High Curie Temperature Achieved in the Ferromagnetic Mn_xGe_1-x/Si Quantum Dots Grown by Ion Beam Co-Sputtering

Ferromagnetic semiconductors (FMSs) exhibit great potential in spintronic applications. It is believed that a revolution of microelectronic techniques can take off, once the challenges of FMSs in both the room-temperature stability of the ferromagnetic phase and the compatibility with Si-based technology are overcome. In this article, the Mn_xGe_1-x/Si quantum dots (QDs) with the Curie temperature (T) higher than the room temperature were grown by ion beam co-sputtering (IBCS). With the Mn doping level increasing, the ripening growth of MnGe QDs occurs due to self-assembly via the Stranski-Krastanov (SK) growth mode. The surface-enhanced Raman scattering effect of Mn sites observed in MnGe QDs are used to reveal the distribution behavior of Mn atoms in QDs and the Si buffer layer. The Curie temperature of Mn_xGe_1-x QDs increases, then slightly decreases with increasing the Mn doping level, and reaches its maximum value of 321 K at the doping level of 0.068. After a low-temperature and short-time annealing, the T value of Mn_0.068Ge_0.932 QDs increases from 321 K to 383 K. The higher Ge composition and residual strain in the IBCS grown Mn_xGe_1-x QDs are proposed to be responsible for maintaining the ferromagnetic phase above room temperature.