Abstract
A device includes a substrate and a recess in the substrate. The recess has a bottom and sidewalls. The device also includes a first epitaxial layer over the bottom of the recess, and a second epitaxial layer over the first epitaxial layer and over the sidewalls of the recess, the second epitaxial layer having a different lattice constant than the substrate. The device further includes a third epitaxial layer over the second epitaxial layer and filling the recess.