Micro-Raman characterization of InxGa1-xN/GaN/Al2O3 heterostructures

InxGa1-xN/GaN/Al2O3 (0001) heterostructures with x=10.5%, 13.5%, 19.0%, 19.6%, and 26.5% are studied, by polarized micro-Raman spectroscopy, under plane and side backscattering geometries. The combination of both scattering geometries, together with variable excitation wavelengths, enabled the possibility to check independently strain-vs-depth distribution and selective-resonance effects from In-rich regions. Several Raman modes have been detected and were attributed to either the GaN or the InGaN films. Particular modes, which are not permitted in the bulk materials, are activated in the InGaN layers. Shifts of the frequencies relative to the ones expected in the bulk materials are explained as due to the elastic strains present in the hetero-structures. The results are evaluated in combination with compositional RBS analysis and the strain values obtained are compared with high resolution x-ray diffraction results including reciprocal space mapping, leading to very good consistency between Raman and XRD. Consequent relaxation values are obtained and the underlying mechanisms are discussed