Silicon nanostructures have interesting possible applications in new generation photovoltaics. The fabrication techniques usually involve a high-temperature post-deposition annealing. Here we report on the spontaneous growth of silicon quantum dots in silicon nitride films by plasma enhanced chemical vapour deposition (PECVD) using silane and nitrogen as reactant gas sources. The growth rate was kept low by working at low discharge power, and a low deposition temperature was selected (300 °C). This approach is particularly interesting because in principle it allows for use of low cost substrates and application of typical device structures adopted in thin film silicon photovoltaics. Photoluminescence and optical properties have been investigated. The evolution of the photoluminescence emission with nitrogen flow rate has been correlated to the change of the dot size. The effect of the strongly absorbing Si nanostructures in the silicon nitride matrix on the absorption properties has been investigated.