Optical spectroscopy of single quantum dots at tunable positive, neutral, and negative charge states - art. no. 165301

We report on the observation of photoluminescence from positive, neutral, a nd negative charge states of single semiconductor quantum dots. For this pu rpose we designed a structure enabling optical injection of a controlled un equal number of negative electrons and positive holes into an isolated InGa As quantum dot embedded in a GaAs matrix. Thereby, we optically produced th e charge states -3, -2, - 1, 0, +1, and +2. The injected carriers form conf ined collective "artificial atoms and molecules" states in the quantum dot. We resolve spectrally and temporally the photoluminescence from an optical ly excited quantum dot and use it to identify collective states, which cont ain charge of one type, coupled to few charges of the other type. These sta tes can be viewed as the artificial analog of charged atoms such as H-, H-2 , H-3 and charged molecules such as H-2(+) and H-3(+2). Unlike higher dimen sionality systems, where negative or positive charging always results in re duction of the emission energy due to electron-hole pair recombination, in our dots, negative charging reduces the emission energy, relative to the ch arge-neutral case, while positive charging increases it. Pseudopotential mo del calculations reveal that the enhanced spatial localization of the hole wave function, relative to that of the electron in these dots, is the reaso n for this effect.