Spectroscopy of positively and negatively charged quantum dots: wave function extent of holes and electrons

We investigate semiconductor quantum dots by optically injecting a well controlled unequal number of electrons and holes into an isolated single dot. The injected carriers form charged complexes of many carriers in the dot. Radiative electron–hole pair recombination takes place after the charged complex relaxes to its ground state. We spectrally and temporally resolve the emission and show that it can be used to unambiguously determine the discrete charge states of the emitting quantum dot. In particular, we identify the emission from both negative and positive charge states of the same dot. We show that while negative charging results in red shifted emission energy, compared with a neutral dot, positive charging results in blue shifted emission energy. We explain this observation in terms of the better confined wave functions of the holes. Due to their smaller volume, the energy associated with hole–hole repulsion is larger than the combined energy associated with electron–hole attraction and electron–electron repulsion.