Study of Vibrational Properties of InGaAsP by Far Infrared Reflectivity

We measured room-temperature reflectivity in the far-infrared region (100-700 cm- ’) of In,-,Ga&P1-, films grown by metalorganic phase epitaxy on InP substrate. The actual As content of the quaternary alloy (ranging from y=O.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.