AlN/Si3N4 multilayers as an interface model system for Al1 − xSixN/Si3N4 nanocomposite thin films

•They are investigated by cross-sectional high resolution transmission electron microscopy.•Expitaxial growth is observed for a Si3N4 thickness of about 1 nm (ca. 3.5 monolayers).•Misfit dislocations provide stress relief between sapphire and (002)-oriented AlN.•The results are used to understand the hardness evolution of co-deposited Al–Si–N films.

AlN transparent coatings alternated with different thicknesses of Si3N4 layers were epitaxially grown on single-crystalline α-Al2O3 (0001) substrates by reactive magnetron sputtering, in order to investigate the possibility of stabilization of a crystalline form of silicon nitride. High resolution transmission electron microscopy study of the cross-sections through these coatings revealed that silicon nitride can be epitaxially stabilized in a crystalline form up to breakdown thickness tSiNyepi of about 0.7–1.0 nm corresponding to ~ 2.5–3.5 monolayers, which is anyway smaller than 1.2 nm (~ 4 monolayers). For tSiNy>tSiNyepi, amorphous Si3N4 is grown on top of epitaxially stabilized 0.7 nm of crystalline Si3N4 resulting in a mixed growth mode. The influence of the epitaxial stabilization of crystalline Si3N4 on hardness enhancement in the case of the Al–SiN system is compared to the TiN/Si3N4 system and some issues for extrapolation to nanocomposites are discussed