The two-dimensional (2D) to three-dimensional (3D) morphology change in the highly strained growth of InAs on GaAs(001) is examined via in-situ, ultra-high vacuum (UHV) scanning tunneling microscopy (STM) and contact-mode atomic force microscopy (C-AFM). The formation of 3D InAs islands (∼2-4nm high) at an InAs delivery, θ∼1.57ML is found to bepreceded by the appearance of small quasi-3D clusters (∼0.6-1.2nm high). The 2D to 3D transition is found to occur over a range of θ from ∼1.45ML to 1.74ML, with a varying and gradual mass transfer from 2D to 3D features with increasing θ. The InAs 3D islands are also examined in this study using non-contact AFM (NC-AFM) in order to assess the usefulness of this technique for imaging 3D features. Unlike the constancy observed in the C-AFM images, the NC-AFM images exhibit a marked imaging condition dependence. The variability observed in the NC-AFM images is qualitatively compared to the outcome of the simplest, force-gradient model of NC-AFM in order to extract a guideline for NC-AFM imaging of 3D features.

Using plan-view and cross-sectional scanning tunneling microscopy, the shape and composition of InAs/GaAs quantum dots are investigated before and after capping by GaAs. During capping, the original pyramidally shaped quantum dots become truncated, resulting in a flat (001) top facet and steeper side facets. The InAs quantum dots are found to be intermixed at their top with GaAs due to material rearrangement. Since the bottom interface of quantum dots and wetting layer is always sharp, this intermixing occurs during capping and not during quantum dot growth. Considering strain energies, a model for the capping is presented.

The photoluminescence (PL) of self-organized InAs/GaAs quantum dots (QD's) shows a decomposition into a set of eight rather narrow lines upon antimony-surfactant mediated growth. This decomposition results from a shell-like growth mode, which means a discrete variation of the QD size in monolayer steps. Based on the PL monolayer splitting, indicative of structurally and chemically well-defined upper interfaces, structural and optical properties can be correlated much more in detail than previously possible for strongly broadened QD ensembles. A comparison of the spectral positions to predictions of eight-band k.p/configuration interaction model calculations for truncated pyramidal InAs/GaAs QD's yields excellent agreement only for the shell-like QD growth mode.

We present a method which increases the versatility of molecular beam epitaxy through multiple application of a stationary shadow mask. The method is based on selected area growth in the overlap of the incidence regions of two molecular beams impinging through different apertures of the mask. The width of the overlap depends on the incidence angles of the beams which can be adjusted in situ. Size-control of CdZnSe quantum-well (QW) islands with a precision of 20 nm by varying the distance between the apertures can be obtained. Without Se beam, a minor quantity of Cd is incorporated in the matrix modulating the band-gap energy by just 5 meV. Cathodoluminescence (CL) of nanoscale QW islands shows dot-like behavior, i.e., sharp emission lines that are attributed to different excited states and a blueshift of the CL on reducing the island dimensions.

We report a detailed study of the electronic properties of self-organized InAs/GaAs quantum dots (QDs) by photoluminescence (PL), time-resolved PL, and PL excitation (PLE) experiments. High-quality InAs/GaAs QDs of tunable size were obtained using the variable deposition amount approach in MBE-growth, yielding ultimately room-temperature emission at 1.3μm for island area densities of ~400μm. The experiments emphasize the role of a slowed down carrier relaxation in the QDs, being important e.g. for energy transfer processes between QDs and the temperature dependence of the carrier capture processes. The quantum size effect of the excited state spectrum is revealed in PLE experiments and shown to be in good agreement to numerical results for pyramidal QDs based on 8 band k⋅p theory. Finally, phononassisted recombination processes are identified demonstrating an enhanced exciton-LO-phonon coupling. Excellent agreement with estimations in the adiabatic approximation suggests that this enhancement is the consequence of the particular quantum confinement and the piezoelectricity in the strained low-symmetry QDs.

The temporal evolution of the size and the shape of self-organized InAs/GaAs quantum dots (QDs) grown using MOCVD is investigated. During a growth interruption after the deposition of the QD material a ripening process is observed, where some QDs grow at the expense of other QDs. A multimodal distribution of the QD ground-state transition energies is observed and attributed to QDs differing in height by entire numbers of atomic monolayers. This distribution is used to track the evolution of the QD ensemble during the growth interruption more detailed. A shape transition from very flat, truncated-pyramid-like QDs to higher, more pyramidal QDs is suggested. An additional antimony flux at the end of the growth interruption leads to an accelerated ripening resulting in a significant red shift of the QD luminescence, which is explained by the surfactant properties of antimony on InAs.

Wavelength selective charging of a QD sub-ensemble is demonstrated, presenting a basis of future wavelength dependent parallel optical memory. High-resolution saturation spectroscopy in two-colour photocurrent experiments is used to investigate the binding energy of the positively charged trion complex as well as the temperature dependence of the coupling to acoustic phonons.

We report photoluminescence (PL) measurements on self-assembled GaSb/GaAs quantum dots. As the laser excitation is increased from very low levels, the PL shows a strong red shift, and then a blue shift, such that it presents a U-shaped curve. Raising the temperature causes a large (<100 meV) blue shift of the PL, and shifts the minimum of the PL energy versus laser excitation curve to higher laser powers. Applying a magnetic field at lasers powers much less than1 W cm(-2) red shifts the PL energy. We explain these effects by population or depopulation of dots that are filled in the dark with holes supplied by carbon acceptors.

