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 gr owth using thermal ammonia and Ga(C2H5)(3) exhibited a rapidly decayin g intensity at the 0001 reflection, characteristic of three-dimensiona l cluster growth. Growth with 30-eV NHx+ ions and Ga(C2H5)(3) exhibite d layer-by-layer intensity oscillations with maxima near odd-integer b ilayers. The mode of nucleation is controlled by the Ga incorporation efficiency on the substrate and GaN islands. The Ga incorporation effi ciency on the substrate is increased by a factor of 4 by low-energy io n irradiation during growth.