Phonon wave effects in the thermal transport of epitaxial TiN/(Al,Sc)N metal/semiconductor superlattices

Abstract

Epitaxial single crystalline TiN/(Al,Sc)N metal/semiconductor superlattice metamaterials have generated significant interest in recent years for their potential applications in high temperature thermoelectric devices, optical hyperbolic metamaterials in the visible and near infrared-spectral range, and as candidates for solar-thermophotovoltaics and high temperature electronic materials. While significant progress in their structural, mechanical, and optical properties has been made, in-depth analysis and detailed understanding of their thermal transport mechanism remain to be addressed. In this article, we show that in short-period epitaxial, lattice-matched TiN/(Al,Sc)N metal/semiconductor superlattices, thermal transport is dominated by phonon wave effects as the wavelengths of phonons that carry significant amounts of heat become comparable to the superlattice period thickness. Due to the increasing contribution of such phonon wave-modes, the cross-plane thermal conductivity at short-periods increases with decreasing period thicknesses resulting in a distinct minimum of thermal conductivity at a period thickness of about 4 nm at room temperature. Thermal conductivity of the superlattices also decreases with an increase in the temperatures due to Umklapp scattering, which supports the wave-nature of the phonon transport mechanism. These results show that the lattice-matched, epitaxial TiN/(Al,Sc)N metal/semiconductor superlattices behave as an effective medium with respect to phonon transport at short-periods, and the wave-nature of phonon dominates the heat conduction mechanism at such length scales.