Far-infrared spectroscopy of CdTe1−xSex(In): Phonon properties

•Optical properties were investigated applying far-infrared spectroscopy.•Dielectric function which includes spatial distribution of free carriers were used.•The long wavelength optical phonons exhibit a two-mode behavior.•The local In mode at about 160cm−1 is observed.•Surface layers with a low concentration of free carriers are formed.