Method for preparing recessed gate structure with protection layer

A method for preparing a recessed gate structure, comprising:

• forming a recessed structure, wherein the recessed structure comprises a substrate with the recess extending into the substrate from a topmost surface of the substrate;
• forming a first functional layer to at least cover a sidewall of a recess of the recessed structure;
• forming a second functional layer to cover the first functional layer;
• performing a rapid thermal treatment to form an interfacial layer extending along an interface between the first functional layer and the second functional layer;
• and
• forming a conductive feature to fill up the recess;
• wherein oxygen in the first functional layer diffuses into a thin portion of the second functional layer in contact with the first functional layer during the rapid thermal treatment, and the thin portion of the second functional layer is oxidized to form the interfacial layer.