Low-temperature/high-resolution photoluminescence spectra of thick freestanding hydride vapor-phase epitaxial GaN substrates show intense sharp lines in the near bandedge spectral region, which has been previously assigned to recombination processes associated with the annihilation of excitons bound to neutral Si and 0 donors. Similar studies carried out on unintentionally doped (UID) and Sidoped homoepitaxial films grown by molecular beam epitaxy and metalorganic chemical vapor deposition methods, respectively, supports the previous identification of Si and 0 as the dominant shallow donors in UlD GaN. The chemical nature of these background impurities and dopants were verified by high sensitivity secondary ion mass spectroscopy. The present work confirms Si and 0 as the pervasive dominant shallow donors in UID GaN and demonstrate the usefulness of photolurninescence as a non-destructive technique to identify impurities in GaN. (c) 2007 Elsevier B.V. All rights reserved.