We report the effects of the nitride passivation layer on the structural, electrical, and interfacial properties of Ge metal-oxide-semiconductor (MOS) devices with a hafnium oxide (HfO₂) gate dielectric layer deposited on p-type 〈100〉 Ge substrates. X-ray photoelectron spectroscopy analysis confirmed the chemical states and formation of HfO₂/Ge₃N₄ on Ge. The interfacial quality and thickness of the layers grown on Ge were confirmed by high-resolution transmission electron microscopy. In addition, the effects of post-deposition annealing (PDA) on the HfO₂/Ge₃N₄/Ge and HfO₂/Ge samples at 400 °C in an (FG+O₂) ambient atmosphere for 30 min were studied. After PDA, the HfO₂/Ge₃N₄/Ge MOS device showed a higher dielectric constant (k) of ~21.48 and accumulation capacitance of 1.2 nF, smaller equivalent oxide thickness (EOT) of 1.2 nm, and lower interface trap density (Dit) of 4.9×1011 cm-2 eV-1 and oxide charges (Qeff) of 7.8×1012 cm-2 than the non-annealed sample. The I-V analysis showed that the gate leakage current density of the HfO₂/Ge₃N₄/Ge sample (0.3-1 nA cm-2 at Vg = 1 V) was half of that of the HfO₂/Ge sample. Moreover, the barrier heights of the samples were extracted from the Fowler-Nordheim plots. These results indicated that nitride passivation is crucial to improving the structural, interfacial, and electrical properties of Ge-based MOS devices.