Nb Electrodes Research Articles

Effect of electrospark deposited surface layers on bond strength of titanium–porcelain

Casting of titanium can be successfully used in prosthodontic applications, but it demands special machines and protection gas to avoid oxidation of the metal. The aims of this study are to investigate the bond compatibility between porcelain and titanium using three-point bending, oxide adherence and thermal expansion tests, and to compare the results with those of a conventional titanium–porcelain system. Titanium alloy surfaces were modified with Nb, YG8 and silicon electrode by electrospark surface modification process. The effect of electrospark surface depositing (ESD) layers on bond strength of titanium to porcelain was evaluated comparatively. Some reasons about bond strength of titanium to porcelain were discussed. Results indicate that ESD modified layer prepared in atmosphere using Si electrode can obtain the strongest bonding to porcelain. The ESD modified layer show metallurgical bond to Ti substrate. In addition, the facts that rough surface can help to improve physic bond, similar nature can also help to chemical link and compact ESD layer represent good high temperature oxidation resistance are the reasons that enhance good bond strength of titanium to porcelain.

Structural and Chemical Analysis of Nanoscale Resistive Switching Devices: Assessment on Nonlinear Properties

ABSTRACTInvestigation into the phenomenon of resistive switching, a reversible change in electrical resistance by the application of a voltage bias, has given rise to the device fabrication, DC electrical testing, and cross sectional TEM/EELS characterization of nanoscale resistive switching devices. Typically, resistive switching devices are composed of a thin oxide layer between two conductive electrodes where applied bias can alter the resistance states. In a cross-bar array, nonlinearity of device I-V relation is a highly desirable characteristic that helps to mitigate the sneak path current leakage issue. Negative differential resistance (NDR) switching behavior offers such nonlinearity and has been observed in TaOx nanoscale devices utilizing certain electrode materials. To investigate this phenomenon, nanodevices were fabricated by sputtering TaOx onto TiN nanovias capped Nb electrodes. Cross sectional TEM/EELS were performed to reveal the physical and chemical changes in these devices to explore possible origins of nonlinear behavior when these top electrode materials are utilized with TaOx films.

Giant magnetoconductance oscillations in hybrid superconductor-semiconductor core/shell nanowire devices.

The magnetotransport of GaAs/InAs core/shell nanowires contacted by two superconducting Nb electrodes is investigated, where the InAs shell forms a tube-like conductive channel around the highly resistive GaAs core. By applying a magnetic field along the nanowire axis, regular magnetoconductance oscillations with an amplitude in the order of e(2)/h are observed. The oscillation amplitude is found to be larger by 2 orders of magnitude compared to the measurements of a reference sample with normal metal contacts. For the Nb-contacted core/shell nanowire the oscillation period corresponds to half a flux quantum Φ0/2 = h/2e in contrast to the period of Φ0 of the reference sample. The strongly enhanced magnetoconductance oscillations are explained by phase-coherent resonant Andreev reflections at the Nb-core/shell nanowire interface.

Current–voltage characteristics of Nb–carbon–Nb junctions

We report on properties of Nb(/Ti)–carbon–(Ti/)Nb junctions fabricated on graphite flakes using e-beam lithography. The devices were characterized at temperatures above 1.8 K where a Josephson current was not observed, but the differential conductivity revealed features below the critical temperature of Nb, and overall metallic conductivity, in spite of a high-junctions resistance. Since the conductivity of graphite along the planes is essentially two-dimensional (2D), we use a theoretical model developed for metal/graphene junctions for interpretation of the results. The model involves two very different graphene “access” lengths. The shorter length characterizes ordinary tunneling between the three-dimensional Nb(/Ti) electrode and 2D graphene, while the second, much longer length, is associated with the Andreev reflections (AR) inside the junction and involves also “reflectionless” AR processes. The relevant transmission factors are small in the first case and much larger in the second, which explains ...

Fabrication of $\hbox{Nb/Al}_{2}\hbox{O}_{3}/\hbox{Nb}$ Josephson Junctions Using In Situ Magnetron Sputtering and Atomic Layer Deposition

Atomic layer deposition (ALD) provides a promising approach for deposition of ultrathin low-defect-density tunnel barriers, and it has been implemented in a high-vacuum magnetron sputtering system for in situ deposition of ALD-Al2O3 tunnel barriers in superconductor-insulator-superconductor (SIS) Josephson junctions. A smooth ALD-Al2O3 barrier layer was grown on a Al-wetted Nb bottom electrode and was followed with a top Nb electrode growth using sputtering. Preliminary low temperature measurements of current-voltage characteristics (IVC) of the Josephson junctions made from these trilayers confirmed the integrity of the ALD-Al2O3 barrier layer. However, the IcRN product of the junctions is much smaller than the value expected from the Ambegaokar-Baratoff formula suggesting a significant pair-breaking mechanism at the interfaces.

