Experimental study of the optical analogue of the Mossbauer effect: Effect of the temperature on the no-phonon line of cobalt impurity luminescence in II-VI compounds

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Abstract
Luminescence spectra due to the 4T2 ( Gamma 8phi ) to 4A2 ( Gamma 8) no-phonon crystal-field transition are analysed in ZnS, ZnSe, ZnTe, CdTe and CdS doped with cobalt. The no-phonon line intensity, extrapolated to 0K, ranges from 0.020 (for CdTe:Co) to 0.24 (for ZnS:Co) of the total luminescence intensity. The effect of temperature on the intensity f and position Omega of the no-phonon line is interpreted according to McCumber's model of the ion-phonon interaction. The numerical value of the quadratic coupling constant alpha h(cross) ranges from 4.5*10-2 cm-1 (for ZnS:Co) to 4.4*10-1 cm-1 (for ZnTe:Co), being greater for systems with smaller f values. However, the effect of temperature on the luminescence line halfwidth 2 Gamma could not be explained within the framework of the McCumber model. Numerical analysis of the experimental data was also made in the Debye approximation. In this model the thermal dependence of f and Omega was reproduced with values of the Debye temperature by some 20-30% lower than those characteristic of pure crystals.