Effects of Mo single-doping and Mo-Al co-doping on ZnO transparent conductive films

Abstract

Molybdenum (Mo) atoms with high valence (+6) can contribute four free electrons to zinc oxide (ZnO) crystals. Therefore, Mo is considered as potential dopant for improving the conductivity of ZnO films. In this paper, using the self-governing ceramic targets, ZnO thin films with different Mo single-doping concentrations (MZO) and Mo-Al co-doped concentrations (MAZO) were prepared by magnetron sputtering method. Combined with the material characterization method and the first principles, the effects of Mo single-doping and Mo-Al co-doping on the optical and electrical properties of ZnO thin films were analyzed. The experimental results show the carrier concentration and conductivity of ZnO films were effectively improved since the doping of Mo atoms increased the concentration of free electrons and decreased the defects of the ZnO lattice. However, the excessive single introduction of Mo6+ free electrons into ZnO lattice was prone to cause the refinement of MZO grain size and the emergence of more potential energy traps in the lattice, resulting in the increase of the absorption and the reduction of transmittance of MZO film. Nevertheless, Mo-Al co-doping made Mo and Al atoms play a synergistic role and effectively improve the optical properties of MAZO films. Astonishingly, with the increase of Mo doping concentration, Mo5+ appeared in MZO and MAZO films for the enhancement of the extranuclear electron localization. Furthermore, based on the electron band theory, it reveals that the Mo-4d impurity states appeared in the valence band of MZO and MAZO films and the valence band moved to a lower energy level, which led to the increase of absorption rate and decrease of transmittance of MZO film. Especially, as Mo-Al co-doping technology is adopted, the co-doping elements can make the impurity state act as a “springboard”, which is beneficial to the formation of photo-generated carriers, and improve the carrier concentration and the conductivity of MAZO films.