The disclosure relates to a semiconductor device. An exemplary structure for a semiconductor device comprises an isolation structure comprising a top surface over a substrate major surface; a cavity having a convex bottom surface below the top surface; a strained material in the cavity and extending above the top surface, wherein the strained material comprises an upper portion having a rhombus shape and a lower portion having substantially vertical sidewalls; and a pair of tapered spacers adjoining a portion of the substantially vertical sidewalls above the top surface.