Epitaxial blocking layer for multi-gate devices and fabrication methods thereof

A semiconductor device, comprising:

• a doped semiconductor substrate having a first lattice constant;
• a dopant blocking layer disposed over the doped semiconductor substrate, wherein the dopant blocking layer has a second lattice constant different from the first lattice constant;
• a buffer layer disposed over the dopant blocking layer, wherein the buffer layer has a third lattice constant different from the second lattice constant;
• an isolation feature disposed on sidewalls of the buffer layer and the dopant blocking layer, wherein a top surface of the isolation feature is lower than a top surface of the buffer layer;
• a plurality of nanostructures suspended over the buffer layer;
• an epitaxial feature abutting the nanostructures;
• and
• a gate structure wrapping around each of the nanostructures.

A semiconductor device, comprising:

• a doped semiconductor substrate having a first lattice constant;
• a dopant blocking layer disposed over the doped semiconductor substrate, wherein the dopant blocking layer has a second lattice constant different from the first lattice constant;
• a buffer layer disposed over the dopant blocking layer, wherein the buffer layer has a third lattice constant different from the second lattice constant;
• a plurality of channel members suspended over the buffer layer;
• an epitaxial feature abutting the channel members, wherein the epitaxial feature is in direct contact with the buffer layer and the dopant blocking layer and extends downwardly below a bottom surface of the dopant blocking layer;
• and
• a gate structure wrapping around each of the channel members.