Large changes in refractive index by synchrotron-radiation-driven compaction of hydrogenated silicon nitride films

When high-energy photons (hν>100 eV) irradiate films of hydrogenated silicon nitride (a-SiNx:H) that have been deposited by plasma-enhanced chemical-vapor deposition, the network of atoms in the material is photolytically rearranged. This reaction proceeds rapidly from the instance the radiation is applied and is almost completed within 200 s. This is in striking contrast with the continuing effect of such radiation on SiO2 (defect formation and decomposition). The result was a change (Δn) in the refractive index at 633 nm of as much as 0.04, and this effect was independent of the temperature of irradiation. By using an effective medium mixture of Si3N4, crystalline Si, and voids to model the dielectric function of a-SiNx:H films, the volumes of the voids and of crystalline Si components were found to be reduced by 44% and 49%, respectively, from their initial values. The resulting 9.8% reduction in the thickness (compaction) is the origin of the large Δn