Aligned Quantum Dots Research Articles

Artificial array of InAs quantum dots on a strain-engineered superlattice

Aligned quantum dots (QDs) were successfully formed along the 〈1 1 0〉 direction on a strain-engineered layer using InAs/GaAs superlattice (SL). The aligned QDs have good quantum confinements without degradation of optical properties. It is found from transmission electron spectroscopy and Raman scattering measurements that the strained InAs/GaAs SL acts as a strained layer having tensile stress for the site control of QDs without the generation of defects acting as nonradiative recombination centers. The artificial array of self-assembled QDs can provide a clue for the easy and high throughput fabrication method for the application of single-electron devices.

Decoherence and quantum entangled exciton states in semiconductor quantum dot molecule

Recently a pair of vertically aligned quantum dots has been suggested as optically driven quantum gate. The later is built when two different particles, an electron and a hole, are created optically. The two particles form entangled states. We study theoretically the dynamics of a single heavy hole exciton confined in a small vertically coupled quantum dot. We use harmonic potentials to model the confining of electrons and holes and calculate the exciton's energy spectrum using the Hund-Mulliken approach, and including the Coulomb interaction. The entanglement may be controlled by the application of external electric field. At certain times of dynamical evolution one can generate a maximally entangled coherent exciton state by initially preparing the quantum dots in a superposition of coherent state. In ideal situations, entangled quantum states would not decohere during processing and transmission of quantum information. However, a real quantum system is inevitably influenced by its environment. Since, we study the decoherence effect caused by the interaction of the confined carriers with the acoustic phonon effects. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electric field control of exciton states in quantum dot molecules

Semiconductor nanostructures have attracted considerable interest during the recent years in view of the potential application in quantum information processing. In particular, quantum dots have been suggested to fulfill an essential requirement for quantum computation: controllable interaction that couples two quantum dot qubits. Previous experiments on two vertically aligned quantum dots have demonstrated the formation of coupled exciton states. We show that this coupling between two In 0.60Ga 0.40As/GaAs quantum dots can be tuned by an electric field applied along the molecule axis. This controllable coupling in such a relatively simple configuration could be implemented in a solid-state-based quantum device.

Replication and alignment of quantum dots in multilayer heteroepitaxial growth

We propose a model to elucidate the self-organization process of islands in multilayer heteroepitaxial growth. The model is based on the preferential nucleation and growth of islands in regions of tensile strain in the total strain field on a spacer-layer surface induced both by embedded islands and by growing surface islands. Surface islands nucleate at the regions of tensile strain induced by embedded islands. The islands grow in size and induce increasingly large compressive strain on the spacer-layer surface. A surface island reaches a stable size after the total strain field around it becomes compressive. The model predicts that islands in successive layers not only form ordered columns but also show uniform distributions of island size and spacing, in agreement with experimental observations.

The effect of inter-dot separation on the finite difference solution of vertically aligned coupled quantum dots

The effect of inter-dot separation in a double pyramidal vertically aligned quantum dot system is investigated using a finite difference technique. It was found that as the separation distance increased beyond a certain limit, 40 Å in this case, the lowest energy electron wave function was localised in the larger dot. When the dots were in close proximity the probability of the carrier existing in the smaller dot increased. The method used here was a finite difference approach which is widely applicable to many scenarios and provided quantitative results for these intuitive concepts.

Effect of elastic anisotropy on the elastic fields and vertical alignment of quantum dots

The elastic fields in the self-organized quantum dot (QD) structures are investigated in details by three-dimensional finite element analysis for an array of lens shaped QDs. Emphasis is placed on the effect of elastic anisotropy of the materials with the anisotropy ratio A ranging from 0.25 to 4.0 for both the QDs and the matrix. It is found that the elastic anisotropy strongly influences the distributions of strain, stress, and strain energy density in the QD structures. It is shown that the elastic interactions among the buried QDs play crucial role in the formation of the satellite energy minima at the cap layer surface, while the materials anisotropy and the cap layer thickness also play important roles. By changing the elastic anisotropy ratio and the cap layer thickness, substantially different distributions of strain energy minima on the cap layer surface are obtained, which may result in various QD ordering phenomena such as vertical alignment, partial alignment, or complete misalignment. Based on the calculation results, a phase diagram is constructed to show the effect of material anisotropy and cap layer thickness on the vertical correlation of QDs.

