Semiconductor device and manufacturing method thereof

A semiconductor device, comprising:

• a first nitride semiconductor layer formed above a substrate;
• a second nitride semiconductor layer formed on the first nitride semiconductor layer;
• a groove penetrating the second nitride semiconductor layer and reaching the first nitride semiconductor layer;
• and
• a gate electrode disposed inside the groove via a gate insulating film,
• wherein the gate insulating film includes a first gate insulating film provided on the first nitride semiconductor layer and a second gate insulating film provided on the first gate insulating film,
• the gate electrode includes a first gate electrode formed on a surface of the second gate insulating film and contacting the surface of the second gate insulating film, and a second gate electrode provided on a surface of the first gate electrode and contacting the surface of the first gate electrode,
• the first gate insulating film is an oxide film containing a first metal or an oxide film containing silicon,
• the second gate insulating film is an oxide film containing a second metal,
• an electronegativity of the second metal is smaller than that of the first metal or silicon,
• the first gate electrode is a nitride film containing a third metal, and
• the second gate electrode is formed of a fourth metal,
• wherein the nitride film containing the third metal comprises titanium nitride having a ratio (N/Ti) of nitrogen (N) to titanium (Ti) which is greater than 1.

A manufacturing method of a semiconductor device, comprising:

• (a) preparing a nitride semiconductor layer;
• (b) forming a first gate insulting film on the nitride semiconductor layer, the first gate insulating film being formed of an oxide film containing a first metal or an oxide film containing silicon;
• (c) forming a second gate insulating film on the first gate insulating film, the second gate insulating film being formed of an oxide film of a second metal;
• (d) forming a first gate electrode on a surface of the second gate insulating film, the first gate electrode contacting the surface of the second gate insulating film and being formed of a nitride film containing a third metal;
• and
• (e) forming a second gate electrode on a surface of the first gate electrode, the second gate electrode contacting the surface of the first gate electrode and being formed of a fourth metal,
• wherein an electronegativity of the second metal is smaller than that of the first metal or silicon,
• wherein the nitride film containing the third metal comprises titanium nitride having a ratio (N/Ti) of nitrogen (N) to titanium (Ti) which is greater than 1.