Fabrication of semiconductor quantum dot structures with a regular in-plane spatial distribution and optimum size uniformity is highly desirable for applications of their novel optical and electronic properties. This remains a challenging subject, especially on the nanometer scale. The strain-driven self-assembled formation of Ge islands on Si during molecular beam epitaxy (MBE) in the Stranski-Krastanov mode appears as the most promising method. An ensemble of Ge islands with random spatial distribution develops on flat Si(001) substrates. We have studied the growth of self-assembled Ge islands on vicinal Si(001) surfaces by MBE. Ge islands on vicinal Si(001) surfaces with a <110> miscut direction are compact pyramids with four equivalent {105} facets. Nearly perfect twodimensional ordering is achieved in a single layer of self-assembled Ge islands on vicinal Si(001) surfaces with a regular ripple caused by step-bunching. Islands with regular wellcontrolled size, shape and lateral separation are achieved by this self-assembling and selfordering process without any nanostructuring required.

Semiconductor quantum dots (QDs) are often referred as artificial atoms or ions due to possible occupation by neutral or charged multi-exciton complexes. Few-particle theory predicts binding energies in the order of some meV. Thus it is possible to manipulate the energetically well separated few particle states by using an external gate electrode and charge the QD states by a discrete and stable number of extra carriers due to Coulomb blockade mechanism. By means of photoluminescence (PL) spectroscopy bias dependent emissions of single QDs in two differently designed photodiode structures are compared. The presented experiments focus on the influence of different tunneling coupling of the QD states to the diode’s n region on the bias dependent PL spectra. For controlled charging of individual QDs two different electric field tunable n-istructures were grown by molecular beam epitaxy. The In0.5Ga0.5As QDs are embedded in an i-GaAs region 40nm above an n-doped GaAs layer which acts as back contact. The growth of the QDs is followed by 320nm intrinsic layer containing a 40 nm Al0.3Ga0.7As blocking layer. As a Schottky gate we use a 5 nm semitransparent Ti layer. In the second sample the distance between QD layer and the n-GaAs is half as wide (20nm). Therefore the samples are referred to as 40nmand 20nm-sample respectively. Both samples were processed as photodiodes combined with shadow masks structured by electron-beam lithography with apertures ranging from 200nm to 500nm. Schematic overviews of the sample and the different samples’ tunneling barriers are shown in Figs. 1 (a) and (b). The occupation of the QD with electrons can be tuned by an external bias voltage VB on the Schottky gate with respect to the back contact. For increasing VB the bands flatten and the electron states of the QD are shifted below the Fermi energy EF of the n-GaAs which leads to single electron charging of the QD. Excitons are optically generated by non-resonant excitation using a HeNe laser. The μ−PL is detected by a cooled CCD. In Fig. 2 (a) we present PL spectra of both samples as a function of VB in the range of -550 to +150mV at a magnetic field of 12T. The PL intensity is displayed as color plot. As a function of VB we find a series of lines with discrete jumps in the emission energy. Those lines result from radiative s-shell transitions of neutral X, single charged X, and double charged X excitons as marked in Fig. 2 (a). Due to the high magnetic field of 12T all lines split into two Zeeman components. The spectra for the two samples exhibit some major differences in their bias dependence. For the 20nm-sample there’s no parallel emission of two differently charged Fig. 1: (a) Photodiode combined with a near field shadow mask. (b) Schematic view of tunneling barriers 20nm sample 40nm sample

On the basis of a single semiconductor quantum dot incorporated in a photodiode we have succeeded in preparing a two-level system with electric contacts, a setup which was previously not attained on the basis of other two-level systems. By means of such a single quantum dot photodiode we demonstrate the transfer of coherent optical excitations into deterministic photocurrents. Rabi oscillations are shown to be directly reflected in the photocurrent. For optical excitation with pi-pulses we observe a quantitative photocurrent given by I=f /spl times/ e, where f is the repetition frequency of the experiment and e is the elementary charge.

Semiconductor quantum dots can be described as quantum mechanical two-level systems. Under the influence of strong electromagnetic driving fields and in the absence of decoherence such systems exhibit Rabi flopping corresponding to a qubit rotation in the context of quantum computing. Based on a single QD incorporated in a photodiode we have prepared a two-level system with electric contacts. By means of this single QD photodiode we demonstrate the transfer of coherent optical excitations into a deterministic photocurrent. The QD photocurrent directly reflects Rabi flopping of the exciton state. Under pi-pulse excitation the exciton occupation is inverted from 0 to 1 leading ideally to the creation of exactly one exciton per laser pulse. For this condition, the photocurrent is determined by the repetition rate of the experiment f and the elementary charge e, resulting quantitatively in I = f . e.

