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

研究了脉冲激光在氢气中烧蚀表面氢化硅纳米晶的形成过程和沉积材料的性能。用这种方法制备的镀层由纳米一次结构和聚集的二次结构组成。一次结构的大小对背景气压不敏感，而二次结构的大小取决于背景气压。通过对分形结构的分析，阐明了羽流尺寸与衬底-靶距离的相关性决定了二次结构。氢气压力作为控制羽流大小的参数。当羽流尺寸小于衬底靶距时，羽流中的团簇聚集导致纤维状聚集的二级结构。这一结果表明，通过氢气压力可以控制羽流的大小来控制聚集的二级结构。纳米晶不是硅和氢的合金，而是表面氢化的硅纳米晶。这意味着氢化发生在纳米晶体形成之后。稳定Si-H键的温度低于硅纳米晶的熔点。因此，氢化反应是在纳米晶形成后开始的。最终，Si-H键在表面稳定下来。氢化样品的孔隙率大于未氢化样品。这是因为表面氢化的纳米晶阻止了团聚。表面氢化样品的光学带隙大于未氢化样品的光学带隙。这种带隙移动对应于样品的孔隙率。表面氢化稳定了纳米晶的表面，减少了纳米晶之间的相互作用。脉冲激光在氢气中烧蚀硅是一种很有前途的控制材料二级结构和光学性质的方法。

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.