Composition dependence of infrared optical phonon modes in AlGaN epilayers grown on sapphire substrates

We reported the systematical study of optical anisotropy of Al_xGa_1-xN epitaxial films grown on sapphire using FTIR measurements. The experimental data can be theoretically fitted by employing a four-phase layered model. Several specific absorption dips were observed when the aluminum composition is larger than 24%. The dip intensity increases with the aluminum compositions. According to the reciprocal space map measurements, these dips may be induced due to the effects of strain relaxation in AlGaN epitaxial layers.