Thermal conductivity of individual Si and SiGe epitaxially integrated NWs by scanning thermal microscopy.

Semiconductor nanowires have demonstrated fascinating properties with application in a wide range of fields including energy and information technologies. In particular, increasing attention has been focused on Si and SiGe nanowires for application in thermoelectric generation after recent successful implementation in miniaturized devices. Despite this interest, an appropriate evaluation of thermal conductivity in such nanostructures still poses a great challenge, especially if the characterization of the device-integrated nanowire is desired. In this work, a spatially resolved technique based on scanning thermal microscopy has been demonstrated for the assessment of the thermal conductivity of Si and SiGe nanowires integrated in thermoelectrical microgenerators. Thermal conductivity values of 15.8 ± 0.8 W m-1 K-1 and 4.2 ± 0.3 W m-1 K-1 were measured for Si and SiGe nanowires, respectively, epitaxially grown on silicon microstructures. Moreover, the range of applicability according to the sample thermal conductance and associated errors are discussed to establish the potential of the novel technique. The results presented here show the remarkable utility of scanning thermal microscopy for the challenging thermal characterization of integrated nanostructures and the development of multiple devices such as thermoelectric generators or photovoltaic cells.

Transitioning from Si to SiGe Nanowires as Thermoelectric Material in Silicon-Based Microgenerators.

The thermoelectric performance of nanostructured low dimensional silicon and silicon-germanium has been functionally compared device-wise. The arrays of nanowires of both materials, grown by a VLS-CVD (Vapor-Liquid-Solid Chemical Vapor Deposition) method, have been monolithically integrated in a silicon micromachined structure in order to exploit the improved thermoelectric properties of nanostructured silicon-based materials. The device architecture helps to translate a vertically occurring temperature gradient into a lateral temperature difference across the nanowires. Such thermocouple is completed with a thin film metal leg in a unileg configuration. The device is operative on its own and can be largely replicated (and interconnected) using standard IC (Integrated Circuits) and MEMS (Micro-ElectroMechanical Systems) technologies. Despite SiGe nanowires devices show a lower Seebeck coefficient and a higher electrical resistance, they exhibit a much better performance leading to larger open circuit voltages and a larger overall power supply. This is possible due to the lower thermal conductance of the nanostructured SiGe ensemble that enables a much larger internal temperature difference for the same external thermal gradient. Indeed, power densities in the µW/cm2 could be obtained for such devices when resting on hot surfaces in the 50-200 °C range under natural convection even without the presence of a heat exchanger.