Nb2O5 Films Research Articles

Fabrication of 500 nm distributed Bragg reflector using Nb2O5-MgF2 multi-layer films

At first, we use an [Formula: see text] analyzer to measure the optical properties (including refractive index [Formula: see text] and extinction coefficient [Formula: see text]) of MgF2 and Nb2O5 single-layer films, in a wavelength range of 200–1700 nm for MgF2 film and in a wavelength range of 350–1500 nm for Nb2O5 film. After the refractive indexes of MgF2 and Nb2O5 single-layer films are measured, we use the measured results to calculate the needed thicknesses of the quarter-wave (1/4 wavelength) MgF2 and Nb2O5 films for the designed green-light (500 nm) distributed Bragg reflectors (DBRs). After that, an E-beam is used to deposit the MgF2-Nb2O5 bilayer films (called as one period) with different periods (two, four, and six periods are deposited in this study) on glass substrates to fabricate the DBRs with a central wavelength of 500 nm. Then we use the field emission scanning electron microscopy (FESEM) to observe the surface images of Nb2O5 films on the different periods of MgF2-Nb2O5 bilayer films. The important novelty is that we use a Focused Ion Beam (FIB) to prepare the samples for the observations of the cross-sections of MgF2-Nb2O5 bilayer films, and those results can be sued to confirm the thicknesses of the bilayer films with different periods. We also compare the reflective ratio of the fabricated DBRs at the designed central wavelength with those calculated values by using the equation investigated by Sheppard. We find that the measured reflective ratios of the fabricated DBRs meet the calculated results obtained from Sheppard’s equation.

Investigations of Al2O3–Nb2O5 bi-layer Bragg reflector investigations for blue and green lights

At first, the single-layer Al2O3 and Nb2O5 films were deposited to find their refractive indices. After the indices of the single-layer Al2O3 and Nb2O5 films were measured, the thickness of 1/4 wavelength ([Formula: see text] for each layer of the Al2O3–Nb2O5 bi-layer Bragg reflectors of blue (450-nm) and green (550-nm) lights could be obtained. E-beam was used to deposit the bi-layer Al2O3–Nb2O films (called as one period) with two, four, and six periods on glass substrates to fabricate the Bragg reflectors of blue and green lights. The measured results had proven that the reflectance ratio increased and the bandwidth and full-width at half-depth (FWHD) decreased with the increase of number of periods. The measured results had also proven that as the designed central wavelength increased (changed from blue to green light) and same periods were deposited, the reflectance ratio decreased and the bandwidth and FWHD values increased.

Tuning the wettability of highly transparent Nb2O5 nano-sliced coatings to enhance anti-corrosion property

In the present work, the deposition temperature-dependent optical, wetting and corrosion properties of niobium oxide (Nb2O5) coatings was investigated. The Nb2O5 thin films were deposited on two different substrates i.e. glass and stainless steel (SS 304) by using sputtering technique at various deposition temperatures (RT, 100 & 200 °C) in Ar:O2 (3:1) atmosphere. To confirm the pertinent growth of the prepared samples, X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were employed. The optical, wetting and corrosion studies of the synthesized samples were executed using UV Vis-spectroscopy (UV), contact angle (CA) and cyclic voltammetry (CV), respectively. The aquatic corrosion studies of Nb2O5 films deposited on SS samples were estimated under two dissimilar ways. The Tafel plot (polarization curve) and EIS (electrochemical impedance spectra) were accomplished in terms of polarization resistance (Rp) under saline water (3.5% NaCl aqueous solution) environment. Nano-sliced Nb2O5 film deposited at 200 °C on SS substrate exhibited excellent optical, good transparency, wetting and material protective behavior i.e. transmittance (~91%), refractive index (n) ~ 2.3045, hydrophobicity (θw) ~ 97.3°, corrosion rate ~ 6.9 × 10−3 mm/y and Rp ~ 2.78 × 106 Ω-cm2 in comparison to the other samples (coated and uncoated).

