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We integrate resonant-cavity light-emitting diodes containing quantum dots onto substrates with giant piezoelectric response. Via strain, the energy of the photons emitted by the diode can be precisely controlled during electrical injection over a spectral range larger than 20 meV. Simultaneously, the exciton fine-structure-splitting and the biexciton binding energy can be tuned to the values required for entangled photon generation., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The cross-plane thermal conductivity κ of multilayers of SiGe nanodots separated either by Si or SiGe can be decreased by reducing the period length or by increasing the nanodot density. It is, however, not clear how far κ can be reduced by using these strategies. In addition, the role of SiGe nanodots on the reduction of j is still not fully understood. In this work, we addressed these issues by studying experimentally the cross-plane j of Ge/Si superlattices with period lengths down to 1.5 nm. Although κ tends to preserve the decreasing trend with reducing the period length, for periods shorter than 2 nm we observed a drastic drop of the average thermal resistance per period. This finding indicates a weakening of the effect of the interfaces on phonon scattering and implies a lower limit for κ. To assess the role played by the nanodots in the reduction of j we studied Ge/Si superlattices with nanodot densities varying from 0 to ~8x1010 cm-2 and a fixed Si spacer thickness of 2.7 nm. The experimental results suggest that SiGe nanodots with "pyramid"-shape have an effect comparable to nominally planar wetting layers on the cross-plane thermal transport. Finally, the comparison of superlattices with nanodots separated by Si1-xGex (with x from 0 to 0.2) shows that spacer alloying is beneficial in reducing the κ by ~20%. The results presented in this work are expected to be relevant to micro/nanoscale energy conversion which requires minimizing the thermal conductivity of superlattice-based thin film thermoelectrics. [ABSTRACT FROM AUTHOR]

Very low density growth of GaAs quantum dots in self-assembled nanoholes created by gallium droplet etching is demonstrated. The emission energy of the quantum dots can be accurately controlled by the GaAs deposition amount, from 1.8 to 1.6 eV, independently of the dot density which can be reproducibly controlled over the range 0.2-2×10-8 cm-2 by the gallium deposition rate. The ensemble full-width-half-maximum is <10 meV and single-dot linewidths of 40 μeV (limited by our spectral resolution) have been measured. Additionally, shallow mounds on the sample surface allow the buried GaAs/AlGaAs dots to be located. A simple method to reliably predict the emission energy based on the shape of the nanohole is presented. [ABSTRACT FROM AUTHOR]

The authors experimentally investigate the effects of atomic layer deposition (ALD) and laser-assisted oxidation on the optical modes in GaAs L3 photonic crystal air-bridge cavities, using layers of InAs quantum dots as internal light source. Four distinct optical mode peaks are observed in the photonic bandgap and they show different wavelength-redshifts (0-6.5 nm) as the photonic crystal surface is coated with an Al2O3 layer (0-5.4 nm thick). Numerical finite-difference time-domain (FDTD) simulations can well-reproduce the experimental result and give insight into the origin of the shifts of modes with different spatial profiles. By combining the ALD coating with in situ laser-assisted oxidation, we are able to both redshift and blueshift the optical modes and we attribute the blueshift to the formation of a GaAs-oxide at the expense of GaAs at the interface between GaAs and the Al2O3 layer. This result can be quantitatively reproduced by including a GaAs-oxide layer into the FDTD model. Selective etching experiments, confirm that this GaAs-oxide layer is mainly at the interface between GaAs and Al2O3 layers. [ABSTRACT FROM AUTHOR]

Single lateral InGaAs quantum dot molecules have been embedded in a planar microcavity in order to increase the luminescence extraction efficiency. Using a combination of metal-organic vapor phase and molecular beam epitaxy samples could be produced that exhibit a 30 times enhanced single-photon emission rate. We also show that the single-photon emission is fully switchable between two different molecular excitonic recombination energies by applying a lateral electric field. Furthermore, the presence of a polarization fine structure splitting of the molecular neutral excitonic states is reported which leads to two polarization split classically correlated biexciton-exciton cascades. The fine structure splitting is found to be on the order of 10 μeV. [ABSTRACT FROM AUTHOR]

