Niobium Oxide Films Research Articles

Electrical switching in metal-oxide-metal structures based on anodic niobium oxide

Nonlinear current-voltage (I–V) characteristics with S-type negative differential resistance, as a characteristic feature of the switching effect, are inherent in a variety of transition metal oxides, niobium oxide included. Although this phenomenon has been known for a long time, recent proposals to use oxide-based switching devices as elements of oscillatory neural networks have resumed the interest in this area. In this work, electrical switching in sandwich structures based on anodic films of niobium oxide is studied. After being electroformed, these structures exhibit S-shaped I–V characteristics. As the temperature increases, the threshold voltage decreases, presumably tending to zero at a critical temperature T0, which correlates with the temperature of metal–insulator phase transition of niobium dioxide. Channels consisting of NbO2 are formed in the initial anodic films during the process of electroforming.

Niobium Oxide Films Deposited by Reactive Sputtering: Effect of Oxygen Flow Rate.

Reactive sputtering is a versatile technique used to form compact films with excellent homogeneity. In addition, it allows easy control over deposition parameters such as gas flow rate that results in changes on composition and thus in the film required properties. In this report, reactive sputtering is used to deposit niobium oxide films. A niobium target is used as metal source and different oxygen flow rates to deposit niobium oxide films. The oxygen flow rate was changed from 3 to 10 sccm. The films deposited under low oxygen flow rates show higher electrical conductivity and provide better perovskite solar cells when used as electron transport layer.

Niobium Oxide Films Deposited by Reactive Sputtering: Effect of Oxygen Flow Rate

Niobium Oxide Films Deposited by Reactive Sputtering: Effect of Oxygen Flow Rate

Effects of preparation conditions on the optical properties of niobium oxide thin films

Major deposition parameters, including oxygen partial pressure and substrate temperature maintained during film deposition, affect the optical properties of electron beam evaporated niobium oxide films. All the films deposited were found to be amorphous as revealed by the X-ray diffraction study. A method requiring measurements at normal incidence of transmission from two films of different thicknesses prepared under identical conditions was used to determine the optical constants. Substantial changes in the optical constants and optical bandgap energy were observed following changes in the preparation conditions. These variations are related to the corresponding compositional changes, as observed by Rutherford Backscattering Spectroscopy, in the atomic ratio of oxygen to niobium in the films prepared under different conditions.

Nitrogen Incorporation Induced Soft-to-Hard Transition Observed in the Mechanical Properties of Amorphous Niobium Oxide Films

The article covers the nitrogen incorporation induced transformation of mechanical behavior of the amorphous Nb-oxide films. Niobium oxynitride (Nb-O-N) films were sputter-deposited using a metallic Nb target in the presence of oxygen and nitrogen. The nitrogen concentration in the Nb-O-N films was varied by adjusting the nitrogen gas flow rate from 0 sccm to 10 sccm while keeping total gas flow (nitrogen + oxygen + argon) constant at 30 sccm. The surface and interface chemical characterization of the samples indicate that, with increasing the nitrogen content, the corresponding mechanical characteristics, namely, hardness (H) and elastic modulus (E), increase from ~ 5 GPa to 15 GPa and 115 GPa to 135 GPa, respectively. The trend observed in H and E values correlates with Nb-O and Nb-N bond formation in Nb-O-N as evidenced in chemical analyses made using x-ray photoelectron spectroscopy. The chemical composition measurements indicate that nitrogen incorporation proceeds with a decrease in relative oxygen content in Nb-O-N films, where the ability to withstand mechanical deformation is enhanced. A correlation between the processing conditions, nitrogen content, and physical/mechanical properties in Nb-O-N films is established.

High Surface Area to Volume Ratio 3D Nanoporous Nb2O5 for Enhanced Humidity Sensing

Sensors based on metal oxide platforms offer ease of device fabrication and simple sensing operation. As a metal oxide platform, highly nanoporous niobium oxide (Nb2O5) films consisting of unique three-dimensional vein-like structures can be efficiently used for developing humidity sensors. In this work, nanoporous Nb2O5 films (with different thicknesses of ∼ 1 μm, ∼ 2 μm, and ∼ 4 μm) were prepared by anodization of niobium foil for 30 min, 60 min, and 120 min. Electron, x-ray, atomic, and vibrational microscopies and spectroscopies were used for characterizing the morphological and structural properties of the Nb2O5 films. The analysis revealed that the nanoporous Nb2O5 exhibited hierarchical vein-like structures with orthorhombic crystalline orientation, and their surface roughness showed a proportional increase with the anodization duration. Metal–semiconductor–metal humidity sensors based on nanoporous Nb2O5 with platinum electrodes were tested in a humidity chamber under conditions of 40% to 90% relative humidity (RH) and different bias voltages. According to the obtained results, the ∼ 4-μm-thick nanoporous Nb2O5 presented the highest relative sensitivity of 216.5 under a bias voltage of 5 V, taking advantage of its extremely porous structure. These sensors provide high surface area to volume ratio, leading to highly effective affinity and interactions between surface-active sites and water molecules.

