We have investigated the transition from small interstitial clusters to 311 defects in ion-implanted Si. Czochralski Si wafers were implanted with 1.2 MeV Si ions to fluences in the range 10(12)-5 x 10(13)/cm(2) and annealed at temperatures of 600-750 degrees C for times as long as 15 h. Photoluminescence and transmission electron microscopy analyses allowed us to analyze the transition of small interstitial clusters, formed by the agglomeration of the excess interstitials introduced by the beam, into 311 defects. It is found that 311 defects form only at fluences greater than or equal to 10(13)/cm(2) and at temperatures above 600 degrees C. When 311 are observed in transmission electron microscopy, the luminescence spectrum is dominated by a sharp signal at 1376 nm which has been correlated with optical transitions occurring at or close to these defects. At lower temperatures or at lower fluence, no extended defects are observed in transmission electron microscopy and the luminescence spectrum present two broad signatures arising from carrier recombination at interstitial clusters. These data strongly indicate that a severe structural transformation occurs in the evolution from small interstitial clusters to extended 311 defects. (C) 2000 American Institute of Physics. [S0003-6951(00)01601-6].