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Excitonic dynamics in a hybrid dot-well system composed of InAs quantum dots (QDs) and an InGaAs quantum well (QW) is studied by means of femtosecond pump-probe reflection and continuous wave (cw) photoluminescence (PL) spectroscopy. The system is engineered to bring the QW ground exciton state into resonance with the third QD excited state. The resonant tunneling rate is varied by changing the effective barrier thickness between the QD and QW layers. This strongly affects the exciton dynamics in these hybrid structures as compared to isolated QW or QD systems. Optically measured decay times of the coupled system demonstrate dramatically different response to temperature change depending on the strength of the resonant tunneling or coupling strength. This reflects a competition between purely quantum mechanical and thermodynamical processes. [ABSTRACT FROM AUTHOR]

Change of the photoluminescence (PL) polarization is studied by changing the excitation intensity and temperature for aligned In(Ga)As quantum dot (QD) structures with varying inter-dot distances grown by molecular beam epitaxy on semi-insulating GaAs (100) substrates. An unusual increase of the polarization ratio is observed by increasing the temperature and/or excitation intensity throughout a low temperature (T < 70 K) and low intensity (Iex < 1 W/cm2) range. This increase as well as the general behavior of the polarized PL are the results of the exciton dynamics and the peculiarities of the system morphology. They are due to the varying inter-dot distances which change the system from zero-dimensional comprised of isolated QDs to one-dimensional comprised of wire-like structures. [ABSTRACT FROM AUTHOR]

We have studied the effects of AlGaAs energy barriers surrounding self-assembled InAs quantum dots in a GaAs matrix on the properties of solar cells made with multiple quantum dot layers in the active region of a photodiode. We have compared the fenced dot samples with conventional InAs/GaAs quantum dot solar cells and with GaAs reference cells. We have found that, contrary to theoretical predictions, the AlGaAs fence layers do not enhance the transport properties of photogenerated carriers but instead suppress the extraction of the carriers excited in the dots by light with wavelengths longer than the cutoff wavelength of the GaAs matrix material. Both the standard quantum dots and the fenced dots were found to give solar cell performance comparable to the GaAs reference cells for certain active region thicknesses but neither showed enhancement due to the longer wavelength absorption or improved carrier transport. [ABSTRACT FROM AUTHOR]

A systematic spectroscopic study of the carrier transfer between quantum dot (QD) and quantum well (QW) layers is carried out in a hybrid dot-well system based on InAs QDs and InGaAs QWs. We observe a strong dependence of the QD and QW photoluminescence (PL) both on the dot-well barrier thickness and height. For thick (or high) barriers QD and QW systems accumulate independently sufficient photogenerated carrier densities to be seen in PL even at low nonresonant excitation power. For thin (or low) barriers it is impossible to detect the PL signal from QW at low excitation densities due to effective carrier transfer from QW to QDs. Strong state-filling effects of the excited QD states influence the carrier transfer efficiencies. By investigating the carrier dynamics using time-resolved spectroscopy and the state-filling effects in the continuous wave excitation regime the basic characteristics of interlevel, intersublevel, and dot-well relaxation are determined. The mechanisms of the dot-well coupling are discussed. [ABSTRACT FROM AUTHOR]

We investigate the optical properties of newly developed InGaAs quantum dot clusters (QDCs). The QDCs are produced using a hybrid growth method that combines droplet homoepitaxy and Stranski–Krastanov growth modes. We focus on a particular geometry, where six individual quantum dots (QDs) spontaneously form a structure morphologically similar to a benzene ring. We observe narrow exciton resonances in microphotoluminescence measurements. Temperature and excitation density dependence of the exciton resonances are investigated. Our experiments suggest that excitons are strongly confined in individual QDs instead of residing in all QDs in the cluster. [ABSTRACT FROM AUTHOR]

