Fabrication of Micro- and Nano-craters on the Surface of GaN Substrates by Using Wet-chemicals Assisted Femtosecond Laser Ablation

We have investigated micro and nano fabrication of wide-bandgap semiconductor Gallium nitride (GaN) using a femtosecond (fs) laser. Nanometer scale crater is successfully formed by wet-chemicals assisted fs laser ablation, in which the laser beam is focused on single-crystal GaN substrates in a hydrochloric acid solution. This method can efficiently remove the ablation debris due to chemical reactions, resulting in high quality ablation. On the other hand, a two-step processing method, i.e., irradiation with fs laser in air followed by wet etching, distorts the shape of crater because of the residual debris. Threshold fluence for wet-chemicals assisted fs laser ablation is lower than that for fs laser ablation in air. This effect is advantageous to the improvement of fabrication resolution. At present, we have achieved the fabrication of crater as small as 510 nm by using a high NA (0.73) objective lens.