A semiconductor substructure with an improved source/drain structure is described. The semiconductor substructure can include an upper surface; a gate structure formed over the substrate; a spacer formed along a sidewall of the gate structure; and a source/drain structure disposed adjacent the gate structure. The source/drain structure is disposed over or on a recess surface of a recess that extends below said upper surface. The source/drain structure includes a first epitaxial layer, having a first composition, over or on the interface surface, and a subsequent epitaxial layer, having a subsequent composition, over or on the first epitaxial layer. A dopant concentration of the subsequent composition is greater than a dopant concentration of the first composition, and a carbon concentration of the first composition ranges from 0 to 1.4 at.-%. Methods of making semiconductor substructures including improved source/drain structures are also described.