Gate oxide structures in semiconductor devices

A method, comprising:

• forming first and second epitaxial layers on a substrate;
• forming a first gate structure with a first gate oxide structure on the first epitaxial layer;
• and
• forming a second gate structure with a second gate oxide structure on the second epitaxial layer,
• wherein forming the first and second gate oxide structures comprises:
• forming first and second oxide layers of substantially equal thicknesses on the first and second epitaxial layers, respectively,
• forming first and second nitride layers of different materials on the first and second oxide layers, respectively,
• growing the first and second oxide layers to form modified first and second oxide layers with thicknesses different from each other,
• removing the first and second nitride layers;
• and
• forming a high-k dielectric layer on the modified first and second oxide layers.

A semiconductor device, comprising:

• a substrate;
• first and second epitaxial layers disposed on the substrate;
• first and second gate oxide structures disposed on the first and second epitaxial layers, respectively, wherein the first and second gate oxide structures comprise:
• first and second oxide layers of different thicknesses disposed on the first and second epitaxial layers, respectively,
• first and second dielectric layers disposed on the first and second oxide layers, respectively, and
• third and fourth oxide layers of different materials with thicknesses that are substantially equal to each other disposed on the first and second dielectric layers, respectively;
• and
• first and second gate metal fill layers disposed on the first and second gate oxide structures, respectively.