Metal insulator semiconductor structures were fabricated from n-Si(100) and n-Ge(100) wafers passivated with thin (4.5–5 nm) films of N-rich BCxNy (0.09≤x≤0.15, 0.38≤y≤0.52) and with atomic layer deposition HfO2 (10 nm) as the gate dielectric. C-V and I-V characteristics of devices with BCxNy films grown at 275–400 °C by chemical vapor deposition showed that lower deposition temperatures resulted in improved electrical characteristics, including decreased hysteresis, lower VFB shift, lower leakage current, and less C-V stretch out. The electrical improvement is attributed to decreased bulk and interfacial defects in lower temperature deposited BCxNy films, which also had a higher optical bandgap [Eg=3.55 eV at 275 °C on Ge(100)], lower subbandgap absorption, lower index of refraction [n(633 nm)=1.84 at 275 °C on Ge(100)], reduced O uptake during ambient exposure, and increased percentage of B. Even for the lowest growth temperature studied (275 °C), BCxNy-passivated Ge(100) devices had considerable hysteresis (1.05 V), and electrical characteristics worsened after a postmetallization anneal. BCxNy-passivated Si(100) devices outperformed similar Ge(100) devices likely due to the higher interface state densities at the BCxNy–Ge(100) interface associated with the higher relative inertness of Ge(100) to thermal nitridation. C-rich BC0.61N0.08 films were also investigated but large amounts of hysteresis and fixed negative charge motivated the abandonment of these films.