Influence of Oxygen Defects and Their Evolution on the Ferromagnetic Ordering and Band Gap of Mn-Doped ZnO Films

We performed a comprehensive investigation on the influence of oxygen vacancies (VO) defects and their evolution under Mn substitution and thermal annealing in different atmospheres on the ferromagnetic ordering and optical energy band gap (Eg) of Mn-doped ZnO thin films. All doped films showed a ferromagnetic phase at room temperature, which became stronger at low temperatures. Our results clearly showed a direct link between the concentration of VO and the observed ferromagnetism (FM). By increasing the Mn concentration (CMn), both photoluminescence and X-ray photoelectron spectroscopy results revealed an enhancement in the concentration of VO, in parallel with a clear improvement in the FM. In addition, the FM in the doped films became stronger after annealing in vacuum and weaker after annealing in air, in accordance with a considerable increase and decrease in the concentration of VO that mediates the observed ferromagnetic order. The results also indicated a direct correlation between the Eg behavior and the concentration of VO. Films with a high CMn having a narrower Eg and higher VO level are found to show stronger FM. On increasing the CMn, a slight shift in the Eg to lower values is detected, indicating weak sp-d hybridization. All doped films showed a single phase with würtzite structure, and the Mn2+ ions are proved to substitute for the Zn2+ in the würtzite lattice.