Comparisons of Theoretical and Experimental Results of Blue Light SiO2–Nb2O5 Distributed Bragg Reflector Fabricated Using E-beam deposition

The crystalline structures and surface morphologies of the SiO2 and Nb2O5 single-layer films were measured using XRD pattern and FESEM and their optical properties (the refractive indices and extinction coefficients) were measured as a function of optical wavelength. After the refractive indices of the SiO2 and Nb2O5 single-layer films were measured, the thicknesses of the SiO2 and Nb2O5 bi-layer films with 1/4 wavelength (λ) for the blue-light (450 nm) distributed Bragg reflectors were obtained. The SiO2–Nb2O5 bi-layer films (called as one period) with two, four, and six periods on glass substrates was deposited using an E-beam system to fabricate the distributed Bragg reflectors (DBRs). The thickness of each layer of SiO2 and Nb2O5 films in the multilayer structure was measured using FESEM equipped with FIB and TEM. The important novelty is that we constructed an overall transfer matrix and calculated the simulated reflectance spectra by incorporating the variable refractive index and thickness of each layer of SiO2 and Nb2O5 films. Another novelty is that we compared the measured maximum reflectance ratios of the fabricated DBRs with the simulation results obtained from the investigated overall transfer matrix method and Sheppard's approximate equation (maximum reflectance ratio at a specific wavelength).

The Impact of ion-beam parameters on the characteristics of Nb2O5 thin films

In this study, the effects of ion-beam parameters on the mechanical, optical and inner structural characteristics of niobium pentoxide (Nb2O5) thin films is investigated. Where a vacuum coating system equipped with a gun of an electron beam at ambient substrate temperature is employed to precipitate Nb2O5 films onto substrates of dissolved silica glass via electron-beam supported by oxygen ions. It is found that the extinction coefficient, the crystalline phase, refractive index, and residual stress are heavily affected by current density and the energy of ion. The transformations in crystallinity, from amorphous to hexagonal transition, revealed in minimum residual stress of –455 *107 psi. The optical band gap and refractive index increased up to 0.96 and 2.32 respectively, as well as a very low extinction coefficient of 10−3 for precipitation at an ion energy of 0.8 KeV and 0.025 mA/cm2 current density.

In Situ Hydrogen Plasma Exposure for Varying the Stoichiometry of Atomic Layer Deposited Niobium Oxide Films for Use in Neuromorphic Computing Applications.

Niobium oxide (NbOx) materials of various compositions are of interest for neuromorphic systems that rely on memristive device behavior. In this study, we vary the composition of NbOx thin films deposited via atomic layer deposition (ALD) by incorporating one or more in situ hydrogen plasma exposure steps during the ALD supercycle. Films with compositions ranging from Nb2O5 to NbO2 were deposited, with film composition dependent on the duration of the plasma exposure step, the number of plasma exposure steps per ALD supercycle, and the hydrogen content of the plasma. The chemical and optical properties of the ALD NbOx films were probed using spectral ellipsometry, X-ray photoelectron spectroscopy, and optical transmission spectroscopy. Two-terminal electrical devices fabricated from ALD Nb2O5 and NbO2 thin films exhibited memristive switching behavior, with switching in the NbO2 devices achieved without a high-field electroforming step. The ability to controllably tune the composition of ALD-grown NbOx films opens new opportunities for realizing a variety of device structures relevant for neuromorphic computing and other emerging electronic and optoelectronic applications.

Observation of a Correlation Between Internal friction and Urbach Energy in Amorphous Oxides Thin Films

We have investigated by spectroscopic ellipsometry (SE, 190–1700 nm) the optical properties of uniform, amorphous thin films of Ta2O5 and Nb2O5 as deposited and after annealing, and after so-called “doping” with Ti atoms which leads to mixed oxides. Ta2O5 and Ti:Ta2O5 are currently used as high-index components in Bragg reflectors for Gravitational Wave Detectors. Parallel to the optical investigation, we measured the mechanical energy dissipation of the same coatings, through the so-called “loss angle” ϕ = Q−1, which quantifies the energy loss in materials. By applying the well-known Cody-Lorentz model in the analysis of SE data we have been able to derive accurate information on the fundamental absorption edge through important parameters related to the electronic density of states, such as the optical gap (Eg) and the energy width of the exponential Urbach tail (the Urbach energy EU). We have found that EU is neatly reduced by suitable annealing as is also perceptible from direct inspection of SE data. Ti-doping also points to a minor decrease of EU. The reduction of EU parallels a lowering of the mechanical losses quantified by the loss angle ϕ. The correlation highlights that both the electronic states responsible of Urbach tail and the internal friction are sensitive to a self-correlation of defects on a medium-range scale, which is promoted by annealing and in our case, to a lesser extent, by doping. These observations may contribute to a better understanding of the relationship between structural and mechanical properties in amorphous oxides.

