Femtosecond Laser Lift-Off with Sub-Bandgap Excitation for Production of Free-Standing GaN Light-Emitting Diode Chips

Laser lift-off (LLO) is commonly applied to separate functional thin films from the underlying substrate, in particular light-emitting diodes (LEDs) on a gallium nitride (GaN) basis from sapphire. By transferring the LED layer stack to foreign carriers with tailored characteristics, for example, highly reflective surfaces, the performance of optoelectronic devices can be drastically improved. Conventionally, LLO is conducted with UV laser pulses in the nanosecond regime. When directed to the sapphire side of the wafer, absorption of the pulses in the first GaN layers at the sapphire/GaN interface leads to detachment. In this work, a novel approach towards LLO based on femtosecond pulses at 520 nm wavelength is demonstrated for the first time. Despite relying on two-photon absorption with sub-bandgap excitation, the ultrashort pulse widths may reduce structural damage in comparison to conventional LLO. Based on a detailed study of the laser impact as a function of process parameters, a two-step process scheme is developed to create freestanding InGaN/GaN LED chips with up to 1.2 mm edge length and approximate to 5 mu m thickness. The detached chips are assessed by scanning electron microscopy and cathodoluminescence, revealing similar emission properties before and after LLO.