VO2 Thin Films Research Articles

Impact of Zn and W Doping Levels on Properties of Thermochromic VO2-Based Thin Films

DC magnetron sputtering at a substrate temperature of 425 °C was used to successfully deposit W/Zn-doped VO2 thin films on soda lime glass (SLG) substrates. The aim was to investigate the influence of Zn doping levels on the transition temperature ( ), luminous transmittance (Tlum) and solar transmittance modulation (ΔTsol) of VO2-based thin films. UV/VIS/NIR spectrometer, X-ray Diffraction (XRD), Atomic Force Microscope (AFM), and Rutherford Backscattering Spectroscopy (RBS) were used to characterise the thin films. It was revealed that W/Zn co-doped VO2 thin films with ~ 10.8 at.% Zn showed a luminous transmittance of ~ 40.4%, with excellent solar transmittance modulation of 10.2%. Furthermore, the transition temperature obtained for the Zn and W co-doped VO2 films with ~ 10.8 at.% Zn was lower at 23.1 °C compared to 25.6 °C and 26.8 °C for thin films with ~ 3.9 at.% Zn and ~ 2.8 at.% Zn, respectively. It was not possible to deposit the films with Zn doping level above 10.8 at.% due to some technical limitations. These findings indicate that thin films with a controlled proportion of Zn in the W/Zn co-doped VO2-based thin films have the potential to be employed for applications such as smart windows.

Ultrathin VO2 grown with oxygen plasma molecular beam epitaxy on TiO2 (001) and Al2O3 (0001)

Bulk VO2 undergoes an insulator-to-metal transition (IMT) with up to five orders of magnitude change in the resistance at 340 K. However, when VO2 is deposited as a film on a substrate, the strain from the substrate can alter the IMT temperature, resistivity ratio of IMT, and hysteresis. Here, we present single-phase VO2 ultrathin films (thickness less than 20 nm) grown using oxygen plasma molecular beam epitaxy (MBE) on TiO2 (001) and Al2O3 (0001) substrates. First, we modify existing recipes employing ozone MBE and reproduce the best films from literature on TiO2 (001), maintaining an almost three orders of magnitude transition in a 12 nm thick film with TIMT of 308 K (296 K) upon warming (cooling). We then extend our recipe to Al2O3 (0001) substrates where we stabilize a 12 nm thin single-phase VO2 film and observe two orders of magnitude transition at 337 K (329 K) upon warming (cooling), expanding the possible growth methods for ultrathin VO2 films on Al2O3 (0001).

Stabilization of the VO2(M2) Phase and Change in Lattice Parameters at the Phase Transition Temperature of WXV1-XO2 Thin Films.

Various methods have been used to fabricate vanadium dioxide (VO2) thin films exhibiting polymorph phases and an identical chemical formula suited to different applications. Most fabrication techniques require post-annealing to convert the amorphous VO2 thin film into the VO2 (M1) phase. In this study, we provide a temperature-dependent XRD analysis that confirms the change in lattice parameters responsible for the metal-to-insulator transition as the structure undergoes a monoclinic to the tetragonal phase transition. In our study, we deposited VO2 and W-doped VO2 thin films onto silica substrates using a high repetition rate (10 kHz) fs-PLD deposition without post-annealing. The XRD patterns measured at room temperature revealed stabilization of the monoclinic M2 phase by W6+ doping VO2. We developed an alternative approach to determine the phase transition temperatures using temperature-dependent X-ray diffraction measurements to evaluate the a and b lattice parameters for the monoclinic and rutile phases. The a and b lattice parameters versus temperature revealed phase transition temperature reduction from ∼66 to 38 °C when the W6+ concentration increases. This study provides a novel unorthodox technique to characterize and evaluate the structural phase transitions seen on VO2 thin films.

