Nb Thin Films Research Articles

Multi-layered Ni64Zr36/M/Ni64Zr36 (M = V, Nb) thin films for hydrogen purification

For hydrogen purification at low temperature, multi-layered hydrogen-permeable thin films with amorphous Ni64Zr36/M/Ni64Zr36 (M = V, Nb) structure were developed and the microstructure and hydrogen permeation performance were examined. The multi-layered thin films were deposited through DC-magnetron sputtering using Ni64Zr36 and V/Nb targets. The overall thickness was adjusted to 2 μm, and that of the internal V or Nb layer was varied between 0.5 and 1.6 μm. The hydrogen permeability was measured in the range of 373–573 K. All the multi-layered thin films showed negative activation energy for hydrogen permeation indicating an increasing permeability with a decreasing temperature. The calculated negative activation energy, −0.43 to −0.60 kJ/mol for Ni64Zr36/V/Ni64Zr36 thin films and −1.19 to −3.90 kJ/mol for Ni64Zr36/Nb/Ni64Zr36 thin films, and the increased hydrogen permeation at low temperature region is obviously due to the V or Nb mid-layer. The observed hydrogen permeability of the prepared thin films was higher than the reported value of amorphous Ni64Zr36 alloy membrane at 573 K and reached the permeability of Pd at 373 K.

Corrosion property and biocompatibility evaluation of Fe–Zr–Nb thin film metallic glasses

The biocompatible thin film metallic glass (TFMGs) materials can be used to modify the substrate material due to their good corrosion resistance and adequate biocompatibility. In this work, five Fe–Zr–Nb TFMGs were fabricated on Si wafer and AISI420 stainless steel disk substrates using three pure targets co-sputtering system. The niobium target power was adjusted to grow TFMGs with different Nb contents ranging from 21.7 to 26.7 at%. Very fine and dense featureless microstructure was discovered for each Fe–Zr–Nb TFMG. The hardness, elastic modulus, H/E and H3/E2 ratios of Fe–Zr–Nb TFMG increased with increasing Nb content. Good adhesion property was revealed for each TFMG. The corrosion resistance of the bare AISI420 stainless steel disk was greatly improved by the deposited Fe–Zr–Nb TFMG. Very good biocompatibility and MG63 cell viability were also found for TFMGs implying their potential application as biomedical materials.

Enhanced superconductivity in Bi2Se3/Nb heterostructures

The superconducting proximity effect in topological insulator and superconductor heterostructures has aroused intensive research interest in the past decade. While many efforts have been made to investigate the induced superconductivity in topological insulators, the exploration of the effect of topological insulators on superconductors is comparatively rare. Here, we report the observation of an enhanced superconductivity in Bi2Se3/Nb heterostructures. The superconducting transition temperature and the critical field of Bi2Se3/Nb heterostructures are higher than those of the pristine Nb thin films. Such enhanced superconductivity is due to the improved superfluid stiffness in Nb thin films by proximity to the metallic Bi2Se3 flakes. Our findings will further advance the understanding of the superconducting proximity effect in the heterostructures.

Characteristics of Very High Q Nb Superconducting Resonators for Microwave Kinetic Inductance Detectors

We have prepared superconducting resonators using high-quality Nb thin films with RRR = 48 and studied their resonance characteristics in detail. It was observed that the measured internal quality factor Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> reached as high as 4×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> , which might be the highest value obtained among the Nb thin film superconducting resonators ever reported. It is demonstrated that the temperature dependence of the internal quality factors can be well explained by considering the temperature dependence of the residual resistance due to phonon scattering, electron-electron scattering, and the Kondo effect of the residual quasiparticle in addition to the extended Mattis-Bardeen theory. It is also found that the change of the resonance frequency of the Nb thin film resonator with respect to temperature can be well explained by considering the temperature dependence of both the Kondo effect and the kinetic inductance of the residual quasiparticles. These results strongly indicate that a finite number of quasiparticles are present in the superconducting gap of the high-quality Nb film even at a sufficiently low temperature of T/T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> <; 0.15 and form a Fermi liquid state.

