An X-ray diffraction and Raman spectroscopy investigation of AlGaN epi-layers with high Al composition

The AlxGa1-xN (0.15 ≤ x ≤ 0.87) epi-layers were grown on c-plane sapphire substrate by metal organic chemical vapor deposition (MOCVD). Based on a combination of X-ray diffraction, optical transmission and Raman spectra experiments, it is shown that the structural and optical properties of AlxGa1-xN epi-layers are governed by the presence of compositional fluctuations associated with strongly localized electronic states. The asymmetric behavior of the Raman E2 mode of AlxGa1-xN epi-layers was attributed to the activation of phonons arising from the disordered state of the alloys. Hydrostatic pressure increases the splitting of A1GaN phonons mode especially for AlGaN epi-layer with high Al composition.