Epitaxial Niobium Research Articles

Epitaxial growth and polarized Raman scattering of niobium dioxide films

We report the structural, electrical, and optical characterization of epitaxial niobium dioxide (NbO2) films fabricated on MgF2(001) substrates. The films were almost stoichiometric, had an indirect bandgap of 0.7 eV, and exhibited a phase transition at ∼1080 K. A polarized Raman scattering study of the films was conducted to investigate the Raman symmetry in the low-temperature phase. Based on the angular-dependent polarized Raman spectra, we assigned 13 modes to Ag symmetry and 14 to Bg symmetry. We also evaluated the Raman tensor elements of the Bg modes and found that the off-diagonal elements were nearly zero in most of the Bg modes, except for a phonon mode at 267 cm−1. This study aids understanding of the lattice dynamics of NbO2, which plays a critical role in the phase transition.

High-resolution x-ray scattering from epitaxial thin films of Y/Nb on Al2O3

During the 1990s, Roger Cowley had a strong interest in the crystal and magnetic structures of rare-earth superlattices as a means to understand the rich and exotic magnetic properties of the rare-earth metals. High-quality samples can be grown by molecular beam epitaxy on sapphire substrates by first depositing a thin epitaxial layer of niobium, then a layer of yttrium or lutetium as a seed. High-resolution x-ray scattering is an excellent probe to characterise the crystal quality and was used to study the structure of the niobium layer. However, relatively little attention was paid to the seed layer. This article summarises some of the x-ray experiments performed by the Cowley group to study the structure of epitaxial niobium on sapphire, and extends the work to report some results on the structure of thin yttrium seed layers. The structure of the yttrium films is shown to have a strong dependence on the thickness of the niobium buffer, with the buffer needing to be thicker than a critical value of ∼80 for the formation of misfit dislocations at the Nb/Al2O3 interface before highly coherent Y films can be grown. Yttrium films grown on Nb buffers thinner than ∼500 show a similar two-peak line shape in scans through their specular Bragg peaks to that seen in the specular Nb Bragg peaks, with a resolution-limited feature on a broader diffuse peak. The resolution-limited feature depends on the thickness of the yttrium film, becoming weaker and having a stronger decay with increasing as the film thickness increases, while the width of the yttrium broad peak evolves as the square root of the width of the niobium Bragg peak. The data are discussed within the context of theories describing the scattering from films with misfit dislocations.

Optical and Tunneling Studies of Energy Gap in Superconducting Niobium Nitride Films

We have prepared an epitaxial niobium nitride (NbN) film and NbN/AlN/NbN tunnel junctions to investigate the energy gaps. By measuring the optical conductivity spectrum of a 41-nm-thick film with the critical temperature (TC) of about 14 K by terahertz time-domain spectroscopy, we obtained the gap frequency of about 1.2 THz. An ill-defined gap was suggested due to the broadening of the onset of absorption as the temperature increases. On the other hand, the current–voltage curve measurement of the tunnel junctions showed the current rise at the gap voltage of 5.6 mV (corresponding to 1.4 THz) with a smeared shape as the temperature increases. We found that both gap broadenings can be explained by introducing a temperature-dependent imaginary energy gap part into the superconducting energy gap, corresponding to a finite quasi-particle lifetime in the NbN films.

Influence of different sputtering conditions on superconductivity of ultra-thin epitaxial niobium nitride film

