We report on the thermal stability of epitaxial cubic-TiN/(Al,Sc)N metal/semiconductor superlattices with the rocksalt crystal structure for potential plasmonic, thermoelectric, and hard coating applications. TiN/Al0.72Sc0.28N superlattices were annealed at 950 and 1050 °C for 4, 24, and 120 h, and the thermal stability was characterized by high-energy synchrotron-radiation-based 2D X-ray diffraction, high-resolution (scanning) transmission electron microscopy [HR(S)/TEM], and energy dispersive X-ray spectroscopy (EDX) mapping. The TiN/Al0.72Sc0.28N superlattices were nominally stable for up to 4 h at both 950 and 1050 °C. Further annealing treatments for 24 and 120 h at 950 °C led to severe interdiffusion between the layers and the metastable cubic-Al0.72Sc0.28N layers partially transformed into Al-deficient cubic-(Al,Sc)N and the thermodynamically stable hexagonal wurtzite phase with a nominal composition of AlN (h-AlN). The h-AlN grains displayed two epitaxial variants with respect to c-TiN and cubic-(Al,Sc)N. EDX mapping suggests that scandium has a higher tendency for diffusion in TiN/(Al,Sc)N than titanium or aluminum. Our results indicate that the kinetics of interdiffusion and the cubic-to-hexagonal phase transformation place constraints on the design and implementation of TiN/(Al,Sc)N superlattices for high-temperature applications.
Scandium Diffraction Spot Superlattice Reflection Increase Annealing Time Inset Image
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J. L. Schroeder and J. Birch acknowledge financial support from Linköping University and the Swedish Research Council (the RÅC Frame Program (2011-6505) and the Linnaeus Grant (LiLi-NFM)). B. Saha and T. D. Sands acknowledge financial support by the National Science Foundation and US Department of Energy (CBET-1048616). The Knut and Alice Wallenberg (KAW) Foundation is acknowledged for the Electron Microscope Laboratory in Linköping. Special thanks to Lina Rogström, Niklas Norrby, and Daniel Ostach for assistance with the synchrotron-radiation measurements.
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