Self-assembled Quantum Dot Structures Research Articles

Resonant Raman scattering from CdTe/ZnTe self-assembled quantum dot structures

We report resonant Raman scattering results of CdTe/ZnTe self-assembled quantum dot (QD) structures. Photoluminescence spectra reveal that the band gap energies of the CdTe QDs decrease with the increase of CdTe thickness from 2.0 to 3.5 monolayers, which indicates that the size of the QDs increases. When the CdTe/ZnTe QD structures are excited by non-resonant excitation, a longitudinal optical (LO) phonon response from the ZnTe barrier material is observed at 206 cm−1. In contrast, when the CdTe/ZnTe QD structures are resonantly excited near the band gap energy of the QDs, additional phonon modes emerge at 167 and 200 cm−1, while the ZnTe LO phonon response completely disappears. The 167 cm−1 mode corresponds to the LO phonon of the CdTe QDs. A spatially resolved Raman scattering from the cleaved edge of the QD sample reveals that the 200 cm−1 mode is strongly localized at the interface between the CdTe QDs and ZnTe cap layer. This phonon mode is attributed to the interface optical (IO) phonon. The analytically calculated value of the IO phonon energy using a dielectric continuum approach, assuming a spherical dot boundary, agrees well with the experimental value.

Photon echo transients from an inhomogeneous ensemble of semiconductor quantum dots

An ensemble of quantum dot excitons may be used for coherent information manipulation. Due to the ensemble inhomogeneity any optical information retrieval occurs in form of a photon echo. We show that the inhomogeneity can lead to a significant deviation from the conventional echo timing sequence. Variation of the area of the initial rotation pulse, which generates excitons in a dot sub-ensemble only, reveals this complex picture of photon echo formation. We observe a retarded echo for {\pi}/2 pulses, while for 3{\pi}/2 the echo is advanced in time as evidenced through monitoring the Rabi oscillations in the time-resolved photon echo amplitude from (In,Ga)As/GaAs self-assembled quantum dot structures and confirmed by detailed calculations.

Multimillion-atom modeling of InAs/GaAs quantum dots: interplay of geometry, quantization, atomicity, strain, and linear and quadratic polarization fields

Electronic structure and optical properties of self-assembled quantum dots strongly depend on an intricate interplay of the quantum mechanical size quantization and the atomistic built-in/internal electrostatic fields in the underlying material system. Specifically, built-in fields in zincblende quantum dots originate mainly from: (1) fundamental crystal atomicity and the interfaces between two dissimilar materials, (2) microscopic distribution of strain, and (3) the piezoelectric polarization. In this paper, we first study the origin and nature of these internal fields in InAs/GaAs quantum dots having three different geometries, namely, box, dome, and pyramid. We then quantify and delineate the impact of these internal fields on the one-particle electronic states in terms of symmetry-lowering and localization in the wavefunctions, shift in the energy states and bandgap, anisotropy and non-degeneracy in the $$P$$P level, and formation of mixed excited bound states near the Brillouin zone center. Finally, we study the geometry and size-dependence of interband optical transitions in the XY and XZ planes treating the quantum size quantization and the internal fields as parameters. The computational framework employs a combination of fully atomistic valence force-field molecular mechanics, 20-band nearest-neighbor $$sp^{3} d^{5} s^{*}$$sp3d5s? tight-binding electronic bandstructure models, and appropriate post-processing tools to obtain the interband optical transition rates. In particular, to model piezoelectricity, four different polarization models (based on the experimental and ab initio coefficients accounting for both linear and non-linear effects) have been considered in increased order of accuracy. With the non-linear piezoelectricity generally opposing the linear counterpart, the net piezoelectric field is found to be negligible in smaller dots but exhibits non-vanishing effects as the dot height is increased.

A performance assessment of type-II interband In0.5Ga0.5Sb QD photodetectors

Self-assembled quantum-dot (QD) structures with type-II band alignment to the surrounding matrix material have been proposed as a III/V material approach to realize small-bandgap device structures suitable for photon detection and imaging in the long-wavelength infrared (LWIR) band. Here, we analyze the photoresponse of In0.5Ga0.5Sb/InAs QD photodiodes and estimate the system performance of type-II QD –based photodetectors. A review of alternative design approaches is presented and the choice of matrix material is discussed in terms of band alignment and its effect on the photoresponse.Photodiodes were fabricated consisting of 10 layers of In0.5Ga0.5Sb QDs grown on InAs (001) substrates with metal–organic vapor-phase epitaxy (MOVPE). The photoresponse and dark current were measured in single pixel devices as a function of temperature in the range 20–230K. The quantum efficiency shows an Arrhenius type behavior, which is attributed to hole trapping. This severely limits the detector performance at typical LWIR sensor operating temperatures (60–120K). A device design with the matrix material InAs0.6Sb0.4 is proposed as a mean to improve the performance by reducing the barrier for hole transport. This can potentially allow type-II InGaSb QDs to be a competitive sensor material for LWIR detection.

