Nanostructure formation on silicon surfaces by high repetition-rate sub-15 femtosecond near-infrared laser pulses

We report on the formation of characteristic surface features on Si(100) surfaces, which were generated by 85 MHz, sub-15 fs pulsed Ti:Sapphire laser light at a centre wavelength of 800 nm. With the Si(100) surface immersed in water, the high peak intensity in the tight focus of a high-numerical aperture objective resulted in profound structural and compositional surface modification at a periodicity of 1.0 mu m. Oxide particles were formed at pulse energies below 0.3 nJ, whereas sub-10 nm hole arrangements surrounded by elevated areas were found at even lower focal intensities. The period of structural modulation was independent of the polarization of the laser light. On removal of SiO(2) by hydrofluoric acid etching, the silicon surface revealed tiny rifts oriented perpendicular to the direction of polarization, which were produced at near-threshold intensities at a period of 130 nm. In areas of higher exposure a random arrangement of structural elements of typically 20 nm in size was observed. In contrast, with the sample immersed in oil significant structural change of the Si(100) surface was not induced. However, filaments of carbon compounds at a diameter of approximately 100 nm were deposited periodically in the illuminated area.