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Very promising capacitance-voltage (C-V) characteristics of capacitors on both p- and n-type Ge substrates are demonstrated by using an in situ O2 plasma passivation and an O2 ambient annealing. The capacitors exhibited a small equivalent oxide thickness of ~1.5 nm and a low Dit (<; 6 * 1011 cm-2eV-1) energy distribution covering the major part of the Ge bandgap (>; 0.51 eV). The surface potential modulation efficiency was estimated to be ~80% from Hatband to strong inversion on both types of Ge. C-V hysteresis was less than 10 mV with positive bias up to 1.5 V. A low gate leakage current density of <; 9 * 10-8 A/cm2 at VFB ± 1 V indicates high quality of the HfO2/GeO2 gate stacks. Excellent comprehensive properties suggest that a combination of O2 plasma passivation and O2 ambient annealing provides a promising technology for GeO2-based CMOS devices.