A semiconductor structure includes a first silicon-containing layer comprising an element selected from the group consisting essentially of carbon and germanium wherein the silicon-containing layer has a first atomic percentage of the element to the element and silicon, a second silicon-containing layer comprising the element over the first silicon-containing layer, and a silicide layer on the second silicon-containing layer. The element in the second silicon-containing layer has a second atomic percentage of the element to the element and silicon, wherein the second atomic percentage is substantially lower than the first atomic percentage.