Transmission electron microscopy investigation of crystalline silicon surface irradiated by femtosecond laser pulses in different background atmospheres

This article aims to obtain structural and compositional characteristics of a crystalline silicon surface irradiated by femtosecond laser pulses in SF₆, N₂, air, and vacuum background atmospheres by performing transmission electron microscopy observation of ??110?? cross-sectional specimens. Conical microstructures covered with defective outer layers were formed in SF₆ gas. The elemental sulfur dopants in the surface microstructure, which located in close proximity to defects, were mainly concentrated at the tip region of the microcones, and about several hundred nanometers thick. In N₂ atmosphere, the defects produced regularly on the silicon surface were of the same types with those formed in SF₆ gas and confirmed to be stacking faults and overlapped twins. Furthermore, silicon crystalline grains with different orientations were observed on the silicon surface irradiated in N₂, air, and vacuum atmospheres. Especially, ??-Si₃N₄ crystalline grains were found to be formed in N₂ and air as chemical products when elemental nitrogen exists, and the SiO₂ amorphous phase was formed in air by the oxidation effect. Based on these experimental results, the relevant interaction mechanisms between pulsed laser and crystalline silicon were suggested to be mainly attributed to laser-assisted chemical etching and laser ablation, i.e., if volatile silicon compounds can be produced in a reactive gas atmosphere (e.g., SF₆), the strong laser-assisted chemical etching dominates over the laser irradiation process. Otherwise, laser ablation is the dominant mechanism such as in N₂, air, and vacuum.