Impact of vacancy defects on optoelectronic and magnetic properties of Mn-doped ZnSe

Based on first principles calculations, we have investigated the electronic, magnetic and optical properties of Mn-doped ZnSe with and without vacancy defects. We find that the incorporation of Mn at Zn site changes the non-magnetic ground state of ZnSe to magnetic ground state with total magnetic moment 5 μ_B. From the investigation of magnetic coupling, it is found that the interaction between Mn spins in Mn-doped ZnSe is AFM which can be explained in term of super-exchange mechanism. The effect of vacancy defects such as Zn or Se vacancy on magnetic coupling between Mn spins were also analyzed and we found that Zn-vacancy promotes double exchange interaction which stabilizes the ferromagnetic state and room temperature ferromagnetism is expected while Se-vacancy doesn't change magnetic ground state from AFM to FM states. The magnetic interactions in Mn-doped ZnSe system with and without Zn vacancy were explained using phenomenological band structure model. Moreover, optical absorptions for all systems were investigated at their stable magnetic states and we found that Mn doping causes the red shifted in absorption edge which is due to the spin forbidden d-d transitions (⁴T₁-⁶A₁) in Mn ions. The new peak in low energy range is related to the acceptor states introduced by Zn-vacancy in Mn-doped ZnSe system. The Se-vacancy generates peaks in visible region which is attributed to the donor states caused by Se-vacancy. Finally, the d-d transition (⁴T₁-⁶A₁) in FM and AFM coupled Mn ions were also studied and we found that the d-d transition peaks in AFM and FM coupled Mn ions are blue and red shifted respectively. Similarly, the blue and red shift in the energy bandgap of ZnSe are also observed in AFM and FM coupled Mn ions which support the agreement between the experimental and theoretical observations.