Nb Thin Films Research Articles

Structure, Optical and Electrical Properties of Nb(Zn) Doped Sol-Gel ITO Films: Effect of Substrates and Dopants.

We present comparative studies of sol-gel ITO multilayered films undoped and doped with Nb or Zn (4 at.%). The films were obtained by successive depositions of five layers using the dip-coating sol-gel method on microscopic glass, SiO2/glass, and Si substrates. The influence of the type of substrates and dopant atoms on the structure and optical properties of the sol-gel ITO thin films is examined and discussed in detail. XRD patterns of these layers showed a polycrystalline structure with an average crystallite size of <11 nm. Raman spectroscopy confirmed the chemical bonding of dopants with oxygen and showed the absence of crystallized Nb(Zn)-oxide particles, indicated by the XRD pattern. Spectroscopic Ellipsometry and AFM imaging revealed a clear dependence of the optical parameters and surface morphology of the ITO and ITO:Nb(Zn) thin films on the type of substrates and dopants. The analysis of the current-voltage and capacitance-voltage characteristics of the Al/ITO/Si structures revealed the presence of charge carrier traps in the ITO bulk and the ITO-Si interface. The densities of these traps are obtained and the character of the current transport mechanism is established.

Laser engineered architectures for magnetic flux manipulation on superconducting Nb thin films

Custom shaped magnetic flux guiding channels have been fabricated on superconducting Nb thin films by laser nanopatterning of their surface. Preferential pathways are defined by suitable combination of imprinted anisotropic pinning domains through laser-induced periodic surface structures (LIPSS). Generated by the selective energy deposition of femtosecond UV laser pulses, quasi-parallel ripple structures are formed under optimized irradiation conditions. On average, each domain is formed by grooves with a lateral period of 260–270 nm and a depth about 80 nm. By combination of scanning and transmission electron microscopy, magneto-optical imaging, and conductive atomic force microscopy techniques, we conclude that the boundaries of the LIPSS-covered domains play a prominent role in the magnetic flux diversion process within the film. This is confirmed by dedicated modeling of the flux dynamics, combined with the inversion of the magneto-optical signal. The created metasurfaces enable control of the flux penetration process at the microscale.

Suppression of cracking induced by epitaxial strain relaxation using a relaxation absorber during the transfer of epitaxial thin films of anatase-type Nb:TiO2

This work developed a technique for fabricating flexible epitaxial thin films of anatase-type Nb:TiO2 (epitaxial TNO) via a transfer process. In our prior work, epitaxial TNO deposited on LaAlO3 (001) single crystal substrates was lifted off and affixed to flexible polymeric sheets. However, many cracks occurred on the surfaces of the transferred thin films, such that the conductivity of the epitaxial TNO was drastically decreased. This loss of conductivity was attributed to the interruption of current paths by the cracks. These numerous cracks were linear and in most cases were oriented either parallel to the [100]anatase∥[100]LAO direction or parallel to the [010]anatase∥[010]LAO direction (where the subscripts anatase and LAO represent the materials for each Miller index). Therefore, cracking likely occurred as a consequence of the relaxation of epitaxial strain and a return to the original axial lengths of the TNO during the transfer process. In the present work, the surface of epitaxial TNO was coated with the ductile polymer SU-8 3050 as a relaxation absorber prior to the transfer. This modified transfer process greatly reduced crack formation in the TNO such that the conductivity of the thin film was improved by two orders of magnitude. Thus, pre-coating with a ductile relaxation absorber was shown to effectively suppress the cracking of flexible epitaxial TNO.

Influence of deposition pressure on microstructure, mechanical and electrical properties of niobium thin films

Niobium (Nb) thin films were deposited on a Si substrate using magnetron sputtering system by varying the deposition pressure. The effect of deposition pressure on the microstructure, residual stresses, and electrical properties was investigated by systematically varying the deposition pressure from 0.15 to 0.60 Pa. The Nb thin films were characterized using hard x-ray reflectivity, grazing incidence x-ray diffraction, four point probe method, and atomic force microscopy. The films grown at lower deposition pressures had smooth surfaces and more compact structures, which are advantageous for lowering electrical resistance but had higher compressive stress. On the other hand, the films grown at higher deposition pressures had columnar type growth with less dense structures, which leads to increase in electrical resistance. However the nature of stresses transform from compressive to tensile. Hard X-ray reflectivity performed on Nb films provides direct insight into the growth and the microstructure which were correlated with the mechanical and electrical properties.

