Semiconductor structure with low defect and method for forming the same

A method for forming a semiconductor structure, comprising:

• forming a first fin structure over a substrate;
• forming an insulating layer around the first fin structure;
• removing a portion of the first fin structure to form a first trench in the insulating layer;
• filling the first trench with a semiconductor material;
• and
• reflowing the semiconductor material to form a first cavity at a bottom portion of the first trench by performing an annealing process.

A method for forming a semiconductor structure, comprising:

• forming a fin structure from a substrate;
• forming an insulating layer around the fin structure;
• removing a portion of the fin structure to form a trench;
• filling the trench with a semiconductor material;
• forming a cavity under the semiconductor material;
• forming a gate structure across the semiconductor material;
• and
• forming source/drain regions in the semiconductor material.