The influence of the substrate misorientation on the structural quality of GaN layers grown by HVPE

The paper describes a Transmission Electron Microscopy study on the structural quality of GaN samples grown by Hydride Vapor Phase Epitaxy (HVPE) on ammonothermal (Am) GaN seeds. The only difference between these samples is the crystallographic direction of misorientation of the Am GaN seeds for the further growth. The studied samples were 1° misoriented in two perpendicular crystallographic directions: the [1 1 0 0] and the [1 1 2 0]. Cross-section samples were studied and showed that growth of homo-epitaxial GaN on the substrate tilted toward the [1 1 0 0] direction appeared to lead to lower defect density than those tilted toward the [1 1 2 0] direction. This was associated with slow growth on {1 0 1 1} planes that are oriented "edge-on" for the growth along [1 1 2 0] direction leading to the formation of facets while the [1 1 0 0] direction is devoid of such facets. The competition between slow and fast growth rates on particular planes would lead to the change of the crystal shape such as triangle or trapezoidal one. Our present study also found that cracks in the samples tilted toward the [1 1 2 0] direction are formed and they are the origin for half loops. Moreover, dislocation propagation from these cracks is observed. Such cracks were not observed for the samples tilted toward the [1 1 0 0] directions, probably due to more uniform growth. There were no dislocations at the sample surface. However, some parasitic growth occurred on the wafer edges for both samples. Cross-section microscopy found high dislocation density in these areas with and a majority of them are edge dislocations.