Investigation of resonant properties of metamaterial THz filters fabricated from vanadium dioxide thin films

Vanadium dioxide (VO2) is a promising candidate for electronic and optical switching applications in the terahertz frequency range due to the metal to insulator transition (MIT). In this study, the use of VO2 patterned as a metamaterial surface or coupled as a homogeneous layer with a metallic metamaterial surface on top is investigated in terms of performance. High-quality VO2 thin films were deposited on c-cut sapphire substrates by using the dc magnetron sputtering technique. A change in resistivity by a factor of 104 MIT in VO2 was observed allowing to investigate its use as a controllable metamaterial. The layer was patterned using a unique geometry (four-cross shaped) that operates in the THz frequency range. To understand its performance as a tunable THz filter, the four-cross structure fabricated from VO2 is compared to one fabricated from Au on VO2 bare film using UV lithography and ion beam etching techniques. The spectral performances of metamaterials were assessed using THz-Time Domain Spectroscopy (THz-TDS) and results were compared with simulations based on CST Microwave Studio. Absence of the resonant effects in the purely developed VO2 device, while clear observation of the MIT behavior shows the strong dependency of the inductive and/or capacitive effects of the four-cross structure on conductivity of the surface metamaterial, which is clearly observable for the Au-based device. In the latter case, the resonant transmittance of the filter can be effectively modulated by change in temperature.

Bilayer vanadium dioxide thin film with elevated transition temperatures and high resistance switching

Despite widespread interest in the phase-change applications of vanadium dioxide (VO2), the fabrication of high-quality VO2 thin films with elevated transition temperatures (TIMT) and high insulator–metal-transition resistance switching still remains a challenge. This study introduces a two-step atmospheric oxidation approach to fabricate bilayer VO2−x/VO2 films on a c-plane sapphire substrate. To quantify the impact of the VO2 buffer layer, a single-layer VO2 film of the same thickness was also fabricated. The bilayer VO2−x/VO2 films, wherein the top VO2−x film was under-oxidized, demonstrated an elevation in TIMT reaching ∼97 °C, one of the highest reported to date for VO2 films and is achieved in a doping-free manner. Our results also reveal a one-order increase in resistance switching, with the optimum bilayer VO2/VO2 film exhibiting ∼3.6 orders of switching from 25 to 110 °C, compared to the optimum single-layer VO2 reference film. This is accompanied by a one-order decrease in the on-state resistance in its metallic phase. The elevation in TIMT, coupled with increased strain extracted from the XRD characterization of the bilayer film, suggests the possibility of compressive strain along the c-axis. These VO2−x/VO2 films also demonstrate a significant change in the slope of their resistance vs temperature curves contrary to the conventional smooth transition. This feature was ascribed to the rutile/monoclinic quasi-heterostructure formed due to the top VO2−x film having a reduced TIMT. Our findings carry significant implications for both the lucid fabrication of VO2 thin film devices as well as the study of phase transitions in correlated oxides.

Structural and Optical Properties of N+ Implanted V2O5 Thin Film on Glass Substrate

In this study, we investigate the effect of nitrogen (16 keV N+) ion implantation on V2O5 thin films with varying thicknesses, focusing on their structural, compositional, and optical characteristics. Through X-ray diffraction, we reveal the coexistence of orthorhombic and tetragonal phases, while UV–vis spectroscopy determines the band gap. Atomic force microscopy unveils the intriguing surface morphology, with the thickest film displaying high smoothness. Notably, grain formation and growth dynamics are intricately linked to roughness (α) and fractal dimension (Df) calculations. Additionally, wetting properties are connected to fractal growth in ion-implanted films. This work underscores the stable optical band gap of V2O5 thin films for optical applications and offers insights into managing wetting behaviors in nanofluidics through fractal patterns, potentially impacting nanotechnology and material engineering.

Zr and W Co-doped VO2 thin films with improved luminous transmittance and transition temperature

Zr and W Co-doped VO2 thin films with improved luminous transmittance and transition temperature

Charge ordering at a dielectric gate in itinerant metallic states with low-field memristor properties in VO2 thin films