All niobia Bragg stacks for optical sensing of vapors

Vapor responsive all Nb2O5 Bragg stacks were prepared using spin-coating of alternating dense and porous films with quarter-wavelength thickness. The required porosity in the films was achieved by soft-templating method using micellar solution of Pluronic copolymer as an organic template. The surface morphology and structure of the films were studied by transmission electron microscopy and selected area electron diffraction, respectively, while optical characterization was performed by combination of ellipsometric and reflectance measurements. Vapor sensing response of the films was tested by reflectance measurements of the samples prior to and after exposure to acetone vapors selected as probe molecules. Incorporation of “defect” layer in the multilayered structures was firstly simulated theoretically and then realized experimentally aiming at improvement of the optical response to volatile organic compounds. Furthermore, the potential of all niobia stacks consisting of alternating dense and porous sol–gel Nb2O5 films as chemical sensors with optical read-out is demonstrated and discussed.

Preparation of high laser-induced damage threshold sol-gel Nb2O5 films with different additives

In this study, different sol-gel Nb2O5 films were prepared by using NbCl5 as precursor, and acetylacetone (ACAC), diethanolamine (DEA) and citric acid (CA) as additive, respectively. The results showed that the film with ACAC as additive presented the highest absorption and stability, corresponding to the lowest laser-induced damage threshold (LIDT) of 18.9 J/cm2, while that with CA as additive had the lowest absorption, corresponding to the highest LIDT of 24.9 J/cm2. The film prepared with DEA as additive had the smallest surface roughness, and the LIDT was 21.1 J/cm2. Moreover, although the films prepared with three different additives were all subjected to typically defect-induced damages, different types of damage features were observed. Finally, three structural evolution models of the films were proposed, accompanied by establishment of the relationship between the additives and the LIDT, which was of great significance to further understand the laser damage mechanism.

Atomic layer deposition and structure optimization of ultrathin Nb2O5 films on carbon nanotubes for high-rate and long-life lithium ion storage

Niobium pentoxides (Nb2O5) as anode materials has attracted tremendous research interest due to its great potential in rate capability and cyclic durability. Nevertheless, the low electrical conductivity and weak structural controllability of Nb2O5 hinders its practical application. Herein, amorphous Nb2O5 thin films are conformally deposited on carbon nanotubes (CNTs) by controlled atomic layer deposition (ALD) and three types of crystalline Nb2O5@CNTs composites are obtained by post-deposition annealing. Moreover, the effects of deposition temperature and annealing temperature on the crystal structure and surface morphology of Nb2O5@CNTs are investigated. Based on these, the electrochemical performances of Nb2O5@CNTs composites as anode materials are compared in lithium half cells. Impressively, ultrathin and uniform Nb2O5 nanocrystal film with hexagonal (TT) phase on CNTs delivers a capacity of 325.1 mAh g−1 at 0.2 A g−1. Even at a high current density of 3 A g−1, a stable capacity of 170 mAh g−1 is still achieved over 4000 cycles. Its high-rate and long-life lithium ion storage is mainly attributed to the synergistic effects of carbon nanotubes, TT-phase Nb2O5, and ultrathin film structure. Therefore, this synthetic strategy facilitates the development of Nb2O5 as anode materials for high-rate and long-life lithium/sodium ion batteries.

