Strain-induced growth-mode transition of V in epitaxial Mo/V(001) superlattices

Epitaxial (001) oriented Mo/V superlattices have been grown on MgO (001) substrates kept at 700 °C by magnetron sputter deposition. Films with different modulation periods Λ and with different fractions, ${\mathit{X}}_{\mathrm{V}}$, of V in the period were investigated (${\mathit{X}}_{\mathrm{V}}$=${\mathit{D}}_{\mathrm{V}}$/Λ, where ${\mathit{D}}_{\mathrm{}\mathrm{V}}$ is the V-layer thickness). The Λ range was 0.313 to 17.7 nm and ${\mathit{X}}_{\mathrm{V}}$ was varied in the range 0.11 to 0.93. The as-deposited films were characterized by cross-sectional transmission-electron microscopy and by x-ray-diffraction techniques. The results show that the superlattices change from a structure with smooth Mo and V layers with sharp and well-defined layer interfaces to a structure where the V layers have a large in-plane thickness fluctuation when the V layers exceed a critical thickness ${\mathit{D}}_{\mathit{c}}$. ${\mathit{D}}_{\mathit{c}}$ increases from ∼0.3 to ∼8 nm as ${\mathit{X}}_{\mathrm{V}}$ is increased from 0.11 to 0.83 and for equally thick Mo and V layers ${\mathit{D}}_{\mathit{c}}$ is ∼2.5 nm. The layer thickness fluctuations are nonaccumulative and disappear if the periodicity of a growing Mo/V superlattice is changed so that the V-layer thickness becomes smaller than ${\mathit{D}}_{\mathit{c}}$. Mo was found to grow in a two-dimensional mode producing layers with uniform thicknesses, following the undulated surface of the V layers. The results are explained in terms of growth above and below the roughening temperatures for V and Mo, respectively. The roughening of V is suggested to be triggered by the surface strain and curvature induced by misfit dislocations. © 1996 The American Physical Society.