Thermal stability of epitaxial cubic-TiN/(Al,Sc)N metal/semiconductor superlattices

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/Al_0.72Sc_0.28N 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/Al_0.72Sc_0.28N 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-Al_0.72Sc_0.28N 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.