Transmission electron microscopy has been applied to study defects in epitaxial doped and undoped GaN layers grown by MOCVD on sapphire and SiC substrates. Samples with InGaN/GaN and AlGaN/InGaN heterostructures have also been investigated. The results of this study show that incorporation of “foreign” atoms increases formation of nano-tubes and pinholes. The highest density of these defects was formed close to the interface with sapphire where oxygen outdiffusion might be expected, or in the subsurface area in the samples where oxygen was added deliberately. Addition of In (or Al) at QW’s also leads to pinhole formation. Increased In fraction or a larger number of QW’s results in a greater density of pinholes and more surface roughness. Many of the “hollow” nanotube defects are terminated during growth.

This paper describes TEM characterization of bulk GaN crystals grown at 1500–1800Kin the form of plates from a solution of atomic nitrogen in liquid gallium under high nitrogen pressure (up to 20 kbars). The x-ray rocking curves for these crystals were in the range of 20–30 arc-sec. The plate thickness along thec axis was about 100 times smaller than the nonpolar growth directions. A substantial difference in material quality was observed on the opposite sides of the plates normal to thec direction. On one side the surface was atomically flat, while on the other side the surface was rough, with pyramidal features up to 100 nm high. The polarity of the crystals was determined using convergent-beam electron diffraction. The results showed that, regarding the long bond between Ga and N along the c-axis, Ga atoms were found to be closer to the flat side of the crystal, while N atoms were found to be closer to the rough side. Near the rough side, within 1/10 to 1/4 of the plate thickness, there was a high density of planar defects (stacking faults and dislocation loops decorated by Ga/void precipitates). A model explaining the defect formation is proposed.

Formation of vertical hollow nano-tubes in GaN grown on different substrates using different growth methods is described. These defects are shown to be of several different types, some related to threading dislocations, but others originating at tubular inversion domains, or crystal inhomogeneities. Kinetic mechanism based on slow growth rate on polar {01̄11} surfaces is proposed to explain the origin of these defects.

The microstructure and characteristic defects of heteroepitaxial GaN films grown on sapphire using molecular beam epitaxy (MBE) and metal-organic-chemical-vapor-deposition (MOCVD) methods and of homoepitaxial GaN grown on bulk substrates are described based on transmission electron microscopy (TEM), x-ray diffraction, and cathodoluminescence (CL) studies. The difference in arrangement of dislocations along grain boundaries and die influence of buffer layers on the quality of epitaxial films is described. The structural quality of GaN epilayers is compared to diat of bulk GaN crystals grown from dilute solution of atomic nitrogen in liquid gallium. The full width at half maximum (FWHM) of the x-ray rocking curves for these crystals was in the range of 20–30 arc sec, whereas for the heteroepitaxially grown GaN the FWHM was in the range of 5–20 arc min. Homoepitaxial MBE grown films had FWHMs of about 40 arc sec. The best film quality was obtained for homoepitaxial films grown using MOCVD; these samples were almost free from extended defects. For the bulk GaN crystals a substantial difference in crystal perfection was observed for the opposite sides of the plates shaped normal to the c direction. On one side the surface was almost atomically flat, and the underlying material was free of any extended structural defects, while the other side was rough, with a high density of planar defects. This difference was related to the polarity of the crystal. A large difference in crystal stoichiometry was also observed within different sublayers of the crystals. Based on convergent beam electron diffraction and cathodoluminescence, it is proposed that GaN antisite defects are related to the yellow luminescence observed in these crystals.