ns laser-Si interaction in air flow: The phenomena of surface etching and hetero-structuration

Abstract

This paper investigates interaction of ns laser pulses with n-type Si under two irradiation conditions: ambient air and flowing air at 16 ms⁻¹. This study was conducted using laser fluence of 1.5–2 J-cm⁻², and laser shots number n (6 ≥ n ≥ 1) at focus and off-focus. Two different topographical phenomena were observed; in flowing air, at focus surface etching dominates, while ripple formation occurred in both conditions. Both of them were strongly dependent on the laser shots and irradiation condition at focus and off-focus. In air flow, at focus or off-focus, randomly distributed etched pits form, increasing in size and population with more laser pulses. Off-focus in air flow, formation of etched patches was reduced and regular ripples transitioned into broken ripples as the shot number increased. Contrary to this, in ambient air, no etching was seen, and persistent ripples were formed, showing localised topographical interruption and fractured features with increased shots. The broken features were more dominant for the increased laser shots due to the incubation impact and the hydrodynamic instabilities of molten silicon. Raman analysis revealed a striking difference in properties of textured Si under two conditions. The distinct topographical features in air flow and ambient air reflect altered melt dynamics and ripple formation driven by laser interaction. It shows a critical role of air flow in selectively altering the surface topography through etching, ripple formation or a combination of both.