Nitrogen Diffusion in Amorphous Silicon Nitride Isotope Multilayers Probed by Neutron Reflectometry

Amorphous silicon nitride is a model system for a covalently bound amorphous solid with a low atomic mobility where reasonable values of self-diffusivities are still lacking. We used neutron reflectometry on isotope enriched ${\mathrm{Si}}_3{}^{14}\mathrm{N}_4/{\mathrm{Si}}_3{}^{15}\mathrm{N}_4$ multilayers to determine nitrogen self-diffusivities ranging from ${10}^{-24}$ to ${10}^{-21}{\mathrm{m}}^2/\mathrm{s}$ between 950 and 1250 °C. Time dependent diffusivities observed at 1150 °C indicate the presence of structural relaxation. For long annealing times (relaxed state) the diffusivities follow an Arrhenius law with an activation enthalpy of $(3.6\pm 0.4)\mathrm{eV}$. The results are indicative of a direct diffusion mechanism without the involvement of thermal point defects.