Superconductors are used in military and commercial applications due to the full diamagnetic and zero resistance properties. Hereinto, niobium nitride (NbN) has been extensively studied for its excellent superconducting properties, such as high transition temperature ( T c), large superconducting energy gap, narrow transition width and good stability. Superconducting NbN films are widely used in low temperature superconducting devices, for instance, hot electron bolometer, superconducting quantum interference devices and rapid single flux quantum. Especially, in 2001 Gol’tsman has successfully detected a single photon response of 810 nm near-infrared using an ultra-thin superconducting NbN, which proved that the high-quality growth of ultra-thin NbN film was the key step for high resolution and sensitivity of the superconducting detector. Up to now, atomic layer deposition (ALD), molecular beam epitaxy (MBE), pulsed laser deposition (PLD) and magnetron sputtering have been successfully used to grow NbN thin films. However, T c is decreasing gradually as the reduced of thickness. Therefore, there is still a challenge to grow ultra-thin superconducting NbN film with high performance. In this work, 5 nm thickness of superconducting epitaxial NbN film has been grown on MgO substrate by magnetron sputtering. We systematically study the effect of different sputtering parameters on the superconductivity properties of the NbN film. The experiment results show that the high vacuum, high sputtering temperature and low working pressure are positively correlated with the superconducting transition temperature of NbN film under the premise of suitable argon-nitrogen ratio or magnetron power. The analyses of physical property measurement system (PPMS) indicate that as-grown 5 nm-thick NbN film has high performance with a superconducting transition temperature as high as 12.5 K under the optimum condition. Morphologic characterizations show that the ultra-thin NbN films have high density and smoothness with 0.116 nm of the surface roughness. Microstructural measurements also illustrate the epitaxial growth of NbN on MgO substrate. This work demonstrates the ultra-thin NbN films can be well controllably grown with high quality and offers a platform to fabricate the ultra-thin superconducting devices in the coming future.

Tayloring surface morphologies and stress states of thin niobium epitaxial films on sapphire substrates

Studying size effects on the modification of thin film physical properties requires the preparation of films with desired microstructures and stress states. For the model system of epitaxial niobium thin films on sapphire [112¯0, miscut < 0.1°] substrates surface topographies and stress states were systematically studied depending on the films' thickness (5 nm – 100 nm) and the deposition temperature (470 °C to 870 °C). Initial stresses, domain structures and textures of the films were characterized using X-ray diffraction techniques. The films' surface topographies were measured by scanning tunneling microscopy and atomic force microscopy, and classified into three different regimes: smooth (2D), rough (3D) and pitted (or ‘meander-like’). Determination of the regimes' transition temperatures enables the tuning of the films microstructures and related stress states for each film thickness.

In Situ Electronic Structure Study of Epitaxial Niobium Thin Films by Angle-Resolved Photoemission Spectroscopy**Supported by the National Key Research and Development Program of China under Grant No 2016YFA0300204, the National Natural Science Foundation of China under Grant Nos 11274332, 11574337, 11404360 and 11227902, the Natural Science Foundation of Shanghai under Grant No 14ZR1447600, the Strategic Priority Research Program (B) of

High-quality single crystalline niobium films are grown on a-plane sapphire in molecular beam epitaxy. The film is single crystalline with a (110) orientation, and both the rocking curve and the reflection high-energy electron diffraction pattern demonstrate its high-quality with an atomically smooth surface. By in situ study of its electronic structure, a rather weak electron-electron correlation effect is demonstrated experimentally in this 4d transition metal. Moreover, a kink structure is observed in the electronic structure, which may result from electron-phonon interaction and it might contribute to the superconductivity. Our results help to understand the properties of niobium deeply.

Pinning effects on flux flow instability in epitaxial Nb thin films

The flux flow properties of epitaxial niobium films with different pinning strengths are investigated by dc electrical resistance measurements and mapped to results derived within the framework of a theoretical model. The cases of weak random pinning in as-grown films, strong random pinning in Ga ion-irradiated films, and strong periodic pinning induced by a nanogroove array milled by a focused ion beam are investigated. The generic feature of the current–voltage curves of the films consists of instability jumps to the normal state at some instability current density j* as the vortex lattice reaches its critical velocity v*. While monotonically decreases for as-grown films, the irradiated films exhibit a non-monotonic dependence attaining a maximum in the low-field range. In the case of nanopatterned films, this broad maximum is accompanied by a much sharper maximum in both and , which we attribute to the commensurability effect when the spacing between the vortex rows coincides with the location of the grooves. We argue that the observed behavior of can be explained by the pinning effect on the vortex flow instability and support our claims by fitting the experimental data to theoretical expressions derived within a model accounting for the field dependence of the depinning current density.

