Temperature dependence of the direct band gap of Si-containing carbon nitride crystalline films

We have measured the temperature dependence of the spectral features in the vicinity of the direct gap $E_g^d$ of Si-containing carbon nitride polycrystalline films in the temperature range between 20 and 500 K using piezoreflectance (PzR). From a detailed line-shape fit to the PzR spectra, the $E_g^d$ and the broadening parameter of direct band-to-band transitions at various temperatures are determined. The temperature dependence of $E_g^d$ are analyzed by the Varshni equation [Physica 34, 149 (1967)] and an empirical expression proposed by O’Donnel and Chen [Appl. Phys. Lett. 58, 2924 (1991)]. The parameters that describe the temperature dependence of the energy gap of the material are evaluated and discussed. The broadening parameter is found to be insensitive to the temperature variation.