Swift heavy-ion irradiation of amorphous silicon dioxide, silicon nitride and silicon oxynitrides composite systems: from ion tracks to nanoparticle shaping

Abstract

This work provides a systematic study of ion irradiation to tailor the physical and chemical properties of a-SiO2, a-Si3N4 and a-SiOxNy materials with a particular focus on three specific areas: (i) the formation of ion tracks by swift heavy-ion (SHI) irradiation in a-SiO2, a-Si3N4, and a-SiOxNy, (ii) the ion beam synthesis of Au nanoparticles (NPs) in a-SiO2 and a-Si3N4; and (iii) the ion shaping process of embedded metallic NPs in a-Si2, a-Si3N4 and at their interface. (i) Ion tracks were created by SHI irradiation of a-SiO2, a-Si3N4 and a-SiOxNy of different composition, with 185 MeV and 2.2 GeV Au ions at fluences between 1 x 10^11 - 1 x 10^13 cm^-2. Small-angle X-ray scattering (SAXS) revealed a cylindrical ion track morphology which resembles, for all materials and compositions considered, an under-dense core surrounded by an over-dense shell with a smooth transition between the two regions, in good agreement with molecular-dynamics simulations. A decrease in the ion track dimensions for samples with higher nitrogen content was determined, accompanied by an increased density change. The latter is attributed to the short-lived thermal spike resulting from the higher thermal conductivity associated with a-Si3N4. IR spectroscopy analysis shows a region of high radiation damage region within the ion track core for a-SiO2 and a-Si3N4. (ii) To study the synthesis of metallic NPs, a-SiO2 and a-Si3N4 were implanted with 2 MeV Au ions at a fluence of 5 x 10^16 cm^-2. The synthesis and growth was promoted by thermal annealing for 60 minutes in either air or N2 gas at temperatures between 1000 and 1100 C. Characterization with SAXS and transmission electron microscopy (TEM) of a-SiO2 showed that the Au NP growth rate exhibits very little difference between thermally grown and plasma enhanced chemical vapour deposited (PECVD) thin layers. In the case of a-Si3N4, when deposited by PECVD the NP growth is very limited, accompanied by the formation of voids at the depth of maximum implantation damage and the formation of NPs with a mean diameter of 3 nm while the a-Si3N4 layer remains amorphous. FTIR showed the presence of Si-H absorbance bands after annealing suggesting a high thermal stability. When the a-Si3N4 layer is deposited by low pressure CVD (LPCVD), the layer undergoes an amorphous-to-poly-crystalline phase transformation. For the study of the ion shaping process of Au NPs embedded in a-SiO2, a-Si3N4 and at the interface of a-SiO2 and a-Si3N4 a good control of the initial NPs size is required. To achieve that, Au NPs were synthesised by depositing two layers of a-SiO2 (or a-Si3N4) of few hundred nanometres in thickness with a 5 nm thick Au layer in-between, followed by subsequent rapid thermal annealing (RTA). (iii) The ion-irradiation induced elongation of embedded Au NPs was carried out by SHI irradiation with 185 MeV Au ions. Transmission electron microscopy (TEM) revealed different elongation rates in a-SiO2 and a-Si3N4. For the former, an evolution from sphere to rod-like NPs with increasing fluence was observed, while for the latter, the Au NPs evolved towards faceted prolate NPs with smaller aspect ratios. When NPs are located at the interface between a-SiO2 and a-Si3N4, preferential elongation towards the a-SiO2 layer was observed with a limited elongation towards the a-Si3N4 layer. Numerical simulations based on the three-dimensional thermal spike (3D-TS) model showed, in the single layer case, a thermal spike duration of about 20 and 10 ps for a-SiO2 and a-Si3N4, respectively. The shorter lifetime and the thermal profile calculated for a-Si3N4 agree with the reduced efficiency of the elongation process observed. In the case of Au NPs located at the interface, the preferential diffusion of Au towards the ion track formed in a-SiO2 can also be explained by the difference of the thermal spike life-time between the two materials, and the reduced efficiency of the elongation process in a-Si3N4. Show more