Effects of Confinement and Surface Reconstruction on the Lattice Dynamics and Thermal Transport Properties of Thin Films

Phonon transport in argon and silicon thin films is examined using harmonic lattice dynamics theory and the Lennard-Jones and Stillinger-Weber potentials. Film thicknesses ranging from 0.8 to 33.5 nm for argon and 0.4 to 8.6 nm for silicon are examined at a temperature of 0 K. Both reconstructed films and films built using the bulk unit cell are considered. Phonon dispersion curves for the in-plane direction and the density of states are computed from lattice dynamics and compared to predictions for a bulk system. The results from the lattice dynamics calculations are used to predict the thermal conductivities of the bulk and thin film structures. NOMENCLATURE a lattice parameter c constant vector cph phonon specific heat h̄ Planck’s constant over 2π i imaginary number î, ĵ, k̂ unit vectors in x-, y-, and z-directions kB Boltzmann’s constant kx thermal conductivity in the x-direction l1, l2 integers m mass ri j separation distance, |r j− ri| r position vector T absolute temperature ∗Address all correspondence to this author u displacement from equilibrium position vg,x x-component of group velocity V volume a, A, B, p, q Stillinger-Weber constants Greek α thermal expansion coefficient γ Grüneisen parameter ε energy scale κ wave vector λ wavelength Λ mean free path σ length scale ν dispersion branch φ interatomic potential energy Φ total potential energy ω angular frequency λ , γ Stillinger-Weber constants