Study of vibrational properties of InGaAsP by far‐infrared reflectivity

We measured room‐temperature reflectivity in the far‐infrared region (100–700 cm−1) of In1−xGaxAsyP1−y films grown by metalorganic phase epitaxy on InP substrate. The actual As content of the quaternary alloy (ranging from y=0.29 to y=1) was derived by combining photoluminescence and high‐resolution x‐ray diffraction results. Over the whole compositional range four different vibrational branches, corresponding to InAs‐like, GaAs‐like, InP‐like, and GaP‐like mode, have been observed, thus confirming the attribution of ‘‘four‐mode behavior’’ to the quaternary alloy. The frequency position of the four main peaks has been calculated within a valence‐force‐field model. The frequency shift of these peaks in the alloy with respect to the bulk values has been explained taking into account anharmonicity and strain effects by means of a simplified elastic model.