Structural and electrical properties of granular metal films

TLDR

In dielectric granular metal films, low‑field resistivity follows an exponential dependence on temperature, while high‑field resistivity follows an exponential dependence on electric field, governed by material constants. The study aims to elucidate how the structural characteristics of granular metal films determine their electrical properties. Granular metal films were fabricated by co‑sputtering Ni, Pt, Au with SiO2 or Al2O3 at varying metal fractions, then characterized by microscopy, diffraction, and resistivity measurements over a wide field and temperature range, revealing percolation‑driven conduction with a threshold near x≈0.5. Tunnelling experiments show that the metallic‑to‑dielectric transition corresponds to fragmentation of the metal continuum, and a simple s/d‑ratio theory accurately predicts the temperature and field dependence of resistivity, yielding a universal scaling of ρ with reduced variables.

Abstract

Abstract Granular metal films (50–200,000 Å 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 ε 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 Å size isolated metal particles in an insulator continuum) is associated with the breaking up of a metal continuum into isolated metal particles. In the dielectric regime the temperature dependence of the low-field resistivity is given by ρL = ρo exp [2√(C/kT)], and the field dependence of the high-field, low-temperature resistivity is given by ρH = ρ∞ exp (εo/ε), where ρo, ρ∞, C, and εo are material constants. A simple theory based on the assumption that the ratio s/d (d-metal particle size and s-separation between particles) is a function only of composition yields expressions for ρ(ε, T) in excellent agreement with experiment. Furthermore, the theory predicts the experimental finding that the resistivity can be expressed in terms of a universal function of the reduced variables kT/C and ε/εo. The inter-relationship between all the important physical properties of granular metals and their structure is also discussed.