Fundamental insight into control of thermal conductivity in silicon-germanium alloy nanowires
- PDF / 1,074,982 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 10 Downloads / 170 Views
Fundamental insight into control of thermal conductivity in silicon-germanium alloy nanowires Yongjin Lee and Gyeong S. Hwang McKetta Department of Chemical Engineering, University of Texas, Austin, Texas 78712, U.S.A. ABSTRACT We present a computational analysis of thermal transport in Silicon-Germanium alloy nanowires (SiGeNWs), particularly focusing on the relative roles of alloy scattering and boundary scattering to the significant reduction of thermal conductivity (κ). Our nonequilibrium molecular dynamics (NEMD) simulations confirm the strong dependence of on Si:Ge ratio, as observed in previous experimental studies. Interestingly, as the amount of impurity increases, the difference in κ between SiGe bulk and SiGeNW becomes smaller. Especially, κSiGeNW and κSiGe have similar κ values when the Ge content is 20-80 %. From a nonequilibrium Green’s function (NEGF)-density functional theory (DFT) analysis, it is suggested that the most reduction in transmission channels is attributed to the strong alloy scattering effect for both Si0.8Ge0.2 bulk and Si0.8Ge0.2 NW. The boundary scattering effect in the SiGe alloy system seems to be unimportant as alloy scattering is dominant. The improved understanding provides fundamental insight into how to modify Si-based materials to enhance their thermoelectric (TE) properties through nanostructuring and alloying. INTRODUCTION SiGe alloys are one of the promising candidates for TE energy conversion, due to its low , as compared to pure Si and Ge counterparts. Besides several attempts to further reduce the of SiGe beyond the so-called alloy limit, recent studies1-4 have shown that synthesizing SiGe into one-dimensional nanostructures such as SiGeNWs successfully improves the TE performance over bulk SiGe due to significantly reduced κ. This significant reduction of κ is attributed to the interplay between the phonon scatterings due to the mass difference between Si and Ge (the socalled alloy scattering) and the surface of NWs (the so-called boundary scattering). However, the relative roles of the boundary and alloying scattering effects still remain unclear, despite their importance in precisely controlling the TE properties of SiGeNWs. In this work, we perform a systematic theoretical analysis of thermal transport in SiGeNWs in order to obtain fundamental insight into control of in SiGeNWs. THEORY Non-Equilibrium Molecular Dynamics To understand the alloying and boundary scattering effects on the thermal conductivity of SiGeNWs, NEMD simulations were performed using LAMMPS (Large-scale Atomic and Molecular Massively Parallel Simulator)5 with a time step of 0.5 fs. In NEMD simulations, using the Müller-Plathe method6, a temperature gradient (T) (in the direction of the flow) was 1
obtained from an imposed heat flux (J) to determine the value of (= – J /T). The heat flux was imposed in the z direction by adding (subtracting) nontranslational kinetic energy to a group of atoms in the heat source (sink) layer. After reaching steady state, the temperature profile was ob
Data Loading...
