Low reflectance sputtered vanadium oxide thin films on silicon

Vanadium oxide thin films on silicon (Si) substrate are grown by pulsed radio frequency (RF) magnetron sputtering technique at RF power in the range of 100–700 W at room temperature. Deposited thin films are characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques to investigate microstructural, phase, electronic structure and oxide state characteristics. The reflectance and transmittance spectra of the films and the Si substrate are recorded at the solar region (200–2300 nm) of the spectral window. Substantial reduction in reflectance and increase in transmittance is observed for the films grown beyond 200 W. Further, optical constants viz. absorption coefficient, refractive index and extinction coefficient of the deposited vanadium oxide films are evaluated.     

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.     

High contrast optical switching in vanadium dioxide thin films

Thermochromic vanadium dioxide thin films prepared by radio-frequency sputtering exhibit controllable transmittance at 1550 nm with a dynamic range exceeding 10³. Efficient optical control with a laser beam at 532 nm is demonstrated with intensities as low as 3 W/cm². Optical switch-on times as short as 100 μs are reported.     

The microstructures and electrical properties of Y-doped amorphous vanadium oxide thin films

One of promising approaches for further improving the sensitivity of microbolometer arrays with greatly-reduced pixel size is using the thermal-sensitive materials with higher performance. In this paper, Y-doped vanadium oxide (VO_x) thin films prepared by a reactively sputtering process exhibit enhanced performance for the microbolometer application compared with frequently-applied VO_x thin films. Both undoped and Y-doped VO_x thin films are amorphous due to the relatively low deposition temperature. Y-doped VO_x thin films exhibit smoother surface morphology than VO_x due to the restrained expansion of particles during depositions. Y-doping increases the temperature coefficient of resistivity by over 20% for the doping level of 1.30 at%. The change rate of resistivity, after aging for 72 h, of thin films was reduced from about 15% for undoped VO_x to 2% due to the introduction of Y. Moreover, Y-doped VO_x thin films have a low 1/f noise level as VO_x ones. Y-doping provides an attractive approach for preparing VO_x thermal-sensitive materials with enhanced performance for microbolometers.     

Effect of fluorine doping on highly transparent conductive spray deposited nanocrystalline tin oxide thin films

The undoped and fluorine doped thin films are synthesized by using cost-effective spray pyrolysis technique. The dependence of optical, structural and electrical properties of SnO₂ films, on the concentration of fluorine is reported. Optical absorption, X-ray diffraction, scanning electron microscope (SEM) and Hall effect studies have been performed on SnO₂:F (FTO) films coated on glass substrates. The film thickness varies from 800 to 1572 nm. X-ray diffraction pattern reveals the presence of cassiterite structure with (2 0 0) preferential orientation for FTO films. The crystallite size varies from 35 to 66 nm. SEM and AFM study reveals the surface of FTO to be made of nanocrystalline particles. The electrical study reveals that the films are degenerate and exhibit n-type electrical conductivity. The 20 wt% F doped film has a minimum resistivity of 3.8 × 10⁻⁴ Ω cm, carrier density of 24.9 × 10²⁰ cm⁻³ and mobility of 6.59 cm² V⁻¹ s⁻¹. The sprayed FTO film having minimum resistance of 3.42 Ω/cm², highest figure of merit of 6.18 × 10⁻² Ω⁻¹ at 550 nm and 96% IR reflectivity suggest, these films are useful as conducting layers in electrochromic and photovoltaic devices and also as the passive counter electrode.     

Mechanical and electrochemical characterization of vanadium nitride (VN) thin films

Vanadium nitride (V-N) thin films were grown using a reactive d.c. magnetron sputtering process, from a vanadium target (99.999%) in an Ar/N₂ gas mixture at different deposition bias voltage. Films were deposited onto silicon (1 0 0) and RUS-3 steel substrates at 400 °C. Structural, compositional, mechanical and electrochemical characterizations were performed by X-ray diffraction (XRD), elastic forward analysis (EFA), nanoindentation, electrochemical impedance spectroscopy (EIS), and Tafel polarization curves, respectively. X-ray diffraction patterns show the presence of (1 1 1) and (2 0 0) crystallographic orientations associated to the V-N cubic phase. Nanoindentation measurements revealed that when the bias voltage increases from 0 V to −150 V the hardness and elastic modulus are increased from 11 GPa to 20 GPa and from 187 GPa to 221 GPa, respectively. EIS and Tafel curves showed that the corrosion rate of steel, coated with V-N single layer films deposited without bias voltage, diminishes 90% compared to the steel without this coating. On the other hand, when the V-N coating was deposited at the highest d.c. bias voltage (−150 V), the corrosion rate was greater than in the steel coated with zero-voltage (0 V) V-N films. This last result could be attributed to the formation of porosities produced by the ion bombardment during the deposition process.     

