Lift-off of GaN LEDs with ultrashort laser pulses

Due to their high efficiency and compact design, GaN-based LEDs are nowadays the light source of choice for many applications. Sapphire is usually taken as a low-cost substrate for epitaxial growth of the InGaN/GaN heterostructure. However, it provides poor electrical and thermal conductivity, limiting its practicability for commercial highpower LEDs as well as for structured light sources. Therefore, a transfer of thin LED structures to more suitable substrates, e.g. rigid silicon or flexible polyimide, is applied. The detachment from sapphire is normally realized with a UV excimer laser, leading to linear absorption in the first layers of GaN.
In this project, we present the successful implementation of a liftoff process based on a versatile femtosecond laser system operating at a wavelength of 520 nm. The corresponding photon energy is too low for direct absorption, but non-linear processes  during the highenergy pulses enable separation of the LED from the original substrate. Luminescence experiments prove the functionality of the lifted chip, whereas SEM measurements reveal local laser induced damage on the surface. Possible mechanisms for these phenomena are discussed.