The native oxides on Ge substrates can be transformed into GeOxNy by in situ NH3 plasma pretreatment. The interfacial and electrical properties of HfO2 caps gate stacks on Ge with and without ultrathin GeOxNy barrier layers have been investigated thoroughly. HfO2/GeOxNy/Ge stacking structure shows a sharp and flat interface between HfO2 and Ge substrates without recognized interfacial layer. In situ NH3 plasma pretreatment effectively improves the electrical properties such as higher accumulation capacitance, smaller frequency dispersion, and lower interface trap density (D-it) than without NH3 plasma pretreatment. It is ascribed to that fact that the GeOxNy barrier layer between HfO2 and Ge substrates shows better thermal stability and suppresses the Ge outdiffusion. The 3-nm-thick HfO2 gate stacks on Ge with 60 s NH3 plasma pretreatment exhibit a capacitance equivalent thickness of 0.96 nm and a leakage current density of 1.12 mA/cm(2) at + 1 V gate bias with acceptable D-it value of 3.42 x 10(12) eV(-1) cm(-2). These results indicate that the surface nitridation by in situ NH3 plasma pretreatment may be a promising approach for the realization of high quality Ge-based transistor devices. (C) 2014 Elsevier B.V. All rights reserved.