Long chains of quantum dots formed in InGaAs/GaAs(100) multiple layers have been systematically investigated by scanning electron, transmission electron, and atomic force microscopies. In addition to the usual two-dimensional wetting layer involved in the Stranski-Krastanov growth, we have directly observed a one-dimensional postwetting layer along the [01-1] direction that strings together the quantum dots in each chain. In sharp contrast with the two-dimensional wetting layer, which exists before the quantum-dot chains form, this one-dimensional postwetting layer develops during the GaAs capping of the existing dot chains. This one-dimensional layer forms through the anisotropic surface diffusion of In atoms that accompanies the change in strain profile during capping and therefore produces the steady-state material distribution that includes a one-dimensional postwetting layer as a result. [ABSTRACT FROM AUTHOR]

Using high-resolution x-ray diffraction (HRXRD), Raman scattering, photoluminescence, and atomic-force microcopy, we investigated InxGa1-xAs/GaAs quantum dot (QD) stacks grown by molecular-beam epitaxy with nominal In contents of 0.30 and 0.35. The analysis of x-ray-diffraction rocking curves using symmetrical (004), asymmetrical (113), and quasiforbidden (002) reflections within the framework of dynamical theory allowed us to determine the average values of strains parallel and perpendicular to the growth direction. We also measured nonuniform In profiles in the InxGa1-xAs layers along the growth direction. This observation confirms the important role of surface segregation of In atoms and interdiffusion of Ga atoms from GaAs layers in the formation of InxGa1-xAs QDs. Both HRXRD and Raman scattering in InxGa1-xAs/GaAs-stacked QD structures demonstrate that the InGaAs inserts in these structures can be modeled effectively as sublayers with two different compositions: sufficiently thick InxGa1-xAs sublayer with the In concentration lower than the nominal one, which includes the thin layer of InGaAs islands with the In concentration much higher than the nominal one. [ABSTRACT FROM AUTHOR]

Temperature-dependent photoluminescence is investigated in bilayer InAs/GaAs quantum dot structures with constant InAs deposition θ1 in the seed layer, but variable deposition θ2 in both the second layer and the GaAs spacer layer. It is shown that interlayer coupling, leading to the formation of asymmetric quantum dot pairs, strengthens the high-temperature photoluminescence and strongly influences carrier relaxation channels. We report that radiative recombination and carrier capture efficiency by the quantum dots in the second layer can be tailored using the deposition θ2 and the GaAs spacer thickness. [ABSTRACT FROM AUTHOR]

We report on an InAs quantum dot (QD) hybrid structure with a top surface QD layer coupled to two buried QD layers that is highly sensitive to surface passivation. After 180 min of passivation, the photoluminescence (PL) peak of the surface QDs shifts from 1545 to 1275 nm while its intensity decreases by one order of magnitude. Time-resolved PL reveals a significant decrease of carrier tunneling between the QD layers because of the surface state modification by chemical treatment. A simple model with rate equations is used to explain the observed optical performance. Our results show that the optical performance of this hybrid structure is very sensitive to the surface environment, making it a potential candidate for sensing applications. [ABSTRACT FROM AUTHOR]

Molecular beam epitaxy growth of multilayer InxGa1-xAs/GaAs(001) structures with low indium content (x = 0.20-0.35) was studied by X-ray diffraction and photoluminescence in order to understand the initial stage of strain-driven island formation. The structural properties of these superlattices were investigated using reciprocal space maps, which were obtained around the symmetric 004 and asymmetric 113 and 224 Bragg diffraction, and ω/2θ scans with a high-resolution diffractometer in the triple axis configuration. Using the information obtained from the reciprocal space maps, the 004 ω/2θ scans were simulated by dynamical diffraction theory and the in-plane strain in the dot lattice was determined. We determined the degree of vertical correlation for the dot position ("stacking") and lateral composition modulation period (LCM) (lateral ordering of the dots). It is shown that initial stage formation of nanoislands is accompanied by LCM only for [110] direction in the plane with a period of about 50 to 60 nm, which is responsible for the formation of a quantum wire like structure. The role of InxGa1-xAs thickness and lateral composition modulation in the formation of quantum dots in strained InxGa1-xAs/GaAs structures is discussed. [ABSTRACT FROM AUTHOR]

