Electronic conduction in thin amorphous MoO3/SiO films deposited by co-evaporation

A study of the effects of changes in composition and temperature on the electrical properties of MoO₃/SiO thin amorphous films is presented. The high-field conduction is probably due to the Poole-Frenkel effect as it is in simple SiO. At low temperature and low field the electron hopping process is dominant but conduction at higher temperatures is a contact-limited process. The decrease in conductivity with increasing concentration of SiO in MoO₃ may be attributed to the increasing number of trapping centres introduced in MoO₃/SiO films during the evaporation process. The increase in conductivity in MoO₃/SiO films with increasing temperature is attributed to the increasing concentration and higher mobility of charge carriers.     

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.     

D.c. conduction studies on thermally evaporated neodymium oxide thin films

Aluminium-neodymium oxide-aluminium thin film capacitors have been prepared by thermal evaporation and the d.c. conduction properties of these films have been studied. The thicknesses of the films have been determined by a multiple beam interferometer. The current-voltage power-law dependence showed that the conduction in these films is space-charge limited. The linear dependence of the current density on the square root of the applied field confirmed the exponential trap distribution. The trap density has been found to be of the order of 10²⁶ m^–3. It has also been observed that the Schottky type of conduction is predominant in the high-field region and the height of the Schottky barrier has been determined. It is seen that the conduction mechanism is an activated process with the activation energy decreasing with increasing field.     

D.c. and a.c. electrical properties of vacuum evaporated thin SiO/GeO2 films

The d.c. and a.c. electrical properties were studied for various compositions of SiO/GeO₂ co-evaporated thin films carrying aluminium electrodes, in the temperature range 193–413 K. A.c. measurements were made over the frequency range 2x10²–10⁶Hz. The value of the d.c. activation energy was found to decrease with increasing GeO₂ content in the SiO. In the region of high applied field (above 10⁶ Vm^–1, the conduction mechanism is governed by Schottky emission at the blocking contact. The a.c. electrical conductivity, (), varies with frequency according to the relation () ^s, where the exponent s was found to be dependent on temperature and frequency. The a.c. conduction at low temperature was due to an electronic hopping process. The number of localized sites was estimated from the a.c. measurements for different compositions of SiO/GeO₂ using the models proposed by Elliott and by Pollak, and the values are compared. The Elliott model satisfactorily accounts for the observed a.c. electrical results. A correlation was found between activation energy, optical band gap, conductivity and number of localized sites for the various compositions of SiO/GeO₂ films. The relative dielectric constant, _r, and loss factor, tan , were found to increase with the increase of GeO₂ content in the films.     

D.c. and a.c. conduction in amorphous titanium dioxide thin films

The d.c. and a.c. measurements were performed with two groups of vacuum- deposited amorphous TiO₂ thin films which had metal-TiO₂-metal sandwich structures. The d.c. conduction process and the frequency dependence of the a.c. conductivity σ_ac of “group 1” samples,which contained numerous oxygen vacancies, showed the three-dimensional Poole-Frenkel effect and σ_ac was proportional to the frequency ? over the range 50 Hz–50 kHz. “Group 2” samples, in which the vacancies were subsequently removed by heat treatment in air, showed the three-dimensional anomalous Poole-Frenkel effect and σ_ac was proportional to ?ⁿ where n was 0.7–0.8.     

D.c. conduction in thin films of SiO/In2O3 before and after electroforming

Electrical measurements on thin co-evaporated SiO/In₂O₃ films before and after electroforming are reported. The high-field conduction (expressed in terms of circulating currentl _c and bias voltageV _b) in thin film sandwich structures of Al–SiO/In₂O₃-Al and Cu-SiO/In₂O₃-Cu is found to obey a relation of the form logI _c V _b ^1/2 . The electroformed samples show voltage-controlled negative resistance, voltage memory, thermal-voltage memory and pressure-voltage memory effects and the results are explained in terms of the filamentary model of electrical conduction.     

Optical constants of thermally evaporated amorphous GeSe3 thin films

The optical transmission spectra of amorphous Ge-Se films of chemical composition GeSe₃, prepared by thermal evaporation, are measured overthe 300 nm to 2500 nm spectral region. A simple, straightforward procedure suggested by Swanepoel, which is based on the use of interference fringes, has been applied in order to derive the real and imaginary parts of the complex refractive index, and also the film thickness. Furthermore, thickness measurements made by a surface-profiling stylus are also carried out to cross-check the results obtained by the present optical method, employing only T(). The dispersion of n is discussed in terms of the single-oscillator Wemple and DiDomenico model, and the optical band gap E _g ^opt has been determined from the absorption coefficient values, using the Tauc procedure.     

