Nanostructures produced by ultraviolet laser irradiation of silicon. II. Nanoprotrusions and nanoparticles

Two-dimensional ordered nanoprotrusion arrays were produced at silicon surface substrates by nanosecond ultraviolet laser irradiation using a Lloyd’s mirror set up. These protrusions are 40 to 70 nm high and have a diameter of ∼ 60 to 100 nm at their base, and in many cases display a regular rectangular lattice. Their origin and evolution were studied using scanning electron microscopy and atomic force microscopy. Nanoprotrusions originate from a subjacent laser-induced periodic surface structure upon continuing irradiation, under the same processing conditions that produced these ripples. Their evolution is discussed in terms of fingering instabilities of melted silicon consistent with a gradient of surface tension due to a temperature gradient. The temperature gradient is produced by the same mechanism responsible for the ripple formation. At slightly higher laser fluences, nanoparticles were observed to form using a single beam of nonpolarized laser light. The nanoparticles also span a linear ordered array, with line spacing that conforms to the grating equation. Their formation mechanism has been described previously as a result of ablation and redeposition, and thus is widely different from the formation of nanoprotrusions. The differences and similarities of nanoprotrusions and nanoparticles, and their connection with nanoripples, were studied in detail. In particular, when the ripple structure was still seen, nanoprotrusions were observed to form on ripple crests while nanoparticles were located in ripple valleys. Nanoprotrusions remained stable under a 2 h thermal annealing at 1073 K . By contrast, the aligned nanoparticle arrays became disordered as a result of a 2 h anneal at a temperature as low as 423 K , as nanoparticles moved on the surface.