超高温度系数V0.97W0.03 O2多晶薄膜的制备研究

将V₂O₅粉体与WO₃粉体均匀混合并压制成靶，用离子束增强沉积加后退火技术在SiO₂衬底上制备掺钨VO₂多晶薄膜.X射线衍射表明，薄膜取向单一，为VO₂结构的［002］相,晶格参数d比VO₂粉晶增大约0.34%；薄膜从半导体相向金属相转变的相变温度约28；室温(300 K)时的电阻-温度系数(TCR)可大于10%/K，是目前红外热成像薄膜TCR的四倍.W离子的半径大于V离子的半径，W的掺入在薄膜中引入了张应力,使薄膜相变温度降低到室温附近，是IBED V_0.97W_0.03O₂薄膜的室温电阻温度系数提高的原因. 关键词： 二氧化钒薄膜 薄膜掺杂 离子束增强沉积     

Oxygen stoichiometry controlled sharp insulator-metal transition in highly oriented VO2/TiO2 thin films

The insulator-metal transition (IMT) in vanadium dioxide (VO₂) which occurs above room temperature (67 °C) is highly sensitive to atomic defects caused by oxygen stoichiometry. The strained growth and the degree of oxygen deficiency in VO₂ epitaxial films result in lowering of transition temperature below room temperature as well as the broadening of transition parameters such as transition width and hysteresis width, which limit its application potential. Here we demonstrate the growth of highly oriented strain-relaxed VO₂ thin films on (001)-oriented TiO₂ substrates at various oxygen partial pressures, exhibiting the narrow transition and hysteresis width. The cross-sectional transmission electron microscopy and x-ray diffraction analyses of the films reveal the highly oriented growth of insulating monoclinic VO₂. The IMT parameters associated with temperature-dependent phase transition vary with the oxygen partial pressure used during the deposition. The presence of multiple and mixed valence states of vanadium in the films was confirmed by Raman and XPS analyses. We have achieved a narrow transition width (2.3 °C) and hysteresis width (1.2 °C) through controlling the oxygen stoichiometry during the growth of VO₂/TiO₂ films.     

Optical data recording with vanadium dioxide-based film reversible media

Thermally induced semiconductor-metal phase transitions taking place in Al/VO₂/insulator and VO₂/insulator film systems upon recording optical data are investigated. Experiments show that the rate of the processes depends on thermophysical parameters of the films, as well as on the energy exposure. The dynamic range of the energy exposure sensitivity is defined by the hysteresis loop width. The shape and width of the hysteresis loop depends on the VO₂ film thickness and the nonstoichiometric ratio profile across the film.     

硅基二氧化钒相变薄膜电学特性研究

本文以原子层沉积超薄氧化铝(Al₂ O₃)为过渡层, 采用射频反应磁控溅射法在硅半导体基片上制备了颗粒致密并具有(011)择优取向的二氧化钒(VO₂)薄膜. 该薄膜具有显著的绝缘体–金属相变特性, 相变电阻变化超过3 个数量级, 热滞回线宽度约为6℃. 基于VO₂薄膜构建了平面二端器件并测试了不同温度下I-V曲线, 观测到超过2个数量级的电流跃迁幅度, 显示了优越的电致相变特性. 室温下电致相变阈值电压为8.6 V, 电致相变弛豫电压宽度约0.1 V. 随着温度升高到60℃, 其电致相变所需要的阈值电压减小到2.7 V. 本实验制备的VO₂薄膜在光电存储、开关、太赫兹调控器件中具有广泛的应用价值.     

离子束增强沉积VO_2多晶薄膜的温度系数

用改进的离子束增强沉积方法和恰当的退火从V2 O5粉末直接制备了VO2 多晶薄膜 .实验测试表明 ,薄膜的取向单一、相变特性显著、结构致密、界面结合牢固、工艺性能良好 ,薄膜的电阻温度系数 (TCR)最高可达 4 2 3% K .从成膜机理出发 ,较详细地讨论了离子束增强沉积VO2 多晶薄膜的TCR高于VOx 薄膜的TCR的原因 .分析认为 ,单一取向的VO2 结构使薄膜晶粒具有较高的电导激活能 ,致密的薄膜结构减少了氧空位和晶界宽度 ,使离子束增强沉积VO2 多晶薄膜结构比其他方法制备的VOx 薄膜更接近于单晶VO2 是其具有高TCR的原因     

