Ferromagnetic Semiconductor Structures based on GeMnTe

Magnetic semiconductors have attracted great interest as possible key elements for realisation of novel spintronic devices that can be directly integrated within conventional semiconductor device architectures. For such applications, a magnetic semiconductor with high Curie temperature TC is required that can be integrated in heteroepitaxial multilayer structures. The major objective of this research project is therefore to fabricate and investigate novel ferromagnetic semiconductor hetero- and nanostructures based on molecular beam epitaxy (MBE) of Ge1-x Mn x Te. In contrary to the Manganese Mn-based III-V compounds, the novelty of our approach is that in our material system the carrier density can be controlled independently of the concentration of the magnetic ions, and that the solid solutions are thermally stable and exists over a very wide composition region. We expect that this stability will allow the fabrication of ferromagnetic heterostructures with TC values above 200 K, while avoiding the problems of magnetic GeMn precipitate formation. The planned research program will encompass the investigations of the growth properties of GeMnTe based heterostructures as well as the investigations of their structural, magnetic, optical as well as electronic properties using a wide range of experimental techniques, including SQUID magnetometry, AFM and TEM studies, x-ray diffraction and advanced synchrotron x-ray scattering, as well as optical spectroscopy and transport measurements. First, we will optimise the Mn concentration, the carrier density as well as the crystalline quality of the Ge1- x Mn x Te epilayers to achieve the highest TC value. Subsequently, we will fabricate magnetic 2D Ge1-x Mn x Te quantum wells within IV-VI heterostructures. The influence of strain on the magnetic properties as well as interlayer coupling effects between the magnetic semiconductor 2D layers will be studied. The lattice mismatch of Ge1-x Mn x Te with respect to different IV-VI materials, such as PbSe and PbTe, will allow to grow self-assembled ferromagnetic semiconductor quantum dots (QDs) using the Stranski-Krastanow growth mode. Under appropriate conditions, the size of these islands will be adjusted in the range of a few nanometers by changing the growth conditions. This will offer the possibility to reveal the influence of reduced dimensionality on the magnetism of the QDs and the possible magnetic coupling between the QDs.

Magnetic semiconductors have attracted great interest as possible key elements for realisation of novel `spintronic` devices that can be directly integrated within conventional semiconductor device architectures. These future devices will not only use the charge of the electrons, but also its magnetic spin for data processing. This would allow an increase of processing power by a reduction of energy consumption. For such applications, a magnetic semiconductor with high Curie temperature TC is required, where the semiconductor depicts still ferromagnetic behaviour. The major objective of this research project was therefore the fabrication and investigation of novel ferromagnetic semiconductor multilayer- and nanostructures. These structures consisted of GeMnTe and GeMn alloys realised by molecular growth. We have grown GeMnTe thin layered structures with quite high transition temperatures of around 200K, comparable to other material systems like GaMnAs. This is still far below room temperature at 300 K, but we believe that this materials system has potential for reaching this value. Moreover, we could show that the ferromagnetic properties could be directly related to the cubic phase of GeMnTe, and not to incoherent precipitates. Furthermore, we investigated the self-organised growth of ferromagnetic GeMn nanomagnets within a Ge semiconductor layer with synchrotron x-ray diffraction studies. This material is of special interest due its compatibility to the today`s silicon-technology. These hexagonal Mn 5 Ge3 nanomagnets were embedded in a cubic Ge matrix and depicted ferromagnetic behaviour up to room temperature. We could reveal the average size and the alignment accuracy of these nanoparticles with respect to the Ge lattice for different growth conditions. A full understanding of their self-organized growth process will be crucial for the tuning of the magnetic properties of the whole semiconductor layer, which is required for possible future device applications.