Ion-assisted nucleation and growth of GaN on sapphire(0001)

We have performed a real-time x-ray scattering study of the nucleation of GaN on sapphire(0001) by gas-source molecular-beam epitaxy. GaN growth using thermal ammonia and Ga(C₂H₅)₃ exhibited a rapidly decaying intensity at the 0001 reflection, characteristic of three-dimensional cluster growth. Growth with 30-eV NHₓ⁺ ions and Ga(C₂H₅)₃ exhibited layer-by-layer intensity oscillations with maxima near odd-integer bilayers. The mode of nucleation is controlled by the Ga incorporation efficiency on the substrate and GaN islands. The Ga incorporation efficiency on the substrate is increased by a factor of 4 by low-energy ion irradiation during growth.