Effects of Co Doping and Point Defect on the Ferromagnetism of ZnO

The effects of different doping proportions of Co doping and oxygen vacancies (V_O) on the ferromagnetism of zinc oxide (ZnO) are controversial. To solve this problem, we investigated the effects of Co doping and point defects (V_O, V_Zn, H_i, and Zn_i) on the ferromagnetism of ZnO through first-principle calculations by using generalized gradient approximation + U (GGA + U) under density functional theory. Results show that the closer the distance between Co and vacancies (V_O or V_Zn), the lower the formation energy and the more stable the system will be. Co-doped ZnO with V_O or V_Zn exhibits long-range ordered ferromagnetism and Curie temperature above the room temperature. Especially, Co-doped ZnO with V_Zn shows a half-metallic behavior, resulting in 100% conduction hole polarizability, which is highly beneficial for dilute magnetic semiconductors (DMSs) as a hole injection source. The ferromagnetism of Zn₁₄CoO₁₆ is caused by the double exchange effects among the electrons of the O–2p, Co–3d, and Zn–3d orbits mediated by the hole carriers after complexes were formed by the Co doping and Zn vacancy. Similarly, the origin of ferromagnetism in Zn₁₅CoO₁₅ is derived from the double exchange effects among the electrons of O–2p, Co–3d, and Zn–4s states mediated by the electron carriers after complexes were formed by the Co doping and O vacancy. In addition, the result shows that Co-doped ZnO with interstitial H (H_i) or Zn (Zn_i) is unfavorable for ferromagnetism and should be avoided experimentally.