Tunnel percolation and current path switching in a granular metal

Abstract

By means of electron beam induced deposition we have prepared a series of granular metals consisting of tungsten nanocrystallites embedded in an insulating matrix. The metal content varied between 19 at% and 34 at%. The samples have been analyzed by voltage-current and temperature-dependent conductivity measurements. Within the insulating matrix the nm-sized metallic nanocrystallites are irregularly distributed so that the classical percolation threshold can be assumed to be at approximately 50 at% metal volume fraction. Consequently, within the classical limit no current transport at low temperatures would be expected. Nevertheless, for samples with metal volume fraction above 24 at% the temperature-dependent conductivity extrapolates to a finite value for T = 0. We argue that clustering of nanocrystallites to larger aggregates of enhanced tunneling propability between the cluster members may be the reason for the observed behaviour. The transport can then be considered as a tunnel percolation process between clusters. As a consequence the current is inhomogeneously distributed in the samples. From analyzing different voltage regions of the voltage-current characteristics, separated by a sample-dependent threshold voltage VC, showing either reversible (at lower voltages) or irreversible (at higher voltages) behaviour, we conclude that only one current path is active at any given moment and that this current path need not be that of the highest possible conductance. In particular, in some instances it proved possible to increase the conductance by a factor of 106 after having passed VC.