Effect of weak electric fields on the conduction in thin metal films
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C PROPERTIES OF SOLIDS
Effect of Weak Electric Fields on the Conduction in Thin Metal Films A. P. Boltaev and F. A. Pudonin Lebedev Physical Institute, Russian Academy of Sciences, Moscow, 119991 Russia e-mail: [email protected]; [email protected] Received February 15, 2006
Abstract—The variation of conduction in island metallic Ti, Co, W, and FeNi films in weak electric field is studied. The variation of the differential conductivity of island metallic films with an electric field at temperatures from T = 77 to 300 K is measured, as well as the temperature dependence of the differential conductivity in the same temperature range. It is shown that a thermally activated conduction mode is realized in such structures. The mechanism of variation of conductivity of island metallic films in a weak electric field is discussed. PACS numbers: 73.22.-f, 73.61.-r DOI: 10.1134/S1063776106090135
1. INTRODUCTION The study of the properties of island metallic films with a thickness smaller than 2–3 nm is stimulated by the general interest in systems with a low dimension (quantum wells, dots, and wires), as well as by the advances made in the technology of preparing various nanostructures and the methods for their investigation. In island metallic films, emission of radiation in an external field was observed [1] and photoconduction was detected in the visible and IR spectral ranges [2]. For a moderate density of metallic islands in such structures, a thermally activated conduction mode can be realized. It was found experimentally that the conductivity in island metallic structures in general varies with temperature in accordance with the law σ ∝ exp[–(T0/T)0.5] [3–6]. However, some authors demonstrated that the exponent (x) in the expression describing the measured value of conductivity in granular systems sometimes differs from 1/2. It was found that the value of x can be 0.75 [7], 0.72 [8], and 1 [9]. Various models explaining the “1/2 law” were mainly analyzed theoretically. These models were modifications of the theory of hopping conductivity in semiconductors, where the 1/2 law is interpreted as the appearance of a Coulomb gap in the electron density of states near the Fermi level. The important role of the Coulomb interaction between charged grains on the formation of the Coulomb gap in granular systems also was noted in [10, 11]. Other models explaining the 1/2 law also exist. For example, the authors of [4] attributed the conduction in granular structures to the correlation between the grain size and the spacing between the grains, while in [12], the conduction in granular structures was associated with an ordered spread in the size
of metallic grains. An attempt at explaining the variation of conductivity with temperature was made in [13] taking into account the deviation of conductivity from the 1/2 law, which was noted in [7–9]. In [13], it was assumed that the conduction mechanism in island systems is associated with the Coulomb interaction between charged islands. It was assumed in [4, 12, 13] that the c
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