Ferromagnetism in self-assembled Ge quantum dots material followed by Mn-implantation and annealing

Ten and twenty layers of self-assembled Ge QDs with 44 and 59-nm-thick Si barrier were grown on high resistivity (1 0 0) p-type Si substrates by rapid thermal chemical vapor deposition followed by Mn ion implantation and post-annealing. A presence of ferromagnetic structure was confirmed in the insulating GeMn diluted magnetic quantum dots (DMQD) and semiconducting GeMn DMQD. The DMQD materials were found to be homogeneous, and to exhibit p-type conductivity and ferromagnetic ordering with a Curie temperature, TC = 350 and 230 K. The X-ray diffraction (XRD) data show that there is a phase separation of Mn5Ge3 from MnGe nanostructure. Temperature dependent electrical resistivity in 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 Mn substituting Ge. Therefore, it is likely that the ferromagnetic exchange coupling of DMQD material with TC = 230 K is hole-mediated due to formation of polarons and the ferromagnetism in sample with TC > 300 K is due to Mn5Ge3 phase.