Individual Self-assembled Quantum Dots Research Articles

Single-color, in situ photolithography marking of individual CdTe/ZnTe quantum dots containing a single Mn2+ ion

A simple, single-color method for permanent marking of the position of individual self-assembled semiconductor Quantum Dots (QDs) at cryogenic temperatures is reported. The method combines in situ photolithography with standard micro-photoluminescence spectroscopy. Its utility is proven by a systematic magnetooptical study of a single CdTe/ZnTe QD containing a Mn2+ ion, where a magnetic field of up to 10 T in two orthogonal, Faraday and Voigt, configurations is applied to the same QD. The presented approach can be applied to a wide range of solid state nanoemitters.

Dissipative preparation of the exciton and biexciton in self-assembled quantum dots on picosecond time scales

Pulsed resonant fluorescence is used to probe ultrafast phonon-assisted exciton and biexciton preparation in individual self-assembled InGaAs quantum dots. By driving the system using large area $(\ensuremath{\ge}10\ensuremath{\pi})$ near resonant optical pulses, we experimentally demonstrate how phonon-mediated dissipation within the manifold of dressed excitonic states can be used to prepare the neutral exciton with a fidelity $\ensuremath{\ge}70%$. By comparing the phonon-assisted preparation with resonant Rabi oscillations we show that the phonon-mediated process provides the higher-fidelity preparation for large pulse areas and is less sensitive to pulse area variations. Moreover, by detuning the laser with respect to the exciton transition, we map out the spectral density for exciton coupling to the bulk LA-phonon continuum. Similar phonon-mediated processes are shown to facilitate direct biexciton preparation via two-photon biexciton absorption, with fidelities $>80%$. Our results are found to be in very good quantitative agreement with simulations that model the quantum dot--phonon bath interactions with Bloch-Redfield theory.

Electro-Elastic Tuning of Single Particles in Individual Self-Assembled Quantum Dots

We investigate the effect of uniaxial stress on InGaAs quantum dots in a charge tunable device. Using Coulomb blockade and photoluminescence, we observe that significant tuning of single particle energies (≈-0.22 meV/MPa) leads to variable tuning of exciton energies (+18 to -0.9 μeV/MPa) under tensile stress. Modest tuning of the permanent dipole, Coulomb interaction and fine-structure splitting energies is also measured. We exploit the variable exciton response to tune multiple quantum dots on the same chip into resonance.

Increased conductance of individual self-assembled GeSi quantum dots by inter-dot coupling studied by conductive atomic force microscopy.

The conductive properties of individual self-assembled GeSi quantum dots (QDs) are investigated by conductive atomic force microscopy on single-layer (SL) and bi-layer (BL) GeSi QDs with different dot densities at room temperature. By comparing their average currents, it is found that the BL and high-density QDs are more conductive than the SL and low-density QDs with similar sizes, respectively, indicating the existence of both vertical and lateral couplings between GeSi QDs at room temperature. On the other hand, the average current of the BL QDs increases much faster with the bias voltage than that of the SL QDs does. Our results suggest that the QDs’ conductive properties can be greatly regulated by the coupling effects and bias voltages, which are valuable for potential applications.

A Waveguide-Coupled On-Chip Single-Photon Source

We investigate single photon generation from individual self-assembled InGaAs quantum dots coupled to the guided optical mode of a GaAs photonic crystal waveguide. By performing confocal microscopy measurements on single dots positioned within the waveguide, we locate their positions with a precision better than 0.5 \mum. Time-resolved photoluminescence and photon autocorrelation measurements are used to prove the single photon character of the emission into the propagating waveguide mode. The results obtained demonstrate that such nanostructures can be used to realize an on-chip, highly directed single photon source with single mode spontaneous emision coupling efficiencies in excess of beta~85 % and the potential to reach maximum emission rates >1 GHz.

Preface

The unique success story of semiconductor physics and technology relies on the ability to highly integrate micro- and nanometer sized functional units on a single chip. Within the last few years single nanostructures, e.g., individual and coupled pairs of quantum dots and single molecules moved more and more into the focus of many research groups to study the exciting physics and to exploit their tremendous potential for device applications. The full advantage of their superior properties can be utilized if a controlled positioning or growth of the nanostructure inside a more complex device structure can be realized, thereby defining precise coupling between the microscopic nanostructure and the macroscopic periphery. single-photon sources coupled quantum-dot gates single quantum-dot tunneling diodes. electroluminescence of single electrically-driven molecules at room temperature controlling quantum-dot emission by integration of semiconductor nanomembranes onto piezoelectric actuators controlled coupling of individual self-assembled quantum dots to plasmonic nanoantennas quantum light emission of two lateral tunnel-coupled (In,Ga)As/GaAs quantum dots controlled by a tunable static electric field realization of vertically stacked and laterally ordered InP and InAs quantum dots for quantum gate applications development of site-controlled SiGe islands on patterned Si(001) and realization of tunneling diodes ultrafast transient reflection spectroscopy of a single self-assembled GaAs quantum dot. We hope that this collection of ideas and results of controlled positioning of single nanostructures stimulates many fascinating experiments and clever applications of quantum objects. We wish to thank all people involved in the research group for their excellent work, the DFG for the financial support and Wiley for the opportunity to publish this volume. On behalf of all applicants being funded by DFG over the years, M. Lippitz and P. Michler Stuttgart, March 2012