An XPS study of amorphous MoO3/SiO films deposited by co-evaporation

X-ray photoelectron spectroscopic (XPS) core-level spectra of MoO₃/SiO thin films are presented. The effects of changes in composition, substrate temperature during deposition and annealing on the binding energy of Mo(3d) and Si(2p) core lines in mixed films are compared with those of MoO₃ and SiO. Appreciable changes in Mo(3d) peak positions and slight changes in Si(2p) peak positions are observed. The change in binding energy of the Mo(3d) doublet may be attributed to the effective incorporation of silicon ions in an MoO₃ lattice which may cause the molybdenum orbital to be a little less tightly bound. This helps in the internal electron transfer from the oxygen (2p) to the molybdenum (4d) level as a result of which the molybdenum is readily changed to lower oxidation states during heat treatment. XPS spectra show that the position of the Si(2p) core state shifts monotonically with increasing oxygen concentration from the value of 101.8 to 102.6 eV.     

Electrical conduction in evaporated terbium fluoride thin films

The current-voltage characteristics of thin-film capacitors with evaporated terbium fluoride dielectric have been studied as a function of temperature (in the range 300 to 418 K). For sufficiently high electric fields (> 10⁴ V cm^–1, the leakage current is found to increase exponentially with the square root of the applied electric field. Analysis of the data suggests an electrode-limited mechanism such as that suggested by Schottky. It is seen that the conduction mechanism is an activated process with the activation energy decreasing with increasing field. Dielectric break-down and its dependence on film thickness have also been investigated. Break-down field strength follows the Forlani-Minnaja relation.     

Electrical conduction mechanisms in thin amorphous semiconductor films

An account is given of the different conduction mechanisms possible in thin amorphous semiconductor films. Special attention is paid to the low temperature regime where variable range hopping between states near the Fermi level is supposed to be most important. An attempt is made to assess the significance of fits of the conductivity versus temperature relation to the much-used T^{-$\text{1}\text{4}$} relation proposed by Mott.     

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 absorption edge in amorphous thin films of MoO3-In2O3

A study of the effects of changes in composition, film thickness, substrate deposition temperature and annealing on the optical properties of MoO₃-In₂O₃ is presented. The results are found to be compatible with the reduction in the value of optical energy gap of these materials as the molar fraction of In₂O₃ in the MoO₃ thin film increases. This decrease of optical gap may be attributed to the incorporation of In(III) ions in an MoO₃ lattice. The decrease in optical band gap with increasing thickness may be interpreted in terms of the incorporation of oxygen vacancies which are also believed to be the source of conduction electrons in the MoO₃-In₂O₃ complex. The decrease of band gap with increasing substrate temperature may be attributed to the enhanced ordering of the samples and the decrease of band gap with annealing may be attributed to a reduction in the concentration of lattice imperfections.     

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.     

Some d.c. properties of evaporated thin aluminium fluoride films

The measurements of some d.c. properties of samples of evaporated aluminium fluoride thin films sandwiched between metal electrodes are reported. The technique of Rutherford scattering of megaelectronvolt alpha particles was used to study the stoichiometry and impurity content of the films. Measurements were made in the temperature range 77–393 K, and the thickness of the films ranged from 800 to 4000 Å. At high temperatures and high fields the conduction process can be explained on the basis of the Poole-Frenkel mechanism in amorphous materials, as described by Hill. At low temperatures the conductivity fits the Mott relation $\text{σ ∝ exp(-T}{}_0\text{/T)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$. The breakdown electric field, measured as a function of film thickness d in the range 700–7500 Å, obeys the Forlani-Minnaja relation.     

Optical properties of evaporated PbSnS3 thin films

PbSnS3 is a ternary sulfide semiconductor material which has been relatively little studied in the literature. Thin films of this material were deposited on glass substrates by a thermal evaporation process and their optical properties investigated. The optical parameters were determined from the analysis of measured transmission spectra, at normal incidence, from 400–1500 nm, at room temperature. The fundamental absorption edge is placed at 1.04±0.05 eV and is forbidden in nature. Other absorption edges are obtained at 1.44 and 1.55±0.05 eV, with a nearby direct one at 1.68±0.05 eV. The peaks at 1.44 and 1.55 eV have been independently confirmed by the spectral photocurrent response of the films. © 1998 Kluwer Academic Publishers