Pulsed magnetic fields are used to study a variety of self-assembled semiconductor nanostructures. We illustrate the power of the technique with two recent examples. In the first, we study confinement in InAs quantum dots on (100) and (311)B oriented GaAs substrates as a function of InAs coverage. We demonstrate that Stranski–Krastanow growth occurs for (100) substrates, but show that for (311)B substrates there is no such transition—rather the dots evolve from fluctuations in the wetting layer. In the second example, we investigate the Coulomb binding of ‘free’ electrons to holes confined to type-II GaSb/GaAs quantum dots. We find that at low laser power the electrons are repelled from the dots (by strain), but that by optical pumping the dots may be multiply charged, attracting the electrons, and more than doubling the binding energy.

The hole confinement in type-II self-organized GaSb/GaAs quantum dots (QDs) was investigated by combining optical excitation and time-resolved capacitance spectroscopy. The experimental results indicate energy-selective charging even for type-II QDs. With increasing excitation energy the apparent hole activation energy decreases, which is attributed to light absorption in sub-ensembles of QDs with decreasing hole localization. The large localization energy of about 450meV and the possibility of optical-multiplexing makes type-II GaSb/GaAs QDs a potential material system for QD memory concepts.

We explore the Coulomb binding of electrons to holes confined to type-II GaSb self-assembled quantum dots. We demonstrate that at low laser power electrons are more weakly bound to holes trapped by the dots than to holes in the wetting layer. On the other hand, at high laser power the hydrogenic binding energy of dot excitons increases by more than a factor of two, and so exceeds that of wetting layer excitons. We attribute this to the strong binding of ‘core’ electrons to dots that are highly charged with holes by optical pumping.

A pronounced modulation is observed in the photoluminescence (PL) spectrum of self-organized InAs/GaAs quantum dots (QDs), recorded at low excitation densities. The clearly distinguishable peaks are identified as a multimodal distribution of the ground state transition energy, originating from a discrete, stepwise variation of the structural properties of the QDs, which is associated with an increase of the QD height in monolayer (ML) steps. The observation of a ML splitting implies a flat QD shape with well-defined upper and lower interfaces as well as negligible indium segregation. The electronic properties of the InAs/GaAs QDs were investigated by PL and PL-excitation spectroscopy and are discussed based on realistic calculations for flat InAs/GaAs QDs with a truncated pyramidal shape based on an extended 8-band k·p model. The calculations predict a red shift of the ground state transition with each additional ML, which saturates for heights above 9ML, is in good agreement with experiment.

For self-organized quantum dots (QDs) the biexciton binding energy depends strongly on the structural properties. It is demonstrated that even a reversal of the sign can occur for a given QDs ensemble. These findings are significant for single-photon emitters and quantum cryptography utilizing single QDs as it offers a way to the renormalization of the exciton/biexciton level system.

Accumulated spectral hole burning is demonstrated for self-organized InAs/GaAs quantum dots (QDs) embedded in the space-charge region of a p–i–n diode. In two-color experiments, selective charging of QDs by resonant optical excitation is proved. A hole storage time of 1.5ms leads to saturation densities of only 0.5mW/cm−2. The results show the potential of inhomogeneously broadened ensembles of self-organized QDs for parallel optical storage. Additionally, spectral hole burning is used to identify the positive trion and to estimate the trion binding energy.

Using scanning nearfield optical microscopy, the photoluminescence characteristics of individual InGaAs quantum dots is investigated. At low temperatures an ensemble of narrow lines is observed, caused by different carrier interactions within a quantum dot. A trion and a biexciton line can be identified in the ground-state region at low excitation power, showing much larger binding energies than previously reported for III-V quantum dots. This behavior can be explained by an inhomogeneous stoichiometry profile in InGaAs quantum dots.

The impact of few-particle interactions on excited states of excitons localized in self-organized InxGa1-xAs/GaAs quantum dots (QD's), charged either with electrons or holes, is investigated. Excited-state absorption is probed size selectively by photoluminescence excitation spectroscopy improving the achieved resolution beyond the inhomogeneous broadening. Charging QD's embedded in suitable diode structures leads to nonlinear changes of the absorption characteristics for the individual excited-state transitions, enabling their unambiguous identification. Few-particle interactions lead to a renormalization of the excited-state transition energies, which ranges from a pronounced red shift to a blue shift depending on the excited-state transition and the type of spectator charge. The most pronounced effects occur charging the ground state of the QD's. The results are supported by eight-band k.p model calculations using a configuration-interaction scheme to account for the Coulomb interaction in the few-particle states.

Lateral carrier transfer is investigated for single CdxZn1-xSe/ZnSySe1-y quantum dots (QD's) in a high-density ensemble by time-resolved spectroscopy. Following nonresonant excitation a significant probability of independent capture of electrons and holes in separate QD's is observed. The subsequent lateral migration of carriers between adjacent QD's leads to a slow decay component of the exciton ground-state luminescence. At low temperatures the lateral carrier transfer is restricted to phonon-assisted inter-QD tunneling, resulting in migration times of the order of several nanoseconds. The role of independent carrier capture is suppressed at high excitation densities or increased temperatures, enabling thermally activated migration.

The exciton-LO-phonon interaction in self-organized quantum dots is investigated emphasizing the impact of realistic structural properties. The possibility to engineer the local charge density via the shape and composition profile of such strained quantum dots provides an unique opportunity to optimize the electronic and optical properties of a semiconductor nanostructure. Size-selective luminescence, resonant Raman scattering, and time-resolved luminescence experiments provide insight into the exciton-LO-phonon and exciton-photon couplings in self-organized quantum dots. The impact of the structural details is analyzed based on eight-band k(.)p model calculations.

We have studied the photoluminescence from type-II GaSb/GaAs self-assembled quantum dots in magnetic fields up to 50 T. Our results show that at low laser power, electrons are more weakly bound to the dots than to the wetting layer, but that at high laser power, the situation is reversed. We attribute this effect to an enhanced Coulomb interaction between a single electron and dots that are multiply charged with holes.