Niobium Thin Films Research Articles

Microwave study of magnetic field penetration parallel to thin niobium films

Complex conductivity of high quality niobium thin films has been investigated by microwave technique in parallel static magnetic field. For the 40 nm thick film no vortices can be formed and the microwave penetration is defined by the strength of the superconducting order parameter which varies with the applied magnetic field. 160 nm thick measured film allows formation of two rows of vortices. Microwave dissipiation is dominated by dynamics of vortices which is strongly affected by size effects. Results have been compared with the generalised models of complex conductivity for low-dimensional superconductor in mixed state following earlier considerations by other authors.

Mixed state conductivity of thin niobium films in perpendicular magnetic fields

Microwave complex frequency shift has been measured for high quality niobium films in perpendicular static magnetic fields up to the upper critical field. Data have been analyzed by the effective conductivity model for type-II superconductors in the mixed state. Depinning frequencies and upper critical fields have been determined from the analysis. The film of thickness 10 nm has κ ≈ 9.5 and depinning frequency ≈20 GHz. It can serve as a model system for high- κ superconductors. The upper critical fields were determined by the flow resistivity in low fields and by the crossover to the normal state. The former method gives slightly higher upper critical field values, reflecting the Ginzburg–Landau coherence length of nonoverlaping vortices.

Characterization of all-Nb nanodevices fabricated by electron beam lithography and ion beam oxidation

The authors present a reliable process that allows them to fabricate all-niobium nanodevices in a reproducible and well controlled way. Deposition of niobium thin films is done through a suspended mask formed in a Ge layer supported by a thermally stable polymer called phenylene-ether-sulfone, while the tunnel barrier is formed by ion beam oxidation of Nb surface. They study the effect of the fabrication conditions on the quality of narrow Nb stripes by measuring their transition temperature. Using dc transport measurements, they characterize tunnel junctions as well as single-electron transistors (SETs) and extract basic junction parameters, such as the barrier height, width, and specific capacitance. Low-frequency charge noise of all-Nb SETs at 50mK has a 1∕f power spectrum and the value of ∼4×10−3e∕Hz1∕2 at 1Hz.

Microwave response of thin niobium films under perpendicular static magnetic fields

The microwave response of high quality niobium films in a perpendicular static magnetic field has been investigated. The complex frequency shift was measured up to the upper critical fields. The data have been analyzed by the effective conductivity model for the type-II superconductors in the mixed state. This model is found to yield consistent results for the coherence lengths in high-kappa superconducting samples, and can be used with HTSC even at temperatures much below T_c. It is shown that for samples with high values of depinning frequency, one should measure both components of the complex frequency shift in order to determine the flow resistivity. The thick Nb film (160 nm) has low resistivity at 10 K, comparable to the best single crystals, and low kappa value. In contrast, the thinnest (10 nm) film has kappa ~ 9.5 and exhibits a high depinning frequency (~20 GHz). The upper critical field determined from microwave measurements is related to the radius of nonoverlaping vortices, and appears to be larger than the one determined by the transition to the normal state.

Aluminium nitride–niobium multilayers and free-standing structures for MEMS

In this article we investigate aluminium nitride–niobium thin films sputtered on Si wafers and free-standing structures made of these layers. The quality of the films is characterized by Rutherford backscattering (RBS), X-ray diffraction and time-of-flight secondary ion mass spectroscopy (TOF SIMS). The thickness and composition of the intermediate layers between the silicon and AlN was found to be crucial for the uniformity and orientation of the AlN. Optimization of growth conditions and layer thicknesses was performed in order to achieve a high quality of the films. Parameters of sputtered AlN layers of different thicknesses sufficient for use in piezoelectric and thermal switches were attained. Subsequently, a sacrificial layer technology was developed to manufacture multilayer free-standing bridges of different lengths and widths from Nb–AlN structures. Bridges down to 20 μm in size were controllably produced and investigated by e-beam microscopy. The technology proved to be reliable and robust at all ranges of investigated thicknesses and lateral sizes of microstructures.