Achieving reversibility of ultra-high mechanical stress by hydrogen loading of thin films

Nano-materials are commonly stabilized by supports to maintain their desired shape and size. When these nano-materials take up interstitial atoms, this attachment to the support induces mechanical stresses. These stresses can be high when the support is rigid. High stress in the nano-material is typically released by delamination from the support or by the generation of defects, e.g., dislocations. As high mechanical stress can be beneficial for tuning the nano-materials properties, it is of general interest to deduce how real high mechanical stress can be gained. Here, we show that below a threshold nano-material size, dislocation formation can be completely suppressed and, when delamination is inhibited, even the ultrahigh stress values of the linear elastic limit can be reached. Specifically, for hydrogen solved in epitaxial niobium films on sapphire substrate supports a threshold film thickness of 6 nm was found and mechanical stress of up to (−10 ± 1) GPa was reached. This finding is of basic interest for hydrogen energy applications, as the hydride stability in metals itself is affected by mechanical stress. Thus, tuning of the mechanical stress-state in nano-materials may lead to improved storage properties of nano-sized materials.

Epitaxial niobium dioxide thin films by reactive-biased target ion beam deposition

Epitaxial NbO2 thin films were synthesized on Al2O3 (0001) substrates via reactive bias target ion beam deposition. X-ray diffraction and Raman spectra were used to confirm the tetragonal phase of pure NbO2. Through XPS, it was found that there was a ∼1.3 nm thick Nb2O5 layer on the surface and the bulk of the thin film was NbO2. The epitaxial relationship between the NbO2 film and the substrate was determined. Electrical transport measurement was measured up to 400 K, and the conduction mechanism was discussed.

Characterization of two different orientations of epitaxial niobium thin films grown on MgO(001) surfaces

Epitaxial Nb thin films deposited onto the same crystalline insulating surface can evolve in very different fashions depending on specific deposition conditions, thereby affecting their microstructure, surface morphology and superconducting properties. Here, we examine and compare the microstructure and ensuing surface morphology from two distinct Nb/MgO series each with its own epitaxial registry—namely Nb(001)/MgO(001) and Nb(110)/MgO(001)—leading to distinct surface anisotropy and we closely examine the dynamical scaling of the surface features during growth. We compare our findings with those in other metal/MgO epitaxial systems and for the first time, general scaling formalism is applied to analyze anisotropic surfaces exhibiting biaxial symmetry. Further, Power Spectral Density is applied to the specific problem of thin film growth and surface evolution to qualify the set of deposition conditions leading to smoother surfaces. We find good correlation between the surface morphology and microstructure of the various Nb films with superconducting properties such as their residual resistance ratio and lower critical field.

Strain Effects on the Crystal Growth and Superconducting Properties of Epitaxial Niobium Ultrathin Films

Superconducting ultrathin films grown epitaxially onto crystalline substrates exhibit strained epitaxial growth due to lattice mismatch, which can have a significant effect on their superconducting properties. We present a complete correlation of the surface morphology, crystal growth, strain, microstructure, and superconducting properties in single-crystal Nb(110) thin films sputter deposited on a-plane sapphire substrates. Notably, we observe that the lattice mismatch between Nb and sapphire induces the formation of a hexagonal surface structure during the first three atomic layers. This is followed by a strained bcc Nb(110) phase whose in-plane lattice parameter progressively relaxes to bulk value. Similar lattice relaxation was also observed in the direction perpendicular to the interface using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Significant perpendicular strain in films up to 30 nm thick was found to ultimately affect the superconducting properties of the Nb thin films...

Hydrogen storage in epitaxial films: a finite element study

In epitaxial niobium (Nb) films on sapphire substrates, there is an enhancement in the solubility of hydrogen (at a given pressure and temperature) due to biaxial tensile stresses. These stresses are partly relieved on growth of the film beyond critical thickness, leading to a reduction in the amount of excess hydrogen that can be stored in the film. Additionally, a depletion of hydrogen from the compressive regions of interfacial misfit dislocations further reduces the hydrogen storage capacity of the film. In this work, a finite-element methodology developed earlier is extended and further utilized to get a spatial map of hydrogen before and after critical thickness is exceeded. Critical thickness for “Nb(111)” ||sapphire(0001) (finite) system is calculated using a three-dimensional numerical model. With the aid of standard theoretical equations, the spatial variation of the enrichment of hydrogen is determined using the stress fields computed from the finite-element model. This calculation is performed in a system with and without an interfacial misfit dislocation, after the critical thickness is exceeded, to evaluate the reduction in the storage capacity of hydrogen in the niobium film.

