Session: 08-01: Micro/Nanoscale Heat Conduction

Paper Number: 130512

130512 - 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. Modeling ballistic transport have relied on phenomenological models, such as Knudsen hydrodynamic equations or optical scattering analogies. 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. The time-space dependent BTE is built upon ShengBTE 3 phonon method, adequate for predicting dynamics in bulk crystalline thin films. In this paper, we demonstrate phonon flux and temperature between acoustic and optical phonons.

Our method is applied to simulate a case in which a GaAs and 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. The ballistic phonon effects are seen in films as thin as 5 nm, and diminish beyond 150 nm thickness. Appearance of non-ballistic conduction away from boundaries occurs at relaxation times more than 100 ps. Under this time scale, acoustic and optical phonons are at more than a magnitude pace nonequilibrium. Overall, this non-grey and non-approximate BTE solver can resolve anticipated ballistic transport regimes with time and spatial precision.

As the size of electronic units, such as resistors and transistors, falls below the mean free path of phonons, or the speed of electric operation is faster than phonon scattering time, and the behavior of heat transfer does not follow Fourier’s law. In this case, we cannot use the thermal properties derived from bulk materials to understand heat transfer properties in these devices. 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.

Presenting Author: Richard Zhang University of North Texas

Presenting Author Biography: Dr. Richard Zhang is an Assistant Professor of Mechanical Engineering at the University of North Texas in Denton, TX specializing in thermal management nano/micro-technology design, fabrication, and testing. Dr. Zhang received his SB in Mechanical Engineering from MIT and PhD in Mechanical Engineering from Georgia Tech. Prior to UNT, he was a Member of Technical Staff at the Aerospace Corporation in El Segundo, CA. His research has been sponsored by the Air Force Office of Scientific Research, Army Research Office, Los Alamos National Laboratory, and NASA. He was a faculty fellow at Air Force Research Laboratory. Dr. Zhang is a member of ASME and AIAA.

Authors:

Richard Zhang University of North Texas