Transport mechanisms in Co-doped ZnO (ZCO) and H-irradiated ZCO polycrystalline thin films

In the present study, the electrical resistivity (r) as a function of the temperature (T) has been measuredin polycrystalline ZnO, Co-doped ZnO (ZCO) and H irradiated ZCO (HZCO) samples, in the 300-20 Krange. The achieved results show impressive effects of Co doping and H irradiation on the ZnO transportproperties. The Co dopant increases the ZnO resistivity at high T (HT), whereas it has an opposite effectat low T (LT). H balances the Co effects by neutralizing the r increase at HT and strengthening its decreaseat LT. A careful analysis of the r data permits to identify two different thermally activated processes asthose governing the charge transport in the three materials at HT and LT, respectively. The occurrence ofsuch processes has been fully explained in terms of a previously proposed model based on an acceptorimpurity band, induced by the formation of Co-oxygen vacancy complexes, as well as known effectsproduced by H on the ZnO properties. The same analysis shows that both Co and H reduce the effects ofgrain boundaries on the transport processes. The high conductivity of HZCO in the whole T-range and itslow noise level resulting from electric noise spectroscopy make this material a very interesting one fortechnological applications.