Carrier-mediated ferromagnetism in Mn(II)-doped ZnTe thin films and their optical properties: A first-principles study

We have investigated the opto-electronic and magnetic properties of Mn(II)-doped ZnTe thin films by employing Density Functional Theory (DFT). In the absence of additional carriers, spin-up Mn-t_a levels are fully occupied, leading to a super-exchange mechanism. The effect of additional p-type doping on the ferromagnetism is studied by considering the addition of hole carriers, Zn vacancies, and C co-doping, and we found that in all cases, coupling of hole carriers with the spin-up Mn-states causes the stability of the ferromagnetism in the Mn(II)-doped ZnTe thin films. We discovered through the optical study that Mn(II) doping at the Zn site widens ZnTe's bandgap and produces spin-forbidden d-d transition peaks on the low energy side of the bandgap. The p-type defects in Mn(II)-doped films produce absorption peaks in the infrared region and improve the absorption efficiency. In addition, the optical bandgap and spin-forbidden d-d transition of Mn(II) to different modes of spin-spin interactions were correlated, and we found a red shift of d-d intra-band transition peaks as well as an optical bandgap in the FM-coupled Mn(II) ions system and a blue shift in the AFM coupled ions system, supporting the experimental observations.