Pure germanium nitride (Ge3N4) thin films were successfully formed on n-type Ge (111) substrate using an atmospheric-pressure (AP) nitrogen plasma. Their film structures and electrical properties were then examined in detail. Synchrotron radiation photoelectron spectroscopy clearly revealed that the Ge3N4 thin films formed by AP plasma had superior oxidative resistance compared to those made using conventional plasma techniques. Films fabricated at 500 °C showed only minor post-oxidation, even after exposure to air, which is a potentially very useful feature for passivation layers at high-permittivity (high-k) dielectric film–Ge interfaces. The films also showed excellent electrical properties. Capacitance-voltage measurements revealed no hysteresis or kinks, indicating that the trap-state density was low at the Ge3N4–Ge interface. The leakage current density is also lower than in films fabricated using other plasma systems. Direct-tunneling current simulations revealed that the effective tunneling mass increased due to the formation of high-quality Ge3N4 thin films, resulting in superior leakage current. These results suggest that our nitridation technique would show major benefits in Ge field-effect transistors.