Role of the carrier density in the transport mechanisms of polycrystalline ZnO films

The transport processes occurring in polycrystalline ZnO have been investigated by measuring theresistivity as a function of temperature in ZnO films with different n-doping levels, obtained by varyingthe oxygen pressure during the deposition process. These films show an electrical resistivity spanningabout two orders of magnitude, from 4 to 8 10^-2 Ohm cm at room temperature, corresponding to lowand high levels of n-type doping, respectively. The present results indicate a relevant role of the carrierdensity in determining the dominant transport mechanisms in these samples by showing that the picturecharacterizing a highly n-doped ZnO sample, where an intra-grain mechanism and a grain-boundarymechanism dominate the high temperature and low temperature transport processes, respectively, isthoroughly overturned in lightly n-doped samples, where a grain-boundary mechanism and an intra-grainmechanism govern the charge transport in the same temperature regimes, respectively. Moreover, thepresent results indicate a critical role of the conditions limiting the occurrence of the Mott variable rangehopping regime. They show indeed that an incomplete check of such conditions can result in erroneousconclusions about the prevalent transport mechanisms