Photoassisted atomic layer deposition of oxides employing alkoxides as single-source precursors

Photoassisted atomic layer deposition (photo-ALD) is a variant of an ALD process where photons of ultraviolet or visible range are utilized to supply energy to, and to modify, the ALD surface reactions. In this paper, the authors report photo-ALD processes for titanium, zirconium, hafnium, niobium, and tantalum oxides by employing the corresponding liquid, volatile metal alkoxides as precursors in a single-source approach, i.e., without any additional reactant. The ALD reactor was equipped with a light source delivering photons over a continuous spectrum between 190 and 800 nm in wavelength. The deposition sequence consisted of a precursor pulse, a purge, a photon exposure, and another purge. The process characteristics and film properties were explored. Nb2O5 and Ta2O5 films were amorphous, whereas TiO2, ZrO2, and HfO2 showed an amorphous and polycrystalline structure, depending on the deposition conditions. With photo-ALD, area-selective deposition is realized by shadow masking. The character of the growth process, i.e., whether the chemistry is driven by photolytic or photothermal mechanism, is discussed based on deposition experiments with patterned substrates and optical filtering. Electrical characterization of photo-ALD HfO2 shows promising dielectric properties.

Effect of indium doping in Nb2O5 thin films for electron transport layers: Investigation of structural, optical, and electrical properties

Nb2O5 films were prepared with various In3+ doping concentrations using a spin coating method. The XRD peaks indicated that the monoclinic phase of Nb2O5 becomes more prevalent with increased In3+ doping. The decrease in transmittance with increasing In3+ doping concentration indicated that the films had better absorptivity in the visible to near-infrared region. A decreased optical band gap of 2.95 eV was obtained at 3.0 wt% In3+ doping. Furthermore, an optimized range of 3.0–7.0 wt% In3+ doping concentrations resulted in decreased sheet resistance and increased figure of merit values from 186 × 107 Ω/□ to 2.82 × 107 Ω/□ and from 0.144 × 10−7 Ω−1 to 5.06 × 10−7 Ω−1, respectively. Thus, the overall results indicate that the optimally In3+-doped Nb2O5 films are potentially valuable as the electron transport layer in perovskite solar cells.

Structural and physical properties of NbO2 and Nb2O5 thin films prepared by magnetron sputtering

NbOx thin films have been deposited on silicon (100) and quartz substrates by magnetron sputtering using a metallic Nb target in an optimized argon–oxygen atmosphere. This work investigates the dependence of structure–property relations on the key deposition parameters towards establishing optimum deposition conditions for the growth of NbOx polycrystalline films. It is found that a sputtering condition corresponding to DC power density 9.87 W cm−2, a substrate temperature of 720 °C, low gas pressures of 8 mtorr, a target to substrate distance of 45 mm gives thin films with good homogeneity and a high degree of crystallinity in the case of both NbO2 and Nb2O5. X-ray diffraction (XRD) and Raman spectroscopy confirmed the tetragonal phase of NbO2 and orthorhombic phase of Nb2O5 for similar deposition temperatures. Scanning electron microscopy (SEM) observations indicate that NbO2 has a unique nanoslice structure while Nb2O5 has a flake-like structure. The optical transmittance of the films has been investigated and found to be dependent on the oxygen gas content during deposition; the optical transmittance decreases with increasing O2 gas content. Optical constants of the films were calculated by fitting a suitable thin film transmittance model to experimental transmittance spectra using a modified Swanepoel technique. The nanohardness and stress in the films were measured by nanoindentation and an optical profilometer respectively. Nanohardness and stress in the film show no large dependence on the oxygen gas content except at high oxygen gas content. The nanohardness value of NbO2 films is approximately 6 GPa, and the Young’s modulus is 150 GPa. The Nb2O5 films exhibit a nanohardness of 5.8–13 GPa and a Young’s modulus of 137–161 GPa.

Thermal conductivity tensor of NbO2

Among several transition metal oxides showing metal to insulator transition, NbO2 has attracted attention as the active material of memory devices due to its high thermal stability. Although clarification of thermal conductivity (κ) is essentially important for optimizing the NbO2 device performance, it has not been reported thus far probably due to the anisotropic crystal structure. Here we show the thermal conductivity tensor of NbO2 crystal. We measured the thermal conductivities of (1 1 0)-, (0 0 1)-, and (1 1 1)-oriented NbO2 films and found 〈1 1 1〉 -direction shows ∼1 W m−1 K−1 higher thermal conductivity compared with the other directions (∼3 W m−1 K−1). Using the measured κ-values, we successfully determined thermal conductivity tensor components κ11, κ13, and κ33. The diagonal components of the thermal conductivity tensor were ∼2.5 W m−1 K−1 while the nonzero off-diagonal component was ∼1 W m−1 K−1. We believe that the present results are of great value in optimizing the performance of NbO2 based devices.