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.     

Atmospheric pressure chemical vapour deposition of vanadium diselenide thin films

Atmospheric pressure chemical vapour deposition (APCVD) of vanadium diselenide thin films on glass substrates was achieved by reaction of [V(NMe₂)₄] and ^tBu₂Se. X-ray diffraction showed that the VSe₂ films were crystalline with preferential growth either along the (1 0 1) or the (1 1 0) direction. Energy-dispersive analysis by X-rays (EDAX) gave a V:Se ratio close to 1:2 for all films. The films were matt black in appearance, were adhesive, passed the Scotch tape test but could be scratched with a steel scalpel. SEM showed that the films were composed of plate-like crystallites orientated parallel to the substrate which become longer and thicker with increasing deposition temperature. Attempts to produce vanadium selenide films were also performed using ^tBu₂Se and two different vanadium precursors: VCl₄ and VOCl₃. Both were found to be unsuitable for producing VSe₂ from the APCVD reaction with ^tBu₂Se. The VSe₂ showed charge density wave transition at 110-115 K.     

Preparation and properties of vanadium dioxide thermochromic thin films

Deposition of vanadium dioxide and the study of its electrical and optical properties at varying deposition conditions have been presented. The materials have been deposited by reactive r.f. magnetron sputtering technique in Ar, O₂ ambient followed by annealing post-treatment. Electrical conductivity measurements indicate that oxygen pressure plays an important role in obtaining VO₂ and atP _{O 2}=2.4% stoichiometric VO₂ can be obtained. The deposition rate of oxides decreases with increasing O₂ pressure and the rate of VO₂ was about 130Å/min. Optical studies show that VO₂ films exhibit thermochromism and it has the potential application for energy efficient solar energy utilization.     

溶胶凝胶法制备参氟二氧化锡薄膜

利用溶胶凝胶方法，在硅碱玻璃底板上制备的透明低电阻SnO2:F薄膜，是一种低辐射导电薄膜。将SnCl4·5H2O 和 NH4F 溶解在50%乙醇和50%水的溶液中。制备条件为底板温度450℃，喷嘴与底板之间的距离60mm,载气流速8 L/min，制备时间5分钟。制成的SnO2:F薄膜面电阻为2Ω/□，可重复性好。并且文中还定性给出了SnO2:F薄膜其红外反射率与面电阻之间的关系。     

Properties of highly oriented spray-deposited fluorine-doped tin oxide thin films on glass substrates of different thickness

Transparent conducting thin films of fluorine-doped tin oxide (FTO) have been deposited onto the preheated glass substrates of different thickness by spray pyrolysis process using SnCl₄·5H₂O and NH₄F precursors. Substrate thickness is varied from 1 to 6 mm. The films are grown using mixed solvent with propane-2-ol as organic solvent and distilled water at optimized substrate temperature of 475 °C. Films of thickness up to 1525 nm are grown by a fine spray of the source solution using compressed air as a carrier gas. The films have been characterized by the techniques such as X-ray diffraction, optical absorption, van der Pauw technique, and Hall effect. The as-deposited films are preferentially oriented along the (2 0 0) plane and are of polycrystalline SnO₂ with a tetragonal crystal structure having the texture coefficient of 6.19 for the films deposited on 4 mm thick substrate. The lattice parameter values remain unchanged with the substrate thickness. The grain size varies between 38 and 48 nm. The films exhibit moderate optical transmission up to 70% at 550 nm. The figure of merit (φ) varies from 1.36×10⁻⁴ to 1.93×10⁻³ Ω⁻¹. The films are heavily doped, therefore degenerate and exhibit n-type electrical conductivity. The lowest sheet resistance (R_s) of 7.5 Ω is obtained for a typical sample deposited on 4 mm thick substrate. The resistivity (ρ) and carrier concentration (n_D) vary over 8.38×10⁻⁴ to 2.95×10⁻³ Ω cm and 4.03×10²⁰ to 2.69×10²¹ cm⁻³, respectively.     

