Infrared Absorption in Some II-VI Compounds Doped with Cr

The optical absorption spectra of substitutional ${\mathrm{Cr}}^{2+}$ ($3d^4$) in single crystals of ZnS (cubic and mixed cubic-and-hexagonal), ZnSe, ZnTe, CdS, and CdTe have been measured for Cr concentrations of ${10}^{18}$-${10}^{20}$ ${\mathrm{cm}}^{-3\mathrm{}}$ and at 2, 15, and 300°K in the wave-number range $5\le \overline{\nu }\le 30000$ ${\mathrm{cm}}^{-1\mathrm{}}$. The room-temperature spectrum in the range $500\le \overline{\nu }\le 30000$ ${\mathrm{cm}}^{-1\mathrm{}}$ is dominated by a broad absorption band at 5500 ${\mathrm{cm}}^{-1\mathrm{}}$ arising from the ${}_{}{}^5{}_{}{}^{}T_2^{}{}_{}{}^{}\to {}_{}{}^5{}_{}{}^{}E$ transition. These two levels originate from the crystal-field splitting of the ${}_{}{}^5{}_{}{}^{}D$ free-ion ground state. At helium temperatures distinct lines appear on the low-energy side of the ${}_{}{}^5{}_{}{}^{}T_2^{}{}_{}{}^{}\to {}_{}{}^5{}_{}{}^{}E_2^{}{}_{}{}^{}$ absorption band in all samples except for CdTe. These lines are associated with zero-phonon transitions and phonon-assisted transitions. As the temperature is increased, the relative intensities of some of the lines change due to the thermal population of low-lying levels belonging to the orbital $T_2$ state. In the case of ZnSe these levels have been studied in the far infrared in the range $5\le \overline{\nu }\le 200$ ${\mathrm{cm}}^{-1\mathrm{}}$, both with and without a magnetic field. An interpretation of the results is made based on crystal-field theory with a strong, static, Jahn-Teller distortion of $E$ symmetry in the ${}_{}{}^5{}_{}{}^{}T_2^{}{}_{}{}^{}$ orbital ground state. There is little or no Jahn-Teller effect in the ${}_{}{}^5{}_{}{}^{}E$ orbital excited state. The optical transitions exhibit a strong coupling to $E$-mode phonons of average energy ∼70 ${\mathrm{cm}}^{-1\mathrm{}}$.