Referring to FIG. 6, the silicon germanium layer 53 comprises four segments labeled as A, B, C, and D, where each of the segments may also be referred to as a sublayer of the silicon germanium layer 53. The interfaces between the segments A, B, C, and D are labeled as 101, 103, and 105, respectively, as illustrated in FIG. 6. The interfaces 101, 103, and 105 may not be seen in Transmission Electron Microscopy (TEM), but can be seen through Secondary Ion Mass Spectrometer (SIMS) analysis, in some embodiments. In the example of FIG. 6, the segment A is a buffer layer with a low concentration of germanium for reducing defects between the silicon germanium layer 53 and the underlying epitaxial material 52 (e.g., epitaxial silicon layer). The segment A may comprise SiGe, SiGe:C, or combinations thereof, as examples. In the illustrated embodiment, the concentration of germanium in the segment A is uniform, and is between about 0 atomic percentage (at %) to about 1.5 at %. A thickness of the segment A, measured along a vertical direction in FIG. 6 (e.g., from the epitaxial material 52 toward an upper surface 53U of the silicon germanium layer 53), is between about 0.5 nm and about 5 nm, in some embodiments.