High-Frequency Nonlinear Response of Superconducting Cavity-Grade Nb Surfaces

Nb Superconducting Radio-Frequency (SRF) cavities are observed to break down and lose their high-Q superconducting properties at accelerating gradients below the limits imposed by theory. The microscopic origins of SRF cavity breakdown are still a matter of some debate. To investigate these microscopic issues temperature and power dependent local third harmonic response was measured on bulk Nb and Nb thin film samples using a novel near-field magnetic microwave microscope between 2.9K-10K and 2GHz-6GHz. Both periodic and non-periodic response as a function of applied RF field amplitude are observed. We attribute these features to extrinsic and intrinsic nonlinear responses of the sample. The RF-current-biased Resistively Shunted Junction (RSJ) model can account for the periodic response and fits very well to the data using reasonable parameters. The non-periodic response is consistent with vortex semi-loops penetrating into the bulk of the sample once sufficiently high RF magnetic field is applied, and the data can be fit to a Time-Dependent Ginzburg-Landau (TDGL) model of this process. The fact that these responses are measured on a wide variety of Nb samples suggests that we have captured the generic nonlinear response of air-exposed Nb surfaces.

Composition Dependence of the β Phase Stability and Mechanical Properties of Ti-Nb Thin Films.

Shape memory alloys (SMAs) are commonly used for various applications, e.g., in the aerospace and automotive industries, robotics, and biomedical sciences. Although Ti-Ni SMAs are commercially available, the low biocompatibility of Ni has stimulated research into the development of Ti-Nb based SMAs as potential replacements of Ti-Ni alloys in biomedical applications. Ti-Nb alloys have attracted attention because of their low stiffness and superelasticity. Superelastic thin films can be used in medical applications, including the fabrication of stents for neurovascular blood vessels, which relies on a thin film and on the use of a Ti-Nb alloy coating for less biocompatible alloys. In this study, Ti-Nb thin films were prepared using magnetron sputtering. A Nb content in the range 12.2-35.9 at.% was used in the films, which was determined using energy-dispersive X-ray spectroscopy. X-ray diffraction measurement was used to analyze the crystal structure of the thin films, and their mechanical properties were investigated using nanoindentation.

Effect of thickness on the structural and optical properties of the niobium‐doped β ‐Ga 2 O 3 films

To investigate the effect of thickness on the structural and optical properties of niobium-doped beta gallium oxide (β -Ga2O3 :Nb) thin films, a series of β -Ga2O3 :Nb thin films with different thicknesses were prepared by radio-frequency magnetron method. The crystalline quality is highly improved when the film thickness exceeds 145 nm. The surface exhibits different morphologies under different film thicknesses. The average transmittance of all the films are over 80% in visible range, and that the ultraviolet absorption edge shifts to longer wavelength indicates the bandgap shrinks with increasing the film thickness. Moreover, the photoluminescence spectrum measurements indicate that fewer defects were formed when the film thickness is around 145 nm.

Angular Dependence of the Critical Current Density of Nb Thin Films of Different Thicknesses with InAs Nanorods as Flux Pinning Centers

Angular Dependence of the Critical Current Density of Nb Thin Films of Different Thicknesses with InAs Nanorods as Flux Pinning Centers

High kinetic inductance microwave resonators made by He-Beam assisted deposition of tungsten nanowires