Quantum mechanical systems, like atoms, molecules, ions, spin-systems and recently also semiconductor quantum dots can be described as two-level systems. Under the influence of strong electromagnetic driving fields and in the absence of decoherence such systems exhibit Rabi oscillations. The Rabi flop of a two-level system by means of an optical π-pulse corresponds to an inversion of the system with respect to its initial state, which is equivalent to a qubit rotation in the context of quantum computing. On the basis of a single semiconductor quantum dot incorporated into a photodiode we have succeeded in preparing a two-level system with electric contacts, a setup which was previously not attained on the basis of other two-level systems, such as atoms. On such a single quantum dot photodiode we perform a photocurrent technique that enables us to monitor the occupation probability of the ground state exciton in a single quantum dot. In a first experiment we show that under the condition of resonant ground state excitation the tunneling-photocurrent saturates for high excitation densities. This photocurrent saturation reflects the incoherent saturation limit of a resonantly driven two-level system (excitonic occupancy =0.5). In a second experiment, we demonstrate the transfer of coherent optical excitations into a deterministic photocurrent. Rabi oscillations are shown to be directly reflected in the photocurrent. For the application of π-pulses we observe a quantitative photocurrent which is given by I=f·e, with the repetition frequency of the experiment f and the elementary charge e.

We report Rabi‐oscillations of the ground state exciton in a single self‐assembled quantum dot (QD). Experimentally, we apply a photocurrent (PC) technique, that enables us to monitor the occupation probability of the exciton state by measuring the dc tunneling current out of the QD. Coherent PC spectrocopy was performed by resonant ps‐excitation of the ground state exciton. Rabi‐oscillations are shown to be directly reflected in the PC. In the case of π‐pulse excitation, we receive a quantitative PC given by I = f × e , with the repetition frequency of the experiment f and the elementary charge e . In our experiment we nearly reach this maximum current showing that our experiment acts as a quantitative proof for Rabi‐flopping in a single QD. In a systematic study of field‐dependence we find Rabi‐oscillations with reduced PC amplitude.

We report the functionalization of semiconductor nanostructures near the surface with the self-assembled monolayer of octadecylthiol (ODT). In the first part of this study, photoluminescence properties of the near-surface quantum dots and quantum wells grown on GaAs [100] substrates were investigated as a function of the distance to the surface, and the luminescence signals were also systematically compared in terms of the confinement potential. In the second part, the ODT monolayer was deposited on the nanostructures based on indium arsenide quantum dots (InAs QDs). The monolayer deposition resulted in a significant enhancement in the photoluminescence from the QDs, which can be attributed to the effective suppression of the surface state densities by arsenide sulfide coupling. It is noteworthy that the enhancement in the luminescence signal was more significant for QDs closer to the surface. Since the hydrophobic surface of the monolayer can be functionalized with polymer films and model cell membranes, this strategy is applicable for the design of local detectors in the very proximity of the surface.

We report about level bleaching in the ground state of a single In0.5Ga0.5As quantum dot, which is observed in terms of a saturation of the photocurrent with increasing excitation power. Our results are explained by a rate equation model for the quantum dot as a two-level system.

Present-day information technology is based mainly on incoherent processes in conventional semiconductor devices 1 . To realize concepts for future quantum information technologies, which are based on coherent phenomena, a new type of ‘hardware’ is required 2 . Semiconductor quantum dots are promising candidates for the basic device units for quantum information processing. One approach is to exploit optical excitations (excitons) in quantum dots. It has already been demonstrated that coherent manipulation between two excitonic energy levels—via so-called Rabi oscillations—can be achieved in single quantum dots by applying electromagnetic fields 3 , 4 , 5 , 6 , 7 . Here we make use of this effect by placing an InGaAs quantum dot in a photodiode, which essentially connects it to an electric circuit. We demonstrate that coherent optical excitations in the quantum-dot two-level system can be converted into deterministic photocurrents. For optical excitation with so-called π-pulses, which completely invert the two-level system, the current is given by I = fe , where f is the repetition frequency of the experiment and e is the elementary charge. We find that this device can function as an optically triggered single-electron turnstile.

By means of photoluminescence spectroscopy we compare the bias-dependent emissions of single-quantum dots which are embedded in two differently designed photodiode structures. Controlled single-electron charging allows to identify neutral, single- and double-charged excitons in the optical spectra of both samples. 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 tunnelling and spin-flip processes. The strength of the tunnelling coupling between quantum dot and back-contact was found to have a strong influence on the observed spectral features—in particular, the parallel appearance of emission lines resulting from the radiative decay of differently charged quantum dot states is suppressed in case of strong tunnelling interaction.

Photoluminescence and structural properties of self-assembled InAs quantum dots, grown on AlyGa1-yAs, are studied for y = 0, 0.3, and 0.5. A maximum blue shift of 150 meV for the ground state: emission energy is determined with increasing y for the samples as grown, Increased surface density and size inhomogeneity is observed for growth on AlGaAs compared to growth on GaAs. Rapid thermal annealing at temperatures ranging from 550 to 850 degreesC is used to further increase the ground state emission energy with respect to the as grown samples. We find a shift up to a maximum emission energy of 1.9 eV for InAs quantum dots (QDs) embedded in Al0.3Ga0.7As compared to 1.4 eV for GaAs matrix material. To combine the higher spectral shift by annealing for InAs QDs embedded in AlGaAs with the lower dot density for the growth on GaAs, favored for single dot spectroscopy, we investigated InAs QDs grown on AlGaAs separated by 2 ML GaAs. For these samples we observe a maximum shift by annealing up 1.4 eV and intense room temperature PL.

