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.

Nonpolar (112¯0) a-plane GaN films have been grown by metal-organic vapor deposition on r-plane (11¯02) sapphire. Lateral growth is favored using a low V:III ratio resulting in films with a smooth surface, while pitted films are grown at a high V:III ratio indicating preferential on-axis growth. High-resolution x-ray diffraction analysis of both film types showed a strong anisotropy in the peak width of the symmetric omega rocking curve with respect to the in-plane orientation, phi. In-plane isotropic behavior of crystallinity with overall reduced omega full width at half maximum values was achieved when the growth was initiated at a high V:III ratio before reducing the V:III ratio for film coalescence. An improvement of crystal quality through initial surface roughening was equally realized by the incorporation of partial-coverage SiNx interlayers.

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In this paper we report on the optical properties of a series of GaN/AlGaN multiple quantum well structures grown on a-plane (112¯0) GaN, which had been deposited on r-plane (11¯02) sapphire substrates, compared to a reference GaN template of the same orientation. The low temperature photoluminescence spectrum of the template layer is dominated by two emission bands, which we attribute to recombination involving excitons in the bulk of the layer and electrons and holes trapped at basal-plane stacking faults, designated X1 and X2, respectively. The photoluminescence spectra from the quantum well structures show similar emission bands except that both X1 and X2 shift to higher energy with decreasing quantum well thickness. The shift to higher energy is due to the effects of quantum confinement on carriers trapped at the stacking faults that intersect the quantum wells, as well as those excitons that are localized within the quantum wells. This assignment is based partly on excitation spectroscopy that reveals exciton transitions associated with electrons from the n=1 and n=2 quantum well confined states.