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An ultrathin equivalent oxide thickness (EOT) Al2O3/ GeOx/Ge gate stack with a superior GeOx/Ge metal-oxide-semiconductor (MOS) interface and p-channel metal-oxide-semiconductor field-effect transistors (pMOSFETs) using this gate stack have been fabricated by a plasma post oxidation method. The properties of the GeOx/ Ge MOS interfaces are systemically investigated, and it is revealed that there is a universal relationship between the interface state density (Dit) at the GeOx/Ge interface and the GeOx interfacial layer thickness. Ge pMOSFETs on a (100) Ge substrate using the Al2O3/GeOx/Ge gate stack have been demonstrated with an EOT down to 0.98 nm. It is found that the Ge pMOSFETs exhibit the peak hole mobility values of 515, 466, and 401 cm2/ V·s at an EOT of 1.18, 1.06, and 0.98 nm, respectively, which has much weaker EOT dependence than the trend of the hole mobility values reported so far, because of low Dit of the present gate stack in the ultrathin EOT region of ~1 nm.