Diluted magnetic semiconductors (DMS), due to the promising applications in future spintronic devices, have attracted enormous attention in the past few decades. Compared with other DMSs, GeMn DMS becomes more attractive in terms of compatibility with current Si technology and easy commercialization. However, in most cases, Mn rich precipitates (e.g., Ge3Mn5 and Ge2Mn5) tend to form during growth due to the low Mn solubility in Ge, which hinders the acquirement of high quality of GeMn DMS for practical spintronic applications. In order to address this challenge, in this thesis, through detailed investigations by transmission electron microscopy (TEM) and superconducting quantum interference device (SQUID), we studied systematically the structures and magnetic properties of GeMn DMS thin films grown by molecular beam epitaxy. The results from this thesis can be summarised as follows.
1. The structural and magnetic properties of Ge1-xMnx thin films grown on Si substrates, with different nominal Mn concentrations (1-4%) and different growth temperatures, have been investigated by TEM and SQUID. We found that, for the samples grown at 70 oC, coherent Mn-rich clusters were verified to exist even at 1% Mn, with increasing the Mn concentration, coherent tadpoles dominate the thin films. As for the thin films grown at 150 oC, however, a significant number of tadpoles formed in the film with only 1% Mn. Mn-rich secondary precipitates such as Mn5Ge3 and Mn11Ge8 became dominant with increasing the doping concentration. Furthermore, coherent interface between Mn11Ge8 precipitates and Ge matrix was first evidenced.
2. The structural variations and magnetic properties of Mn doped Ge thin films grown on Ge substrates have been studied by TEM and energy dispersive spectroscopy. We demonstrated that the morphology of coherent Mn-rich clusters can be controlled from tadpole to nanocolumn by altering the nominal Mn concentration. For a 3.5% Mn doped Ge1-xMnxthin film grown at 70 oC, a critical thickness of ~12 nm has been identified to form Mn-rich clusters. As Mn-rich clusters and precipitates exist in all samples, the GeMn thin films show a complex magnetic behaviours, which are mainly originated from the clusters and precipitates. Our results also reveal that a flat buffer layer surface plays a vital role to obtain vertically aligned tadpoles and nanocolumns.
3. We have successfully developed a novel approach to fabricate extraordinarily coherent and self-organized MnGe nanodot arrays embedded in the Ge and GaAs matrixes by low temperature MBE. A high yield of such aligned nanodot arrays was confirmed on different substrates, showing an ideal controllability and reproducibility. We anticipate that our studies will advance the development of MnGe magnetic semiconductors and/or other similar systems. The obtained coherent and self-assembled nanostructures could be potentially used as the building blocks in the high-density magnetic memories, sensors and spintronic devices, enabling a new generation of low dissipation magnetoelectronic devices.
4. The effect of substrate, GeMn/Ge thickness, Mn concentration and growth temperature on the structure of the GeMn/Ge superlattices grown by molecular beam epitaxy have been investigated by MBE and TEM. We found that, by varying the growth parameters, the structure of GeMn/Ge superlattices can be changed from disordered GeMn nanodots to ordered GeMn nanodot arrays, then to well aligned GeMn nanocolumns.