HfO2/GeOxNy/Ge gate stacks with sub-nanometer capacitance equivalent thickness and low interface trap density by in situ NH3 plasma pretreatment

The native oxides on Ge substrates can be transformed into GeO_xN_y by in situ NH₃ plasma pretreatment. The interfacial and electrical properties of HfO₂ caps gate stacks on Ge with and without ultrathin GeO_xN_y barrier layers have been investigated thoroughly. HfO₂/GeO_xN_y/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 GeO_xN_y 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/cm² at +1 V gate bias with acceptable D_it value of 3.42 × 10¹² 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.