Isotope heterostructures, 28Si314N4|natSi315N4|28Si314N4, produced by reactive magnetron sputtering and subsequent in situ crystallization in combination with secondary ion mass spectrometry were used to study self-diffusion. The Si and N diffusivities were measured in polycrystalline α-Si3N4 by means of isotope interdiffusion at between 1200 and 1700C (figure 35). The diffusivities of the two elements coincided within error limits over the whole temperature range investigated and obeyed the Arrhenius law:

D (m2/s) = 6 x 10-6 exp[-5.0(eV)/kT]

Isotope heterostructures which were composed of a phase mixture of α-Si3N4 and β-Si3N4 exhibited diffusivities which were not significantly lower and which had nearly the same activation enthalpy (4.9eV); indicating similar diffusivities in both polymorphic phases of the nitride. The entropy of self-diffusion was calculated to be ΔS ≈ 2k, which implied that diffusion was mediated by localized point defects; in contrast to the extended point defects usually found in semiconductors such as Si, Ge and GaAs.

Simultaneous Diffusion of Si and N in Silicon Nitride. H.Schmidt, U.Geckle, M.Bruns: Physical Review B, 2006, 74[4], 045203

Figure 35

Diffusivity of N and Si in Si3N4

(Black: N diffusion, white: Si diffusion)