Magnetic-field dependence of hole levels in self-assembled InGaAs quantum dots

Recent magnetotransport experiments of holes in InGaAs quantum dots [D. Reuter, P. Kailuweit, A. D. Wieck, U. Zeitler, O. Wibbelhoff, C. Meier, A. Lorke, and J. C. Maan, Phys. Rev. Lett. 94, 026808 (2005)] are interpreted by employing a multiband k¿p Hamiltonian, which considers the interaction between heavy hole and light hole subbands explicitly. No need of invoking an incomplete energy shell filling is required within this model. The crucial role we ascribe to the heavy hole-light hole interaction is further supported by one-band local-spin-density functional calculations, which show that Coulomb interactions do not induce any incomplete hole shell filling and therefore cannot account for the experimental magnetic field dispersion.

Magnetic-field dependence of hole levels in self-assembled InGaAs quantum dots

Recent magnetotransport experiments of holes in InGaAs quantum dots [D. Reuter, P. Kailuweit, A. D. Wieck, U. Zeitler, O. Wibbelhoff, C. Meier, A. Lorke, and J. C. Maan, Phys. Rev. Lett. 94, 026808 (2005)] are interpreted by employing a multiband k¿p Hamiltonian, which considers the interaction between heavy hole and light hole subbands explicitly. No need of invoking an incomplete energy shell filling is required within this model. The crucial role we ascribe to the heavy hole-light hole interaction is further supported by one-band local-spin-density functional calculations, which show that Coulomb interactions do not induce any incomplete hole shell filling and therefore cannot account for the experimental magnetic field dispersion.

Room temperature photoluminescence of InGaAs Surface Quantum Dots

Self-assembled InGaAs quantum dots show unique physical properties such as three dimensional confinement, high size homogeneity, high density and low number of dislocations. They have been extensively used in the active regions of laser devices for optical communications applications [1]. Therefore, buried quantum dots (BQDs) embedded in wider band gap materials have been normally studied. The wave confinement in all directions and the stress field around the dot affect both optical and electrical properties [2, 3]. However, surface quantum dots (SQDs) are less affected by stress, although their optical and electrical characteristics have a strong dependence on surface fluctuation. Thus, they can play an important role in sensor applications

Electron dephasing scattering rate in two-dimensional GaAs/InGaAs heterostructures with embedded InAs quantum dots

We report a comprehensive study of weak-localization and electron-electron interaction effects in a GaAs/InGaAs two-dimensional electron system with nearby InAs quantum dots, using measurements of the electrical conductivity with and without magnetic field. Although both the effects introduce temperature dependent corrections to the zero magnetic field conductivity at low temperatures, the magnetic field dependence of conductivity is dominated by the weak-localization correction. We observed that the electron dephasing scattering rate tau(-1)(phi), obtained from the magnetoconductivity data, is enhanced by introducing quantum dots in the structure, as expected, and obeys a linear dependence on the temperature and elastic mean free path, which is against the Fermi-liquid model. (c) 2008 American Institute of Physics. [DOI: 10.1063/1.2996034]

High optical sensitivity to ambient conditions of uncapped InGaAs surface quantum dots

The influence of the environment on the optical properties of self-assembled In0.5Ga0.5As surface quantum dots is studied as a function of different ambient conditions for sensing applications. Their room temperature photoluminescence (PL) quenches under vacuum and decreases strongly under dry O2 or N2 environments. Nevertheless, they have a strong signal at 1.55 lm in air or in a wet atmosphere. The presence of water molecules in the environment improves the PL intensity likely due to its polar character and therefore its easier adsorption by the surface dangling bonds, leading to a suppression of the non-radiative recombination centers.

Engineering of 3D self-directed quantum dot ordering in multilayer InGaAs/GaAs nanostructures by means of flux gas composition

Lateral ordering of InGaAs quantum dots on the GaAs (001) surface has been achieved in earlier reports, resembling an anisotropic pattern. In this work, we present a method of breaking the anisotropy of ordered quantum dots (QDs) by changing the growth environment. We show experimentally that using As(2) molecules instead of As(4) as a background flux is efficient in controlling the diffusion of distant Ga adatoms to make it possible to produce isotropic ordering of InGaAs QDs over GaAs (001). The control of the lateral ordering of QDs under As(2) flux has enabled us to improve their optical properties. Our results are consistent with reported experimental and theoretical data for structure and diffusion on the GaAs surface.

In-plane mapping of buried InGaAs quantum rings and hybridization effects on the electronic structure

This work reports the investigation on the structural differences between InAs quantum rings and their precursor quantum dots species as well as on the presence of piezoelectric fields and asymmetries in these nanostructures. The experimental results show significant reduction in the ring dimensions when the sizes of capped and uncapped ring and dot samples are compared. The iso-lattice parameter mapped by grazing-incidence x-ray diffraction has revealed the lateral extent of strained regions in the buried rings. A comparison between strain and composition of dot and ring structures allows inferring on how the ring formation and its final configuration may affect optical response parameters. Based on the experimental observations, a discussion has been introduced on the effective potential profile to emulate theoretically the ring-shape confinement. The effects of confinement and strain field modulation on electron and hole band structures are simulated by a multiband k.p calculation. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4733964]

Optimization of InGaAsN(Sb)/GaAs quantum dots for optical emission at 1.55 µm with low optical degradation

Low optical degradation in GaInAsN(Sb)/GaAs quantum dots (QDs) p–i–n structures emitting up to 1.55 μm is presented in this paper. We obtain emission at different energies by means of varying N content from 1 to 4%. The samples show a low photoluminescence (PL) intensity degradation of only 1 order of magnitude when they are compared with pure InGaAs QD structures, even for an emission wavelength as large as 1.55 μm. The optimization studies of these structures for emission at 1.55 μm are reported in this work. High surface density and homogeneity in the QD layers are achieved for 50% In content by rapid decrease in the growth temperature after the formation of the nanostructures. Besides, the effect of N and Sb incorporation in the redshift and PL intensity of the samples is studied by post-growth rapid thermal annealing treatments. As a general conclusion, we observe that the addition of Sb to QD with low N mole fraction is more efficient to reach 1.55 μm and high PL intensity than using high N incorporation in the QD. Also, the growth temperature is determined to be an important parameter to obtain good emission characteristics. Finally, we report room temperature PL emission of InGaAsN(Sb)/GaAs at 1.4 μm.

