GaN surface ablation by femtosecond pulses: atomic force microscopy studies and accumulation effects

Laser-induced ablation and surface processing are investigated in GaN epitaxial layers irradiated by 130 - 150 fs pulses at wavelength of 398 and 795 nm. These layers are important materials in optoelectronics and microelectronics. GaN is an inert and hard substance with very limited possibility of wet etching. Laser processing seems to be suitable for different steps of fabrication of nitride-based devices (mesa shaping, grooving, scribing, mirror and surface grating preparation, etc.). Atomic force microscopy is used for a high-resolution investigation of initial and irradiated GaN surface. Imperfections of the mirror-like as-grown (0001) surface are identified (monolayer steps, open-core dislocations). The laser-induced damage threshold (LIDT) is determined at 398 and 795 nm. The dependence of LIDT(N) on the number of shots, N, is tested in accordance with the phenomenological relation LIDT(N) equals LIDT(1)N^((gamma -1)) in order to reveal the defects accumulation in an ablation process of GaN surface ((gamma) equals 1 corresponds to an accumulation-free ablation). The damage of GaN is found to be free of accumulation effect at low pulse energy below 0.6 X LIDT(1) at 795 nm and some effect is found at higher energy with (gamma) equals 0.86 at multi-shot irradiation conditions. A successful surface processing of GaN epitaxial layer (grown by MOCVD technique) is demonstrated for a single-pulse laser ablation of a typical energy 4 - 6 times higher than LIDT(1). The ablation depth up to 500 nm is achievable. The edge is almost vertical and the rim of a laser-ablated channel is free of debris and peel-off. There is an experimental evidence of a successful laser processing by two-photon absorption of femtosecond pulses at GaN surface with photon energy in a transmission region of the material. Photoluminescence (PL) of the defect-related Y-band at 550 - 600 nm was not enhanced at the rim of ablated region when energy of ablated pulses was 3 X LIDT(1). The PL of GaN was excited by two-photon absorption of 110 +/- 10 fs illumination at 726 nm.