Vibrational frequencies of hydrogenated silicon carbonitride: A DFT study

Abstract

Due to their good chemical and thermal inertness, SiCxNy:H films are suitable for a variety of applications in electronic, tribology, optic, photovoltaic and more recently gas separation membranes. For these applications, film evolution can be attractively probed by FTIR spectroscopy. In this work, a systematic quantum mechanical study of the vibrational modes position in the pattern of a-SiCxNy(O):H is presented. Vibrational frequencies of SiC, SiN, CN, SiH, CH and NH moieties have been calculated at DFT/B3LYP level of theory using the 6-311++G(3df,3pd) basis set. Characteristic absorption domains have been compared with FTIR data from the literature. In particular, DFT calculations provide guidelines to discriminate SiH from CN stretching bands, which are calculated to lie below and above ~2230cm−1, respectively. As an example, the oxidation of a microwave PECVD SiCxNy:H films during ageing was evidenced in this work through the progressive increase of SiO stretching band (~1040cm−1). The simultaneous decay of the band centered at 2170cm−1 was attributed to the vanishing of SiH bond upon oxidation. This example illustrates that unambiguous band assignment is required to provide a molecular description of the ageing process, which is in turn required to optimize the material composition and stability. Results of these calculations will be helpful to identify both the chemical moieties and their environment in future investigations on a-SiCxNy:H materials but also on materials containing additional elements such as B- or O-doped SiCN-based systems.