Fabrication of single- or double-row aligned self-assembled quantum dots by utilizing SiO2-patterned vicinal (001) GaAs substrates

We investigated the formation of In0.8Ga0.2As self-assembled quantum dots (SAQDs) grown on SiO2-patterned 1°, 2°, and 5°-off (001) GaAs substrates by selective area metalorganic vapor phase epitaxy technique. The SiO2 patterns were filled with various stripe opening windows along the misorientation direction of the substrates. During the growth of the GaAs buffer layer on the opening regions, the steps on the (001) top facet was affected by the widths of the (001) top facet and the misorientation angles of the substrates. Single- or double-row aligned In0.8Ga0.2As SAQDs having definite interval were successfully fabricated on the (001) top facet with optimized top width and periodicity of step bunching. These results indicate that the selective growth technique of SAQDs by utilizing SiO2-patterned vicinal substrates is promising for nanoelectronic device applications such as single-electron memory devices.

Alignment of InAs quantum dots on a controllable strain-relaxed substrate using an InAs/GaAs superlattice

We fabricated InAs self-assembled quantum dots on a strained layer using molecular beam epitaxy. The controllable strained layer consisted of an InAs/GaAs superlattice and a GaAs spacer layer on a GaAs (001) substrate. We formed two-dimensional arrays of quantum dots along the 〈110〉 directions on the partially strain-relaxed layer that is formed using the superlattice system. The increase in the thickness of the partially strain-relaxed layer resulted in stronger alignment of the quantum dots. The aligned quantum dots are applicable to quantum devices, because they confine carriers well, in spite of the existence of dislocation networks. Strongly aligned quantum dots have a lower carrier transition energy because of their larger size and increased relaxation.

Coupled-stripe quantum-well-assisted AlGaAs–GaAs–InGaAs–InAs quantum-dot laser

Data are presented on the coupled-stripe laser operation (continuous wave, 300 K) of a single InAs quantum-dot (QD) layer coupled via a thin (5 Å) GaAs barrier to an auxiliary strained InGaAs quantum well (QW) grown (confined) in an AlGaAs–GaAs heterostructure. Because of strain-induced (QW strain) improvement of the QD growth and QD alignment along diagonal (reflecting) ridges, the InGaAs-QW+InAs-QD crystals exhibit high gain along and across laser stripes, which is advantageous for coupled-stripe laser operation. A twin-stripe single-QD-layer QW+QD laser (4 μm stripes on 6 μm centers) of usual cleaved length, < 500 μm, is capable of continuous 300 K operation, with only probe heat-sink clamping and testing, at >50 mW.

Entangled states of electron–hole complex in a single InAs/GaAs coupled quantum dot molecule

We summarize here results of calculations and experiments on electron and valence hole states in a single pair of vertically stacked and electronically coupled InAs self-assembled quantum dots. In perfectly aligned quantum dots one can relate an electron–hole complex to a pair of entangled qubits. The information carried by individual qubit is related to the quantum dot index (isospin) of individual carrier. The quality of fabricated quantum dot molecules is identified from the exciton fine structure in a magnetic field.

