Semiconductor device and method for forming the same

A method, comprising:

• forming a dummy gate structure over a substrate;
• depositing a semiconductive layer along a sidewall of the dummy gate structure;
• performing an implantation process to dope the semiconductive layer;
• forming a source/drain epitaxial layer adjacent to the dummy gate structure;
• and
• removing the dummy gate structure to form a gate trench;
• and
• forming a metal gate structure in the gate trench, wherein after performing the implantation process and prior to forming the metal gate structure, a portion of the semiconductive layer is transferred to a dielectric layer.

A semiconductor device, comprising:

• a substrate;
• a gate structure over the substrate;
• gate spacers on either side of the gate structure, the gate spacers each comprising:
• a first spacer layer, wherein along a horizontal direction, the first spacer layer has a core region and surface regions on opposite sides of the core region, and oxygen concentrations of the surface regions are greater than an oxygen concentration of the core region;
• and
• a second spacer layer over the first spacer layer;
• and
• source/drain epitaxy structures on either side of the gate structure and separated from the gate structure by the gate spacers.