SELECTIVE UV-LASER PROCESSING FOR LIFT-OFF OF GaN THIN FILMS FROM SAPPHIRE SUBSTRATES

Gallium nitride (GaN) thin films on sapphire substrates were successfully separated and transferred onto Si substrates by pulsed UVlaser processing. A single 600 mJ/cm, 38 ns KrF excimer laser pulse was directed through the transparent substrate to induce a rapid thermal decomposition of the GaN at the GaN/sapphire interface. The decomposition yields metallic Ga and N2 gas that allows separation of the GaN film from the substrate. Three-micron-thick free-standing GaN membranes were also fabricated using the laser lift-off technique. Surface roughness of the exposed interfacial layer was measured to be ~24 nm (rms) by atomic force microscopy. Photoluminescence measurements of the GaN membranes showed no optical degradation of the GaN after liftoff from the sapphire. Based on a 10 meV red-shift of the donor-bound exciton peak, an estimated biaxial compressive stress of ~0.4 GPa in the GaN film was relieved by separation from the sapphire growth substrate.