The surface morphology and optical properties of the (In,Ga)As/GaAs(100) multilayer structures with self-organized quantum dots and quantum wires, which were grown by molecular-beam epitaxy, are investigated. It is found that the ordered arrangement of quantum dots in the heterointerface plane starts to form during the growth of the first periods of the multilayer structure. As the number of periods increases, quantum dots line up in series and form wires along the $$[0\bar 11]$$ direction. An increase in the lateral ordering of the structures under consideration correlates with an increase in the optical emission anisotropy governed by relaxation anisotropy of elastic strains and by the shape of nano-objects. A possible mechanism of lateral ordering of quantum dots and wires in multilayer structures, which includes both anisotropy effects of the strain fields and adatom diffusion, as well as the elastic interaction of neighboring quantum dots, is discussed. [ABSTRACT FROM AUTHOR]

The electronic states and direct interband absorption of light in quantum dot molecule (QDM) consisting 4 or 5 QDs are investigated. The electron energy levels and wave functions are calculated. It is shown that the energy levels are split in azimuth direction. In the case of the presence of the central QD in QDM an additional splitting in the radial direction is revealed. The suggested method can be used to calculate the electronic states and light absorption of arbitrary number QDM. The results are useful for the application of the QDM to photoelectric devices. [ABSTRACT FROM AUTHOR]

A detailed growth scenario of surface quantum dot molecules (QDM) in the system (In,Ga)As/GaAs(001) has been investigated in terms of shape and elastic strain evolution. QDMs are grown by a combined approach using droplet epitaxy for initial homoepitaxial GaAs mounds, which subsequently serve as nucleation spots for surrounding (In,Ga)As surface quantum dots. Atomic force micrographs trace a detailed pathway toward the final QDM containing up to six quantum dots with perfect inherent symmetry. Synchrotron-based grazing incidence diffraction together with grazing incidence small angle x-ray scattering reveal a relaxation behavior, which for all growth stages comprises a strained lattice along [formula] and partial elastic relaxation along [110]. Numerical finite element calculations on the three-dimensional strain profile support the experimental findings. [ABSTRACT FROM AUTHOR]

We have investigated the self-organized, step bunch assisted formation of In0.19Ga0.81As/GaAs quantum dots in vertical superlattices consisting of one, four, eight, and ten periods. Samples were grown by molecular beam epitaxy on vicinal 2°A and 2°B GaAs(001) substrates. Those with miscut along the [110] (2°B) exclusively show step bunches of an aspect ratio larger than 10 but without the formation of quantum dots. This highly linear pattern is improved during subsequent periods as proved by high resolution x-ray diffraction and grazing incidence diffraction. On the other hand, a miscut along the [110] (2°A) initially causes a crosslike pattern of step bunches, which finally becomes a two-dimensional arrangement of individual quantum dots. [ABSTRACT FROM AUTHOR]

The self-organized formation of In0.40Ga0.60As quantum dot chains was investigated using x-ray scattering. Two samples were compared grown on GaAs(100) by molecular beam epitaxy. The first sample with a single layer of In0.40Ga0.60As dots shows weak quantum dot alignment and a corresponding elongated shape along [011], while the top layer of a multilayered In0.40Ga0.60As/GaAs sample exhibits extended and highly regular quantum dot chains oriented along [011]. Numerical calculations of the three-dimensional strain fields are used to explain the initial stages of chain formation by anisotropic strain relaxation induced by the elongated dot shape. [ABSTRACT FROM AUTHOR]

The authors report on the use of GaAs islands, formed by the droplet epitaxy growth technique, as a template for the growth of clusters of InAs quantum dots. Surface morphology measurements show that the shape and dimensions of the GaAs islands and consequently the formation of InAs quantum dots depend strongly on the annealing temperature and annealing time. This can be explained by the diffusion of gallium atom during the annealing process and the selective formation of InAs quantum dots on the misoriented GaAs island sidewalls. [ABSTRACT FROM AUTHOR]

