Simulation and experimental research on top-hat shaped femtosecond laser processing of GaN

The wide bandgap semiconductor material gallium nitride (GaN) has a wide range of applications in optoelectronic devices. As a consequence of the evolution of the device, the conventional processing techniques are no longer capable of fulfilling the necessary requirements. The utilisation of a top-hat femtosecond laser can lead to a significant enhancement in the quality and efficiency of the processing, due to the benefits conferred by the short pulse, high peak power and uniform energy distribution. In this paper, a multi-physics field model is established using COMSOL Multiphysics® to study the thermal field dynamics and the spatial and temporal variations in free electron density on the surface of GaN following irradiation by a top-hat femtosecond laser. The aim is to gain insight into the influence of this process on the optical properties and ablation morphology. The model is then validated by comparing the results with experimental data, which demonstrate the grooving process of GaN with a flat surface edge, a smooth bottom of the groove, and a perpendicular sidewall. The findings indicate that alterations in the free electron density impact the optical characteristics of the material surface. The distribution of free electrons within the material is a determining factor in the ablation morphology. The experimental results support the hypothesis that the top-hat laser has significant potential for use in the preparation of GaN devices.