CeO 2 and Y 0.9 La 0.2 Ba 1.9 Cu 3 O y (La-YBCO) were examined as buffer layer materials. The buffer layers were prepared on STO substrates by pulsed laser deposition or off-axis magnetron sputtering methods. (Hg,Re)-1212 thin films were fabricated on the buffered STO substrates by a two-step process. c -Axis oriented and in-plane aligned films were obtained on CeO 2 and La-YBCO buffer layers. The films had a smooth surface and exhibited T c above 115 K. The J c values at 77 K in a self-field were 4.4 × 10 6 and 9.1 × 10 6 A/cm 2 for the films on the CeO 2 and La-YBCO buffer layers, respectively.

We demonstrate high performance microfuel cells (μFC) operating at room temperature. The smallest μFC has a reaction surface of 0.11 cm 2 and has an output power density equal to 22.9 mW cm −2 . Methanol and air are supplied using microchannels etched into silicon wafers using microfabrication techniques which can accurately determine the μFC surface and the microchannel dimensions. The insertion of a novel hydrophilic fibrous layer into the anode diffusion layer stack produces 9.25 mW cm −2 for an input fuel flow rate of 550 nL min −1 . The benefits of size-scaling and architecture optimization in μFC are demonstrated. Our observations and conclusions are by no means unique to methanol μFC but could be applied to other microfluidic liquid fuel μFC based on, e.g. microbial fuel cells, bio-ethanol and glucose solution.

International audience; Miniature high-performance micro fuel cells (μFC) operating at room temperature have been fabricated using microsystems technologies. The smallest μFCs have a reaction surface of 0.11 cm2 (volume ∼18.6 mm3; mass ∼35.6 mg) and produce an output power density of 22.9 mW/cm2. The insertion of a hydrophilic macroporous layer into the anode diffusion layer stack, an input fuel flow rate as low as 550 nL min−1 produces 9.25 mW/cm2 at a fuel use efficiency of ∼75%. By optimizing the microfluidic architecture, e.g. the microchannel dimensions and the diffusion layer stack, we demonstrate of the smallest, highest performance μFCs reported to date

We demonstrate state-of-the-art room temperature operation of silicon microchannel-based micro-direct methanol fuel cells (μDMFC) having a very high fuel use efficiency of 75.4% operating at an output power density of 9.25 mW cm −2 for an input fuel (3 M aqueous methanol solution) flow rate as low as 0.55 μL min −1 . In addition, an output power density of 12.7 mW cm −2 has been observed for a fuel flow rate of 2.76 μL min −1 . These results were obtained via the insertion of novel hydrophilic macroporous layer between the standard hydrophobic carbon gas diffusion layer (GDL) and the anode catalyst layer of a μDMFC; the hydrophilic macroporous layer acts to improve mass transport, as a wicking layer for the fuel, enhancing fuel supply to the anode at low flow rates. The results were obtained with the fuel being supplied to the anode catalyst layer via a network of microscopic microchannels etched in a silicon wafer.

This paper reports the design, fabrication and characterization of high performance miniaturized micro direct methanol fuel cells (microDMFC) functioning at room temperature under a forced low input fuel flow rate (

This paper shows a self-exchange system to supply liquid fuel and to exhaust CO2 spontaneously for micro direct methanol fuel cell (muDMFC). The system performs effectively and continuously without any circulation loops and external pumps. The fuel is carried to the anode due to capillary force. The system is consisted of a hydrophobic porous membrane and two comb-shape channels. It was confirmed that the muDMFC works with a less differential pressure than1 kPa between a fuel inlet and a CO2 outlet. The fuel supply rate of only several mul/min/cm2 was needed for the DMFC to work. Furthermore, the fuel supply rate was spontaneously varied along the electric current with constant pressure.

[100]-tilt grain boundary Josephson junctions with misorientation of the c-axis directions have been fabricated using epitaxial (Hg0.9Re0.1)Ba2CaCu2Oy [(Hg,Re)-1212] thin films grown on SrTiO3 bicrystal substrates. Two types of the [100]-tilt junctions, that is, the mountain-type junction and the valley-type junction, of which a-b planes formed a mountain shape and a valley shape along the grain boundaries, respectively, were examined for various misorientation angles (θ). Both the types of junctions showed resistively shunted junction-type current-voltage (I-V) characteristics with relatively high characteristic voltages in a wide temperature range from 4.2to110K. The valley-type junctions exhibited properties superior to those for the mountain-type junctions, such as very low excess current and much homogeneous current distribution, which is attributed to different film growth mode near the grain boundaries. They also exhibited the products of the critical current (Ic) and the normal-state resistance (R...

Approximately 160-nm-thick (Hg0.9Re0.1)Ba2CaCu2Oy [(Hg,Re)-1212] epitaxial thin films were fabricated on 2°–18.4°-tilt SrTiO3 vicinal substrates with thin buffer layers. The vicinal films had superior crystallinity and superconducting properties comparable to those for conventional films with no tilt. X-ray diffraction measurements revealed that the tilt angles of the c axes of the (Hg,Re)-1212 are slightly larger by 0.5°–1.5° than the substrate tilt angles, which is attributed to deformation of crystal structure due to lattice mismatch in the c-axis direction. The resistivity and critical current density along the c-axis direction (ρc and Jc-c) were estimated to be ρc=500mΩcm at room temperature and Jc-c=(1.0–1.4)×105A∕cm2 at 10K from transport measurements for different in-plane directions of the vicinal films. The ρc clearly exhibited semiconductor-like temperature dependence below 180K. The temperature dependence of Jc-c almost agreed with the theoretical relationship for superconductor-insulator-supe...