Unusually low thermal conductivity of gallium nitride nanowires

We report measurements of thermal conductivity kappa on individual gallium nitride nanowires (GaN NWs) with diameters ranging from 97 to 181 nm grown by thermal chemical vapor deposition. We observed unexpectedly small kappa values, in the range of 13-19 W/m K at 300 K, with very weak diameter dependence. We also observe unusual power law kappa similar to T-n behavior with n= 1.8 at low temperature. Electron-energy-loss-spectroscopy measurements indicate Si and O concentrations in the ranges of 0.1-1 and 0.01-0.1 at. %, respectively. Based on extensive numerical calculations, we conclude that both the unexpectedly low kappa and the T-1.8 dependence are caused by unusually large mass-difference scattering, primarily from Si impurities. Our analysis also suggests that mass-difference scattering rates are significantly enhanced by the reduced phonon group velocity in nanoscale systems. Planar defects running the length of the NW, previously characterized in detail, may also play a role in limiting the phonon mean free path. (c) 2008 American Institute of Physics.