Extremely smA Sn nanodots embedded in the subsurface of SiO2, i.e., Sn-SiO2 quantum dot composites, have been formed by ion implantation of the Sn-120(+) isotope into (0001) Z-cut quartz at a low kinetic energy of 9 keV at room temperature. Transmission electron microscopy images show that the Sn nanodots have an average size of about 3 nm, and both single-crystal and amorphous nanodots have been observed at room temperature. The nanodots are randomly distributed in the lateral direction but confined in a narrow layer in the depth direction. The bimodal size distribution that is often observed in high energy implantation was not observed in the present study. The spatial morphology and crystallinity of the Sn nanodots can be modified by thermal annealing, and the extent of the modification is. dependent on the annealing temperature. The distribution and crystallinity of the Sn nanodots in single-crystal bulk SiO2 are different from those of Sn nanocrystals in thin SiO2 films on silicon substrates. The mechanisms that could account for these phenomena are discussed. (c) 2007 American Vacuum Society.