Exploring the Properties of Niobium Oxide Films for Electron Transport Layers in Perovskite Solar Cells.

In this work, niobium oxide films were deposited by reactive magnetron sputtering under different oxygen flow rate and applied as electron transport layer in perovskite solar cells. It was found that the deposition made using 3.5 sccm of oxygen flow resulted in films with better electrical properties which helped the extraction of the photogenerated electrons to the external circuit, improving the Jsc and consequently the device efficiency. In addition, by photoluminescence measurements, we found a better charge transfer from perovskite to TiO2/niobium oxide film deposited at 3.5 sccm of oxygen flow.

Low temperature solution deposited niobium oxide films as efficient electron transport layer for planar perovskite solar cell

In high efficient planar perovskite solar cells (PSC), electron transportation layer (ETL) plays an important role in its photovoltaic performance. Except for TiO2, Nb2O5 has also been considered as ideal electron selective layer material for planar perovskite solar cell due to its excellent optical transmittance and similar Femi level with TiO2. Herein, we report niobium oxide film deposited by a simple and low temperature solution method, as an effective alternative to high temperature sintered TiO2 film in the regular architecture of PSC. Nb2O5 modification of FTO has successfully tuned the surface work function from 4.59 eV to 4.28 eV without obviously affecting the surface roughness and optical transmittance, which could significantly decrease energy level offset and improve the open voltage. Appropriate UV ozone treatment could adjust the surface hydrophilicity of Nb2O5 film benefiting the growth of perovskite films on low temperature solution deposited Nb2O5 coated FTO substrate. Relatively high efficiency over 17% with less J–V hysteresis has been achieved by optimizing the precursor solution, spin rate, annealing temperature and UV ozone treatment time. The results indicate low temperature solution deposited Nb2O5 film can be a low cost and easy-prepared ETL material for high performance PSC.

Surface chemistry and the corrosion behavior of magnetron sputtered niobium oxide films in sulfuric acid solution

In this work, magnetron sputtered Nb2O5 films were obtained at two different deposition times. The surface chemistry of the deposited layers was investigated by X-ray photoelectron spectroscopy (XPS). The corrosion behavior was assessed by electrochemical impedance spectroscopy, potentiodynamic polarization and potentiostatic polarization tests in 0.5 M H2SO4 + 2 ppm HF solution at room temperature. The Nb5+/Nb4+ ratio decreased with the deposition time, leading to a higher surface activity for the film obtained at 30 min. This result was confirmed by its lower impedance values and higher current densities when compared to the 15′-film. Surface chemistry played a major role on the corrosion behavior of the sputtered films and can be tailored by the deposition time.

Large-size niobium disulfide nanoflakes down to bilayers grown by sulfurization

Atomically thin layers of group VB transition metal dichalcogenides (TMDs) provide a unique platform for studying two-dimensional (2D) superconductivity and charge density waves. Thus far, the bottom-up synthesis of these 2D TMDs has often involved precursors that are corrosive or toxic, and their lateral sizes are typically only a few micrometers. In this paper, we report the growth of NbS2 nanoflakes with a thickness down to bilayers and a lateral dimension up to tens of micrometers without using harsh chemical species. NbS2 nanoflakes either standing or lying with respect to the sapphire substrate were obtained by sulfurization of niobium oxide films that were prepared via pulsed laser deposition. Standing nanoflakes are considered to grow epitaxially on the sapphire substrate according to their ordered orientation, while lying nanoflakes with random orientations were grown directly on top of the niobium oxide films. The Raman spectra of the 3R-phase exhibit strong dependence on the layer thickness, where the A1 mode softens as the layer number decreases. In contrast to the stable bulk NbS2, the ultra-thin nanoflakes were oxidized on their top surfaces after prolonged exposure to air, as revealed by X-ray photoelectron spectroscopy. Our work explores an important route to synthesize large-size NbS2 nanoflakes and studies the oxidation process, which is a critical factor to consider if practical applications should be realized in the future.

