Semiconductor device and manufacturing method thereof

A method comprising:

• depositing a Ge-containing channel layer over a substrate;
• depositing a semiconductive protection layer over the Ge-containing channel layer, wherein a germanium atomic percentage of the semiconductive protection layer is lower than a germanium atomic percentage of the Ge-containing channel layer;
• performing an oxidation process on the semiconductive protection layer at a first temperature to form germanium oxide in a top portion of the semiconductive protection layer;
• removing the germanium oxide at a second temperature higher than the first temperature;
• and
• forming a gate structure over the semiconductive protection layer after removing the germanium oxide.

A method comprising:

• depositing a channel layer over a substrate;
• depositing a semiconductive protection layer over the channel layer, wherein a germanium atomic percentage of the channel layer is higher than a germanium atomic percentage of the semiconductive protection layer;
• performing an oxidation process to the semiconductive protection layer in an oxygen-containing environment;
• performing an annealing process to the semiconductive protection layer after performing the oxidation process in an oxygen-free environment;
• repeatedly performing the oxidation process and performing the annealing process;
• and
• forming a gate structure over the semiconductive protection layer after repeatedly performing the oxidation process and performing the annealing process.