The present disclosure provides a method of fabricating a nonplanar circuit device. The method includes receiving a substrate having a first semiconductor layer of a first semiconductor material and a second semiconductor layer of a second semiconductor material on the first semiconductor layer, wherein the second semiconductor material is different from the first semiconductor material in composition. The method further includes patterning the first and second semiconductor layers to form a fin structure in the first and second semiconductor layers. The method further includes performing a selective oxidization process to the first semiconductor layer such that a bottom portion of the first semiconductor layer is oxidized.