Comparison of thermal and plasma-enhanced atomic layer deposition of niobium oxide thin films

Niobium pentoxide was deposited using tBuN=Nb(NEt2)3 as niobium precursor by both thermal atomic layer deposition (ALD) and plasma-enhanced atomic layer deposition (PE-ALD) with H2O and O2 plasma as coreactants, respectively. The deposition temperature was varied between 150 and 350 °C in both ALD processes. Amorphous films were obtained in all cases. Self-limiting saturated growth was confirmed for both ALD processes along with high uniformity over a 200 mm Si wafer. The PE-ALD process enabled a higher growth per cycle (GPC) than the thermal ALD process (0.56 Å vs 0.38 Å at 200 °C, respectively), while the GPC decreases with increasing temperature in both cases. The high purity of the film was confirmed using Rutherford backscattering spectrometry, elastic recoil detection, and x-ray photoelectron spectroscopy, while the latter technique also confirmed the Nb+5 oxidation state of the niobium oxide films. The thermal ALD deposited films were substoichiometric due to the presence of oxygen vacancies (VO), of which a more dominant presence was observed with increasing deposition temperature. The PE-ALD deposited films were found to be near stoichiometric for all investigated deposition temperatures.

Effective passivation of silicon surfaces by ultrathin atomic-layer deposited niobium oxide

This letter reports on effective surface passivation of n-type crystalline silicon by ultrathin niobium oxide (Nb2O5) films prepared by atomic layer deposition (ALD) and subjected to a forming gas anneal at 300 °C. A champion recombination parameter J0 of 20 fA/cm2 and a surface recombination velocity Seff of 4.8 cm/s have been achieved for ultrathin films of 1 nm. The surface pretreatment was found to have a strong impact on the passivation. Good passivation can be achieved on both HF-treated c-Si surfaces and c-Si surfaces with a wet-chemically grown interfacial silicon oxide layer. On HF-treated surfaces, a minimum film thickness of 3 nm is required to achieve a high level of surface passivation, whereas the use of a wet chemically-grown interfacial oxide enables excellent passivation even for Nb2O5 films of only 1 nm. This discrepancy in passivation between both surface types is attributed to differences in the formation and stoichiometry of interfacial silicon oxide, resulting in different levels of chemical passivation. On both surface types, the high level of passivation of ALD Nb2O5 is aided by field-effect passivation originating from a high fixed negative charge density of 1–2 × 1012 cm−3. Furthermore, it is demonstrated that the passivation level provided by 1 nm of Nb2O5 can be further enhanced through light-soaking. Finally, initial explorations show that a low contact resistivity can be obtained using Nb2O5-based contacts. Together, these properties make ALD Nb2O5 a highly interesting building block for high-efficiency c-Si solar cells.

Influence of RF sputtering power on structural and optical properties of Nb2O5 thin films

In this research, Niobium oxide (Nb2O5) films were deposited on glass substrates using RF sputtering technique. The effect of RF sputtering powers (150, 175 and 200 W) on the structural, and optical properties of as-deposited and annealed Nb2O5 thin films were examined using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), UV–vis-NIR double beam spectrophotometer and Photoluminescence (PL) measurements. X-ray diffraction results revealed that the as-deposited and annealed films have polycrystalline structure. The optical parameters such as absorption coefficient, optical band gap energy, refractive index, dielectric constants and oscillators’ parameters are estimated. The dispersion parameters are evaluated and analyzed according to a single oscillator model.

Surface Characterization of NbO Islands Formed on Nb(100) by X-Ray Photoelectron Diffraction

An ordered thin film of niobium oxide (NbO) was formed on the Nb(100) surface by heating the niobium single crystal at 1023 K for 2 min under an oxygen atmosphere. The surface composition was analyzed by X-ray photoelectron spectroscopy (XPS), and the surface structure was characterized by low energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD) generated by synchrotron radiation. Annealing at 1223 K in ultra-high vacuum yielded a reconstructed (3 × 1) LEED pattern. XPS measurements indicated that the surface comprised of metallic and oxidized niobium species. The comparison between experimental and theoretical XPD results indicated that 60% of the Nb(100) surface was covered by two-layered nanoislands of NbO, and 40% had bare patches.

