Superficial Si nanostructure synthesis by low-energy ion-beam- induced phase separation

Silicon nanocrystals (SiNCs) have a promising application in photonic devices. The disproportionation reaction of silicon suboxide is a synthesis pathway for the production of SiNCs. In this paper, synchrotron infrared nanospectroscopy (SINS) and atomic force microscopy (AFM) are used to investigate the superficial silicon-suboxide phase separation induced by ion irradiation. A 2 keV Cs+ ion beam produces preferential oxygen sputtering on the surface of a 50 nm SiO2 film, and a consequent localized change in the stoichiometry to SiOx with x < 2. The ion-beam direct energy deposition induces, concurrently to physical etching, a disproportionation reaction that transforms SiOx into a mixture of Si nanostructures and SiO2. With increasing beam fluence, a dense pattern of Si nanoplates emerge, texturing the surface. Multi-layer finite dipole model (FDM) is used for the theoretical analysis at atomic scale to predict the determinant factors of the infrared near-field behavior at the surface. The model shows that the combination of vacancies and oxygen atoms present in Si crystal produces Surface Phonon Polaritons. The measured SINS spectra show structures consistent with FDM calculations for defective Si.