Rocksalt structure scandium nitride films have been grown on magnesium oxide (0 0 1) substrates by molecular beam epitaxy using a radio frequency plasma source for nitrogen. The case of Sc-rich growth conditions, which occurs when the scandium flux J(Sc) exceeds the nitrogen flux J(N), is discussed. Despite the excess Sc during growth, reflection high-energy electron diffraction and X-ray diffraction (XRD) show that these films have only a single orientation which is (00 1), and ion channeling confirms the good crystallinity. Rutherford backscattering shows that these films are off-stoichiometric, and this is found to be directly related to variations in the nitrogen, not the scandium, content by secondary ion mass spectrometry. High-resolution XRD reciprocal lattice mapping shows that these variations in the nitrogen content are related to the existence of the N-vacancies. It is concluded that Sc-rich growth leads to the incorporation of N-vacancies into the crystal structure, the concentration of which depends on the Sc/N flux ratio. Additionally, excess scandium conditions at the surface are explored by in situ scanning tunneling microscopy. The observed wider terrace widths as compared to N-rich growth are due to an increased surface diffusion which is attributed to a Sc-rich, metallic surface structure. Combined with the large dislocation density, the enhanced diffusion results in a predominant spiral growth mode. (C) 2002 Elsevier Science B.V. All rights reserved.