Precise control of strain field for the selective formation of self-assembled InAs/GaAs quantum dots

Abstract We have investigated the alignment of self-assembled InAs quantum dots (QDs) using the strained InxGa1−xAs/GaAs superlattice system in which we can control the strain field precisely. Through the estimation of critical thickness of strained layer, an in situ alignment of quantum dots is obtained along 〈1 1 0〉 direction on the controlled strained layer. The anisotropic alignments along [1 1 0] and [ 1 1 0 ] directions are closely related with the anisotropy of strain-relief for both directions, which affect the anisotropic diffusivity of indium adatoms on the strained superlattice layer. The strong alignment appears on the highly strained layer, resulting in the formation of chained quantum dots. These results indicate that aligned InAs quantum dots formed by changing the periods of strained superlattice hold promise for potential applications in nanoelectronic devices, such as single electron tunneling devices.

Study of GaAs spacer layers in InAs/GaAs vertically aligned quantum dot structures

We investigated vertically aligned InAs/GaAs QD structures, grown by atomic layer molecular beam epitaxy, with a number N of layers and with spacer thicknesses d. QD alignment and structure quality were checked by transmission electron microscopy. The dependencies of carrier capture, decay dynamics and existence of quenching channels on the design parameters N and d were studied by time resolved photoluminescence (PL), PL excitation (PLE) and PL temperature-dependent measurements. Our results show that the carrier capture and the radiative efficiency of the QDs are negatively affected by increasing the number of QD layers and by reducing the spacer thicknesses; this effect is likely to be related to the increase of defect concentrations in GaAs spacers, due to relaxation of an increasingly large strain.

Alignment of InAs Quantum Dots on GaAs Using the Manipulation of Strain Fields

ABSTRACTWe fabricated InAs self-assembled quantum dots (QDs) on strained layer using molecular beam epitaxy. The strained layer consisted of InAs/GaAs superlattice(SL) and GaAs barrier layer on (001) GaAs substrate. Through controlling thickness of the strained layer, we formed two-dimensional alignments of QDs on misfit dislocation arrays along <110> directions made by strained layer. The increase of the strained layer thickness resulted in a stronger alignment of QDs, which were observed by atomic force microscopy studies. The aligned QDs were confirmed to confine carriers well and have different size distributions by photoluminescence measurement.

X-Ray diffraction analysis of multilayer InAs-GaAs heterostructures with InAs quantum dots

Multilayer InAs-GaAs structures with an array of vertically aligned InAs quantum dots in a GaAs matrix, grown by molecular-beam epitaxy, were investigated by crystal truncation rods and high-resolution x-ray diffractometry methods. It was shown that the formation of scattering objects such as vertically aligned quantum dots in the structures strongly influences the mechanism of diffraction scattering of x-rays and changes the spatial distribution of the diffracted radiation. This is explained by the appearance of additional long-range order in the lateral arrangement of the scattering objects in the periodic structures, by the curving of the crystallographic planes in the periodic part of the structure, and by the quasiperiodicity of the deformation profile due to the vertically coupled quantum dots. The observed spatial distribution of the diffracted intensity can be explained qualitatively on the basis of a new model where the scattering layers with quantum dots consist of defect-free, coherently coupled, InAs and GaAs clusters.

Self-organization of the InGaAs/GaAs quantum dots superlattice

The mechanism of self-organization of quantum dots (QDs) during the growth of InGaAs/GaAs multilayers on GaAs (1 0 0) was investigated with cross-sectional transmission electron microscopy (XTEM), and double-crystal X-ray diffraction (DCXD). We found that the QDs spacing in the first layer can affect the vertical alignment of QDs. There seems to exist one critical lateral QD spacing, below which merging of QDs with different initial size is found to be the dominant mechanism leading to perfect vertical alignment. Once the critical value of QDs spacing is reached, the InGaAs QDs of the first layer are simply reproduced in the upper layers. The X-ray rocking curve clearly shows two sets of satellite peaks, which correspond to the QDs superlattice, and multi-quantum wells (QW) formed by the wetting layers around QDs.

