Distinguishing Between Coherent Interdiffusion and Incoherent Roughness in Synthetic Multilayers Using X-Ray Diffraction

Abstract

We have developed a method to separate coherent interfacial interdiffusion from incoherent interfacial roughness by extending an electromagnetic dynamical theory to calculate the reflectivity of a multilayer having an arbitrary interfacial profile with a variable degree of randomness in the repeating layer thicknesses. We find that the intensity of the subsidiary maxima are extremely sensitive to incoherent roughness while the intensity of the Bragg maxima are largely determined by the interfacial electron density profiles. Experimental data are modeled in a manner similar to that used by Warren and Averback to determine domain size of crystallites. We divide the multilayer into coherent domains differing from one another by small deviations from the average layer thicknesses. The diffraction intensity from each of these domains is then added to obtain the experimental pattern. The diffraction spectra of a set of Pt/Co multilayers with similar layer thicknesses but prepared with different sputtering gases illustrates the ability to separate the effects of coherent interdiffusion from incoherent roughness. The extent of incoherent roughness obtained using this model to analyze the diffraction data of these Pt-Co multilayers is in good agreement with TEM and STM results from the same samples. The diffraction patterns could not be simulated with abrupt concentration profiles and the extent of interdiffusion was found to be correlated with the energy of reflected neutrals present during the synthesis of the multilayers.