Defect structure and phase transitions in epitaxial metastable cubic Ti0.5Al0.5N alloys grown on MgO(001) by ultra-high-vacuum magnetron sputter deposition

Abstract

Ti0.5Al0.5N alloy films, typically 1.5 μm thick, were grown on MgO(001) at temperatures Ts between 400 and 850 °C by ultra-high-vacuum reactive magnetron sputtering in pure N2. Films grown at Ts between ≂480 and 560 °C were single crystals in which the lattice misfit strain was partially relieved by glide of 〈001〉 misfit dislocations, with Burgers vector =a0/2〈011〉, on {011̄} planes. Cross-sectional transmission electron microscopy investigation showed no evidence of residual extended defects in the films until thicknesses of ≂150 nm at which point threading dislocations, oriented along the [001] growth direction, were observed. Surface-initiated spinodal decomposition, resulting in the formation of compositionally modulated NaCl-structure platelets along [001] with width ≂1 nm, occurred over a narrow growth temperature range between 540 and 560 °C as a precursor to bulk phase separation of wurtzite-structure AlN at Ts≥560 °C. The alloy was continuously depleted of AlN at higher growth temperatures until the equilibrium two-phase structure, cubic TiN and wurtzite AlN, was obtained at Ts≥750 °C.