We evaluate the performance of hybrid microwave resonators made by combining sputtered Nb thin films with Tungsten nanowires grown with a He-beam induced deposition technique. Depending on growth conditions, the nanowires have a typical width w∈[35−75] nm and thickness t∈[5−40] nm. We observe a high normal state resistance R◻∈[65−150] Ω/◻ which together with a critical temperature Tc∈[4−6] K ensures a high kinetic inductance making the resonator strongly nonlinear. Both lumped and coplanar waveguide resonators were fabricated and measured at low temperature exhibiting internal quality factors up to 3990 at 4.5 GHz in the few photon regime. Analyzing the wire length, temperature, and microwave power dependence, we extracted a kinetic inductance for the W nanowire of LK≈15 pH/◻, which is 250 times higher than the geometrical inductance, and a Kerr non-linearity as high as KW,He/2π=200±120 Hz/photon at 4.5 GHz. The nanowires made with the helium focused ion beam are thus versatile objects to engineer compact, high impedance, superconducting environments with a mask and resist free direct write process.

Substrate temperature effect on growth behavior and microstructure-properties relationship in amorphous Ni[sbnd]Nb thin films

The effect of substrate temperature on growth behavior and microstructure-properties correlation of magnetron sputtered Ni60Nb40 films were systematically investigated. With substrate temperature increasing from 293 K to 493 K, and thickness changing from 50 nm to 1 μm, all films kept in amorphous state. The characteristic of columnar growth was clearly seen, with reduced interfacial fraction between adjacent columns and increased granule size at high substrate temperature. Through scaling analysis, films deposited at all temperatures grow in an anomalous roughening mode, and at 293 K for films thicker than 200 nm, a difficult roughening phenomenon was revealed. High substrate temperature can facilitate the surface kinetic roughening. Both decreased interface fraction-to-volume ratio by particle coarsening and reduced free volume by structure relaxation caused the dense atomic atomic packing and increased density, which may be the crucial reason for enhanced mechanical properties and thermal stability against crystallization.

Effect of annealing atmosphere on the structural and optical properties of the Nb‐doped β‐Ga 2 O 3 films

The Nb-doped β-Ga 2 O 3 (β-Ga 2 O 3 :Nb) thin films have been deposited on the Si and quartz substrates by radio-frequency magnetron technique in argon ambient. The effects of annealing atmosphere on the structural and optical properties of β-Ga 2 O 3 :Nb thin films have been investigated. The crystallinity of β-Ga 2 O 3 :Nb film is improved obviously after annealing. An increase in surface roughness is observed on annealed films. The bandgap from 5.19 to 5.26 eV is obtained after annealing in different atmosphere, which is larger than the 5.09 eV before annealing. Moreover, the red-shift of photoluminescence emission peak is observed after annealing, and the annealing atmosphere has an influence on the peak intensity.

Investigation of Dayem Bridge NanoSQUIDs Made by Xe Focused Ion Beam

Superconducting QUantum Interference Devices (SQUIDs) based on nanobridge junctions have shown increasing promise for single particle detection. This paper describes the development of the fabrication of improved and reproducible nanobridge junctions fabricated by focused ion beam (FIB) milling from niobium thin films. Although the very low noise properties of nanobridge SQUIDs are well known, the nature of the milling process is little understood at the level of local superconducting properties. In this paper, we report the results for nanobridge Josephson devices and SQUIDs, which we believe are the first to be made by Xenon (Xe) FIB milling. Temperature-dependent current–voltage behavior, microwave-induced Shapiro steps, and SQUID response to magnetic fields have been measured. We make preliminary comparisons with nominally identical devices milled from Nb thin films using either Xe or Ga ions.

Magnetic order and disorder in nanomagnets probed by superconducting vortices

We have studied two nanomagnet systems with strong (Co/Pd multilayers) and weak (NdCo alloy films) stray magnetic fields by probing the out-of-plane magnetic states with superconducting vortices. The hybrid samples are made of array of nanomagnets embedded in superconducting Nb thin films. The vortex motion detects relevant magnetic state features, since superconducting vortices are able to discriminate between different magnetic stray field strengths and directions. The usual matching effect between the superconducting vortex lattice and the periodic pinning array can be quenched by means of disorder magnetic potentials with strong stray fields at random. Ordered stray fields retrieve the matching effect and yield asymmetry and shift in the vortex dissipation signal. Furthermore vortices can discriminate the sizes of the nanomagnet magnetic domains, detecting magnetic domain sizes as small as 70 nm. In addition, we observe that the vortex cores play the crucial role instead of the supercurrents around the vortex.

