Growth of single-crystal CrN on MgO(001): Effects of low-energy ion-irradiation on surface morphological evolution and physical properties

Abstract

CrN layers, 0.5 μm thick, were grown on MgO(001) at Ts=570–775 °C by ultrahigh vacuum magnetically unbalanced magnetron sputter deposition in pure N2 discharges at 20 mTorr. Layers grown at Ts⩽700 °C are stoichiometric single crystals exhibiting cube-on-cube epitaxy: (001)CrN||(001)MgO with [100]CrN||[100]MgO. At higher temperatures, N2 desorption during deposition results in understoichiometric polycrystalline films with N fractions decreasing to 0.35, 0.28, and 0.07 with Ts=730, 760, and 775 °C, respectively. The surface morphologies of epitaxial CrN(001) layers were found to depend strongly on the incident ion-to-metal flux ratio JN2+/JCr which was varied between 1.7 and 14 with the ion energy maintained constant at 12 eV. The surfaces of layers grown with JN2+/JCr=1.7 consist of self-organized square-shaped mounds, due to kinetic roughening, with edges aligned along orthogonal 〈100〉 directions. The mounds have an average peak-to-valley height 〈h〉=5.1 nm and an in-plane correlation length of 〈d〉=0.21 μm. The combination of atomic shadowing by the mounds with low adatom mobility results in the formation of nanopipes extending along the growth direction. Increasing JN2+/JCr to 14 leads, due to increased adatom mobilities, to much smoother surfaces with 〈h〉=2.5 nm and 〈d〉=0.52 μm. Correspondingly, the nanopipe density decreases from 870 to 270 μm−2 to <20 μm−2 as JN2+/JCr is increased from 1.7 to 6 to 10. The hardness of dense CrN(001) is 28.5±1 GPa, but decreases to 22.5±1 GPa for layers containing significant nanopipe densities. The CrN(001) elastic modulus, 405±15 GPa, room-temperature resistivity, 7.7×10−2 Ω cm, and relaxed lattice constant, 0.4162±0.0008 nm, are independent of JN2+/JCr.