非化学计量比靶材溅射制备Cu-Al-O薄膜的光学电学性质研究

考虑到铜铝溅射速率的差别,使用铜铝比例为0.9 ∶1的多晶CuAlO₂靶材,用射频磁控溅射法制备Cu-Al-O薄膜.研究不同衬底温度对薄膜光学电学性能的影响.在衬底温度500 ℃附近,薄膜在可见光范围内具有很好的透光性,达到70%,计算拟合得到直接帯隙为3.52 eV,与CuAlO₂相的理论值符合较好.在室温附近,薄膜导电符合半导体热激活机理,在衬底温度为500 ℃附近薄膜电导率达到2.48×10^-3 S·cm^-1. 关键词： Cu-Al-O 衬底温度 透过率 电导率     

溶胶-凝胶VO2薄膜转换特性研究

利用溶胶凝胶法在SiO2Si衬底上沉积高取向的V2O5薄膜,在压强低于2Pa,温度高于400℃的条件下,对V2O5薄膜进行真空烘烤,获得了电阻率变化3个数量级以上、弛豫宽度为62℃的VO2多晶薄膜.以X射线衍射(XRD)、扫描电子显微镜(SEM)图和电阻率转换特性等实验结果为依据,详细分析了溶胶凝胶薄膜在真空烘烤时从V2O5向VO2的转化,它经历了从VnO2n+1(n=2,3,4,6)到VO2的过程.实验证明,根据选择合适的成膜热处理条件和真空烘烤条件是实现溶胶凝胶V2O5结构向VO2结构成功转换的关键 关键词： 溶胶-凝胶法 氧化钒薄膜 VO2膜转换特性     

氧化钒薄膜微观结构的研究

采用直流磁控反应溅射在Si(100)衬底上溅射得到(001)取向的V₂O₅薄膜.x射线衍射(XRD)、扫描电镜(SEM)和傅里叶变换红外光谱(FTIR)的结果表明，氧分压影响薄膜的成分和生长取向，在氧分压0.4Pa时溅射得到(001)取向的纳米V₂O₅薄膜，即沿c轴垂直衬底方向取向生长的薄膜.V₂O₅薄膜经过真空退火得到(001)取向的VO₂薄膜，晶体颗 关键词： 微观结构 氧化钒薄膜 择优取向 直流磁控溅射     

Surface oxidation of vanadium dioxide films prepared by radio frequency magnetron sputtering

This paper reports that the thermochromic vanadium dioxide films were deposited on various transparent substrates by radio frequency magnetron sputtering, and then aged under circumstance for years. Samples were characterized with several different techniques such as x-ray diffraction, x-ray photoelectron spectroscopy, and Raman, when they were fresh from sputter chamber and aged after years, respectively, in order to determine their structure and composition. It finds that a small amount of sodium occurred on the surface of vanadium dioxide films, which was probably due to sodium ion diffusion from soda-lime glass when sputtering was performed at high substrate temperature. It also finds that aging for years significantly affected the nonstoichiometry of vanadium dioxide films, thus inducing much change in Raman modes.     

氧化钒薄膜的微结构及阻变特性研究

采用射频反应溅射法于室温下在Cu/Ti/SiO₂/Si基底上制备了氧化钒薄膜. X-射线衍射、X射线光电子能谱分析仪及原子力显微镜结果表明, 室温下制备的氧化钒薄膜除微弱的V₂O₅ (101)和V₂O₃ (110)峰外, 没有明显的结晶取向, 是VO₂, V₂O₅, V₂O₃及VO的混合相薄膜, 且薄膜表面颗粒大小均匀, 表面均方根粗糙度约为1 nm. 采用半导体参数分析仪对薄膜的电开关特性进行测试. 结果表明薄膜具有较低的开关电压(V_Set<1 V, V_Reset<-0.5 V), 并且具有稳定的可逆开关特性. 薄膜从低阻态转变为高阻态的电流(I_Reset)随限流的增大而增大.通过高低阻态时I-V对数曲线的拟合(高阻态斜率>1, 低阻态斜率=1), 认为Cu离子在薄膜中扩散形成的导电细丝是该体系发生电阻转变的主要机制. 关键词： 氧化钒薄膜 电阻开关 电阻式非挥发存储器 导电细丝     

