Large Thermoelectric Figure of Merits from SiGe Nanowires by Simultaneously Measuring Electrical and Thermal Transport Properties
Journal
NANO LETT
Date
2012.05.14
Abstract
Thermoelectric devices directly convert heat into electricity or vice versa through simple
structures without moving parts. Such features have attracted significant research efforts to
improve their efficiencies, which are typically described by the thermoelectric figure-of-merit
(ZT). However, thermoelectric properties are strongly correlated, making ZT improvement difficult.
One possible approach to avoid such correlation is to suppress phonon transport by scattering
phonons at the surface of confined nanowire structures.1-5 In crystalline solids, phonon
characteristic lengths such as wavelengths and mean free paths are often broad (e.g., Si: from a
few nanometers to tens of microns even at 300 K)6, which makes the confined boundary of nanowires
insufficient to minimize heat transport. In this work, we employed Ge alloying with Si (Ge
concentration: 6~86 ％) as well as nanowire structures in order to maximize the depletion of heat-
carrying phonons. This results in a thermal conductivity as low as ~1.2 W/m-K at 450 K, showing a
large ZT of ~0.46 in comparison to those of SiGe bulks. More importantly, the three key
thermoelectric properties were ‘simultaneously’ measured from the same nanowires to facilitate
more accurate ZT prediction because the composition, crystallinity, and thermoelectric properties
of nanowires might vary considerably even for the samples that have been grown at the same time.
We also found that the surface boundary scattering is prominent when the nanowire diameter is ~100
nm or larger, whereas Ge alloying plays a more important role in suppressing thermal conductivity
for smaller diameter samples, which agrees with the computational results based on non-equilibrium
molecular dynamics.