Effect of deposition rate and annealing on Nb thin film growth on Cu substrate: Molecular dynamics simulation

Molecular dynamics (MD) simulations are performed to study the growth mechanisms of Nb thin film on Cu substrate. In this investigation, we provide insights and information about the quality of the deposited Nb thin film by analyzing the growth mode, stress, and evolution of its microstructural and morphological properties by considering different deposition rates and thermal annealing processes. The results show that the growth of Nb atoms on the Cu substrate follows the island mode. The interface analysis reveals the presence of a nucleation phenomenon, moreover, the results indicate that the first layer of Nb film remains incomplete until approximately the amount of 4 monolayers of Nb is deposited. The analysis of interdiffusion indicates that Nb penetration is initially limited to the first Cu layer. However, after annealing, there is a significant increase in Nb atom penetration and a clear rearrangement of Nb within each layer of the film. The film adopts a BCC structure, with crystallinity rates varying based on the deposition rate. The highest crystallinity rate is observed at 5 atoms/ps, and annealing further increases these percentages. Deposition at different rates leads to different levels of compressive normal and biaxial stress.

Porous-anodic-alumina-templated Ta-Nb-alloy/oxide coatings via the magnetron-sputtering/anodizing as novel 3D nanostructured electrodes for energy-storage applications

The Ta-52 at.%Nb thin alloy films were magnetron sputter-deposited over a low-aspect-ratio nanoporous anodic-alumina template formed in 0.05 M tartaric acid solution at 250 V and modified by the pore-widening technique to enlarge the pores up to ∼500 nm. The alloy coated the pores evenly, thus forming a 3D continuous conducting nanofilm on the template. Partially anodizing the templated alloy in a borate buffer solution of pH 7.5 generated a compact amorphous mixed-oxide anodic film thickening proportionally to the applied voltage. It was revealed that the oxide on the Ta-52 at.%Nb alloy grows with a slower migration of Ta5+ ions relative to Nb5+ ions, resulting in mixing Ta2O5 and Nb2O5 in the film depth and forming a few-nm-thick Nb2O5 outmost layer. The unique migration of Ta4+, Ta3+, Nb4+, and Nb3+ ions is assumed accountable for forming corresponding suboxides in the 3D anodic film in contrast to a flat Ta-52 at.%Nb alloy film used as a reference. The 3D anodic films behave as an n-type semiconductor with a low donor density Nd = ∼2 × 1018 cm−3, appropriate for dielectric applications. An unusual two-layered structure with a sharp electrical interface revealed in the 3D oxide films anodized to 30–130 V, comprising a low-resistivity layer superimposed on the high-resistivity layer, is explained by an immobile negative space charge in the outer film part. The air-annealing at moderate temperatures releases the space charge and transforms the two layers into a high-resistivity single layer having substantially improved dielectric properties and thermostable (up to 250 °C) capacitance of 1.2 μF cm−2 achieved for the film anodized to practical 50 V. The 3D films having up to 4.5 times enlarged effective surface area can be utilized as novel metal/oxide nanostructured electrodes for electrolytic microcapacitors suitable for classical electronic circuits and energy-storage applications.

