Irradiation effects in Ag-Fe bilayers: Ion-beam mixing, recrystallization, and surface roughening

Ag/Fe bilayers deposited onto SiO2/Si substrates were irradiated at 20, 77, and 300 K with 300-750 keV Ar and Xe ions in order to study the ion-beam-induced mixing and phase formation in this thermally immiscible system. A combination of Rutherford backscattering spectroscopy (RES), channeling, conversion electron Mossbauer spectroscopy (GEMS), and scanning tunneling microscopy (STM) was used to analyze the atomic transport at the interface and the resulting microstructure and morphology changes of the samples. In the CEMS measurements, a 13 nm thin Fe-57 marker layer at the Ag/Fe interface was introduced in order to enhance the sensitivity to alterations of the interfacial composition. From the small amount of Ag atoms found to be dissolved in Fe and from the sharpness of the element profiles at the interface, we derived a very small mixing efficiency, which is significantly smaller than the prediction of the ballistic model. Since ballistic mixing is expected in any case, we argue that demixing and phase separation occur in the relaxation stage or thermal spike phase of the collision cascade, as a consequence of the positive heat of mixing. On the other hand, ion irradiation induces a large surface roughening of the Ag top layer as proven by STM. This effect is obviously due to recrystallization of Ag, which results in grain growth and texture formation along the direction of the impinging ion, as demonstrated by RBS/channeling measurements.