Theoretical investigation of optoelectronic and magnetic properties of Co-doped ZnS and (Al, Co) co-doped ZnS

In this work, electronic, magnetic and optical properties of Co-doped and (Co, Al) co-doped ZnS have been studied using the first principles technique based on density functional theory (DFT). The non-spin polarized state of ZnS changes to spin-polarized state when doping with Co. The Co ions interact antiferromagnetically which can be discussed on the basis of the super-exchange mechanism. The additional electron introduced by Al co-doping changes the magnetic order from antiferromagnetic to ferromagnetic state. Moreover, optical properties such as dielectric functions, reflectivity, absorption coefficients and transmissivity for undoped ZnS, Co; ZnS and (Co, Al); ZnS were also discussed. We found that the optical absorption edges shift to lower energy after Co doping in ZnS. After Al co-doping in Co; ZnS, a new peak is observed in infrared region. Thus, we found all curves of optical properties are broadened after Co doping and (Co, Al) co-doping in ZnS. The correlation between the magnetic coupling and intra-band d-d transition was also investigated and we found that d-d (⁴T₁-⁶A₁) transition peak for AFM coupled Co ions is higher than that for FM coupled Co ions, which is in line with theoretical and experimental observations. The improved optical and magnetic results indicate that (Co, Al) co-doped ZnS can be used as a promising candidate for photonic and spintronic devices in the future.