The formation of germanium (Ge) nanocrystals under rapid thermal annealing was attributed mainly to the reduction of Ge suboxides by silicon (Si) diffused from the Si substrate. It was observed that the nanocrystals could be uniformly distributed in the bulk of the oxide when annealed at 800 degrees C, or could result in regions with different nanocrystal densities and size distributions in the bulk of the oxide separated by a region void of nanocrystals when annealed at 900 degrees C. For annealing at 1000 degrees C, nanocrystals were only observed at the silicon-silicon dioxide interface and these have significant size variation, with the rest of the oxide being void of nanocrystals. The nanocrystals formed at 900 and 1000 degrees C were generally found to be defective. (c) 2005 Published by Elsevier B.V.