Microstructural, Optical and Electrical Properties of Molybdenum Doped ZnO Films Deposited by Magnetron Sputtering

The undoped and molybdenum doped ZnO (MZO) films were deposited on quartz substrates by radio frequency magnetron sputtering. Mo-doping influences the grain size of the films and leads to a compressive stress in MZO films. XPS reveals that there is only Mo6+, no Mo5+ or Mo4+, in the MZO films. Mo doping introduces two effects on Zn 2p(3/2) XPS spectra, one is the decrease of binding energy of Zn 2p(3/2), and the other is the emergence of a smaller peak at 1022.0 eV in Zn 2p(3/2) spectrum. The optical bandgap of MZO films is in the range of 3.30-3.31 eV, larger than 3.29 eV of the undoped ZnO films. The resistivity of MZO films, which is in the range of 0.062-6.18 Omega cm, decreases first and then increases with the increase of Mo content. The resistivity of MZO films reaches the minimum when the Mo doping concentration is 2 wt.%. According to our observation, it can be supposed that proper doping level of Mo can stimulate the formation of Zn-i and prompt the conductivity of MZO films. Annealing in air causes a significant increase in resistivity of the MZO films, which can be attributed to the adsorption of oxygen and the concentration change of the donor defects Zn-i and its complexes defects in the annealing process.