Formation of decorated LIPSS on GaN thin film by UV picosecond laser beam in air and under vacuum

The formation of laser induced periodic surface structures (LIPSS) on GaN thin film (thickness ~ 1.6 μm) was investigated using a picosecond laser (pulse duration about 40 ps) at a wavelength of 355 nm. Through a parametric study (variations of pulse number from 10 to 1000 and of fluence from 130 up to 300 mJ.cm^− 2), the existence domain of low spatial frequency LIPSS (LSFL with period Λ = 265 nm) was determined. Optimized parameters were found to be a beam fluence of 230 mJ.cm^− 2 for 100 pulses to obtain highly regular LIPSS without damaging the film. Moreover, original LSFL structures on GaN thin film were found to be decorated with circular nano-structures (diameter ⌀ about 70 nm) referred to as nano-puddles. Investigation of the nano-puddles on the LIPSS was performed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Finally, the morphologies of the decorated LIPSS formed under vacuum (pressure P = 10^− 4 mbar) were compared to the one formed under atmospheric pressure and the phenomena leading to the formation of the nano-puddles are discussed.