X-ray diffraction (XRD) is widely used for the rapid evaluation of the structural quality of thin films. In order to determine how defect densities relate to XRD data, we investigated a series of heteroepitaxial nonpolar a-plane GaN films with different densities of dislocations and basal plane stacking faults (determined by transmission electron microscopy). Factors influencing XRD data include surface roughness effects, limited lateral coherence lengths, lateral microstrain, mosaic tilt, and wafer curvature, in addition to the defects present. No direct correlation between defect densities and any measured XRD parameter was found. However, the structural imperfections dominating XRD data can be identified by specific analysis of each individual broadening factor. This reductive approach permits full explanation of the in-plane rotational anisotropy of symmetric ω-scan widths for both a-plane and m-plane films: in these samples, mosaic tilt is the dominant factor.

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The microstructure of nonpolar, a-plane (11-20) GaN grown on r-plane (1-102) sapphire, using a three dimensional (3D)-two dimensional (2D) growth transition, has been studied at different stages of metal organic vapor phase epitaxy. The microstructure and morphology of GaN islands formed at the initial stages of growth, as well as the fully coalesced film, were characterized using transmission electron microscopy (TEM) and atomic force microscopy (AFM). The growth of GaN islands (bounded by {10-11} and (000-1) facets) was established under reactor conditions of relatively high pressure and high V/III ratio, whereas the island coalescence was achieved at lower pressure and low V/III ratio, leading to pit-free films with shallow striations along ⟨0001⟩. Cross-sectional TEM studies, in combination with the AFM studies of the uncoalesced films, showed that there was a correlation between the point at which partial dislocation line direction changed and the point at which growth conditions changed from the 3D to 2D mode. Lengthening the 3D growth stage obviously increased the size of the GaN islands and was also found to increase the basal plane stacking fault (BSF) length and decrease the density of partial dislocations in the coalesced films: It is suggested that BSFs in adjacent islands merge when islands are sufficiently large to impinge before the 2D growth step. Their merging necessitates the annihilation of some of the partial dislocations, and this causes the decrease in dislocation density.

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The effect of ScN interlayer thickness on the defect density of (112¯2) semipolar GaN grown on m-plane sapphire was studied by transmission electron microscopy. The interlayers comprised Sc metal deposited on a GaN seedlayer that was nitrided before GaN overgrowth by metal-organic vapor-phase epitaxy. Both interlayer thicknesses reduced the dislocation density by a factor of 100 to low-108 cm−2. The 8.5 nm interlayer produced regions that were free from basal plane stacking faults (BSF) and dislocations. The overall BSF density here was reduced by a factor of 5, to (6.49±0.07)×104 cm−1, without the need for an ex situ mask patterning step.