A method for forming a semiconductor structure includes forming a gate stack over a semiconductor substrate in a wafer; forming a recess in the semiconductor substrate and adjacent the gate stack; and performing a selective epitaxial growth to grow a semiconductor material in the recess to form an epitaxy region. The step of performing the selective epitaxial growth includes performing a first growth stage with a first growth-to-etching (E/G) ratio of process gases used in the first growth stage; and performing a second growth stage with a second E/G ratio of process gases used in the second growth stage different from the first E/G ratio.