Growth mode and asymptotic smoothing of sputtered Fe/Au multilayers studied by x-ray diffuse scattering

Abstract

The interfacial roughness of sputtered Fe[15 \AA{}]/Au[21 \AA{}] crystalline multilayers on MgO(001) was studied using x-ray specular and nonspecular reflectivity. The nonspecular scattering was collected using an image plate detector that allowed us to map the very weak x-ray diffuse intensity in one in-plane ${(q}_{x})$ and one out-of-plane ${(q}_{z})$ momentum-transfer direction. We have evaluated the interfacial static and dynamic roughness exponents from the in-plane diffuse scattering and the roughness conformality, and found them to be $\ensuremath{\alpha}=0.43\ifmmode\pm\else\textpm\fi{}0.05$ and $z=1.61\ifmmode\pm\else\textpm\fi{}0.15,$ respectively. From this result we show that the growth of this metallic thin film is described well by the Kardar-Parisi-Zhang (KPZ) model. By following the evolution of the interfacial roughness of this multilayer, starting from the MgO/Fe/Au heterostructure used to initiate a coherent lattice, we observe a tendency toward saturation in the growth and interfacial smoothing after the deposition of 40 bilayers. This asymptotic smoothing is induced by the mechanisms of the KPZ model and results in a reduction of the interfacial slope \ensuremath{\rho} as a function of time, according to $\ensuremath{\rho}(t)\ensuremath{\propto}{t}^{\ensuremath{-}0.31}.$ We have also found that the in-plane roughness cutoff length remains finite throughout the multilayer and does not increase to the lateral length of the sample, even after 100 bilayers have been grown.