Structural Relaxation And Self-Diffusion In Covalent Amorphous Solids: Silicon Nitride As A Model System

Neutron reflectometry and isotope multilayers were used to investigate self-diffusion in covalent amorphous solids during isothermal annealing and its correlation to structural relaxation. Amorphous silicon nitride was chosen as a model system. Neutron reflectometry is a superior method to measure very low self-diffusivities, occurring in covalent solids, by applying only short time anneals. This allows one to determine time dependent changes of diffusivity over a broad temperature range before crystallization of the metastable solid occurs. The measured nitrogen diffusivities decrease by more than one order of magnitude during annealing between 950 and 1150 degrees C, while at the same time also a decrease of the atomic density is observed. We interpret this behavior as a structural relaxation of the amorphous network structure that is governed by annihilation of interstitial-like defects. (c) 2007 American Institute of Physics.