Microstructural evolution and Poisson ratio of epitaxial ScN grown on TiN(001)/MgO(001) by ultrahigh vacuum reactive magnetron sputter deposition

Abstract

ScN layers, 60–80 nm thick, were grown at 800 °C on 220-nm-thick epitaxial TiN(001) buffer layers on MgO(001) by ultrahigh vacuum reactive magnetron sputter deposition in pure N2 discharges. The films are stoichiometric with N/Sc ratios, determined by Rutherford backscattering spectroscopy and x-ray photoelectron spectroscopy, of 1.00±0.02. Plan-view and cross-sectional transmission electron microscopy analyses showed that the films are single crystals which appear defect free up to a critical thickness of ≃15 nm, above which an array of nanopipes form with their tubular axis along the film growth direction and extending to the free surface. The nanopipes are rectangular in cross section with areas of ≃1.5×5 nm2 and are self-organized along 〈100〉, directions with an average separation of ≃40 nm. Their formation is the result of periodic kinetic surface roughening which leads to atomic self-shadowing and, under limited adatom mobility conditions, to deep cusps which are the origin of the nanopipes. The ScN layers are nearly relaxed, as determined from x-ray diffraction θ-2θ scans in both reflection and transmission, with only a small residual compressive strain due to differential thermal contraction. The Poisson ratio of ScN was found to be 0.20±0.04, in good agreement with ab initio calculations.