Fabrication and ferromagnetism of Si–SiGe/MnGe core–shell nanopillars

Abstract

Si-Si0.5Ge0.5/Mn0.08Ge0.92 core-shell nanopillar samples were fabricated on ordered Si nanopillar patterned substrates by molecular beam epitaxy at low temperatures. The magnetic properties of the samples are found to depend heavily on the growth temperature of the MnGe layer. The sample grown at a moderate temperature of 300 °C has the highest Curie temperature of 240 K as well as the strongest ferromagnetic signals. On the basis of the microstructural results, the ferromagnetic properties of the samples are believed to come from the intrinsic Mn-doped amorphous or crystalline Ge ferromagnetic phase rather than any intermetallic ferromagnetic compounds of Mn and Ge. After being annealed at a temperature of 500 °C, all the samples exhibit the same Curie temperature of 220 K, which is in sharp contrast to the different Curie temperature for the as-grown samples, and the ferromagnetism for the annealed samples comes from Mn5GeSi2 compounds which are formed during the annealing.

Growth and Magnetism of MnxGe1−x Heteroepitaxial Quantum Dots Grown on Si Wafer by Molecular Beam Epitaxy

“…However, the self-assembly growth process of QDs requires high growth temperatures for atom migration. In the structure of Mn-doped Ge QDs, intermetallic compound phases between Mn and Ge can form, as we reported in previous papers [24,25]. Therefore, a proper growth temperature is important for the growth of QDs.…”

“…(1) When the growth temperature was low, the quality of the crystal in the QD was poor, which was amorphous or polycrystalline. In our previous study of Si-SiGe/MnGe core-shell nanopillars, the uniform distribution of the Mn dopant and the amorphous state of the MnGe layer were observed by High Resolution Transmission Electron Microscope (HRTEM) images and Energy Dispersive Spectrometer (EDS) mappings in MnGe layer grown at a low temperature [25]. Mn cannot reach the lattice substitution site in the lattice, and the energy band arrangement of the QD and the substrate is also distorted.…”

“…The rod-shaped nanostructures can be attributed to the intermetallic compound phase[18]. In our previous work, Mn x Ge 1−x QDs were grown on a SiGe virtual substrate, where Mn 5 Ge 3 might form in small dome-shaped QDs[24], and the presence of Mn 11 Ge 8 was observed in the top Mn-rich region of the Si-SiGe/MnGe core-shell nanopillars, which we fabricated earlier[25]. These results indicated that an intermetallic compound phase easily forms under improper growth conditions.…”