Zone center optical phonons in AlxGa1−xN mixed crystals

Abstract

Infrared reflectivity and Raman spectroscopy were used as two complementary experiments to probe the zone center optical phonons in the aluminum gallium nitride (AlxGa1−xN) systems. A careful analysis of the infrared reflectivity data using the Kramers-Kronig technique and classical dispersion theory gives several resonance modes shifting in frequency with the mole fraction of the aluminum content in the alloy. A signature of an additional resonance mode appears for x∼0.58. A geometrical arrangement was performed to observe the E2 and A1(LO) phonons from the first order Raman scattering. The variation of the frequencies of these phonons was followed in the compositional range investigated. In addition to these two phonons observed from the Raman scattering, a new phonon mode shows up for x∼0.4. Our experimental results were compared with a theoretical study of the lattice dynamics of random AlxGa1−xN alloy based on the generalization of the modified random-element isodisplacement and rigid-ion model in order to interpret the experimentally observed resonance modes. The two-mode behavior in the AlxGa1−xN alloy was observed only for the E1(TO) optical mode.