The concentration of germanium in the segment B of FIG. 6 is uniform (e.g., uniform within limitations of manufacturing) and has a value between about 25 at % and about 28 at %. A thickness of the segment B, measured along the vertical direction of FIG. 6, is between about 10 nm and about 20 nm, in some embodiments. The concentration of germanium in the segment B is higher than that in the segment A. A width of the fin 58, which is measured between opposing sidewalls of the fin in FIG. 6 (e.g., between opposing sidewalls of the silicon germanium layer 53), is between 3 nm and about 6 nm, in the illustrated embodiment.

Still referring to FIG. 6, the concentration of germanium in the segment C increases continuously along the vertical direction from the interface 105 toward the interface 103. In other words, the segment C has a gradient concentration of germanium, and therefore, the segment C is also referred to as a gradient sublayer of the silicon germanium layer 53. In some embodiments, the concentration of germanium in the segment C proximate the interface 105 is between about 15 at % and about 20 at %, and the concentration of germanium in the segment C proximate the interface 103 is between about 25 at % and about 28%. A thickness of the segment C, measured along the vertical direction of FIG. 6, is between about 15 nm and about 30 nm. In the illustrated embodiment, the concentration of germanium in the segment B is equal to the highest concentration of germanium in the segment C (e.g., at the interface 103).