Impact of Channel Orientation on Electrical Properties of Ge p- and n-MOSFETs With 1-nm EOT Al2O3/GeOx/Ge Gate-Stacks Fabricated by Plasma Postoxidation

The impact of surface orientations on electrical characteristics of Ge p-and n-channel MOSFETs with ultrathin equivalent oxide thickness Al2O3/GeOx/Ge gate-stacks is systematically investigated. It is found that (100), (110), and (111) GeOx/p-Ge metal-oxide-semiconductor (MOS) interfaces have a similar interface trap density (D-it) level, but (100) GeOx/n-Ge MOS interfaces exhibit the lowest D-it. As a result, the highest peak mobility is obtained in (110) Ge pMOSFETs for holes and in (100) Ge nMOSFETs for electrons. The higher interface state density is observed inside valence band of Ge for (110) GeOx/Ge MOS interface and inside conduction band of Ge for (110) and (111) GeOx/Ge MOS interfaces, leading to the significant effective mobility reduction in high normal field for (110) Ge pMOSFETs, and (110) and (111) Ge nMOSFETs. It is also confirmed that decrease of plasma oxidation temperature is also effective in reduction of the interface roughness at (100) to (111) GeOx/Ge interfaces, resulting in clear enhancement of both hole and electron mobility in high normal field region.