Twin symmetry texture of energetically condensed niobium thin films on sapphire substrate (a-plane Al2O3)

An energetic condensation technique, cathodic arc discharge deposition, is used to grow epitaxial Niobium (Nb) thin films on a-plane sapphire (hexagonal-closed-packed Al2O3) at moderate substrate heating temperature (&amp;lt;400 °C). The epitaxial Nb(110)/Al2O3(1,1,-2,0) thin films reached a maximum residual resistance ratio (RRR) value 214, despite using a reactor-grade Nb cathode source whose RRR was only 30. The measurements suggest that the film’s density of impurities and structural defects are lower when compared to Nb films produced by other techniques, such as magnetron sputtering, e-beam evaporation or molecular-beam-epitaxy. At lower substrate temperature, textured polycrystalline Nb thin films were created, and the films might have twin symmetry grains with {110} orientations in-plane. The texture was revealed by x-ray diffraction pole figures. The twin symmetry might be caused by a combination effect of the Nb/Al2O3 three-dimensional epitaxial relationship (“3D-Registry” Claassen’s nomenclature) and the “Volmer-Weber” (Island) growth model. However, pole figures obtained by electron backscattering diffraction (EBSD) found no twin symmetry on the thin films’ topmost surface (∼50 nm in depth). The EBSD pole figures showed only one Nb{110} crystal plane orientation. A possible mechanism is suggested to explain the differences between the bulk (XRD) and surface (EBSD) pole figures.

Quantitative assessment of pinning forces and magnetic penetration depth in NbN thin films from complementary magnetic force microscopy and transport measurements

Epitaxial niobium nitride thin films with a critical temperature of ${T}_{\mathrm{c}}=16$ K and a thickness of 100 nm were fabricated on MgO (100) substrates by pulsed laser deposition. Low-temperature magnetic-force-microscopy (MFM) images of the supercurrent vortices were measured after field cooling in a magnetic field of 3 mT at various temperatures. The temperature dependence of the penetration depth has been evaluated by a two-dimensional fitting of the vortex profiles in the monopole-monopole model. Its subsequent fit to a single $s$-wave-gap function results in the superconducting gap amplitude, $\ensuremath{\Delta}(0)=(2.9\ifmmode\pm\else\textpm\fi{}0.4)$ meV $=(2.1\ifmmode\pm\else\textpm\fi{}0.3){k}_{\mathrm{B}}{T}_{\mathrm{c}}$, which perfectly agrees with the previous reports. The pinning force has been independently estimated from the local depinning of individual vortices by the lateral forces exerted by the MFM tip and from transport measurements. A good quantitative agreement between the two techniques shows that for low fields, $B\ensuremath{\ll}{\ensuremath{\mu}}_{0}{H}_{\text{c2}}$, MFM is a powerful and reliable technique to probe the local variations of the pinning landscape. We also demonstrate that the monopole model can be successfully applied even for thin films with a thickness comparable to the penetration depth.

Effect of low-energy ion bombardment on the crystal structure and superconductivity of niobium films

The effect of ion bombardment on the growth of Nb films and their crystal structure is investigated. Epitaxial niobium films with the (001) orientation are grown on (01i2) Al2O3 substrates heated to 600°C and biased at −20 V. Niobium films with pronounced axial texture in the [110] direction are grown on water-cooled Sitall (devitrified glass such as Pyroceram) substrates. In Nb films biased at −50 V, which are in the superconducting state, the motion of individual magnetic vortices is observed with a magnetooptic indicator.

MBE growth of (1 1 0) refractory metals on a -plane sapphire

The initial stages of growth of epitaxial (1 1 0) niobium, tantalum, and molybdenum films on (1 1 2 0) sapphire by molecular beam epitaxy, MBE, are analyzed in situ by reflection high-energy electron diffraction (RHEED), and ex situ by high-resolution electron microscopy (HREM) and X-ray diffraction (XRD) techniques. The RHEED analysis shows that both Nb and Ta initially deposit (for approximately the first two monolayers in the case of Nb and the first four monolayers for Ta) with an hexagonal surface symmetry that is not consistent with the normal bcc structure of these metals, in agreement with the previous result of Oderno et al. [1] for Nb on sapphire. On further deposition, the films are observed to relax into the normal bcc (1 1 0) structure. Intensity oscillations of the RHEED specular beam are observed during both stages of growth. The RHEED oscillations are damped out after approximately 20 monolayers of Nb (≈30 monolayers of Ta) as a steady-state surface roughness is reached. HREM analysis reveals that the initial hexagonal structure has transformed completely to the normal bcc structure in the thicker films. The epitaxial relationships at each stage of growth are identified and a model is suggested for the development of the initial hexagonal structure based on Nb–O bonding at the interface. As the thickness of the bcc film increases, strain relief is observed by the formation of misfit dislocations. The growth mode of Mo is found to be different from that of Nb and Ta. The initial deposit grows in a three-dimensional mode with poor atomic order; the RHEED patterns are not sufficiently distinct to identify whether the hexagonal structure is formed. However, the surface becomes progressively smoother as growth proceeds and thicker films of Mo are comparable in crystalline quality, as measured by X-ray rocking curves, to Nb and Ta. The different growth mode of Mo is attributed to greater mismatch with the sapphire substrate and possibly different M–O ionic bonding at the substrate interface.

