Role Of Energy Distribution In Contacts On Thermal Transport In Si: A Molecular Dynamics Study

We use molecular dynamics simulations to investigate how the energy input and distribution in contacts affect the thermal transport in silicon as described by the Stillinger-Webber potential. We create a temperature difference across a Si specimen by maintaining the temperature of two contacts (also made of Si) using widely used thermostats: the deterministic Nose-Hoover approach and a stochastic Langevin bath. Quite surprisingly, the phonon thermal conductivity of the channel obtained using the two thermostats but under otherwise identical conditions can differ by a factor of up to three. The discrepancy between the two methods vanishes as the coupling strength between the thermostat and material is reduced and for long channels. A spectral analysis of the contacts and channel shows that increasing the coupling of the stochastic Langevin thermostat affects the spectral energy distribution in the contacts away from that based on the vibrational density of states, broadening peaks and smoothening the distribution. This results in contacts injecting phonons preferentially in low frequency modes and in transport through the channel away from local equilibrium. A comparison of the MD results with Boltzmann transport equation simulations provides an additional insight into the role of contacts on thermal transport in nanoscale specimens. These results stress the importance of contacts in nanoscale thermal transport in simulations and in the interpretation of experimental data. Published by AIP Publishing.