Infrared absorption in some II-VI compounds doped with Cr

The optical absorption spectra of substitutional Cr2+ (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) cm(-3) and at 2, 15, and 300 degrees K in the wave-number range 5 <=(nu) over bar <= 30 000 cm(-1). The room-temperature spectrum in the range 500 <= (nu) over bar <= 30 000 cm(-1) is dominated by a broad absorption band at 5500 cm(-1) arising from the T-5(2) -->E-5 transition. These two levels originate from the crystal-field splitting of the D-5 free-ion ground state. At helium temperatures distinct lines appear on the low-energy side of the T-5(2) --> E-5(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 <= (nu) over bar <= 200 cm(-1), 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 T-5(2) orbital ground state. There is little or no Jahn-Teller effect in the E-5 orbital excited state. The optical transitions exhibit a strong coupling to E-mode phonons of average energy similar to 70 cm(-1).