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M. Seifner:
"Synthesis and Characterisation of One-Dimensional Ge-based Nanostruktures with Metastable Composition";
Betreuer/in(nen), Begutachter/in(nen): S. Barth, A. Lugstein, J. Fleig; Institut für Materialchemie, 2019.

Problems concerning the native oxide of Ge hampered the integration of Ge into the Si-based semiconductor industry. After solving these problems and the successful integration of Ge on Si platforms, the superior properties of Ge in terms of hole and electron mobility when compared to Si have been used to develop novel high-performance devices. Nowadays, efficient doping of Ge which is already well-established for Si is now in the focus of this research field. In addition, metastable compounds based on Ge are of great interest due to the possibility of transforming Ge from an indirect bandgap semiconductor with very poor absorption and emission of light in the mid-IR range into a direct bandgap semiconductor. The down-scaling of material´s dimensions towards the nanometre regime enables the growth of materials with special properties and compositions which cannot be observed and reached for bulk materials.
This work focuses on the growth of anisotropic, metastable alloys in a metal-assisted bottom-up growth process by the SLS/VLS-mechanism. The incorporation of the metal growth promoter by solute trapping during the growth of the Ge crystal is targeted which requires a kinetically controlled process at very low temperatures. In the first part of the results section the incorporation of Ga in a Ge crystal matrix via a vapour-phase process is described. The metastable composition of the material contains approximately four times the solubility limit at high temperatures and 50 times the composition limit at the actual growth temperatures. The material is highly conducting and shows metal-like behaviour.
Furthermore, the incorporation of Sn in anisotropic Ge crystals is realised via a solution-based microwave-assisted process. The metastable Ge1-xSnx alloy (x = 0.17 - 0.28) nanostructures are formed via homogeneous nucleation and without the use of a template. Several new findings such as the stability of α-Sn at high temperatures, the Ge1-xSnx material´s stability with and without the presence of metallic Sn as well as the solid-state diffusion mechanism for the material degradation are observed. Based on these results, a phase map is suggested including the kinetic influence on phase evolution. In addition, the physical properties such as conductivity and indirect observation of a direct bandgap material are presented. The results obtained by this microwave approach are used to implement a vapour-phase process for the epitaxial growth of anisotropic, metastable Ge1-xSnx alloys with high Sn contents of 19 at. % on a Ge substrate. The size-dependence of a transition zone to accommodate the lattice mismatch between the growing material and the substrate is described. Finally, a direct bandgap of the material (0.29 eV) is demonstrated by photoluminescence. This is a significant step towards the integration of this compound on a Si platform.
The obtained materials are characterised by electron microscopy, X-ray diffraction, IR absorption, and photoluminescence measurements. Furthermore, single NW devices are fabricated to evaluate the electronic properties of metastable Ge1-xSnx and Ge1-xGax nanowires. Results obtained in this thesis provide a deeper understanding of synthesis parameters of metastable compounds and demonstrate new approaches for the synthesis of materials achieved under kinetically controlled growth conditions.