中文摘要：

采用两步AlN缓冲层（一层低温AlN和一层高温AlN）在r面蓝宝石衬底上生长了非极性的a面GaN,并利用高分辨X射线衍射和光致荧光谱对所生长的材料进行了研究。两步AlN缓冲层在我们之前的工作中已被证明比单步高温AlN或低温GaN缓冲层更有利于减小材料各向异性和提高晶体质量,本文进一步优化了两步AlN缓冲层的结构,并得到了各向异性更小,晶体质量更好的a面GaN薄膜。分析表明,两步AlN缓冲层中的低温AlN层在减小各向异性中起着关键作用。低温AlN层能抑制了优势方向（c轴）的原子迁移,有利于劣势方向（m轴）的原子迁移,从而减小了Al原子在不同方向迁移能力的差异,并为其后的高温AlN缓冲层和GaN层提供＂生长模板＂,以得到各向异性更小、晶体质量更好的a面GaN材料。

英文摘要：

Nonpolar（1120） a-plane GaN films with two-step AlN buffer（a low-temperature（LT） and a high-temperature（HT） AlN layers） were grown on（1102） r-plane sapphire by metalorganic chemical vapor deposition（MOCVD）.The as-grown films were investigated by high-resolution X-ray diffraction（XRD） and photoluminescence（PL）.The two-step AlN buffer has been proved to be advantageous in crystal quality compared with one-step LT-GaN or HT-AlN buffers in our early works.In this report,the thickness of the two-step buffer was further optimized,and much less anisotropic a-plane GaN films were achieved.It was found that the LT-AlN layer of the two-step buffer played a key role in reduction of anisotropy of the GaN film grown.