Some embodiments of the present disclosure provide a semiconductor structure including a substrate and an epitaxy region partially disposed in the substrate. The epitaxy region includes a substance with a lattice constant that is larger than a lattice constant of the substrate. The concentration profile of a substance in the epitaxy region is monotonically increasing from a bottom portion of the epitaxy region to a of the epitaxy region. A first layer of the epitaxy region has a height to width ratio of about 2. The first layer is a layer positioned closest to the substrate, and the first layer has an average concentration of the substance from about 20 to about 32 percent. A second layer disposed over the first layer. The second layer has a bottom portion with a concentration of the substance from about 27 percent to about 37 percent.