Micro-Raman imaging of GaN hexagonal island structures

We use Raman scattering to obtain a stress map of lateral epitaxy overgrown GaN. Isolated hexagonal islands are grown by selective area overgrowth without a seed layer. Stress mapping is obtained from shifts in the $E2$ phonon. GaN in the aperture area has the greatest biaxial compressive stress, ≈0.18 GPa. The overgrowth region is under slightly smaller stress, about 0.15 GPa. We attribute marked variations in the $A1(LO)$ phonon intensity to spatial variations in the free carrier concentration. This is found to be small in the aperture region and high in the lateral overgrowth. The position-dependent presence of the lower coupled plasmon–phonon band is consistent with this interpretation.