In this work, the investigation results of amorphous hydrogenated silicon nitride layers (a-SiNx:H) obtained by plasma-enhanced chemical vapour deposition technology have been presented. Chemical compositions, surface morphology and surface topography studies were performed on the layers using Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive spectrometry and atomic force microscopy methods. By using optical methods: spectroscopic ellipsometry and thermal spectroscopic ellipsometry; the optical parameters (refractive index, extinction coefficients), optical band gap of a-SiNx:H layers and film thickness in the spectral range of 190-1700 nm have been determined. Additionally, we carried out thermo-optical studies to determine the temperature hysteresis of optical parameters. a-SiNx:H layers exhibit similar properties as non-stoichiometric Si-rich silicon nitride layers. They exhibited amorphous structure and their main components were silicon. After heating and cooling processes values of refractive index n and extinction coefficient k return to the baseline. In our paper we present thermal dependence of a-SiNx:H films refractive index. Presented layers showed similar optical constants to silicon substrate but they exhibiting much better dielectric properties than Si. a-SiNx:H layers exhibited very high refractive indices (about 3.8). The layers, presented in this work may potentially have broad applications in optoelectronics, photovoltaics and photonics.