Swift Heavy Ion-Induced Reactivity and Surface Modifications in Indium Thin Films

As an energetic ion traverses a target material, it loses its energy through the processes of electronic energy loss (S _e) and nuclear energy loss (S _n). Controlled swift heavy ion (SHI) irradiation on solid targets produces its effects through both of these mechanisms, as a consequence of which modifications occur in the structure, surface morphology, and magnetic and optical properties, apart from ion implantation and ion-induced reactivity. A systematic investigation of these effects can be useful in developing standard protocols for creating desired effects in materials using specific ion beams. In this study, indium films of thickness 25 nm were deposited on silicon substrates and were subjected to 100 MeV O⁷⁺ and 100 MeV Si⁷⁺ ion irradiation, with the fluences varying from 1 × 10¹¹ to 1 × 10¹³ ions/cm². The pristine and SHI-irradiated films were then characterized using glancing incidence X-ray diffraction (GIXRD), Rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The motive was to identify the effects of irradiation with different ion species having large variations in electronic and nuclear energy losses. While the RBS results suggest that sputtering is extremely low and that there are no major changes in the film composition due to ion beam-induced mixing, the GIXRD analysis indicates that increasing the ion fluence reduces the crystallinity of the film for both the ions. Ion beam irradiation with O⁷⁺ ions, however, results in beam-induced reactivity, as the GIXRD scan shows characteristic peaks from indium oxide (In₂O₃), which become the predominant peaks at the highest fluence used here. Si⁷⁺ ion irradiation results in a narrowing of the particle size distribution on the surface, with no evidence of reactivity. SEM results indicate fusion and fragmentation of grains with the increase in the ion fluences, and AFM images reveal an increase in the surface roughness of a few percent when irradiated with both 100 MeV O⁷⁺ and 100 MeV Si⁷⁺ ions.