Electron Current Through Thin Mica Films

Abstract

Thin films of mica have unique attributes that are exceptionally good for studies of high-field conduction mechanisms in thin-film insulators and the quantum mechanical tunneling of electrons from metal to metal. The principal advantages of using mica films are that the films are crystalline and the cleavage planes occur every 10A. This property results in films whose thicknesses are integral multiples of lOÅ and whose surfaces are uniformly parallel over sizable areas. Hence, very well-defined metal-mica-metal strutures are possible. Furthermore, the fact that the insulator is split from a bulk sample allows the index of refraction, dielectric constant, forbidden energy gap, and trapping levels and their density to be obtained directly from measurements performed on thick samples of mica rather than requiring that these properties be inferred from the conduction characteristics alone. In the work to be described, all the cleaving was done in a high vacuum just prior to the evaporation of metal electrodes so as to avoid air contamination at the interfaces. Results of these studies indicate that the current through the 30 and 40Å films exhibited quantitative agreement with the theoretical voltage and temperature dependence derived by Stratton for the tunneling of electrons directly from metal to metal. Thicker films at room temperature exhibited volt-ampere curves suggesting Schottky emission of electrons from the cathode into the conduction band of mica. However, the thermal activation energy was smaller than that found from other measurements, and the experimental Schottky dielectric constant was larger than the square of the index of refraction. These facts would indicate that the electrons were being injected into polaron states in the insulator. At low temperatures and high fields, the current through the thicker films did not exhibit the Fowler-Nordheim dependence as would be predicted by a simple extention of the theory of field emission into a vacuum.

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