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.     

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.     

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.     

Electron spin resonance and electrical properties of co-evaporated SiO/SnO2 thin films

Electron spin resonance (ESR) measurements are reported for various compositions of SiO/SnO₂ thin films and indicate a decrease in the value of spin density for increasing SnO₂ content in the SiO. Annealing of the device further reduces the value of the spin density. The electrical activation energy and resistivity have been found to increase after annealing of the device and the results are compared with the optical and ESR measurements.     

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.     

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.     

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.     

Correlation between electron spin resonance,electrical conductivity and optical absorption edge of co-evaporated thin films of the dielectric system SiO/V2O5

Measurements of electron spin resonance, d.c. and a.c. electrical conduction and optical absorption of thin film samples of SiO/V₂O₅ prepared by a co-evaporation process at a pressure in the range 8×10⁻⁶ to 3×10⁻⁵ torr are reported. It is found that the spin density of the mixed system decreases by about one order of magnitude compared with that of an SiO film deposited under similar conditions. This reduction correlates with the d.c. electrical conductivity which also decreases as the V₂O₅ content of the complex SiO/V₂O₅ films increases. At the same time the optical energy gap also decreases. At lower temperatures (down to 198 K), a.c. conductance measurements give evidence of hopping conduction. D.c. conductance shows a transition from hopping conduction to free-band (extended state) conduction at about 263 K.     

D.C. conduction in thin films of SiOx/V2O5 co-evaporated assemblies prior to electroforming

D.C. conduction in MIM sandwich structures based on SiO _x /V₂O₅ as a dielectric has been investigated before electroforming. The electrodes make a blocking contact to the dielectric with a barrier height, ₀, dependent on the type of electrode material used. The voltage-current characteristic is studied between 165 and 413 K. Below room temperature and at low fields hopping conduction is dominant; at intermediate temperatures a transition to free band conduction is observed. At higher temperatures and fields the conduction is enhanced by Schottky barrier lowering associated with an activation energy E 0.15eV. Hopping conduction has also been found to be dominant above room temperature in thin films having a high density of trapping states.     

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.     

A.c. conduction mechanism and dielectric properties of co-evaporated SiO/B2O3 amorphous thin films

The a.c. electrical properties on Al-SiO/B₂O₃-Al sandwich devices with various compositions of SiO/B₂O₃ were studied over a frequency range of 2×10² to 1×10⁶ Hz and in the temperature range 158 to 463 K. The a.c. conductance G varies with frequency according to the relation G ^s, where the exponent s was found to be 0.92 (at 158 K) in the frequency range 8×10² to 2×10⁴ Hz and above 10⁵ Hz the conductance shows a square law dependence on frequency. These results suggest that the a.c. conduction mechanism in SiO/B₂O₃ thin films is due to an electronic hopping process. The numbers of localized sites were calculated using the conductivity relations given by Elliott and by Pollak and the results are compared. The effects of composition, temperature and annealing on the dielectric constant and loss factor were studied. The relative dielectric constant and loss factor were found to decrease with the increase of B₂O₃ content in SiO. Annealing of the samples reduces the values of the dielectric constant, loss factor tan , temperature coefficient of capacitance and a.c. conductivity. The variation in capacitance with the composition of SiO/B₂O₃ was investigated at room temperature, the results being normalized to 10 kHz. The dispersion was found to decrease with the addition of B₂O₃ into SiO.     

Electron spin resonance study in P2O5-MoO3 and P2O5-MoO3-CaO glasses

ESR spectra of the glass systems P₂O₅-MoO₃ and P₂O₅-MoO₃-CaO are presented. The electron spin resonance results showed a strongly exchange-narrowed interaction with the [Mo⁵⁺]/[Mo⁶⁺] ratio and line shapes independent of temperature over the range 77 to 300 K. This result supports the concept of temperature-dependent mobility of the carriers and an unchanged paramagnetic site over this temperature range. The more important difference between the spectra of the binary and ternary glasses is that in the former with an increase in MoO₃ content the concentration of Mo⁵⁺ ions decreases but in ternary glasses it is reversed, i.e. the ion concentration of Mo⁵⁺ increases with increase in MoO₃ content.     

Characterization of laser-ablated V2O5 thin films

Vanadium pentoxide thin films have been prepared by the pulsed laser deposition technique. The influence of substrate temperature on the growth of V₂O₅ films was studied. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements have been carried out in order to understand the growth mechanism. The crystallization in V₂O₅ thin films starts at deposition temperatures as low as 473 K and the grain size increased with deposition temperature. The films exhibited predominantly (0 0 1) orientation, representing the orthorhombic layered structure. The infrared (IR) and Raman measurements supported the above data.     

Electron spin resonance and some electrical and optical properties of GeO2/SiO x thin films

Electron spin resonance measurements show a reduction in the spin density associated with unpaired electron dangling bonds of a mixed complex of co-evaporated GeO₂/SiO _x films compared with that of SiO _x films investigated under similar conditions. Unlike the behaviour of BaO/GeO₂ complex films, these samples show a decrease in the optical gap as the thickness increases for films of fixed compositions. However, it increases as the SiO percentage increases in films of the same thickness. D.C. measurements at low applied fields below 10⁵ V cm^–1 agree well with the spin density results.     

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.     

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.