First principles calculations of optoelectronic and magnetic properties of Co-doped and (Co, Al) co-doped ZnO

Based on first principles calculations within spin polarized-density functional theory, we investigate optoelectronic and magnetic properties of Co-doped and (Co, Al) co-doped ZnO. Our results demonstrate that Co substitution changes the nonspin polarized state of pure ZnO to spin polarized with total magnetic moment of 3 μ_B. The origin of antiferromagnetic (AFM) coupling between Co spins in the ZnO lattice is guided mainly by a super-exchange mechanism without any additional defects. The effect of Al codoping on the ground state of Co-doped ZnO is also investigated, and we find that the additional electron introduced by Al codoping changes the magnetic ground state of Co-doped ZnO from AFM to FM state, and the estimated Curie temperature is expected to be higher than room temperature. The optical absorption spectra of pure ZnO and Co-doped systems for all compositions are investigated, and we find that with an increase in Co concentration, the bandgap of ZnO and the position of spin-allowed d-d transition peaks of Co spins exhibit a blueshift and redshift behavior, respectively, which are consistent with the experimental results. Al codoping produces absorption peaks near infrared light and visible-light regions and increases transition energy due to the Burstein-Moss effect. Furthermore, the bandgap and d-d transition peaks of Co spins are correlated with magnetic coupling, and we observed a redshift of fundamental bandgap and d-d transition peaks of Co ions for FM coupled Co spins, and blueshift for AFM coupled Co spin systems.