Surface morphological evolution of epitaxial CrN(001) layers

CrN layers, 57 and 230 nm thick, were grown on MgO(001) at T-s=600-800 degrees C by ultrahigh-vacuum magnetron sputter deposition in pure N-2 discharges from an oblique deposition angle alpha=80 degrees. Layers grown at 600 degrees C nucleate as single crystals with a cube-on-cube epitaxial relationship with the substrate. However, rough surfaces with cauliflower-type morphologies cause the nucleation of misoriented CrN grains that develop into cone-shaped grains that protrude out of the epitaxial matrix to form triangular faceted surface mounds. The surface morphology of epitaxial CrN(001) grown at 700 degrees C is characterized by dendritic ridge patterns extending along the orthogonal < 110 > directions superposed by square-shaped super mounds with < 100 > edges. The ridge patterns are attributed to a Bales-Zangwill instability while the supermounds form due to atomic shadowing which leads to the formation of epitaxial inverted pyramids that are separated from the surrounding layer by tilted nanovoids. Growth at 800 degrees C yields complete single crystals with smooth surfaces. The root-mean-square surface roughness for 230-nm-thick layers decreases from 18.8 to 9.3 to 1.1 nm as T-s is raised from 600 to 700 to 800 degrees C. This steep decrease is due to a transition in the roughening mechanism from atomic shadowing to kinetic roughening. Atomic shadowing is dominant at 600 and 700 degrees C, where misoriented grains and supermounds, respectively, capture a larger fraction of the oblique deposition flux in comparison to the surrounding epitaxial matrix, resulting in a high roughening rate that is described by a power law with an exponent beta > 0.5. In contrast, kinetic roughening controls the surface morphology for T-s=800 degrees C, as well as the epitaxial fraction of the layers grown at 600 and 700 degrees C, yielding relatively smooth surfaces and beta <= 0.27. (c) 2005 American Institute of Physics.