Optical anisotropy and photoluminescence polarization in single InAlAs quantum dots

We have investigated the optical anisotropy in individual self-assembled quantum dots. The linear polarization analysis of the positive trion photoluminescence reveals the effect of the strain-induced valence band mixing since the positive trion has the spin-paired holes and therefore exchange interaction has no influence. Meanwhile, the neutral exciton indicates the complex polarization states due to both the in-plain asymmetries of the dot shape and the strain distributions. The experimental and theoretical polarization analysis has been performed for tens of InAlAs quantum dots and the correlation between the important parameters was investigated.

Bias-dependent conductive characteristics of individual GeSi quantum dots studied byconductive atomic force microscopy

The bias-dependent electrical characteristics of individual self-assembled GeSi quantumdots (QDs) are investigated by conductive atomic force microscopy. The results reveal thatthe conductive characteristics of QDs are strongly influenced by the applied bias. At low (−0.5 to − 2.0 V) andhigh (−2.5 to − 4.0 V) biases, the current distributions of individual GeSi QDs exhibit ring-like and disc-likecharacteristics respectively. The current of the QD’s central part increases more quicklythan that of the other parts as the bias magnitude increases. Histograms of the magnitudeof the current on a number of QDs exhibit the same single-peak feature at low biases, anddouble- or three-peak features at high biases, where additional peaks appear at large-currentlocations. On the other hand, histograms of the magnitude of the current on the wettinglayers exhibit the same single-peak feature for all biases. This indicates the conductivemechanism is significantly different for QDs and wetting layers. While the small-currentpeak of QDs can be attributed to the Fowler–Nordheim tunneling model at low biasesand the Schottky emission model at high biases respectively, the large-currentpeak(s) may be attributed to the discrete energy levels of QDs. The results suggestthe conductive mechanisms of GeSi QDs can be regulated by the applied bias.

Temperature dependence of the emission spectra of individual self-assembled quantum dots

We have developed a quantum-mechanical theory for the interaction of light and electron-hole excitations in semiconductor quantum dots. Our theoretical analysis results in an expression for the photoluminescence intensity in the non-linear regime. The validity of the theoretical results is tested analyzing experimental data reported for the temperature dependence of the emission spectra of an individual lens-shaped In0.4Ga0.6As self-assembled quantum dot in a wide temperature range up to 300 K. Our theoretical predictions for the redshift of the emission peak with increasing temperature, in the range 2–300 K, agree with the experiment.

Enhanced Sequential Carrier Capture into Individual Quantum Dots and Quantum Posts Controlled by Surface Acoustic Waves

Individual self-assembled quantum dots and quantum posts are studied under the influence of a surface acoustic wave. In optical experiments we observe an acoustically induced switching of the occupancy of the nanostructures along with an overall increase of the emission intensity. For quantum posts, switching occurs continuously from predominantly charged excitons (dissimilar number of electrons and holes) to neutral excitons (same number of electrons and holes) and is independent of whether the surface acoustic wave amplitude is increased or decreased. For quantum dots, switching is nonmonotonic and shows a pronounced hysteresis on the amplitude sweep direction. Moreover, emission of positively charged and neutral excitons is observed at high surface acoustic wave amplitudes. These findings are explained by carrier trapping and localization in the thin and disordered two-dimensional wetting layer on top of which quantum dots nucleate. This limitation can be overcome for quantum posts where acoustically induced charge transport is highly efficient in a wide lateral matrix-quantum well.

Transient Hole Trapping in Individual GeSi Quantum Dot Grown on Si(001) Studied by Conductive Atomic Force Microscopy

Electrical properties of individual self-assembled GeSi quantum dots grown on Si substrates are investigated by using conductive atomic force microscopy at room temperature. By controlling the bias voltage sweep in a certain fast sweep rate range, a novel current peak is observed in the current–voltage characteristics of the quantum dots. The current peaks are detectable only during the backward voltage sweep immediately after a forward sweep. The current peak position and intensity are found to depend strongly on the voltage sweep conditions. This kind of current–voltage characteristic under fast sweep is very different from the ordinary steady state current behaviour of quantum dots measured previously. The origin of this phenomenon can be attributed to the transient hole trapping in the potential well formed by the quantum dot sandwiched between the native oxide layer and the bottom Si substrate.

Room temperature photoluminescence of individual self-assembled quantum dots

We investigate the emission spectra of individual lens-shaped self-assembled quantum dots (QDs) in the high-temperature regime in order to contribute to the fine structural analysis and to the appreciation of the QDs’ optical response. Our theoretical analysis results in an expression for the photoluminescence (PL) intensity of QDs in the linear regime, which reproduces satisfactorily the experimentally observed PL signal of individual lens-shaped In 0.5Ga 0.5As self-assembled QDs. Using the appropriate material parameters, the theoretical predictions for the interlevel spacing as well as for the dephasing time caused by electron-longitudinal optical phonon interactions are in good agreement with experiment.

