Vanadium Oxide Films Research Articles

Hole-injection role of solution-processed thermally treated VOx thin films in Si nanowire-based solar cells

We have studied the hole-injection workability of dopant-free vanadium oxide (VOx) semiconductor thin films. Solution-processable VOx films were annealed in different atmospheres at 450 ˚C. X-ray diffraction showed that different crystal structures of V2O5, VO2 (M) and VO2 (B) were obtained after annealing in oxygen, nitrogen and under vacuum respectively. Gap states that originated from oxygen deficiencies located inside the band gap of the annealed VOx were demonstrated using X-ray photoelectron spectroscopy. These VOx thin films act effectively as transparent hole-injection layers owing to their high work function, wide band gap, and presence of oxygen deficiencies. The different ambient annealed VOx films were employed in Si nanowire (SiNW)-based solar cells. The vacuum-annealed oxide film VO2–x (B) showed the best hole-injection ability. SiNWs/VOx solar cells are noticeably more stable than SiNWs/PEDOT:PSS hybrid solar cells, which decay swiftly under ambient conditions unless encapsulation is provided. The utilization of VOx films to act as hole-injection layers opens a pathway toward the development of cheap and durable electronics and optoelectronics devices.

Tunable thermochromism in V2O5 films deposited by cathodic vacuum arc method by tailoring the oxygen deficiency

Theromochromism and electrochromic coatings have promising applications in smart windows, IR sensors, and bio-medical devices. In this paper, we report the growth of vanadium oxide films using cathodic vacuum arc method for thermochromic application. Successive annealing of the as grown film shows the formation of yellow color V2O5 films. The color of the as grown films suggest the growth of highly disordered black VxOy film which after annealing gets converted into yellow V2O5 as confirmed by the X-ray diffraction and Raman spectroscopy. The scanning electron microscopy equipped with energy dispersive X-ray analysis was used to investigate the morphology and chemical composition of V2O5 phase. Although, the films gets completely converted into the stable phase after annealing but heating cycling of temperature significantly changes the color of the films in a reversible manner that confirms the thermochromic behavior. We propose a breathing of oxygen (tailoring of Oxygen within the films) responsible for such reversible change in the color. These studies open new pathways for investigating these films to electrochromic properties by locally heating the materials through electric current (through Joule heating) and change the optical properties.

Phase Control of Multivalent Vanadium Oxides VOx by Ion‐Beam Sputter‐Deposition

Vanadium–oxygen materials are of interest for various applications and fields of solid‐state physics owing to the unequaled plethora of different phases. The wealth of phases and complexity of its phase diagram infer a strong sensitivity on the growth parameters for each phase. Thus, the reproducible growth of vanadium‐oxide thin‐films of defined phases by nonequilibrium techniques is challenging. Here, it is shown that ion‐beam sputter‐deposition (IBSD) is a powerful tool to reproducibly deposit defined polycrystalline vanadium oxide films by precisely controlling oxygen flux and substrate temperature in the growth process. Hence, it is demonstrated that IBSD has the potential to reliably produce binary phases (including unstable phases) from the vanadium–oxygen phase space. X‐ray diffraction (XRD) and Raman spectroscopy are used to establish a map of the different crystalline phases dependent on the growth parameters. In particular, it is proved that thin‐film V3O7 can be realized by IBSD and its Raman fingerprint is unambiguously identified.

Optimization of temperature coefficient of resistance of Al-doped vanadium oxide thin film prepared by atomic layer deposition for uncooled microbolometer

Vanadium oxide (VOX) is an excellent thermal sensitive candidate for uncooled microbolometers. However, undoped VOX prepared by atomic layer deposition (ALD) has a temperature coefficient of resistance (TCR) of ca. −2 ∼ −3%/K. For improving its TCR, our deposition approach based on the combination of ALD and rapid post-deposition annealing (RTA) is proposed. Besides, aluminum-doping into the VOX films is performed via that approach, and the number of Al2O3 cycles is adjusted for varying the dopant loadings. Changes in physical, chemical, and electrical characteristics of the VOX films due to Al-doping are discussed in detail. The advantage of introducing Al3+ dopants is to hinder the thermally activated phase transition of VO2 phases, leading to an improvement in TCR of Al-doped VOX up to −4.2%/K, which remains stable over a wide temperature range of 298–328 K. However, the excessive Al doping also carries an adverse effect on TCR. The reason for that is discussed for further understandings of doping effects.

