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S. Barth, M. Seifner, P. Pertl:
"Synthesis of metastable Semiconductors using Molecular Sources";
Talk: Chemiedozententagung 2016, D-69117 Heidelberg; 2016-03-21 - 2016-03-23.

Group IV semiconductor nanowires are interesting building blocks for electronic and optoelectronic devices.[1] However, the light emission and absorption characteristics change dramatically, when a heavier homologue is incorporated in the crystal structure. Pure germanium is a semiconductor with an indirect bandgap, which can be converted in a tuneable narrow direct bandgap material by alloying with Sn to form a metastable Ge1−xSnx phase. This group IV alloy can be used for band structure engineering by varying the tin content. The Ge1−xSnx alloy is compatible with CMOS processing and therefore an ideal candidate for infrared optoelectronics and optical devices, such as infrared lasers and high speed transistors. Ge1−xSnx alloys are usually formed in gas phase processes taking advantage of homo- or heteroepitaxy between the growing layer and the substrate. The transition from an indirect to a direct bandgap is expected in the range of 5.3-11 % Sn in Ge1−xSnx according to theoretical calculations and experimental data, which support these findings. An incorporation of Sn in the Ge lattice in a bottom-up synthesis should be carried out under kinetic control, because the binary phase diagram reveals a very low equilibrium solubility of Sn in Ge (<1%).
This contribution will address our strategy for the first bottom-up synthesis of Ge1 xSnx nanowires in a microwave supported, solvent-based growth process without the use of a substrate. The microwave assisted procedure allows us to grow Ge1−xSnx nanowires with tin contents of >11 % (up to 28 %), which is well above the limits of other bottom up approaches in liquids described in literature (<4.3 %).[2] The growth mechanism as well as the active metalorganic species involved in the growth will be discussed.[3] Different growth stages during the formation of nanowires can be distinguished and a diameter dependence of the Sn content in Ge1−xSnx nanowires is observed.