Niobium Thin Films Research Articles

Influence of structural defects on 1/f in thin niobium films

The 1/f noise in thin niobium films manufactured by various technologies and exposed to γ-quantum irradiation and current annealing has been studied. The evaporation conditions (the sublayer material and the substrate temperature) are shown to strongly affect the noise level. Irradiation adds to the film noise, whereas current annealing leads to decrease of the latter. The current dependence of the voltage fluctuations is found to be nonlinear. The fluctuations are proportional to the current in the region of low currents, and they are proportional to the square of the current flowing through the specimen in the region of high currents. The results are interpreted based on the relation of the 1/f noise to the mobile defects of the crystal structure.

High-resolution x-ray-scattering study of the structure of niobium thin films on sapphire.

The structure of two thin films of Nb grown epitaxially on sapphire has been studied using high-resolution x-ray-scattering techniques. The films, of nominal thickness 400 and 150 \AA{}, were grown using molecular-beam epitaxy on a (112\ifmmode\bar\else\textasciimacron\fi{}0) sapphire substrate. They were of exceptionally high quality, with rocking curves of \ensuremath{\approxeq}0.004\ifmmode^\circ\else\textdegree\fi{} for the (110${)}_{\mathrm{Nb}}$ Bragg peak. The x-ray scattering observed in transverse scans of the wave-vector transfer through the (110${)}_{\mathrm{Nb}}$ Bragg peak exhibits two other unusual features. First, weak satellite Bragg peaks are observed. These are shown to arise from a regular sinusoidal distortion of the Nb thin film. Using a frozen phonon model for this distortion, we relate the position of the satellites to the length of a terrace on the vicinal sapphire surface, and the intensity of the satellites to the lattice mismatch between the two materials in the surface normal direction. Second, in addition to the sharp Bragg peaks, there is a broader, diffuse component. This is isotropic in the plane of the film and has a Lorentzian-squared line shape. The diffuse component is shown to arise from a random network of two-dimensional Nb domains, with the size of domain related to the in-plane lattice mismatch. By combining these results with x-ray-reflectivity measurements from the thin films, and x-ray-reflectivity and crystal-truncation-rod measurements from the bare substrates, we develop a detailed model of the structure of Nb films on sapphire.

Thin-film technology and dc-SQUID instrumentation for a superheated superconducting detector

We report on improvements of the dc-SQUID instrumentation developed to study superheating-supercooling transitions of thin-film indium dots. In previous measurements on ordered arrays of dots with diameters larger than 100µm the flip signal of an individual dot in perpendicular magnetic field has been detected. In order to use these dots as superheated superconducting detectors it is necessary to reduce the dot diameter to lower the energy threshold and to avoid intermediate states. For these reasons, our effort has been to develop small and thick dots with diameter down to10 µm and thickness up to3.5 µm. At the same time the coupling with the dc-SQUID readout was improved using a thin-film niobium pickup coil integrated on the same chip containing the dots. Phase transitions of individual dots were observed. The superheating phase transition spread in temperature of the samples was about 0.15 %.

Structure of niobium thin films on sapphire

The structure of Nb thin films grown by molecular beam epitaxy on sapphire has been studied using high-resolution x-ray scattering techniques. Transverse scans of the x-ray wave vector transfer through the (110)Nb Bragg peak show diffuse scattering with a Lorentzian-squared profile, and satellite Bragg peaks for certain orientations of the crystal. The former feature arises from a random, two-dimensional network of Nb domains, and the latter from a periodic distortion of the Nb films at the terrace edges of the vicinal sapphire substrate.

Electron holography of vortex lattices in superconductors

The recent success in observing magnetic vortices in superconductors by means of transmission electron microscopy techniques has opened a new window to investigate the behavior of vortex lattices. We report the first direct observation of 2-D vortex lattices in niobium thin films taking advantage of the high resolution and sensitivity of electron holography.The experiments were conducted in a 300 keV field emission electron microscope equipped with a cold stage that allows magnetic fields to be applied while the specimen can be maintained from 30 K down to 4.5 K. The Nb specimen is tilted by 45° to the electron beam and the magnetic field is applied in the horizontal direction. The microscope also has a rotatable electron biprism used to form holograms. The thin (70±20 nm) foil specimens were prepared by chemically polishing 2 mm × 2 mm × 30 μm sections of Nb that had been annealed to ∼2000°C in a vacuum of 10-6 Pa.

Properties of r.f. sputtered niobium thin films for metrological applications

Niobium superconductive films for metrological applications (refractory Josephson tunnel junctions, superconductive bolometers and radiation sensors), have been fabricated via r.f. magnetron sputtering. Their electrical and structural properties have been measured on a wide range of film thicknesses (10–1000 nm). The data indicate the growth of a first layer at the substrate interface, which has a different behaviour with respect to the remaining of the film. The electrical data of resistivity at various temperatures show the relevant role of this layer at thicknesses < 200 nm and the effect of pre-deposition treatments of the reduction of this interfacial layer. Experimental resistivity data can be fitted for these thicknesses only using a modified version of the Mayadas-Shatkaz model, which keeps into account not only the reflection at grain boundaries but also the altered composition layer.

