The stability of diffused Cu in silicon crystal was evaluated by measuring the changes of photoluminescence intensity of the 1.014 eV Cu center with long storage time. Although the solubility of Cu is known to be low (≪0.1 atom/cm3), high concentrations (≫1013 atom/cm3) of mobile Cu in various Cu complexes (including the Cu center) were always maintained during more than a two-year storage at room temperature without Cu precipitation for float-zone and Czochralski (Cz) crystals when carbon concentrations were not so high for the latter. The Cu center for Cz crystals decreased shortly after annealing due to oxygen-enhanced nucleation of Cu sinks. Different behaviors of the Cu center in different crystals were reasonably explained by analyzing the transformation of Cu atoms between the Cu complexes and sinks. © 2001 American Institute of Physics.