Simultaneous diffusion of Si and N in silicon nitride

Silicon nitride is a covalently bound solid of scientific and technological importance with a very low atomic mobility where reliable self-diffusivities are still lacking. We used ²⁸Si₃¹⁴N₄∕ⁿᵃᵗSi₃¹⁵N₄∕²⁸Si₃¹⁴N₄ isotope heterostructures produced by reactive magnetron sputtering and subsequent in situ crystallization in combination with secondary ion mass spectrometry to study self-diffusion. Si and N diffusivities were measured in polycrystalline α-Si₃N₄ by isotope interdiffusion at temperatures between 1200 and 17000.2em0ex^∘C. The diffusivities of the two elements coincide within error limits over the whole temperature range investigated and follow an Arrhenius law with an activation enthalpy of ΔH=5.00.3em0exeV and a pre-exponential factor of D₀=6×10^-60.3em0exm²∕s. Isotope heterostructures which are composed of a phase mixture of α-Si₃N₄ and β-Si₃N₄ show diffusivities which are not significantly lower and which have nearly the same activation enthalpy of ΔH=4.90.3em0exeV, indicating similar diffusivities in both polymorphic phases of silicon nitride. The entropy of self-diffusion is calculated to ΔS≈2 kB, which is a hint that diffusion is mediated by localized point defects, in contrast to extended point defects usually found in other semiconductors, such as Si, Ge, and GaAs. Possible diffusion mechanisms are discussed.