Vanadium dioxide (VO2) − a non-stoichiometric oxide semiconductor (SC) offers exotic properties at a self-confined structure of correlated 3d1-electrons (spins) useful for non-volatile memory devices, smart switches, and human brain-inspired neuromorphic devices. Poor chemical stability and fragile nature limit its technologies of thin films. In view of resolving some of these issues, we developed polymer stabilized VO2 films (thickness t ≤ 100 nm), using VO2 nanocolloids in poly(vinylpyrrolidone) (PVP) (as a VO2 dispersoid, a molecular template, and a film former) in water, at a (100) Si(p++) substrate. Using a nano SiO2/TiO2 gate (t ≤ 10 nm), VO2 is grown (011) preferentially in a confined shape of nanoplates (nanocrystals) along the films, mostly of 15 to 40 nm widths at 20–30 nm crystallite size. The results are described with X-ray diffraction, surface topologies, lattice images, and X-ray photoelectron spectroscopy (XPS) of films in the variable charges 2V4+ → V3+ + V5+ order at the itinerant metallic states. A significant V5+-3d0, ≤ 33 at%, is shown in the XPS bands, which induces metallic states at conducting ‘V4+ → V5+ + e−’ channels. So, a charge-regulated SC → metal transition incurs via an induced M1 → R-VO2 metallic state near room temperature. A memristor VO2@TiO2/Si so made renders a wide current-voltage (I-V) loop at room temperature, with a leakage current that is well controlled at a high-k TiO2 gate. It exhibits a reversible switching at a duly small threshold field, Vt ≤ 0.2 V. This is the smallest Vt tuned so far beneficial for the low field, ≤ 1.0 V, devices. The charge models corroborate the effect of field-induced charge order at the interfaces of ‘conducting through channels’, regulating a reversible I-V hysteresis in an on-off cycle.

Impact of the crystallographic variants of VO2 thin films on c- and r-cut sapphire on structural phase transition and radiofrequency properties

Vanadium dioxide exhibits a metal to insulator transition close to room temperature, making it very interesting in particular for radio frequency (RF) device applications. Here, we compare the structural and RF properties of VO2 thin films grown by magnetron sputtering on c-cut and r-cut sapphire substrates. The epitaxial growth of VO2 on c-cut sapphire gives rise to several crystallographic variants for the insulating M1 phase. Moreover, during the structural transition, simultaneous presence of both metallic and insulating phases is evidenced by x-ray diffraction over a large temperature range. Films grown on r-cut sapphire exhibit only two variants and present a very narrow temperature range of their structural transition. Interestingly, such structural differences of the films grown on c- and r-cut sapphire substrates have very little influence on their dc resistivity, while the transmission of the RF signal through the metallic phase was found much lower on c-cut than on r-cut sapphire. This supports the fact that r-cut sapphire is preferable for VO2-based RF device fabrication.

Photothermally Activated Artificial Neuromorphic Synapses.

Biological nervous systems rely on the coordination of billions of neurons with complex, dynamic connectivity to enable the ability to process information and form memories. In turn, artificial intelligence and neuromorphic computing platforms have sought to mimic biological cognition through software-based neural networks and hardware demonstrations utilizing memristive circuitry with fixed dynamics. To incorporate the advantages of tunable dynamic software implementations of neural networks into hardware, we develop a proof-of-concept artificial synapse with adaptable resistivity. This synapse leverages the photothermally induced local phase transition of VO2 thin films by temporally modulated laser pulses. Such a process quickly modifies the conductivity of the film site-selectively by a factor of 500 to "activate" these neurons and store "memory" by applying varying bias voltages to induce self-sustained Joule heating between electrodes after activation with a laser. These synapses are demonstrated to undergo a complete heating and cooling cycle in less than 120 ns.

Resistive switching localization by selective focused ion beam irradiation

Materials displaying resistive switching have emerged as promising candidates for implementation as components for neuromorphic computing. Under an applied electric field, certain resistive switching materials undergo an insulator-to-metal transition through the formation of a percolating filament, resulting in large resistance changes. The location and shape of these filaments are strongly influenced by hard-to-control parameters, such as grain boundaries or intrinsic defects, making the switching process susceptible to cycle-to-cycle and device-to-device variation. Using focused Ga+ ion beam irradiation, we selectively engineer defects in VO2 and V2O3 thin films as a case study to control filament formation. Using defect pre-patterning, we can control the position and shape of metallic filaments and reduce the switching power significantly. A greater than three orders of magnitude reduction of switching power was observed in V2O3, and a less than one order of magnitude reduction was observed in VO2. These experiments indicate that selective ion irradiation could be applied to a variety of materials exhibiting resistive switching and could serve as a useful tool for designing scalable, energy efficient circuits for neuromorphic computing.