We tune the emission energy of self-assembled InAs/GaAs(001) quantum dots (QDs) by partial GaAs capping and annealing. During the annealing step, the surface above the QDs flattens substantially. The existence of In-rich cores below the thin GaAs cap is directly probed by using in situ selective etching. The blueshift of the QD emission energy, resulting from the reduction of the QD height, is tuned by increasing the annealing time or by reducing the capping layer thickness. For long annealing times, the ensemble photoluminescence (PL) displays a multipeak behavior which is attributed to monolayer fluctuations of the QD height. Single-QD micro-PL shows that the quality of the QDs can be improved by desorbing the In accumulated around the QDs by performing the annealing at high substrate temperatures. [ABSTRACT FROM AUTHOR]

We report on optical investigations of MOVPE-grown InGaAs/GaAs quantum dots emitting at the telecom O-band that were integrated onto uniaxial piezoelectric actuators. This promising technique, which does not degrade the emission brightness of the quantum emitters, enables us to tune the quantum dot emission wavelengths and their fine-structure splitting. By spectrally analyzing the emitted light with respect to its polarization, we are able to demonstrate the cancelation of the fine structure splitting within the experimental resolution limit. This work represents an important step towards the high-yield generation of entangled photon pairs at telecommunication wavelength, together with the capability to precisely tune the emission to target wavelengths. [ABSTRACT FROM AUTHOR]

Independent tuning of emission energy and decay time of neutral excitons confined in single selfassembled In(Ga)As/GaAs quantum dots is achieved by simultaneously employing vertical electric fields and lateral biaxial strain fields. By locking the emission energy via a closed-loop feedback on the piezoelectric actuator used to control the strain in the quantum dot, we continuously decrease the decay time of an exciton from 1.4 to 0.7 ns. Both perturbations are fully electrically controlled and their combination offers a promising route to engineer the indistinguishability of photons emitted from spatially separated single photon sources. [ABSTRACT FROM AUTHOR]

Spatially and temporally overlapping double potential wells are realized in a hybrid optical microtube cavity due to the coexistence of an aggregate of luminescent quantum dots embedded in the tube wall and the cone-shaped tube's geometry. The double potential wells produce two independent sets of optical modes with different sets of mode numbers, indicating phase velocity separation for the modes overlapping at the same frequency. The overlapping mode position can be tuned by modifying the tube cavity, where these mode sets shift with different magnitudes, allowing for a vernier-scale-like tuning effect. [ABSTRACT FROM AUTHOR]

We combine micro-photoluminescence (PL) with terahertz excitation to investigate the response of single self-assembled InAs/GaAs quantum dots to intense terahertz pulses tuned to the s-to-p transition. Spectra and transients of single photoluminescence lines reveal the dynamics of electrons upon excitation and subsequent relaxation back into the initial state. Under certain circumstances, the terahertz pulse can release trapped charge carriers, which relax into the quantum dot. Furthermore, we demonstrate near-total depletion of the positive trion PL by an intense terahertz pulse. [ABSTRACT FROM AUTHOR]

We have performed time-resolved measurements of the time scale for conversion of excitons in dark states to bright (light-emitting) states in GaAs quantum dots. The dark states are pumped using two-photon absorption, while the bright state emission is observed in single-photon emission. This conversion time is connected to the spin flip time for carriers in the quantum dots. The time scale is found to be of the order of several hundred picoseconds. [ABSTRACT FROM AUTHOR]

We investigate the growth of self-assembled Ge nanostructures on top of embedded Ge nanowires on Si(001) substrates. Ge nanostructures, such as nanodashes, nanodumbbells, and dot chains are observed simply by tuning the growth temperature and thickness of the Si spacer between the Ge layers. The self-assembly process is governed by the surface strain fields generated by the embedded Ge nanowires and is well-described by our theoretical calculations. The catalyst-free and horizontal growth of such Ge nanostructures directly on Si(001) is attractive for investigating exotic transport properties through Si/Ge-based quantum devices. [ABSTRACT FROM AUTHOR]

