Niobium Layers Research Articles

Magnetic filters in UHV arc-discharges: constructions, field modelling and tests of efficiency

The paper concerns modelling of three different configurations of magnetic channels, designed for the elimination of micro-droplets emitted from a cathode during plasma-arc discharges. Since arc discharges under high-vacuum conditions may be used for the deposition of thin films (e.g. metal or superconducting layers) upon different substrates, the elimination of the micro-droplets is of importance for improvement of the deposited layers. Distributions of magnetic field lines inside the considered magnetic filters were calculated by means of a Maxwell 2D-code. The computed distributions were compared with regard to their correctness. According to the modelled configurations, three magnetic filters were constructed and tested experimentally. The efficiency of the magnetic filtering has been estimated on the basis of an optical microscopy of the deposited samples and measurements of ion currents. The best results were achieved with a T-type filter, which ensured the lowest number of transmitted micro-particles (3.3 × 105 micro-droplets cm−2 upon the sample surface) and the largest deposition rate of the niobium layer (0.95 nm s−1).

Progress report on the development of a very high sensitivity parametric transducer for the Mario Schenberg Gravitational wave detector

We have constructed the Mario Schenberg gravitational wave detector at the Physics Institute of the University of Sao Paulo as programmed by the Brazilian Graviton Project, under the full support of FAPESP (the São Paulo State Foundation for Research Support). We are ready to do a first test run of the spherical antenna at 4.2K with three parametric transducers and an initial target sensitivity of h∼10−21Hz−½ in a 60Hz bandwidth around 3.2kHz. The parametric transducers to be used on the Mario Schenberg detector consist of reentrant klystron copper-aluminum cavities covered with a thin layer of niobium on the walls and on the oscillating membrane. The gap between the central conical post and the membrane is about 40µm. Here we present a progress report on this transducer development related with niobium layer deposition in the transducer cavities and on the design of silicon membranes for the last transducer mechanical mode.

Atomic spin decoherence near conducting and superconducting films

We derive scaling laws for the spin decoherence of neutral atoms trapped near conducting and superconducting plane surfaces. A result for thin films sheds light on the measurement of Y. J. Lin, I. Teper, C. Chin, and V. Vuleti\ifmmode \acute{c}\else \'{c}\fi{} [Phys. Rev. Lett. 92, 050404 (2004)]. Our calculation is based on a quantum-theoretical treatment of electromagnetic radiation near metallic bodies [P. K. Rekdal, S. Scheel, P. L. Knight, and E. A. Hinds, Phys. Rev. A 70, 013811 (2004)]. We show that there is a critical atom-surface distance that maximizes the spin relaxation rate and we show how this depends on the skin depth and thickness of the metal surface. In the light of this impedance-matching effect we discuss the spin relaxation to be expected above a thin superconducting niobium layer.

Nanometric deformations of thin Nb layers under a strong electric field using soft x-ray laser interferometry

We present measurements of in situ nanometric-resolution topographical modifications of thin niobium layers subjected to strong electric fields. The Nb layers, deposited on a fused silica substrate, are interferometrically flash probed using soft x-ray laser (XRL) at the wavelength of 21.2nm. Its pulses are reflected by the probed sample under grazing incidence angle, and the information about surface deformation is obtained by a Fresnel wave-front-division interferometer. It was experimentally established that the probing pulses at the soft x-ray wavelength do neither produce any measurable photoelectric-field emission, nor alter the topographical features of the probed surface. The examined Nb electrodes were periodically probed while the electric field was increased up to 80MV∕m, and alterations of their topographical characteristics with a resolution of ∼2nm in the relief elevation were obtained. It was found that behavior of the Nb layer strongly depends on the polarity of the applied voltage. Only small modifications are observed with the Nb surface at the positive potential, whereas the negative potential induces significant transient surface perturbations, with peak-to-valley elevation differences ∼50nm. The qualitative nature of these modifications was observed to be unrelated to intermittent parasitic breakdowns.