Effect of ZnO dopant on V2O5:ZnO films electrochromic properties

Due to its unique physical and chemical properties, vanadium pentoxide has become a popular material for electrochromic devices, and using doping with other transition metals can enhance its overall performance. Therefore, the peroxovanadate sol-gel method was used to produce ZnO-doped V2O5 thin films. The results of this work show how ZnO doping affected the electrochromic and structural characteristics of thin films of V2O5. The main finding was that adding ZnO to the V2O5 films improved their cyclic stability and reversibility. Besides that, the incorporation of the dopant resulted in a mixture of crystalline phases, according to the X-ray diffraction. The charge density has increased from 28.79 for undoped to 41.68 mC/cm2 for doped films. The UV–Vis transmittance values between bleached and colored states - ΔT at 633 nm - changed from 14.4 to 55.5% for undoped and doped films, respectively. The roughness (root mean square, RMS) of the film remained the same, i.e., 23.5 ± 4.3 for undoped to 23.8 ± 5.1 nm for doped, as revealed by atomic force microscopy. Finally, the doping of V2O5 with ZnO improved the electrochemical and electrochromic characteristics of these new films, making them promising candidates for electrochromic device applications.

Tunable IR perfect absorbers enabled by tungsten doped VO2 thin films

The temperature tunability of complex dielectric constants of vanadium dioxide (VO2) makes it a promising phase-change material for use in active, dynamic, tunable photonics applications. Specifically, the semiconductor-to-metal phase transition in VO2 enables reversible, broadband, and large complex refractive index variation and paves the way for a plethora of applications. Although the critical temperature for phase-transition is 68 °C for VO2 films, its transition temperature can be reduced to room temperature by tungsten-doping of vanadium dioxide. Such a degree of freedom in controlling the critical temperature through tungsten doping provides further tunability of the thermochromic behavior. In this work, we investigate a variety of W-doped VO2 thin films deposited by laser ablation of targets with increasing W doping content and report detailed infrared characterization together with numerical simulations. Our experimental results indicate that the perfect absorption can be achieved at different temperatures, within the VO2 insulator-to-metal phase transition process, as a function of W doping content. Tunable subwavelength layers allow perfect absorption under different temperature conditions around λ = 12 µm. We show that a high dynamic range of reflectivity can be achieved when the temperature is increased above the phase transition temperature. Furthermore, we observe perfect absorption at 11.8 µm at room temperature for a W content of 0.75%. We believe that W-doped VO2 thin films with tunable and controllable perfect absorption will open the way for a class of promising thermo-optical devices including thermos-photovoltaics, infrared filters, radiative cooling devices, and thermal emitters.

Structural, optical properties of V2O5 and NiO thin films and fabrication of V2O5/NiO heterojunction

The exceptional stability of metal oxide heterojunctions makes them worthy of the future. Here we report V2O5/NiO heterojunction device fabricated utilizing both physical and chemical deposition techniques. Orthorhombic V2O5 thin films were grown by thermal evaporation technique using the V2O5 nanostructures synthesized via reverse micelle method. Nickel oxide (NiO) thin films were deposited using the solution-processed spin coating technique. The structural, optical and morphological properties of the V2O5 and NiO thin films were studied in detail. Highly transparent V2O5/NiO heterojunction was fabricated on FTO coated glass substrate with a device geometry of FTO/NiO/V2O5/Ag. The electrical properties were studied and the J-V curve shows a rectifying nature with a rectification ratio of 12 at a bias voltage of 0.7 V and a knee voltage of 0.58 V. The variation of the ideality factor is studied in different linear regions of the ln (J)- V curve. From the C-V characteristics of the device, a built-in potential of 0.54 V was obtained.