The vectorial four-wave mixing response of an individual strongly confined exciton-biexciton system with fine-structure splitting in a GaAs/AlGaAs quantum dot is measured by dual-polarization heterodyne spectral interferometry. The results are compared with theoretical predictions based on the optical Bloch equations. The system is described by a fourlevel scheme, which is a model system of the nonlinear excitonic response in low-dimensional semiconductors. We measure its coherence properties and determine the underlying dephasing mechanisms. An impact of the inhomogeneous broadening by spectral wandering on the coherent response is investigated. We further discuss the different four-wave mixing pathways, polarization selection rules, the time-resolved polarization state, the vectorial response in two-dimensional four-wave mixing and ensemble properties. [ABSTRACT FROM AUTHOR]

Hybrid interfaces between semiconductor quantum dots and atomic systems could be of potential fundamental and technological interest, because they can combine the advantages of both constituents. Semiconductor quantum dots are tunable and deterministic sources of single and entangled photons. Atomic vapours are widely used as slow-light media and quantum memories. Merging both systems could enable the storage of quantum dot emission-an important step towards the implementation of quantum memories and quantum repeaters. Here, we show a hybrid semiconductor-atomic interface for slowing down single photons emitted from a single quantum dot. We use a double absorption resonance in rubidium vapour to create a slow-light medium in which a single photon is stored for 15 times its temporal width. Our result is the first demonstration of non-classical light storage, where single photons are generated on demand from a semiconductor source. [ABSTRACT FROM AUTHOR]

The three-dimensional composition profiles of individual SiGe/Si(001) islands grown on planar and pit-patterned substrates are determined by atomic force microscopy (AFM)-based nanotomography. The observed differences in lateral and vertical composition gradients are correlated with the island morphology. This approach allowed us to employ AFM to simultaneously gather information on the composition and strain of SiGe islands. Our quantitative analysis demonstrates that for islands with a fixed aspect ratio, a modified geometry of the substrate provides an enhancement of the relaxation, finally leading to a reduced intermixing. [ABSTRACT FROM AUTHOR]

The radiative biexciton-exciton decay in a semiconductor quantum dot (QD) has the potential of being a source of triggered polarization-entangled photon pairs. However, in most cases the anisotropy-induced exciton fine structure splitting destroys this entanglement. Here, we present measurements on improved QD structures, providing both significantly reduced inhomogeneous emission linewidths and near-zero fine structure splittings. A high-resolution detection technique is introduced which allows us to accurately determine the fine structure in the photoluminescence emission and therefore select appropriate QDs for quantum state tomography. We were able to verify the conditions of entangled or classically correlated photon pairs in full consistence with observed fine structure properties. Furthermore, we demonstrate reliable polarization-entanglement for elevated temperatures up to 30 K. The fidelity of the maximally entangled state decreases only a little from 72% at 4K to 68% at 30 K. This is especially encouraging for future implementations in practical devices. [ABSTRACT FROM AUTHOR]

Reversible biaxial strains are used for tuning the emission wavelengths of high quality GaAs/AlGaAs quantum dots (QDs) in the spectral range of the 87Rb D2 lines. The strain is transferred by integrating free standing (Al)GaAs nanomembranes, containing QDs, onto piezoelectric actuators. Narrow excitonic emission lines as sharp as 25 μeV are shown, and a tuning range larger than 5 nm is demonstrated. This range corresponds to an induced anisotropic biaxial strain of the order of 0.15%, as evaluated from the shift in the emission of the GaAs from the nanomembranes. The presented on-chip technology is potentially interesting for future quantum memories based on hybrid semiconductor-atomic interfaces. [ABSTRACT FROM AUTHOR]

