Optical excitations of a self-assembled artificial ion

By use of magnetophotoluminescence spectroscopy, we demonstrate bias-controlled single-electron charging of a single quantum dot. Neutral, single, and double charged excitons are identified in the optical spectra. At high magnetic fields one Zeeman component of the single charged exciton is found to be quenched, which is attributed to the competing effects of tunneling and spin-flip processes. Our experimental data are in good agreement with theoretical model calculations for situations where the spatial extent of the hole wave functions is smaller as compared to the electron wave functions. Semiconductor quantum dots ~QD’s ! are often referred to as artificial atoms. Different levels of neutral occupancies in QD’s have been obtained in the last years by power dependent optical excitation. The associated few-particle states were intensively studied by multiexciton photoluminescence ~PL! spectroscopy and corresponding theoretical investigations. 1‐7 Occupancies with different numbers of electrons and holes result in charged exciton complexes. In analogy to QD’s with neutral occupancy—artificial atoms— charged exciton complexes may be considered as artificial ions. In the field of low-dimensional semiconductors charged excitons were first observed in quantum-well structures. 8 In QD’s, charged excitons were studied in inhomogeneously broadened ensembles by PL ~Ref. 9! as well as in interband transmission experiments, 10 and recently also in single, optically tunable QD’s, 11 as well as in electrically tunable quantum rings by PL. 12 Few-particle theory predicts binding energies for charged QD excitons on the order of some meV. 11,13 This allows for the controlled manipulation of energetically well-separated few-particle states under the action of an external gate electrode. Discrete and stable numbers of extra charges are thereby possible via the Coulomb blockade mechanism. In future experiments and possible applications, the resonant optical absorption and emission of such systems is expected to be tunable between discrete and characteristic energies. Moreover such few-particle systems are expected to exhibit an interesting variety of spin configurations, which can be controlled by an external magnetic field, gate-induced occupancy, and spin-selective optical excitation. In the present paper we present, for the first time to our knowledge, experimental and theoretical results on the gate-controlled charging of a single InxGa12xAs QD with an increasing number of electrons probed by magneto-PL. For controlled charging of individual QD’s a special electric-field tunable n-i structure has been grown by molecular-beam epitaxy. In0.5Ga0.5As QD’s are embedded in an i-GaAs region 40 nm above an n-doped GaAs layer (5 310 18 cm 23 ) which acts as a back contact. The growth of the QD’s is followed by 270-nm i-GaAs, a 40-nm-thick Al0.3Ga0.7As blocking layer, and a 10-nm i-GaAs cap layer. As a Schottky gate we use a 5-nm-thick semitransparent Ti layer. The samples were processed as photodiodes combined with electron-beam-structured shadow masks with apertures ranging from 200 to 800 nm. Schematic overviews of the sample and the band diagram are shown in Figs. 1~a! and 1~b!. The occupation of the QD with electrons can be controlled by an external bias voltage VB on the Schottky gate with respect to the back contact. For increasing VB the band flattens, and the electron levels of the QD are shifted below the Fermi energy of the n-GaAs back contact, which results in successive single-electron charging of the QD. In our experiments excitons are generated optically at low rate and form charged excitons together with the VB induced extra electrons in the QD. We used a HeNe laser ~632.8 nm! for