Correlation between Natural Oxidation Process and Photoluminescence Properties of Hydrogenated Si Nanocrystallites Prepared by Pulsed Laser Ablation

Abstract

Natural oxidation processes of hydrogenated Si nanocrystallites were investigated to clarify effects of surface oxidation on photoluminescence wavelength. Hydrogenated Si nanocrystallites were prepared by pulsed laser ablation in hydrogen gas ambient. The Si–H bonds on the surface of the nanocrystallites enable us to estimate the local configuration of Si–O bonds using infrared frequency shifts. The natural oxidation process was investigated by measuring the density and local configuration of Si–O bonds. The oxidation process can be classified into first and second stages. The first stage is due to the diffusion of oxygen molecules in the nanocrystallites through voids in the porous structure, and the second stage is due to the oxidation of each nanocrystallite from the top surface to the sub-surface. The configurations of Si–O bonds in the first and second stages are silicon-rich and oxygen-rich compositions, respectively. The photoluminescence wavelength was blue-shifted with increasing Si–O bond density. This PL peak shift was not continuous, but three PL peak regions at 800, 600–700, and 400–500 nm were observed. This result indicates that the origin of this PL peak shift is not due to quantum confinement because of decreased diameter of Si nanocrystallites, but is due to the existence of surface oxide. A photoluminescence peak at 800 nm was observed in fresh specimens, and those at 600–700 and 400–500 nm were observed from the first and second stages of oxidation, respectively.