Current instabilities in GaAs/InAs quantum dot structures

Unexpected excess current was obtained in GaAs/InAs quantum dot structures at low temperatures and low current levels. This excess current exhibited instabilities with changing the bias, and over the time. It has been concluded that the excess current is a minority injection current connected with recombination through defects originated from the formation of QDs. The instabilities are connected with unstable occupation of energy levels induced by the above defects, which depend on temperature and on the current level. Excess currents have also been obtained in annealed AlGaAs/GaAs structures. These excess currents exhibited memory effect, which was probably connected with formatin of defects during annealing.

Semiconductor quantum dot (QD) structures can be used as a model for understanding the effect of the microscopic structure, symmetry of crystals, and molecules on their macroscopic properties. In this work, the results of two theoretical approaches demonstrate that the spin dynamics in ordered QD structures depends on the size, spatial configuration, and topology of the object built of QDs. It was shown that the spin dynamics in QD structures with the hopping regime of conductivity significantly differs from the spin dynamics in two-dimensional (2D) and three-dimensional (3D) structures being at the other side of the metal-insulator transition. The special character of the effective magnetic field $\ensuremath{\delta}\mathbf{H}$ fluctuations appearing only during tunneling between quantum dots is responsible for the insensitivity of spin relaxation times to the magnitude of the external magnetic field in infinite QD structures (2D square lattice and 1D linear QD chain). In finite QD structures (QD rings and linear chains), an external magnetic field ${\mathbf{H}}_{0}$ is directly involved in the spin relaxation process and spin is lost due to interaction with a special combination of fields $\mathrm{\ensuremath{\Delta}}\mathbf{H}\ensuremath{\sim}[{\mathbf{H}}_{\mathbf{0}}\ifmmode\times\else\texttimes\fi{}\ensuremath{\delta}\mathbf{H}]/\ensuremath{\delta}H$ that leads to an unusual orientation dependence of ESR linewidth, recently observed for QD chains. It was shown that the ordering of QD structures can be used for the conservation of spin orientation. For 1D finite quantum dot chains, the ordering can provide the stabilization of all spin components ${S}_{x},{S}_{y}$, and ${S}_{z}$, while for ringlike molecules only ${S}_{z}$ polarization can be stabilized. The results obtained in this work can be useful for development of novel semiconductor devices and in quantum information processing.

Capture, relaxation and recombination in quantum dots

The effects of growth temperature on the optical properties of InAs/InGaAs/GaAs quantum dot (QD) structure were investigated at the important temperature range of 25degC to 80degC. Results from QD samples grown at low growth temperature of 450degC suggest better thermal stability as compared to that grown at 510degC. This work suggests the feasibility of obtaining better thermal stability at a low growth temperature for InAs/InGaAs/GaAs QD structures.

Semiconductor quantum dot (QD) structures are considered as promising building block for spintronic applications with the advantage of prolonged spin relaxation time owing to 0D character of confined carriers or excitons. However, feasible application is haunted by severe spin injection loss from its adjacent barrier layers and its mechanism is still not fully understood. Here, we show that exciton spin injection in self-assembled InAs/GaAs QD molecular structures (QMSs) is dominated by localized excitons confined within the QD-like regions of the wetting layer (WL) and GaAs barrier layer surrounding QD structures. The origin of spin injection loss is attribute to finite anisotropic exchange interaction (AEI) of the localized excitons subjected to asymmetric confinement potential in the injection layers. As a result, the AEI of the injected excitons and, thus, the spin injection efficiency is determined to be correlated with the overall geometric symmetry of QMSs, which hold strong influence on the confinement potential of the localized excitons in the surrounding barrier layers. Our results shed light on the microscopic origin of the spin injection loss in QD structures. More importantly, they offer a useful guideline to significantly improve spin injection efficiency by optimizing the lateral arrangement of QMSs and overcome a major challenge in the QD based spintronic device applications.

