A method includes forming isolation features on a substrate, thereby defining an active region on the semiconductor substrate; recessing the active region to form a fin trench; forming a fin feature on the fin trench by growing a first semiconductor layer on the substrate and a second semiconductor layer on the first semiconductor layer; performing a first recessing process; forming a dummy gate stack over the fin feature and the isolation feature; performing a thermal oxidation process to selectively oxidize the first semiconductor layer to form a semiconductor oxide feature on sidewalls of the first semiconductor layer; performing a second recessing process such that a portion of the isolation feature is recessed to below the second semiconductor layer, resulting in a dented void overlying the semiconductor oxide feature and underlying the second semiconductor layer; and forming a gate stack including a gate dielectric layer extending to the dented void.