A method includes forming a semiconductor fin, forming a dummy gate on a top surface and sidewalls of the semiconductor fin, and removing the dummy gate to form a recess. The semiconductor fin is exposed to the recess. After the dummy gate is removed, an oxidation is performed on the semiconductor fin to form a condensed germanium-containing fin in the recess, and a silicon oxide layer on a top surface and sidewalls of the condensed germanium-containing fin. The method further includes forming a gate dielectric over the condensed germanium-containing fin, and forming a gate electrode over the gate dielectric.