Excitons and multi-excitons in single CdTe quantum dots probed by near-field spectroscopy

A near-field optical spectroscopy study of a single CdTe/ZnTe quantum dot at low temperatures is presented. While the photoluminescence spectrum at low excitation power reveals only one single sharp peak due to the radiative recombination of excitons (X) in the single dot, several additional sharp peaks appear with increasing excitation density. The dominant features are ascribed to exciton complexes and charged exciton complexes such as negatively charged excitons (X−), neutral (2X and 3X) and negative (2X− and 3X−) biexcitons and triexcitons. Exciton charging arises due to efficient hole trapping by residual acceptors in the barrier material. This partly inhibits the formation of biexcitons and triexcitons. A spectral feature appearing close to the X− peak is tentatively assigned to X2− negative excitons. This feature is found to shift to the red with increasing power: two possible explanations for this unexpected behaviour are proposed.