We study single GaAs quantum dots with optical transitions that can be brought into resonance with the widely used D2 transitions of rubidium atoms. We achieve resonance by Zeeman or Stark shifting the quantum dot levels. We discuss an energy stabilization scheme based on the absorption of quantum dot photoluminescence in a rubidium vapor. This offers a scalable means to counteract slow spectral diffusion in quantum dots. [ABSTRACT FROM AUTHOR]

The authors report on the fabrication and detailed structural characterization of ordered arrays of vertically stacked SiGe/Si(001) island pairs. By a proper choice of growth parameters, islands which have both large sizes and high Ge fraction are obtained in the upper layer. Finite element method calculations of the strain distribution reveal that (i) the Si spacer between a pair of islands can act as a lateral quantum dot molecule made of four nearby dots for electrons and (ii) the tensile strain in a Si cap deposited on top of the stack is significantly enhanced with respect to a single layer. [ABSTRACT FROM AUTHOR]

The authors present steps toward the experimental realization of indistinguishable single photon sources based on independent unstrained GaAs quantum dots (QDs), which are embedded in planar cavities to improve the light collection efficiency. The emission lines of two QDs are brought into resonance and overlapped at a beam splitter. The coherence properties of the emitted photons are investigated by measuring the first-order field correlation function. Despite the fact that the short dephasing time of the selected QDs prevents us to observe quantum interference between the two photons, the approach could be applied to other QDs. [ABSTRACT FROM AUTHOR]

Selective area wavelength tuning of InAs quantum dots (QDs) in GaAs matrix is achieved by patterning TiO2 and SiO2 layers on the sample surface followed by rapid thermal processing. After heat treatment, the QD emission under the SiO2 capped areas shows pronounced blueshifts compared with regions capped with TiO2/SiO2, where interdiffusion is strongly suppressed. Finite element calculations of the strain generated by the different thermal expansion coefficients of GaAs, SiO2, and TiO2 at high temperature are used to interpret the results. This method may provide a simple route to achieve monolithic integration of optoelectronic devices based on QDs. [ABSTRACT FROM AUTHOR]

The authors investigate the formation of quantum-dot crystal (QDC) defects during the growth of two-dimensional QDCs on flat surfaces. The flat surface is modulated by the periodic strain fields of a buried quantum-dot (QD) seed layer, which in turn is grown onto a patterned substrate surface. Two types of QDC defects, QD vacancies, and QD interstitials are observed whenever the QD density controlled by the growth conditions does not match the areal density of the patterned sites. Our kinetic Monte Carlo growth simulations take into account the realistic strain profile of buried QDs and are able to describe the occurrence of both types of QDC defects during the growth of seeded QDCs. [ABSTRACT FROM AUTHOR]

Site-controlled growth of single and pairs of InAs quantum dots is demonstrated on ex situ electron-beam patterned (001) GaAs substrates using in situ Ga-assisted deoxidation prior to overgrowth. 6–8 ML of gallium deposited at a substrate temperature of 460 °C in the absence of arsenic followed by a brief anneal under arsenic is used to remove the surface oxide without damaging a pattern consisting of ∼100 nm wide, ∼20 nm deep holes. Single dot luminescence is shown from a dilute array (10 μm spacing) of such site-controlled dots, located only 8 nm from the regrowth interface. [ABSTRACT FROM AUTHOR]

The morphological evolution of both pits and SiGe islands on patterned Si(001) substrates is investigated. With increasing Si buffer layer thickness the patterned holes transform into multifaceted pits before evolving into inverted truncated pyramids. SiGe island formation and evolution are studied by systematically varying the Ge coverage and pit spacing and quantitative data on the influence of the pattern periodicity on the SiGe island volume are presented. The presence of pits allows the fabrication of uniform island arrays with any of their equilibrium shapes. [ABSTRACT FROM AUTHOR]

