Photoluminescence (PL) properties of SiO_(2) films containing Si nanocrystals (nc-Si) and Er were studied. The average size of nc-Si was changed in a wide range in order to tune the exciton energy of nc-Si to the energy separations between the discrete electronic states of Er~(3+). PL from exciton recombination in nc-Si and the intra-4f shell transition of Er~(3+) were observed simultaneously. At low temperatures, periodic features were observed in the PL spectrum of nc-Si. The period agreed well with the optical phonon energy of Si. The appearance of the phonon structure implies that nc-Si which satisfy the energy conservation rule during the energy transfer process can resonantly excite Er~(3+). For the PL from Er~(3+), a delay was observed after the pulsed excitation of nc-Si hosts. The rise time of the PL showed strong size dependence. The effects of the quantum confinement of excitons in nc-Si on the energy transfer process are discussed.
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