Formation of carbon-substituted single crystal silicon layer is prone to generation of large number of defects especially at high carbon concentration. The present invention provides structures and methods for providing low defect carbon-substituted single crystal silicon layer even for high concentration of carbon in the silicon. According to the present invention, the active retrograde profile in the carbon implantation reduces the defect density in the carbon-substituted single crystal silicon layer obtained after a solid phase epitaxy. This enables the formation of semiconductor structures with compressive stress and low defect density. When applied to semiconductor transistors, the present invention enables N-type field effect transistors with enhanced electron mobility through the tensile stress that is present into the channel.