Diameter-dependent thermal conductivity of ultrathin GaP nanowires: A molecular dynamics study

Abstract

The diameter dependence of the thermal conductivity of nanowires is usually modeled using Matthiessen's rule, by putting the mean free path of phonons equal to the diameter $d$ of the nanowire. This results in a thermal conductivity $\ensuremath{\kappa}$ that decreases with decreasing $d$, due to the increase in boundary scattering. Recent molecular dynamics studies of heat transport in thin silicon nanowires have shown a nonmonotonic diameter dependence of $\ensuremath{\kappa}$, where a decrease with decreasing $d$ is followed by an increase to a value of $\ensuremath{\kappa}$ exceeding the bulk thermal conductivity. This increase of $\ensuremath{\kappa}$ was explained by an increase of the importance of hydrodynamic transport effects in the thinner wires, where the normal scattering by phonon-phonon interaction increases, but the Umklapp scattering decreases [Y. Zhou, X. Zhang, and M. Hu, Nano Lett. 17, 1269 (2017)]. Here, we study heat transport in thin nanowires of the compound semiconductor gallium-phosphide in the wurtzite crystal structure, using molecular dynamics simulations. A similar nonmonotonic $d$ dependence of $\ensuremath{\kappa}$ is found as in silicon nanowires, but with a minimum in $\ensuremath{\kappa}$ occurring at a much larger diameter of $d\ensuremath{\approx}8\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ instead of 2--3 nm.