Search

The interplay between structural properties and charge transfer in self-assembled quantum ring (QR) chains grown by molecular beam epitaxy on top of an InGaAs/GaAs quantum dot (QD) superlattice template is analyzed and characterized. The QDs and QRs are vertically stacked and laterally coupled as well as aligned within each layer due to the strain field distributions that governs the ordering. The strong interdot coupling influences the carrier transfer both along as well as between chains in the ring layer and dot template structures. A qualitative contrast between different dynamic models has been developed. By combining temperature and excitation intensity effects, the tuning of the photoluminescence gain for either the QR or the QD mode is attained. The information obtained here about relaxation parameters, energy scheme, interlayer and interdot coupling resulting in creation of 1D structures is very important for the usage of such specific QR-QD systems for applied purposes such as lasing, detection, and energy-harvesting technology of future solar panels. [ABSTRACT FROM AUTHOR]

The in-plane photoconductivity and photoluminescence are investigated in quantum dot-chain InGaAs/GaAs heterostructures. Different photoconductivity transients resulting from spectrally selecting photoexcitation of InGaAs QDs, GaAs spacers, or EL2 centers were observed. Persistent photoconductivity was observed at 80K after excitation of electron-hole pairs due to interband transitions in both the InGaAs QDs and the GaAs matrix. Giant optically induced quenching of in-plane conductivity driven by recharging of EL2 centers is observed in the spectral range from 0.83 eV to 1.0 eV. Conductivity loss under photoexcitation is discussed in terms of carrier localization by analogy with carrier distribution in disordered media. [ABSTRACT FROM AUTHOR]

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]

A set of high quality single quantum well samples of GaAs1-xBix with bismuth concentrations not exceeding 6% and well widths ranging from 7.5 to 13 nm grown by molecular beam epitaxy on a GaAs substrate at low temperature is studied by means of photoluminescence (PL). It is shown that the PL line shape changes when the exciton reduced mass behavior changes from an anomalous increase (x < 5%) to a conventional decrease (x > 5%). Strongly non-monotonous PL bandwidth dependence on the excitation intensity is revealed and interpreted in terms of optically unresolved contributions from the saturable emission of bound free excitons. [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]

Using molecular beam epitaxy, we prepared an anisotropic media consisting of InGaAs quantum wires epitaxially grown on GaAs (311)A. Anisotropy is observed in the lateral conductivity and photoluminescence polarization. However, an isotropic Hall effect is observed in the same samples. We show that the Hall effect in this anisotropic heterostructure remains isotropic regardless of the change of the doping in GaAs barriers and regardless of the InGaAs coverage, whereas the conductivity anisotropy experiences a strong change under these actions. In addition, we observed an anomalous increase in carrier density, 'freeze-in,' at low temperatures. In order to explain this, we generalized the theory of Look [D. C. Look, Phys. Rev B 42, 3578 (1990)] by considering the low field magneto-transport in anisotropic media. This theory confirms that the Hall constant remains isotropic in anisotropic semiconductor heterostructures, agreeing with our experiment and explains the anomalous behavior of carriers as a result of multi-band conductivity. [ABSTRACT FROM AUTHOR]

Spectroscopic study of self-assembled InAs/GaAs quantum dots (QDs) capped with an InxGa1-xAs quantum well (QW) is carried out under variable excitation intensity and temperature. The QW reduces strain in the QDs, and it is demonstrated that the cap layer shifts the transition energy of the QD lower, while maintaining the strong quantization of the QD electronic states which develops in photoluminescence as a series of excited state transitions at higher excitation intensity. An additional spectral band is also revealed below the energy of the InGaAs QW exciton, which is unambiguously assigned to the emission of a strain-induced QD layer created by the InAs stressors within the InGaAs QW. [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 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]

