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

CrN layers, 0.5 mum thick, were grown on MgO(001) at T-s=570-775 degreesC by ultrahigh vacuum magnetically unbalanced magnetron sputter deposition in pure N-2 discharges at 20 mTorr. Layers grown at T(s)less than or equal to700 degreesC are stoichiometric single crystals exhibiting cube-on-cube epitaxy: (001)(CrN)parallel to(001)(MgO) with [100](CrN)parallel to[100](MgO). At higher temperatures, N-2 desorption during deposition results in understoichiometric polycrystalline films with N fractions decreasing to 0.35, 0.28, and 0.07 with T-s=730, 760, and 775 degreesC, respectively. The surface morphologies of epitaxial CrN(001) layers were found to depend strongly on the incident ion-to-metal flux ratio J(N2)(+)/J(Cr) which was varied between 1.7 and 14 with the ion energy maintained constant at 12 eV. The surfaces of layers grown with J(N2)(+)/J(Cr)=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 mum. The combination of atomic shadowing by the mounds with low adatom mobility results in the formation of nanopipes extending along the growth direction. Increasing J(N2)(+)/J(Cr) to 14 leads, due to increased adatom mobilities, to much smoother surfaces with [h]=2.5 nm and [d]=0.52 mum. Correspondingly, the nanopipe density decreases from 870 to 270 mum(-2) to <20 mu m(-2) as J(N2)(+)/J(Cr) 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, 7.7x10(-2) Omega cm, and relaxed lattice constant, 0.4162+/-0.0008 nm, are independent of J(N2)(+)/J(Cr). (C) 2002 American Institute of Physics.