Nanocrystalline thin films of copper nitride were grown on Si (100) wafers at a low substrate temperature by reactive magnetron sputtering of Cu target with the mixture of nitrogen and argon. The influence of nitrogen deficiency upon the structural, optical and electrical properties of as-deposited films was investigated. X-ray diffraction confirms the presence of cubic Cu3N and Cu biphases irrespective of carefully optimized processing parameters. With a Cu content approaching the stoichiometry for Cu3N, the films assume a smooth morphology with densely-packed nanocrystallites of about 40-60nm in size. Those deposits containing more than 79% Cu are metallic conductors with excellent electrical conductivity via a percolation mechanism, whereas the slightly substoichiometric Cu3N films show a typical behavior of deficit semiconductor, with an optical gap of about 1.85eV as revealed by photoreflectance measurement. All the observations are discussed in terms of nitrogen reemission from the growing film. (c) 2005 Elsevier B.V. All rights reserved.