Semiconductor devices with enhanced carrier mobility

A method, comprising:

• forming an active region comprising a plurality of channel layers interleaved by a plurality of sacrificial layers over a substrate;
• forming a dummy gate stack over the substrate;
• forming a gate spacers extending along sidewalls of the dummy gate stack and in direct contact with a topmost channel layer of the plurality of channel layers;
• performing a pre-amorphization implantation (PAI) process to convert a portion of the active region into an amorphous region, wherein a bottom surface of the amorphous region is below a bottommost surface of the plurality of channel layers;
• forming a tensile stress film over the substrate;
• performing an annealing process to recrystallize the amorphous region, the recrystallized region of the substrate comprising dislocations;
• selectively removing the tensile stress film;
• after the selectively removing of the tensile stress film, removing portions of the recrystallized region to form source/drain openings, a dislocation core of one of the dislocations, wherein, upon formation of the source/drain openings, a dislocation core of one of the dislocations is disposed laterally adjacent to one of the source/drain openings and above a bottom surface of the one of the source/drain openings;
• forming source/drain features in the source/drain openings;
• and
• replacing the dummy gate stack and the plurality of sacrificial layers with a gate structure, wherein a bottommost portion of the gate structure is disposed under a bottommost channel layer of the plurality of channel layers.

A method, comprising:

• forming a fin-shaped active region over a substrate;
• forming a dummy gate stack intersecting the fin-shaped active region;
• forming a gate spacer adjacent to the dummy gate stack;
• performing an implantation process to amorphize a portion of the fin-shaped active region not covered by the dummy gate stack into an amorphous region;
• performing an annealing process to recrystallize the amorphized portion, the recrystallized portion of the substrate comprising dislocations, wherein one of the dislocations has a dislocation core;
• after the performing of the annealing process, recessing the recrystallized portion to form a source/drain opening, wherein a portion of the dislocations is disposed laterally adjacent to the source/drain opening, wherein a bottom surface of the source/drain opening is below the dislocation core;
• after the recessing of the recrystallized portion, forming a source/drain feature in the source/drain opening, wherein a bottom surface of the source/drain feature is below the dislocation core;
• selectively removing the dummy gate stack;
• and
• forming a gate structure adjacent to the source/drain feature.