A. Schliwa

Size-dependent fine-structure splitting in self-organized InAs/GaAs quantum dots

A systematic variation of the exciton fine-structure splitting with quantum dot size in single quantum dots grown by metal-organic chemical vapor deposition is observed. The splitting increases from to as much as with quantum dot size. A change of sign is reported for small quantum dots. Model calculations within the framework of eight-band theory and the configuration interaction method were performed. Different sources for the fine-structure splitting are discussed, and piezoelectricity is pinpointed as the only effect reproducing the observed trend.

Interrelation of structural and electronic properties in InxGa1-xN/GaN quantum dots using an eight-band k · p model

We present an eight-band kp-model for the calculation of the electronic structure of wurtzite semiconductor quantum dots (QDs) and its application to indium gallium nitride (InxGa1 xN) QDs formed by composition fluctuations in InxGa1 xN layers. The eight-band kp-model accounts for strain effects, piezoelectric and pyroelectricity, spin-orbit and crystal field splitting. Exciton binding energies are calculated using the self-consistent Hartree method. Using this model, we studied the electronic properties of InxGa1 xN QDs and their dependence on structural properties, i.e., their chemical composition, height, and lateral diameter. We found a dominant influence of the built-in piezoelectric and pyroelectric fields, causing a spatial separation of the bound electron and hole states and a redshift of the exciton transition energies. The single-particle energies as well as the exciton energies depend heavily on the composition and geometry of the QDs.

106years extrapolated hole storage time in GaSb∕AlAs quantum dots

A thermal activation energy of 710meV for hole emission from InAs∕GaAs quantum dots (QDs) across an Al0.9Ga0.1As barrier is determined by using time-resolved capacitance spectroscopy. A hole storage time of 1.6s at room temperature is directly measured, being three orders of magnitude longer than a typical dynamic random access memory (DRAM) refresh time. The dependence of the hole storage time in different III–V QDs on their localization energy is determined and the localization energies in GaSb-based QDs are calculated using eight-band k⋅p theory. A storage time of about 106years in GaSb∕AlAs QDs is extrapolated, sufficient for a QD-based nonvolatile (flash) memory.

Many-particle effects in type II quantum dots

Many-particle effects are investigated in the photoluminescence of type II GaSb/GaAs quantum dots (QDs). With increasing excitation density, i.e., exciton occupation, the photoluminescence shows first a blueshift and then saturates developing a plateau region. The peculiar behavior is attributed to Coulomb charging and state filling of the localized holes to dominate the many-particle regime. A high temperature stability makes the GaSb/GaAs QDs suitable for room-temperature devices.

Single-photon emission from InGaAs quantum dots grown on (111) GaAs

In this letter, we demonstrate that self-organized InGaAs quantum dots (QDs) grown on GaAs (111) substrate using droplet epitaxy have great potential for the generation of entangled photon pairs. The QDs show spectrally sharp luminescence lines and low spatial density. A second order correlation value of g(2)(0)<0.3 proves single-photon emission. By comparing the power dependence of the luminescence from a number of QDs we identify a typical luminescence fingerprint. In polarization dependent microphotoluminescence studies a fine-structure splitting ranging ≤40 μeV down to the determination limit of our setup (10 μeV) was observed.

Electron localization by self-assembled GaSb/GaAs quantum dots

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.

The structural, electronic and optical properties of GaSb/GaAs nanostructures for charge-based memory

The potential for GaSb nanostructures embedded in GaAs to operate as charge-based memory elements at room temperature is introduced and explored. Cross-sectional scanning-tunnelling microscopy is employed to directly probe and optimize the growth of nanostructures by molecular beam epitaxy. The results of structural analysis are combined with electrical measurements made with deep-level transient spectroscopy, showing excellent agreement with theoretical calculations which model the electronic structure of the nanostructures using 8-band kp theory. Hole-localization energies exceeding 600 meV in quantum dots and near-100% material contrast between GaSb-rich quantum rings (QRs) and the surrounding GaAs matrix are revealed (no intermixing). Optical measurements confirm the depth of the hole localization, and demonstrate substantially lower inhomogeneous broadening than has previously been reported. Multiple peaks are partially resolved in ensemble photoluminescence of GaSb/GaAs QRs, and are attributed to charge states from discrete numbers of confined holes.