Compare the phase transition properties of VO2 films from infrared to terahertz range

VO₂ with reversible semiconductor–metal phase transition properties is particularly available for the application in smart opto-electrical devices. However, there are rare reports on comparing its phase transition properties at different ranges. In this study, the VO₂ films are designed with the similar crystalline structure and stoichiometry, but different morphologies by inorganic and organic sol-gel methods, and their phase transition characteristics are compared both at infrared and terahertz range. The results indicate that the VO₂ film prepared by inorganic sol-gel method shows more compact nanostructure. It results in larger resistivity change, infrared and terahertz switching ratio in the VO₂ film. Moreover, it presents that the phase transition intensity of VO₂ film in terahertz range is more sensitive to its microstructure. This work is helpful for understanding the susceptibility of terahertz switching properties of VO₂ to its microstructure. And it can provide insights for the applications of VO₂ in terahertz smart devices.     

Effect of oxygen partial pressure and VO<Subscript>2</Subscript> content on hexagonal WO<Subscript>3</Subscript> thin films synthesized by pulsed laser deposition technique

We report on the effect of oxygen partial pressure and vacuum annealing on structural and optical properties of pulsed laser-deposited nanocrystalline WO₃ thin films. XRD results show the hexagonal phase of deposited WO₃ thin films. The crystallite size was observed to increase with increase in oxygen partial pressure. Vacuum annealing changed the transparent as-deposited WO₃ thin film to deep shade of blue color which increases the optical absorption of the film. The origin of this blue color could be due to the presence of oxygen vacancies associated with tungsten ions in lower oxidation states. In addition, the effects of VO₂ content on structural, electrochemical, and optical properties of (WO₃)_1−x (VO₂) _x nanocomposite thin films have also been systematically investigated. Cyclic voltammogram exhibits a modification with the appearance of an extra cathodic peak for VO₂–WO₃ thin film electrode with higher VO₂ content (x ≥ 0.2). Increase of VO₂ content in (WO₃)_1−x (VO₂) _x films leads to red shift in optical band gap.     

The characteristics of Au:VO2 nanocomposite thin film for photo-electricity applications

Au nanoparticles have been fabricated on normal glass substrates using nanosphere lithography (NSL) method. Vanadium dioxide has been deposited on Au/glass by reactive radio frequency (rf) magnetron sputtering. The structure and composition were determined by X-ray diffraction and X-ray photoelectron spectroscope. Electrical and optical properties of bare VO₂ and Au:VO₂ nanocomposite thin films were measured. Typical hysteresis behavior and sharp phase transition were observed. Nanopartical Au could effectively reduce the transition temperature to 40 °C. The transmittance spectrum for both Au:VO₂ nanocomposite thin film shows high transmittance under transition temperature and low transmittance above transition temperature. The characteristics present the Au:VO₂ nanocomposite thin film can be used for applications, such as “smart window” or “laser protector”.     

离子束增强沉积VO2多晶薄膜的温度系数

用改进的离子束增强沉积方法和恰当的退火从V2O5粉末直接制备了VO2多晶薄膜.实验测试表明，薄膜的取向单一、相变特性显著、结构致密、界面结合牢固、工艺性能良好，薄膜的电阻温度系数（TCR）最高可达4.23%/K.从成膜机理出发，较详细地讨论了离子束增强沉积 VO2多晶薄膜的TCR高于VOx薄膜的TCR的原因.分析认为，单一取向的VO2结构使薄膜晶粒具有较高的电导激活能，致密的薄膜结构减少了氧空位和晶界宽度，使离子束增强沉积 VO2多晶薄膜结构比其他方法制备的VOx薄膜更接近于单晶VO2是其具有高TCR的原 关键词： VO2多晶薄膜 离子束增强沉积 热电阻温度系数     

Thermally tunable optical constants of vanadium dioxide thin films measured by spectroscopic ellipsometry

Smart materials with reversible tunable optical constants from visible to near-infrared wavelengths could enable excellent control over the resonant response in metamaterials, tunable plasmonic nanostructures, optical memory based on phase transition and thermally tunable optical devices. Vanadium dioxide (VO₂) is a promising candidate that exhibits a dramatic change in its complex refraction index or complex dielectric function arising from a structural phase transition from semiconductor to metal at a critical temperature of 70 °C. We demonstrated the thermal controllable reversible tunability of optical constants of VO₂ thin films. The optical/dielectric constants showed an abrupt thermal hysteresis which confirms clearly the electronic structural changes. Temperature dependence of dielectric constants as well as optical conductivity of sputtered VO₂ thin films was also reported and compared to previous theoretical and experimental reports.     