Planar deposition of Nb thin films by HiPIMS for superconducting radiofrequency applications

Cost efficiency and sustainability are critical challenges for future accelerator machines. Nb-coated Cu (Nb/Cu) superconducting radiofrequency (SRF) accelerating cavities, while requiring R&D to meet high acceleration gradient performance standards, show potential for addressing these challenges. Defects in the Nb layer responsible for the deterioration of cavity performance have been linked to the underlying Cu substrate. This study investigates a novel optimization method by mitigating the substrate surface’s impact on deposited Nb, using High Power Impulse Magnetron Sputtering alongside a DC substrate bias (from −50 to −300 V). Trenched silicon substrates are chosen to emulate substrates with exceptionally rugged surface characteristics. Film deposition is investigated both experimentally and through kinetic Monte Carlo simulations. Higher DC voltages are shown to eliminate self-shadowing during the coating process via improved incident ionic flux directionality, enhanced adatom mobility and increased re-sputtering rates, ultimately yielding flat and densely-packed films. The influence of the substrate’s shape on the film’s surface was found to decrease exponentially with increasing ion bombardment energies. Films sputtered under high DC bias conditions (−300 V) exhibited complete planarization with a 30% re-sputtering rate. Strategies for applying this technique to Nb/Cu SRF cavities, enhancing their viability for future particle accelerators, are also discussed.

Formation and Microwave Losses of Hydrides in Superconducting Niobium Thin Films Resulting from Fluoride Chemical Processing

AbstractSuperconducting niobium (Nb) thin films have recently attracted significant attention due to their utility for quantum information technologies. In the processing of Nb thin films, fluoride‐based chemical etchants are commonly used to remove surface oxides that are known to affect superconducting quantum devices adversely. However, these same etchants can also introduce hydrogen to form Nb hydrides, potentially negatively impacting microwave loss performance. Here, comprehensive materials characterization of Nb hydrides formed in Nb thin films as a function of fluoride chemical treatments is presented. In particular, secondary‐ion mass spectrometry, X‐ray scattering, and transmission electron microscopy reveal the spatial distribution and phase transformation of Nb hydrides. The rate of hydride formation is determined by the fluoride solution acidity and the etch rate of Nb2O5, which acts as a diffusion barrier for hydrogen into Nb. The resulting Nb hydrides are detrimental to Nb superconducting properties and lead to increased power‐independent microwave loss in coplanar waveguide resonators. However, Nb hydrides do not correlate with two‐level system loss or device aging mechanisms. Overall, this work provides insight into the formation of Nb hydrides and their role in microwave loss, thus guiding ongoing efforts to maximize coherence time in superconducting quantum devices.

Tailoring optical response in nanostructured bilayers: Effects of surface roughness and layer thickness on NbN/Nb and TiN/Ti

Numerical calculations were performed on NbN/Nb and TiN/Ti nanostructured bilayers. Discrete dipole approximationimplemented in the DDSCAT code was utilized. In order to design a TUC (Target Unit Cell) with realistic surfaceroughnesspattern, Atomic Force Microscope images of 100 ±10 nm Nb thin films were used. The optical properties of nanostructured bilayers (60-100 nm thickness with a ±10 nm surface roughness) were calculated, keeping the interfacebetween the layers as smooth flat surfaces. For the NbN/Nb nanostructure with a thickness between 100 ±10 nm, and 70 ±10 nm, in a wavelength range between 200 and 350 nm, the reflectance and absorptance show a nearly flat spectrumwith an intensity of 20% and 80%, respectively. For larger wavelengths, absorptance decays smoothly. In contrast, for the 60 ±10 nm bilayer, the absorptance decays faster. For the TiN/Ti nanostructure, at 415 nm, a relative maximum in the reflectance, and a minimum in absorptance, are observed. The critical value of the absorbed light spectra begins to shift to higher wavelengths as the thickness of the TiN layer increases. We demonstrate that adding a rough nitride layer on top of its metallized layer drastically modifies the optical response of the nanostructured bilayer, an interesting result forplasmonics and nanoelectronics applications.

Superconductivity in Nb: Impact of Temperature, Dimensionality and Cooper-Pairing.

The ability to realistically simulate the electronic structure of superconducting materials is important to understand and predict various properties emerging in both the superconducting topological and spintronics realms. We introduce a tight-binding implementation of the Bogoliubov-de Gennes method, parameterized from density functional theory, which we utilize to explore the bulk and thin films of Nb, known to host a significant superconducting gap. The latter is useful for various applications such as the exploration of trivial and topological in-gap states. Here, we focus on the simulation's aspects of superconductivity and study the impact of temperature, Cooper-pair coupling and dimensionality on the value of the superconducting pairing interactions and gaps.

