Formation process of surface hydrogenated Si nanocrystallites and its optical properties.

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Formation process of surface hydrogenated Si nancrystallites by pulsed laser ablation in hydrogen gas and properties of deposited material are clarified in this research. The deposit prepared by this method consists of nanocrystalline primary structure and aggregated secondary structures. The size of primary structure is insensitive to the background gas pressure, while the structure of secondary structure depends on the background gas pressure. From the analysis of the fractal structure, it is clarified that correlation between plume size and substrate-target distance determines the secondary structure. Hydrogen gas pressure acts as parameter to control the plume size. When the plume size is smaller than the substrate target distance, cluster-cluster aggregation in the plume results in fiber like aggregated secondary structure. This result indicates that the aggregated secondary structure can be controlled by the plume size though the hydrogen gas pressure. The nanocrystallites are not an alloy of silicon and hydrogen but are surface hydrogenated silicon nanocrystal. This means that the hydrogenation takes place after the formation of the nanocrystal. The temperature to stabilize Si-H bond is lower than the melting point of the silicon nanocrystal. Therefore, hydrogenation starts after the formation of the nanocrystallites. Eventually, the Si-H bonds are stabilized on the surface. The porosity of the hydrogenated sample is larger than that of the nonhydrogenated one. This is because surface hydrogenated nanocrystal prevent aggregation. The optical band gap of the surface hydrogenated sample is larger than that of nonhydrogenated one. This band gap shift corresponds to the porosity of the sample. The surface hydrogenation stabilizes the surface and reduces the interaction between the nanocystallites. The pulsed laser ablation of silicon in hydrogen gas is a promising method to control the secondary structure and optical properties of the material.