Preparation and electrical properties of Bi<Subscript>3.25</Subscript>Pr<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ferroelectric thin films

Bi_3.25Pr_0.75Ti₃O₁₂ (BPT) ferroelectric thin films have been prepared by chemical solution deposition on platinized Si substrates. Well-crystallized BPT films can be achieved by 600 °C rapid thermal annealing. The film surface is smooth and crack-free, composed of uniform spherical grains around 90–100 nm in diameter. The electrical properties of Pt/BPT/Pt thin film capacitors were characterized by hysteresis and impedance measurements. The remanent polarization of 700 °C annealed BPT films is around 20 C/cm² at 120-kV/cm stimulus field. The dielectric constant is around 380 at 10 kHz, 100-mV amplitude. The remanent polarization of BPT film showed a slight reduction, 10% of its original value, after 2.8×10⁹ cycles, while a 30% reduction of non-volatile polarization was observed. PACS 81.15.-z; 77.55.+f; 77.22.Gm     

Effect of buffer layer on VOx film fabrication by reactive RF sputtering

Vanadium oxide VO_x films were fabricated by RF magnetron sputtering on various metal buffer layers or silica glass substrates at a substrate temperature of 400 °C. V₂O₅ film was fabricated on a silica glass substrate, and VO₂ films were fabricated on V, W, Fe, Ni, Ti, and Pt metal buffer layers. The transition temperature of the sample on the V buffer layer was 68 °C and that on the W buffer layer was 53 °C. The VO₂ film was also fabricated on the V buffer layer by non-reactive sputtering using a V₂O₅ target at a substrate temperature of 400 °C.     

VO2-WO3 nanocomposite thin films synthesized by pulsed laser deposition technique

Pure VO₂ and VO₂-WO₃ composite thin films were grown on quartz substrate by pulsed laser deposition (PLD) technique. The influence of varying WO₃ molar concentration in the range from x = 0.0 to x = 0.4 on structural, electrical and optical properties of VO₂-WO₃ nanocomposite thin films has been systematically investigated. X-ray diffraction studies reveal the single crystalline monoclinic VO₂ phase (m-VO₂) up to 10% of WO₃ content whereas both m-VO₂ as well as h-WO₃ (hexagonal WO₃) phases were present at higher WO₃ content (0.2 ≤ x ≤ 0.4). Optical transmittance spectra of the films showed blue shift in the absorption edge with increase in WO₃ content. Temperature dependence of resistivity (R-T) measurements indicates significant variation in metal-insulator transition temperature, width of the hysteresis, and shape of the hysteresis curve. Cyclic Voltammetry measurements were performed on VO₂-WO₃ thin films. A direct correlation between V/W ratio and structure-property relationship was established. The present investigations reveal that doping of WO₃ in VO₂ is effective to increase the optical transmittance and to reduce the semiconductor to metal phase transition temperature close to room temperature.     

Preparation of the La0.8Sr0.2MnO3 films on STO and LAO substrates by excimer laser-assisted metal organic deposition using the KrF laser

La_0.8Sr_0.2MnO₃ films were prepared on SrTiO₃ (STO) and LaAlO₃ (LAO) substrates using excimer laser-assisted metal organic deposition (ELAMOD). For the LAO substrate, no epitaxial La_0.8Sr_0.2MnO₃ film was obtained by laser irradiation in the fluence range from 60 to 110 mJ/cm² with heating at 500 °C. On the other hand, an epitaxial La_0.8Sr_0.2MnO₃ film on the STO substrate was formed by laser irradiation in the fluence range from 60 to 100 mJ/cm² with heating at 500 °C. To optimize the electrical properties for an IR sensor, the effects of the laser fluence, the irradiation time and the film thickness on the temperature dependence of the resistance and temperature coefficient of resistance (TCR: defined as 1/R·(dR/dT)) of the LSMO films were investigated. An LSMO film on the STO substrate that showed the maximum TCR of 3.9% at 265 K was obtained by the ELAMOD process using the KrF laser.     

