Octahedral cavities have recently been found in the Czochralski silicon (CZ‐Si) substrate surface layer just under oxide defects. We investigate the effect that adding HCl to oxygen during oxidation has on the oxide defect density. The effect of intentionally introducing impurities onto a Si surface on the oxide defect density is also examined. Our experimental results suggest that impurities are closely related to the generation of oxide defects. A model is presented in which impurities are incorporated into the growing octahedral cavities during Si crystal growth, and then introduced into the growing oxides during thermal oxidation. These impurities in the oxides then act as a conductive path (oxide defects) in the insulator on the CZ‐Si. ©1996 American Institute of Physics.