Session: 08-02: Micro/Nanoscale Heat Conduction

Paper Number: 131799

131799 - Observations of Nonequilibrium Phonon Transport Near Nanoscale Hotspots 

Abstract: 

Nanoscale hotspots exist widely in many engineering applications, including nanoscale electronics, optoelectronics, and quantum nanodevices. Understanding thermal transport driven by phonons near nanoscale hotspots is vital for quantitatively predicting and improving the performance of these applications. Prior studies have found phonons to deviate from thermal equilibrium near nanoscale hotspots, which significantly affects the hot carrier transport and thermal management in nanoelectronics. However, they were focused on two-dimensional materials, leaving this phonon nonequilibrium overlooked in three-dimensional electronic and optoelectronic semiconductors.

In this work, we develop a refined tip-enhanced Raman thermometry approach, which for the first time reveals an unusually substantial phonon nonequilibrium in gallium nitride (GaN) near sub-10 nm laser-excited hotspots. Large temperature differences are characterized among longitudinal optical (LO) phonons, traverse optical (TO) phonons, and acoustic phonons. To quantitatively analyze this nonequilibrium phonon transport at the nanoscale, previous studies have usually adopted the macroscale heat diffusion equation, the multitemperature model or the gray phonon Boltzmann transport equation (BTE) to investigate phonon transport near nanoscale hotspots. However, these theoretical methods are based on certain simplifying assumptions that are inadequate for capturing the intricacies of phonon transport near a nanoscale hotspot. Here, we employ the more rigorous nongray phonon BTE combined with the first-principles calculations to accurately determine phonon temperatures under nanoscale laser excitations. Quantitative agreements between the tip-enhanced Raman measurements and accurate first-principles based phonon BTE calculations are also obtained for the first time. Based on the theoretical framework of the phonon BTE, we reveal that the phonon nonequilibrium near nanoscale hotspots originates from two effects, i.e., selective electron-phonon interaction and ballistic phonon transport. For relatively large hotspots, phonon nonequilibrium is primarily contributed by selective electron-phonon interaction. When the hotspot size further reduces, the contribution from ballistic phonon transport becomes increasingly more important. This phonon nonequilibrium is illustrated to cause higher temperature rise near these hotspots, leading to significantly smaller effective thermal conductivity. Furthermore, for the GaN, the large phonon nonequilibrium originates from the fact that the strong Fröhlich coupling of hot carriers with longitudinal optical phonons causes significant selective excitation and the large acoustic-optical phonon frequency gap causes insufficient energy transfer between acoustic and optical phonons. These two mechanisms are found to be common in some III-V semiconductors, as pronounced phonon nonequilibrium is further illustrated in both aluminum nitride (AlN) and boron arsenide (BAs).

This work provides an experimentally and theoretically unified evidence of phonon nonequilibrium in three-dimensional electronic and optoelectronic materials at the nanoscale, which is an important step toward the understanding of phonon dynamics in electronic and optoelectronic materials.

Presenting Author: Jiaxuan Xu Shanghai Jiao Tong University

Presenting Author Biography: Ph.D student in Shanghai Jiao Tong University

Authors:

Jiaxuan Xu Shanghai Jiao Tong University
Hua Bao Shanghai Jiao Tong University