High temperature decomposition and age hardening of single-phase wurtzite ${\mathrm{Ti}}_{1-x}{\mathrm{Al}}_{x}N$ thin films grown by cathodic arc deposition

Wurtzite $T_m\mathrm{AlN}$ ($T_m=\mathrm{transition}$ metal) themselves are of interest as semiconductors with tunable band gap, insulating motifs to superconductors, and piezoelectric crystals. Characterization of wurtzite $T_m\mathrm{AlN}$ is challenging because of the difficulty to synthesize them as single-phase solid solution and such thermodynamic, elastic properties, and high temperature behavior of wurtzite ${\mathrm{Ti}}_{1-x}{\mathrm{Al}}_{x}N$ is unknown. Here, we investigated the high temperature decomposition behavior of wurtzite ${\mathrm{Ti}}_{1-x}{\mathrm{Al}}_{x}N$ films using experimental methods combined with first-principles calculations. We have developed a method to grow single-phase metastable wurtzite ${\mathrm{Ti}}_{1-x}{\mathrm{Al}}_{x}N$ ($x=0.65$, 0.75, 085, and 0.95) solid-solution films by cathodic arc deposition using low duty-cycle pulsed substrate-bias voltage. We report the full elasticity tensor for wurtzite ${\mathrm{Ti}}_{1-x}{\mathrm{Al}}_{x}N$ as a function of Al content and predict a phase diagram including a miscibility gap and spinodals for both cubic and wurtzite ${\mathrm{Ti}}_{1-x}{\mathrm{Al}}_{x}N$. Complementary high-resolution scanning transmission electron microscopy and chemical mapping demonstrate decomposition of the films after high temperature annealing (${950}^{\circ }C$), which resulted in nanoscale chemical compositional modulations containing Ti-rich and Al-rich regions with coherent or semicoherent interfaces. This spinodal decomposition of the wurtzite film causes age hardening of 1–2 GPa.