Narayan J, Pant P, Wei W, Narayan R J, Budai J D
Department of Materials Science and Engineering, EB-1, Centennial Campus, North Carolina State University, Raleigh, North Carolina 27695-7907, USA.
J Nanosci Nanotechnol. 2007 Aug;7(8):2719-25. doi: 10.1166/jnn.2007.670.
This paper addresses the formation of nanostructured gallium nitride nucleation (NL) or initial layer (IL), which is necessary to obtain a smooth surface morphology and reduce defects in h-GaN layers for light-emitting diodes and lasers. From detailed X-ray and HR-TEM studies, researchers determined that this layer consists of nanostructured grains with average grain size of 25 nm, which are separated by small-angle grain boundaries (with misorientation approximately 1 degrees), known as subgrain boundaries. Thus NL is considered to be single-crystal layer with mosaicity of about 1 degrees. These nc grains are mostly faulted cubic GaN (c-GaN) and a small fraction of unfaulted c-GaN. This unfaulted Zinc-blende c-GaN, which is considered a nonequilibrium phase, often appears as embedded or occluded within the faulted c-GaN. The NL layer contained in-plane tensile strain, presumably arising from defects due to island coalescence during Volmer-Weber growth. The 10L X-ray scans showed c-GaN fraction to be over 63% and the rest h-GaN. The NL layer grows epitaxially with the (0001) sapphire substrate by domain matching epitaxy, and this epitaxial relationship is remarkably maintained when c-GaN converts into h-GaN during high-temperature growth.
本文探讨了纳米结构氮化镓成核层（NL）或初始层（IL）的形成，这对于获得光滑的表面形貌以及减少用于发光二极管和激光器的h-GaN层中的缺陷是必要的。通过详细的X射线和高分辨率透射电子显微镜研究，研究人员确定该层由平均晶粒尺寸为25nm的纳米结构晶粒组成，这些晶粒由小角度晶界（取向差约为1度）分隔，称为亚晶界。因此，NL被认为是具有约1度镶嵌性的单晶层。这些纳米晶大多是有缺陷的立方氮化镓（c-GaN）和一小部分无缺陷的c-GaN。这种被认为是非平衡相的无缺陷闪锌矿c-GaN，通常以嵌入或包裹在有缺陷的c-GaN中的形式出现。NL层包含面内拉伸应变，推测是由于在伏尔默-韦伯生长过程中岛状合并产生的缺陷所致。10L X射线扫描显示c-GaN的比例超过63%，其余为h-GaN。NL层通过畴匹配外延与（0001）蓝宝石衬底外延生长，并且当c-GaN在高温生长过程中转变为h-GaN时，这种外延关系得到显著维持。