Size induced metal–insulator transition in nanostructured niobium thin films:intra-granular and inter-granular contributions

With a reduction in the average grain size in nanostructured films of elemental Nb, weobserve a systematic crossover from metallic to weakly insulating behaviour. An analysis ofthe temperature dependence of the resistivity in the insulating phase clearly indicates theexistence of two distinct activation energies corresponding to inter-granular andintra-granular mechanisms of transport. While the high temperature behaviour isdominated by grain boundary scattering of the conduction electrons, the effect ofdiscretization of energy levels due to quantum confinement shows up at low temperatures.We show that the energy barrier at the grain boundary is proportional to the widthof the largely disordered inter-granular region, which increases with a decreasein the grain size. For a metal–insulator transition to occur in nano-Nb due tothe opening up of an energy gap at the grain boundary, the critical grain size is≈8 nm and the corresponding grain boundary width is≈1.1 nm.

Preparation of arrays of metallic nanoparticles exhibiting coulomb blockade at room temperature: an approach based on the self‐organization of metal‐loaded diblock‐copolymers

AbstractThe self‐organization of HAuCl4 loaded poly (styrene)‐block‐poly (2‐vinylpyridine) (PS‐b‐P2VP) diblock‐copolymers resulting in hexagonally ordered arrays of Au nanoparticles is exploited to prepare a system exhibiting single electron charging effects at room temperature. For this purpose, the metallic nanoparticles are deposited on top of an Al2O3 tunnel barrier separating them from a Niobium thin film backelectrode grown on a c‐cut Sapphire substrate. The single electron system is completed by the metal tip of a scanning tunneling microscope (STM), which is positioned right above the nanoparticle and serves as the second electrode. The STM additionally allowed taking current‐voltage I (V) characteristics revealing a large gap around zero bias typical of a Coulomb blockade. This interpretation is supported by the good agreement between experimental I (V) curves and their theoretical description. Copyright © 2006 John Wiley & Sons, Ltd.

The ac effect of vortex pinning in the arrays of defect sites on Nb films

Niobium thin films with spacing-graded array of submicrometer-scaled holes had been fabricated using electron beam lithography through a lift-off technique. The magnetoresistance measurements and current-voltage characteristics were carried out with the external magnetic field applied perpendicular to the film plane, in which commensurable effects were observed in both experiments. The magnetoresistance with positive/negative directions of dc current revealed identical curves except the dips at matching fields separated. Two distinct current-voltage curves, which resulted from the positive and negative applied current directions, respectively, were discerned when the external magnetic field was fixed at the matching field, which is believed to be due to asymmetry pinning potential in the spacing-graded array of holes. In addition, ac current-voltage curve measured at matching field showed a ratchet bump along with another extra peak associated with incommensurable effect.

Building blocks for a polarimeter-on-a-chip

For the “Primordial Anisotropy Polarization Pathfinder Array (PAPPA)” balloon flight project, we have designed and made thin-film niobium microstrip circuits as building blocks for a “polarimeter-on-a-chip” in which superconducting transmission lines are used to couple millimeter wave signals from planar antennas to superconducting transition edge sensor (TES) detectors. Our goal is to demonstrate technology for precision measurements of the polarization of the cosmic microwave background. To enable characterization and verification of our microstrip components, we have incorporated waveguide probes on each chip that can bring millimeter wave signals from a room temperature vector network analyzer to the superconducting circuits on the chip and back again for S-parameter measurements. We have designed a planar antenna and RF choke on the probes to efficiently couple radiation between waveguide and thin-film microstrip. To support the probe antennas in waveguides, we sculpted thin silicon cantilevers that extend from an edge of each silicon chip into a pair of waveguides within a specially designed split-block mount. This technique will allow us to make calibrated measurements at low temperatures of the velocity, impedance, and loss properties of our niobium transmission lines, the frequency response of microstrip filters, hybrid couplers, or terminations, and the performance of integrated detectors.