Electronic coupling and structural ordering of quantum dots using InAs quantum dot columns

In this article, recent investigations of vertically aligned quantum dot columns conducted at Stanford University are reviewed. The quantum dots are InAs in a matrix of GaAs. Both the quantum dots and quantum dot columns are formed through strain-induced islanding, without lithography. Two aspects of these columns are discussed. First, the electronic coupling of quantum dots within columns of up to ten quantum dots is demonstrated. The coupling is adjusted and improvements to a simple light-emitting diode are shown. Second, increased uniformity of a surface quantum dot layer is shown when a subsurface layer of these columns are used. The most impressive results occur when the columns contain a large number of islands. Reduced variations in average ensemble height and diameter, called size uniformity, and average nearest neighbor distances, called structural uniformity, are shown. A surface unit cell of islands is demonstrated and the lack of a surface lattice is discussed.

Ripening suppression and large photoluminescence blueshift in aligned InGaAs quantum dots on a vicinal (100) GaAs substrate

We observe ripening suppression in aligned InGaAs quantum dots (QDs) along multiatomic steps on a vicinal (100) GaAs substrate. By varying the growth interruption times, a study of QD morphologies by atomic force microscope reveals a clear Ostwald ripening suppression effect on QD formation. On the other hand, we observe a regular ripening for InGaAs QDs on an exact (100) substrate. In addition, n-QD chains aligned along multiatomic steps are observed. An inhomogeneously broadened photoluminescence (PL) spectrum with larger spectral width is obtained for a QD ensemble on a vicinal (100) substrate as compared to those on an exact (100) substrate although statistical analyses of QD size distributions show similar size dispersions. The spectral broadening is attributed to the lateral quantum coupling of aligned n-QD chains. PL spectra for QDs on vicinal (100) substrates show a larger rate of blueshift with increasing interruption times, which is interpreted as a lack of the PL compensation effect by size enlargement of the QDs during growth interruption. Thus the result supports the ripening suppression effect which is observed in the morphology study.

MBE Growth and Characterization of Composite InAlAs/In(Ga)As Vertically Aligned Quantum Dots

ABSTRACTIn the present work we study the effect of vertical alignment in the quantum dot array formed by successive deposition of several rows of InAlAs and InGaAs quantum dots separated by thin AIGaAs spacer layers. Transmission electron microscopy and photoluminescence studies revealed that the InAlAs QDs characterized by high areal density force InGaAs to be transformed into the denser array as compared to the case of spontaneous transformation. Using denser array of composite quantum dots in the active region of a diode laser leads to the increase in modal gain, decrease in internal loss, and decrease in the threshold current density for short cavity diodes. Room temperature continuous wave output power as high as 3.3 W at 0.87 µm is achieved.

Electronic coupling effects in self-assembled InAs quantum dots

Electronic coupling effects in self-assembled InAs quantum dots are investigated using capacitance and far-infrared (FIR) spectroscopy. Coupling between different dot-layers is studied using capacitance spectroscopy on samples with double-layers of vertically aligned quantum dots. A strong electrostatic dot–dot interaction reveals itself in distinct shifts of the many particle ground state energy. The fact that the line widths of the FIR resonances are typically a factor of 3 narrower than expected from the inhomogenous broadening due to size fluctuations of the dots is attributed to lateral coupling between the dots within the plane of the dot ensemble.

Electroluminescence in vertically aligned quantum dot multilayer light-emitting diodes fabricating by growth-induced islanding

A molecular beam epitaxy (MBE) growth-induced islanding process has been used to form self-organized multiple quantum dot layers, in which quantum dots from different layers are vertically aligned in columns and are electronically coupled. These structures are used as the active region in light-emitting diodes operating at room temperature. Light-emitting diodes are investigated using quantum dot columns containing single, 5 and 10 InAs quantum dots. These diodes emit light over a broad band with typical spectral linewidths of 120 nm, peaked between 1000 and 1100 nm. In addition to the quantum dot spectral feature, a spectral feature from a thin quantum well region, integral to the quantum dot formation process, is seen in the single quantum dot diode, but is eliminated in diodes with active regions containing columns of multiple quantum dots.