Red-Emitting Semiconductor Quantum Dot Lasers

Enhancement in electro-optic performance of InAlGaAs/GaAs quantum dot lasers by ex situ thermal annealing

This Letter reports the growth, fabrication, and characterization of molecular beam epitaxy (MBE)-grown quaternary InAlGaAs/GaAs quantum dot (QD) lasers emitting at sub-900 nm. The presence of Al in QD-based active regions acts as the origin of defects and non-radiative recombination centers. Applying optimized thermal annealing annihilates the defects in p-i-n diodes, thus lowering the reverse leakage current by six orders of magnitude compared to as-grown devices. A systematic improvement in the optical properties of the devices is also observed in the laser devices with increasing annealing time. At an annealing temperature of 700°C for 180 s, Fabry-Pérot lasers exhibit a lower pulsed threshold current density at infinite length of 570 A/cm².

Edge States and Strain-Driven Topological Phase Transitions in Quantum Dots in Topological Insulators

We present here a theory of the electronic properties of quasi two-dimensional quantum dots made of topological insulators. The topological insulator is described by either eight band k→·p→ Hamiltonian or by a four-band k→·p→ Bernevig-Hughes-Zhang (BHZ) Hamiltonian. The trivial versus topological properties of the BHZ Hamiltonian are characterized by the different topologies that arise when mapping the in-plane wavevectors through the BHZ Hamiltonian onto a Bloch sphere. In the topologically nontrivial case, edge states are formed in the disc and square geometries of the quantum dot. We account for the effects of compressive strain in topological insulator quantum dots by means of the Bir-Pikus Hamiltonian. Tuning strain allows topological phase transitions between topological and trivial phases, which results in the vanishing of edge states from the energy gap. This may enable the design of a quantum strain sensor based on strain-driven transitions in HgTe topological insulator square quantum dots.

Electronic and optical properties of Janus black arsenic-phosphorus AsP quantum dots under magnetic field

By utilizing the tight-binding method, the electronic spectrum and states distribution of square Janus monolayer black arsenic phosphorus (b-AsP) quantum dots (QDs) in the presence of a perpendicular magnetic field are explored. Strong in-gap states of b-AsP QDs, whose probability densities are distributed on the armchair boundary (armchair edge states) appear in the energy gap of host perfect two-dimensional b-AsP. The corresponding energy levels of the armchair edge states can degenerate to the Landu energy levels upon applying a perpendicular magnetic field. When an in-plane polarized light is introduced, due to the presence of armchair edge states, the edge-to-edge transitions are mainly induced from the armchair edge (hole) states to zigzag edge (electron) states. The optical absorption undergoes blue shift as a function of the magnetic field. Our work suggests tunable optical properties via modulating the armchair edge states of a b-AsP QD and provides a theoretical basis for the design of b-AsP-based optoelectronic devices.

Recent Developments of Quantum Dot Materials for High Speed and Ultrafast Lasers

Owing to their high integration and functionality, nanometer-scale optoelectronic devices based on III-V semiconductor materials are emerging as an enabling technology for fiber-optic communication applications. Semiconductor quantum dots (QDs) with the three-dimensional carrier confinement offer potential advantages to such optoelectronic devices in terms of high modulation bandwidth, low threshold current density, temperature insensitivity, reduced saturation fluence, and wavelength flexibility. In this paper, we review the development of the molecular beam epitaxial (MBE) growth methods, material properties, and device characteristics of semiconductor QDs. Two kinds of III-V QD-based lasers for optical communication are summarized: one is the active electrical pumped lasers, such as the Fabry–Perot lasers, the distributed feedback lasers, and the vertical cavity surface emitting lasers, and the other is the passive lasers and the instance of the semiconductor saturable absorber mirrors mode-locked lasers. By analyzing the pros and cons of the different QD lasers by their structures, mechanisms, and performance, the challenges that arise when using these devices for the applications of fiber-optic communication have been presented.

