The effect of the thermal history of Czochralski silicon crystals on the generation of crystallographic defects and refresh time degradation in high density memory devices was investigated. The result from an electron microscopic analysis shows that the oxide precipitates with a ring-type pattern around the crystal periphery, typically observed in the bottom section, have a polyhedral-shaped morphology. The grown-in oxide polyhedra are thermodynamically stable during the high temperature device processing steps and hence could be nucleated at temperatures above 1150 degrees C. The present result also indicates that the refresh time degradation in trench-type memory devices is directly related to the grown-in oxide polyhedra located in the device active region. Based upon the present analysis, the formation of the oxide precipitate ring in the bottom section of the crystal is hypothesized to be caused by radiative heating during and after the crystal tailing-off process.