Supercurrent in Nb/InAs-nanowire/Nb Josephson junctions

We report on the fabrication and measurements of planar mesoscopic Josephson junctions formed by InAs nanowires coupled to superconducting Nb terminals. The use of Si-doped InAs-nanowires with different bulk carrier concentrations allowed to tune the properties of the junctions. We have studied the junction characteristics as a function of temperature, gate voltage, and magnetic field. For junctions with high doping concentrations in the nanowire, Josephson supercurrent values up to 100 nA are found. Owing to the use of Nb as superconductor, the Josephson coupling persists at temperatures up to 4 K. In all junctions, the critical current monotonously decreased with the magnetic field, which can be explained by a recently developed theoretical model for the proximity effect in ultra-small Josephson junctions. For the low-doped Josephson junctions, a control of the critical current by varying the gate voltage has been demonstrated. We have studied conductance fluctuations in nanowires coupled to superconducting and normal metal terminals. The conductance fluctuation amplitude is found to be about 6 times larger in superconducting contacted nanowires. The enhancement of the conductance fluctuations is attributed to phase-coherent Andreev reflection as well as to the large number of phase-coherent channels due to the large superconducting gap of the Nb electrodes.

Improved microstructure and ohmic contact of Nb electrode on n-type 4H-SiC

Niobium was deposited as an electrode material on an n-type SiC wafer for power device application. The reaction microstructure and electrical contact property were investigated after annealing at 700 to 1000°C and compared with the results for an Ni electrode. Microstructure-related problems of the Ni electrode could be resolved without sacrificing ohmic contact behavior with a low contact resistivity of 1.53×10−4Ωcm2. Carbon precipitation was completely eliminated with Nb by the formation of carbides, leading to good adhesion upon wire bonding process. At the reaction interface, Nb5Si3 was formed in an epitaxial relationship with SiC, leading to a good interface contact property as well as good interface adhesion.

Observation of meander pattern in signals from superconducting MgB2 detector by scanning pulsed laser imaging

Superconducting MgB2 meander detector has been imaged by scanning a spot of 1.5-μm focused pulsed laser. The superconducting detector using high-quality 10B-enriched MgB2 thin films at higher operating temperatures has been fabricated to utilize a resistance change induced by the nuclear energy of 10B and neutron. The MgB2 detector consists of a 200-nm-thick MgB2 thin-film meander line, a 300-nm-thick SiO protective layer, and 150-nm-thick Nb electrodes with 1-μm MgB2 wires. The devices were placed in a 4K refrigerator to control at a certain temperature below Tc. A scanning laser spot can be used by the combination of the XYZ piezo-drive stage and an optical fibre with an aspheric focused lens. The measurement system is fully controlled by LabVIEW based software. We succeeded in observing a line-and-space image of a meandering pattern by analysing response signals.

Structural and transport characterization of ultra thin Ba0.05Sr0.95TiO3 layers grown over Nb electrodes for the development of Josephson junctions

A phenomenological approach was used to obtain critical information about the structure and electrical properties of ultra thin Ba0.05Sr0.95TiO3 (BSTO) layers over Nb electrodes. The method allows, in a simple way, to study and to optimize the growth of the barrier in order to improve the performance and application of Josephson junctions. A very good control of the layer thickness with a low roughness was achieved during the deposition process. The BSTO layers present an energy barrier of 0.6 eV and an attenuation length of 0.4 nm, indicating its good insulating properties for the development of Josephson junctions with improved performance.

Soft X-ray Detection Performance of Superconducting Tunnel Junction Arrays with Asymmetric Tunnel Junction Layer Structure

Superconducting tunnel junction (STJ) array detectors with an asymmetric tunnel junction layer structure have been fabricated for the soft X-ray region between 100 eV and 1 keV. The asymmetric layer design was effective in solving the problem of double peak response to monochromatic X-rays, which is commonly observed in STJ detectors. The sensitive area was patterned by a lift-off technique that ensured no contamination on the top Nb electrode surface. The performance of a 100-pixel STJ array detector was investigated through fluorescent X-ray analysis of oxides and nitrides for the energy region of the K-lines of oxygen, nitrogen, and boron. The STJ array detector exhibited a high energy resolution of <15 eV, which cannot be achieved by semiconductor detectors, and an energy nonlinearity of <1%. It was demonstrated that the performance is suitable for fluorescence-yield X-ray absorption fine structure (XAFS) spectroscopy for light trace elements.

