(705e) Thermoelectric Characterization of Suspended Single Silicon-Germanium Alloy Nanowires

The use of nanowires for thermoelectric energy generation has gained momentum in recent years as an approach to improve the figure of merit (ZT) due in part to larger phonon scattering at the boundary resulting in reduced thermal conductivity while electrical conductivity is not significantly affected. Silicon-germanium (SiGe) alloy nanowires are promising candidates to further reduce thermal conductivity by phonon scattering because bulk SiGe alloys already have thermal conductivity comparable to reported Si nanowires1. In this work, we show that thermal and electrical conductivity can be measured for the same single nanowire eliminating the uncertainties in ZT estimation due to measuring the thermal conduction on one set of wires and the electrical conduction on another set. In order to do so, we use nanomanipulation to place vapor-liquid-solid boron-doped SiGe alloy nanowires on predefined surface structures. Furthermore, we developed a contact-annealing technique to achieve negligible electrical contact resistance for the placed nanowires that allows us, for the first time, to measure electrical and thermal properties on the same device. We observe that thermal conductivity for SiGe nanowires is dominated by alloy scattering for nanowires down to 100 nm in diameter between the temperature range 40-300 ºK. The estimated electronic contribution of the thermal conductivity as given by the Wiedemann-Franz relationship is about 1 order of magnitude smaller than the measured thermal conductivity which indicates that phonons carry a large portion of the heat even at such small dimensions.

1.Li D. et al. Thermal conductivity of Si/SiGe superlattice nanowires. APL 2003.

This work was performed at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility at Los Alamos National Laboratory (Contract DE-AC52-06NA25396) and Sandia National Laboratories (Contract DE-AC04-94AL85000).