The quantum-confined Stark effect of excitonic states in self-assembled (In,Ga)As/GaAs quantum dots was studied by microphotoluminescence spectroscopy. A similar Stark-shift behavior for excitons, biexcitons, and a charged state was observed. Investigations suggest the absence of a permanent dipole moment in the lateral quantum dot plane. Values of the polarizability could be derived for all the investigated states. Furthermore, high-resolution Fabry-Pérot interferometry was applied to resolve the excitonic fine structure splitting and to investigate the influence of a lateral electric field. For a single dot, the splitting could be tuned to zero, thus affording the possibility to create electrically controlled entangled photon pairs. [ABSTRACT FROM AUTHOR]

The authors investigate the composition profile of SiGe islands after capping with Si to form quantum dots, using a two step etching procedure and atomic force microscopy. Initially, the Si capping layers are removed by etching selectively Si over Ge and then the composition of the disclosed islands is addressed by selectively etching Ge over Si. For samples grown at 580 °C the authors show that even when overgrowth leads to a flat Si surface and the islands undergo strong morphological changes, a Ge-rich core region is still preserved in the dot. At high growth and overgrowth temperatures (740 °C), the experiments show that the newly formed base of the buried islands is more Si rich than their top. Furthermore, the authors find that for the growth conditions used, no lateral motion takes place during capping. [ABSTRACT FROM AUTHOR]

The authors employ a focused laser beam both as a probe and as a local heat source to tune in situ, over a broad range and with resolution-limited accuracy, the quantized energy states of single optical microcavities and self-assembled quantum dots (QDs). The approach is demonstrated by bringing an optical mode of a microdisk into resonance with the emission of a single QD and by tuning spatially separated QDs in mutual resonance. This processing method may be used, e.g., to fabricate arrays of perfectly resonant QDs. [ABSTRACT FROM AUTHOR]

The authors apply selective wet chemical etching and atomic force microscopy to reveal the three-dimensional shape of SiGe/Si(001) islands after capping with Si. Although the “self-assembled quantum dots” remain practically unaffected by capping in the temperature range of 300–450 °C, significant morphological changes take place on the Si surface. At 450 °C, the morphology of the capping layer (Si matrix) evolves toward an intriguing semifacetted structure, which we call a “ziggurat,” giving the misleading impression of a stepped SiGe island shape. [ABSTRACT FROM AUTHOR]

Ordered GaAs/AlGaAs quantum dots (QDs) are fabricated on patterned GaAs(001) substrates and their optical properties are investigated by microphotoluminescence (PL) spectroscopy. QDs exhibit sharp excitonic lines with typical single QD emission features. Photon-correlation spectroscopy shows single photon emission for the neutral exciton transition. Polarization-dependent PL measurements reveal a sharp exciton line and a fine structure exchange splitting of about 70 μeV. [ABSTRACT FROM AUTHOR]

We present microphotoluminescence investigations of InAs quantum dots (QDs) integrated into self-rolling InGaAs/GaAs strained layers. The emission signal from the QDs is redshifted due to strain relaxation and increased in intensity after the strained layers are released from the substrate and rolled up into tubes. We detect waveguided light at the tube ends, which originates from the QDs at the laser excitation spot. The possibility of integrating quantum emitters into the tube walls acting as waveguides represents a major step toward the realization of flexible high quality factor optical resonators based on rolled-up nano- and microtubes. [ABSTRACT FROM AUTHOR]

We investigate the photoluminescence (PL) properties of three-dimensional InAs/GaAs quantum-dot (QD) crystals grown on shallow modulated periodic hole arrays patterned on GaAs(001). We find that the PL spectra become narrower and more intense with increasing number of QD layers. A deconvoluted PL linewidth of 14.9 meV is obtained from a defect-free QD crystal consisting of 11 stacked QD layers. The PL spectra obtained for QD crystals containing QD vacancies show significantly broader spectra. The PL peak energy and linewidth of the QDs across the whole pattern (100×100 μm2) remain constant within 1.278±0.001 eV and 21.0±1.7 meV, respectively. From power-dependent PL measurement, we can resolve up to seven excited-state PL peaks confirming the remarkable size homogeneity of our QD crystals. This experimental result can be reasonably fitted by a calculation based on random population theory and on a simple model for the QD confinement potential. [ABSTRACT FROM AUTHOR]

