Interplay between GaN and AlN sublattices in wurtzite AlxGa1-xN alloys revealed by Raman spectroscopy

The large frequency shift displayed by the longitudinal optical (LO) phonons A1(LO) and E1(LO) when going from GaN to AlN promises an accurate determination of the composition in AlxGa1−xN bulk layers by Raman spectroscopy. However, this determination is affected by a large uncertainty for low Al mole fractions (x<0.20), due to the broadened spectral line shape exhibited by these modes. A detailed study of Raman spectra recorded on layers with x<0.27, grown either on sapphire or on silicon substrates, has been performed in order to elucidate the origin of that broadening. The influence on the A1(LO) line shape of the sapphire substrate modes, compositional inhomogeneities, residual strain, and those effects inherent to the lattice dynamics of ternary alloys, is analyzed. We conclude that the broadening is caused by intrinsic inhomogeneities of the microscopic polarization fields resulting from alloying. This effect is usually obscured in other III–V compounds, such as arsenides or phosphides, due to their...