Laser induced formation of periodic nanostructures in silicon covered by SiO2

Three-dimensional (3D) and two-dimensional (2D) periodic silicon nanostructures formed by polarized focused Nd:YAG laser irradiation (532 nm) with spot size less than 3 mu m on Si covered by SiO2 are presented in this paper. We observed that at a low laser intensity 1 range, from I=0.9 to 1.08 W, 2D periodic coexisting of liquid and solid exists, while for 1.08 < 1 < 1.44 W, 3D periodic ripples were formed. However, when the light intensity is out of those ranges, either no melting was created (I < 0.9 W) or the periodicity was destroyed (1 > 1.44 W). The periodicity of these periodic structures is 359 nm related to the wavelength of frequency doubledNd:YAG laser and the index of refraction of SiO2. We propose a model based on the fact that as the oxygen is diffusing locally from SiO2 into the melted Si, thus forming SiO beta with a lower melting point, successive pulses melt preferentially these regions giving rise to a positive feedback. This dynamic nanoscale modeling, based on variations of melting points of Si and dielectric and reflection coefficient, confirms the experimental results.