Optical and electrical resistivity studies of isovalent and aliovalent 3d transition metal ion doped ZnO

In this paper, we report on the optical and electrical transport studies of TM ion (Ni, Ti, V, Fe, Cr, Mn, and Co) doped ZnO polycrystalline samples. Diffuse reflectance spectroscopy of doped ZnO showed the existence of absorption bands which were attributed to the d-d transitions of respective dopants. Electrical resistivity was found to decrease in case of Ti-, V-, Fe-, and Ni-doped ZnO bulk samples as compared to undoped. We explain the above behavior on the basis of impurity d-band splitting model. It is observed that with increase in dopant content the temperature range where variable-range hopping is valid shrinks to lower values and the activation energy lowers. Increase in characteristic temperature and decrease in localization length was observed with increase in TM ion content pointing towards the delocalization of electrons that sets in with doping.