Development of Laser Lift-off Process with a GaN/Al0.7Ga0.3N Strained-Layer Superlattice for Vertical UVC LED Fabrication

David Doan, Shinji Nozaki, Kazuo Uchida

A laser lift-off (LLO) process with a GaN/Al_0.7Ga_0.3N strained-layer superlattice was newly developed for use in the fabrication of a vertical UVC LED without the use of UVC incompatible materials such as epoxy to suppress cracking. Since the UVC-LED epitaxial structures grown by Metal-Organic Vapor Phase Epitaxy contain AlGaN layers with high Al contents, it is often grown on an AlN buffer layer. In blue LEDs, GaN buffer layers are used for growth. However, GaN-based films often present a problem for UVC growth, resulting in cracking caused by lattice mismatch.  AlN layers are transparent to UV lasers utilized in the LLO process and thus making lift-off of the sapphire substrate very challenging. This GaN/Al_0.7Ga_0.3N strained layer superlattice was employed to absorb the UV laser during the LLO process and suppress the dislocations climbing to the UVC-LED epitaxial structure grown on this layer allowing for a highly uniform and crack-free surface. UVC-LED structures were grown utilizing a GaN/Al_0.7Ga_0.3N strained layer superlattice inside a horizontal flow metal-organic vapor phase epitaxy reactor. Copper substrates were then deposited onto the back surface of the wafers. LLO was achieved by employing a laser fluence of 1 J/cm² from a 248 nm excimer laser through the sapphire substrate. Successful LLO of a 2” sapphire substrate was attained without any cracking introduced when using this process. No deterioration of crystal quality in UVC-LED epitaxial structure such as dislocations and intermixing of atoms by LLO was also confirmed by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy analysis.