Semiconductor devices and methods of forming the same are provided. In one embodiments, the method includes providing a structure that includes a substrate, a first gate structure and a second gate structure over the substrate, a first source/drain (S/D) feature comprising silicon adjacent to the first gate structure, a second S/D feature comprising silicon germanium (SiGe) adjacent to the second gate structure; and one or more dielectric layers over sidewalls of the first and second gate structures and over the first and second S/D features. The method further includes etching the one or more dielectric layers to form openings exposing the first and second S/D features, forming a masking layer over the first S/D feature, implanting gallium (Ga) into the second S/D feature while the masking layer is over the first S/D feature, removing the masking layer; and etching the first and second S/D features with an oxygen-atom-containing etchant.