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.     

A.C. and D.C. conduction in lightly substituted anthracene Langmuir films

The a.c. and d.c. properties of short-chain derivatives of anthracene prepared by using the Langmuir-Blodgett technique are reported; these thin films display interesting conduction effects on account of their unusual molecular structure. Emphasis is placed on the 9-alkyl-10-anthrylpropionic acid derivatives in which the alkyl chain is either C₄H₉ (C4 anthracene) or C₆H₁₃ (C6 anthracene). As expected, the dark conduction properties are markedly different from those of unsubstituted crystalline anthracene. For example, there is a marked anisotropy in the conductivity of C4 anthracene, the in-plane value exceeding that in a direction perpendicular to the layers by a factor of 10⁸. By use of appropriate electrodes double injection effects accompanied by blue light emission were found. The admittance data for these novel films differ slightly from those of films of conventional fatty acid salts.     

Stability of plasma-deposited amorphous hydrogenated boron films

An investigation of radio-frequency plasma-deposited amorphous hydrogenated boron films was conducted in order to determine the influence of the substrate temperature T_S and d.c. self-bias U_SB on the physical properties and film stability. We found three different regions of stability depending on T_S and U_SB. Films prepared at high d.c. self-bias and substrate temperature are mechanically unstable due to internal stress. They peel off during the deposition process or on first contact with the ambient atmosphere. Films deposited at low self-bias and substrate temperature were found to be chemically unstable. Exposed to the ambient atmosphere, they undergo chemical changes and incorporate large amounts of oxygen and carbon. These two different regions of instability are separated by chemically and mechanically stable films. The chemical composition and the structural properties of chemically stable and unstable films were determined by Fourier transform infrared spectroscopy, X-ray induced photoelectron spectroscopy and ion beam analysis. These measurements show that chemical stability correlates with the boron density. Chemically stable films reveal densities of more than 68% up to 99% of the density of crystalline boron. In general, elevated substrate temperature and ion energy cause densification of the films and increasing internal stress. Densification leads to chemical stability, while internal stress is the reason for mechanical instability.     

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 in evaporated MoO3/SiO amorphous thin films

The dielectric properties of vacuum-deposited MoO₃/SiO films of different compositions studied in the frequency range 10² to 10⁶ Hz at various temperatures (193 to 393 K) are reported. The properties of the film capacitor are found to be temperature and frequency dependent. The decrease in a.c. conductance with increasing concentration of SiO in MoO₃ may be attributed to the increasing number of trapping centres generated in MoO₃/SiO films during the evaporation process.     

Noise measurements in electron-beam-evaporated amorphous silicon thin films

The low frequency noise of vacuum-evaporated amorphous silicon films, both intrinsic and prepared under a partial pressure of hydrogen, was measured over a range of temperatures. The current noise spectral density shows a predominance of 1/?^α noise at room temperature, where α lies between 0.8 and 1.04, and the conduction proceeds primarily by the hopping of carriers near the Fermi energy level. As the temperature is raised towards the region of extended states conduction the noise spectrum changes to more of a generation-recombination type with α having values from 1.66 to 2.04 at the highest temperatures. These results imply that no extremely large, very narrow band of gap states was formed under our fabrication conditions and they therefore agree with earlier work on the mixed conduction process and with field effect data.     

Raman characterization of boron doped tetrahedral amorphous carbon films

Boron doped tetrahedral amorphous carbon films, having boron content from 0.59 to 6.04 at.%, have been prepared by a filtered cathodic vacuum arc system using boron mixed graphite targets. The influence of boron on the surface morphologies and microstructures of the films was studied by atomic force microscopy and Raman spectroscopy. The surface images showed that the irregular tops on the surface of the films tended to form larger clusters as boron content increased. The Raman spectra of the films were, respectively, deconvoluted using Gaussian and Breit-Wigner-Fano line shapes. The Raman parameters, including the intensity ratios, peak positions, peak widths and coupling coefficients, obtained from both line shapes were described and compared. It was found that both line shapes could produce consistent results except the peak widths of G bands. The major effect of boron introduction was to increase the clustering of the sp² phase in the films.     

D.c. conductivity of amorphous CulnSe2 films

Journal of Materials Science Letters -     

Optical properties and chemical reactivity of hydrogenated amorphous boron thin films

Hydrogenated boron thin films have been deposited at temperatures in the range 50–100 °C with the radiofrequency plasma decomposition of diborane B₂H₆ diluted in hydrogen. The chemical composition of the films has been determined by elastic recoil detection and by the Castaing microprobe techniques. We have found that the as-deposited films contain 10–20 at % H and that they react with the ambient atmosphere within a few days and reach a final composition close to B_0.64H_0.12O_0.12C_0.06N_0.06. The optical properties of the as-deposited films studied by spectroscopic phase-modulated ellipsometry in the range 1.7–5.0 eV are characteristic of the high optical gap semiconductors. The electron diffraction measurements performed onin situ annealed samples show that the films are amorphous up to 950 °C. Infrared spectroscopy investigations performed on the as-deposited films have revealed two hydrogen bonding sites: B-H terminal bonds and B-H-B bridge bonds, along with some B-O-B groups. Upon exposure to the atmosphere, the vanishing of B-H-B and B-O-B bridge bonds and the increase of B-H and B-O-H absorption bands after 1 day, are observed. On leave from Physics Department, CNEA, Avenida Libertador 8250, 1429 Buenos-Aires, Argentina, and CONICET.     

D.C. conduction mechanisms of certain perovskite ceramics

A series of a complex perovskite Ca _x Sr_{(1 – x)}TiO₃ (CST) where (0 x 1) has been prepared by mechanical mixing of their oxides (CaTiO₃ and SrTiO₃). X-ray diffraction showed that no traces of other compounds and the line widths indicated that the products were homogeneous. The d.c. conductivity of the sample were measured over the range of temperature between 303–800 K. Temperature dependent of d.c. measurement revealed that the conductivity is thermally activated. Over the low temperature range between (333–573 K) for CaTiO₃, (353–483 K) for Ca_0.7Sr_0.3TiO₃ and (373–423 K) for Ca_0.5Sr_0.5TiO₃ a semiconductor—metal behavior was observed. From the relationship between activation energy E _a and the compositions, it has been found that E _a goes through a minimum around 50% Sr—fraction. The field lowering coefficient is evaluated experimentally.     

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.