Epitaxial V0.6W0.4N/MgO(001): Evidence for ordering on the cation sublattice

Abstract

V0.6W0.4N alloys are grown on MgO(001) by ultrahigh vacuum reactive magnetron sputtering from V and W targets in 10 mTorr pure-N2 atmospheres at temperatures Ts ranging from 600 to 900 °C. Based on x-ray diffraction and transmission electron microscopy results, all films have the B1-NaCl crystal structure and grow with a cube-on-cube epitaxial relationship to the substrate, (001)VWN‖(001)MgO and [100]VWN‖[100]MgO. Rutherford backscattering spectrometry analyses show that the N content in V0.6W0.4Nx alloys decreases with increasing Ts from overstoichiometric with x = 1.13 at 600 °C, to approximately stoichiometric with x = 1.08 at 700 °C, to understoichiometric at 800 °C (x = 0.80) and 900 °C (x = 0.75). High-resolution scanning transmission electron microscopy, Z-contrast, and selected-area electron diffraction investigations of V0.6W0.4N(001) alloys grown at 600 and 700 °C reveal the onset of W ordering on adjacent 111 planes of the metal sublattice; no ordering is observed for understoichiometric films grown at higher temperatures.