Fabrication of High-aspect-ratio Nanohole Arrays on GaN Surface by Using Wet-chemical-assisted Femtosecond Laser Ablation

A periodic array of nanoholes with high aspect ratios was successfully fabricated by using a multi-scan laser irradiation technique, in which each position in the array is irradiated one time per scan with a femtosecond laser pulse. This technique can easily avoid interference to the laser pulses by micro-bubbles generated in the wet-chemical-assisted ablation process. As a result, we obtained nano-sized ablation holes with an aspect ratio of 1.6, which is much higher than that possible with single pulse ablation. The depth of the ablation craters formed by multi-scan irradiation technique was found to saturate at higher pulse number, presumably due to the limited depth of focus. We also revealed that the ablated depth strongly depends on the refractive index of the medium facing the GaN substrate. In fact, compared to ablation in air followed by HCl etching, the wet-chemicalassisted ablation process enhances the aspect ratio by about 30 % because of the high refractive index of the HCl solution. In addition, by controlling the synchronization of the laser pulses and the motion of the piezo-stage, an almost perfectly periodic arrangement of nanoholes was fabricated on the GaN substrate. The technique developed in this study is likely to make a significant contribution towards realization of two-dimensional photonic crystal structures on GaN-based blue lightemitting diodes.