The integrated circuit includes a gate structure over a substrate. The integrated circuit further includes a first silicon-containing material structure in a recess adjacent to the gate structure. The first silicon-containing material structure includes a first layer having an uppermost surface below a top surface of the substrate and a bottommost surface in contact with the substrate. The first silicon-containing material structure further includes a second layer over the first layer, wherein an entirety of the second layer is co-planar with or above the top surface of the substrate. A first region of the second layer closer to the gate structure is thicker than a second region of the second layer farther from the gate structure. Thickness is measured in a direction perpendicular to the top surface of the substrate.