Time-domain thermoreflectance measurement of the thermal diffusivity of Nb thin films

Nb-coated Cu superconducting radiofrequency (SRF) cavities are of interest as a possible alternative to bulk Nb cavities for particle accelerators due to the high thermal conductivity of Cu and its lower cost than Nb. Studying the thermal diffusion properties in Nb thin films on Cu is important for developing Nb-coated Cu SRF cavities. In the present study, the thermal diffusivity of 100‒800 nm Nb films on Cu is measured by time-domain thermoreflectance (TDTR). The thermal diffusivity is obtained from a curve fit of the TDTR signal to a one-dimensional Fourier heat diffusion model over a time extending from 25 to 985 ps after laser-surface interaction. The film was examined by atomic force microscopy and scanning electron microscopy. The grain size and thermal diffusivity increase with film thickness. The thermal diffusivity reaches a value similar to that reported for bulk Nb for the 400 and 800 nm films, for which the average grain size is 47 ± 13 nm and 68 ± 18 nm, respectively. TDTR measurement for the 400-nm film taken over the temperature range of 293 to 40 K showed that the thermal diffusivity increases as the temperature is reduced. The results indicate that grain boundaries reduce the thermal diffusivity of the Nb film and, therefore, must be considered in Nb-coated Cu SRF cavities.

Magnetization dynamics and spin-glass-like origins of exchange-bias in Fe–B–Nb thin films

The phenomenon of exchange bias has been extensively studied within crystalline materials, encompassing a broad spectrum from nanoparticles to thin-film systems. Nonetheless, exchange bias in amorphous alloys has remained a relatively unexplored domain, primarily owing to their inherently uniform disordered atomic structure and lacking grain boundaries. In this study, we present a unique instance of exchange bias observed in Fe–B–Nb amorphous thin films, offering insights into its origins intertwined with the system's spin-glass-like behavior at lower temperatures. The quantification of exchange bias was accomplished through a meticulous analysis of magnetic reversal behaviors in the liquid-helium temperature range, employing a zero-field cooling approach from various initial remanent magnetization states (±MR). At reduced temperatures, the appearance of asymmetric hysteresis, a hallmark of negative exchange bias, undergoes a transformation into symmetric hysteresis loops at elevated temperatures, underscoring the intimate connection between exchange-bias and dynamic magnetic states. Further investigations into the magnetic thermal evolution under varying probe fields reveal the system's transition into a spin-glass-like state at low temperatures. We attribute the origin of this unconventional exchange bias to the intricate exchange interactions within the spin-glass-like regions that manifest at the interfaces among highly disordered Fe-nuclei. The formation of Fe-nuclei agglomerates at the sub-nanometer scale is attributed to the alloy's limited glass-forming ability and the nature of the thin-film fabrication process. We propose that this distinctive form of exchange bias represents a novel characteristic of amorphous thin films.

Transport properties of 2H-NbSe2 synthesized by selenization of Nb thin films

A novel method for the synthesis of 2H-NbSe2 thin films by selenization of precursor Nb thin films is reported. The polycrystalline films grow predominantly in the hexagonal 2H-NbSe2 phase with bulk lattice constants. Their remarkable microstructure consists of a three-dimensional network of flake-like grains substantially stacked vertically on the substrate. The electronic transport between 1.2 K and 300 K in zero and applied magnetic fields up to 14 T has been extensively studied. The study comprises resistivity, magnetoresistance, Hall coefficient, upper critical field, and critical current density. The results are discussed taking account of the coexisting charge-density-wave and superconducting phases.

