二维GaN非极性面的结构与热输运特性研究

Light emitting diodes (LED) has energy-saving, environment-friendly, high efficiency and other advantage. However, the thermal effect of the chip is significant. Especially, The internal heat flux density of the chip increases rapidly with the miniaturization of LED device, which becomes the key factor that impedes the design and development of the high power and micro-nano LED devices. The thermal transport properties at the chip-level in the nanoscale are inherently different from that in the bulk, which depend on the specific structure and size. GaN-based material is commonly used for the blue LED, however, the electrons and holes have poor overlapping in the band structure in the polar c-plane, which leads to the decrease of luminescence efficiency, and also, the thermal conductivity of the two dimensional polar c-plane is much lower than that of the bulk. Therefore, in this proposal, we will study the thermal transport of the two dimensional GaN in the non-polar faces, m- and a- planes. We will use the first principle calculations with the phonon non-equilibrium Green function or the phonon Boltzmann equation to carry out the research. In addition, we will compare the dependence of thermal conductivity on the number of two-dimensional GaN layers with the dependence of thermal conductivity on the film thickness in the experiments. The study results will predict the structure dependence of the two dimensional GaN non-polar faces on the number of layers, applied external stress, decorated atoms, etc., and how to improve the thermal transport by controlling the structure. These results will provide theoretical and technical guidance for the chip-level thermal management by controlling the structure of two-dimensional GaN in nonpolar faces, which will further improve the reliability of the LED chips.

发光二极管（LED）具有节能、环保、高效等特点，然而芯片的热效应显著，特别是伴随着LED器件的小型化，芯片内部热流密度迅速增大，成为阻碍高功率微纳尺度LED设计和应用的关键因素。处于纳米尺度的芯片层热输运特性不同于块体，与具体的结构和尺寸有很大关系。基于此，本项目从蓝光LED常用基体材料GaN出发，针对常用的极性面c-面的电子空穴交叠变小、发光效率变低以及二维GaN非极性面的热导率远低于块体等特点，基于第一性原理结合声子非平衡格林函数和声子玻尔兹曼方程两种方法，研究二维GaN非极性面m-和a-面的热输运机理，并将热导率随层数的变化关系与实验上热导率随薄膜厚度的变化关系进行比较。研究结果将揭示二维GaN非极性面的结构随层数，外应力，修饰原子等的变化关系以及如何通过改变结构提高热导率，为基于二维GaN非极性面的结构调控实现芯片层级热管理给予理论和技术指导，进一步提高LED芯片的可靠性。

发光二极管（LED）具有节能、环保、高效等特点，其发光特性对电子带隙类型和大小敏感，另外，伴随着LED器件的小型化，芯片内部热流密度迅速增大，成为阻碍高功率微纳尺度LED设计和应用的关键因素。处于纳米尺度的发光部件其带隙大小和热输运特性不同于块体，与具体的结构和尺寸有很大关系。基于此，本项目从蓝光LED常用基体材料GaN出发，基于第一性原理结合声子玻尔兹曼方程和分子动力学两种方法，研究了二维GaN非极性面m-和a-面的结构调控及对应的电子结构和热输运性质。研究结果显示（1）通过增加二维GaN非极性面的层数，m-面的电子能带带隙逐渐由间接带隙向直接带隙过渡，带隙大小先减小再增大，a-面的热导率随层数增大很快趋近于块体；（2）氢原子修饰能够将表面的悬键饱和，从而改变了电子能带带隙类型且对带隙大小由较大的调整，但氢原子修饰对热导率的调节有限；（3）通过施加外加应力，电子能带带隙大小和热导率均可以在较大范围内调控，且各存在一个最优应力对应最大带隙宽度和最大热导率。本研究为基于二维GaN非极性面的结构调控实现原子层级热管理给予理论和技术指导，从而进一步提高LED芯片的可靠性。