Direct observation of the initial nucleation and epitaxial growth of metastable cubic GaN on (001) GaAs

We report on the direct observation of the initial nucleation and the resulting microstructure of GaN deposited on GaAs(001) by plasma-assisted molecular beam epitaxy. Using high-resolution transmission electron microscopy, we demonstrate that, despite the extreme lattice mismatch between these two materials, GaN nucleates in the metastable cubic phase with a well-defined orientation relationship to the substrate and a sharp heteroboundary. The preference of the metastable phase in the initial stage of growth is discussed in connection with a coincidence lattice for the epitaxy-induced interface structure of the initial GaN nuclei.