The surface irregularities of molecular-beam-epitaxy-grown cubic GaN on GaAs substrates were characterized by micro-Raman spectroscopy. Some surface irregularities are found to be the result of the mixed phases of cubic (zinc-blende) and hexagonal (wurtzite) GaN, while others originate from the alloying of GaN with the GaAs substrate. The polarization scattering of the surface irregularities suggests that misoriented wurtzite GaN clusters sit on or are imbedded in the cubic GaN layer. It is revealed that the crystalline defects created during the growth of cubic GaN induce a growth of hexagonal GaN and, therefore, the surface irregularities consist of a phase mixture of cubic and hexagonal GaN polytypes.
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