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

Abstract

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.

Keywords: MEMS; SiGe nanowires; Silicon nanowires; VLS-CVD; energy harvesting; thermoelectricity.

Figure 1

(a) Sketch of the fabricated devices consisting of a suspended (typically 1 mm²) platform connected to the bulk part of the device by a thin structured nitride membrane on top of which the metal leg is placed, and by Si-based nanowires (NWs) along the other three sides of the platform. In these regions, the NWs bridge a given number of 10 μm wide trenches (from 1 to 4 in the current design) defined by micromachined vertical Si walls. These Si-based NWs arrays configure the laterally extended nanostructured semiconductor leg of the thermocouple; (b) SEM image of the device corner opposite the membrane where grown dense arrays of nanowires can be appreciated bridging the trenches and growing freely inside the corner cavity.

Figure 2

Seebeck coefficient as a function of temperature measured in devices with Si NWs legs, SiGe NWs legs, bulk Si legs bridging the suspended platform and its surrounding Si rim.

Figure 3

(a) Seebeck voltage (open circuit voltage) as a function of hotplate temperature measured for Si and SiGe NWs devices; (b) Maximum electrical power density obtained in both types of devices in the same temperature conditions.

Figure 4

Estimated ΔT across Si and SiGe NWs inferred from the Seebeck coefficients and the open circuit voltages measured for them as a function of the hotplate temperature.

Figure 5

(a) Maximum power density obtained for Si and SiGe NW devices as a function of the open circuit voltage (showing the different device resistance at play); (b) internal ΔT (showing the different electrical resistance and Seebeck coefficient at play). Notice that for clarity, only the data corresponding to the lower ΔT portion available for SiGe has been represented.