Deep levels in metamorphic InAs/InxGa1−xAs quantum dot (QD) structures are studied with deep level thermally stimulated conductivity (TSC), photoconductivity (PC) and photoluminescence (PL) spectroscopy and compared with data from pseudomorphic InGaAs/GaAs QDs investigated previously by the same techniques. We have found that for a low content of indium (x = 0.15) the trap density in the plane of self-assembled QDs is comparable or less than the one for InGaAs/GaAs QDs. However, the trap density increases with x, resulting in a rise of the defect photoresponse in PC and TSC spectra as well as a reduction of the QD PL intensity. The activation energies of the deep levels and some traps correspond to known defect complexes EL2, EL6, EL7, EL9, and EL10 inherent in GaAs, and three traps are attributed to the extended defects, located in InGaAs embedding layers. The rest of them have been found as concentrated mainly close to QDs, as their density in the deeper InGaAs buffers is much lower. This an important result for the development of light-emitting and light-sensitive devices based on metamorphic InAs QDs, as it is a strong indication that the defect density is not higher than in pseudomorphic InAs QDs.

We have compared the photoluminescence (PL) spectra of InAs/InGaAsP/InP quantum dots (QD) structures with that of InGaAs/InP quantum well (QW) structures by changing the incident angle for TE polarized excitation light. From the results, we have found the shift of the PL peak wavelength is different between QD and QW. Then, we have measured the change in the FWHM of the PL spectrum against the excitation light intensity. The increment rate of the FWHM for QD is larger than that for QW. From the above, we have supposed each QD couples electrically to form a continuous band structure and the quantum levels broaden, due to adjacent QD whose size are variant. It means the band filling effect for the bulk sample occurs in QD samples.

This study presents an band-alignment tailoring of a vertically aligned InAs/GaAs(Sb) quantum dot (QD) structure and the extension of the carrier lifetime therein by rapid thermal annealing (RTA). Arrhenius analysis indicates a larger activation energy and thermal stability that results from the suppression of In-Ga intermixing and preservation of the QD heterostructure in an annealed vertically aligned InAs/GaAsSb QD structure. Power-dependent and time-resolved photoluminescence were utilized to demonstrate the extended carrier lifetime from 4.7 to 9.4 ns and elucidate the mechanisms of the antimony aggregation resulting in a band-alignment tailoring from straddling to staggered gap after the RTA process. The significant extension in the carrier lifetime of the columnar InAs/GaAsSb dot structure make the great potential in improving QD intermediate-band solar cell application.

Improving the characteristics of intermediate-band solar cell devices using a vertically aligned InAs/GaAsSb quantum dot structure

Quantum structures for multiband photon detection

The polarization dependence of the photosensitivity (PS) of the Schottky barrier diodes based on the InGaAs/GaAs quantum well (QW) and quantum dot (QD) structures has been studied. The PS polarization dependence follows the theoretical one with good agreement. The effect of the hole state mixing in the QW on the PS polarization dependence has been studied. The asymmetry of the PS polarization dependence of the QDs in the [110] and [11̄0] directions has been observed attributed to the effect of piezoelectric field induced by the elastic strain.

Electrical and structural characterization of InAs/InGaAs quantum dot structures on GaAs

