Comparison of 30Si diffusion in amorphous Si–C–N and Si–B–C–N precursor-derived ceramics

We carried out silicon self-diffusion studies in amorphous precursor-derived ceramics of composition Si3BC4.3N2 and Si2.6C4.1N2.3 to compare the atomic transport properties of a boron containing and of a boron free material in the system Si–(B)–C–N. Ion implanted stable 30Si isotopes were used as tracers and secondary ion mass spectrometry (SIMS) was used for depth profiling. The experimentally determined diffusivities are lower by a factor of 10 for Si3BC4.3N2 than for Si2.6C4.1N3.3 in the whole temperature range investigated. This might be a hint that the mobility of the constituent elements plays an important role in the stabilization of the amorphous state of Si3BC4.3N2 at high temperatures. Both ceramics obey an Arrhenius behaviour with activation enthalpies and pre-exponential factors of ΔH=5.7 eV and D0=0.001 m2/s for Si3BC4.3N2, and ΔH=5.55 eV and D0=0.003 m2/s for Si2.6C4.1N3.3, respectively. These results are consistent with a diffusion mechanism mediated by vacancy like defects operating in these amorphous ceramics, which is analogous to the related crystalline silicon based non-oxide ceramics Si3N4 and SiC.