Transition-metal doping of semiconducting chalcopyrites: half-metallicity and magnetism

An extensive ab initio investigation of the electronic and magnetic structure of transition-metal-doped chalcopyrites is reported, focusing on Cr, Mn and Fe in CuAlS2, AgGaS2, ZnGeP2 and CdGeAs2. Our results show that the electronic properties, in terms of defect-induced density of states, largely depend on the specific combination of host, transition-metal dopant and substituted cation. In particular, for AgGaS2 doped with Cr, Mn and Fe in both cationic sites, we show that the transition metal substituting silver gives rise to strongly localized orbitals in proximity to the Fermi level, whereas generally a much higher hybridization occurs when the transition metal substitutes for Ga. On the other hand, if we fix the dopant, namely Cr, and introduce it into different hosts, Cr-doped CuAlS2 and AgGaS2 show more localized defect-induced states, whereas in more covalently bonded hosts, such as ZnGeP2 and CdGeAs2, a stronger hybridization between Cr d states and the host valence band occurs. Our findings have important consequences on the exchange interaction range, that, along with the exchange interaction strength, must be taken into account in a careful materials design aiming at optimizing the ferromagnetic properties. Finally, exploiting a simple model to predict magnetic moments in diluted magnetic semiconductors, we achieve half-metallic antiferromagnetism in chalcopyrites by co-doping CuAlS2 with Mn and V; indeed, we obtain a 100% spin-polarized density of states, with a mixed impurity band due to both Mn and V d states, ferrimagnetically coupled in the ground state.