The effects of electronic wave function hybridization are studied in In(Ga)As/GaAs dot chain samples by means of continuous wave and time resolved photoluminescence (PL). A set of wetting layer (WL) states strongly coupled with the quantum dot (QD) heavy hole states is revealed in the PL. The hybridization of states is favored by the presence of a specific one-dimensional postwetting layer that strings together the QDs in each chain. It is demonstrated that the strength of hybridization is significantly weakened for states deep in the WL gap. The hybridization of the WL states and the QD hole states substantially affects the rates of carrier trapping by the QDs and the carrier distribution. Specific convex shape of the PL transients is related to the presence of long living localized states in the WL energy gap. [ABSTRACT FROM AUTHOR]

GaAs and AlAs thin capping layers as well as postgrowth rapid thermal annealing (RTA) were applied to InAs quantum dots (QDs) grown by molecular beam epitaxy to study the tunability of optical properties of QDs by photoluminescence (PL) methods. The PL of AlAs-capped QDs shows double-peak structure, as opposed to GaAs-capped QDs, which is due to the formation of two families of QDs in the AlAs-capped sample confirmed by the power dependent PL measurements. The PL peak of the GaAs-capped samples subjected to RTA showed blueshift and narrowing with an increase in RTA temperature. This is the result of thermally enhanced In–Ga intermixing. More complex changes in the PL spectrum of AlAs-capped QDs during the RTA procedure were found and explained by the different In compositions in two branches of QDs. The features observed in the temperature dependences of PL peak energy of GaAs- and AlAs-capped samples were interpreted in terms of thermal escape of carriers from smaller QDs with further redistribution between larger QDs and different InAs content in two families of QDs. [ABSTRACT FROM AUTHOR]

Remotely doped In0.35Ga0.65As layers of different coverages 6, 9, 11, and 13 ML were grown by molecular beam epitaxy on (100) GaAs. Quantum dot (QD) nucleation was observed in situ by reflection high-energy electron diffraction at 8 ML growth of In0.35Ga0.65As, while for 6 ML, only two-dimensional (2D) growth was observed. Atomic force microscopy, low temperature photoluminescence, and Hall effect measurements confirmed this transition from 2D to three-dimensional growth. Low-frequency noise studies have been performed to probe defects in such heterostructures throughout the transition from a highly strained quantum well to QDs. Results were compared to a bulk n-type GaAs reference sample. We revealed three main defects in GaAs with activation energies of 0.8, 0.54, and 0.35 eV. These defects with the same activation energies were found in all samples. However, structures containing In0.35Ga0.65As QDs show an additional peak at low temperatures due to the presence of defects which are not observed for reference GaAs and quantum well samples. Detailed analysis shows that for 9 and 11 ML In0.35Ga0.65As QD samples this peak corresponds to the well known M1 defect in GaAs with an activation energy of 0.18 eV, while for a coverage of 13 ML the defect was found to have an activation energy of 0.12 eV. All defects were characterized quantitatively in terms of their activation energy, capture cross section, and density. These studies indicate that noise spectroscopy is a very sensitive tool for electronic material characterization on the nanoscale. [ABSTRACT FROM AUTHOR]

Spectroscopic study of the InAs quantum dot (QD) formation in GaAs ringlike nanostructures is carried out. Ga droplet epitaxy is used to form GaAs ringlike nanostructures. Subsequently InAs is deposited to obtain InAs QDs by self-assembly inside the holes of the nanostructures. Regularly spaced bands in the photoluminescence spectra exhibit state filling properties under increased excitation power. However, it is demonstrated that these bands do not represent excited states of a single ensemble of dots, but are separate ensembles with individual ground state energies, which are coupled through the GaAs ring structure on which they form. The most likely cause of these uniformly spaced ensemble energies is monolayer differences in the effective height of the dots. Temperature, excitation power density, and time dependent photoluminescence measurements are used to demonstrate the significance of the interdot coupling. The photoluminescent properties of these novel nanostructures make them candidates for optoelectronic applications. [ABSTRACT FROM AUTHOR]