Phase Stability and Properties of Ti-Nb-Zr Thin Films and Their Dependence on Zr Addition.

Ternary Ti-Nb-Zr alloys were prepared by a magnetron sputtering method with porous structures observed in some of them. In bulk, in order to control the porous structure, a space holder (NH4HCO3) is used in the sintering method. However, in the present work, we show that the porous structure is also dependent on alloy composition. The results from Young’s modulus tests confirm that these alloys obey d-electrons alloy theory. However, the Young’s modulus of ternary thin films (≈80–95 GPa) is lower than that for binary alloys (≈108–123 GPa). The depth recovery ratio of ternary Ti-Nb-Zr thin films is also higher than that for binary β-Ti-(25.9–34.2)Nb thin film alloys.

Effect of Au nanoparticles on the optical and electrical properties of Nb-doped β-Ga2O3 film

The Nb-doped β-Ga2O3 (β-Ga2O3:Nb) thin films have been deposited on the p-Si and Au nanoparticles decorated p-Si substrates by radio frequency magnetron technique in argon ambient. All the annealed β-Ga2O3:Nb films are composed of similar crystallite sizes obtained by XRD and SEM measurements. The β-Ga2O3:Nb thin film grown on the Au nanoparticles decorated substrate shows lower transmittance and narrower band gap compared to that of grown on p-Si reference substrate. Photoluminescence intensity was quenched because of the short separation distance between semiconductor and the Au nanoparticles. The current density was enhanced and the barrier height of the β-Ga2O3:Nb/p-Si heterojunction was reduced by inserting Au nanoparticles in the interface of β-Ga2O3:Nb/p-Si heterojunction.

Nb and Ta Co-Doped TiO2 Transparent Conductive Thin Films by Magnetron Sputtering: Fabrication, Structure, and Characteristics

Nb and Ta co-doped anatase titanium dioxide (NTTO) nanocrystalline thin films were deposited on quartz and Si (100) substrates by RF magnetron sputtering. The influence of RF power on the growth, structure, morphology, and properties of the samples are discussed in detail. X-ray diffraction measurements show that the films are polycrystalline with anatase tetragonal structure, which is further confirmed by Raman spectroscopy analysis. Meanwhile, Raman spectroscopy results indicate that the peak width of Eg(1) mode, which is directly correlated to the carrier density, changes obviously with RF power. It is found that the substitution of Nb5+ and Ta5+ at Ti site is significantly improved with the increase of RF power from 150 W to 210 W. For the sample deposited at 210 W, the optical transmittance is above 82% in the visible range and the electrical resistivity is as low as 1.3 × 10−3 Ω cm with carrier density of 1.1 × 1021 cm−3 and Hall mobility of 4.5 cm2 V−1 s−1. The optical and electrical properties of NTTO thin films can be compared to those of Nb or Ta doped anatase TiO2. However, co-doping with Nb and Ta gives a possible platform to complement the limitations of each individual dopant.

Flux Pinning by the InAs Nanorods in Nb Thin Films

We investigate the feasibility of using the semiconductor InAs nanorods for flux pinning in Nb thin films. The InAs nanorods are grown in the vertical direction on Si (111) substrates by metal-organic chemical vapor deposition. The grown nanorods are about 80 nm in diameter, 5 μm in height, and 2 × 10 9 /cm 2 in density. The four-terminal strip pattern of Nb films is fabricated on nanorod-grown substrates and, for the reference, on nanorod-free substrates by sputtering through a metal mask. We observe clear enhancement of the upper critical fields and the critical current densities for field oriented along the nanorod axis in the Nb film with embedded nanorods. We also confirm the flux pinning by InAs nanorods in the measurements of the angular dependence of critical current densities. Our results indicate that the InAs nanorods are promising linear defects for flux pinning research in superconductors.

