Far UV resonant Raman scattering in hexagonal Ga 1- xAl xN alloys

UV excited (244 nm) Raman spectra of Ga _{1- x}Al _xN layers (0< x≤0.85) grown on sapphire substrates by metal-organic vapor phase epitaxy have been acquired. In contrast with photoluminescence governed by deep center recombinations in aluminum-rich layers ( x > 0.55), strong multiple A ₁(LO)-phonon scattering mediated by Fröhlich electron-phonon interaction gives evidence for electronic excitations across the band gap as intermediate states of the Raman process over the whole compositional range. Alloying and temperature effects are used to obtain the resonant profile of the A ₁(LO) line intensity. The shift in line position, within spectra recorded under visible and ultraviolet excitation, is attributed to the probing of local compositional fluctuations by the long-range Fröhlich interaction.