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.

Formation of distorted rutile-type NbO2, MoO2, and WO2 films by reactive sputtering

We report the film growth of distorted rutile-type NbO2 (4d1 system), MoO2 (4d2), and WO2 (5d2) by radio-frequency (RF) magnetron reactive sputtering. Through optimization of growth conditions for the three oxides on Al2O3(0001), we found that the increase/decrease in the RF power had an equivalent role to that of the decrease/increase in the oxygen ratio in the Ar–O2 sputtering gas. X-ray photoelectron spectroscopy supported the d1 electronic configuration of NbO2 and d2 electronic configurations of MoO2 and WO2. An electrical transport measurement confirmed that NbO2 was insulating, while MoO2 and WO2 were metallic, consistent with the d-electron filling in the molecular orbital bonding band. The growth scheme presented in this study will be useful for valence control in various oxide thin films with a simple sputtering technique.

An All-Solid-State Electrochromic Device Based on WO3–Nb2O5 Composite Films Prepared by Fast-Alternating Bipolar-Pulsed Reactive Magnetron Sputtering

In this study, WO3–Nb2O5 electrochromic films and an ITO/WO3–Nb2O5/Nb2O5/NiVOx/ITO all-solid-state electrochromic device were deposited using fast-alternating bipolar-pulsed magnetron sputtering using tungsten and niobium targets. The influence of different sputtering powers from the niobium target on the refractive index, extinction coefficient, optical modulation, coloration efficiency, reversibility, and durability of the WO3–Nb2O5 films is discussed. The aim of this work is to find the suitable Nb proportion to increase durability and less negative effect in the electrochromic performance of Nb2O5-doped WO3 films. The lifetime of the WO3–Nb2O5 films is 4 times longer than pure WO3 films when the sputtering power of the Nb target is higher than 250 W. The results show that WO3–Nb2O5 composite films used for an all-solid-state electrochromic device can sustain over 3 × 104 repeated coloring and bleaching cycles while the transmission modulations can be kept above 20%. The coloring and bleaching response times are 7.0 and 0.7 s, respectively.

Structural and electrical characterization of polycrystalline NbO2 thin film vertical devices grown on TiN-coated SiO2/Si substrates

We report on the electrical properties of polycrystalline NbO2 thin film vertical devices grown on TiN coated SiO2/Si substrates using pulsed laser deposition. First, we analyzed the thickness and contact size dependences of threshold switching of NbO2 films grown in 10 mTorr Ar/O2 mixed growth pressure, where 25.1%/74.9% of NbO2/Nb2O5 surface composition content was estimated by ex-situ x-ray photoelectron spectroscopy. Then the threshold switching and self-sustained current oscillatory behavior of films with different NbO2/Nb2O5 composition ratios was measured and analyzed. The current-voltage measurement revealed that the leakage current property in the insulating state was dominated by the trap-charge assisted Poole-Frankel conduction mechanism. All films showed threshold switching behavior in agreement with the previously proposed Joule heating mechanism. The second film was grown in lower (1 mTorr) growth pressure, which resulted in a higher (34.2%/65.8%) NbO2/Nb2O5 film surface composition. The film grown in higher growth pressure demonstrated lower off-state leakage current, faster switching, and self-sustained oscillations with higher frequency than the film grown in lower growth pressure.

Synthesis of T-Nb2O5 thin-films deposited by Atomic Layer Deposition for miniaturized electrochemical energy storage devices

Atomic Layer Deposition has been used to grow 30 to 90 nm-thick amorphous Nb2O5 films onto Pt current collectors deposited on Si wafer. While T-Nb2O5 polymorph is obtained by further annealing at 750 °C, the film thickness and the resulting electrode areal capacity are successfully controlled by tuning the number of ALD cycles. The electrochemical analysis reveals a lithium ion intercalation redox mechanism in the T-Nb2O5 electrode. An electrode areal capacity of 8 µAh cm-² could be achieved at 1 C, with only 40% capacity loss at 30 C (2 minutes discharging time). This paper aims at demonstrating the use of Atomic Layer Deposition method in the fabrication of Nb205-based on-chip micro-devices for Internet of Things (IoT) applications.

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.