Nanoscale thin films of niobium oxide on platinum surfaces: creating a platform for optimizing material composition and electrochemical stability

A nanoscale thin film of niobium oxide on a platinum substrate was evaluated for its influence on the electronic and chemical properties of the underlying platinum towards the oxygen reduction reaction with applications to proton exchange membrane fuel cells. The nanoscale thin film of niobium oxide was deposited using atomic layer deposition onto the platinum substrate. A film of niobium oxide is a chemically stable and electronically insulating material that can be used to prevent corrosion and electrochemical degradation when layers are several nanometers thick. These layers can be insulating if sufficiently thick and may not be sufficient to protect the platinum from corrosion if too thin. An ∼3 nm thin film of niobium oxide was fabricated on the platinum surface to determine its influence on the electronic and chemical properties at the interface of these materials. The atomic layer deposition process enabled a precise control over the material composition, structure, and layer thickness. The niobium oxide film was evaluated using cyclic voltammetry and electrochemical impedance spectroscopy to evaluate whether a balance could be found between the inhibition of platinum degradation and electronic insulation of the platinum for use in proton exchange membrane fuel cells. The 3 nm thin niobium oxide film was found to be sufficiently thin to permit electronic conductivity while reducing the incidence of platinum dissolution.

XPS study of multilayer multicomponent films

In the paper, we propose an XPS-based quantitative method for depth profile analysis of chemical and phase composition of multi-component and multi-layer samples. The method includes: (1) new method for background subtraction accounting for depth non-uniformity of electron energy losses; (2) new method for photoelectron line decomposition into elementary peaks, which accounts for physical nature of the decomposition parameters; (3) joint solving of both background subtraction and photoelectron line decomposition problems; (4) criterion for assessing of line decomposition accuracy; (5) simple formula for layer thickness extraction for multi-element and multi-layer sample. We apply the developed method for analysis of multilayer niobium oxide and sub-oxide films before and in course of ion milling.

Phase modification and morphological evolution in Nb2O5 thin films and its influence in dye- sensitized solar cells

Thermal energy plays a crucial role on the phase evolution of niobium oxide (Nb2O5) thin films and when employed as a blocking layer these films can manoeuvre charge transfer process in a dye sensitized solar cell (DSSC). Niobium oxide film, prepared by RF magnetron sputtering process, endured phase transitions successively from amorphous to orthorhombic and finally to monoclinic phases when subjected to post-deposition annealing. The co-existence of orthorhombic and monoclinic phases with an interesting surface morphology is perceived at an annealing temperature of 900°C. Nb2O5 blocking layer at the FTO/TiO2 interface strongly influenced the photovoltaic parameters of the DSSC and the blocking layer in the orthorhombic phase is found to be most effective in suppressing charge recombination and delivered a maximum efficiency of 7.33%. The improvement in open circuit voltage can be foreseeable as shifting of the Fermi level towards the conduction band edge of the TiO2 as a result of structural modification of the Nb2O5 blocking layer. The thermal stability of the FTO is also investigated and found that the electrical and optical properties of FTO were remarkably stable up to 600°C and begin to change appreciably from 700°C onwards.

Pulse Reverse Voltage Electropolishing of Superconducting Radio Frequency Niobium Cavities in Aqueous, Low Concentration Sulfuric Acid Electrolytes without Hydrofluoric Acid