Laser nanostructured metasurfaces in Nb superconducting thin films

Superconducting Nb thin films have been nanostructured by means of a femtosecond UV laser. Laser induced periodic surface structures (LIPSS) with lateral modulation of ≈250nm and depth smaller than amplitude (≲20nm from crest to trough) are obtained for optimized laser scanning conditions over the film surface, i.e. power, frequency, scanning speed and polarization. This provides a fast and scalable procedure of surface control at the nanoscale. In thin films, control over the kinetics of the LIPSS formation process has been crucial. Untreated and laser-patterned samples have been characterized by electron and atomic force microscopy as well as by local and global magnetometry. The superconducting properties reveal anisotropic behavior in accordance with the observed topography. The imprinted LIPSS define channels for anisotropic current flow and flux penetration. At low temperatures, magnetic flux avalanches are promoted by the increased critical current density, though flux tends to be channeled along the LIPSS. In general, directional flux penetration is observed, being a useful feature in fluxonic devices. Scalability allows us to pattern areas of the order of cm2/min.

Effect of Nb and V doped elements on the mechanical and tribological properties of CrYN coatings

One of the most promising approaches to enhancing the tribological properties of engineering coatings is to add transition elements to the structure. In this study, Nb-doped CrYN and V-doped CrYN thin films were deposited by pulsed DC reactive sputtering in a closed-field unbalanced magnetron sputtering (CFUBMS) system. The deposition parameters examined were target current (1, 1.5 and 2 A), deposition pressure (0.15, 0.25 and 0.35 Pa), pulse frequency (100, 200 and 350 kHz) and duty cycle (85 %, 70 % and 50 %). A Taguchi L9 orthogonal design was used to define the deposition process parameters for each doped film. The Nb and V-doped CrYN thin films were characterized in terms of their microstructure, thickness, composition, hardness and tribological properties by X-ray diffraction (XRD), scanning electron microscopy (SEM), Energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), nanohardness and pin-on-disc testing, respectively. The bond strength between the substrate and the films (adhesion) was analyzed by scratch testing. For the Nb-doped thin films, a maximum hardness value of 21.4 GPa and the lowest friction coefficient of 0.36 were obtained. On the other hand, in the V-doped thin films, the maximum hardness value was 16.1 GPa, while the lowest friction coefficient obtained was 0.11. In addition, Nb-doped and V-doped CrYN thin films exhibited extraordinary adhesion properties. The effect of the selected deposition parameters (target current, pulse frequency, and duty cycle) in relation to the film thickness, hardness, and coefficient of friction properties of the Nb and V-doped CrYN thin films were investigated using the Taguchi approach and optimum operating conditions were identified and confirmed.

Determination on the Metastable Interface Structure of Nb/MgO(100) Interface

The metastable interface structures between Nb thin films and MgO(100) substrate are investigated using first‐principles calculation. Based on the calculation results of interface separation works for 25 interface models, three metastable interfaces and one stable interface are identified. The growth orientations of Nb films in both the metastable and stable interfaces are found to be Nb[110] and Nb[100], respectively, which align with previous experimental findings. This suggests that it is feasible to predict the metastable interfaces in metal/oxide interface systems by calculating interface energy. Additionally, a comparison is conducted between the stable and metastable interfaces. The metastable interface exhibits a lower interface separation work and lower system energy compared to the stable interface. Herein, it is indicated that the interface separation work plays a key role in determining the metastable interface structure of Nb/MgO(100), rather than the total energy of the entire interface system.

Effect of surface characteristics of sputtered titanium and niobium thin alloy films on cell growth and adhesion

Titanium (Ti)–niobium (Nb) thin alloy films were prepared by on-chip sputtering. Their surface properties were characterized, and their cell growth and adhesion properties were investigated. Protrusions were observed on the surface of a thin film made of Ti mixed with a small amount of Nb; these protrusions were smaller and more numerous than those on Ti and Nb thin films. Addition of a small amount of Nb enhanced cell adhesion and cell growth compared to those of Ti or Nb thin films. In addition, cell growth and adhesion decreased with an increase in Nb content. Therefore, the stable morphology, negative zeta potential and suitable protrusion size of these Ti–Nb thin alloy films enhance cell adhesion and cell growth on their surfaces.

