Ultrafast laser lift-off technology for delamination of GaN layers

Most of nowadays light-emitting diodes (LEDs) based on gallium nitride (GaN) production use sapphire wafers as the growth substrate. However, sapphire exhibits limitations in terms of electrical and thermal conductivities, therefore, GaN must be transferred to more appropriate substrates to fulfill diverse requirements. At present, the transfer of GaN is mainly achieved by means of laser lift-off (LLO), which uses pulses of UV laser light in the nanosecond range. In this study, femtosecond LLO (fs-LLO) technique has been used for GaN delamination. Femtosecond pulsed laser with a wavelength of 800 nm (1.55 eV), and a pulse width of 50 fs was utilized for conducting the laser lift-off experiments, employing photon energy below the GaN band gap (3.4 eV). The reliance on multiphoton absorption and ultrashort pulses in fs-LLO minimizes structural damage compared to conventional LLO approaches. The effect of different laser power and different laser scanning speeds on the fs-LLO processing of GaN has been studied in detail. Various characterization methods, including optical microscopy, scanning electron microscopy, X-ray diffraction, and photoluminescence spectroscopy were used to observe the structural quality of the materials after fs-LLO. The results show that there is less thermal damage on the GaN surface when scanned with a smaller laser power. The scanning speed of the displacement stage affects the surface smoothness of the GaN. The outcome on the flexible tape yielded GaN with high surface quality with no noticeable degradation.