Formation of nanoscale fine-structured silicon by pulsed laser ablation in hydrogen background gas

The formation of nanoscale fine structures during pulsed laser ablation of a silicon target in a hydrogen atmosphere has been studied by analyzing the deposited silicon fine structures prepared under different conditions. Transmission electron microscopy, scanning electron microscopy (SEM), Raman scattering, and infrared absorption studies on the deposited samples indicate that silicon nanocrystallites are produced when the background gas pressure is higher than a critical value. The deposited substance is found to show hierarchical structure having surface hydrogenated silicon nanocrystallites as the primary structure and aggregates of the nanocrystallites as the secondary structure. The secondary structure depends on the hydrogen background gas pressure, while the size of the primary nanocrystallites is 4-5 nm independent of the pressure. These results suggest that the fine structure is formed in two steps; the silicon nanocrystallites having a stable surface are initially formed and they are subsequently aggregated to form the secondary structure. Analysis of surface free energy suggests that the stability is acquired by termination of the surface by creation of Si - H bonds. We carried out fractal analysis of the SEM image of the deposits and found that the secondary structure shows good self-similar structure when deposited at higher background gas pressure. The fractal dimension of aggregated secondary structure varies from 1.7 to more than 2.0 with decreasing background gas pressure. Comparison of these values with reported results for the fractal growth simulation indicates that the region at which aggregation of the nanocrystallites takes place changes from in the plume to on the substrate with decreasing background gas pressure. Effects of the hydrogen background gas on the nanocrystallization process and spatial distribution of formed nanocrystallites in the plume are discussed. The formation of surface stabilized Si nanocrystallites and their spatial confinement by background gas in the first and second steps determine the hierarchical structure of deposited substance.