Material and electrical properties of TiO2/HfO2 bi-layer gate stacks were investigated for germanium (Ge) based metal-oxide-semiconductor devices. In situ NH3 plasma treatment was employed to passivate the Ge surface and promising performance including low capacitance-voltage hysteresis and interface trap density was achieved. It shows a superior dielectric breakdown voltage (4.2-3.4 V) for the TiO2/HfO2 bi-layer stacks than HfO2 single layer stack at a similar capacitance equivalent thickness (CET) of 1.6 nm. A minimum CET of 1.4 nm was obtained for capacitors on both p and n-type Ge (100) with a gate leakage current density < 4 x 10(-7) A/cm(2) at V-FB +/- 1 V. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3559770] All rights reserved.