Transition metal doping and clustering in Ge

Ab initio calculations of clusters in Ge, up to three-impurity atoms in substitutional and/or interstitial sites, are discussed. Interstitial defects, with usually high formation energies when isolated, are stabilized thanks to the interaction with substitutional sites nearby. Mn impurities show a tendency to cluster and magnetic alignment strongly dependent on the sites; however, the occurrence of some Mn clusters is seen not to degrade magnetism, though reducing the total magnetic moment. Cr codoping leads to large formation energy and antiferromagnetic alignment. Co codoping lowers the energy cost of Mn incorporation without disrupting ferromagnetic ordering, thus representing a possible route to help Mn incorporation in Ge.