Methods for defining an isolation region of a semiconductor structure are provided. The method includes, for instance: providing a semiconductor structure with a recess therein; disposing an insulator layer conformally within the recess in the semiconductor structure to partially fill the recess; modifying at least one material property of the insulator layer to obtain a densified insulator layer within the recess, where the modifying reduces a thickness of the densified insulator layer compared to that of the insulator layer; and depositing at least one additional insulator layer within the recess over the densified insulator layer, where the densified insulator layer within the recess defines, at least in part, an isolation region of the semiconductor structure.