When 9:30 AM - 11:30 AM Jan 24, 2014
Where 1690 Beyster Building
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Formation and Properties of Ion-Induced Nanoparticles in SiNx


Justin Canniff
Thesis Defense

Rachel S. Goldman, advisor

 

For many decades, there has been interest in low-dimensional structures (nanostructures) due to their expected unique physical properties. Due to the size dependence of their band gap energy, light emission, and free carrier confinement, NCs are promising for tandem solar cells, optical amplification medium, and memory applications. In this thesis, we investigate the formation ion-induced nanostructures in SiNx and their effect on coherent acoustic phonon damping. The formation and spatial positioning of Si, SiN, Ga, and GaN embedded nanocrystals (NCs) and near-surface Ga nanoparticles (NPs) in SiNx by Ga+ focused ion beam (FIB) irradiation and rapid thermal annealing (RTA) were investigated. The effect of Ga+ irradiation and membrane thickness on the acoustic phonon resonance in SiNx was also investigated.


We have investigated the formation of embedded nanocrystals (NCs) in SiNx. During irradiation, redeposition is enhanced by developing side walls, leading to enhanced near-surface [Ga] and [Si]. Subsequent RTA leads to the formation of Si and Ga NCs embedded in SiNx. When the ratio of the irradiated area to the sidewall area is increased, redeposition is limited, and SiNx and GaN NCs are also apparent. When the irradiation dose is decreased, Ga-rich fractals are apparent, with surface coverage and fractal dimension dependent on irradiation dose. We discuss the effect of limited redeposition on NC formation and the catalytic effect of Ga on Si nucleation and growth.


We have also investigated the formation and coarsening of near-surface Ga nanoparticles (NPs) in SiNx. For surfaces with minimal curvature, dose-limited diffusive growth is apparent, leading to nearly close packed arrays with NP diameters as small as 3 nm and densities as high as ~4x1012 cm-2. Following annealing at elevated temperatures, the diffusive flux is increased, leading to NP coarsening by Ostwald ripening. For surfaces with increased curvature, the driving force for diffusion towards the valleys also increases, leading to Ga NP coalescence and a bi-modal distribution of NP sizes.


Finally, we have investigated the effect of embedded NCs, with dimensions of up to ~500 nm, on coherent acoustic phonon damping in SiNx. The phonon wavelength is expected to be in the range of ~1 μm to ~100 nm for phonons with a frequency ranging from 10 to 100 GHz, leading to enhanced phonon scattering due to the similarity between the nanostructure dimensions and the phonon wavelength. Similar acoustic phonon damping was observed for the 200 nm unimplanted, unimplanted-annealed, and as- implanted samples. For thinner 100 nm samples, a ~30% increase in damping was observed. Both the FWHM as a function of frequency and the FWHM data suggest increased damping for the nanocrystalline sample relative to the amorphous samples. Further experiments could directly correlate nanostructures to damping of specific acoustic phonon frequencies, enabling engineering of the phonon mean free path that can allow for thermal conductivity minimization.