Infrared optical responses of wurtzite In_xGa_{1 - X}N thin films with porous surface morphology

Room temperature infrared (IR) optical responses of wurtzite indium gallium nitride (In_xGa_{1 - x}N) in the composition range of 0.174 ≤ x ≤ 0.883 were investigated by the polarized IR reflectance spectroscopy. Analyses of the amplitudes of oscillation fringes in the non-reststrahlen region revealed that the high frequency dielectric constants of the samples were unusually smaller than the values predicted from the Clausius-Mossotti relation. This odd behavior was attributed to the porous surface morphology of the In_xGa_{1 - x}N samples. The E₁ optical phonon modes of the In_xGa_{1 - x}N were deduced from the composition dependent reststrahlen features. The obtained values were compared to those calculated through the modified random element iso-displacement (MREI) model. The deviation between the measured data and the MREI model prediction were explained in detail from the aspects of strain, thermal expansion and anharmonic phonon-coupling. Finally, it was found that the large discrepancy of the E₁(LO) mode is mainly attributed to the effects of the longitudinal phonon-plasmon coupling.