Distributed feedback organic lasing in photonic crystals

Considerable research efforts have been devoted to the investigation of distributed feedback (DFB) organic lasing in photonic crystals in recent decades. It is still a big challenge to realize DFB lasing in complex photonic crystals. This review discusses the recent progress on the DFB organic laser based on one-, two-, and three-dimensional photonic crystals. The photophysics of gain materials and the fabrication of laser cavities are also introduced. At last, future development trends of the lasers are prospected.

Electric field tuning of spin splitting in topological insulator quantum dots doped with a single magnetic ion

We investigate theoretically the electron spin states in a disk-shaped topological insulator quantum dot (TIQD) containing a single magnetic Mn²⁺ ion. We demonstrate that the energy spectra and the density distributions of the symmetry-protected edge states in a HgTe TIQD can be modulated effectively by a single magnetic impurity Mn²⁺. Additionally, when an in-plane external electric field is applied., it not only tunes the spin splittings of edge states via the s(p)-d exchange interaction between the electron (hole) and the magnetic Mn²⁺ ion respectively, but also gives rise to the bright-to-dark transitions in the optical transition spectrum. Such spin properties of TIQDs with single Mn²⁺ ion as illustrated in this work could offer a new platform for topological electro-optical devices.

Direct Determination of 3D Distribution of Elemental Composition in Single Semiconductor Nanoislands by Scanning Auger Microscopy

An application of scanning Auger microscopy with ion etching technique and effective compensation of thermal drift of the surface analyzed area is proposed for direct local study of composition distribution in the bulk of single nanoislands. For GexSi1 - x-nanoislands obtained by MBE of Ge on Si-substrate gigantic interdiffusion mixing takes place both in the open and capped nanostructures. Lateral distributions of the elemental composition as well as concentration-depth profiles were recorded. 3D distribution of the elemental composition in the d-cluster bulk was obtained using the interpolation approach by lateral composition distributions in its several cross sections and concentration-depth profile. It was shown that there is a germanium core in the nanoislands of both nanostructure types, which even penetrates the substrate. In studied nanostructures maximal Ge content in the nanoislands may reach about 40 at.%.

A Nanowire-Based Plasmonic Quantum Dot Laser

Quantum dots enable strong carrier confinement and exhibit a delta-function like density of states, offering significant improvements to laser performance and high-temperature stability when used as a gain medium. However, quantum dot lasers have been limited to photonic cavities that are diffraction-limited and further miniaturization to meet the demands of nanophotonic-electronic integration applications is challenging based on existing designs. Here we introduce the first quantum dot-based plasmonic laser to reduce the cross-sectional area of nanowire quantum dot lasers below the cutoff limit of photonic modes while maintaining the length in the order of the lasing wavelength. Metal organic chemical vapor deposition grown GaAs-AlGaAs core-shell nanowires containing InGaAs quantum dot stacks are placed directly on a silver film, and lasing was observed from single nanowires originating from the InGaAs quantum dot emission into the low-loss higher order plasmonic mode. Lasing threshold pump fluences as low as ∼120 μJ/cm(2) was observed at 7 K, and lasing was observed up to 125 K. Temperature stability from the quantum dot gain, leading to a high characteristic temperature was demonstrated. These results indicate that high-performance, miniaturized quantum dot lasers can be realized with plasmonics.

General route for the decomposition of InAs quantum dots during the capping process

The effect of the capping process on the morphology of InAs/GaAs quantum dots (QDs) by using different GaAs-based capping layers (CLs), ranging from strain reduction layers to strain compensating layers, has been studied by transmission microscopic techniques. For this, we have measured simultaneously the height and diameter in buried and uncapped QDs covering populations of hundreds of QDs that are statistically reliable. First, the uncapped QD population evolves in all cases from a pyramidal shape into a more homogenous distribution of buried QDs with a spherical-dome shape, despite the different mechanisms implicated in the QD capping. Second, the shape of the buried QDs depends only on the final QD size, where the radius of curvature is function of the base diameter independently of the CL composition and growth conditions. An asymmetric evolution of the QDs' morphology takes place, in which the QD height and base diameter are modified in the amount required to adopt a similar stable shape characterized by a averaged aspect ratio of 0.21. Our results contradict the traditional model of QD material redistribution from the apex to the base and point to a different universal behavior of the overgrowth processes in self-organized InAs QDs.

