Time-resolved photoluminescence (PL) decays have been measured for silicon nanocrystallites embedded in silicon dioxide. The nanocrystallites were formed by implanting Si ions at various doses into thermally grown SiO2 films, followed by thermal annealing at 1100 degrees C. Silicon dioxide films of various thickness in the range 100-1000 Angstrom were used. The results show that PL could be detected even for films as thin as 100 Angstrom. The spectral and time-dependence characteristics are very similar to those of porous Si, exhibiting a clear stretched exponential behavior. The extracted parameters tau and beta are, however, somewhat higher in value for the Si nanocrystallites. This may tentatively be attributed to a high degree of passivation of the Si nano crystallites in the oxide matrix. An observed spectral redshift for thinner oxides and higher implantation doses may be explained considering the excess Si concentration along the implantation profile. The stretched exponential decay suggests that carrier trapping and diffusion occurs between adjacent nanocrystallites, most likely in the form of excitons.