Raman scattering by the E2h and A1(LO) phonons of InxGa1−xN epilayers (0.25 < x < 0.75) grown by molecular beam epitaxy

Abstract

We use Raman scattering to investigate the composition behavior of the E2h and A1(LO) phonons of InxGa1−xN and to evaluate the role of lateral compositional fluctuations and in-depth strain/composition gradients on the frequency of the A1(LO) bands. For this purpose, we have performed visible and ultraviolet Raman measurements on a set of high-quality epilayers grown by molecular beam epitaxy with In contents over a wide composition range (0.25 < x < 0.75). While the as-measured A1(LO) frequency values strongly deviate from the linear dispersion predicted by the modified random-element isodisplacement (MREI) model, we show that the strain-corrected A1(LO) frequencies are qualitatively in good agreement with the expected linear dependence. In contrast, we find that the strain-corrected E2h frequencies exhibit a bowing in relation to the linear behavior predicted by the MREI model. Such bowing should be taken into account to evaluate the composition or the strain state of InGaN material from the E2h peak frequencies. We show that in-depth strain/composition gradients and selective resonance excitation effects have a strong impact on the frequency of the A1(LO) mode, making very difficult the use of this mode to evaluate the strain state or the composition of InGaN material.