Ferromagnetism in Mn-implanted Ge/Si nanostructure material

Multistacked Ge quantum dots (QDs) with Si spacers of different thicknesses have been grown on (100) Si substrates by rapid thermal chemical vapor deposition followed by Mn ion implantation and post-annealing. The presence of a ferromagnetic structure was confirmed in the insulating (Si0.45Ge0.55)Mn0.03 diluted magnetic quantum dots (DMQD) and semiconducting (Si0.45Ge0.55)Mn0.05 DMQD. The DMQD materials were found to be homogeneous, and to exhibit p-type conductivity and ferromagnetic ordering with Curie temperatures TC = 350 and 160 K respectively. The x-ray diffraction (XRD) data show that there is a phase separation of Mn5Ge3 from the MnGe nanostructure. Temperature-dependent electrical resistivity in the semiconducting DMQD material indicates that manganese introduces two acceptor levels in germanium at 0.14 eV from the valence band and 0.41 eV from the conduction band implying that Mn substitutes for Ge. Therefore, it is likely that the ferromagnetic exchange coupling of DMQD material with TC = 160 K is hole-mediated due to formation of bound magnetic polarons and the ferromagnetism in the sample with TC > 300 K is due to Mn5Ge3 phase.