FIG. 6 further illustrates the segment D over the segment C. In the illustrated embodiment, the segment D has a uniform (e.g., uniform within limitations of manufacturing) concentration of germanium that is between about 15 at % and about 20 at %. In other words, the concentration of germanium in the segment D is equal to the lowest concentration of germanium of the segment C (e.g., at the interface 105). A thickness of the segment D, measured along the vertical direction of FIG. 6, is between about 5 nm and about 10 nm. In the example of FIG. 6, the uniform concentration of germanium in the segment D is smaller than the uniform concentration of germanium in the segment B. In some embodiments, the thickness of the segment C is larger than the thickness of the segment D, and is also larger than the thickness of the segment B.

In addition, FIG. 6 illustrates in phantom a capping layer 55 over the silicon germanium layer 53. The capping layer 55 may be a silicon capping layer that is substantially free of germanium. The capping layer 55 may be formed in subsequent processing (see FIG. 13), before a dummy dielectric layer 60 (see FIG. 14) is formed over the fin 58. Although the capping layer 55 in FIG. 6 is illustrated to be on top of the segment D, the capping layer 55 may also extend along sidewalls of the silicon germanium layer 53, as illustrated in FIG. 13.