Infrared optical responses of wurtzite In_xGa_1−xN thin films with porous surface morphology

Room temperature infrared (IR) optical responses of wurtzite indium gallium nitride (In_xGa_1−xN) 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−xN samples. The E₁ optical phonon modes of the In_xGa_1−xN 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. - Highlights: • Composition dependence of E₁ modes of In_xGa_1−xN alloys (x = 0.174 to 0.883) • Dependence of porous morphology on infrared (IR) optical properties • Longitudinal phonon–plasmon coupling obscures determination of E₁(LO) mode.