Samples were prepared by implanting 100keV Ge ions into amorphous SiO2 films to fluences of 3 x 1016 or 1017/cm2. They were then annealed at up to 1000C in a N2 atmosphere. The structure of the films was studied by transmission electron microscopy, energy dispersive X-ray spectroscopy, secondary ion mass spectrometry and X-ray photo-electron spectroscopy. The most striking observation was that spherical voids with diameters of up to tens of nm were observed in the films after annealing (1000C, 1h). The volume fraction of voids increased with Ge fluence. The mechanism behind void formation was indicated by the evolution of the sample structure upon increasing annealing time or temperature; the Ge first segregated into nanocrystals which then increased in size by diffusion and Oswald ripening. The Ge was quite mobile in SiO2 and, as O or moisture from the annealing atmosphere diffused in from the surface, the Ge bonded in an oxide closer to the surface than the precipitate. There was thus a net flux of Ge out of the nanoprecipitate and into an oxide closer to the surface. The volume occupied by the Ge precipitate became a void. It was concluded that this model fitted the observations. Filling of the voids by beam-induced migration under transmission electron microscope beam exposure was also reported.

Mechanisms of Void Formation in Ge Implanted SiO2 Films. E.S.Marstein, A.E.Gunnæs, U.Serincan, S.Jørgensen, A.Olsen, R.Turan, T.G.Finstad: Nuclear Instruments and Methods in Physics Research B, 2003, 207[4], 424-33