The Ti-Si-N system is attracting interest for the fabrication of wear-protective thin film materials in the form of nanocomposites, multilayers, and solid solutions [1]. We recently showed that cubic SiNx can be stabilized on TiN(001) surfaces [2]. Here, we present results from a detailed investigation of the nucleation and growth of SiNx layers in SiNx/TiN multilayers. TiN/SiNx multilayer films were deposited by reactive dual target magnetron sputtering onto MgO(001) substrates kept at T = 300 to 800 °C in a mixed N2 and Ar atmosphere. A combination of XRD, HREM, STEM, RHEED, RBS, and ERDA was used to characterize the as-deposited films. XRD showed low- and high-angle superlattice peaks corresponding to a well-defined compositional modulation and crystal quality. The c-SiNx layers grow to a thickness of 5-13 Å before transition to amorphous Si3N4 as seen in HREM. RHEED studies on SiNx/TiN(001) bilayers confirm the phase transformation and revealed 1x4 surface reconstruction along <110>, similar to our recent STM study [3].
Nanoindentation experiments show the highest hardness of 40.8 ±0.8 GPa in TiN20Å/c-SiN5Å multilayers. For comparison, monolithic TiN films have a hardness of 20.2 ± 0.9 GPa. Contributing hardening mechanisms for the superlattices are Koehler hardening, grain size hardening as well as coherency strain between TiN and c-SiNx. ERDA shows increasing N concentration, in the range of 1.0 to 1.33, but also Ar incorporation toward 2 at.% for a SiNx layer thickness above ~15-25 Å.
[1] A. Flink et al., Surf. Coat. Technol. 200 1535 (2005)
[2] H. Söderberg et al., Appl. Phys. Lett. 88, 191902, (2006)
[3] L. Hultman et al., PRB in press (2007)
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