Polarized infrared (IR) reflectance measurement was carried out to investigate the optical phonon modes of wurtzite structure In0.92Ga0.08N thin film grown by molecular beam epitaxy. Composition dependence of IR reststrahlen features was observed. Theoretical polarized IR reflectance spectrum was simulated using the standard multilayer optics technique with a multi-oscillator dielectric function model. By obtaining the best fit of experimental and theoretical spectrum, the Brillouin zone center E1 optical phonon modes together with the dielectric constant, layer thickness, free carriers concentration and mobility were extracted non-destructively. The extracted E1 optical phonon modes were compared with those generated from modified random element isodisplacement (MREI) model.

The lattice vibrational properties of wurtzite ternary mixed crystal aluminium indium nitride (AlxIn1-xN) are investigated thoroughly using modified random element iso-displacement (MREI) model and Born-Huang procedure. MREI model, which considers the nearest neighbour interactions, is used to predict the composition dependence of longitudinal and transverse optical phonon frequencies. For AlxIn1-xN alloy, oscillator strength of its weak mode is sufficiently significant for composition range of 0 < x < 0.4. As a result, AlxIn1-xN alloy is deduced to exhibit mixed-mode behaviour. Finally, the calculated dielectric functions for the entire composition range (0 < x < 1) are used to simulate the surface phonon polariton characteristics of the AlxIn1-xN alloy.