We report on the formation of ordered and size homogeneous InAs quantum dot (QD) arrays on patterned GaAs(001) substrates. A material depletion region is observed around the patterned area while a long-range homogeneous distribution of QDs is found inside the patterned area. A sample with less InAs deposition shows a gradient in the material distribution over the patterned area. Based on these observations we propose a simple model to describe the QD formation in the patterned area. The QD shape evolution is also investigated and discussed. [ABSTRACT FROM AUTHOR]

Single quantum dots (QDs) reproduce into pairs of QDs with increasing separation distance during the growth of a three-dimensional QD crystal. Kinetic Monte Carlo simulations—that rely on strain profiles deduced from experiment—can describe this lateral replication process, which is triggered by a distinct ridge structure that evolves during the overgrowth of two-dimensional periodic QD arrays. [ABSTRACT FROM AUTHOR]

The model systems for self-organized quantum dots formed from elemental and compound semiconductors, namely Ge grown on Si(001) and InAs on GaAs(001), are comparatively studied by scanning tunneling microscopy. It is shown that in both material combinations only two well-defined families of faceted and defect-free nanocrystals exist (and coexist). These three-dimensional islands, pyramids, and domes show common morphological characteristics, independent of the specific material system. A universal behavior is further demonstrated in the capping-passivation process that turns the nanocrystals in true quantum dots. [ABSTRACT FROM AUTHOR]

Self-assembled InP quantum dots, embedded in Ga[SUB0.52]In[SUB0.48]P and grown by solid source molecular-beam epitaxy, exhibit strong structural and optical anisotropy. Photoluminescence measurements reveal that the quantum dots are elongated in [1&1bar;0] crystal direction and the optical transitions of both the dots and the surrounding GaInP material dominate for light polarized along this direction, whereas embedded compressively strained Ga[SUBx]In[SUB1-x]P quantum wells behave isotropically. The comparison of the optical gain of a strained Ga[SUBx]In[SUB1-x]P quantum well laser and a threefold stacked quantum-dot laser in [110] and [1&1bar;0] directions (edge emission) emphasizes this difference. The gain of the quantum-well laser shows no directional dependence. The quantum-dot laser reveals significantly larger gain for light propagating perpendicular to the dot elongation. Thus, particular care has to be taken to align the cavities of InP/GaInP quantum-dot lasers in [110] direction. [ABSTRACT FROM AUTHOR]

InAs/GaAs(001) quantum dots are grown at high temperature and extremely low flux and analyzed by in situ scanning tunneling microscopy. A bimodal distribution of dots is observed, composed of "small" and "large" islands. While the former show a broad distribution of sizes and shapes, the latter appear to be highly uniform and have a truncated pyramid shape with irregular octagonal base. (110) and (111) facets are identified and atomically resolved showing (1 × 1) and (2 × 2) surface reconstructions, respectively. The shape of the large quantum dots is in excellent agreement with recent theoretical predictions, proving that the chosen deposition conditions are close to thermodynamic equilibrium. [ABSTRACT FROM AUTHOR]

We present a systematic study of closely stacked InAs/GaAs quantum dots (QDs) grown at low growth rates. Transmission electron microscopy reveals that for thin spacer layers vertically aligned QDs merge into one large QD. After capping the initial QD layer the GaAs surface is decorated with well-developed nanostructures, which act as nucleation centers for the QDs deposited in the second layer. Despite the size increase, photoluminescence (PL) experiments show a systematic blueshift up to 103 meV of the QD related signal with decreasing spacer thickness. We explicitly show that this significant blueshift cannot fully be ascribed to specific growth phenomena, but instead is caused by the actual presence of the second dot layer. We report a PL linewidth as narrow as 16 meV at low temperature for a sample with 5 nm spacer thickness. [ABSTRACT FROM AUTHOR]