Quantum Dots for Single- and Entangled-Photon Emitters

The efficient generation of polarized single or entangled photons is a crucial requirement for the implementation of quantum key distribution (QKD) systems. Self-organized semiconductor quantum dots (QDs) are capable of emitting one polarized photon or an entangled photon pair at a time using appropriate electrical current injection. We realized a highly efficient single-photon source (SPS) based on well-established semiconductor technology: In a pin structure, a single electron and a single hole are funneled into a single InAs QD using a submicron AlOx current aperture. Efficient radiative recombination leads to emission of single polarized photons with an all-time record purity of the spectrum. Non-classicality of the emitted light without using additional spectral filtering is demonstrated. The out-coupling efficiency and the emission rate are increased by embedding the SPS into a micro-cavity. The design of the micro-cavity is based on detailed modeling to optimize its performance. The resulting resonant single-QD diode is driven at a repetition rate of 1 GHz, exhibiting a second-order correlation function of g(2)(0) = 0. Eventually, QDs grown on (111)-oriented substrates are proposed as a source of entangled photon pairs. Intrinsic symmetry-lowering effects leading to the splitting of the exciton bright states are shown to be absent for this substrate orientation. As a result, the XX rarr X rarr 0 recombination cascade of a QD can be used for the generation of entangled photons without further tuning of the fine-structure splitting via QD size and/or shape.

Lateral positioning of InGaAs quantum dots using a buried stressor

We present a “bottom-up” approach for the lateral alignment of semiconductor quantum dots (QDs) based on strain-driven self-organization. A buried stressor formed by partial oxidation of (Al,Ga)As layers is employed in order to create a locally varying strain field at a GaAs(001) growth surface. During subsequent strained layer growth, local self-organization of (In,Ga)As QDs is controlled by the contour shape of the stressor. Large vertical separation of the QD growth plane from the buried stressor interface of 150 nm is achieved enabling high optical quality of QDs. Optical characterization confirms narrow QD emission lines without spectral diffusion. V C 2012 American Institute of Physics .[ http://dx.doi.org/10.1063/1.3691251] The deterministic alignment of quantum dots (QDs) during an epitaxial growth process is mandatory for electronic and optoelectronic devices 1 based on single QDs, for example, single photon detectors 2 and non-classical light emitters. 3 The self-organized formation of coherently strained islands, e.g., QDs, by the growth of strained layers in the “Stranski-Krastanow” growth regime is a consequence of the total energy minimization of the strained layer system. 4–6 QDs are formed if the strain energy relieved by island formation surpasses the energy cost associated with newly formed surfaces and edges. 7 Therefore, a selective formation of QDs on a surface will occur if the surface exhibits sites of increased strain energy, higher strain energy relief, or lower facet formation energy during growth of a strained layer. Current techniques for QD positioning generally deploy nanometer-scale lithography techniques like electron beam lithography, 8 focused ion beam lithography, 9 local oxidation, 10 or nano-imprinting 11 in order to define nanometersized areas as exclusive nucleation sites prior to the growth of quantum dots. All these “top-down” approaches share a number of difficulties, which impact the structural and optical properties of the quantum dots. First, deterministic quantum dot nucleation is possible only within very close vertical proximity to the structural patterning. An often reported problem is the missing of QDs at shallow holes patterned on a growth surface. 8,9 Since the patterning involves etching of the surface or other invasive means, the quantum dots will be surrounded by defect sites which degrade their structural and optical quality. 12 Even though sophisticated cleaning

Manifestation of unconventional biexciton states in quantum dots

Although semiconductor excitons consist of a fermionic subsystem (electron and hole), they carry an integer net spin similar to Cooper-electron-pairs. While the latter cause superconductivity by forming a Bose-Einstein-condensate, excitonic condensation is impeded by, for example, a fast radiative decay of the electron-hole pairs. Here, we investigate the behaviour of two electron-hole pairs in a quantum dot with wurtzite crystal structure evoking a charge carrier separation on the basis of large spontaneous and piezoelectric polarizations, thus reducing carrier overlap and consequently decay probabilities. As a direct consequence, we find a hybrid-biexciton complex with a water molecule-like charge distribution enabling anomalous spin configurations. In contrast to the conventional-biexciton complex with a net spin of s=0, the hybrid-biexciton exhibits s=±3, leading to completely different photoluminescence signatures in addition to drastically enhanced charge carrier-binding energies. Consequently, the biexcitonic cascade via the dark exciton can be enhanced on the rise of temperature as approved by photon cross-correlation measurements.