Characterization of pyrolytically deposited V2O3 thin films

V₂O₃ films were deposited using the pyrolysis technique. The surface composition and structure of the deposited V₂O₃ films were investigated using scanning electron microscopy (SEM), Auger electron spectroscopy (AES) and reflection high energy electron diffraction (RHEED). Electronic conductivity measurements of the films were also carried out as a function of temperature from room temperature to about 430 K.The SEM analysis showed a smooth film surface which on exposure at high intensity showed some form of restructuring. The AES, however, indicated the presence of carbon in the deposited film while the RHEED showed a streaked pattern which could result from the restructing observed in the SEM analysis.The electrical conductivity measurements showed metallic-type behaviour in the region investigated. A smooth second-order transition was observed at about 400 K.     

Optical absorption spectra of evaporated V2O5 and co-evaporated V2O5/B2O3 thin films

The optical absorption spectra of evaporated V₂O₅ and co-evaporated V₂O₅/B₂O₃ thin films have been studied. For higher photon energies, the absorption is found to be due to a direct forbidden electronic transition process from the oxygen 2p band to the vanadium 3d band in a similar way to that observed in crystalline V₂O₅. The exponential behaviour of absorption edge for lower photon energies is attributed to the electronic transitions between the tailed-off d-d states corresponding to V⁴⁺ ions. For co-evaporated V₂O₅/B₂O₃ films the optical energy gap is observed to increase with the increase in V₂O₅ content of the composite films.     

Optical absorption in co-evaporated V2O5-TeO2 thin films

The electron diffraction pattern shows that co-evaporated V₂O₅-TeO₂ thin film samples are amorphous at room temperature and become polycrystalline at temperatures higher than about 513 K. This behaviour is similar to that of amorphous V₂O₅ thin films. The optical absorption edge of amorphous thin films of V₂O₅-TeO₂ is studied in the wavelength range 200 to 900 nm and the FTIR spectra are studied in the wave number range 400 to 4000 cm^–1. The FTIR spectra of amorphous V₂O₅ thin film are found to be similar to those of amorphous V₂O₅-TeO₂ thin films. This suggests that the coordination number of the vanadium ion in V₂O₅-TeO₂ is the same as that in crystalline V₂O₅, and thus the optical absorption edge of amorphous V₂O₅-TeO₂ thin films can be described by direct forbidden transitions.     

Infrared absorption spectra of evaporated V2O5 and co-evaporated V2O5/B2O3 thin films

The Fourier transform infrared spectra of different compositions of evaporated V₂O₅/B₂O₃ thin films have been investigated. Most of the absorption bands corresponding to V₂O₅ and B₂O₃ films coincide with those reported by other authors. The short-range order in amorphous V₂O₅ films is found to be conserved. The absorption spectra indicate a boroxol ring structure for B₂O₃ films. In co-evaporated V₂O₅/B₂O₃ films the boron is observed to substitute in the V₂O₅ network such that the coordination number of vanadium ion remains unchanged. The presence of a number of bands corresponding to -OH groups indicates the hygroscopic character of the films.     

Electron spin resonance studies of evaporated V2O5 and co-evaporated V2O5/B2O3 thin films

Electron spin resonance studies of evaporated V₂O₅ and co-evaporated V₂O₅/B₂O₃ amorphous thin films have been made. For lower molar contents of B₂O₃, the co-evaporated V₂O₅/B₂O₃ films show poorly resolved hyperfine structure, whereas for higher content of B₂O₃ the hyperfine spectra are well resolved. This behaviour of films is attributed to the increase in the lifetime of a particular V⁴⁺ ion due to Anderson localization of charge, as the degree of disorder increases with increase in the molar content of B₂O₃. The unpaired electron at a given time is localized on a single ⁵¹V nucleus. The low intensity of ESR signal for higher concentration of V₂O₅ in the co-evaporated V₂O₅/B₂O₃ films has been related to the less effective concentration of V⁴⁺ ions due to antiferromagnetic coupling of the V⁴⁺ ions.     

Electrical conduction through MIM structures of evaporated V2O5 and V2O5/B2O3 amorphous thin films

The electrical conduction through vacuum-evaporated thin films of V₂O₅ and V₂O₅/B₂O₃ in MIM structures has been investigated. The high-field behaviour of both types of film is in accordance with the Poole-Frenkel type of mechanism. The increase in B₂O₃ content in co-evaporated V₂O₅/B₂03 films results in a decrease in the conductivity of the composite films. This is attributed to the expansion of the resultant film structure due to the network-forming effect of B₂O₃. The covaporated thin films of V₂O₅/B₂O₃ with a molar content of B₂O₃ larger than 40% are observed to be unstable because of their hygroscopic nature.     