The authors report on x-ray diffuse scattering at nominally strain-free GaAs(001) quantum dot molecules (QDMs). Al0.3Ga0.7As deposited by molecular beam epitaxy on GaAs(001) acts as barrier layer between the GaAs(001) substrate and subsequently grown QDMs; the adjusted thickness of 50 nm preserves the in-plane lattice parameter. Pairs of lenselike quantum dots are created with preferential orientation along [110] placed on shallow hills. Grazing incidence diffraction along with kinematical scattering simulations indicate completely strain-free QDs which prove a strongly suppressed intermixing between QDMs and the underlying AlGaAs barrier layer. [ABSTRACT FROM AUTHOR]

Laterally ordered multilayered arrays of InGaAs quantum dots are investigated by photoluminescence as a function of high index GaAs substrates. Different laser wavelengths are used to investigate the photoluminescence from quantum dots layer-by-layer. High optical quality is demonstrated for laterally ordered quantum dot arrays. GaAs(511)B is identified as the optimum high index substrate for growth of InGaAs/GaAs multilayered quantum dots, demonstrating strong photoluminescence with a narrow full width at half maximum linewidth of 23 meV in spite of the potential for misfit dislocations. [ABSTRACT FROM AUTHOR]

We report selective chemical etching as a promising procedure to study the buried quantum dots in multiple InGaAs/GaAs layers. The dot layer-by-dot layer etching is demonstrated using a mixed solution of NH4OH:H2O2:H2O. Regular plan-view atomic force microscopy reveals that all of the exposed InGaAs layers have a chain-like lateral ordering despite the potential of significant In–Ga intermixing during capping. The vertical self-correlation of quantum dots in the chains is observed. [ABSTRACT FROM AUTHOR]

Transient photoluminescence from a series of asymmetric InAs quantum-dot bilayers with a GaAs barrier layer thickness varying from 30 to 60 monolayers between the quantum-dot planes is investigated. The interdot carrier transfer process is analyzed. In the framework of a three-level system, interdot carrier transfer times between 200 and 2500 ps are derived and compared with similar data from the literature. Within the semiclassical Wentzel–Kramers–Brillouin approximation, the observed “transfer time-barrier thickness-relation” supports nonresonant tunneling as the microscopic carrier transfer mechanism. [ABSTRACT FROM AUTHOR]

We report results of time-resolved terahertz (THz) pulse spectroscopy experiments on laterally ordered chains of self-assembled InGaAs quantum dots photoexcited with 400 nm, 100 fs laser pulses. A large anisotropy in the transient photoconductive response is observed depending on the polarization of the THz probe pulse with respect to the orientation of the dot chains. Fast (3.5–5 ps) and efficient carrier capture into the dots and one-dimensional wetting layers underneath the dot chains is observed below 90 K. At higher temperatures, thermionic emission into the two-dimensional wetting layers and barriers becomes significant and the anisotropy in the photoconductive signal is reduced. [ABSTRACT FROM AUTHOR]

Utilizing the naturally curved surface contours provided by oval defects on a GaAs(100) surface, we demonstrate that alignment of quantum-dot chains formed during the growth of (In,Ga)As multilayers is unyielding to a modest deviation of surface orientation from (100) of about 0.7° along [01-1] and 8° along [011]. This finding suggests that the strain-driven kinetic anisotropy responsible for the formation of the quantum dot chains dominates over selective island formation at steps due to surface misorientation. The robustness of the quantum dot chain adds to its potential for its future application. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Generally (In,Ga)As strained growth on GaAs surfaces results in zero-dimensional quantum dots. The formation of one-dimensional quantum wires is demonstrated during (In,Ga)As molecular-beam-epitaxial growth on GaAs(311)A at high temperature. The wires are running along the [-233] direction. Atomically resolved scanning tunneling microscopy images reveal that the wires are triangular-shaped in cross section and the two side bonding facets are {11,5,2}. These results are discussed in terms of a mechanism of strain-driven facet formation. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]