Ballistic Phonon Heat Conduction Under Non-Equilibrium in Nanoscale Heterogeneous Semiconductor Thin Films

Abstract Thermal management of microelectronic components is becoming critical as devices become smaller and faster. Developing our understanding of size-dependent thermal properties impacts the semiconductor industry. At the nanoscale, phonon thermal transport is challenged by ballistic scattering as mean free path energy exceeds that of the thickness of the thin film. In this approach, we bypass models and directly integrate first principles phonon collision methods in a lattice. We have developed a method that solves the time-dependent Boltzmann Transport Equation (BTE) for phonons with heterogeneous spatial structures of different materials. Our method is applied to simulate a case in which InAs semiconductor nanometer-thin films sandwiched between a heat source and sink. The equilibration in fraction of picosecond time resolution showed nonlinear heat flux normally linear in Fourier scales. Our results demonstrate how energy is transferred in time and space by acoustic and optical phonon modes. Contact thermal resistivity is observed at the material interfaces with the source and sink. This validates boundary-effect scattering of phonons, where the effect does not rely on surface correlations or optical specularity. This “brute-force” BTE method is necessary for observing non-uniformity in heat flux and temperature distribution in thin films. While the computational intensity is relatively high compared to its native process, our relatively simple first principles-in-a-lattice approach can down the road resolve mismatched solid interfacial phonon transmittance, contact resistance, and defect scattering without reliance on thermodynamic parameters and approximations.