Low-temperature epitaxial growth of Si--Ge heterostructures opens possibilities for synthesizing very small and abrupt low-dimensional structures due to the low adatom surface mobilities. We present photoluminescence from Ge quantum structures grown by molecular-beam epitaxy at low temperatures which reveals a transition from two-dimensional to three-dimensional growth. Phononless radiative recombination is observed from ‹105› faceted Ge quantum dots with height of approximately 0.9 nm and lateral width of 9 nm. Postgrowth annealing reveals a systematic blueshift of the Ge quantum dot's luminescence and a reduction in nonradiative recombination channels. With increasing annealing temperatures Si--Ge intermixing smears out the three-dimensional carrier localization around the dot. [ABSTRACT FROM AUTHOR]

We present gain measurements and calculations for InAs/GaAs quantum dot injection lasers. Measurements of the modal gain and estimation of the confinement factor by transmission electron microscopy yield an exceptionally large material gain of 6.8(±1)×104 cm-1 at 80 A cm-2. Calculations including realistic quantum dot energy levels, dot size fluctuation, nonthermal coupling of carriers in different dots, and band filling effects corroborate this result. A large maximum differential gain of 2×10-12 cm2 at 20 A cm-2 is found. The width of the gain spectrum is determined by participation of excited quantum dot states. We record a low transparency current density of 20 A cm-2. All experiments are carried out at liquid nitrogen temperature. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This article presents information on quantum dot arrays. Researchers have reported on a technique to realize long-range ordered quantum dot (QD) arrays with pronounced size homogeneity. The technique comprises patterning of a GaAs substrate surface with an anodic aluminum oxide membrane and subsequent self-assembled InAs QD growth. The researchers developed this technique because they claimed that an effective means to synthesize three-dimensional confined nanostructures, which possess a high degree of spatial ordering did not exist before.

We report on hole g-factor measurements in three terminal SiGe self-assembled quantum dot devices with a top gate electrode positioned very close to the nanostructure. Measurements of both the perpendicular as well as the parallel g-factor reveal significant changes for a small modulation of the top gate voltage. From the observed modulations, we estimate that, for realistic experimental conditions, hole spins can be electrically manipulated with Rabi frequencies in the order of 100 MHz. This work emphasises the potential of hole-based nano-devices for efficient spin manipulation by means of the g-tensor modulation technique. [ABSTRACT FROM AUTHOR]

We prepare symmetry-controlled GaAs/AlGaAs quantum dots (QDs) on (001) GaAs substrates by infilling GaAs into AlGaAs nanoholes. For the most symmetric QDs, we measure an average excitonic fine structure splitting (FSS) of only (3.9 ± 1.8) μeV. The FSS and polarization direction of the two bright excitonic recombination lines directly reflect the degree of the QD symmetry. Since the FSS is comparable to typical homogeneous linewidths of excitonic recombination, these strain-free GaAs/AlGaAs QDs might offer a practical platform to generate entangled photons in future quantum devices. [ABSTRACT FROM AUTHOR]

A systematic investigation of the stacking behavior of InAs quantum dots (QDs) with varying GaAs interlayer thickness d is presented. We compare two-fold stacks of large QDs (approx. 25 nm base width), which emit at 1.30 μm, to small QDs (approx. 20 nm base width) emitting at 1.14 μm. For large islands photoluminescence yields an energetic blueshift of the second layer islands with decreasing d, although transmission electron microscopy clearly reveals a approx. 70% larger dot size in the second layer, whereas for small islands a similar size increase of the dots in the upper layer and an energetic redshift are observed. A detailed analysis of confinement and material intermixing effects suggests that for large QDs strain driven material intermixing is dominant. For small QDs the confinement effect plays the major role and causes the observed photoluminescence energy redshifts. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]