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Electrical conduction in very thin metal films deposited onto single-crystal sodium chloride substrates has been investigated. The conduction for annealed films is found to be thermally activated with a negative temperature coefficient of resistance. Inspection of the films in an electron microscope shows them to consist of separated microparticles. Electron tunneling through the vacuum between microparticles is generally assumed to be the mechanism of electrical conduction. In this work an attempt has been made to show that conduction can occur in the substrate surface. The conduction is believed to be electron tunneling between regions in the substrate surface immediately under the metallic particles. The current carriers can be contributed to the insulator surface regions by the metallic microparticles. In addition, an activation energy is thought to be necessary before tunneling can occur. This energy is electrostatic in nature and is dependent upon the average size and separation of the microparticle regions.
Herman et al. (1966) studied this question.