In situ thin film differential thermal analysis of amorphous V2O5 thin films

In situ differential thermal analysis (DTA) experiments were performed on highly quenched amorphous V₂O₅ thin films directly evaporated onto a thin film differential thermocouple microprobe. Irreversible low temperature relaxations were thus evidenced, a result impossible to obtain with a classical DTA apparatus using microcrucibles such as Setaram or Mettler TA 2000 devices.     

Electrical and thermal properties of highly quenched amorphous V2O5 thin films

Amorphous thin films of V₂O₅ have been prepared by vapour deposition in high vacuum ( 10^–6 torr). In order to study the role of quenching, various temperatures, ranging from –196 to 260° C, have been selected for the substrate. Differential thermal analysis. X-ray diffraction and conductivity measurements clearly divide the material into two sets, depending on the efficiency of the quenching. Whereas the least-quenched samples resemble those previously obtained by splat-cooling, the better quenched are only barely stable and, as a consequence, exhibit unique features, such as the occurrence of a glassy transition and the highest crystallization temperature ever found for V₂O₅.     

A.c. conduction through MIM sandwich samples of evaporated thin films of V2O5 and V2O5/B2O3

The a.c. electrical properties of evaporated V₂O₅ and co-evaporated V₂O₅/B₂O₃ films have been investigated. The conductivity in both types of film follows the theory of Elliott for single polaron hopping. The behaviour of capacitance, tangent of loss angle and dielectric loss agrees with the model proposed by Goswami and Goswami. The co-evaporated films of V₂O₅/B₂O₃ show better dielectric properties than simple V₂O₅ with increasing content of B₂O₃. Because of their hygroscopic character, the films with B₂O₃ content larger than 40% are observed to be unstable in the atmosphere.     

The optical properties of amorphous V2O5 and SiO thin films and of the mixed dielectric system SiO/V2O5

The optical absorption of amorphous thin films of V₂O₅, SiO and of SiO/V₂O₅ is studied in the photon energy range 0.42 to 6.53 eV. The optical absorption edge of evaporated V₂O₅ films can be described by direct forbidden transitions while that of SiO films follows the non-direct transitions in k-space. The data of the SiO/V₂O₅ oxide mixtures are fitted to new values of the exponent in the well-known absorption equation and the corresponding optical band gaps are determined. Experimental data on the wavelength dependence of the refractive index of SiO films are presented. The dispersion of the refractive index follows a single oscillator model. The infrared spectra show that some bonding occurs between the two oxides so that the mixed dielectric system SiO/V₂O₅ cannot be considered as a simple physical mixture.     

RF sputtered electrochromic V2O5 films

The properties of vanadium pentoxide (V₂O₅) films deposited by r.f. reactive sputtering from V₂O₅ target are investigated. In particular the composition and structure of these films are analysed by Rutherford backscattering spectrometry (RBS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) together with their electrochromic properties. Information on electrochromic behaviour of these films, like the injected charge and the width of transition from bleached to coloured state when lithium was electrochemically injected, is obtained by voltammetric and spectrophotometric measurements. The dependence of all these properties on the oxygen flow used in the argon–oxygen gas mixture during deposition is analysed. The highest value of injected charge and the greatest differences in transmittance values due to electrochromic transition are shown by samples deposited at low O₂ flow. The films appear to be amorphous when the O₂ flow is lower than 20% and the stoichiometry is not greatly affected by the O₂ flow.     

Structure and morphology of laser-ablated Pb thin films

The growth of polycrystalline Pb thin films on Si substrates at room temperature by near infrared laser ablation of pure Pb target is presented. Several diagnostic techniques were used to deduce the structure, the morphology and the thickness of the deposited films. We investigated the above properties as a function of laser fluence (1-8 J/cm²). Detailed morphological studies were also carried out on the irradiated target surface to deduce the laser ablation process. According to our results, the increase of laser fluence improves the crystallinity of the deposited films but not the deposition rate and the droplet density. The measured deposition rate was extremely low and varied between 0.04 and 0.09 Å per pulse. The final goal of this research activity is the realization of photocathodes based on Pb-thin-film to be used in superconducting Nb radio-frequency cavity.     

Influence of deposition temperature on the growth of vacuum evaporated V2O5 thin films

V₂O₅ films were deposited on silicon (111) substrates by vacuum evaporation technique at various deposition temperatures of 300, 473, 573, 623 and 673 K. X-ray characterization revealed that the films deposited at T_s≤473 K are amorphous and the film deposited at T_s≥573 K is polycrystalline. It is interesting to note that the film deposited at T_s=573 K is strongly oriented with (001) planes parallel to the substrate and the degree of preferred orientation towards (001) planes found to decrease with further increase in the deposition temperature. The influence of deposition temperature on the growth of the V₂O₅ films has been studied by Raman scattering spectroscopy. The films deposited on the silicon substrates maintained at 573 K are found to have better structural quality.     

