Formation and Properties of Ion-Induced Nanoparticles in SiNx.

Abstract

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 addition, NCs may scatter acoustic phonons, thereby enabling engineering of the phonon mean free path and the resulting thermal conductivity. In this thesis, we examine the formation of metallic and semiconducting nanostructures using various focused-ion-beam irradiation doses followed by rapid-thermal-annealing. The mechanisms for nanocomposite formation and their influence on acoustic phonon resonances in SiNx are examined. For a range of high dose Ga+ irradiations of SiNx, we report on the formation of Si, SiN, Ga, and GaN embedded nanocrystals (NCs) as well as Ga-rich fractal morphologies. During ultra-high dose Ga+ 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. We discuss the effect of limited redeposition on NC formation and the catalytic effect of Ga on Si nucleation and growth. For a combination of low and medium dose Ga+ irradiation into SiNx, we report on the formation and coarsening of near-surface Ga nanoparticles (NPs). For surfaces with minimal curvature, diffusive growth is apparent. 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 on coherent acoustic phonon damping in SiNx membranes. Similar acoustic phonon damping was observed for the as-received, annealed, and as-implanted SiNx membranes whose thickness exceeds 200nm. For the thinner SiNx membranes, an increase in acoustic phonon damping is observed for both as-received membranes and those with nanocrystallites.