Strong correlation effects in vanadium oxide films

In the present work the metal-insulator transition in doubly orbitally degenerated model of quasi-two-dimensional material based on V2O3 film, in which a crucial role is played by on-site Coulomb interaction and correlated hopping of electrons, has been investigated. With use of a projection procedure in the Green function method the energy spectrum of electrons has been calculated to model variations of the material properties at the temperature changes, the external pressure application and doping. The obtained expressions for thermodynamic potential and the energy gap widths allow analyzing the possible phase transitions in a system, the dependency of characteristics on the external actions for this strongly correlated material.

Nanostructured and columnar vanadium and vanadium oxides films synthesized by means of magnetron-based gas aggregation source

In this study the possibility to employ magnetron-based gas aggregation source for the effective and facile production of mesoporous vanadium films composed of individual vanadium nanoparticles is investigated. It is demonstrated that due to the high deposition rate of V nanoparticles under optimized conditions, their low impact energies, and the directionality of the deposition process, it is possible not only to produce porous nanoparticle coatings, but also coatings with columnar structures. Furthermore, it is shown that such produced coatings may be easily transformed into vanadium pentoxide ones by their annealing in air at the temperature of 550 °C, without loosening the mesoporous character of resulting films. • Mesoporous vanadium nanoparticle films were produced by a gas aggregation source. • The architecture of the films may be changed to columnar one by substrate tilting. • The films are transformed to V 2 O 5 ones upon annealing in air at 550 °C. • The structure of the nanoparticle films is preserved during the annealing.

Effect of the RF-power and annealing on the structural, optical, morphological and electrical properties of RF-sputtered V2O5 thin films

Thin films of vanadium oxide were deposited on glass substrates by the radio frequency reactive sputtering from a high purity metallic vanadium target (99.7%) with a diameter of 10 cm. The reactive sputtering was carried out in an argon-oxygen gas mixture containing 10% of O2 and 90% of Ar. The films were deposited at different RF powers (150 W, 200 W, 250 W and 300 W) for a fixed deposition time of 150 min. X-ray diffractograms showed that the deposited thin films crystallized in an orthorhombic V2O5 phase. It was found that the crystallite size varies with the RF power and is maximized using 300 W as an RF power. Scanning Electron Microscopy and Raman scattering analyzes have confirmed the formation of V2O5 thin films. In addition, optical transmittance measurements were performed using a Shimadzu UV-PC spectrophotometer in the 220–2000 nm range. It was observed that the optical band gap of the films decreases with increasing the RF power. Electrical resistivity was found to decrease with increasing the RF power from 150 to 250 W, and then it increases.

STUDY OF THE EFFECT OF ANNEALING AND IRRADIATION OF HIGH ENERGY IONS ON VANADIUM OXIDE THIN FILMS

Vanadium due to multiple oxidation states (+2 to +5) has various oxides like vanadium dioxide, vanadium pentoxide, and hexavanadium tredecaoxide. Thin films of vanadium oxide have been fabricated using the reactive radio frequency sputtering technique. Deposited films are then annealed in Argon (Ar) environment at 500°C and then irradiated with 100 MeV Ag8+ high-energy ions. The vibrational properties of the thin films have been analyzed by Raman scattering and Fourier transform infrared spectroscopy (FTIR). Results (Raman and FTIR spectra) of the annealed film show that the V6O13 phase is converted into a mixed phase (V6O13, VO2, and V2O5). The annealed irradiated film shows no change in the Raman lines while a shift towards the lower wave number is noticed in FTIR spectra peaks. This shifting of peaks towards lower wave numbers in FTIR spectra is attributed to the weakening of vanadium oxygen bonds. It is considered that the weakening of bonds could occur when high-temperature cylindrical zones are created along the ion track.

Phase transition in vanadium oxide films formed by multistep deposition

VOx films deposited using the multistep method have been investigated. These films were deposited by repeating the two-stage method of low-temperature deposition – low-temperature annealing. The structure and characteristics of VOx thin films have been studied. Taking into account the obtained results, theoretical modeling of the structure was performed and the parameters of the metal-insulator transition have been calculated.