Quantum Dots As Biophotonics Tools.

This chapter provides a short review of quantum dots (QDs) physics, applications, and perspectives. The main advantage of QDs over bulk semiconductors is the fact that the size became a control parameter to tailor the optical properties of new materials. Size changes the confinement energy which alters the optical properties of the material, such as absorption, refractive index, and emission bands. Therefore, by using QDs one can make several kinds of optical devices. One of these devices transforms electrons into photons to apply them as active optical components in illumination and displays. Other devices enable the transformation of photons into electrons to produce QDs solar cells or photodetectors. At the biomedical interface, the application of QDs, which is the most important aspect in this book, is based on fluorescence, which essentially transforms photons into photons of different wavelengths. This chapter introduces important parameters for QDs' biophotonic applications such as photostability, excitation and emission profiles, and quantum efficiency. We also present the perspectives for the use of QDs in fluorescence lifetime imaging (FLIM) and Förster resonance energy transfer (FRET), so useful in modern microscopy, and how to take advantage of the usually unwanted blinking effect to perform super-resolution microscopy.

Measuring The Hydrodynamic Radius Of Quantum Dots By Fluorescence Correlation Spectroscopy.

Fluorescence Correlation Spectroscopy (FCS) is an optical technique that allows the measurement of the diffusion coefficient of molecules in a diluted sample. From the diffusion coefficient it is possible to calculate the hydrodynamic radius of the molecules. For colloidal quantum dots (QDs) the hydrodynamic radius is valuable information to study interactions with other molecules or other QDs. In this chapter we describe the main aspects of the technique and how to use it to calculate the hydrodynamic radius of quantum dots (QDs).

Measurement Of The Hydrodynamic Radius Of Quantum Dots By Fluorescence Correlation Spectroscopy Excluding Blinking.

One of the most important properties of quantum dots (QDs) is their size. Their size will determine optical properties and in a colloidal medium their range of interaction. The most common techniques used to measure QD size are transmission electron microscopy (TEM) and X-ray diffraction. However, these techniques demand the sample to be dried and under a vacuum. This way any hydrodynamic information is excluded and the preparation process may alter even the size of the QDs. Fluorescence correlation spectroscopy (FCS) is an optical technique with single molecule sensitivity capable of extracting the hydrodynamic radius (HR) of the QDs. The main drawback of FCS is the blinking phenomenon that alters the correlation function implicating in a QD apparent size smaller than it really is. In this work, we developed a method to exclude blinking of the FCS and measured the HR of colloidal QDs. We compared our results with TEM images, and the HR obtained by FCS is higher than the radius measured by TEM. We attribute this difference to the cap layer of the QD that cannot be seen in the TEM images.

Growth and capping of InAs/GaAs quantum dots investigated by x-ray Bragg-surface diffraction

An x-ray diffraction method, based on the excitation of a surface diffracted wave, is described to investigate the capping process of InAs/GaAs (001) quantum dots (QDs). It is sensitive to the tiny misorientation of (111) planes at the surface of the buffer layer on samples with exposed QDs. After capping, the misorientation occurs in the cap-layer lattice faceting the QDs and its magnitude can be as large as 10 degrees depending on the QDs growth rates, probably due to changes in the size and shape of the QDs. A slow strain release process taking place at room temperature has also been observed by monitoring the misorientation angle of the (111) planes.

Hyperfine interactions in silicon quantum dots

A fundamental interaction for electrons is their hyperfine interaction (HFI) with nuclear spins. HFI is well characterized in free atoms and molecules, and is crucial for purposes from chemical identification of atoms to trapped ion quantum computing. However, electron wave functions near atomic sites, therefore HFI, are often not accurately known in solids. Here we perform an all-electron calculation for conduction electrons in silicon and obtain reliable information on HFI. We verify the outstanding quantum spin coherence in Si, which is critical for fault-tolerant solid state quantum computing.

Dispersion of electron g-factor with optical transition energy in (In,Ga)As/GaAs self-assembled quantum dots

The electron spin precession about an external magnetic field was studied by Faraday rotation on an inhomogeneous ensemble of singly charged, self-assembled (In,Ga)As/GaAs quantum dots. From the data the dependence of electron g-factor on optical transition energy was derived. A comparison with literature reports shows that the electron g-factors are quite similar for quantum dots with very different geometrical parameters, and their change with transition energy is almost identical. (C) 2011 American Institute of Physics. [doi:10.1063/1.3588413]

Carrier transfer in the optical recombination of quantum dots

We report a study of dynamic effects detected in the time-resolved emission from quantum dot ensembles. Experimental procedures were developed to search for common behaviors found in quantum dot systems independently of their composition: three quantum dot samples were experimentally characterized. Systems with contrasting interdot coupling are compared and their sensitivity to the excitation energy is analyzed. Our experimental results are compared and contrasted with other results available in literature. The optical recombination time dependence on system parameters is derived and compared to the experimental findings. We discuss the effects of occupation of the ground state in both valence and conduction bands of semiconductor quantum dots in the dynamics of the system relaxation as well as the nonlinear effects.