Femtosecond laser modification of vanadium dioxide for color display applications

Vanadium dioxide (VO2) has been one of the most significant functional materials for its prominent property change during phase transition. Here the modulation in crystallinity and morphology of VO2 thin film is achieved by femtosecond laser direct writing. Both the local crystallization from an amorphous phase and self-organized periodical structures are generated due to the well-controlled femtosecond laser energy deposition and controlled accumulation. The dynamic evolution of either the volumetric or surface nano-/micro-structure depends strongly on the femtosecond laser conditions, revealed by changes of the Raman bands and reflectance spectra at visible wavelengths. Based on the distinguishable color display performance of differently processed regions, a four-color-level image has been drawn on a VO2 thin film. This work proposes an elaborate “annealing by light” and optical property modulation method for the VO2, paving the way to producing complex integrated multi-functional devices for color display and data storage.

Thermochromic VO[formula omitted] thin film from electron beam evaporated V[formula omitted]O[formula omitted]/Ni/V[formula omitted]O[formula omitted] and V[formula omitted]O[formula omitted]/Co/V[formula omitted]O[formula omitted] multilayer stack system

The oxygen–vanadium system presents different equilibrium phases amongst which monoclinic and tetragonal forms of VO2 exist below and above the phase transition temperature of 68 °C. The structure can be explained by pure Mott phase transition with Peierls electron-lattice distortion giving ultrafast transformation of the semiconducting monoclinic VO2 to metallic rutile structure. Hence, practical applications in technology can be explored such as; switching devices, thermal relays and energy management, sensors and actuators, electrochromic and photochromic memories. In this study, VO2 thermochromic thin films were deposited on a quartz glass substrate by electron beam evaporation technique in ultrahigh vacuum at room temperature using sandwich structured V2O5/metal (Ni, Co)/V2O5 multilayer target followed by post-annealing at 500 °C under vacuum. The morphology and crystal structure were investigated by X-ray diffraction, Ultraviolet/Visible/Near Infrared spectrophotometry, and scanning electron microscopy analyses. The sandwich structure effectively regulates the stoichiometry and polymorphs of the material. It was observed that the polymorph of the films switch from VO2 (B) to VO2 (M) when the Ni and Co middle layer reached 7 nm and 10 nm respectively. For the aforementioned thicknesses of the Ni and Co layers, the V2O5/Ni/V2O5 and V2O5/Co/V2O5 multilayers showed thermochromic properties. For V2O5/Ni (7 nm)/V2O5 and V2O5/Co (10 nm)/V2O5 multilayer films, the temperature-dependent spectral transmission measurements show the change in optical range of ∼ 65% and ∼ 43% respectively after the phase transition at wavelength 2500 nm. Transmission measurements of VO2 thin films, were determined at temperatures below and above the phase transition temperature in both the visible and the near-infrared ranges of the electromagnetic spectrum. The temperature-dependent optical transmittance measurement for V2O5/Co (10 nm)/V2O5 multilayer films shows that the multilayer exhibits thermochromic properties and has higher phase transition temperature.

Characterization of the VO2 thin films grown on glass substrates by MOD

AbstractThis paper describes the deposition of Nb doped vanadium dioxide (VO2) films on a glass substrate by metal organic decomposition (MOD) and their thermochromic properties. The difference in thermochromic properties of VO2 thin films on a glass substrate was investigated with and without a buffer layer of Hf0.5Zr0.5O2 (HZO). The phase transition temperature of VO2 thin film successfully reduced from 83°C to 43°C on a glass substrate with a buffer layer of HZO. Without a buffer layer of HZO, the thermochromic properties of VO2 thin films deteriorated comparing with a buffer layer of HZO. HZO buffer layer effectively suppresses the miniaturization of VO2 crystallite size of thin film due to Nb doping. Moreover, it would block out the diffusion of Al, Na, and Ca impurity ions from a glass substrate and the partial oxidation of VO2 thin films judging from XPS O1s spectra and XPS depth profile analysis. We conclude that the insertion of the HZO buffer layer is a useful technique for controlling the transition temperature of VO2 for the smart window applications by MOD.

