CDS&E: Coupled Thermal, Piezoelectric, and Hot Carrier Effects in AlGaN/GaN HEMTs: Multiscale Modeling of Time Evolution of Device Degradation

基于氮化物材料系统的高电子迁移率晶体管(HEMT)具有高击穿电压、高输出功率、高效率、宽带宽、低噪声、耐温度和抗辐射等特点，在无线通信、国土安全、雷达和卫星系统以及新兴的恶劣环境计算、传感、云网络和功率转换电子设备等领域具有巨大的应用潜力。然而，与这些设备的长期可靠性相关的问题仍然是一个主要问题。可靠性可以定义为系统在特定条件下在给定时间内运行而不发生故障的概率。对于半导体器件，器件参数的不可恢复变化(例如输出电流的退化)可被认为是故障。正如最近报道的应力或加速测试所证明的那样，高温度梯度(热效应)、高电场和内置或诱导的机械应力(逆压电效应)以及高电流密度(热载流子效应)都可能导致组成材料的损伤或缺陷，并最终导致器件失效。然而，控制缺陷形成的确切物理机制以及这些缺陷的性质和分布仍然不清楚。这不仅对解释实验结果，而且对于预测或推断设备寿命，例如对于远程应用中使用的设备来说是关键的任务，都提出了重大挑战。众所周知，要研究在实验上难以解决的物理过程，数值模拟变得至关重要。本项目旨在开发一个多尺度和多物理模拟框架，用于模拟AlGaN/GaN HEMT退化的时间演化和物理机制。开发的模拟器将使设备设计能够提高可靠性。该模拟器和相关的教学材料将在NanHUB.org上部署并免费提供，供更广泛的社区在研究和课堂活动中使用。这项研究的目的是开发一个多尺度、多物理模拟框架(HEMT 3-D)来模拟AlGaN/GaN HEMT中的器件退化机制。需要解决的具体基本问题包括：a)金属扩散、极化和感应电荷密度之间的关系，b)缺陷的来源、空间和时间分布，以及它们如何影响静电、能带结构、栅漏、载流子去束缚和陷阱-去陷阱以及接触电阻，c)晶格加热和热电子注入势垒材料之间的相关性，d)应变和逆压电效应及其温度依赖性，以及e)通过工程几何形状、材料组成、沟道取向和通过微观调节界面特性来增强势垒-沟道和缓冲器-衬底界面的热传递来优化器件。为了正确处理纳米结构活动区的原子对称性以及复杂的、非线性的、高度随机的、在不同长度和时间尺度上动态耦合的潜在物理过程，模拟器将采用集成第一性原理分子动力学、晶格动力学蒙特卡罗和量子修正的电子-声子输运核的模块化方法。模拟器的可移植性和运行效率将通过使用开源科学软件、编译器和库，以及纳入GPGPU平台的优化模型、算法和扩展而实现。将在软件开发工作的每个阶段根据文献中提供的实验数据以及通过与研究实验室、学术界和行业的实验者合作来考虑对计算结果的验证。

High electron mobility transistors (HEMTs) based on nitride material systems feature a unique combination of high breakdown voltage, high output power, high efficiency, wide bandwidth, low noise, and temperature and radiation hardness and have great potential in applications such as wireless communication, homeland security, radar and satellite systems, as well as emerging harsh-environment computing, sensing, cloud-networking and power conversion electronics. However, issues related to long-term reliability of these devices still remain a major concern. Reliability can be defined as the probability of operating a system for a given time under specified conditions without failure. For semiconductor devices, unrecoverable change of a device parameter (such as degradation in the output current) may be considered as a failure. As evidenced in recently reported stress or accelerated tests, high temperature gradients (thermal effect), high electric fields and built-in or induced mechanical stresses (inverse piezoelectric effect), and high current densities (hot carrier effect) may all induce damages or defects in the constituent materials and ultimately lead to device failure. Yet, the exact physical mechanisms governing the defect formation as well as the nature and distribution of these defects are still not clearly understood. This poses a significant challenge to not only interpreting the experimental results but also predicting or extrapolating the device lifetime, tasks that are critical, for example, for devices used in remote applications. It is well acknowledged that to study physical processes that are experimentally intractable, numerical modeling becomes essential. This project sets out to develop a multiscale and multiphysics simulation framework for modeling the time evolution of and the physical mechanisms responsible for AlGaN/GaN HEMT degradation. The developed simulator will enable device design for improved reliability. The simulator and the related instructional materials will be deployed and made freely available on nanoHUB.org for the broader community to use in research and classroom activities. The objective of the proposed research is to develop a multiscale, multiphysics simulation framework (HEMT 3-D) for modeling device degradation mechanisms in AlGaN/GaN HEMTs. Specific fundamental issues to be addressed include: a) correlation between metal diffusion, polarization, and induced charge density, b) origin, spatial and temporal distribution of defects, and how they affect electrostatics, band structure, gate leakage, carrier deconfinement and trapping-detrapping and contact resistances, c) correlation between lattice heating and hot-electron injection into the barrier material, d) strain and inverse-piezoelectric effects and their temperature dependence, and e) device optimization through engineering geometry, material composition, channel orientation, and enhancement of heat transfer at barrier-channel and buffer-substrate interfaces via microscopic tuning of the interface characteristics. To properly treat the atomistic symmetry in the nanostructured active region as well as the underlying physical processes that are complex, nonlinear, highly stochastic and dynamically-coupled at different length and time scales, the simulator will employ a modular approach integrating first-principles molecular dynamics, lattice kinetic Monte Carlo, and quantum-corrected electron-phonon transport kernels. Portability and run-time efficiency of the simulator will be achieved through the use of open-source scientific software, compilers and libraries, as well as incorporating optimized models, algorithms and extensions for GPGPU platforms. Verification of the computational results will be considered at every stage of the software development effort against experimental data available in literature as well as through collaboration with experimentalists in research laboratories, academia and industries.