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

The interfacial roughness of sputtered Fe[15 Angstrom]/Au[21 Angstrom] 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 alpha = 0.43 +/- 0.05 and z = 1.61 +/- 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 rho as a function of time, according to rho(t)proportional to t(-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.