Ge-Si based Nanowire Thermoelectric Materials in the Phonon Confinement Regime

For over a decade, nanotechnology has allowed rational synthesis of nanodot, nanowire and nanotube materials with different structure, composition, and with size control at the atomic precision. These nanoscale building blocks make it possible to explore electronic and energy harvesting devices with completely new operating principles. Similar to photonic and electronic confinement regimes where the device size approaches the wavelength of photon and electron wavefunctions, nanoscale semiconductor materials have enabled a new era of phononic engineering to turn waste heat into power and to explore the greener side of silicon. In particular, we have devised a highly sensitive differential bridge thermometry method to characterize thermal transport properties of individual Ge and heterostructure core-shell Ge/Si nanowires with diameters < 20 nm[1].  The resulting drastically suppressed thermal conductance beyond the diffusive boundary scattering limit is the first experimental evidence for the role of confinement effect on acoustic phonon dispersions in nanowires[2]. Lately we have also explored the high carrier mobility and concentration modulation in the semiconductor Ge/Si nanowires using gate modulation, and presented the first field-effect modulated thermoelectric power factor measurement on nanowires with diameters down to 11 nm[3]. The power factor was found to strongly correlate with carrier mobility and can exceed that of bulk Ge in nanowires with high hole mobility. Taken together, the  well-established tunable electronic property, excellent thermoelectric power in Ge/Si nanowire materials, combined with thermal transport in the phonic confinement regime, are expected to open up a new level of thermoelectric performance beyond classical avenues.

1. Wingert, M. C.; Chen, Z. C. Y.; Dechaumphai, E.; Moon, J.; Kim, J. H.; Xiang, J.; Chen, R. K. Nano Lett. 2011, 11 (12), 5507−5513.

2. Zou, J.; Balandin, A. J. Appl. Phys. 2001, 89 (5), 2932−2938.

3. J. Moon, J-H. Kim, Z.C.Y. Chen, J. Xiang, and R. Chen, Nano Lett. 2013, 13 (3), 1196–1202.