Structure and composition of Nb and NbC layers on graphite

Niobium layers of 6 to 8 μm in thickness have been deposited on an ATJ graphite substrate by radio-frequency (rf) magnetron sputtering. After deposition, the layers were annealed under vacuum (6.67×10 −4 Pa) at a temperature in the range of 1373–2073 K for 0.5 to 3 h. The effects of the negative substrate bias voltage on the deposited niobium layers and niobium carbide layers derived by heat treatment were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Vickers microhardness test. Depending on the negative dc bias voltage, V b, applied to graphite substrates, three distinct categories of Nb and of NbC layers were formed: (a) the Nb layers deposited on graphite substrates biased to V b<50 V, exhibited a singularly nucleated columnar structure and these layers were converted by annealing treatment into highly porous NbC layers of low hardness, (b) the Nb layers deposited at V b in the range of 50–80 V were composed of a continuously nucleated sub-columnar structure and were transformed into dense NbC layer exhibiting the highest microhardness value equal to ∼13 GPa; and (c) the Nb layers deposited at V b≥80 VDC possessed an imperfect structure and were transformed into an NbC layer with the highest compactness characterized by a brittle fracture and a moderately high microhardness value.

Superconducting Pinning by Magnetic Domains in a Ferromagnet-Superconductor Bilayer

The local flux profile and the critical current are studied using an array of Hall sensors in a ferromagnetic-superconducting bilayer which consists of niobium film covering ferromagnetic Co/Pt multilayer with perpendicular magnetic anisotropy. The results indicate about threefold enhancement of the flux pinning in niobium layer caused by the isolated magnetic domains which are created during the magnetization reversal of the Co/Pt multilayer. The geometrical barrier is absent, and the critical current is strongly peaked in close vicinity to the sample center, suggesting that the critical state differs from that predicted by the Bean model

Analysis of anodic films on Nb and NbNx by glow discharge optical emission spectroscopy

AbstractSputter‐deposited niobium layers containing nitrogen form anodic films with resultant capacitance and leakage current density little influenced by annealing up to 523 K. The nitrogen species incorporated into the anodic oxide films at the alloy/film interface are distributed in the inner 70–75% of the film thickness, with fine bubbles containing high‐pressure N2O gas being evident. Glow discharge optical emission spectroscopy (GDOES) analysis reveals their continued presence after annealing at 523 K. The insignificant changes in GDOES depth profiles also suggest that the formation of low‐valent oxide at the metal/film interface as a result of annealing is not responsible for the behaviour. For thermal treatments above 400 K, capacitances and leakage currents increase significantly for niobium, with nitrogen additions diminishing the effects. Such behaviour is suggested to arise from suppression of mechanical damage to films through the presence of highly pressurized gas in the inner part of the films. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Cross-type submicron josephson junctions using SNS technology for josephson voltage standard applications

We have developed a very simple method for manufacturing submicrometer Josephson junctions (JJs) based on niobium and titanium layers. Together with the use of a microwave circuit consisting of modified coplanar strips this type of JJs offers the possibility of easy integration of several thousand submicrometer junctions in a microwave circuit. Up to this point we have manufactured and successfully tested arrays with 2000 super-/normal-/super-conductor junctions with areas from 0.3 /spl mu/m/spl times/0.3 /spl mu/m to 2.0 /spl mu/m/spl times/2.0 /spl mu/m. First measurements with microwave irradiation showed stable Shapiro steps at voltages up to 40 mV and, thereby, the proper operation of all 2,000 junctions was seen.

Josephson junctions and superconducting quantum interference devices made by local oxidation of niobium ultrathin films

We present a method for fabricating Josephson junctions and superconducting quantum interference devices (SQUIDs) which is based on the local anodization of niobium strip lines 3–6.5 nm thick under the voltage-biased tip of an atomic force microscope. Microbridge junctions and SQUID loops are obtained either by partial or total oxidation of the niobium layer. Two types of weak link geometries are fabricated: lateral constriction (Dayem bridges) and variable thickness bridges. SQUIDs based on both geometries show a modulation of the maximum Josephson current with a magnetic flux periodic with respect to the superconducting flux quantum h/2e. They persist up to 4 K. The modulation shape and depth of SQUIDs based on variable thickness bridges indicate that the weak link size becomes comparable to the superconducting film coherence length ξ which is of the order of 10 nm.

