Initial stage of growth of GaN/GaAs(001) in plasma-assisted molecular beam epitaxy

The nucleation and subsequent growth of cubic GaN on GaAs(001) in molecular beam epitaxy are studied by reflection high-energy electron diffraction (RHEED), transmission electron microscopy (TEM), and X-ray diffraction. We find that the layer finish is strongly dependent on the conditions chosen for the nucleation of the first monolayers of GaN on GaAs. For optimized growth conditions, TEM demonstrates 10 nm thick GaN films to be epitaxial and connected layers regardless of the 20% lattice mismatch. Furthermore, RHEED reveals an increasing lateral domain size in the course of growth for this case. This approach towards two-dimensional growth manifests itself in a steady improvement of the crystal quality with increasing thickness as determined by X-ray diffraction. However, this improvement does not take place for non-optimum conditions in the initial stage of growth regardless the conditions chosen in the later stage of growth, from which we conclude that control over nucleation is a necessity for the successful fabrication of GaN-based heterostructures.