Probing the onset of wurtzite phase formation in (V,Al)N thin films by transmission electron microscopy and atom probe tomography

Abstract

Single-phase metastable cubic (V,Al)N thin films with columnar microstructure were grown by high power pulsed magnetron sputtering at 440 °C and the thermal decomposition mechanisms were systematically investigated by post-deposition vacuum annealing from 600 to 900 °C. The onset of spinodal decomposition into isostructural V- and Al-rich cubic nitride phases is demonstrated after cyclic vacuum annealing at 700 °C. Moreover, at this temperature, evidence for aluminum diffusion to grain boundaries and triple junctions is provided by correlation of transmission electron microscopy and atom probe tomography data. The formation of Al-rich regions can be understood by the more than 25% lower activation energy for bulk diffusion of aluminum compared to vanadium as obtained from ab initio calculations. It is reasonable to assume that these Al-rich regions are precursors for the formation of wurtzite AlN, which is unambiguously identified after annealing at 800 °C by microscopy and tomography. The significantly larger equilibrium volume of wurtzite AlN compared to the cubic phase explains its initial formation exclusively at triple junctions and grain boundaries. In contrast, twin boundaries are enriched in vanadium. Interestingly, the formation of the wurtzite phase at grain boundaries and triple junctions can be tracked by resistivity measurements, while X-ray diffraction and nanoindentation data do not support an unambiguous wurtzite phase formation claim for annealing temperatures <900 °C. Hence, it is evident that previously reported formation temperatures of wurtzite AlN in transition metal aluminum nitrides, determined by other characterization techniques than chemical and structural characterization at the nanometer scale and/or resistivity measurements, are overestimated.