Tailoring the thermochromic properties of sputter-deposited VO2 films by O2+ ion irradiation

Vanadium dioxide (VO2) thin film is a well-known phase transition material showing extensive modification in their physical properties, such as electronic, magnetic, and optical characteristics when the metal–insulator transition (MIT) occurs. Many approaches (including doping, micro-structure, external stress, and defect engineering) can enhance the MIT characteristics of vanadium dioxide thin films. More recently, low-energy ion irradiation has been well-established to modify the electrical, optical, and physical properties of metal oxide thin films. In this paper, the effect of low-energy O2+ ion irradiation on the thermochromic properties of VO2 thin films was investigated. From the temperature-dependent near infrared transmittance measurements, both MIT temperature and thermal hysteresis width of VO2 thin films showed an opposite trend and decreased as the ion fluence increased. The values of solar modulation and luminous transmittance of irradiated VO2 films were significantly altered by introducing low-energy ion irradiation, compared to the pristine VO2 thin films. According to the XPS analyses, the vanadium ion are in mixed of two valence states (V4+ and V5+). Moreover, the concentration of oxygen vacancies in VO2 films increases as the fluence of low-energy O2+ ion increases. Our study concludes that low-energy O2+ ion irradiation is an effective means to optimize the MIT characteristics of VO2 thin films to a certain extent.

Evaluation of low-voltage-driven multi-colored electrochromic device based on dry-deposited V2O5

A V2O5 film fabricated by a nanoparticle deposition system (NPDS) was used to demonstrate a highly durable electrochromic device (ECD). The V2O5 film deposited on both the electrodes functioned as a complementary layer during the redox reaction to balance the charge inside the device. Thereby, it mitigated the load applied to the ECD by enabling its electrochromic reaction within the range of ±1 V. The fabricated ECD showing different colors at opposite voltages were applied to both the electrodes. Meanwhile, a TiO2-based opaque electrolyte was used to show different colors of V2O5 thin films independently. The fabricated ECD can display three colors: yellow, green, and blue. Furthermore, each color variation is completed within 10 s. According to the International Commission on Illumination (CIE) data, the color differences (ΔE) between yellow and green, green and blue, and yellow and blue were measured to be approximately 27, 17, and 44, respectively. Meanwhile, the fabricated ECD showed almost a similar level of electrochromic performance even after 500 repeated runs. Moreover, the charge capacity of the ECD after a 10,000-cycle cyclic voltammetry (CV) analysis was 49.01 mC/cm2 (the initial charge capacity was measured to be 47.31 mC/cm2). This verified the high durability of the device. Thus, an ECD capable of coloration under a low voltage range of ±1 V was fabricated using a dry deposition method with a TiO2-based opaque electrolyte, to vividly display different colors at both sides of the device simultaneously. The unique device can independently display different colors on both the electrodes. The dry deposition method was applied to deposit an identical V2O5 electrochromic film on the electrodes and an opaque electrolyte. The device has potential applications in functional electrochromic displays.

Editors’ Choice—Equilibrium Boundaries of VO2 Phase Stability: Determination by Solid Electrolyte EMF Measurements and Effect on Optical Changes during Metal-Insulator Transition

Vanadium dioxide is widely known for its metal-insulator transition (MIT), in which drastic changes in resistivity and IR-transparency occur. This makes VO2 thin films promising materials for high-frequency optoelectronic devices. To get the most MIT sharpness, thin films should not contain impurities of hyper-oxygen or hypo-oxygen phases arising during VO2 synthesis. To ascertain the conditions of single-phase VO2 existence, the equilibrium boundaries of VO2 with neighboring phases were determined using the electromotive force method (EMF) with a solid electrolyte ZrO2(Y2O3). Our data for the high-oxygen boundary of VO2 existence in equilibrium with the V6O13 phase agree with the only data known in the literature. We established that VO2 is, in equilibrium with the V9O17 phase at the low-oxygen boundary, which forms V8O15 under further reduction. The temperature of the peritectoid decomposition of V9O17 is established, and the corresponding corrections to the phase diagram of the vanadium-oxygen system are introduced. The Gibbs energies for V9O17, V8O15, and V6O13 formation reactions are calculated. It is also shown that the IR reflectance of VO2 films brought to equilibrium at the high-oxygen boundary is much greater than that of films equilibrated at the low-oxygen boundary.