溶胶-凝胶V2O5薄膜的制备及其电化学特性研究

优化了以V2O5粉末、苯甲醇和异丙醇为原料制备V2O5薄膜的溶胶一凝胶技术工艺,使用XRD、FTIR、AFM、UV-VIS-NIR分光光度计和电化学分析等方法研究了薄膜的特性。实验结果表明：采用经过优化的工艺制备的V2O5溶胶为V^4 离子和V^5 离子的混合体,具有很好的稳定性和重复性;随热处理温度的升高,薄膜结构收缩,晶粒长大,V^4 离子逐渐被氧化为V^5 离子;V2O5薄膜具有良好的电化学伏安循环特性。     

Nonlinear high-field conductivity in Nb2O5 thin films

The conduction mechanism in thin amorphous films of Nb₂O₅ with Nb counterelectrodes has been investigated for undoped films and for films doped with Nb by a low energy implantation-diffusion process occurring during sputtering of the Nb counterelectrodes. For films with evaporated electrodes, the conductance G versus the applied voltage V showed three regions: (a) an ohmic region at low fields up to a field E₁ = 1.33 × 10⁵ V/cm; (b) a nonlinear dependence of the form I∝V^m, where m = 3.8, at intermediate fields; and (c) for fields E>1.2×10⁶ V/cm an exponential dependence of G on V corresponding to the Poole-Frenkel field-assisted emission mechanism. Films with sputtered Nb electrodes exhibited ohmic regions up to a field E₁ = 4×10⁴ V/cm (where E₁ decreases with increasing sputtering voltage), a relation G ∞ V⁴ at intermediate fields up to E = 8.3 × 10⁴ V/cm, and then an exponential dependence of G and V corresponding to the Poole-type emission mechanism. The conductance and capacitance increased in films with sputtered Nb electrodes and were found to increase, while the thermal activation energy decreased, with increasing sputtering voltage. The above results indicate an increase of coulombic donor sites in Nb₂O₅ films as a consequence of the Nb implantation-diffusion process.     

热处理对V2O5-TiO2复合薄膜表面结构与光学性能的影响

以钛酸丁脂、V2O5粉末为前驱体,采用溶胶-凝胶法制备了V2O5-TiO2复合薄膜,并采用X射线光电子能谱(XPS)和Uv-vis-nir分光光度计等方法研究了热处理对薄膜性能的影响.结果表明,随热处理温度的升高,复合薄膜中钛离子的价态不变,仍为Ti4 ;而V4 离子逐渐被氧化为V5 离子;并采用氢氧基团、碳氧键与多种钒离子进行叠加拟合,拟合结果与实验曲线非常符合;V2O5-TiO2复合薄膜在紫外光波段的透射率减小,吸收带边出现红移.     

X-ray photoelectron spectroscopic studies of evaporated V2O2and co-evaporated V2O5/B2O3 films

X-ray photoelectron spectra of evaporated V₂O₅ and co-evaporated V₂O₅/B₂O₃ thin films have been investigated. The photoelectron spectrum of a simple V₂O₅ film shows the splitting of the V 2p level in accordance with the spins. The values of binding energies corresponding to V 2p and O1s are comparable with those reported previously. For co-evaporated V₂O₅/B₂O₃ films a chemical shift in the O 1s level has been observed which has been attributed to the changed chemical environment of oxygen as a result of the presence of boron and vanadium atoms. The values of binding energies for V 2p_3/2 and O 1s corresponding to simple evaporated V₂O₅ and co-evaporated V₂O₅/B₂O₃ show the presence of V₂O₄ species in the films.     

Enhanced dielectric properties of modified Ta2O5 thin films

 Tantalum oxide (Ta₂O₅) is a promising high dielectric constant material for the DRAM applications because of its ease of integration compared to other complex oxide dielectrics. The dielectric constant and thermal stability characteristics of bulk Ta₂O₅ samples were reported to enhance significantly through small substitutions of Al₂O₃. However, this improvement in the dielectric constant of (1-x)Ta₂O₅-xAl₂O₃ is not clearly understood. The present research attempts to explain the higher dielectric constant of (1-x)Ta₂O₅-xAl₂O₃ by fabricating thin films with enhanced dielectric properties. A higher dielectric constant of 42.8 was obtained for 0.9Ta₂O₅–0.1Al₂O₃ thin films compared to that reported for pure Ta₂O₅ (25–30). This increase was shown to be closely related to a-axis orientation. Pure Ta₂O₅ thin films with similar a-axis orientation also exhibited a high dielectric constant of 51.7, thus confirming the orientation effect. The leakage current properties and the reliability characteristics were also found to be improved with Al₂O₃ addition. Received: 24 November 1998 / Reviewed and accepted: 7 December 1998