Probing proton irradiation effects in GaN by micro-Raman spectroscopy

Thick freestanding GaN substrates with low background free-carrier concentrations were subjected to 6 MeV beam proton irradiations. The beam penetration depth, incident perpendicularly to the c-axis, and the change in the carrier concentrations were accessed by monitoring the phonon-plasmon coupled mode by micro-Raman spectroscopy. Reduction of the Raman shift of the A ₁(LO)-plasmon coupled mode confirmed that the free-carrier concentrations decreased after proton irradiations. The penetration depth of 6 MeV protons was inferred from the point where phonon-plasmon coupled mode frequencies changed abruptly. The carrier concentrations in the GaN templates before irradiations were 4×10¹⁶ cm^-3. However, after proton irradiation, the concentration decreased to less than 1×10 ¹⁵ cm^-3 resulting from the incorporation of defects trapping free carriers. It was estimated from the lowest phonon frequencies that most of the irradiated protons stop near 180 μm, which was consistent with the results of Monte Carlo numerical simulations (SRIM). Most of the irradiation-induced lattice damage was recovered after 2 minutes of thermal annealing at 900 °C, as verified by the spectra of the A₁(LO)- plasmon coupled mode.