Fabrication of superconducting tunnel junctions for soft X-ray spectroscopy

Superconducting tunnel junction (STJ) array detectors with a new design, which has a minimum junction edge coverage of an SiO2 insulation, passivation layer and an asymmetric tunnel junction layer structure, have been fabricated for a soft X-ray region between 100 eV and 1 keV. The sensitive area was patterned by removing the SiO2 deposition layer by a lift-off technique that ensured no contamination layer on the top Nb electrode surface. The width of the passivation rim was as narrow as 0.5 µm at the junction edge. The clean Nb surface and the narrow SiO2 rim resulted in almost no artifact photon events in a low-energy region. The asymmetric layer design is effective in solving a problem of double peak response to monochromatic X-rays, which is commonly observed in STJ detectors. The performance of a 100 pixel array detector was investigated by the fluorescent X-ray analysis of oxides and nitrides: an energy resolution of about 30 eV for the total absorption of the Kα lines of oxygen and nitrogen. We plan to realize an energy resolution of better than 20 eV and a counting rate of over 1 Mcps for fluorescence-yield X-ray absorption spectroscopy for light trace elements. Copyright © 2011 John Wiley & Sons, Ltd.

Evaluation of spin polarization inp-In0.96Mn0.04As using Andreev reflection spectroscopy including inverse proximity effect

We report on carrier transport across a superconductor/ferromagnetic semiconductor junction with a highly transparent metallic contact, Nb/$p$-In${}_{0.96}$Mn${}_{0.04}$As. Below $~$10 K, $p$-In${}_{0.96}$Mn${}_{0.04}$As becomes ferromagnetic, as evidenced by the hysteretic transverse resistance caused by the anomalous Hall effect. Below the superconducting critical temperature $T$${}_{c}$ of the Nb electrodes (8.2 K), a conductance reduction occurs within the bias voltage that is comparable to the Nb superconducting energy gap. A rather moderate slope in the differential conductance curves within the gap region indicates the partial suppression of the Andreev reflection caused by spin-polarized carriers in $p$-In${}_{0.96}$Mn${}_{0.04}$As. Spin polarization $P$ in $p$-In${}_{0.96}$Mn${}_{0.04}$As has been extracted by fitting the measured differential conductance curves with a newly modified Blonder-Tinkham-Klapwijk model with both spin polarization and the inverse proximity effect (mod2-BTK model). The extracted $P$ value is $P$ $=$ 0.725 at 0.5 K, and it decreases gradually with increasing temperature.

Growth of Two-Dimensional Carbon Nanostructures and Their Electrical Transport Properties at Low Tempertaure

We report on a systematic electrical transport study of carbon nanowalls using both the normal metal and superconducting electrodes. The nonlinear transport and corresponding anomalous dI/dV versus bias curves below ∼2 K observed in samples with both Ti and Nb electrodes is accounted for by the formation of charge density waves due to enhanced density of states at the Fermi level at edges or extended defects. This phase competes with superconducting instability at very low temperature, as manifested by distinctive resistance-temperature behaviors and associated dV/dI characteristics observed in different samples.

Evaluation of spin polarization in p-In0.96Mn0.04As using Andreev reflection spectroscopy

We report on the transport behavior and spin polarization P of carriers for superconductor/ferromagnetic semiconductor (S-F) junctions. We fabricated Nb/ferromagnetic semiconductor p-In0.96Mn0.04As junctions with Hall voltage probes. Below ~15 K, p-In0.96Mn0.04As becomes ferromagnetic and then the anomalous Hall effect is observed. Below the TC of the Nb electrodes (~8.2 K), we observed a conductance reduction within the Nb superconducting energy gap voltage owing to the suppression of Andreev reflection by spin polarization in p-In0.96Mn0.04As. We evaluated the degree of spin polarization in p-In0.96Mn0.04As experimentally by comparing the measured differential conductance with that obtained with Strijkers' model extended for spin-polarized Andreev reflection including the inverse proximity effect. Consequently, the P value extracted experimentally for p-In0.96Mn0.04As at 0.5 K was 0.795 assuming Z = 0.

