The Electrical Transport Properties Of Ag-SnO₂Nanogranular Films

Granular thin films, consisting of nanometer metal particles, are consideredpotential in fundamental research. Trying to understand the influence of tunnelingeffect and the electron-electron interaction on granular system, a series of Ag-SnO2granular films about500nm in thickness were deposited on glass substrates bycosputtering method. The Ag volume fraction x (0.20x0.90) in each sample wasobtained from the energy-dispersive X-ray spectroscopy analysis. The film thickness,microstructure and electrical properties were measured by a surface profiler,transmission electron microscopy and physical property measurement system,respectively.Two percolation transitions are observed in Ag-SnO2nanogranular films with Agvolume fraction x ranging from~0.20to~0.90. In the vicinity of each percolationthreshold xci(i=1,2), the variation in σ with x obeys a power law for x> xci. The originof the first percolation transition at xc1(xc1> xc2) is similar to that of the classical one,while the second transition is explained as originating from the tunneling to thesecond-nearest neighboring Ag particles. These observations provide strongexperimental support for the validity of current theories concerning tunneling effect inconductor-insulator nanogranular composites.We have also measured the electrical conductivity σ and the hall coefficient RHofAg-SnO2granular films in the strongly coupled regime (gT>1) withxc1x0.65.We found a ln Tlaw over a wide rang of temperature from2to~100K. ThislnT dependence is explained as originating from the electron-electron interactioneffect in the presence of granularity. Furthermore, a logarithmic in T correction to thehall coefficient is observed from liquid-helium temperature up to room temperature,which also arose from the local effects of the Coulomb interaction, i.e., the virtualdiffusion correction. The results indicate validity of the predicted universalconductivity and hall resistivity behavior in the current theories concering theelectron-electron interaction in granular metal.In the vicinity of the first percolation threshold, the value of hall coefficient atxc10.50is about7times as large as that at high x part (such as x0.90), andGiant Hall effect (GHE) is not present in the Ag-SnO2granular films. The reason is that the dephasing length of electron is found to be comparable to the grain size, andthe quantum-interference effect occurs within a single grain only, while theenhancement of hall coefficient of Ag-SnO2granular films near the percolationthreshold can be well explained by the classical percolation theory.