Niobium superconducting radio frequency cavities (SRF) are required for the International Linear Collider as well as other high energy physics projects. In order for these cavities to achieve the required particle acceleration gradients, electropolishing is conducted as a final surface finishing operation. Conventional electropolishing of SRF cavities is based on the well-established viscous salt film paradigm [[1]] and utilizes a viscous electrolyte consisting of a mixture of sulfuric acid (95-98%) and hydrofluoric acid (49%) in a 9:1 volume ratio [[2]]. The concentrated is included to establish a thick, viscous boundary layer to result in surface brightening or smoothing [[3]]. The hydrofluoric acid is included to remove the niobium oxide film formed during electropolishing [[4]]. Previously we described efforts directed towards demonstrating an innovative electropolishing process for niobium coupons in low concentration (5%) aqueous sulfuric acid without hydrofluoric acid enabled by pulse reverse voltage waveforms electrolyte [[5],[6]]. The advantages of pulse reverse voltage electropolishing are summarized in in Figure 1. Specifically, the forward (anodic) pulse on-time and peak voltage are tuned to eliminate the need for concentrated sulfuric acid; the off-time is adjusted to dissipate heat, and the reverse (cathodic) pulse on-time and peak voltage are tuned to eliminate the need for hydrofluoric acid. In this contribution, we review and describe continuing efforts leading to the scale-up of the electropolishing process to single-cell and three-cell niobium SRF cavities [[7],[8]]. Finally, we present a first order economic comparison of the pulse reverse voltage low concentration aqueous sulfuric acid process compared to baseline concentrated sulfuric acid-hydrofluoric acid process [[9]]. Acknowledgements: The financial support of Faraday corporate, DOE P.O. No. 594128 and DOE Contract No. DE-SC0004588 is acknowledged. [1] Jacquet, P.A. (1936) On the Anodic Behavior of Copper in Aqueous Solutions of Orthophosphoric Acid. Trans. Electrochem. Soc. 69(1) 629-655. [2] Tian, H., Corcoran, S., Reece, C. and Kelly, M. (2008) The Mechanism of Electropolishing of Niobium in Hydrofluoric-Sulfuric Acid Electrolyte. J. Electrochem. Soc., 155, D563-568. [3] D. Landolt,(1987) “Fundamental Aspects of Electropolishing” Electrochimica Acta 32(1) 1-11 (1987). [4] MacDougall, B. (1995) “The Importance of Surface Oxide Films in Corrosion, Semiconductor and Environmental Research” Proceedings of the Symposium on High Rate Metal Dissolution Processes, Vol. 95-19, (Eds. M. Datta, B. MacDougall and J. Fenton) The Electrochemical Society, Pennington, NJ, pp 16-31. [5] M. Inman, E.J. Taylor, T.D. Hall “Electropolishing of Passive Materials in HF-Free Low Viscosity Aqueous Electrolytes” J. Electrochem. Soc., 160(9) E94-E98 (2013). [6] E.J. Taylor, M.E. Inman, T.D. Hall (2015) “Electrochemical system and method for electropolishing superconductive radio frequency cavities” U.S. Patent No. 9,006,147 issued April 14, 2015. [7] E.J. Taylor, T.D. Hall, M. Inman, S. Snyder (2013) “Electropolishing of Niobium SRF Cavities in Low Viscosity Aqueous Electrolytes without Hydroflouric Acid” Paper No. TUP054, Presented SRF2013, Paris, FRANCE. [8] A.M. Rowe, A. Grassellino, T.D. Hall, M.E. Inman, S.T. Snyder, E.J. Taylor (2013) “Bipolar EP: Electropolishing without Flourine in a Water Based Electrolyte” Paper No. TUIOC02, Presented SRF2013, Paris, FRANCE. [9] E.J. Taylor, M. Inman, T. Hall, S. Snyder, A. Rowe, D. Holmes (2015) “Economics of Electropolishing Niobium SRF Cavities in Eco-Friendly Aqueous Electrolytes without Hydrofluoric Acid” Paper No. MOPB092, Presented SRF2015, Whistler, CANADA. Figure 1

Threshold switching and electrical self-oscillation in niobium oxide films

Electrical self-sustained oscillations have been observed in a broad range of two-terminal systems and are of interest as possible building blocks for bio-inspired neuromorphic computing. In this work, we experimentally explore voltage-controlled oscillations in NbOx devices with a particular focus on understanding how the frequency and waveform are influenced by circuit parameters. We also introduce a finite element model of the device based on a Joule-heating induced insulator-metal transition. The electroformed device structure is represented by a cylindrical conductive channel (filament) comprised of NbO/NbO2 zones and surrounded by an Nb2O5−x matrix. The model is shown to reproduce the current-controlled negative differential resistance observed in measured current-voltage curves, and is combined with circuit elements to simulate the waveforms and dynamics of an isolated Pearson–Anson oscillator. Such modeling is shown to provide considerable insight into the relationship between the material response and device and circuit characteristics.

Influence of Post-Deposition Annealing on Structure and Optical Characteristics of Niobium Oxide Thin Films

In this work, niobium oxide thin films were deposited on silicon wafer (100) and glass slide substrate by reactive magnetron sputtering. The niobium oxide films were further annealed and effect of post annealing on the crystallinity, microstructure and optical properties was studies. In order to identify the crystalline structure and microstructure, X-ray diffraction (XRD) and scanning electron microscope (SEM) measurement carried out. The optical property was determined by UV-Vis spectrophotometer. It has been observed that with increase in annealing temperature films become microcrystalline films. In particularly, the optical property of niobium oxide thin film also improves with annealing temperature.