Synthesis and Characterisation of Nanostructured Germanium-Tin Alloys

Many electronic devices are based on a chemical element silicon. The properties of germanium are very similar to silicon. Both elements can be used in a broad field of applications except devices which rely on a luminescence of the material. Under normal circumstances, pure silicon and germanium do not luminesce effectively and therefore the use in these areas of applications is restricted. This limitation can be circumvented by a homogeneous mixing of germanium with tin in a single material, which is a challenging task. Typically the synthesis of this germanium-tin material is based on sophisticated gas-phase techniques at low temperatures and highly defined crystal surfaces. To date, simpler techniques are not capable to achieve high quality nanostructures of this material. In most synthetic approaches mixtures of both components are obtained instead of a homogeneous distribution of the elements. This project is based on the development of techniques to form material with a defined percentage of tin in a germanium crystal and to control the shape of the structures in at the nanometre scale. To date, small and defined structures can be obtained by downsizing larger coatings into the desired shape. The current project avoids such costly procedures and targets the formation of particles and hair-like crystals from a solution. These solvent-based procedures are cheaper and provide additional knowledge on the crystallisation and growth of this material to tailor its properties. The material formation targets especially microwave-based synthesis procedures, which are well known to heat up food in everyday life. The gained knowledge shall be transferred to more elaborated processes to form germanium-tin nanostructures with defined shapes on different surfaces and in powder form. The sources for the material synthesis will be specific chemical compounds, which can be synthesised in the PIs laboratory. Modification and tailoring of the process can be achieved by chemical design of the compounds used in the process. This already shows, why it is important for this project to be based on different knowledge spanning form nanotechnology, materials chemistry and physics background. The control of the composition of the material and its shape is detrimental, because both affect the physical properties. The physical properties, such as the luminescence, of this germanium tin alloy will be investigated in this project. These measurements will provide information on how to modify the process and composition to optimize the materials properties. Multiple areas of applications can be envisioned for this material including light sources, such as lasers, electronic devices, biomedical diagnosis etc Especially medically important imaging techniques could benefit from these materials because the emitted light does not harm the surrounding tissue due to its low energy, which van be modulated by the composition of the germanium tin alloy.

Modern electronics is generally based on silicon technology. In this regard, Germanium can be used for similar purposes either in combination with silicon or on its own for a large variety of applications. Electronic components are based on semiconductors with specific additives that substitute the semiconductor atoms in the crystal lattice, the so called dopants. The physical properties of a material such as silicon are tailored to specific applications by the type and concentration of specific dopants. This strategy of mixing of elements in predefined compositions while retaining the same crystal structure is not possible for every combination and thus some physical properties cannot be observed by standard processing techniques even though these properties can be predicted. For instance, the pure silicon and germanium cannot provide efficient light emission limiting the use of these semiconductors for specific applications. In the course of this project specific methods have been developed that allow the preparation of nanostructured materials with previously not achieved composition and crystal quality. Moreover, the approaches presented are compatible with silicon technology and could be implemented in established processes. The resulting compositions of the crystals lead to altered physical properties by the incorporation of unusually high concentrations of tin (up to 32 %) and gallium (3-4 %) in the germanium crystals. These compositions cannot be achieved by melting of the components due to a separation of the individual metals upon crystallisation and thus low temperature low temperature processing is required. The alloying resulted in strongly altered conductivity and the possibility to obtain efficient light emission from Ge/Sn alloy nanostructures in the mid-infrared. Thus, these nanostructures materials could be used in future technologies for optical on-chip communication, optoelectronic computing, photodetectors and light emitting diodes etc.