Single-crystal line layers of GaN and related alloys such as AlGaN and InGaN were grown on Al2O3 (0001) substrates by radio-frequency magnetron sputter epitaxy. The crystalline structures of these layers were studied as functions of substrate temperature, N-2 composition ratio in N-2/Ar mixture source gas and gas pressure during the growth. Surface structure of GaN layer depended on Ga/N ratio in flux density, and nitrogen-rich growth condition resulted in pyramid-type facet structure whereas Ga-rich growth produced flat surface. The crystalline quality of GaN layer improved at relatively low N-2 composition ratios, and the GaN layer grown at 30% N-2 condition was transparent and colorless. AlxGa1-xN layers with x=0.06-0.08 and InxGa1-xN layers with x=0.45-0.5, were obtained at 30-40% and 30-50% N-2 composition ratios, respectively. The AlN and InN molar fractions in these layers were considerably different from Al and In molar fractions in starting metal alloys (x=0.15 in both AlxGa1-x and InxGa1-x alloys). (C) 2007 Elsevier B.V. All rights reserved.