LED芯片老化过程中有源区的缺陷演化机理研究

The proliferation of various types of defects during the ageing is the main reason of the long-term luminous decay and EQE droop of light-emitting diodes (LEDs). The interplays among different kinds of defects and carriers lead to the growth of dark spots/regions in the surfaces of chips, which furtherly lower the optical power as well as life-span. It is therefore critical to study the evolution of defects of different kinds to improve the performance and life-span of devices. In this project, we consider the photoluminescence, electroluminescence, and the external quantum efficiency vs. forward current curves as telltales of the defect status in the active region, and investigate the defect evolution in LEDs by monitoring the shift of these characters during ageing. This project is divided into four steps as follows: a) The interplays among various types of defects and carriers under different injection levels and the respective growth speed of defects of various types during ageing; b) Mechanism of dark spots and the inhomogeneous growth of defects in different regions; c) Carry out simulation by COMSOL, and combine the theories obtained from previous steps, understanding the mechanism of defect evolution d) The inner connection among low-temperature optical parameters, life-span (ageing speed) and EQE Droop of a same chip. This project aims both to give a main reason of long-term luminous decay and to explain the mechanism of EQE droop, which would contribute to the life-span and performance of high-brightness LEDs.

发光二极管（LED）老化过程中有源区产生的各种类型缺陷是光衰和效率降低的主因之一。掌握有源区各种类型缺陷在LED工作过程中的演化规律是提升器件性能和寿命的必由之路。本项目以观测光致发光与电致发光谱（常温与低温下）和外量子效率-正向电流曲线参数在老化中的变化为手段，研究LED芯片老化过程中有源区缺陷演化问题。具体为（1）不同激发密度条件下,不同类型缺陷与载流子相互作用机制的区别和不同类型缺陷随着老化时间各自的增长速率（2）芯片发光面暗斑生成机制及芯片内不同区域缺陷的生长趋势差异。（3）利用COMSOL软件进行载流子动力学仿真模拟，并建立物理模型统一以上两个阶段的理论成果，探索有源区缺陷演化机理。（4）在以上结果的基础上，研究芯片低温光学参数特性与芯片寿命（光衰速度）和效率降低三者之间的内在联系。解释引起光衰和效率降低现象的机制，对制作高性能、长寿命的芯片提供理论参考。

本项目以观测光致发光与电致发光谱（常温与低温下）和外量子效率-正向电流曲线参数在老化中的变化为手段，研究LED芯片老化过程中有源区缺陷演化问题。项目基本达到了预期目的，对以下两个科学问题做了系统性研究和解释（1）不同激发密度条件下,不同类型缺陷与载流子相互作用机制的区别和不同类型缺陷随着老化时间各自的增长速率（2）芯片发光面暗斑生成机制及芯片内不同区域缺陷的生长趋势差异。通过研究不同发光波长的LED器件的老化特性，发现在老化过程中点缺陷是引起发光下降的重要因素，尽管载流子局域化会起到一定的缓解作用。同时，我们引进了显微高光谱相机对芯片表面进行空间分辨光谱检测，发现电流密度对器件局域老化也存在极大的关联性。我们认为，各种老化因素最终可以归结为引起局部晶格温度升高对晶格的破坏性。我们对micro-LED的光电热特性做了试探性研究，为今后对micro-LED的深入研究打下基础。