GaSb quantum dots (QDs) in a GaAs matrix have attracted much attention for the potential ability to extend the accessible wavelength of QD emitters towards a wavelength of > 1.3 μm owing to their unique electronic and optical properties caused by their staggered (type-II) band alignment. North et al. calculated the electronic structure of self-assembled GaSb/GaAs dots by using the simple one-band model (effective mass model). Therefore it is interesting to carry out the detailed calculations using a more accurate method, in particular in the view of the new experiments. The aim of this work is to study the electronic properties of GaSb/GaAs self-assembled quantum dots (SAD) by using the sp 3 s * empirical tight-binding method. The influence of strain on the electronic structure is determined by minimizing the elastic energy within the Valence Force Field Approach and the piezoelectric potential is calculated by solving the corresponding 3D Poisson equation. Single-particle bound-state energies are computed as a function of the dot sizes. Finally we have found that the theoretical PL spectra are in a good agreement with the experiment and k.p method. empirical models are widely implemented to study a large quantum dots. There are three empirical models, the k.p approximation (9), the pseudopotential model (10) and the tight-binding model (11), (12). The k.p approximation treats a quantum dot as a confined bulk and continuum system, while the pseudopotential and tight-binding models treat the system with the atomistic description. The distinction between the two atomistic models is the degree of atomic detail included in the model. Within the tight-binding model, the atomistic detail is limited to a small basis set, while in the pseudopotential model the feature of wave functions is described with a large basis set. Therefore the tight-binding model is computationally less costly than the pseudopotential model. The tight-binding model is a good candidate for the study of relatively big and complicated systems. At the moment, the theoretical calculations of the type-II quantum dots have not been extensively investigated and there are only few papers that we can mention here. North and co-authors (13) investigated the electronic structure of type-II GaSb/GaAs quantum dots by using the multiband effective-mass calculation. K. Gradkowski and et al. (2) reported the results of the room temperature photoreflectance (PR) and photoluminescence (PL) measurements of molecular beam epitaxy (MBE) grown GaAsSb/GaAs quantum dot structure: one with a capping quantum well and one without it. PL technique is employed to probe the ground-state transition energies. Using the experimental data, they calculated a band structure of these dots by using the 8-band k.p approach. The theoretical analysis accounts well in both GaAsSb/GaAs quantum dot with a capping quantum well and one without it. In this paper, we report the tight-binding calculations of the type-II GaSb/GaAs quantum dots. The strain distribution of the quantum dot is calculated by using the Valence Force Field method. The piezoelectricity is also obtained by solving the 3D Poisson's equation. Then we numerically calculate the electronic structure of the quantum dot as a function of the dot sizes. Finally we compare our calculated results with the experiment and another numerical analysis.

In III-nitride quantum dots (QDs), optical transition rate is very low because of the large built-in electrostatic field caused by the spontaneous polarization (SP) and piezoelectric (PZ) effects. In this work, we study the screening potential which is a solution of the self-consistent Hartree equation taking into account the built-in electrostatic field and its effect on light emission characteristics of non-polar wurtzite (WZ) (112¯0) GaN/AlN QD. It is found that the light emission intensity of the non-polar (112¯0) GaN/AlN QD structure is expected to be about four times larger than that of the c-plane (0001) GaN/AlN QD structure because the y-polarized matrix elements in the non-polar QD are larger than that in the c-plane QD. These predictions indicate that non-polar GaN/AlN QD structure have strong potential for highly efficient opto-electronic devices.

A commonly accepted picture of the transition dipole structure of colloidal semiconductor quantum dots is the so-called 2D degenerate dipole. Optical transitions in CdSe-ZnS quantum dot occur primarily for the low energy state for which the total spin angular momentum projection axis is J=±1. These transitions involve X+iY and X-iY combinations of the (hole) Bloch functions along the QD "X" and "Y" axis; in a spherical approximation, there are equal dipole strengths along two orthogonal axes in the QD frame resulting in a 2D degenerate dipole. This 2D dipole model has been used by Bawendi et al.[1] in their studies of quantum dot emission properties and Enderlein et al.[2] in their studies of quantum dot radiation patterns, and also provides a useful prototype of the complicated dipole structure for chiral nanosystems. Despite reliance on this model, a definitive study demonstrating the dipole structure of CdSe-ZnS quantum dots has not yet been performed. However, as demonstrated by Efros[3], transitions between the |1Se > z electron state and |1Se3/2 > hole state contain projections along the quantum dot Z-axis. During excitation, this transition is forbidden; however, non-radiative energy transfer can make this transition allowed during emission. This 3rd dipole component can, in principle, produce a noticeable effect in the observed linear polarization anisotropy of the light emitted from the quantum dot and observed interference patterns.