Defects in nanocrystalline Nb films: Effect of sputtering temperature

Thin niobium (Nb) films (thickness 350–400 nm) were prepared on (1 0 0)Si substrate in a UHV chamber using the cathode beam sputtering. The sputtering temperature T s was varied from 40 up to 500 °C and the influence of the sputtering temperature on the microstructure of thin Nb films was investigated. Defect studies of the thin Nb films sputtered at various temperatures were performed by slow positron implantation spectroscopy (SPIS) with measurement of the Doppler broadening of the annihilation line. SPIS was combined with transmission electron microscopy (TEM) and X-ray diffraction (XRD). We have found that the films sputtered at T s = 40 °C exhibit elongated, column-like nanocrystalline grains. No significant increase of grain size with T s (up to 500 °C) was observed by TEM. The thin Nb films sputtered at T s = 40 °C contain a high density of defects. It is demonstrated by shortened positron diffusion length and a high value of the S parameter for Nb layer compared to the well-annealed (defect-free) bulk Nb reference sample. A drastic decrease of defect density was found in the films sputtered at T s ≥ 300 °C. It is reflected by a significant increase of the positron diffusion length and a decrease of the S parameter for the Nb layer. The defect density in the Nb layer is, however, still substantially higher than in the well-annealed reference bulk Nb sample. Moreover, there is a layer at the interface between the Nb film and the substrate with very high density of defects comparable to that in the films sputtered at T s < 300 °C. All the Nb films studied exhibit a strong (1 1 0) texture. The films sputtered at T s < 300 °C are characterized by a compressive macroscopic in-plane stress due to lattice mismatch between the film and the substrate. Relaxation of the in-plane stress was observed in the films sputtered at T s ≥ 300 °C. The width of the XRD profiles of the films sputtered at T s ≥ 300 °C is significantly smaller compared to the films sputtered at lower temperatures. This is most probably due to a lower defect density which results in reduced microstrains in the films sputtered at higher temperatures.

Defect studies of hydrogen-loaded thin Nb films

Hydrogen interaction with defects in thin niobium (Nb) films was investigated using slow positron implantation spectroscopy (SPIS) combined with X-ray diffraction (XRD) and transmission electron microscopy (TEM). Thin Nb films on Si substrates were prepared using cathode beam sputtering at room temperature. Initially, the microstructure of the virgin (hydrogen-free) films was characterized. Subsequently, the films were step-by-step electrochemically charged with hydrogen and the evolution of the microstructure with increasing hydrogen concentration was monitored. Hydrogen loading leads to a significant lattice expansion which was measured by XRD. Contrary to free-standing bulk metals, thin films are highly anisotropic. The in-plane expansion is prevented because the films are clamped on the elastically hard substrate. On the other hand, the out-of-plane expansion is substantially higher than in the bulk samples. Moreover, an enhanced hydrogen solubility in the α-phase was found in nanocrystalline Nb films. It was found that most of positrons in the films are trapped at open-volume defects at grain boundaries (GBs). These defects represent trapping sites also for hydrogen atoms. Hydrogen trapping at vacancy-like defects like GBs leads to a local increase of the electron density and is reflected by a pronounced decrease of the S parameter in the hydrogen-loaded samples. In addition, it was found that new defects are introduced at higher concentrations of hydrogen due to the formation of NbH (β-phase) particles.

Thin-Film Persistent Current Switch

We have developed a fast, low power heat switch for switching a niobium thin film between the normal and superconducting state. The sputtered niobium film (400 nm thick, 100 /spl mu/m wide) has a critical current density of 5/spl times/10/sup 10/ Am/sup -2/. Switching is produced by joule heating a small section of the niobium film with a titanium thin-film resistor. With the heat switch in vacuum, the minimum heater power needed to switch to the normal state was 4.5/spl times/10/sup -5/ W. A simple three-dimensional thermal model shows that the minimum power is primarily determined by the thermal conductivity of the substrate. We have achieved response times less than 10/sup -6/ s.

Dependence of magnetic penetration depth on the thickness of superconducting Nb thin films

In this paper we present the results of a systematic study on the magnetic field penetration depth of superconducting niobium thin films. The films of thicknesses ranging from $8\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}300\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ were deposited on a Si substrate by dc magnetron sputtering. The values of the penetration depth $\ensuremath{\lambda}(0)$ were obtained from the measurements of the effective microwave surface impedance by employing a sapphire resonator technique. Additionally, for the films of thickness smaller than $20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, the absolute values of $\ensuremath{\lambda}(0)$ were determined by a microwave transmission method. We found that the reduction of the film thickness below $50\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ leads to a significant increase of the magnetic field penetration depth from about $80\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ for $300\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thick film up to $230\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ for a $8\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thick film. The dependence of the penetration depth on film thickness is described well by taking into account the experimental dependences of the critical temperature and residual resistivity on the thickness of the niobium films. Structural disordering of the films and suppression of superconductivity due to the proximity effect are considered as mechanisms responsible for the increase of the penetration depth in ultrathin films.