Fluxons in thin-film superconductor-insulator superlattices

In a system of thin alternating layers of superconductors and insulators the equations describing static and dynamic fluxon solutions are derived. The approach, represented by a useful compact matrix form, is intended to describe systems fabricated for example of niobium or niobium-nitride thin films; in the limit of ultrathin superconductor films it may give a model for describing fluxon motion in layered high-Tc superconductors. Numerical examples of current versus voltage curves to be expected in such an experiment are presented.

Critical temperature of thin niobium films on heavily doped silicon.

Measurements of the critical temperature are presented for 20--40 nm niobium films on heavily doped n- and p-type silicon, for a wide range of silicon doping concentrations and sample preparation methods. Contrary to a normal-metal--superconductor bilayer, a depression of the critical temperature does not occur in the semiconductor-superconductor system. We show the absence of a depression to be consistent with conventional theories for the proximity effect between a superconductor and a normal metal. It is due to the large mismatch in material parameters between the two layers.

Superconducting niobium thin film sputtering onto copper quarter wave resonators for heavy ion accelerators

Niobium sputtered copper quarter wave resonators (QWRs) represent an innovative and promising alternative to lead electroplated copper cavities, or to those made of niobium, both bulk and explosively bonded on copper. The authors describe the R&D efforts of the first niobium sputtered copper prototypes. The results obtained and the progressive improvements achieved test by test make Nb sputtered QWRs an intriguing possibility for the acceleration of heavy ions whenever the need for changing from Pb to Nb is encountered. A prototype with a Q-value of 7*10/sup 8/ and maximum accelerating field of 4 MV/m has been produced. >

Superconducting niobium infrared thermal detectors and circuits on back-etched substrates

The authors report on highly sensitive thermal infrared detectors, the result of combining the two technologies of superconducting low-T/sub c/ thin-film electronics and back-etched or 'micromachined' substrates. Functioning detectors have been fabricated consisting of a thin-film niobium Josephson junction thermometer and infrared-absorbing panel, integrated together with a DC superconducting quantum interference device, (SQUID) amplifier on a single back-etched silicon substrate. A detector made with one of two fabricated back-etched geometries has a calculated thermal conductance of 1*10/sup -7/ W/K at 4.2 K. On another device, a measured value of 9* was obtained for the amplification of the thermometer responsitivity by the integrated DC SQUID. Prospects are discussed for operating similar arrays at higher and lower temperatures than 4.2 K.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Preparation and Properties of Niobium Thin Films

أعدت أغشية النيبيوم الرقيقة (Nb) في صورة أقراص بأقطار 3مم ، 12 مم بوساطة التبخير الحراري في وسط فائق التفريغ وذلك باستخدام قاذفة إلكرونيات. الغرض من هذا التبخير هو الحصول على أغشية رقيقة ذات درجة نقاء من الشوائب عالية جدا ولا تقل عن 99.999% وقد تم فحص هذه الأغشية كهربائيا في ظروف مختلفة وفي درجات حرارة متفاوتة 42 ، 77 ، 300درجة كلفن ووجد أن الأغشية الرقيقة التي يقل سمكها عن 500 أنجشتروم غير مناسبة للتطبيقات العملية وذلك لرداءة توصيلها الكهربي وعدم ثباتها في حالة مستقرة. أما الأغشية التي يزيد سمكها عن ذلك فقد وجد أنها تظل ثابتة الخواص كهربائيا وفيزيائيا ولا تتأثر بالعيوب المحتمل وجودها في سطح القاعدة.

Microwave characterization of a low TC superconducting microstrip

Abstract A resonator structure has been used in conjunction with a novel measurement system to accurately characterize the microwave performance of a low TC superconducting microstrip. The performance of the thin film niobium on sapphire structures could be measured without the need for calibration with the cryostat. Unloaded Q at room temperature was found to be so low that the above structure cannot be used as a microstrip medium. However, below TC a rapid and dramatic reduction in the surface losses leads to an unloaded Q > 104 at 2 GHp and still > 103 above 10 GHz. The frepuency dependence of the loss was found to be of the form ƒ2.11 at 4.2 K, supporting surface loss calculations [Phys. Rev.111, 412 (1958)].

Real-time observation of vortex lattices in a superconductor by electron microscopy

THE dynamic behaviour of the quantized vortices of magnetic flux that penetrate type II superconductors, and specifically their interaction with pinning sites, is a topic of both scientific and technological interest, particularly since the discovery of high-transition-temperature (high- Tc) superconductors1. Until now, however, it has not been possible to study this behaviour in 'real time'. The Bitter technique2, scanning tunnelling microscopy3 and scanning electron microscopy4 have so far provided only static images, whereas a magneto-optical technique that provides time-resolved information5 does not resolve individual vortices. Here we report the real-time observation of vortices in a thin film of niobium, using the technique of Lorentz microscopy6. The coherent and penetrating beam of a recently developed 300-kV field-emission electron microscope6 allows us to observe, at 30 frames per second, the motion of thermally activated vortices, and their response to an applied magnetic field.