Characteristics of Superconducting Nb Layer Fabricated Using High-Vacuum Electron Beam Evaporation

We have fabricated a niobium (Nb) layer for a Josephson junction by means of a high-vacuum electron beam (EB) evaporation technique. The high-vacuum EB-evaporated Nb layer is deposited under the base pressure of 4×10-9 Pa and the deposition rate of 1.2 nm/min. In spite of the extremely low deposition rate, Nb layers of over 50 nm thickness have the transition temperature of 9.3 K and the ratio of residual resistivity of more than 3. The surface roughness and the grain size of the high-vacuum EB-evaporated Nb layer are studied using atomic force microscopy and X-ray diffraction analysis. The stress in the high-vacuum EB-evaporated Nb layers is examined by measuring the change in the radius of curvature of the substrate before and after Nb layer deposition, and the X-ray diffraction patterns of the Nb layers. The high-vacuum EB-evaporated Nb layer exhibits a weak tensile stress.

Field emitter array fabricated using focused ion and electron beam induced reaction

A dual beam system consisting of focused ion and electron beams was used for manufacturing of Nb-gated silicon field emitter arrays. Gate opening was produced either by reactive focused ion beam etching of both the niobium and the silicon dioxide layer or by physical sputtering of the niobium layer by focused ion beam and subsequent wet etching of the underlying silicon dioxide layer. Platinum tips were deposited into the gate opening using an electron beam induced chemical reaction. Prototype devices, which exhibit field emission, were produced successfully. Several process parameters such as ion dose, beam diameter, and etch duration were systematically varied to identify the optimum condition for the fabrication of field emitter arrays.

Investigation of Toughening and Resistance‐Curve Behavior in Hybrid Molybdenum Disilicide

This paper presents the results of a study of the toughening and resistance‐curve behavior in a hybrid molybdenum disilicide (MoSi2) composite that has been reinforced with 2 mol% yttria partially stabilized zirconia particles (TZ‐2Y) and niobium layers. Toughening and resistance‐curve behavior occur as a result of the combined effects of crack bridging and transformation toughening. Crack‐tip shielding that is due to large‐scale crack bridging and transformation toughening also is modeled using micromechanics concepts. The models show that the overall crack‐tip shielding from transformation toughening is very limited. However, large‐scale bridging by the ductile niobium layers promotes significant levels of toughening and resistance‐curve behavior.

Corrosion protection of titanium by deposition of niobium thin films

Titanium is a promising material for medical implants, replacing bones and teeth. However, at pH values below 2, which occur in the dental environment, the corrosion resistance is compromised. The deposition of niobium layers onto titanium is a possibility to increase the corrosion resistance, as measured in 5 N HCl solution.

Niobium tunnel junctions with multi-layered electrodes

The current-voltage characteristics of the niobium - aluminum oxide niobium tunnel junctions have been studied systematically and are compared with numerical simulations based on the microscopic theory of the proximity effect. The thickness of the base niobium layer is varied from 35 to 500 mm while the thickness of the aluminum layer is kept constant (about 9 nm). In a separate series of experiments the aluminum thickness is varied from 2 to 30 nm for two fixed thicknesses of the base electrode: 50 and 200 nm. The appropriate conditions for a full suppression of the so called knee structure at the gap voltage in the current-voltage characteristic are experimentally determined and theoretically interpreted in the framework of the microscopic theory. The influence of the additional aluminum layer in a composite base electrode on the properties of the tunnel junction have been studied in dependence on the aluminum thickness and distance of this layer from the barrier. The obtained results demonstrate that the current-voltage characteristics of tunnel junction can be engineering by an appropriate layer thickness of compound base electrode.

