Low fraction of hexagonal inclusions in thick and bulk cubic GaN layers

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We report present a study of the formation of hexagonal inclusions in thick (∼5μm) and bulk (∼50μm) cubic GaN (c-GaN) layers grown on GaAs (001) substrates. Surface analysis of both samples revealed the evidence of hexagonal grains on the surface. However, larger grains were formed in the bulk sample. In the XRD measurement, the signature of the h-GaN in the bulk sample was more visible than the thick sample. This is consistent with the PL measurement, of which the h-GaN peak emission was found to be more intense in the bulk sample with respect to the thick sample. Such observations showed that the fraction of hexagonal inclusions had increased by increasing the thickness of the c-GaN layer. As presented in the Raman spectroscopy, the signals of the h-GaN were dominant, but the transverse optical (TO) and longitudinal optical (LO) modes of c-GaN were still observable at ∼559cm−1 and ∼739cm−1, respectively. Finally, a micro-PL measurement showed that the average content of the hexagonal inclusions in the c-GaN layer up to ∼50μm was less than 22%, suggesting that our samples have a lower hexagonal GaN content than some reported thin c-GaN samples.