Comparative Study of 1.3 and 1.5 m Light-Emitting Diodes GaAs-Based InAs/InGaAs and InAs/(Ga, In) (N, As) Self-Assembled Quantum Dots

This study presents self-assembled quantum dots structures made on GaAs substrate. The samples were grown by Molecular Beam Epitaxy (MBE) in the Stranski-Krastanow (SK) growth mode. Two types of quantum dots structures are performed under different growth conditions and are compared: the first type is composed of two structures with InAs quantum dots encapsulated with (Ga, In) (N, As) and the second one with boxes wrapped with InGaAs. The influence of encapsulation of quantum dots is highlighted and as already shown, the redshift of emission wavelength depends on nitrogen doping. Further investigations are done through the incorporation of indium and nitrogen in the quantum dots structures in order to understand their optimal doping level on electrical and optical properties. The effects of temperature, an important growth parameter, are examined through images of nanostructures obtained by Atomic Force Microscopy (AFM) and Scanning Electronic Microscopy (SEM) techniques.

The effect of strained confinement layers in InP self-assembled quantum dot material

We investigated a series of self-assembled InP quantum dot structures. The Ga concentrations of the GaxIn(1−x)P upper confining quantum well layers were varied from x = 0.43 to x = 0.58, centering on a strain compensated structure. All were grown on a (Al0.3Ga0.7)0.52In0.48P lower confinement layer. Across this range of composition the dot emission wavelength changed approximately linearly with the Ga fraction. Of the compositions used, x = 0.54 gave the best defined and largest magnitude dot absorption spectrum, corresponding to the largest dot density. This resulted in the lowest overall threshold current density with a reduced threshold current temperature sensitivity at room temperature and above. These results are confirmed over two growth series.

Structural characterization of GaAs self-assembled quantum dots grown by Droplet Epitaxy on Ge virtual substrates on Si

The structure of self-assembled quantum dots (QDs) grown by Droplet Epitaxy on Ge virtual substrates has been investigated by TEM. The QDs have a pyramidal shape with base and height of 50nm. By (002) dark field TEM it was seen that the pyramid top is Ga poor and Al rich most likely because of the higher mobility of Ga along the pyramid sides down to the base. The investigated QDs contain defects identified as As precipitates by Moirè fringes. The smallest ones (3–5nm) are coherent with the GaAs lattice suggesting that they could be a cubic phase of As precipitation. It seems to be a metastable phase since the hexagonal phase is recovered as the precipitate size increases above ∼5nm.

Multilayered type-II GaSb/GaAs self-assembled quantum dot structure with [formula omitted] light emission at room temperature

We have investigated the growth of multilayered type-II GaSb/GaAs self-assembled quantum dot (QD) structure. We show that the key point for the growth of the multilayed GaSb/GaAs QDs lies in how to switch the V elements: Sb and As. It is found that, after the dot deposition, a short exposure to As atmosphere smoothes out the GaSb 3D islands and transforms the structure into 2D-like while the 3D islands remain in an Sb soak. There is a large PL energy separation of 214meV between the QDs and the corresponding 2D-like structure due to the smoothing-out effect. The PL peak of the multilayered dots is at 1.35μm at room temperature (RT) and only digresses from 1.3μm insignificantly for a broad range of temperature from 13K to RT.

Molecular Dynamics Study on Mechanical Properties in the Structure of Self-Assembled Quantum Dot

Stress and strain in the structure of self-assembled quantum dots constructed in the Ge/Si(001) system is calculated by using molecular dynamics simulation. Pyramidal hut cluster composed of Ge crystal with {105} facets surfaces observed in the early growth stage are computationally modeled. We calculate atomic stress and strain in relaxed pyramidal structure. Atomic stress for triplet of atoms is approximately defined as an average value of pairwise (virial) quantity inside triplet, which is the product of vectors between each two atoms. Atomic strain by means of atomic strain measure (ASM) which is formulated on the Green’s definition of continuum strain. We find the stress (strain) relaxation in pyramidal structure and stress (strain) concentration in the edge of pyramidal structure. We discuss size dependency of stress and strain distribution in pyramidal structure. The relationship between hydrostatic stress and atomic volumetric strain is basically linear for all models, but for the surface of pyramidal structure and Ge-Si interface. This means that there is a reasonable correlation between atomic stress proposed in the present study and atomic strain measure, ASM.

