Bragg diffraction imaging characterization of crystal defects in GaN (0001) substrates: Comparison of the growth method and the seed approach

We investigate the defect structure of gallium nitride (GaN) substrates grown by hydride vapor phase epitaxy (HVPE) and ammonothermal method, with emphasis on the seeding approach ("foreign seed" or "native seed"). X-ray Bragg diffraction imaging techniques (laboratory X-ray Lang topography (L-XRT) and synchrotron monochromatic rocking curve imaging (RCI)) were used to study the defects of the GaN substrates. The efficiency of the in-process L-XRT method, whereas being strongly dependent on the structural perfection of the crystals, is important because it provides a good overview of the defect structure for entire substrates. But it remains qualitative, or semi-quantitative. RCI, on the other hand, allows obtaining complete quantitative information about lattice misorientation and distortion with sub-µm resolution. The contrast of the diffraction images of defects such as grain boundaries, dislocations, dislocation bundles, planar defects and others, is discussed, with emphasis on the influence of threading dislocation density on the contrast of the Bragg diffraction imaging. We complemented the diffraction studies with defect selective etching analyses and, to determine the level of impurities in the GaN substrates, by time-of-flight secondary ion mass spectrometry (ToF-SIMS). The main finding of this study is that a native seed approach is essential for crystallizing GaN with high structural perfection and low threading dislocation density. This is true whether the GaN crystals are grown by HVPE or ammonothermal methods. A potential route to low-defect, low-impurity GaN substrates is outlined as a fundamental element for realizing GaN-based devices with high performance, life-time, and reliability.