Plasmon-phonon interaction and surface optical mode in Cd1-xFexTe1-ySey single crystals

The interaction between electrons and phonons represents a notable phenomenon in the realm of condensed matter physics, exerting a substantial influence on diverse electronic and optical characteristics of materials. Within this context, an exhaustive investigation of the Raman and Far – Infrared reflectivity spectra of Cd_1-xFe_xTe_1-ySe_y single crystals can yield valuable insights into the various impacts of Plasmon – phonon interactions on the fundamental physics of II – IV semiconductors. Spectral analysis was executed employing a suitable fitting procedure. In the analysis of Far – infrared spectra, a dielectric function incorporating the presence of Plasmon – LO phonon interaction was employed. Three principal lines in the spectra, contingent upon the composition, were discerned at approximately 140 cm⁻¹, 170 cm⁻¹ and 200 cm⁻¹. 140 cm⁻¹ feature corresponds to the longitudinal – transverse (LO – TO) splitting of the CdTe – like mode. The 170 cm⁻¹ feature is associated with CdSe, while the 200 cm⁻¹ feature is linked to the local Fe mode. These features were elucidated within the framework of the modified random – element – isodisplacement (MREI) mode. The calculated phonon frequencies demonstrated a high level of agreement with experimentally determined values. Additionally, in all samples, a surface layer characterized by a low concentration of free carriers (depleted region) was formed. Consequently, a surface optical mode (SOP) was registered at approximately 150 cm⁻¹ in samples with a predominant CdTe component (y less that 15 %) and at around 190 cm⁻¹ in samples with a majority CdSe content (y greater than 95 %).