Fabrication of high-quality VO2 thin films by ion-assisted dual ac magnetron sputtering

Surface and Electrical Characterization of Bilayers Based on BiFeO3 and VO2

Thin films of BiFeO₃, VO₂, and BiFeO₃/VO₂ were grown on SrTiO₃(100) and Al₂O₃(0001) monocrystalline substrates using radio frequency and direct current sputtering techniques. To observe the effect of the coupling between these materials, the surface of the films was characterized by profilometry, atomic force microscopy, and X-ray photoelectron spectroscopy. The heterostructures, monolayers, and bilayers based on BiFeO₃ and VO₂ grew with good adhesion and without delamination or signs of incompatibility between the layers. A good granular arrangement and RMS roughness between 1 and 5 nm for the individual layers (VO₂ and BiFeO₃) and between 6 and 18 nm for the bilayers (BiFeO₃/VO₂) were observed. Their grain size is between 20 nm and 26 nm for the individual layers and between 63 nm and 67 nm for the bilayers. X-ray photoelectron spectroscopy measurements show a higher proportion of V⁴⁺, Bi³⁺, and Fe³⁺ in the films obtained. The homogeneous ordering, low roughness, and oxidation states on the obtained surface show a good coupling in these films. The I-V curves show ohmic behavior at room temperature and change with increasing temperature. The effect of coupling these materials in a thin film shows the appearance of hysteresis cycles, I-V and R-T, which is typical of materials with high potential in applications, such as resistive memories and solar cells.

Tunable and polarization-sensitive perfect absorber with a phase-gradient heterojunction metasurface in the mid-infrared

Inspired by the growing family of Van der Waals materials, hBN supported phonon polaritons have attracted much attention due to their inherent hyperbolic dispersion properties in the mid-infrared. However, the lack of tunability imposes a severe restriction on the diversified, functional and integrated applications. Here, we propose a phase-gradient heterostructure metasurface to realize a dynamically tunable and polarization-sensitive perfect absorber in the mid-infrared through combining hBN and phase change VO₂. Narrow-band perfect absorption at 7.2 µm can be switched to broadband around 11.2 µm through controlling the temperature of VO₂. The governed physics of the bandwidth and absorption differences are demonstrated. Phonon polaritons in hBN phase-gradient configurations and plasmon polaritons in periodic VO₂ blocks are respectively excited. We also investigate the absorption dependence on the polarization states of designed absorber. The method of engineering the absorption through controlling the temperature and polarization states opens up a new avenue for tunable applications such as data storage and integrated optical circuits.

Enhanced Metal-Insulator Transition Performance in Scalable Vanadium Dioxide Thin Films Prepared Using a Moisture-Assisted Chemical Solution Approach

Vanadium dioxide (VO₂) is a strong-correlated metal-oxide with a sharp metal-insulator transition (MIT) for a range of applications. However, synthesizing epitaxial VO₂ films with desired properties has been a challenge because of the difficulty in controlling the oxygen stoichiometry of VO _x, where x can be in the range of 1 < x < 2.5 and V has multiple valence states. Herein, a unique moisture-assisted chemical solution approach has been developed to successfully manipulate the oxygen stoichiometry, to significantly broaden the growth window, and to significantly enhance the MIT performance of VO₂ films. The obvious broadening of the growth window of stoichiometric VO₂ thin films, from 4 to 36 °C, is ascribed to a self-adjusted process for oxygen partial pressure at different temperatures by introducing moisture. A resistance change as large as 4 orders of magnitude has been achieved in VO₂ thin films with a sharp transition width of less than 1 °C. The much enhanced MIT properties can be attributed to the higher and more uniform oxygen stoichiometry. This technique is not only scientifically interesting but also technologically important for fabricating wafer-scaled VO₂ films with uniform properties for practical device applications.