Structure, morphology and selected mechanical properties of magnetron sputtered (Mo, Ta, Nb) thin films on NiTi shape memory alloys

In this study, pseudo-elastic Ni 50.8 at.%-Ti alloy sheets of 1000 μm thickness were coated with 10 μm thick refractory metal thin films (Mo, Ta and Nb thin films, respectively), by nonreactive d.c. magnetron sputtering. These thin films were characterized with regard to their microstructure, morphology and selected mechanical properties. Microstructural characterization of the thin films included X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) analyses. SEM images show, that Mo thin films grow in a densely packed structure with columnar grains. Pole figure analyses reveal a distinct (110) texture of the deposited Mo thin films on the NiTi substrate. The Mo thin films exhibit relatively large crystallite sizes, 290–300 nm. Ta and Nb thin films show XRD reflections of various lattice planes, i.e. polycrystalline b.c.c. structures, with preferential (110) orientation. Both of these thin films grow with much smaller crystallite size in comparison to the Mo thin films, 30–40 nm.The Young's Modulus of both the thin films and thin film/substrate composites were investigated on various scales. For this purpose, indentation experiments were performed using both nano- and microindentation techniques. A comparison of the values of the Young's Modulus of the thin film materials (in bulk, literature data) and the results obtained from nanoindentation shows a very good agreement between the measured data and bulk data. Progressive scratch tests were used to evaluate the adhesion of the thin film materials on the NiTi substrate. These scratch tests suggest that the metallic thin films undergo plastic deformation at larger normal forces. However, no thin film delamination is observed, independent of the material combinations and testing conditions applied.

Surface morphology control of Nb thin films by biased target ion beam deposition

One of the many challenges for niobium (Nb) based superconducting devices is the concurrent improvement over the surface morphology without undermining superconducting properties. In this work, the authors demonstrated the modification to the surface morphology of 10–100 nm Nb thin films deposited by bias target ion beam deposition (BTIBD). Target current (IT) and target bias (VT) were the main parameters explored and were found to be critical to the control over crystallinity and surface morphology of Nb films. The optimized growth conditions yielded a ∼50 nm thick Nb film with the root-mean-square roughness of 0.4 nm, which was an order of magnitude smoother than that of Nb films by sputtering process. The critical temperature for superconductivity was close to the value of the bulk Nb (∼9.3 K) with a very homogeneous transition (the superconducting transition width ΔTc of 0.1 K). The quality of Nb film was evident in the presence of a very thin proximity layer (∼0.75 nm). The experimental results demonstrated that the preparation of smooth Nb films with expected superconductivity by BTIBD could serve as a base electrode for the in situ magnetic layer or insulating layer for superconducting electronic devices.

Room-temperature growth of thin films of niobium on strontium titanate (0 0 1) single-crystal substrates for superconducting joints

With the eventual aim of forming joints between superconducting wires of YBa2Cu3O7−δ (YBCO), thin films of Nb were grown at room-temperature on SrTiO3 (STO) (0 0 1), a single-crystal substrate that shows good lattice matching with YBCO. The crystallinity, surface morphology, and superconducting properties of the Nb thin films were investigated and compared with those of similar films grown on a silica glass substrate. The Nb thin films grew with an (hh0) orientation on both substrates. The crystallinity of the Nb thin films on the STO substrate was higher than that on the silica glass substrate. X-ray diffraction measurements and observation of the surface morphology by atomic-force microscopy indicated that Nb grew in the plane along the [1 0 0] and [0 1 0] directions of the STO substrate. This growth mode relaxes strain between Nb and STO, and is believed to lead to the high crystallinity observed. As a result, the Nb thin films on the STO substrates showed lower electric resistivity and a higher superconducting transition temperature than did those on the silica glass substrates. The results of this study should be useful in relation to the production of superconducting joints.