Effects of volume mismatch and electronic structure on the decomposition of ScAlN and TiAlN solid solutions

Abstract

Thin solid films of metastable rocksalt structure (c-) ${\text{Sc}}_{1\ensuremath{-}x}{\text{Al}}_{x}\text{N}$ and ${\text{Ti}}_{1\ensuremath{-}x}{\text{Al}}_{x}\text{N}$ were employed as model systems to investigate the relative influence of volume mismatch and electronic structure driving forces for phase separation. Reactive dual magnetron sputtering was used to deposit stoichiometric ${\text{Sc}}_{0.57}{\text{Al}}_{0.43}\text{N}(111)$ and ${\text{Ti}}_{0.51}{\text{Al}}_{0.49}\text{N}(111)$ thin films, at $675\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ and $600\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$, respectively, followed by stepwise annealing to a maximum temperature of $1100\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. Phase transformations during growth and annealing were followed in situ using x-ray scattering. The results show that the as-deposited ${\text{Sc}}_{0.57}{\text{Al}}_{0.43}\text{N}$ films phase separate at $1000--1100\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ into nonisostructural c-ScN and wurtzite structure (w-) AlN, via nucleation and growth at domain boundaries. ${\text{Ti}}_{0.51}{\text{Al}}_{0.49}\text{N}$, however, exhibits spinodal decomposition into isostructural coherent c-TiN and c-AlN, in the temperature interval of $800--1000\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. X-ray pole figures show the coherency between c-ScN and w-AlN, with $\text{AlN}(0001)\ensuremath{\parallel}\text{ScN}(001)$ and $\text{AlN}⟨0\overline{1}10⟩\ensuremath{\parallel}\text{ScN}⟨\overline{1}10⟩$. First-principles calculations of mixing energy-lattice spacing curves explain the results on a fundamental physics level and open a route for design of novel metastable pseudobinary phases for hard coatings and electronic materials.