Temporal relaxation of photoinduced nonequilibrium in niobium.

Transient photoimpedance responses (TPR) of niobium thin films are examined in the normal, transition, and superconducting states. The niobium sample's impedance is modulated by the photoabsorption of 300-fs, \ensuremath{\sim}2-eV laser pulses with 0.002--1 \ensuremath{\mu}J fluence. The samples, biased with a dc current, exhibit transient voltage signals corresponding to the photoinduced impedance transient. The TPR-signal amplitude and temporal dependence are analyzed in terms of the electron-photon, electron-phonon, electron-electron interactions and phonon trapping. Data interpretation is facilitated by comparing measurements of the TPR signal in all three states. Typically, the normal- and transition-state TPR signals are bolometric and are a manifestation of resistive impedance changes. At low laser fluences and in the zero-resistance superconducting state, the TPR signal is primarily due to kinetic inductance changes produced by the nonequilibrium quasiparticle generation and recombination processes strongly influenced by phonon trapping in the sample. At higher fluence, the niobium samples undergo temporary phase transitions that lead to more complicated TPR-signal responses, which we also analyze.

ChemInform Abstract: Thin Films of Niobium and Niobium Oxides by PECVD.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Hybrid DC resistive SQUID

The construction, technology and experimental results of a double-junction hybrid resistive interferometer (DC R-SQUID) are described. The DC R-SQUID consists of a thin-film niobium chip containing two contacts and a bulk construction with a replaceable resistive part. Maximum signal is about 5 μV with a R-SQUID loop inductance of 40±15pH, with the resistance of the resistive part ranging from ≈10 -12 to ≈10 -5 μ.

High resolution microscopy study of niobium-sapphire interfaces modified by ion beam mixing

Thin polycrystalline niobium films were electron beam deposited at 773 K onto α-Al 2O 3 single crystal with (0001) surface orientation. The samples were subsequently irradiated at room temperature with 1.5 MeV xenon ions to a fluence of 5 × 10 15 ions cm −2. Niobium-sapphire interface modifications were investigated by Rutherford backscattering spectroscopy and X-ray diffraction at glancing incidence. The specimens were characterized by high resolution transmission electron microscopy performed on cross-sectional specimens. An intermediate layer of 5 nm thickness with an amorphous structure near the interface was observed after irradiation. The thickness of this layer suggests that the mixing is mainly governed by ballistic processes.

A planar gradiometer based on a microwave RF-SQUID

A type of gradiometer, working on the basis of a planar microwave RF SQUID, is described. In this type of RF SQUID, a superconducting half-wavelength stripline resonator serves as tank circuit, into which the SQUID is integrated. If a gradiometer configuration is used for the SQUID (i.e. two loops), a certain asymmetry of the stripline resonator should be provided to ensure sufficient coupling between gradiometer and resonator. Gradiometers were prepared from thin niobium films on sapphire substrates, having either microbridges or tunnel junctions as Josephson elements. When operated in hysteretic mode, modulation voltages of about 100 mu V were measured for both microbridge and tunnel junction SQUIDs.

Photoresponse of Nb films; observation of biexponential recovery times of the superconducting state

The authors present a systematic study of the photoresponse of superconducting Nb films over ranges of fluence and bias current and for temperatures of 6.5 K<T<300 K. The response of thin niobium films to fast pulses of 665-mn laser light was studied over a temperature range that spans the superconducting transition temperature. Below T/sub c/, the results are consistent with those of L.R. Testardi (1971), but the improved time resolution obtained in the present work (10 ps compared with 0.1 mu s) has made it possible to resolve more subtle features of the response. At very low fluences, the quasi-particle recombination time tau /sub r/ is measured in the range of reduced temperature T/T/sub c/ from 0.75 to 0.95; tau /sub r/ is so short in Nb that it has previously been measured only once, by M. Johnson and R.H. Silsbee (1987). That experiment used a branch imbalance diffusion length measurement and unexpectedly observed a biexponential decay. The present measurement agrees with that result, and the fast decay time is used to obtain a value for tau /sub r/, winch agrees with theory. At higher fluences, the response scales in a manner consistent with a T* model. Above T/sub c/, a small signal of unknown origin is observed and is compared with the photoresponses of a gold and nickel film. The results on a single YBa/sub 2/Cu/sub 3/O/sub 7 /sample are briefly compared with those of Nb.

7541. Investigation of defects in boron implanted silicon by means of p-n junction : R Lenhard and S Luby,Vacuum,41, 1990, 856–858

The sorption of hydrogen by thin niobium films, evaporated at 10−9 torr, has been studied at low pressures and at temperatures of 77° and 293° K. The measurements have been carried out employing a Bayard-Alpert gauge and an omegatron type mass spectrometer, both supplied with lanthanum hexaboride coated filaments. For the very thin films considered (< 0.1 mg|cm2) the sorption capacity was found to be independent of the temperature and directly proportional to the evaporated niobium. The measured capacity was found to correspond to an atomic ratio H:Nb of about 1:2. The initial sorption rate seems to be independent of the film thickness. The shape of the curve relating sorption rate to quantity sorbed is different at the two different temperatures.