Flux flow properties of niobium thin films in clean and dirty superconducting limits

Flux flow properties for clean and dirty superconducting limits in strong pinning niobium films are studied. Measurements of electric field vs current density characteristics at high $J$ values $(J\ensuremath{\simeq}{10}^{6}\phantom{\rule{0.3em}{0ex}}\mathrm{A}∕{\mathrm{cm}}^{2})$ are successfully compared to theoretical models of flux flow and of flux flow instability in a large range of temperatures and magnetic fields. The nonlinear regime at high dissipation is analysed in the frame of a modified Larkin Ovchinnikov model that takes into account a quasiparticles heating effect. This model elaborated by Bezuglyj and Shklovskij (BS) defines a transition magnetic field ${B}_{t}$ above which the quasiparticles distribution became nonuniform due to a finite heat removal from the substrate. From the BS model, we can deduce values of the nonequilibrium lifetime of quasiparticles ${\ensuremath{\tau}}_{\mathrm{qp}}$, which are 10 to 100 times shorter in the dirty sample compared to the clean one and whose temperature dependance is specific to the electronic nature of the Nb film. The study of the nonlinear regime provides also a quantitative determination of the thermal transparency of the film-substrate interface.

Niobium layers on graphite: Growth parameters and thermal annealing effects

Thin films of niobium (∼8 μm thick) were deposited on an ATJ graphite substrate by radio-frequency magnetron sputtering at low temperatures (420±50 K) under argon pressure of 0.67 Pa. After deposition, the layers were thermally annealed in a vacuum of 6.67×10−4 Pa at temperatures of 1073–2073 K for 10 to 480 min. Nb2C and NbC phases were identified by X-ray diffraction after the thermal annealing, scanning electron microscopy and wavelength dispersive analyses. The carbide layer thicknesses were measured and the growth rate constants were determined at different temperatures. It was found that the activation energies are 190 kJ/mol for Nb2C, 164 kJ/mol for NbC, and 169.4 kJ/mol for the combined carbide layers. This study also presents the specific features of phase formation, adhesion and microhardness of the carbide layers.

Studies of niobium thin film produced by energetic vacuum deposition

An energetic vacuum deposition system has been used to study deposition energy effects on the properties of niobium thin films on copper and sapphire substrates. The absence of a working gas avoids the gaseous inclusions commonly seen with sputtering deposition. A biased substrate holder controls the deposition energy. Transition temperature and residual resistivity ratio of the niobium thin films at several deposition energies are obtained together with surface morphology and crystal orientation measurements by atomic force microscope inspection, X-ray diffraction analysis and transmitted electron microscope (TEM) analysis. The results show that niobium thin films on a sapphire substrate exhibit the best cryogenic properties at a deposition energy of around 123 eV. The TEM analysis revealed that epitaxial growth of film was evident when the deposition energy reached 163 eV for a sapphire substrate. Similarly, niobium thin films on copper substrates show that the film grows more oriented with higher deposition energy and the grain size reaches the scale of the film thickness at a deposition energy of around 153 eV.

Vertically Aligned Carbon‐Nanotube Arrays Showing Schottky Behavior at Room Temperature

Vertically aligned carbon-nanotube (CNT) arrays were fabricated in the thin-film anodic aluminum oxide (AAO) templates on silicon wafers utilizing a niobium (Nb) thin film as the source electrode. The average diameter of the CNTs was 25 nm, and the number density was 3 x 10(10) cm(-2). The CNT arrays synthesized at 700 degrees C and above exhibited Schottky behavior even at 300 K, with energy gaps between 0.2 eV and 0.3 eV. However, individual CNTs obtained by removal of the template behaved as resistors at 300 K. The CNT/Nb oxide/Nb junction is thought to be responsible for the Schottky behavior. This structure can be a useful cornerstone in the fabrication of nanotransistors operating at room temperature.

