The density of silicon islands trapped in buriedSiO2layers produced by the implantation of oxygen into (001)Si substrates is monitored by atomic force microscopy for the oxygen dose range just above that required for continuous oxide formation. In addition to an exponential increase in the Si island density with oxygen dose, the regularly shaped remnants of anSiO2phase with a reduced HF etch rate were found. The formation of this additional oxide phase as a result of enhanced internal pressure inside the Si crystal is proposed to account for the retention of Si islands in the buried oxide. ©1997 American Institute of Physics.