Transition metal impurities in Ge: Chemical trends and codoping studied by electronic structure calculations

Ab initio electronic structure calculations within density functional theory have been performed to study the structural, electronic, and magnetic properties of transition metal impurities into a Ge matrix. We examine impurities of single isolated transition metal (TM) over the 3d series (TM=V,Cr,Mn,Fe,Co,Ni), occupying the substitutional and interstitial sites and interpret the electronic and magnetic properties in terms of a simple analysis based on atomic orbitals and electron filling. We show that Mn is indeed the most promising TM dopant to make Ge suitable for spintronic applications since it would grant ferromagnetic alignment, rather high local magnetic moment and hole doping. Then, we study the tendency of the most interesting TM impurities to cluster in the Ge matrix and investigate the case of (Mn,Co) and (Mn,Cr) codoping in Ge, both experimentally achieved. Analyzing the structural, electronic, and magnetic properties and giving insights regarding the expected solubility and the properties of the alloys, we find that while Mn impurities show a tendency to cluster, the presence of Co and overall Cr could favor a more uniform TM impurity distribution. However, both Co and Cr result to be far less efficient than Mn in promoting ferromagnetic alignment and high magnetic moment.