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Structural analysis of ELO-GaN grown on sapphire using the x-ray micro-beam of an 8-GeV storage ring

Abstract

We investigated the micrometer-scale structure of epitaxially laterally overgrown GaN (ELO-GaN) on a sapphire without a SiO2 mask using the 2×4 μm2 micro-beam x-ray of an 8-GeV storage ring. The GaN (0 0 0 12) rocking curve of a wing region had a sharp peak with a FWHM of 46 arcsec, while that of a seed region had several broad peaks. This peak-narrowing indicates that the density of threading dislocations (TD) is reduced and the grain size is extended in the wing region by the lateral growth. The observed grain size in the wing region was 10 μm × 5 μm. The mapping of the rocking curves shows that the wing regions hang down from the seed region at a tilting angle of 81.5 arcsec, although there is no SiO2 mask which is a cause of c-axis tilting in ELO-GaN. After comparing the results of ELO-GaN with and without coalescence, we conclude that the small c-axis tilting is caused by the compressive stress induced by the difference in the thermal expansion coefficient of the GaN and the sapphire substrate. Because a shrinkage of grain size was observed after the coalescence of the wing regions, it is thought the coalescence is one of the causes of the generation of the few threading dislocations in the wing regions. We believe that the reduction of the threading dislocations and the expansion of gain size in the wing regions are important for the further improvement of the device characteristics of GaN-based LDs grown on ELO-GaN.

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