Comprehensive analysis of the temperature dependence of Fe:ZnSe, Fe:ZnMnSe, and Fe:CdMnTe absorption over the 4 to 400K range

The temperature dependence of Fe²⁺ ions in single crystals ZnSe, Zn1-xMnxSe (x ≈ 0.2), and Cd1-xMnxTe (x ≈ 0.375) was systematically investigated over a wide temperature range of 4 to 400K, allowing for a detailed comparative analysis. The study focused on the structure of individual absorption peaks, examining temperature-induced variations in both the absolute values of the absorption coefficient and the full width at half maximum (FWHM) of the peaks. Two cryostats were used: one cooled with liquid helium (4 to 300K) and the other with liquid nitrogen (77 to 400K). Both were equipped with CaF₂ windows to facilitate mid-infrared transmission measurements. The absorption spectrum reveals distinct line features that gradually broaden as the host temperature increases. At 4K, the absorption spectrum of Fe:ZnSe exhibits several sharp zero-phonon lines corresponding to purely electronic transitions that occur without coupling to the lattice vibrations of the ZnSe host. On the contrary, much of the remaining absorption structure arises from phonon-assisted transitions, which are broadened because of phonon dispersion. Notably, these phonon-assisted transitions become the dominant contribution to the absorption spectrum at temperatures exceeding 30 to 40K. In comparison to ZnSe, phonon-assisted transitions were dominant in the mixed host even at very low temperatures.