Paper NS+BI-MoA8
Estimation of Thermal Conductivity of Si and Oxide-Covered-Si Nanowires by Molecular Dynamics Simulation

Monday, November 9, 2009, 4:20 pm, Room L

Nanowires (NWs) have attracted much attention because they are expected to be applicable to various devices such as the field-effect transistors, solar cells, and thermoelectric devices. The reason is that in nanostructures, we can see some peculiar phenomena which are not observed in bulk materials. One of the phenomena is the lower thermal conductivity of NWs [1]. The thermal conductivity of NWs is one to two orders of magnitude lower than the bulk value (~168 W/mK at 300K). This is appreciated to occur by confinement of the phonon propagation. The phonons are prevented by surface scattering due to the small wire radius. This means that surface atoms may have a great influence on material properties. Experimental observations of the thermal conductivity by P. Yang’s group [1] as well as theoretical studies by different approaches such as Boltzmann transport equation, Monte Carlo (MC) method for phonon propagation [2], and molecular dynamics (MD) simulations [3] have been published. In addition, Hochbaum et al. experimentally observed the reduction of thermal conductivities in Si-NWs fabricated by electroless etching, where silicon NWs were coated by native oxides [4]. These studies indicate thermal conductivities are very sensitive to surface structures.

In this paper, we present the thermal conductivity of naked Si-NWs and oxide-coated Si-NWs at 300 K, evaluated by employing direct nonequilibrium MD simulation with the Stillinger-Weber interatomic potential model for Si/O systems. The thermal conductivity was obtained for various cross sections (2-8 nm^2) and lattice orientations (<100>, <111>, and <110>). The thermal conductivity for naked Si-NWs with cross sections of 2-8 nm² at 300 K was 2-10 W/mK, which was in good agreement with that for <100>Si-NWs (1-2 W/mK) obtained from equilibrium MD simulation by Volz et al. [3] and from MC simulation by Chen et al. [2]. We found the small dependency of thermal conductivity on the lattice orientation. In case of oxide-coated Si-NWs, the thermal conductivity was reduced as the thickness of Si-oxides increased. To check our simulation results, we also analyzed the strain in Si-NW due to surface oxidized layers. Detailed simulation results and analytical approach for various surface structures or cross-sections will be shown at the conference.

[1] D. Li et al., Appl. Phys. Lett. 83, 2934(2003)

[2] Y. Chen et al., Journal of Heat Transfer, Transaction of ASME, 127, 1129(2005)

[3] S.G.Volz et al., Appl. Phys. Lett. 75, 2056(1999)

[4] A.I.Hochbaum et al., Nature, 451, 163(2008)