One and two-dimensional arrays of nanoripples and nanoprotrusions can be generated by pulsed laser irradiation. Nanoprotrusions were produced on the surface of silicon when a Lloyd’s mirror configuration was used, and could develop to a height of 80 nm. Atomic force and high resolution scanning electron microscopy studies revealed that these structures are preceded by the formation of extended ripples that can reach a height of up to 12 nm. It is shown that one or two identifiable arrays of mutually orthogonal ripples may form. Out of the four differently-spaced ripple arrays that were found, three indicated a very close connection with the production of laser-induced periodic surface structures (LIPSS). Interference of the incoming or refracted laser beam and the laser light scattered by surface undulations has been recognized as the cause of LIPSS formation. The present experiments show that the Lloyd’s mirror configuration strongly enhances the formation of ripples and that nanoprotrusions form, sometimes at the intersection of two mutually orthogonal sets of ripples. Each of the beams, the directly incident and the mirror-reflected beam, independently produce an interference pattern with their corresponding scattered beams. However, these two independently generated patterns coincide because their periodicity is only a function of the angle of incidence. Thus, the two interference patterns reinforce each other’s effects on the substrate. Single beam, direct irradiation of the substrate failed to induce nanoprotrusions, although 1-D ripple arrays developed.
Analyses of the data indicated that the formation of the ripples is due to a hydrodynamic process while a very thin layer of silicon remains melted. It was concluded that the gradient of surface tension is responsible for the formation of the ripple structure and that the break-down of the ripples into aligned protrusions is due to a temperature modulation along the ripple lines. The intersection of two mutually perpendicular ripple structures and the very high reflectivity of the nanoprotrusion tips are two of the causes that promote this secondary modulation of laser light.
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Guan, Y., Pedraza, A. Synthesis and Characterization of Self-Organized Nanostructure Arrays Generated by Laser Irradiation. MRS Online Proceedings Library 818, 112–117 (2004). https://doi.org/10.1557/PROC-818-M11.47.1