Chemical equilibrium between excitons, electrons, and negatively charged excitons in semiconductor quantum wells

The principle of chemical equilibrium is found to govern formation of negatively charged excitons ${(X}^{-})$ by excitons (X) and free electrons ${(e}^{-}).\mathrm{}$ An optical spectroscopy study of undoped ${\mathrm{C}\mathrm{d}\mathrm{T}\mathrm{e}/\mathrm{C}\mathrm{d}}_{0.88}{\mathrm{Mn}}_{0.12}\mathrm{Te}$ double quantum wells (QW’s) revealed formation of a negatively charged exciton state in the thicker QW due to the interwell tunneling of photoexcited electrons. A peculiar, nonmonotonic behavior of the ratio of X to $X^{-}$ densities as a function of the excitation density evidenced that the chemical law of action of masses corresponding to the reaction ${X+e}^{-}X^{-}$ is fulfilled.