Off-Stoichiometry Spectroscopic Investigations of Pure Amorphous Silica and N-Doped Silica Thin Films

Cathodoluminescence spectroscopy and X-ray photoelectron spectroscopy were concurrently used to investigate the local physicochemical nature of the amorphous lattice in pure SiO2 and N-doped SiO2 thin films prepared by radiofrequency magnetron sputtering (the latter samples deposited under a set of different conditions of N-2 partial pressure). The main aim of this investigation was twofold: (i) to extend our knowledge of the physical and chemical structure of SiO2 films and (ii) to explore our capacity of manipulating, fine tuning, and measuring their stoichiometry characteristics. The presence of nitrogen atoms in the amorphous host structure was confirmed to significantly affect the formation of oxygen-deficient centers, nonbridging oxygen hole centers, and other kinds of defect complexes. The main challenge here was to relate the variations in type and concentration of these peculiar defects to the processing conditions and to the amount of nitrogen incorporated in the SiO2 amorphous matrix. The evolution of both pure and doped systems was monitored with increasing the temperature of an annealing cycle following film deposition (1 h in air, at temperatures ranging between 50 and 1200 degrees C, with 50 degrees C step). Stoichiometry changes could thus be clarified and temperature thresholds found for the annihilation of N sites and for the formation of a pseudoequilibrium stoichiometric structure in silica glass.