Ellipsometry and transmission electron microscopy study of MoSi2 coatings after oxidation at high temperature in air

MoSi2 coatings were deposited on Si (100) substrates by means of magnetron sputtering. The structural and optical properties of the coatings were then investigated through transmission electron microscopy (TEM) and UV-visible-near IR ellipsometry after heat treatments in air at two temperatures (1100 K and 1600 K). After annealing at 1100 K, no silica layer could be observed by TEM at the surface of the MoSi2 coating, whereas annealing at 1600 K gave rise to a protective silica layer. As observed by TEM, this silica layer contained nanometric molybdenum silicide crystals. Increasing the annealing time at 1600 K from 20 min to 2 h resulted in both a higher concentration and growth of these molybdenum silicide crystals in the silica layer. An optical model, taking into account the observed microstructure and allowing a good fit of the ellipsometry data, was developed. It is also shown that the dielectric constants of the non-oxidized part of the MoSi2 layer became modified when the annealing temperature (1100 K versus 1600 K) and the annealing time at 1600 K (20 min versus 2 h) were increased.