Macro-defect-free homoepitaxial GaN growth through halogen-free vapor-phase epitaxy on native GaN seeds

Although halogen-free vapor-phase epitaxy (HF-VPE) is suitable for GaN bulk-crystal growth, the macrodefects (hexagonal and embedded hexagonal pits) have been observed on the as-grown surfaces of HF-VPE GaN layers. In this study, the formation mechanism of these defects is clarified using a combination of scanning transmission electron microscopy, cathodeluminescence, and three-dimensional atom-probe tomography. These analyses indicated that the formation mechanisms of both hexagonal and embedded hexagonal pits were related to the dislocations, and the hexagonal pit was identified as a hollow-core super dislocation. Moreover, some of hollow-core dislocations might dissociate into multiple closed-core elementary dislocations, as a result the embedded hexagonal pits were formed. These dislocations are generated by misoriented GaN on the regrowth interface owing to an unintentional Ga vapor supply at the initial stage of HF-VPE regrowth. To suppress the dislocation generation at the regrowth interface, growth conditions are optimized at the initial growth stage; thereby, the optimized HF-VPE-GaN layer exhibits no hexagonal pits on the grown front. The full widths at half maximum of the ω-scan X-ray rocking curves at the optimized HF-VPE-GaN layer were almost the same as those of the underlying GaN substrate. Thus, we could grow macrodefect-free and low-dislocation-density homoepitaxial thick (∼100 μm) GaN layers at a growth rate of ∼160 μm/h on a native GaN substrate.