界面形核时间对GaN薄膜晶体质量的影响

利用金属有机化学气相沉积技术系统研究了界面形核时间对c面蓝宝石衬底上外延生长GaN薄膜晶体质量的影响机理. 用原子力显微镜、扫描电子显微镜、高分辨X射线衍射仪以及光致发光光谱仪表征材料的晶体质量以及光学性质. 随着形核时间的延长, 退火后形成的形核岛密度减小、尺寸增大、均匀性变差, 使得形核岛合并过程中产生的界面数量先减小后增大, 导致GaN外延层的螺位错和刃位错密度先减小后增大, 这与室温光致发光光谱中得到的带边发光峰与黄带发光峰的比值先增大后降低一致. 研究结果表明, 外延生长过程中, 界面形核时间会对GaN薄膜中的位错演变施加巨大影响, 从而导致GaN外延层的晶体质量以及光学性质的差异.

Effect of interface nucleation time of the GaN nucleation layer on the crystal quality of GaN film

In this paper, the influences of the growth time of low-temperature (LT) GaN nucleation layer on the crystal quality and optical properties of GaN film are investigated. It is found that the optimal LT nucleation layer growth time can effectively reduce the crystal defects and is favorable to forming the annihilation of dislocations. GaN films are grown on c-plane sapphire substrates by metal-organic chemical vapor deposition. Crystal quality and optical properties are characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), high-resolution X-ray diffraction (HRXRD), and photoluminescence spectra, respectively. In the AFM images, the island density decreases as growth time increases, while the size of island becomes larger and the uniformity of island size deteriorates as growth time increases, leading to the phenomenon that the number of interfaces formed during the nucleation island coalescence, first decrease and then increase as detected by SEM, which also induces the screw dislocation density and edge dislocation density to first decrease and then increase as measured by HRXRD. This first-decrease-and-then-increase variation trend is consistent with the first-increase-and-then-decrease variation trend of the ratio of the band edge emission peak intensity to the yellow luminescence peak intensity tested by photoluminescence, which is confirmed by HRXRD. It is shown that GaN islands with different sizes and densities could lead to different mechanisms of dislocation evolution, thereby forming GaN epitaxial layers with different dislocation densities and optical properties. Through controlling the nucleation time, GaN films with the smallest dislocation density could be obtained.