I. Balberg

Deposition of device quality, low H content amorphous silicon

Device‐quality hydrogenated amorphous silicon containing as little as 1/10 the bonded H observed in device‐quality glow discharge films have been deposited by thermal decomposition of silane on a heated filament. These low H content films show an Urbach edge width of 50 mV and a spin density of ∼1/100 as large as that of glow discharge films containing comparable amounts of H. High substrate temperatures, deposition in a high flux of atomic H, and lack of energetic particle bombardment are suggested as reasons for this behavior.

PERCOLATION THEORY AND ITS APPLICATION TO GROUNDWATER HYDROLOGY

The theory of percolation, originally proposed over 30 years ago to describe flow phenomena in porous media, has undergone enormous development in recent years, primarily in the field of physics. The principal advantage of percolation theory is that it provides universal laws which determine the geometrical and physical properties of the system. This survey discusses developments and results in percolation theory to date, and identifies aspects relevant to problems in groundwater hydrology. The methods of percolation theory are discussed, previous applications of the theory to hydrological problems are reviewed, and future directions for study are suggested.

Recent developments in continuum percolation

Abstract While most natural systems to which percolation theory has been applied are continuum systems, most of the developments in percolation theory have been made through the study of lattice models. Recently, two major developments have shown, however, that continuum percolation warrants investigation in its own right, having features with no counterpart on lattices. The first development was in understanding the dependence of the threshold on the particle structure; the second was in finding a relation between critical exponents of physical properties and the local geometrical properties of the system. These developments explained observations in the four major groups of continuum systems: porous media, doped semiconductors, microemulsions and composite materials.

Tunneling and percolation in metal-insulator composite materials

In many composites, the electrical transport takes place only by tunneling between isolated particles. For a long time, it was quite a puzzle how, in spite of the incompatibility of tunneling and percolation networks, these composites conform well to percolation theory. We found, by conductance atomic force microscopy measurements on granular metals, that it is the apparent cut off of the tunneling to non-nearest-neighbors that brings about this behavior. In particular, the percolation cluster is shown to consist of the nearest-neighbors subnetwork of the full tunneling network.

Solution of the tunneling-percolation problem in the nanocomposite regime

We noted that the tunneling-percolation framework is quite well understood at the extreme cases of percolation-like and hopping-like behaviors but that the intermediate regime has not been previously discussed, in spite of its relevance to the intensively studied electrical properties of nanocomposites. Following that we study here the conductivity of dispersions of particle fillers inside an insulating matrix by taking into account explicitly the filler particle shapes and the inter-particle electron tunneling process. We show that the main features of the filler dependencies of the nanocomposite conductivity can be reproduced without introducing any a priori imposed cut-off in the inter-particle conductances, as usually done in the percolation-like interpretation of these systems. Furthermore, we demonstrate that our numerical results are fully reproduced by the critical path method, which is generalized here in order to include the particle filler shapes. By exploiting this method, we provide simple analytical formulas for the composite conductivity valid for many regimes of interest. The validity of our formulation is assessed by reinterpreting existing experimental results on nanotube, nanofiber, nanosheet and nanosphere composites and by extracting the characteristic tunneling decay length, which is found to be within the expected range of its values. These results are concluded then to be not only useful for the understanding of the intermediate regime but also for tailoring the electrical properties of nanocomposites.

High‐contrast optical storage in VO2 films

When vanadium dioxide, VO2, undergoes its semiconductor‐to‐metal transition at 68 °C its reflectivity decreases by about a factor of 2 throughout the visible range. By using a thin VO2 film on a vanadium substrate we were able to achieve a much higher contrast between the reflectivities of the two VO2 phases. The VO2 film was grown as an antireflecting coating for one of the phases and thus the film thickness was used to determine the light wavelength at which the contrast will reach its maximum value. For example, with an 800‐A‐thick film a contrast of 16 (rather than 2) was obtained at a light wavelength of 7000 A. In utilizing the present system for optical storage the high contrast obtained yielded an order of magnitude higher read‐out efficiency than the efficiency reported for VO2 films sputtered on glass. To demonstrate information recording in the present system, both a pulsed laser and a cw laser were used for spatially selective’’writing’’ in the film. It was found that the present system has hi...

Percolation in a composite of random stick-like conducting particles

Abstract We report measurements of the conductivity, σ, of carbon-PVC composite as a function of the carbon weight %, ω, in the composite. Below some critical carbon content, ω c , the conductivity drops sharply while above this ω c we find a power-low behavior σ ∝ (ω - ω c ) t . The best fits of the data with this power-law show that 1.5 t ω c >9.0. This is in good agreement with the predicted universal value, t = 1.7, and the experimental value obtained for a system containing sphere-like conducting particles, t = 1.9 ± 0.2.

The optical absorption of iron oxides

Abstract The first comprehensive study of the optical absorption spectra of the iron oxides FeO, Fe3O4 and Fe2O3 is reported. Combining the present results with previous data enables a reliable common one-electron energy level diagram for all three materials.

Crystalline phases at the p‐ to n‐type transition in Cu‐ternary semiconducting films

We report here a study of the Raman spectra of ternary Cu–In–S and Cu–In–Se polycrystalline film compounds as a function of the x=[In]/{Cu]+[In]} ratio. Using these spectra we were able to identify, with high resolution in x, the phases present in the films. We found that the single phase of chalcopyrite CnInSe2 exists over the fairly wide composition range of 0.48⩽x⩽0.55, and that the lattice disorder increases with the increase of In content. No such single phase range was found for the Cu–In–S films. Considering the electrical properties of these materials around x=0.5, it is concluded that the native defect model accounts for the electrical properties of the Cu–In–Se films but does not account simply for the electrical properties of the Cu–In–S films.

Electrical transport mechanisms in three dimensional ensembles of silicon quantum dots

In this review, we try to derive a comprehensive understanding of the transport mechanisms in three dimensional ensembles of Si quantum dots (QDs) that are embedded in an insulating matrix. This understanding is based on our systematic electrical measurements as a function of the density of Si nanocrystallites as well as on a critical examination of the available literature. We conclude that in ensembles of low density QDs, the conduction is controlled by quantum confinement and Coulomb blockade effects while in the high density regime, the system behaves as a simple disordered semiconductor. In between these extremes, the transport is determined by the clustering of the QDs. In view of the clustering, two types of transitions in the electrical and optical properties of the system are identified. In order to understand them, we introduce the concept of “touching.” The application of this concept enables us to suggest that the first transition is a local carrier deconfinement transition, at which the conce...