Controlled grain-size thermochromic VO2 coatings by the fast oxidation of sputtered vanadium or vanadium oxide films deposited at glancing angles

An original, simple and cost-effective oxidation strategy to attain thermochromic vanadium dioxide thin films is reported. This two-step procedure comprises the initial deposition of DC magnetron-sputtered vanadium or vanadium oxide films by the combination of glancing angle deposition and, if needed, reactive gas pulsing process, followed by fast oxidation of such layers in air atmosphere at high temperatures. Thanks to the careful control of the thermal treatment parameters, and taking advantage of the superior reactivity of the high surface-to-volume porous deposited structures, the formation of pure VO2 (M1) layers was achieved. The comprehensive characterization of such oxidized systems by means of scanning electron microscopy, Raman spectroscopy and scanning-transmission electron microscopy techniques such as electron energy-loss spectroscopy, not only confirmed the presence of the VO2 (M1) phase, but also allowed to shed light on the key role that reaction time plays in the selective formation of vanadium dioxide films of adjustable grain size and crystallinity. The optimal conditions to stabilize thermochromic VO2 consists in using large deposition angles (85 °) and short oxygen pulses (≤ 2 s) during the growth, followed by fast and short thermal treatments (≤ 45 s with a heating rate of 42 °C s−1) in air atmosphere at 550 °C. The metal-to-insulator response of the accomplished VO2 layers was finally evaluated by means of temperature dependent Kelvin probe force microscopy measurements, evidencing surface potential drops at heating, greater than those reported in the literature to date for VO2 thin films.

Femtojoule‐Power‐Consuming Synaptic Memtransistor Based on Mott Transition of Multiphasic Vanadium Oxides (Adv. Funct. Mater. 46/2021)

Memtransistors In article number 2102567, Hyungtak Seo and co-workers present a highly stable Mott memtransistor based on a multiphasic vanadium oxide film, which possesses not only tunable synaptic functions through controlling either drain or gate biases, but also ultralow power consumption. Their study promotes the potential of memtransistors for use in practical memory applications, including artificial synapse and neuromorphic processing.

Effects of natural aging on the surface compositions and optical properties of thermochromic vanadium oxide films

We experimentally investigate the long-term atmospheric durability of thermochromic vanadium oxide (VOX) films. The VOX films were deposited on glass substrates by reactive direct current magnetron sputtering and were then in-situ annealed at the preparation temperature ≤350 °C. The X-ray photoelectron spectroscopy (XPS) was applied to investigate the surface compositions of the natural aged films, while the optical properties of the films were characterized by the spectrophotometer and spectroscopic ellipsometry. The XPS results reveal that the oxygen-deficient VOX (X < 2.5) components possessing remarkable chemical changes is preferred to react with the O2 and H2O in the air and become high oxidation state compounds. However, it is worth to note that the optical properties of the films shows extra-low variation performance (e.g. linear increasing rate of transmittance ~0.00448% per day @ λ = 600 nm). The aging mechanism of the VOX films in air was interpreted in terms of the interface aging and the diffusion-reaction aging on basis of the above experimental results. This study would provide valuable experience of the natural aging of VOX films.

Hole Transport Layer based on atomic layer deposited V2Ox films: Paving the road to semi-transparent CZTSe solar cells

This work explores the use of very thin transparent vanadium oxide films deposited by atomic layer deposition (ALD) technique as hole transport layer for CZTSe solar cells as alternative of opaque molybdenum based contacts. Contact resistivity between the CZTSe absorber and the ALD V2Ox contact was measured. In order to improve contact quality, a cleaning bath using hydrofluoric acid (HF) dip, was also analyzed and its influence on kesterite surface was studied. Elementary material characterization and composition analysis of the V2Ox layers was performed. Contact quality was assessed yielding contact resistivity values below 9 and 30 mΩcm2 for ALD and thermal evaporated V2Ox films respectively.The proposed ALD V2Ox based hole transport layer was deposited onto a glass covered with a transparent conductive oxide forming part of the rear contact scheme of a vertical CZTSe solar cell with a conventional ITO/CdS stack as electron transport layer. The impact of subsequent thermal post-annealing treatments in the cell performance was also analyzed yielding efficiency up to 3.9% on a semi-transparent CZTSe solar cell without any additional optimization process. In this way, a CZTSe solar cell with both transparent electrodes has been demonstrated paving the way to obtain in the future high efficiency bifacial and/or semitransparent Building –integrated photovoltaic devices.

