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This thesis concerns the fabrication and characterization of SiGe nanostructures (such as quantum dots (QDs) and nanowires (NWs)) as well as related thermoelectric devices.
For thermoelectric application, the major advantage of SiGe-based material is the compatibility to prevailing complementary metal-oxide-semiconductor (CMOS) integrated circuits (IC) technology. We propose thermally oxidizing SiGe on Si-containing layer, for generating precise numbering, placing and size-tunable Ge QD array, and study their optical and thermoelectric properties in this thesis. Interestingly, unique ‘explosive’ behavior of Ge QDs through Si3N4 layers into the underlying Si substrate is observed during high-temperature oxidation for a long time; the QDs regress almost to their origins as individual Ge nuclei are formed during the oxidation of the original nanopatterned SiGe structures that were used for their generation. A kinetic model is proposed to explain the anomalous migration behavior and morphological changes of the Ge QDs, based on the Si flux that is generated during the oxidation of Si-containing layers. This finding enhances control over the location and quantity of Ge QDs. Moreover, we observed the transition energy of Ge QD appears to have a systematic blue-shift with a decrease in Ge QD size, indicating the transition of Ge from indirect to direct band-gap materials, thanks to quantum confinement effects. The temperature-dependent thermal conductivity of the Ge QD array is systematically investigated. The experimental results appear to reveal an inverse dependence of the thermal conductivity of Ge QDs on dot size in a SiO2 system. Clearly, surface/boundary scattering of nanostructures and alloy scattering cause phonon drag and considerably reduce thermal conductivity, owing to quantum confinement.
The thermoelectric properties of Si1-xGex nanopillars were systematically investigated. The thermal conductivities of Si1-xGex nanopillars with Ge mole fractions of 0.12-0.36 are between 0.8 and 2 W/m•K, and depend inversely on Ge content. An increase in the Ge content significantly increases the electrical conductivity of both N- and P-type Si1-xGex NWs, because of a reduction in the effective mass and, therefore, an increase in carrier mobility. The inclusion of Ge into the nanopillar structures can support the decoupling of the resistances to phonon and charge transport.
NiSi1-xGex nanocontacts are used to connect pairs of N- and P-type pillars at the bottom of each in the Si1-xGex microcooler. Therefore, obtaining a detailed understanding of the thermal stability, electrical property and phase formation of NiSi1-xGex NWs is important due to the injected high current and voltage in Si1-xGex microcoolers. The experimental finding suggests the geometry of Si NWs is key factor to affect the phase, volumetric expansion and electrical resistivity of the NixSiy NWs. The phase transition from NiSi with large NWs (WSi NW = 250-450 nm) to Ni2Si with small NWs (WSi NW = 40-100 nm) is closely correlated associated with the observed variation in electrical resistivity with NW width after silicidation at 500oC, and the resistivities of the various NixSiy phases are NiSi - 12.5 -cm and Ni2Si - 30 -cm. Interestingly, an Ni-rich phase of the NixSiy NWs is formed, in sharp contrast to the formation of nickel monosilicide for the NiSi film. A new, plastic deformation mechanism is proposed to explain it.
Electron-beam lithography and plasma etching were used to generate SiGe nanopillars with a diameter of 250 nm and a height of 1000 nm. Then, the Si1-xGex (x = 0 and 0.24) P/N-pair nanopillar arrays with sizes of 17×12 μm^2, 37×32 μm^2, and 56×52 μm^2 for use in thermoelectric microcoolers is developed. The effects of Ge content and the size of the array of SiGe nanopillars on cooling capability are systematically studied. An array of Si0.76Ge0.24 nanopillars with an area of 37×32 µm^2 exhibits a cooling efficiency as high as 1.36oC/μA at 90oC, such that cooling by more than 15oC is achieved at 90oC. | en_US |