Hydrothermal Synthesis of VO2 Polymorphs: Advantages, Challenges and Prospects for the Application of Energy Efficient Smart Windows

Vanadium dioxide (VO₂ ) is a widely studied inorganic phase change material, which has a reversible phase transition from semiconducting monoclinic to metallic rutile phase at a critical temperature of τ_c ≈ 68 °C. The abrupt decrease of infrared transmittance in the metallic phase makes VO₂ a potential candidate for thermochromic energy efficient windows to cut down building energy consumption. However, there are three long-standing issues that hindered its application in energy efficient windows: high τ_c , low luminous transmittance (T_lum ), and undesirable solar modulation ability (ΔT_sol ). Many approaches, including nano-thermochromism, porous films, biomimetic surface reconstruction, gridded structures, antireflective overcoatings, etc, have been proposed to tackle these issues. The first approach-nano-thermochromism-which is to integrate VO₂ nanoparticles in a transparent matrix, outperforms the rest; while the thermochromic performance is determined by particle size, stoichiometry, and crystallinity. A hydrothermal method is the most common method to fabricate high-quality VO₂ nanoparticles, and has its own advantages of large-scale synthesis and precise phase control of VO₂ . This Review focuses on hydrothermal synthesis, physical properties of VO₂ polymorphs, and their transformation to thermochromic VO₂ (M), and discusses the advantages, challenges, and prospects of VO₂ (M) in energy-efficient smart windows application.

Facile Phase Control of Multivalent Vanadium Oxide Thin Films (V2O5 and VO2) by Atomic Layer Deposition and Postdeposition Annealing

Atomic layer deposition was adopted to deposit VO_x thin films using vanadyl tri-isopropoxide {VO[O(C₃H₇)]₃, VTIP} and water (H₂O) at 135 °C. The self-limiting and purge-time-dependent growth behaviors were studied by ex situ ellipsometry to determine the saturated growth conditions for atomic-layer-deposited VO_x. The as-deposited films were found to be amorphous. The structural, chemical, and optical properties of the crystalline thin films with controlled phase formation were investigated after postdeposition annealing at various atmospheres and temperatures. Reducing and oxidizing atmospheres enabled the formation of pure VO₂ and V₂O₅ phases, respectively. The possible band structures of the crystalline VO₂ and V₂O₅ thin films were established. Furthermore, an electrochemical response and a voltage-induced insulator-to-metal transition in the vertical metal-vanadium oxide-metal device structure were observed for V₂O₅ and VO₂ films, respectively.

Influence of Discharge Current on Phase Transition Properties of High Quality Polycrystalline VO₂ Thin Film Fabricated by HiPIMS

To fabricate high-quality polycrystalline VO₂ thin film with a metal-insulator transition (MIT) temperature less than 50 °C, high-power impulse magnetron sputtering with different discharge currents was employed in this study. The as-deposited VO₂ films were characterized by a four-point probe resistivity measurement system, visible-near infrared (IR) transmittance spectra, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy. The resistivity results revealed that all the as-deposited films had a high resistance change in the phase transition process, and the MIT temperature decreased with the increased discharge current, where little deterioration in the phase transition properties, such as the resistance and transmittance changes, could be found. Additionally, XRD patterns at various temperatures exhibited that some reverse deformations that existed in the MIT process of the VO₂ film, with a large amount of preferred crystalline orientations. The decrease of the MIT temperature with little deterioration on phase transition properties could be attributed to the reduction of the preferred grain orientations.

Ultrathin Films of VO2 on r-Cut Sapphire Achieved by Postdeposition Etching

The metal-insulator transition (MIT) properties of correlated oxides thin films, such as VO2, are dramatically affected by strain induced at the interface with the substrate, which usually changes with deposition thickness. For VO2 grown on r-cut sapphire, there is a minimum deposition thickness required for a significant MIT to appear, around 60 nm. We show that in these thicker films an interface layer develops, which accompanies the relaxation of film strain and enhanced electronic transition. If these interface dislocations are stable at room temperature, we conjectured, a new route opens to control thickness of VO2 films by postdeposition thinning of relaxed films, overcoming the need for thickness-dependent strain-engineered substrates. This is possible only if thinning does not alter the films' electronic properties. We find that wet etching in a dilute NaOH solution can effectively thin the VO2 films, which continue to show a significant MIT, even when etched to 10 nm, for which directly deposited films show nearly no transition. The structural and chemical composition were not modified by the etching, but the grain size and film roughness were, which modified the hysteresis width and magnitude of the MIT resistance change.

Solid-state-reaction synthesis of VO2 nanoparticles with low phase transition temperature, enhanced chemical stability and excellent thermochromic properties

Solid-state-reaction synthesis of VO₂ nanoparticles at 500 °C with different time.