Raman and photoluminescence spectroscopy of free-standing GaN separated from sapphire substrates by 532 nm Nd:YAG laser lift-off

Gallium nitride (GaN) thin-films grown on sapphire substrates were lifted onto silicon wafer using a laser lift-off (LLO) process. The pulsed second harmonic (532 nm) Nd:YAG laser used in our LLO experiments had a photon energy (2.33 eV) much smaller than the band gap energy of hexagonal GaN (3.41 eV). The free carriers in GaN absorbed the laser subsequently heated up the material. This caused the lift-off of GaN from the sapphire substrate. Raman spectroscopy performed on the as-grown and lift-off GaN materials revealed that LLO processed GaN had changed some GaN from hexagonal to cubic form. Low temperature photoluminescence (PL) also showed a peak at 376 nm, which further confirms the existence of cubic GaN. While the exact mechanism for causing this phase transformation is yet to be identified, we tentatively attribute this unexpected phenomenon to the highly non-equilibrium heating and cooling cycles taking place during the pulsed LLO process.