B. Abeles

Structural and electrical properties of granular metal films

Abstract Granular metal films (50–200,000 A thick) were prepared by co-sputtering metals (Ni, Pt, Au) and insulators (SiO2, Al2O3), where the volume fraction of metal, x, was varied from x = 1 to x = 0.05. The materials were characterized by electron micrography, electron and X-ray diffraction, and measurements of composition, density and electrical resistivity at electric fields e up to 106 V/cm and temperatures T in the range of 1.3 to 291 K. In the metallic regime (isolated insulator particles in a metal continuum) and in the transition regime (metal and insulator particles in a metal continuum) and in the transition regime (metal and insulator labyrinth structure) the conduction is due to percolation with a percolation threshold at x⋍0.5. Tunnelling measurements on superconductor-insulator-granular metal junctions reveals that the transition from the metallic regime to the dielectric regime (10–50 A size isolated metal particles in an insulator continuum) is associated with the breaking up of a metal ...

Optical properties and selective solar absorption of composite material films

Abstract Composite material films are widely used as selective solar absorbers. We investigated the cermets Auue5f8MgO and Wue5f8MgO and the semiconductor-insulator composites Siue5f8CaF 2 and Siue5f8MgO prepared by co-sputtering. For Au-MgO the dielectric constant does not exhibit the resonance structure near 0.6 μm that is characteristic of other systems of gold particle dispersions, and furthermore the infrared absorption is much larger than that predicted by theory. This anomalous behavior could be due to the textured surface of the films. For the Wue5f8MgO films the observed dielectric constants are in good agreement with the Maxwell Garnett theory. In the films with Si dispersions strong absorption bands appear in the infrared; these are due to compounds formed by chemical reaction between Si and the matrix materials. The photothermal conversion efficiency of solar energy of these materials was estimated and compared with other selective solar absorbers.

Phase transition in reactively co-sputtered films of VO2TiO2

Abstract Films of mixed oxides of VO 2 and TiO 2 were prepared by reactive co- sputtering. With the addition of TiO 2 to VO 2 the metal-non-metal resistive transition is shifted to higher temperatures and broadened considerably. In films containing more than 50 vol.% TiO 2 there is no evidence for the transition.

Numerical Simulation of Hopping Conductivity in Granular Metal Films

Abstract We numerically simulate the temperature dependence of thermally activated tunnelling conductance in granular metal films. By mapping the grains on to a simple-cubic lattice and considering hops up to third-nearest neighbours, it is shown that the hopping conductivity α follows - In σ α 1/T1/2 accurately over several orders of magnitudes. These results support the interpretation that this widely observed behaviour results from interpolating the high-temperature activation behaviour and the low-temperature - Inσ α 1/T1/4 behaviour.

Minimum metallic conductivity in granular metal films

Granular metal films, consisting of fine mixtures of immiscible metals and insulators, are found to exhibit a minimum metallic conductivity, σm, which varies inversely as the average metal grain size d. It is shown that, as a consequence of localization on metal grains due to the electrostatic charging energy associated with the grain, σm?e2/2zh/d (z is the number of nearest neighbor grains), in order of magnitude agreement with observations.

Optical properties of granular magnesium films

Abstract We studied the optical properties of magnesium cermets formed by co- sputtering magnesium metal and MgF 2 . An absorption peak was observed at about 2500 A, in good agreement with the predictions of the Maxwell-Garnett theory. The weakness and large width of the absorption is indicative of very small metal grains.

Numerical Simulation of Hopping Conductivity in Granular Metals

Electrical conduction in granular metals is simulated by mapping the hopping conductance between pairs of metal grains onto a simple cubic lattice with bonds between neighbors. By considering a log-normal distribution of grain sizes and the effect of disorder potential, the numerically calculated network conductance exhibit clear deviation from simple activation. Plotting -log a vs. T-½, where σ denotes conductivity and T the temperature, gives good straight line behavior with slopes comparable to those measured experimentally. Our results are noted to differ from those of Adkins et al.

Electron Localization in Granular Metals

Granular metals (GM) consist of a mixture of finely divided metals and dielectric particles. When the volume fraction x of the metal is large the GM’s exhibit metallic behavior—a positive temperature coefficient of resistivity, superconductivity, and ferromagnetism. In this limit the dielectric acts as impurity scattering centers. As x is reduced below about 0.7, nonmetallic behavior sets in, and properties such as superconductivity,1 ferromagnetism,2 and electrical conductivity3 undergo drastic change. In this work we have used electron tunneling into GM’s to observe directly the development of an energy gap in the tunneling density of states associated with the metal-nonmetal transition.