高铝组分AlGaN半导体量子结构中的光生载流子动力学性质研究

AlGaN semiconductor with high Al content is ideal material for fabrication of solar-blind ultra-violet detectors and light-emitting diodes. The material is featured by its high density of defects and compositional inhomogeneity of Al content, which introduce inhomogeneity of inner electric field and band structures. Such inhomogeneity with the effect on the excitation, transmission and recombination of carriers may degrade the efficiency of devices. On the other hand, the inhomogeneity may also act as quantum structures with local electric field, which can improve the gain of detectors and the light emitting efficiency. So the thoroughly exploring of the physical mechanisms will help to design novel structures in devices with proper modulation of these features. However, the normally applied characterization methods without high spatial resolution are hard to reveal the relationship between the carrier dynamics and the inhomogeneity at nanometer scale. In this project, we applied the deep ultra-violet photovoltage spectrum measured by a photo-assisted Kelvin force microscope to study the opto-electric properties of single defects. The method has a spatial resolution of atomic level at sub-nanometer. We can also quantitatively measure the influence of defects and compositional inhomogeneity on the carrier dynamics including diffusion lengths and recombination velocities. By alternating the impurities and the composition of the material, the micro mechanism of opto-electric properties can be systematically studied by first-principle calculations, which are the foundation of designing novel deep ultra-violet devices with high efficiency.

高铝组分AlGaN半导体材料是制备波长280nm以下日盲紫外探测和发光器件的理想材料。高密度缺陷和铝组分不均匀性是这类材料的重要特征，其导致能带结构的不均匀性，影响载流子的激发、输运和复合，是制约器件效率的关键因素。但另一方面，缺陷和不均匀的组分本身作为量子结构，所构造的局域电场也可以改善探测增益和发光效率。因此如果能深入认识其中物理机制，揭示这些纳米尺度的结构和载流子动力学性质的对应关系，无疑可以有针对性的进行调控，突破效率瓶颈。本项目拟应用自主研发的基于光辅助扫描开尔文探针显微镜的深紫外光电压谱方法，以亚纳米的原子级空间分辨率研究单个缺陷的电荷状态和光电响应特性，定量测量强极化场下缺陷和组分不均匀性对载流子扩散长度、复合速率等动力学性质的影响。与第一性原理计算相结合，系统地探究不同掺杂和组分的材料中局域场和电子空穴对的相互作用微观过程和物理机制，为设计新型高效的深紫外光电器件奠定基础。

本项目建立了一套有效的纳米尺度紫外光电压谱测量和分析方法，在自主研制的超高真空原子力显微镜上，增加针对AlGaN材料表面清洁处理的实验装置。发展了基于双频单次扫描的表面电势测量方法，采用双锁相在两个频率上分辨进行形貌和电势的反馈，相比通常的表面电势测量方法，由于针尖不需要抬高，因此表面电势的空间分辨率和稳定性可以得到极大的提高。采用该方法可获得7nm的电势空间分辨率。创新性的在测量光电势的扫描开尔文探针显微镜基础上集成了200-300nm激发波长可调的深紫外飞秒脉冲激光系统，从而可原位测量局域荧光寿命和高分辨的局域光电势，获取局域载流子扩散寿命、长度等输运性质的信息。通过这种方法获得了不同铝组分下，AlGaN中表面缺陷的类型、电荷状态、能级位置和光电响应特性；揭示了了组分不均匀性引起的畴界边缘、对应局域内电场和能带结构的分布，明确了这种局域化效应对载流子寿命、扩散长度等动力学性质的影响。例如在Al组分为50%的AlGaN材料中，在不同区域出现了明显的相分离，对应了261和312nm的发光峰，高分辨的光电压测试结果显示表面的大量缺陷和边缘态是出现相分离的可能来源。