Smoothing, roughening and sputtering: the complex evolution of immiscible Fe/Bi bilayer system

Abstract

In this work, swift heavy ion (SHI) induced surface smoothing, roughening and sputtering of thermally immiscible Fe/Bi bilayer system has been investigated. The pristine and irradiated samples were analysed by Rutherford backscattering spectrometry (RBS), grazing angle x-ray diffraction (XRD), atomic force microscopy and scanning electron microscopy including x-ray dispersive energy analyzer. RBS analysis revealed that steepness of the low energy edge of the Bi signal increases at a fluence of 3 × 1013 ions cm−2, beyond which the slope of the rear edge decreases. The increased steepness is due to smoothing induced at initial fluence; however, the decrease in the slope of rear edge beyond 3 × 1013 ions cm−2 fluence is a result of surface roughening. XRD reveals the increase in the crystalline nature of Bi after irradiation at 3 × 1013 ions cm−2. Irradiation at higher fluences from 6 × 1013 to 1 × 1014 ions cm−2 leads to a decrease in the crystalline nature of Bi. Surface roughness of pristine and irradiated samples from AFM analysis revealed that initially roughness decreases with a fluence of 3 × 1013 ions cm−2. However, at higher fluences, beyond 3 × 1013 ions cm−2, the agglomeration of smaller grains has been observed due to the shear flow mechanism, which results in surface roughening. The observed behaviour of surface smoothing and roughening under SHI irradiation may be explained on the basis of the thermal spike model.