YOxNy dielectric films were grown by rf-magnetron sputtering on a n-Si(1 0 0) substrate at different deposition temperatures from 300 to 550 °C. The interfacial properties of YOxNy films with different deposition temperatures were investigated using x-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy. It was found that an interfacial layer of yttrium silicate was formed between the YOxNy films and the Si substrate during deposition, and the higher the deposition temperature, the thicker the interfacial layer. The x-ray diffraction analysis shows that the phase transition of the YOxNy films occurs at substrate temperatures between 400 and 550 °C. The spectroscopic ellipsometry results indicate that the deposition temperature has a strong effect on the optical properties of the YOxNy films. The band gap was found to shift to higher energy at higher deposition temperature, which is likely due to the change in the crystalline structure of the YOxNy films.