1042 分子動力学法による自己組織化量子ドットの積層構造と機械的特性の評価(OS9,OS10 合同ポスターセッション,OS・一般セッション講演)

1042 分子動力学法による自己組織化量子ドットの積層構造と機械的特性の評価(OS9,OS10 合同ポスターセッション,OS・一般セッション講演)

Current quantization in an optically driven electron pump based on self-assembled quantum dots

The electronic structure of self-assembled semiconductor quantum dots consists of discrete atom-like states that can be populated with a well-defined number of electrons. This property can be used to fabricate a d.c. current standard that enables the unit of ampere to be independently defined. Here we report an optically pumped current source based on self-assembled InAs/GaAs quantum dots. The accuracy obtained so far is 10−1 and is limited by the uncertainty in the number of dots. At 10 K the device generates a current difference of 2.39 nA at a frequency of 1 kHz. The accuracy could be improved by site-selective growth techniques where the number of dots is fixed by pre-patterning. The results are promising for applications in electrical metrology, where a current standard is needed to close the so-called quantum metrological triangle. The atom-like electronic structure of semiconductor quantum dots makes them ideal for storing well-defined numbers of electrons. This in turn can be used in the development of standards for current, to independently define the ampere.

Correlating structure, strain, and morphology of self-assembled InAs quantum dots on GaAs

We report on the use of a direct x-ray phase retrieval method, coherent Bragg rod analysis, to characterize self-assembled InAs quantum dots (QDs) grown epitaxially on GaAs substrates. Electron density maps obtained close to the x-ray absorption edges of the constituent elements are compared to deconvolute composition and atomic spacing information. Our measurements show no evidence of a wetting layer and reveal bowing of the atomic layers throughout the QD, extending from the QD-substrate interface. This leads to a half-layer stacking shift which may act to partially decouple the QDs electronically from the substrate.

Nano-crystalline Sb-based compound semiconductor formed on silicon

A self-assembled InGaSb quantum dot (QD) structure as a nano-structured Sb-based compound structure is fabricated on a silicon substrate using a solid-state molecular beam epitaxy (MBE). A small (dimensions of less than a few tenths of a nanometer) and high-density (>1010/cm2) InGaSb QD structure can be obtained on a Si wafer surface under optimal growth conditions at a low temperature, which is compatible for use with Si-CMOS processes. Using high-resolution transmission electron microscope (TEM) observation, a clear crystalline lattice image in the fabricated InGaSb QD and the interface between the InGaSb and the Si are successfully observed. Based on these results, we expect that a nano-crystalline Sb-based compound semiconductor on a Si surface will become a useful and novel material system with high crystalline quality, enabling the development of a new generation of integrated photonics and high-speed electron devices on the Si platform.

Composition of the “GaAs” quantum dot, grown by droplet epitaxy

Self-assembled strain-free quantum dot (QD) structures were grown on AlGaAs surface by the droplet epitaxal method. The QDs were developed from pure Ga droplets under As pressure. The QDs were investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Both techniques show that the QDs are very uniform in size and their distribution on the surface is also homogeneous. The high resolution cross-sectional TEM investigation shows perfect lattice matching between the QD and the substrate, and also the faceting of the side walls of QD can be identified exactly by lattice planes. Analytical TEM (elemental mapping by EELS) unambiguously identifies the presence of Al in the QD.

Energy level scheme ofInAs/InxGa1−xAs/GaAsquantum-dots-in-a-well infrared photodetector structures

A thorough investigation of quantum-dots-in-a-well structures for infrared photodetector applications has been performed employing different experimental techniques. The electronic structure of self-assembled InAs quantum dots embedded in an In0.15Ga0.85As/GaAs quantum well (QW) was deduced from photoluminescence (PL) and PL excitation (PLE) spectroscopy. From polarization-dependent PL it was revealed that the quantum dots hold two electron energy levels and two heavy-hole levels. Tunnel capacitance spectroscopy confirmed an electron energy level separation of about 50 meV, and additionally, that the conduction-band ground state and excited state of the dots are twofold and fourfold degenerates, respectively. Intersubband photocurrent spectroscopy, combined with simultaneous interband pumping of the dots, revealed a dominant transition at 150 meV (8.5 mu m) between the ground state of the quantum dots and the excited state of the QW. Results from detailed full three-dimensional calculations of the electronic structure, including effects of composition intermixing and interdot interactions, confirm the experimentally unravelled energy level scheme of the dots and well.

