Correlated-interfacial-roughness anisotropy inSi1−xGex/Si superlattices

Interfacial roughness in ${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ge}}_{\mathit{x}}$/Si superlattices grown by molecular-beam epitaxy on vicinal Si(001) surfaces has been investigated using low-angle x-ray-diffraction diffuse-intensity measurements and atomic-force microscopy. The vertically correlated-interfacial roughness is, in specific situations, highly anisotropic and oriented with respect to the substrate miscut. The lateral length scale of the roughness is many times greater than the average separation of the substrate steps. The presence of the anisotropy depends on Ge concentration. A thermodynamic model for the interface morphology is presented.