Hopping Conduction in Granular metals

• Main Authors: Chien-Han Lin, 林振漢
• Other Authors: Yu-Shu Wu
• Format: Others
• Language: en_US
• Published: 2001
• Online Access: http://ndltd.ncl.edu.tw/handle/81710905321345132858

博士 === 國立清華大學 === 電機工程學系 === 89 === In this thesis, we present a theoretical of hopping conduction in granular metal systems. In Chapter 1, we give a brief introduction to the properties of granular metal systems, variable-range-hopping(VRH) conduction theory, resistor network model, and percolation model concepts. In Chapter 2, we modify some previous transport theories which consider only the nearest-neighbor(n.n.) hopping, and compare the modified theory with the experiment of Abeles et al. We carry out a critical path(CP) analysis, and show that, with practical material parameters, the modified theory gives better agreement with the experiment, for the temperature-dependent conductivity σ~exp[-(T0(x)/T)^n], where x = metal volume fraction and n = 1/2. We also present a resistor network simulation, and show that the temperature range over which the exponent n = 1/2 is rather wide and compares reasonably with the experiment of Abeles et al. In Chapter 3, we develop a percolation method for hopping resistances of granular metals with non-nearest neighbor hopping included. The method is based on the VRH model and neglects, for non-nearest neighbor hopping, the potential barrier presented by intermediate grains. This neglect of potential barrier is incorporated as a local constraint in the calculation. We compare our result with experiments, and found that the inclusion of non-nearest neighbor hopping in the VRH model results in reasonable agreement, in both the characteristic temperature T0 and the temperature range over which the “ ” law is observed for the hopping resistance. This quantitative comparison provides the evidence that hopping transport phenomena in granular metals can be described by the VRH model. In Chapter 4, we investigate the multi-path effect with the forgoing percolation model. This effect lowers the system resistance and the characteristic temperature T0. We analyze the multi-path effect within the percolation model, and the analysis can be extended to any random or regular lattice, e.g., sc, bcc, fcc, and so on. In Chapter 5, we present a study of quantum diffusion effect in granular metals. This effect results from the reduction, due to quantum diffusion, in the probability of a coherent hopping process, and is shown to affect the temperature dependence of hopping conductivity. The effect is sizable in the low temperature range where the transport is dominated by long-range hopping.