Interplay between microscopic and macroscopic disorder in percolating Pd films

(Received 9 September 1991, accepted in final form 21 January 1992)

Abstract
Thin metallic Pd films (100 Å) were submitted to irradiation with 100 KeV Xe ions. After an initial increase of the atomic disorder, a decrease of the metallic coverage occurs as holes are created due to sputtering. This leads to a percolating transition. The modifications of the electronic mean free path, film thickness and metallic surface coverage were measured in situ up to the percolation threshold by means of resistive measurements, Rutherford Backscattering and Transmission Electron Microscopy experiments. The resistance at 300 K is found to diverge near the percolation threshold with the typical 2D exponent value. A phenomenological model accounts for the evolution of the topological parameters as the fluence (number of incoming ions per unit surface) increases. We present low temperature electrical measurements on films irradiated at different fluences, thus lying at different distances from the percolation threshold. Our results show a strong interaction between the microscopic and the macroscopic disorder near the percolation threshold : the conductance temperature dependence crosses over from a logarithmic shape (i.e. characteristic of weak localization in 2D metallic films) to a power lam dependence, due to the non-Euclidean (fractal) dimensionality of the percolating metallic structure. Moreover, very close to the threshold but always above it, the conductance varies exponentially with temperature, revealing the appearance of an Anderson insulator, although a continuous metallic path exists across the samples. At 4.2 K, the resistance approaches the percolation threshold with exponent 2, (different from the classical percolation value 1.3 observed at 300 K). This new exponent must contain information on the observed Metal-Insulator Transition.