Optical transitions in InAs/InP quantum dot structures are studied and interpreted in terms of heavy- (hh) and light-hole (lh) excitons originating in InAs valence band. Appearance of a step-like absorption and two activation energies in photoluminescence spectra of quantum dots under temperature elevation is described as a transition from the excitonic states to the two-dimensional-like states realized in the InAs/InP quantum dots. A mixture of hh and lh states determines the anisotropy of the transmission spectra and may result in rotation of the polarization plane for light transmitted through the InAs/InP quantum dot system. [ABSTRACT FROM AUTHOR]

The structural and optical properties of GaAs1−xBix quantum wells (QWs) symmetrically clad by GaAs barriers with and without additional confining AlGaAs layers are studied. It is shown that a GaAs/GaAs1−xBix/GaAs QW with x ∼ 4% and well width of ∼4 nm grown by molecular beam epitaxy demonstrates efficient photoluminescence (PL) that becomes significantly more thermally stable when a cladding AlGaAs layer is added to the QW structure. The PL behavior for temperatures between 10 and 300 K and for excitation intensities varying by seven orders of magnitude can be well described in terms of the dynamics of excitons including carrier capture in the QW layer, thermal emission and diffusion into the cladding barriers. Understanding the role of these processes in the luminescence of dilute GaAs1−xBix QW structures facilitates the creation of highly efficient devices with reduced thermal sensitivity and low threshold current. [ABSTRACT FROM AUTHOR]

The structural and optical properties of a set of high-quality GaAs1-xBix/GaAs quantum well (QW) heterostructures with Bi concentrations ranging from 3.5% to 6.7% are studied. The energies of the excitonic ground state transitions are determined as a function of Bi concentration and spatial confinement. The influence of material disorder on the optical properties of QWs is investigated. It is determined that trap-related luminescence responds differently to temperature changes depending on whether the Bi concentration is more or less than 5%. Below 5% it contributes significantly to the overall photoluminescence line shape whereas above 5%, it is insignificant. [ABSTRACT FROM AUTHOR]

The photoluminescence spectra of BiI3(Mn) and BiI3(Cr) single crystals were studied. The new bands at λ = 1.1 and λ = 1.83 pm were registered at the temperature 4.2 K. It was shown that the first and second bands arise due to Mn2+ and Cr3+ ions, accordingly. The experimental results were explained using the configuration model. The energies between minima of configuration curves of excited and basic states for both centres were estimated. [ABSTRACT FROM AUTHOR]

We performed the computer modelling of the dispersion dependences of real ε1 (E) and imaginary ε2 (E) parts of complex dielectric function ε(E) for 2H-PbI2 crystals with atomically clean surface at the temperature 5 K and the light polarization E ⊥ C and determined the energy position of the exciton bands and the parameters of the critical points. The obtained data allowed us to conclude the exciton spectra of this semiconductor can be described in terms of a single Wannier series with a large ground-state anomaly caused by a repulsive central-cell correction due to the cationic character of the exciton. [ABSTRACT FROM AUTHOR]

A high-performance 1.3 μm InAs/GaAs quantum-dot laser directly grown on Si substrates has been achieved by using InAlAs/GaAs strained-layer superlattice serving as dislocation filter layers (DFLs). The Si-based laser achieves lasing operation up to 111°C with a threshold current density of 200 A/cm2 and an output power exceeding 100 mW at room temperature. [ABSTRACT FROM AUTHOR]

We report the growth of InAs(1-x)Sb(x) self-assembled quantum dots (QDs) on GaAs (100) by molecular beam epitaxy. The optical properties of the QDs are investigated by photoluminescence (PL) and time-resolved photoluminescence (TRPL). A type I to type II band alignment transition is demonstrated by both power-dependent PL and TRPL in InAs(1-x)Sb(x) QD samples with increased Sb beam flux. Results are compared to an eight-band strain-dependent k x p model incorporating detailed QD structure and alloy composition. The calculations show that the conduction band offset of InAs(1-x)Sb(x)/GaAs can be continuously tuned from 0 to 500 meV and a flat conduction band alignment exists when 60% Sb is incorporated into the QDs. Our study offers the possibility of tailoring the band structure of GaAs based InAsSb QDs and opens up new means for device applications.