Electrical conduction and Meyer-Neldel Rule in nanocrystalline silicon thin films

Electrical conductivity has been studied as a function of temperature in amorphous silicon containing varying amounts of Si crystallites (nc-Si:H), prepared by plasma enhanced chemical vapor deposition (PECVD) of silane mixed with hydrogen. HRTEM shows the presence of nanocrystals in the films, prepared under high hydrogen dilution. The relative fraction of crystallinity, chi(c) is estimated using Raman spectroscopy and has been changed by varying the hydrogen dilution and RF power. The room temperature conductivity at first changes very little as chi(c) increases but shows a sharp increase as chi(c) crosses a certain threshold value. Our findings are consistent with the percolation theory calculations. The conductivity is found to be thermally activated and also gives a good fit to the T-1/4 law, for variable range hopping. Further, the prefactors and the slopes in both cases are found to be correlated, through a Meyer-Neldel type relationship, whose origin is not clear at present. This latter relationship (hopping MNR) is like the conventional MNR and many materials having diverse conduction mechanisms, obey it. We take a look at the derivation of the T-1/4 law and try to see if the hopping MNR can be explained in all materials in general and in nc-Si:H in particular. No explanation is available in either case, at present. (C) 2013 Elsevier B.V. All rights reserved.