Experimental realization of a 3D integrated RSFQ T-flip-flop using stacktrons

Superconducting circuits based upon Rapid Single Flux Quantum (RSFQ) Logic are an interesting approach for future digital electronics with clock frequencies in the range of 100 GHz. Our intention is to increase the integration density of such circuits. Therefore we have successfully fabricated and tested an RSFQ-circuit based on a new kind of stacked three terminal device, henceforth called a "stacktron". A stacktron consists of two vertically arranged all-niobium tunnel junctions, each separately shunted by a thin film resistor, and an access to the intermediate niobium layer. An RSFQ frequency divider (T-flip-flop) including two stacktrons has been tested up to 20 GHz. As a shunt resistor we used Pd/Au, so that /spl beta//sub c/<1 and the IV-curve is nonhysteretic. The junctions showed a current density of approximately 250 A/cm/sup 2/. The I/sub c/R/sub n/-product was 100 /spl mu/V. These first steps toward a three dimensional (3D) RSFQ architecture will be discussed in the framework of highly integrated and complex RSFQ-circuits.

Proximity effect, Andreev reflections, and charge transport in mesoscopic superconducting/semiconducting heterostructures

In the quasi-two-dimensional (Q2D) electron gas of an InAs channel between an AlSb substrate and superconducting niobium layers, the proximity effect induces a pair potential so that a Q2D mesoscopic superconducting/normal/superconducting (SNS) junction forms in the channel. The pair potential is calculated with quasiclassical Green’s functions in the clean limit. For such a junction, alternating Josephson currents and current–voltage characteristics (CVCs) are computed, using the nonequilibrium quasiparticle wavefunctions which solve the time-dependent Bogoliubov–de Gennes equations. The CVCs exhibit features found experimentally by the Kroemer group: a steep rise of the current at small voltages (‘foot’) changes at a ‘corner current’ to a much slower increase of current with higher voltages, and the zero-bias differential resistance increases with temperature. Phase-coherent multiple Andreev reflections and the associated Cooper pair transfers are the physical mechanisms responsible for the oscillating Josephson currents and the CVCs. Additional experimental findings not reproduced by the theory require model improvements, especially a consideration of the external current leads which should give rise to hybrid quasiparticle/collective-mode excitations.

Niobium Plating Processes in Alkali Chloride Melts

Niobium deposits were prepared from alkali chloride melts on nickel and AISI316 stainless steel substrates both by constant current and by pulse current methods. The influence of electrolysis conditions on the nature, morphology and purity of the deposits was investigated by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis. No metallic niobium was obtained at temperatures below 500 ∘C. At temperatures between 550 and 650 ∘ C, the deposits were dendritic and non-adherent, whereas pure niobium layers could be obtained at 750 ∘ C. Detailed analysis showed that a large negative overpotential during the pulse current period lead to the presence of suboxides, such as Nb6O, in the metallic phase. Suitable electrolysis conditions gave pure oxygen-free niobium. Cross section analysis showed that on nickel a thin layer of niobium–nickel alloy such as NbNi3, was formed at the metal interface. In contrast no alloys were detected at the niobium-stainless steel interface, where homogenous adherent layers of thickness around 50 μm were obtained.

Superconducting tunnel junction base electrode planarization

We have investigated the planarizing effects of multilayered superconducting thin films. Atomic force microscopy measurements indicate that coating a standard niobium base electrode with alternating layers of aluminum and niobium significantly reduces the film’s overall surface roughness. Planarized films such as these were used as the base electrodes of superconducting tunnel junctions that show vastly improved leakage characteristics over conventional junctions fabricated under the same conditions.

Effect of a barrier layer of niobium and copper on physico-chemical processes of interaction between solid molybdenum and liquid steel

Effect of a barrier layer of niobium and copper on physico-chemical processes of interaction between solid molybdenum and liquid steel

Fracture-free release of CVD diamond

Vapor-deposited diamond sometimes fractures during or after deposition. This cracking may be caused by differential stresses between the substrate and the deposited diamond. The yield of fracture-free CVD diamond may be greatly enhanced when the deposition surface is pre-coated with a thin layer of a temperature-dependent hydride former such as niobium. During cooling from the deposition temperature in a hydrogen atmosphere, hydrogen reacts between 300 and 500 °C with the niobium layer to form a weak embrittled layer of niobium hydride. Thermal differential strains generated by cooling cause the hydride to fracture at stress levels well below the fracture stress of the deposited diamond. With this technique, large thick sheets of crack-free CVD diamonds are reproducibly released from their deposition substrate.