Phase formation and microstructure of Nb 1 − xAl xN alloy films grown on MgO (001) by reactive sputtering: a new ternary phase

Abstract

Ternary Nb 1 − x Al x N films were grown on single-crystal MgO(001) substrates by reactive triode sputtering in an Ar + N 2 gas mixture. A phase spread deposition technique was employed to fabricate films with a composition gradient along the length of a substrate. For a deposition temperature of 500 °C, X-ray diffraction (XRD) showed that the compound films had the cubic B1 structure for Al fractions in the range from 0 < x < 0.60. The lattice parameter decreased with increasing x, from 4.41 Å at x = 0 to approximately 4.26 Å for x = 0.59. A new ternary NbAlN phase was discovered in a narrow composition range around 63% Al. This phase crystallized in an fcc structure with a unit cell size of 4.15 ± 0.02 A ̊ and was stabilized possibly by epitaxy to the single crystal substrate. For Al fractions beyond 65% XRD indicated nanocrystalline films having the hexagonal wurtzite structure. Determination of elastic strain, crystallographic tilt and microstructural quality was performed by high resolution XRD reciprocal space mapping. Epitaxial films with x < 0.50 grew essentially unstrained, with the mismatch accommodated by means of misfit dislocations giving the film a mosaic character. Growth modes and phase decomposition paths of the metastable BI films at x ⩽ 0.60, as well as the structure of the new phase at x = 0.63, were assessed by planar and cross sectional transmission electron microscopy.