Refractive index of multiline nanosecond laser-induced periodic surface structures and porous silicon

Abstract

To study the effect of multiline laser processing in the optical response of silicon, a set of p-type single-crystalline silicon wafers with 0.01 to 0.02 $\Omega$m resistivity, 525 $\mu$m thickness, and [111] orientation, was irradiated with a multiline Nd:YAG pulsed laser (1064, 532 and 355 nm) applying energies from 310 to 3100 J. A group of those surfaces was produced using argon gas blowing, while other group was manufactured in free atmosphere. Using confocal microscopy, it was observed that the gas-protected samples shown surface periodic structures in the form of ripples with an average pitch of 547 nm. Trough diffuse reflectance tests it was determined that proportionally to the energy supplied in the laser processing, these surfaces reflect between 10% and 30% in the UV region and between 60% and 80% in the IR region. On the other hand, the free atmosphere-made surfaces presented structures and diffraction properties characteristic of porous silicon (PS). The refractive index of the surfaces with periodic structures was calculated based on the diffuse reflectance measures while that of PS surfaces was calculated using the surface voids fraction (pores) determined with the confocal microscope image analysis software.