Band gap engineering of ZnO via doping with manganese: effect of Mn clustering

The effect of Mn doping on optical properties of zinc oxide ZnO has been studied theoretically. The dependence of the Mn concentration and distribution on the optical band gap was investigated at density-functional level applying a hybrid functional. Supercells of varying size were used to model different Mn concentrations. Possible point defects such as oxygen vacancies and zinc interstitials were taken into account. The thermodynamic stability of defect clustering in ZnO was studied. The magnetic coupling between the Mn ions was studied in dependence of the Mn-Mn distance and the distance to lattice defects. As a main result, we find that Mn clustering in the ZnO host lattice is energetically preferred, and leads to pronounced changes in the electronic structure. In agreement with previous theoretical studies we obtain antiferromagnetic ground states in the absence of point defects. The energy difference between ferromagnetic and antiferromagnetic coupling decreases if electron donating defects such as interstitial Zn are close to Mn ions. The strong dependence of the optical band gap from the Mn-Mn and Mn-defect distances is in line with earlier experiments.