Deposition of hybrid structures of reduced graphene oxide and tin dioxide thin films,and persistent photoconductivity observation

Reduced graphene oxide (rGO) is deposited on glass substrate by dripping and sol-gel-coating methods giving rise to nanostructures. When in combination with thin films of SnO₂, they form a heterostructure SnO₂:2 at% Eu/rGO, which alters the surface electrical conductivity. SnO₂ and rGO were also combined as a composite, with conductivity strongly affected by ultraviolet excitation, and shows persistent photoconductivity (PPC) phenomenon even very close to room temperature. Both sort o hybrid structures can be applied in electronic devices. The SnO₂ films are deposited via chemical route by sol-gel or by a mixed technique that combines powders generated by drying the sol-gel solution with resistive evaporation of this powder. Resistivity measured as a function of temperature show that the SnO₂:2 at%Eu sample behaves very similarly to the SnO₂:2 at%Eu/rGO heterostructure sample, with the same energy level for the dominant defect, 172 meV, coincident with ionization of oxygen vacancies. Despite not changing the position of this level, the presence of rGO on the surface of the SnO₂ film induces a decrease in conductivity in vacuum, demonstrating the surface interaction.     

Laser scribing of indium tin oxide (ITO) thin films deposited on various substrates for touch panels

In this study, a Nd:YAG laser with wavelength of 1064 nm is used to scribe the indium tin oxide (ITO) thin films coated on three types of substrate materials, i.e. soda-lime glass, polycarbonate (PC), and cyclic-olefin-copolymer (COC) materials with thickness of 20 nm, 30 nm, and 20 nm, respectively. The effect of exposure time adjusted from 10 μs to 100 μs on the ablated mark width, depth, and electrical properties of the scribed film was investigated. The maximum laser power of 2.2 W was used to scribe these thin films. In addition, the surface morphology, surface reaction, surface roughness, optical properties, and electrical conductivity properties were measured by a scanning electron microscope, a three-dimensional confocal laser scanning microscope, an atomic force microscope, and a four-point probe. The measured results of surface morphology show that the residual ITO layer was produced on the scribed path with the laser exposure time at 10 μs and 20 μs. The better edge qualities of the scribed lines can be obtained when the exposure time extends from 30 μs to 60 μs. When the laser exposure time is longer than 60 μs, the partially burned areas of the scribed thin films on PC and COC substrates are observed. Moreover, the isolated line width and resistivity values increase when the laser exposure time increases.     

Excimer laser ablation of thin titanium oxide films on glass

Thin titanium dioxide films are deposited on glass substrates by magnetron sputter deposition. They are irradiated in air, by means of a KrF excimer laser. The ablation rate is measured as a function of the laser fluence per pulse, F, and of the number of pulses, N. Above a fluence threshold, the films are partially ablated. The ablated thickness does not vary linearly with N. This is the signature of a negative feedback between the film thickness and the ablation rate. The origin of this negative feedback is shown to be due to either thermal or electronic effects, or both. At high F, the film detachs from the substrate.     

Effect of solution molarity on the characteristics of vanadium pentoxide thin film

Vanadium pentoxide (V₂O₅) thin films have been prepared by spray pyrolysis technique. The influence of solution molarity on the characteristics of the V₂O₅ has been investigated. X-ray diffraction analysis (XRD) showed that, the films deposited at ≥0.1 M were orthorhombic structure with a preferential orientation along 〈0 0 1〉 direction. Moreover, the crystallinity was improved by increasing solution molarity. The microstructure parameters have been evaluated by using a single order Voigt profile method. The optical band gaps, determined by using Tauc plot, have been found to be 2.50 ± 0.02 and 2.33 ± 0.02 eV for the direct and indirect allowed transition, respectively. Also the complex optical constants for the wavelength range 300-2500 nm are reported. At room temperature, the dark conductivity as a function of solution molarity showed the range of 5.74 × 10⁻² ± 0.03 to 3.36 × 10⁻¹ ± 0.02 Ω⁻¹ cm⁻¹. While at high temperature, the behaviour of electrical conductivity dominated by grain boundaries. The values of activation energy and potential barrier height were 0.156 ± 0.011 and 0.263 ± 0.012 eV, respectively.