Photolithographic structuring of a thin metal film coil on a Zerodur cylinder

Photolithographic structuring is a common method for generating micro structures (<approximately 20 μm) on flat substrates, especially on silicon in the mass production of integrated circuits. After applying a thin, homogeneous, UV-sensitive photoresist film to the substrate, a mask (generally a transparent film) with the desired structure is positioned in the beam path. After a photochemical reaction has taken place in the photoresist, a chemical base is used to remove the exposed (photo positive process) or unexposed (photo negative process) layer. Subsequently, a PVD (Physical Vapour Deposition) process based on cathode sputtering is used to apply the metal layer. In the last step, the metal layer on the photoresist structure as well as the photoresist structure itself are removed by a chemical process. Applying a metal layer (50–200 μm wide and 800 nm thick) to a Zerodur cylinder, the conventional photolithographic process leads to an unacceptably high reject rate in the fabrication process on cylindrical structures. The weak point of this process is the area in which the two sides of the transparent film overlap, as these have to be adjusted very precisely to the Zerodur substrate. An alternative method is to write the structure directly on the photoresist using a UV-beam, or to ablate the photoresist and to proceed then with the same fabrication steps as in the previously described process. A direct-writing photolithographic process in combination with a cathode-sputtering PVD method has made it possible to fabricate an accurate pair of thin niobium film coils on a Zerodur cylinder. Superconductive properties of the niobium film were obtained by the use of a low-power PVD process. Final tests confirmed the superconductive behaviour of the niobium thin film structure.

Molecular precursors for the CVD of niobium and tantalum nitride

Treatment of NbCl 5 with excess HN(SiMe 2Ph) 2 in toluene resulted in the formation of crystalline [NbCl 3(NSiMe 2Ph)(NH 2SiMe 2Ph)] 2. In the presence of excess 3,5-lutidine (3,5-Me 2C 5H 3N), the reaction of TaCl 5 with 2 equivalents of HN(SiMe 3) 2 resulted in the isolation of the monomeric complex [TaCl 3(NSiMe 3)(NC 5H 3Me 2-3,5) 2]. The X-ray crystal structure of [TaCl 3(NSiMe 3)(NC 5H 3Me 2-3,5) 2] has been determined. Low pressure chemical vapour deposition of [NbCl 3(NSiMe 2Ph)(NH 2SiMe 2Ph)] 2 and [TaCl 3(NSiMe 3)(NC 5H 3Me 2-3,5) 2] forms thin films of niobium and tantalum nitride, respectively, at 600 °C.

Superconducting quantum interference devices: State of the art and applications

Superconducting quantum interference devices (SQUIDs) are sensitive detectors of magnetic flux. A SQUID consists of a superconducting loop interrupted by either one or two Josephson junctions for the RF or dc SQUID, respectively. Low transition temperature (T/sub c/) SQUIDs are fabricated from thin films of niobium. Immersed in liquid helium at 4.2 K, their flux noise is typically 10/sup -6//spl Phi//sub 0/ Hz/sup -1/2/, where /spl Phi//sub 0//spl equiv/h/2e is the flux quantum. High-T/sub c/ SQUIDs are fabricated from thin films of YBa/sub 2/Cu/sub 3/O/sub 7-x/, and are generally operated in liquid nitrogen at 77 K. Inductively coupled to an appropriate input circuit, SQUIDs measure a variety of physical quantities, including magnetic field, magnetic field gradient, voltage, and magnetic susceptibility. Systems are available for detecting magnetic signals from the brain, measuring the magnetic susceptibility of materials and geophysical core samples, magnetocardiography and nondestructive evaluation. SQUID "microscopes" detect magnetic nanoparticles attached to pathogens in an immunoassay technique and locate faults in semiconductor packages. A SQUID amplifier with an integrated resonant microstrip is within a factor of two of the quantum limit at 0.5 GHz and will be used in a search for axions. High-resolution magnetic resonance images are obtained at frequencies of a few kilohertz with a SQUID-based detector.