Se-doped Nb2O5–Al2O3 composite-ceramic nanoarrays via the anodizing of Al/Nb bilayer in selenic acid

Novel arrays of Nb2O5-based ceramic nanostructures of various sizes (9–210 nm) and morphologies (dots, goblets, rods) aligned on substrates are fabricated via the anodizing of a thin Nb film through the initially formed porous anodic alumina (PAA) film in 1.5 M selenic acid (H2SeO4) – a new aqueous electrolyte generating extraordinarily thinner PAA pores than any other solutions. Accordingly, the nanostructures formed in the selenic acid are 1.3-fold thinner and better self-ordered than their counterparts formed from the same Al/Nb precursor bilayer in a reference oxalic-acid electrolyte. The nanostructures have a dual (core/shell) composition: the inner material (the core) is stoichiometric Nb2O5, whereas the outer layer (the shell) is a few nm-thick substoichiometric NbOx mixed with Al2O3. The composite-ceramic nanoarrays grow doped with selenium species such as selenate (SeO42−) and selenide (Se2−) anions originating from the electrolyte and migrating inward under the high electric field. The incorporated Se species do not contribute to photoluminescence emission nor hinder the Raman signal from the nanoarrays, which makes them highly promising as Nb2O5-based SERS biosensing substrates. The planar PAA-inbuilt Se-doped Nb2O5–Al2O3 nanostructured ceramic film performs like a high-k low-loss low-leakage-current dielectric promising for on-chip integration. More potential applications of the Se-doped ceramic nanoarrays developed here include biomedical antibacterial coatings, advanced superhydrophobic surfaces, gas-sensing, and catalytic layers.

Methodology to Characterize Thermal Properties of Thin Film Superconductors Using a DynaCool Physical Property Measurement System

We describe a methodology to characterize heat capacity <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C<inline-formula><tex-math notation="LaTeX">$_{p}$</tex-math></inline-formula></b> of 1 μm thick Nb thin films at a temperature range of 60 K to 1.8 K. We used a commercially available relaxation calorimeter device by Quantum Design DynaCool Physical Property Measurement System (PPMS). The mass of thin films is negligible compared to the minimum coarse estimate required by the heat capacity module of the PPMS. To overcome this limitation, a vertical stack of multiple diced substrates of the same material with uniform dimensions of 2.5 mm x 2.5 mm was joined by cryogenically compatible grease to form a stack resulting in a larger effective mass of the target film. The critical behavior of heat capacity in the region of the transition temperature is demonstrated. The heat capacity of Nb thin films was measured by subtracting the total heat capacity of the joining grease and stack of substrate used as addenda. Results in this work prove that a vertical stacking approach is viable for measuring the behavior of deposited thin films.

Numerical Calculation of Magnetic Field Enhancement and Impact of Surface Defects on Premature Entry of Magnetic Field in Thin Nb Films for SRF Cavities

In this work, we have investigated two series of Nb/Cu samples deposited by HiPIMS technique, and differing in Nb deposition conditions, Nb film thickness and Cu substrate polishing techniques. All the films were additionally irradiated by Nd:YAG laser to smooth their surfaces. The impact of the magnetic field enhancement at the surface defects on the premature start of magnetic field penetration into the superconducting film was studied, combining experiments and numerical calculations. Compared to previous study, improved numerical calculations by using the Finite Element Method (FEM) served to calculate the maximum field enhancement factor β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> , reflecting impact of the most crucial surface defects found in the samples. Magnetization measurements at 4.2 K in DC magnetic field, oriented parallel to the film, were employed to determine the start of the field penetration <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">en</sub> . The SEM and AFM analyses served to investigate the Nb surface morphology. In some samples, deviations from the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">en</sub> (β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> ) dependence were observed. It was found that the magnetic field penetration could start from the Nb/Cu interface rather than from the free Nb surface due to visibly better quality of the free Nb surface observed by SEM analysis, and that could lead to the deterioration of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">en</sub> (β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> ) dependence.