Polycrystalline GaN (poly-GaN) films were grown on silica glass substrates by metal-organic chemical vapor deposition. An n-type conductivity in all the undoped poly-GaN films was observed and background impurities such as silicon and carbon were detected. The Si impurity was found to be diffused from the silica substrate and thought to be a dominant source of the n-type conductivity. It was also assumed that the C impurity as a compensating acceptor slightly affected the background electron concentration. It was thought that deep level emissions around 2.2 eV were associated with gallium vacancies and carbon impurity complexes at grain boundaries of the poly-GaN. (C) 2003 Elsevier Science B.V. All rights reserved.