Deep levels in GaAs(001)/InAs/InGaAs/GaAs self-assembled quantum dot structures and their effect on quantum dot devices

Currently lattice mismatch strain-driven three-dimensional coherent island based quantum dots, dubbed self-assembled quantum dots (SAQDs), constitute the most developed class of quantum dots with successful applications to lasers and considerable potential for infrared detectors in the 1–12 μm regime. This is in no small part a consequence of the extensive studies on the formation and control of the islands and on their capping by appropriate overlayer materials under optimal growth conditions. By contrast, surprisingly few studies have been reported on the presence and nature of the deep levels in SAQD structures, much less direct studies of the impact of deep levels on SAQD based device characteristics. The latter is of particular significance to devices such as detectors that require large numbers of SAQD layers [i.e., multiple quantum dot (MQD) structures] and are thus increasingly prone to accumulating strain-induced defect formation with increasing numbers of quantum dot layers. In this paper, we report the results of a study of the density, energy profile, and spatial profile of deep levels in different regions of GaAs(001)/InAs/InGaAs/GaAs SAQD structures in which the InGaAs/GaAs capping layers have been grown at different growth conditions. Different types of deep levels are found in different regions and, as expected, their densities are found to increase in the presence of the SAQDs. The study shows that it is the density of deep levels in the GaAs capping layer, forced to be grown at the low temperature of ∼500 °C to suppress In outdiffusion, which has a significant adverse impact on quantum dot device characteristics. Their density can be reduced by growth conditions such as migration enhanced epitaxy that permit high quality overgrowths at temperatures as low as ∼350 °C. Nevertheless, the ultimate performance limitation of thick MQD based devices resides in the ability to realize low density of the deep levels relative to the density of SAQDs.

Dependence of Curie temperature on surface strain in InMnAs epitaxial structures

Pairs of self-assembled InMnAs quantum dot structures and reference epitaxial layers (0 < x < 0.13) were prepared on GaAs substrates by low-pressure metal organic vapour phase epitaxy. Magnetic moment measurements indicated that reference epitaxial layer had a Curie temperature of 343 K independent on the composition. On the other hand, the quantum dots prepared under Stranski–Krastanov growth mode from the identical gas phase composition showed a lower value of Curie temperature. This value varied from 41 to 235 K in relation to the material composition. Moiré fringes at transmission electron microscopy plan view were used for characterization of strain in InMnAs quantum dot structures.

Self-assembled quantum-dot superluminescent light-emitting diodes

The development of low-cost, compact, high-power and broadband superluminescent light-emitting diodes is an important research subject for a wide range of applications. We describe how self-assembled quantum-dot structures can provide an efficient means of realizing such devices utilizing a number of their unique physical properties. Such quantum dot superluminescent diodes are leading to a revolution in the development of broadband emitters for widespread medical, biological and telecommunications applications.

Role of segregation in InAs/GaAs quantum dot structures capped with a GaAsSb strain-reduction layer

We report a combined experimental and theoretical analysis of Sb and In segregation during the epitaxial growth of InAs self-assembled quantum dot structures covered with a GaSbAs strain-reducing capping layer. Cross-sectional scanning tunneling microscopy shows strong Sb and In segregation which extends through the GaAsSb and into the GaAs matrix. We compare various existing models used to describe the exchange of group III and V atoms in semiconductors and conclude that commonly used methods that only consider segregation between two adjacent monolayers are insufficient to describe the experimental observations. We show that a three-layer model originally proposed for the SiGe system [D. J. Godbey and M. G. Ancona, J. Vac. Sci. Technol. A 15, 976 (1997)] is instead capable of correctly describing the extended diffusion of both In and Sb atoms. Using atomistic modeling, we present strain maps of the quantum dot structures that show the propagation of the strain into the GaAs region is strongly affected by the shape and composition of the strain-reduction layer.

Optical Properties of Self-assembled InAs Quantum-dot Superluminescent Diodes

We report broad-band superluminescent diodes (SLDs) with different active layers using a selfassembled InAs quantum-dot (QD) structure grown by using atomic layer molecular beam epitaxy. The photoluminescence and the electroluminescence measurements show a relationship between the emission properties and different activelayer structures of the QD-SLD These results explain the possibility of QD-based SLDs exceeding the performance of quantumwell-based SLDs.