Origin of hydrogen-inclusion-induced critical current deviation in Nb/AlOx/Al/Nb Josephson junctions

We investigated the mechanisms that change the critical current density (Jc) of Nb/AlOx/Al/Nb Josephson junctions due to the inclusion of hydrogen in the Nb electrodes. Our investigations were performed according to three aspects: the superconductivity change, the change in thickness of the barrier layer, and the change in the barrier height due to the electronic effect. The results are as follows: (a) the hydrogen-inclusion-accompanied changes in the superconductivity parameters, such as the junction gap voltage, were much less than those of the critical current density, (b) the effect of hydrogen inclusion on Jc varied depending on the electrodes, i.e., the upper electrode above the barrier layer was the most affected, (c) the junctions with increased Ics due to hydrogen exclusion showed the identical amount of decrease in the junction resistance measured at room temperature, and (d) the hydrogen exclusion from the junction electrodes had no influence on the Nb/Al/AlOx/Al/Nb junctions, which had an extra Al layer. Based on these results we conclude that the Jc change is mainly caused by the change in junction resistance. A one order of magnitude smaller effect is caused by the superconductivity change. We believe the Jc change is caused by a Nb work function increase due to the hydrogen inclusion, resulting in an increase in barrier height.

Observation of Spin-Triplet Superconductivity in Co-Based Josephson Junctions

We have measured a long-range supercurrent in Josephson junctions containing Co (a strong ferromagnetic material) when we insert thin layers of either PdNi or CuNi weakly ferromagnetic alloys between the Co and the two superconducting Nb electrodes. The critical current in such junctions hardly decays for Co thicknesses in the range of 12-28 nm, whereas it decays very steeply in similar junctions without the alloy layers. The long-range supercurrent is controllable by the thickness of the alloy layer, reaching a maximum for a thickness of a few nm. These experimental observations provide strong evidence for induced spin-triplet pair correlations, which have been predicted to occur in superconducting-ferromagnetic hybrid systems in the presence of certain types of magnetic inhomogeneity.

Superconducting transport in an LED with Nb electrodes

Superconducting transport is measured between two superconducting electrodes at the n-type semiconductor side of a superconductor-based LED where a Josephson junction is formed. The characteristics of the Josephson junction are found to be modulated by applying voltage to the normal electrode at the p-type semiconductor side. The Josephson junction characteristics show an extraordinary sensitivity to the radiative recombination process, which we estimate as the recombination efficiency.

Scanning laser microscopy of an MgB2 superconducting sensor

The MgB2 detector consists of a 200-nm-thick MgB2 thin-film meander line, a 300-nm-thick SiO protective layer, and 150-nm-thick Nb electrodes. We prepared a straight MgB2 stripline of width 3μm and length 100μm. To understand the position-dependent sensitivity of the MgB2 detector, a scanning pulsed laser spot can be achieved by the combination of the XYZ piezo-drive stages and an optical fiber with an aspheric focusing lens. The observed scanning XY image indicates that the improvement in sensor homogeneity is very important to improve the sensitivity of the MgB2 sensor as a neutron detector.

Measurement of spin polarization in [formula omitted]- In0.96Mn0.04As using Andreev reflection spectroscopy

We report on the transport behavior and spin polarization of carriers P for superconductor/ferromagnetic semiconductor (S-F) junctions. We fabricated Nb/ferromagnetic semiconductor p-In0.96Mn0.04As junctions with Hall voltage probes. Below ∼15 K, p- In0.96Mn0.04As becomes ferromagnetic and then the anomalous Hall effect is observed. Below the TC of the Nb electrodes (∼8.2 K), we observed a conductance reduction within the Nb superconducting energy gap voltage owing to the suppression of Andreev reflection by spin polarization in p- In0.96Mn0.04As. We have evaluated the degree of spin polarization in p- In0.96Mn0.04As experimentally by comparing the measured differential conductance with that obtained with the Strijkers model extended for spin-polarized Andreev reflection including the inverse proximity effect. Consequently, the P value extracted experimentally for p- In0.96Mn0.04As at 0.5 K was P=0.33∼0.795.

Doppler shift in Andreev reflection from a moving superconducting condensate in Nb/InAs Josephson junctions

We study narrow ballistic Josephson weak links in a InAs quantum wells contacted by Nb electrodes and find a dramatic magnetic-field suppression of the Andreev reflection amplitude, which occurs even for in-plane field orientation with essentially no magnetic flux through the junction. Our observations demonstrate the presence of a Doppler shift in the energy of the Andreev levels, which results from diamagnetic screening currents in the hybrid Nb/InAs-banks. The data for conductance, excess and critical currents can be consistently explained in terms of the sample geometry and the McMillan energy, characterizing the transparency of the Nb/InAs-interface.