In this communication, results are presented of the application of etching in molten E+M etch (KOH-NaOH eutectic mixture with 10% MgO) for studying defects in GaN. The method was used to study defects on differently oriented cleavage and basal planes of GaN single crystals, MOCVD-, MBE- and HVPE-grown epitaxial layers and LD and LED structures. Dislocations, dislocation loops and stacking faults have been revealed on $(10\, \bar{\text{\scriptsize 1}}\, 0)$, $(1\,\bar{\text{\scriptsize 2}}\,10)$ and $\{0001\}$ Ga- and N-polar planes. Diversified etch pit morphology was observed depending on the crystallographic orientation of the etched samples and was correlated with the crystallographic symmetry of the GaN lattice. Etching results were calibrated using TEM analysis.

Heteroepitaxial GaN layers grown on sapphire by metal organic vapour phase epitaxy (MOVPE) have been characterised by conventional transmission electron microscopy (TEM) on planar and cross-sectional samples, Large Angle Convergent Beam Electron Diffraction (LACBED) and by high-resolution transmission electron microscopy (HRTEM). Hollow tubes termed nanopipes were resolved on planar view and cross-sections of heteroepitaxial GaN. For advanced studies of the nature of nanopipes the LACBED method was employed. The recognition between perfect structure and screw distortion around nanopipes was performed with high accuracy using Zone Axis LACBED images.

We used single crystals of GaN, obtained from high-pressure synthesis, as substrates for Metalorganics Vapor Phase Epitaxy growth of violet and UV laser diodes. The use of high-quality bulk GaN leads to the decrease of the dislocation density to the low level of 105 cm−2, i.e. two orders of magnitude better than typical for the Epitaxial Lateral Overgrowth laser structures fabricated on sapphire. The low density and homogeneous distribution of defects in our structures enables the realization of broad stripe laser diodes. We demonstrate that our laser diodes, having 15 μm wide stripes, are able to emit 1.3-1.9 W per facet (50% reflectivity) in 30 ns long pulses. This result, which is among the best ever reported for nitride lasers, opens the path for the development of a new generation of high power laser diodes.