Electrochromic Behavior of Vanadium Pentoxide Thin Films Prepared by a Sol–Gel Spin Coating Process

Herein, the synthesis of vanadium oxide thin films with high electrochromic coloration efficiency through a simple sol–gel spin coating technique is reported. The structural studies confirm the formation of V2O3V2O5 mixed phase at 250 °C and the pure β‐V2O5 phase at 350 and 450 °C. The decrease in optical bandgap, refractive index, and extinction coefficient improves the transmission ability of the films at higher temperatures. The photoluminescence emission peak observed at 553 nm is due to the recombination of electron–hole pair intrinsic transition from the lowest split‐off V 3d band to the O 2p valence band. The tuning of surface morphology from rod to platelet structure may be due to the higher annealing temperature which assists the disorder movement of the particles. The observed V 2p core levels confirm the existence of V5+ oxidation state with the splitting of 7.2 eV. The electrochromic behavior of vanadium oxide film is detected in 0.5 m LiClO4 in propylene carbonate electrolyte solution. The results show that the film annealed at 450 °C exhibits high coloration efficiency of 110.98 cm2 C−1, good reversibility of 99%, and faster switching speed for bleaching (2.1 s) and coloration (2.4 s) processes.

A Ca‐Ion Electrochromic Battery via a Water‐in‐Salt Electrolyte

AbstractMultivalent‐ion batteries with electrochromic functionality are an emerging green technology for development of low‐carbon society. Compared to Mg2+, Zn2+ and Al3+, Ca2+ has a low polarization strength similar to that of Li+, therefore Ca2+ for electrochromism and battery can avoid kinetic issues caused by other multivalent‐ions with high polarization strength. Here, by exploiting Ca‐ion carriers for electrochromism and a water‐in‐salt (WIS) Ca(OTF)2 electrolyte for the first time, a new and safe aqueous Ca‐ion electrochromic battery (CIEB) has been demonstrated. The WIS Ca(OTF)2 electrolyte demonstrates enhanced anion‐cation interactions and decreased water activity. Vanadium oxide (VOx) and indium hexacyanoferrate (InHCF) films are respectively developed as anode and cathode because of their stable and high‐rate Ca2+ insertion/extraction, as well as matched electrochromism. The CIEB demonstrates a stable and high‐rate capability, a high energy density of 51.4 mWh m−2 at a power density of 1737.3 mW m−2, and a greenish yellow‐to‐black electrochromism. The presented results are beneficial for understanding redox kinetics in WIS electrolytes, and inspire researches on batteries and electrochromism with multivalent‐ions.

Low metal–insulator transition temperature of Ni-doped vanadium oxide films

Elemental doping is the main means to regulate the phase transition of vanadium oxide (VO2); however, the effects of low valence elemental (<4+) doping on the phase transition of VO2 are still controversial. In the present work, Ni-doped VO2 films were prepared on quartz glass by direct current reactive magnetron sputtering and subsequent annealing. With the increase of the Ni doping content, the phase transition temperature of heating (TH) of the VO2 films decreased from 73.4 °C to 52.4 °C. The temperature required for the occurrence of phase transition (Tb) was lower than TMIT. Different from the undoped VO2 film, the Ni-doped VO2 films had a Tb of around 30 °C. XRD and Raman results revealed that some rutile VO2 microcrystals appeared in the vanadium oxide films because of the lattice distortion by incorporated Ni. Hence, rutile VO2 micro-crystallinities significantly facilitated the phase transition of monoclinic VO2 to rutile one.

