Gallium nitride thin films were deposited on sapphire or zinc oxide substrates with a molecular beam epitaxial method. Thin films with a (GaN)-N-14/(GaN)-N-15/(GaN)-N-14 isotopic heterostructure were also grown. A CAMECA-type secondary ion mass spectrometry (SIMS) was employed to analyze impurities such as substrate elements. Nitrogen isotopes were also analyzed. Some samples were annealed in a nitrogen atmosphere, and the diffusivities of the elements and isotopes were evaluated. Although the crater surface after using Cs+ ions as the primary ion beam became smoother than after using O-2(+), preferential sputtering was observed. It is concluded that this preferential sputtering causes the isotope distribution to be abnormal. Elements of the substrates diffused into the thin films. (c) 2006 Elsevier B.V. All rights reserved.