Phonon-assisted absorption of a single InGaAs/GaAs quantum dot is investigated by multichannel photoluminescence excitation spectroscopy as a function of magnetic field. GaAs LO phonon-assisted generation of exciton and biexciton states is observed. Magnetic field induced detuning of the resonance conditions results in a twofold diagonal splitting of the exciton and a symmetric fourfold splitting of the biexciton resonances. Those findings are explained in terms of sequential phonon-assisted biexciton generation followed by sequential biexciton decay.

We have fabricated single-quantum-dot photodiodes by embedding InGaAs quantum dots in the intrinsic region of an n-i-Schottky diode combined with near-field shadow masks. As a function of the bias voltage, we study one and the same quantum dot in the two complementary regimes of photocurrent and photoluminescence. The Stark shift of the exciton ground state continues monotonically in both regimes, confirming nicely the observation of the same quantum dot in photoluminescence and photocurrent. In the limit of high electric fields, we observe a broadening of the photocurrent linewidth from which we determine a strongly reduced exciton lifetime of below 1 ps.

Photoluminescence (PL) of near surface InAs quantum dots (QDs) has been studied as a function of the distance to the surface (30, 20, 10, 6 nm). We observe a strong decrease in the QD PL intensity with decreasing barrier thickness. Nevertheless, the QDs still show reasonably strong PL intensity even when they are only 10 nm beneath the surface. After the deposition of self-assembled monolayers of octadecylthiol, we observe an increase in PL intensity up to a factor of 1.87. Such an enhancement is attributed to a decrease in the density of surface states. This demonstrates that near surface InAs QDs are very sensitive to changes of the surface conditions and the deposition of octadecylthiol monolayer may be used to increase their sensitivity, which is promising toward future bio-sensor applications.

By means of photoluminescence spectroscopy we compare the bias dependent emissions of single quantum dots which are embedded in two differently designed photodiode structures. Controlled single-electron charging allows to identify neutral, single- and double-charged excitons; in the optical spectra of both samples. The strength of the tunneling coupling between the quantum dots and the diode's n region is found to have a strong influence on the observed spectral features-in particular, the parallel appearance of emission lines resulting from the radiative decay of differently charged quantum dot states is suppressed in case of strong tunneling interaction.

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.

We report about magneto-optical experiments on single self-assembled InGaAs quantum dots. In the first part of our contribution we concentrate on phonon assisted absorption studied by photoluminescence excitation spectroscopy. We observe phonon assisted absorption via InGaAs and GaAs LO phonons, as well as sequential phonon assisted biexciton generation followed by sequential biexciton decay. The magnetic field dependence of the observed phonon resonances exhibits distinct differences between exciton and biexciton lines, which are caused by spin conservation and decay statistics. In the second part, we report about controlled single electron charging of a single quantum dot investigated by magneto-photoluminescence spectroscopy on electric field tunable structures. We observe the emission lines of neutral, single, and double charged exciton states for different bias conditions. The application of high magnetic fields results in fully resolved Zeeman splittings and diamagnetic shifts. The upper Zeeman component of the single charged exciton is found to be quenched at higher electric fields. This behavior is explained in terms of an enhanced tunneling probability of the triplet versus the singlet configuration.

We report about optical experiments on single self-assembled quantum dots. In power-dependent low-temperature magneto-photoluminescence experiments we have analysed the emission spectra of single dots for increasing exciton occupation numbers. Decays from different configurations (up to 4 excitons) lead to a renormalization of the emission lines in the region of the s- and p-shell of the dot. Photoluminescence excitation spectroscopy further allows us to explore the absorption properties of a single-quantum dot. Both interband absorption and comparably strong phonon-assisted absorption via InGaAs and GaAs LO phonons are observed, as well as sequential phonon-assisted biexciton generation followed by sequential biexciton decay. Working towards applications of single quantum dots, local charge injection/extraction into/from dots is performed on special p–i–n structures via a STM-tip. STM-induced luminescence from a single-dot results in single-line emission from the quantum dot ground state under the condition of low injection currents. Reverse operation of such a single quantum dot LED allows in addition for spectrally resolved photocurrent experiments. Characteristic sharp peaks in the STM tip current vs. excitation energy are attributed to resonant quantum dot interband absorption processes.

Gate-controlled charging of a single self assembled quantum dot is investigated by photoluminescence spectroscopy, For different bias conditions neutral, single and double charged quantum dot states are identified by analysis of the corresponding exciton emission lines.