Structural and electrical properties of granular metal films

Author： Abeles B. Sheng Ping Coutts M.D. Arie Y.

ISSN： 1460-6976

Source： Advances In Physics, Vol.24, Iss.3, 1975-05, pp. : 407-461

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Abstract

Granular metal films (50–200,000 Å thick) were prepared by co-sputtering metals (Ni, Pt, Au) and insulators (SiO 2 , Al 2 O 3 ), 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 10 6 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 &rgr;L = &rgr;o exp [2√( C/kT )], and the field dependence of the high-field, low-temperature resistivity is given by &rgr;H = &rgr;∞ exp (εo/ε), where &rgr;o, &rgr;∞, 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 &rgr;(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.