Semiconductor devices with strained source/drain structures

A device, comprising:

• a semiconductor substrate;
• a gate structure over the semiconductor substrate;
• spacers overlying opposite sidewalls of the gate structure;
• lightly doped source/drain (LDD) regions in the semiconductor substrate, interposed by the gate structure;
• source/drain (S/D) regions in the semiconductor substrate, adjacent to the LDD regions and partially under the spacers, wherein each of the S/D regions comprises:
• a first portion adjacent to a top surface of the semiconductor substrate;
• a second portion surrounding the first portion;
• and
• a third portion between the second portion and the semiconductor substrate, wherein the second portion has a composition different from the first and third portions;
• and
• contact features over the S/D regions, wherein
• the first, second, and third portions are formed in a trench of the semiconductor substrate,
• the trench comprises:
• a first sidewall section extending obliquely downward from the top surface of the semiconductor substrate and away from a center of the trench,
• a second sidewall section further extending obliquely downward from the first sidewall section and toward the center of the trench, and
• a bottom connected to the second sidewall section, and
• the third portion includes:
• an upper part included in a tip portion of the trench, the tip portion defined by the first and second sidewall sections, and
• a lower part on the bottom of the trench, the lower part physically discontinuous from the upper part.