Formation and investigation of characteristics of thermoresistive structures based on vanadium oxide films

The processes of reactive magnetron sputtering of a V target in Ar/O2 gas mixture are investigated. It was found that when using a pulsed current for sputtering and a pressure in the chamber less than 0.06 Pa, the intensities of the emission lines of vanadium at 437.922 nm, argon at 750.386 nm, and oxygen at 777.417 nm with a change in the oxygen concentration in Ar/O2 gas mixtures (ГO2) have no hysteresis and unambiguously depend on the parameters of the sputtering process, which makes it possible to stabilize the process without using feedback systems. By monitoring the sputtering process by optical emission spectroscopy and depositing films on a rotating substrate of diameter 100 mm, vanadium oxide (VOx) films with nonuniformity thickness less than ±2.4 % and surface resistance less than ±2.5 % were obtained. Studies by transmission line method of the influence of the parameters of the reactive magnetron sputtering and subsequent annealing at O2 pressure of 0.04 Pa on the characteristics of thermoresistive structures based on VOx films showed that when the contacts are deposited without ion cleaning, the current-voltage characteristics (IV) and the dependence of the resistance on the length of resistors R(L) are nonlinear, which indicates the presence of a potential barrier in the contacts. Preliminary ion cleaning can significantly improve the linearity of the IV characteristic. The most linear IV characteristics were obtained for Ti contacts. However, the specific contact resistance of the VOx/Ti contact increases with an increase in the oxidation state of the VOx films and reaches ρc = 0.1 Ohm·m2 at the specific resistance of vanadium oxide ρ = 0.1 Ohm·m. The analysis of the dependences of the temperature coefficient of resistance (TCR) and ρ of VOx films on the annealing temperature showed that, upon annealing, ρ and TCR slightly decrease, i. e. there occurs a partial deoxidation of the films. However, unlike annealing at atmospheric pressure, there are no temperature regions at which a sharp decrease in the resistivity and TCR occurs.

Atomic Layer Deposition and Thermal Transformations of Thin Titanium–Vanadium Oxide Films

Atomic Layer Deposition and Thermal Transformations of Thin Titanium–Vanadium Oxide Films

Low threshold voltage, highly stable electroforming-free threshold switching characteristics in VOx films-based device

Threshold switching (TS) devices have evolved as one of the most promising elements in memory circuit due to their important significance in suppressing crosstalk current in the crisscross array structure. However, the issue of high threshold voltage (Vth) and low stability still restricts their potential applications. Herein, the vanadium oxide (VOx) films deposited by the pulsed laser deposition (PLD) method are adopted as the switching layer to construct the TS devices. The TS devices with Pt/VOx/Pt/PI structure exhibit non-polar, electroforming-free, and volatile TS characteristics with an ultralow Vth (+0.48 V/−0.48 V). Besides that, the TS devices also demonstrates high stability, without obviously performance degradations after 350 cycles of endurance measurements. Additionally, the transition mechanism is mainly attributed to the synergistic effect of metal-insulator transition of VO2 and oxygen vacancies. Furthermore, the nonvolatile bipolar resistance switching behaviors can be obtained by changing oxygen pressure during the deposition process for switching films. This work demonstrates that vanadium oxide film is a good candidate as switching layer for applications in the TS devices and opens an avenue for future electronics.

Influence of annealing on structure, phase and electrophysical properties of vanadium oxide films

The aim of this work was to study the effect of the parameters of deposition process and subsequent annealing on the properties of vanadium oxide VO x films deposited by the pulsed reactive magnetron sputtering of a V target in an Ar/O 2 gas mixture. The dependences of the structure, phase, temperature coefficient of resistance (TCR), resistivity p , band gap Egof the films on the oxygen concentration in Ar/O 2 gas mixture during the deposition Г O2 , and the temperature of annealing in an O 2 atmosphere were obtained. The films were found to have an amorphous structure after deposition. Crystallization processes are observed at temperatures above 275 °C. In this case, depending on the temperature, polycrystalline films with a monoclinic, cubic or mixed crystal lattice are formed and a transition occurs from the intermediate oxide V 4 O 9 to the mixed phase VO 2 /VO x /V 2 O 5 and then to the higher oxide V 2 O 5 . The character of changes in p , TCR and Egof films coming from the change in the annealing temperature is complex and largely determined by Г O2 . It was established that with the view of using VO x films as thermosensitive layers, the following conditions of deposition and annealing would be preferable: films deposited at the oxygen concentration 25 % in Ar/O 2 gas mixture and annealed at a temperature of 250–275 °C in an O 2 atmosphere for 10 min. Under these conditions VO x films with the following properties were obtained: p = (1.0 – 3.0) . 10 -2 Ohm . m, TCR = 2.05 %/°C, and E g = 3.76–3.78 eV.