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The propagation behavior of threading dislocations (TDs) and the effects of Burgers vectors in hydride vapor phase epitaxy (HVPE) GaN bulk crystals generated on Na-flux-grown GaN and in a commercially available HVPE-grown GaN bulk crystal were investigated. Analyses based on chemical etching and transmission electron microscopy (TEM) revealed a close correlation between the etch pit sizes and the Burgers vector of these TDs. The existence of TDs with the unique Burgers vector b = 1m + 1c was observed for the first time ever using a large-angle convergent-beam electron diffraction technique and plan-view bright-field scanning TEM. Multi-photon excitation photoluminescence microscopy observations showed that TDs with b = 1c had a meandering morphology in contrast to the linear morphology of TDs with b = 1a, 1a + 1c, or 1m + 1c in both types of HVPE-grown GaN crystals. The inclinations of TDs with b = 1a and 1a + 1c in HVPE-grown GaN on Na-flux-grown GaN were greatly affected by large symmetrical hexagonal hillocks. The TDs with b = 1a were inclined in the slope directions of the hillock planes, while those with b = 1a + 1c were inclined in the a directions parallel or antiparallel to the a component in their Burgers vector. These inclinations were readily explained by the isotropic elastic theory of an individual TD. The hillocks were produced around pairs of parallel mixed TDs for which the a components were opposite to one another. This phenomenon was attributed to inclusions at the interface between the Na-flux-grown GaN substrate and the HVPE-grown layer. The origins of other TDs including unusual ones having b = 1m + 1c are also discussed herein.

The local piezoelectricity of a Na-flux GaN crystal grown on a multipoint-seed-GaN template is investigated using piezoresponse force microscopy. The piezoresponse is critically dependent on two types of growth regions that are dominantly formed in the Na-flux GaN crystal: the c-growth sector (cGS), which is grown on top of the point-seed GaN surface with a growth front of ( 0001 ) planes, and the facet-growth sector (FGS), which is grown on the side of cGS with { 10 1 ¯ 1 } facets. Quantitative analyses reveal the GaN surface displacements at cGS that result from the piezoresponses increase with the applied AC voltage: the measured values well reflect the piezoelectric constant of d33 in GaN. The piezoresponses at the FGS and the boundary between the cGS and FGS are less sensitive than that at the cGS. A combination of cathodoluminescence and multiphoton excitation photoluminescence techniques clarifies that a local reduction of the piezoresponse observed in cGS is attributed to microscale FGSs that exist randomly in cGS. The dependence of the piezoresponse on the growth regions is quantitatively discussed from three possible viewpoints that potentially affect the polarization properties of GaN: residual strain, local crystallographic tilting, and inherent carrier distribution. As a result, a carrier screening effect is the most probable candidate to induce reduction of the piezoresponse in the FGSs of GaN crystals.

We have investigated the local electrical and structural properties of threading dislocations (TDs) in Na-flux-grown GaN bulk single crystals that consist entirely of c-plane growth sectors. Conductive atomic force microscopy analysis of Schottky-like contacts locally fabricated on dislocation-related etch pits showed a variety of leakage current under reverse bias conditions. Microstructural analysis around TDs revealed a correlation between the current leakage and the structure of the TDs and their surroundings. Microscale faceted growth sectors with a higher oxygen impurity concentration locally formed in the vicinity of TDs had a larger influence on the current leakage than the structure of TDs.

The mechanisms associated with electrical conduction through individual threading dislocations (TDs) in a Na-flux GaN crystal grown with a multipoint-seed-GaN technique were investigated by conductive atomic force microscopy (C-AFM). To focus on individual TDs, dislocation-related etch pits (DREPs) were formed on the Na-flux GaN surface by wet chemical etching, after which microscopic Pt electrodes were locally fabricated on the DREPs to form conformal contacts to the Na-flux GaN crystal, using electron beam assisted deposition. The C-AFM data clearly demonstrate that the leakage current flows through the individual TD sites. It is also evident that the leakage current and the electrical conduction mechanism vary significantly based on the area within the Na-flux GaN crystal where the TDs are formed. These regions include the c-growth sector (cGS) in which the GaN grows in the [0001] direction on top of the point-seed with a c-plane growth front, the facet-growth sector (FGS) in which the GaN grows with {...