Ultrafast processes simulation under femtosecond laser irradiation of Gallium Nitride thin films

Abstract

Lateral epitaxial growth is an effective method to reduce dislocations in Gallium Nitride (GaN) films. Compared to heteroepitaxy, the homoepitaxial growth films have lower stress. However, its application is limited due to the complex processes and high cost. In recent years, femtosecond laser technology has been applied in wide bandgap semiconductor processing increasingly due to its high instantaneous power density and excellent photon absorption rate. In the homoepitaxial growth of GaN, the femtosecond laser technology used for processing in-situ patterns is an effective method to reduce dislocations. Moreover, the laser equipment is relatively simple and compact, easy to be integrated into an in-situ growth cavity. However, there is still a lack of research on the mechanism of ultrafast laser processing of pattern windows on GaN substrates. In our study, the model of the transient nonlinear evolutions of femtosecond laser acting on GaN is established to solve partial differential equations with coefficients of the GaN film and the laser. The optical and electrical properties, such as the laser intensity and reflectivity, can be obtained. In addition, the ablation morphology of GaN can be predicted. The results show that under the action of the femtosecond laser with a power intensity of 0.6 J/cm2 and a wavelength of 1030 nm, numerous free electrons are generated on the GaN surface, and the reflectivity increases to 0.94. The electron temperature rises to 9750 K and then decreases. While the lattice temperature rises slowly, reaching 2060 K around 4 ps, which is in equilibrium with the electron temperature. The ablation pit on the thin film surface has a radius of 1 μm and a depth of 0.2 μm. The simulation results are consistent with the current theoretical and experimental results, which are helpful in further revealing the transient nonlinear mechanism of ultrafast laser acting on GaN. Under our simulation conditions, the femtosecond laser technology can be applied to process the sub-micron-sized patterns, which provides a more theoretical basis for preparing the GaN in-situ growth patterns.