博士论文答辩：基于模式级别玻尔兹曼输运方程的纳米尺度非平衡热输运研究
日期：2025/05/26 - 2025/05/26
博士论文答辩：基于模式级别玻尔兹曼输运方程的纳米尺度非平衡热输运研究
主讲人：Jiaxuan Xu, Ph.D. candidate at UM-SJTU Joint Institute
时间：2025年5月26日（周一）下午2:00-3:30
地点：密西根学院龙宾楼403会议室
讲座摘要
With modern semiconductor technology entering the more-than-Moore era, advanced electronic devices have been scaled down to the nanoscale. At this scale, thermal transport governed by phonons and electrons determines how the heat energy is generated and transferred within semiconductor devices, which plays a crucial role in advancing transistor performance, enhancing circuit functionality, and improving overall reliability from devices to systems. Different from the macroscale, a common phenomenon of nanoscale thermal transport is the deviation from the local thermal equilibrium state. This phenomenon manifested as significant differences in equivalent temperature between electrons and phonons, as well as between different phonon modes, i.e., the electron-phonon nonequilibrium and mode-level phonon nonequilibrium. However, to date, the underlying mechanisms of such nanoscale nonequilibrium thermal transport and how to quantitatively analyze its impact on devices are still elusive. Additionally, direct evidence of nonequilibrium thermal transport in device semiconductors at the nanoscale is still lacking. Consequently, these gaps further hinder the development of effective strategies for manipulating nonequilibrium thermal transport. Therefore, the objective of this dissertation is to bridge these gaps and investigate nanoscale nonequilibrium thermal transport in realistic semiconductor devices based on phonon and electron mode-level Boltzmann transport equation (BTE).
The mechanisms of nonequilibrium thermal transport in typical nanostructures within semiconductor devices are first analyzed. Significant temperature nonequilibrium among ballistic-to-diffusive phonons is found in thin semiconductor films after considering the phonon-phonon coupling that has been overlooked in the literature. The selective electron-phonon interaction and the ballistic phonon transport are demonstrated to be the origin of the nanoscale phonon nonequilibrium. Quantitative analysis illustrates that it is this phonon nonequilibrium that leads to less efficient thermal transport at the nanoscale, and thus significantly higher peak temperature and effective thermal resistance in silicon FinFETs. The revealed fundamental mechanisms lay the foundation for exploring methods to manipulate nonequilibrium thermal transport in nanoscale devices.
A theoretically and experimentally unified evidence of nonequilibrium thermal transport at the nanoscale in semiconductors are then achieved. By comparing the accurate first-principles based phonon BTE calculations with tip-enhanced Raman thermal measurements conducted with the collaborators, substantial phonon nonequilibrium in GaN near sub-10 nm laser-excited hotspots is directly revealed for the first time, as supported by quantitative agreements between theoretical predictions and Raman measurements. This large phonon nonequilibrium is demonstrated to be universal in three-dimensional device semiconductors at the nanoscale, especially for III-V polar semiconductors. This establishes a theoretically and experimentally unified approach for investigating nanoscale phonon nonequilibrium in electronics.
Next, an effective method to manipulate the mode-level phonon nonequilibrium and enhance nanoscale thermal transport is proposed. It is found that introducing defects into the nanoscale heating region can suppress phonon nonequilibrium and thus enhance effective thermal conductivity. Building on this finding, a novel strategy for reducing the high channel temperature within nanoscale transistors through isovalent channel doping is demonstrated. Rigorous electro-thermal coupled simulations reveal that doping germanium or carbon atoms into a silicon-based FinFET reduces the average channel temperature by over 20%. This strategy is compatible with current semiconductor technology and shows promise for integration with silicon strain engineering to achieve co-optimization of electrical and thermal performance in advanced devices.
Finally, a promising pathway for manipulating electron-phonon nonequilibrium transport to prolong the hot electron cooling process is revealed. By applying the electron-phonon coupled BTE, mode-level phonon nonequilibrium induced by external excitations is shown to significantly perturb the carrier relaxation dynamics in semiconductors. While high-frequency optical phonon nonequilibrium rapidly reheats electrons and elevates electron temperatures, the long-lived low-frequency acoustic phonon nonequilibrium significantly prolongs hot electron relaxation from 1~2 picoseconds to tens of picoseconds, which is valid in GaN, AlN, and Si. This method offers a general mechanism for manipulating hot electron relaxation to reduce heat losses and improve the energy efficiency of nanoscale devices.
This dissertation promotes the fundamental understanding of nanoscale thermal transport in semiconductor devices, which provides theoretical guidance and promising pathways for manipulating the nonequilibrium transport of energy carriers and improving energy efficiency in future advanced electronics.
主讲人介绍
Jiaxuan Xu received the B.S. degree in Energy and Power Engineering from Shandong University, Jinan, China, in 2020. He is currently pursuing the Ph.D. degree with the Thermal Properties and Energy Conversion Laboratory, Shanghai Jiao Tong University, Shanghai, China. His research interests include micro/nanoscale thermal transport, nonequilibrium thermal transport, and electron-phonon interactions.