Sharp-line electroluminescence from individual quantum dots by resonant tunneling injection of carriers

We report sharp electroluminescence lines from individual self-assembled InAs quantum dots (QDs) excited by resonant tunneling injection of carriers from the n- and p-doped GaAs layers of a p-i-n diode. Bias-tunable tunneling of carriers into the dots provides a means of controlling injection and light emission from a small number of individual dots within a large ensemble. We also show that the extent of carrier energy relaxation prior to recombination can be controlled by tailoring the morphology of the QD layer.

Stark Effect on Exciton Complexes of a Single Quantum Dot Embedded in a p–n Junction

The emission spectrum of exciton complexes formed in individual self-assembled quantum dots (QDs) embedded in a p–n junction is theoretically studied using an effective mass model. We calculate the particle Coulomb interactions, electron–hole overlaps and transition energies of exciton complexes for various strengths and directions of an applied electric field. Both redshifts and blue shifts are observed in excitons, positive and negative trions, and biexcitons. We show that the Stark effect can be used to manipulate the spontaneous emission rate of individual QDs embedded in microcavities.

Fano interference effect on the transition spectrum of single-electron transistors

We theoretically study the intraband transition spectrum of single electron transistors (SETs) composed of individual self-assembled quantum dots. The polarization of SETs is obtained by using the nonequilibrium Green's function technique and the Anderson model with three energy levels. Owing to nonradiative coupling between two excited states through the continuum of electrodes, the Fano interference effect significantly influences the peak position and intensity of infrared wavelength single-photon spectrum.

Strain effects on individual quantum dots: Dependence of cap layer thickness

We have studied the effects of strain on individual self-assembled quantum dots (QDs) exemplified by InP dots embedded in GaInP. The quantum dot sample was etched from the top and in this way the amount of capping material was reduced. In a sequence of etch cycles, the cap layer was thinned, and the photoluminescence from several individual QDs could be followed as a function of cap layer thickness. The evolution of the emission spectra clearly depended on the quantum dot size. We interpret this as arising from differences in the aspect ratio for quantum dots of different sizes. The influence of the capping layer, for different QD geometries, was modeled using deformation potential theory with the strain calculated using a full three-dimensional linear elasticity model. The results agree well with the experimental observations. (Less)

Geometrical Effects on the Optical Properties of Quantum Dots Doped with a Single Magnetic Atom

The emission spectra of individual self-assembled quantum dots containing a single magnetic Mn atom differ strongly from dot to dot. The differences are explained by the influence of the system geometry, specifically the in-plane asymmetry of the quantum dot and the position of the Mn atom. Depending on both these parameters, one has different characteristic emission features which either reveal or hide the spin state of the magnetic atom. The observed behavior in both zero field and under magnetic field can be explained quantitatively by the interplay between the exciton-manganese exchange interaction (dependent on the Mn position) and the anisotropic part of the electron-hole exchange interaction (related to the asymmetry of the quantum dot).

Systematic reduction of the permanent exciton dipole for charged excitons in individual self-assembled InGaAs quantum dots

The nature of the few particle wavefunctions for neutral and positively charged excitons is probed in individual InGaAs quantum dots using Stark-effect perturbation spectroscopy. A systematic reduction of the vertical component of the permanent excitonic dipole ( p z ) is observed as additional holes are added to the dot. A comparison with calculations reveals that this reduction (Δ p z /e∼15–20%) is accompanied by a significant lateral expansion of the hole (∼2 nm) and contraction (∼1 nm) of the electron wavefunctions. We suggest that this lateral redistribution of the charged exciton wavefunctions provides an optical means to probe the lateral composition profile of the dot.

Selective excitation of self-assembled quantum dots by using shaped pulse

We carried out optical selective excitation of individual self-assembled quantum dots by using phase-modulated pulses. Based on scattered photoluminescence excitation resonances in individual QDs, the excitation pulses modulated in the spectral region allows for addressing individual ground states emission. The photoluminescence spectra including several QDs showed intensity changes according to both the modulation energies and phases. The results also suggested the individual control of selective QDs even in collective excitation.

Electric field effects in single semiconductor quantum dots observed by scanning tunneling luminescence

Scanning tunneling microscopy (STM) and scanning tunneling luminescence were used to correlate the topography with the emission spectra from individual self-assembled InP quantum dots (QDs). We have investigated in detail how the electric field induced by the STM tip affects the emission from the QDs. This was done when exciting a QD, by altering the bias for constant current, by altering the current for constant bias, or by changing the tip position. An increased bias (increased electric field) leads to Stark shift of the QD emission, whereas a larger tunneling current results in state filling of the emission. Furthermore, when exciting the QD, the position of the STM tip is shown to have large effects on the QD luminescence.