Silicon nitride (SiNx:H) thin films were grown on silicon by the plasma-enhanced chemical vapor deposition (PECVD) method at low temperature in order to study their optical, electrical properties and correlate these properties to the chemical composition of the layers, so that films with desired properties may be achieved for silicon solar cells. By varying the silane (SiH4) to ammonia (NH3) ratio in the plasma gas we have been able to modify the index of refraction (from 1.9 to 2.3) and also the silicon surface state passivation properties of the films. Our results indicate that the mid-gap surface state density in silicon can be reduced down to 1.1 x 10(10) cm-(2) eV(-1) for the SiN chi:H layer deposited under optimized silane to ammonia ratio. Also, an extensive study has been carried out on the effect of rapid thermal annealing (RTA) on the carrier lifetime, reflectance, chemical composition, refractive index and interface states which decides the final output of the solar cell. (C) 2007 Elsevier B.V. All rights reserved.