During a study of the annealing of damage produced by high‐dose (1015–1016ions/cm2) arsenic implantations into Si, a stable high‐defect‐density structure was observed. It resulted from implantations through thin SiO2films covering the Si. Formation of the stable defect structure is related to the presence of the SiO2film during implantation, but not during annealing. Subsequent experiments indicate that knock‐on of oxygen by the As ions is not directly responsible for the effect.