Improvement of Resolution in Nano-fabrication of GaN by Wet-chemical-assisted Femtosencond Laser Ablation

Nano-fabrication technique for wide band-gap semiconductor gallium nitride (GaN) using a second harmonic of near infrared femtosecond (fs) laser (lambda= 387 nm, 150 fs) has been investigated. We have carried out a wet-chemical-assisted fs laser ablation method, in which the laser beam is focused onto a single-crystal GaN substrate immersed in a hydrochloric (HCl) or a phosphoric (H3PO4) acid solution. The highest quality and best uniformity with less residual debris were obtained by using a concentrated HCl solution and thus nanoscale craters as small as 320 nm at full width at half maximum were formed by this technique. The second harmonic used in this work enhanced the fabrication resolution compared with the case of a fundamental beam of fs laser used in the previous report. We have also demonstrated the formation of two-dimensionally (2D) periodic nanostructures on surface of a GaN substrate using the second harmonic single fs-laser pulse of 110 nJ. Ablation craters show a symmetrical-round shape with a diameter as small as 670 nm and with a depth of 60 nm. Such structures are applicable to fabrication of 2D photonic crystals which improve the light extraction efficiency for blue LEDs in the near future.