Electrical,optical and structural properties of pure and gold-coated VO2 thin films on quartz substrate

In the present study, vanadium dioxide films were grown on quartz glass substrate by reactive KrF laser ablation technique using a vanadium dioxide target. The gold films of various thicknesses were then deposited on the VO₂ film by sputtering technique. Films were characterized by X-ray diffraction to determine crystallography, by four-point probe to determine the electrical property and by double-beam spectrophotometry to determine optical reflection and transmission behaviour in the 200–2500 nm spectral region. The resistance per square of VO₂ thin film decreases by two orders of magnitude across the metal insulator transition (MIT). The optical transmittance and reflectance exhibits, strong temperature dependence in the infrared region without a significant change in the visible region for VO₂ thin films. The presence of gold layer on VO₂ films significantly reduces the resistance per square, the critical temperature and percentage transmittance of the materials.     

Pulse laser deposition of vanadium dioxide films

Thin amorphous and crystalline films of VO₂ are obtained on (0001) sapphire substrates via pulsed laser deposition with the speed separation of particles under a variety of deposition conditions. The electrical and optical properties of the films in the vicinity of the phase transition in the temperature range of 20 to 100°C are studied. The temperature of transition (T_c) and the width of the hysteresis are found to be 67.5 and 3°C, respectively.     

Conducting antimony-doped tin oxide films derived from stannous oxalate by aqueous sol-gel method

Nanocrystalline SnO₂:Sb films were prepared by a sol-gel route using C₆H₈O₇-triethanolamine (TEA) mixing aqueous solution with pH 6.5-7.0. Stannous oxalate and antimony trichloride were used as tin and antimony sources. IR, XRD FESEM, FETEM, UV-vis and four-point probe measurement were used to characterize sol-gel chemistry, structure, morphologies, optical and electrical properties. Mechanism of sol-gel reaction illuminated that existence of TEA supplied large numbers of active tin hydrate and ionized state carboxyl groups for tin and antimony chelation through the amido association with the ionized H⁺ on -COOH of H₃L and H₂C₂O₄. The 6 at.% Sb-doped films with film thickness of 600 nm had sheet resistance as low as 42.85 Ω/ when annealed at 450 °C for 10 min. Annealing temperature intensively altered sheet resistance and optimum was in the range of 450-500 °C. The longer annealing time caused Sb volatilization which led to the optimum doping level shifted from 6 to 12 at.%.     

Crystal and magnetic structure of CeVO<Subscript>3</Subscript>

The crystal structure and the magnetic ordering pattern of the electrically insulating perovskite CeVO₃ was investigated by high-resolution powder X-ray diffraction and single-crystal neutron diffraction. A structural phase transition from an orthorhombic to a monoclinic structure (with space groups Pbnm and P2₁/b, respectively) was observed upon cooling below T _s = 136 K. This transition is associated with a strong distortion of the VO₆-octahedra and can be attributed to orbital ordering. A magnetic ordering transition driven by exchange interactions between vanadium moments is observed at T _N = 124 K, and antiferromagnetic interactions between magnetic moments on vanadium and cerium ions induce a progressive magnetic polarization of the cerium sublattice at lower temperatures. The full magnetic structure is described by a superposition of the modes (C _x , F _y , −) and (F _x , C _y , −). The unit cell volume and the tilt angles of the VO₆-octahedra in the CeVO₃-crystal structure are anomalous compared to those of other members of the series RVO₃ (R = lanthanide atom), and the ordered magnetic moments on both vanadium and cerium sublattices at low temperatures are considerably smaller than the free-ion values of V³⁺ and Ce³⁺. Possible origins of this behavior are discussed.     

Oxygen pressure induced structure,morphology and phase-transition for VO2/c-sapphire films by PLD

To investigate the effects of oxygen pressure on the structural and phase transition properties for VO₂/c-sapphire, highly orientated VO₂ thin films were grown on (0001) sapphire substrates by pulsed laser deposition (PLD) with different oxygen pressures. The crystal structure, morphology and component of the films were systematically investigated. The temperature-dependent resistance (R-T) measurement was conducted, which showed the distinct phase transition characteristic for the prepared films. The results indicate that the oxygen pressure plays an important role for the VO₂ film preparation. The film grown at 1.7 Pa has lower phase transition temperature, higher film strain, and smaller grain size than that at 5.4 Pa, while no obvious crystal phase transition is observed. The experiment suggests that even a small change in oxygen pressure can influence the structure, morphology and phase-transition behavior of VO₂ films obviously, and its potential causes are mainly attributed to the reduction of the kinetic energy to the substrate for target atoms caused by the oxygen pressure, the resulting grain aggregation and interfacial stress.