SIZE-CONTROLLED PERCOLATION PATHWAYS FOR ELECTRICAL-CONDUCTION IN POROUS SILICON

Silicon shows photo-and electroluminescence at visible wavelengths whe n chemically etched into a microporous network of 'wires' several nano metres thick(1). This raises the possibility of a silicon-based optoel ectronic technology. The luminescence properties may be understood on the basis of the injection or creation of electrons and holes in the i nterconnected network of wires which recombine radiatively with high e fficiency(1,2). Elucidating the electron-transport mechanisms has been held back by several difficulties, particularly that of making stable , high-quality contacts to the porous material. Here we report experim ents that probe the conduction process using photoemission stimulated by hard-ultraviolet/X-ray synchrotron radiation, obviating the need fo r good electrical contacts. We find that the conductivity of porous si licon films is temperature-dependent, and that the films become insula ting at low temperatures. We suggest that these results may be underst ood in terms of a percolation process occurring through sites in the p orous network in which conductivity is thermally activated, and we pos tulate that this activation may be the consequence of a Coulomb blocka de effect(3,4) in the nanoscale channels of the film. This is consiste nt with our observation of optical 'unblocking' of conducting pathways . These results imply that the size distribution of the nanowires in t he silicon backbone plays a key role in determining the conduction pro perties, and that porous-silicon light-emitting diodes may use only a small (and the least efficient) fraction of the material. Improvements in electroluminescence efficiency may be possible by taking into acco unt the percolative nature of the conduction process.