Structural relaxation and self-diffusion in covalent amorphous solids: Silicon nitride as a model system

Abstract

Structure and dynamics of silicon-oxygen pairs and their role in silicon self-diffusion in amorphous silicaNeutron 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°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.

Atomic transport in metastable compounds: Case study of self-diffusion inSi−C−Nfilms using neutron reflectometry

“…Neutron reflectometry 28 is an advanced method to determine self-diffusivities. [29][30][31][32][33][34] It has the advantage over other methods to be nondestructive and is able to determine even sub-nanometer diffusion lengths. [32][33][34] Following the annealing time dependence of bonding configurations by Fouriertransform infrared spectroscopy ͑FTIR͒ we identified a metastable transient state in the system Si-C-N and applied neutron reflectometry to quantify nitrogen diffusivities in this state.…”

Neutron Reflectometry: A Tool to Investigate Diffusion Processes in Solids on the Nanometer Scale

“…The measured diffusivities decrease by more than one order of magnitude, which can be interpreted as structural relaxation of the amorphous network structure that is governed by annihilation of interstitial-like defects. [17] Carrying out experiments with SIMS at the same temperature a sufficiently larger annealing time of 167 h was necessary to obtain a diffusivity in the amorphous state of about 1 Â 10 À22 m 2 s À1 . [18] However, no time dependence could be detected due to the fact that a diffusivity averaged over this long annealing time was measured, roughly corresponding to the diffusivity in the relaxed state.…”

“…[17,19] Diffusivities in the relaxed amorphous state as a function of temperature are given in Figure 4. In comparison to diffusivities measured for polycrystalline Si 3 N 4 (grain size 80 nm).…”

“…The grains have diameters of about 30 nm and a thickness which is approximately the film thickness. [17]). The line is a guide for the eye.…”

“…4. Nitrogen diffusivities in polycrystalline Si3N4 (SIMS) [19] and amorphous SiN1.33 (NR) [17] as a function of reciprocal temperature. A typical reflectivity pattern and the corresponding simulation of the experimental data with the program Parratt32 (based on Parratt's formalism) [22] is given in Figure 5(a).…”