Abstract

The origin of the red shift of the excitonic luminescence with respect to the absorption peak (Stokes shift) is explained in terms of a thermal population of the inhomogeneous distribution of excitonic states, contrary to the usually accepted picture of exciton trapping.^1-4 We suppose that the photogenerated carriers, after momentum and energy relaxation, populate the excitonic states with a quasi-equilibrium distribution described by an effective temperature T_c. Then, if the inhomogeneous broadening is small compared with the thermal energy KT_C each exciton state has essentially the same thermal population: the PL reflects the absorption and the SS is not observable. On the contrary, if the excitonic linewidth exceeds the thermal energy, the low energy side of the inhomogeneous exciton band is enhanced by the thermal population and the SS appears. On the basis of such a simple model we derive a general relationship between the Stokes shift S and the absorption FWHM Δ, namely S = 0.18 Δ²/KT_C, which gives a quadratic dependence on Δ, in contrast with the linear behaviour found in Refs. 3 and 4 as a consequence of the exciton trapping at each local minima. Further-more we predict a (KT_C)^–1 dependence on the photogenerated carrier temperature T_c, against the thermally activated detrapping proposed in Ref. ².

© 1994 IEEE