Kinetics of hydrogen absorption and desorption in epitaxial thin niobium films

We have investigated the static and kinetic properties upon hydrogen absorption desorption in epitaxial niobium films by in situ and time-resolved resistivity measurements and compare equilibrium values with respective lattice-parameter changes. We find for the excess resistivity a strong deviation from Nordheim's rule, which is usually applied for describing the resistance change as a function of hydrogen concentration. Instead of a maximum for half of the saturation concentration, we observe a shift of the maximum to higher hydrogen concentrations. Furthermore, we find that the relaxation times for reaching an equilibrium concentration when charging or decharging Nb films with hydrogen exhibit three different regimes with characteristically different time scales. For low hydrogen pressures (concentrations) the relaxation time at a temperature of 473 K is on the order of ${10}^{3} \mathrm{s},$ for an intermediate hydrogen pressure (concentration) the relaxation time increases by more than an order of magnitude, and beyond 100 mbars it drops to less than 10 s.

Optical and near-infrared photon counting detector using superconducting tunnel junctions

The properties of Superconducting Tunnel Junctions (STJ) make them very suitable for low light level astronomical observations. The applications are of great importance for the future in the field of high-angular resolution astronomy where STJ surpass all other detectors. STJ detectors will be used to improve the sensitivity of such a system to be implemented on ground-based interferometer like the GI2T in France or IOTA in USA. All this instrumentation is developed in the framework of the Very Large Telescopes Interferometer built by the European Southern Observatory in Chile. We have developed a modified version of the “SNEP” fabrication process for making Nb/Al-AlO x -Al/Nb and Ta/Al-AlO x -Al/Ta/Nb STJs, showing very low sub-gap leakage current at 0.2 K by using double thin aluminium trapping layers and the higher energy gap of the Ta or Nb absorber grown epitaxially on the R-plane sapphire substrate. The key points of the junction fabrication process have been studied: experimental parameters suitable to grow high quality, (2 0 0) oriented epitaxial niobium films with ρ 300 K /ρ 10 K >20 by DC-magnetron sputtering on 3 in.-diameter R-plane sapphire substrates heated at 600°C and AlO x barrier oxidation conditions. The influence of such processing parameters and trapping layer thickness have been analysed on the [ I– V] curves measured down to 0.2 K.

Atomic Structure of the Interface between Epitaxial Niobium Films and α-Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Substrates

Atomic Structure of the Interface between Epitaxial Niobium Films and α-Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Substrates

Structural and electrical properties of laser ablated epitaxial niobium doped lead-zirconium-titanate thin films

Epitaxial Niobium doped PbZr 0.52Ti 0.48(PNZT) YBa 2Cu 3O 7−x thin film heterostructures consisting of ferroelectric PNZT films with thicknesses over the range d = 1.05μm to 0.02μm have been successfully fabricated in situ on single crystal LaAlO 3(100) using a Nd:YAG pulsed laser depositiontechnique. Characterization by x-ray diffraction techniques including θ–2θ scan, rocking curves and oblique ϕ-scan reveals the epitaxial quality of PNZT thin films down to a thickness of 60 nm. Thickness dependence of ferroelectric and dielectric properties of the PNZT films show that the coercive field E c has a d − 2 3 dependence that is in good agreement with a domain nucleation model. For films in the thickness range d = 1.05μm to 0.06μm the remanent polarization is found to be independent of film thickness. On the other hand the dielectric permittivity is found to decrease monotonically with decreasing film thickness.