A method includes providing a substrate having an n-type fin-like field-effect transistor (NFET) region and forming a fin structure in the NFET region. The fin structure includes a first layer having a first semiconductor material, and a second layer under the first layer and having a second semiconductor material different from the first semiconductor material. The method further includes forming a patterned hard mask to fully expose the fin structure in gate regions of the NFET region and partially expose the fin structure in at least one source/drain (S/D) region of the NFET region. The method further includes oxidizing the fin structure not covered by the patterned hard mask, wherein the second layer is oxidized at a faster rate than the first layer. The method further includes forming an S/D feature over the at least one S/D region of the NFET region.