Homogeneous growth of self-assembled Si nanopillar array via an asymmetric overlap of a spot beam laser

Nanosecond ultraviolet (UV) laser irradiation of Si thin films with a Gaussian beam was employed to study the transition from laser-induced periodic surface structures (LIPSS) to nanopillar formation. Single-pulse irradiation produced concentric modification regimes, ranging from weak melting to complete nanoparticle coverage and ablation, depending on local fluence. Under multi-pulse one-dimensional (1D) scanning, incubation was quantified via the evolution of the modified diameter, enabling determination of an incubation-corrected threshold fluence F th and an upper fluence F r for ripple stability. By combining these thresholds with analytical expressions for the effective modification width, we derived conditions that distinguish single- and double-strip nanoparticle-ripple regions under overlapped Gaussian spots. Two-dimensional (2D) raster scanning revealed that well-defined ripples and vertically aligned nanopillars form only within a confined fluence window and an optimal accumulated fluence F acc = 16 - 17 J/cm 2 . Higher accumulated fluence leads to overmelting and irregular morphologies. These results demonstrate that asymmetric 2D scanning - initial 1D x-scans to form uniform ripple seeds followed by y-shifts for controlled sequential growth - enables deterministic, large-area fabrication of uniform Si nanopillars using industrially relevant nanosecond laser processing.