Resonant Raman Scattering in Strained and Relaxed In_xGa_1-xN/GaN Multiple Quantum Wells

We present resonant Raman scattering measurements on strained and relaxed InxGa1-xN/GaN multiple quantum wells. The pseudomorphic sample does not show significant deviation of the A1(LO) phonon frequency with respect to GaN value due to a strong compensation of composition and strain effects which makes the frequency of this mode almost independent on In concentration. In contrast, the relaxed sample shows a marked decrease of the Raman frequency. Raman spectra excited in the energy range of sample emission have been recorded at room temperature. The resonant conditions have been attained using tuneable lasers in the blue-green spectral region. Resonant profiles are significantly blue-shifted with respect to the photoluminescence emission as a result of an inhomogeneous In distribution. In relaxed multiple quantum well, the Raman shift of the A1(LO) mode and the maximum of the resonant Raman profile give a direct estimate of the In concentration and its variation range.