Suppression of dislocations by Sb spray in the vicinity of InAs/GaAs quantum dots

The effect of Sb spray prior to the capping of a GaAs layer on the structure and properties of InAs/GaAs quantum dots (QDs) grown by molecular beam epitaxy (MBE) is studied by cross-sectional high-resolution transmission electron microscopy (HRTEM). Compared to the typical GaAs-capped InAs/GaAs QDs, Sb-sprayed QDs display a more uniform lens shape with a thickness of about 3 ~ 4 nm rather than the pyramidal shape of the non-Sb-sprayed QDs. Particularly, the dislocations were observed to be passivated in the InAs/GaAs interface region and even be suppressed to a large extent. There are almost no extended dislocations in the immediate vicinity of the QDs. This result is most likely related to the formation of graded GaAsSb immediately adjacent to the InAs QDs that provides strain relief for the dot/capping layer lattice mismatch.

PACS

81.05.Ea; 81.07.-b; 81.07.Ta.

Model of a realistic InP surface quantum dot extrapolated from atomic force microscopy results

We report on numerical simulations of a zincblende InP surface quantum dot (QD) on In₀.₄₈Ga₀.₅₂ buffer. Our model is strictly based on experimental structures, since we extrapolated a three-dimensional dot directly by atomic force microscopy results. Continuum electromechanical, [Formula: see text] bandstructure and optical calculations are presented for this realistic structure, together with benchmark calculations for a lens-shape QD with the same radius and height of the extrapolated dot. Interesting similarities and differences are shown by comparing the results obtained with the two different structures, leading to the conclusion that the use of a more realistic structure can provide significant improvements in the modeling of QDs fact, the remarkable splitting for the electron p-like levels of the extrapolated dot seems to prove that a realistic experimental structure can reproduce the right symmetry and a correct splitting usually given by atomistic calculations even within the multiband [Formula: see text] approach. Moreover, the energy levels and the symmetry of the holes are strongly dependent on the shape of the dot. In particular, as far as we know, their wave function symmetries do not seem to resemble to any results previously obtained with simulations of zincblende ideal structures, such as lenses or truncated pyramids. The magnitude of the oscillator strengths is also strongly dependent on the shape of the dot, showing a lower intensity for the extrapolated dot, especially for the transition between the electrons and holes ground state, as a result of a relevant reduction of the wave functions overlap. We also compare an experimental photoluminescence spectrum measured on an homogeneous sample containing about 60 dots with a numerical ensemble average derived from single dot calculations. The broader energy range of the numerical spectrum motivated us to perform further verifications, which have clarified some aspects of the experimental results and helped us to develop a suitable model for the spectrum, by assuming a not equiprobable weight from each dot, a model which is extremely consistent with the experimental data.

Composition-tunable nonlinear optical properties of ternary CdSexS1−x (x = 0–1) alloy quantum dots

The lattice spacing and UV/vis absorption spectra of CdSe_xS_1−x quantum dots exhibit composition-tunable features upon changing x values.

Single-Nanocrystal Photoluminescence Spectroscopy Studies of Plasmon-Multiexciton Interactions at Low Temperature

Using thick-shell or "giant" CdSe/CdS nanocrystal quantum dots (g-NQDs), characterized by strongly suppressed Auger recombination, we studied the influence of plasmonic interactions on multiexciton emission. Specifically, we assessed the separate effects of plasmonic absorption and plasmonic emission enhancement by a systematic analysis of the pump fluence dependence of low-temperature photoluminescence (low-T PL) derived from individual CdSe/CdS g-NQDs deposited on nanoroughened silver films. Our study reveals that (1) the multiexciton (MX) emissions in g-NQD coupled to silver films were enhanced not only through the creation of more excitons via enhancement of absorption but also through the direct modification of the competition between the radiative and nonradiative recombination processes of MXs; (2) strong enhancement in absorption is not necessary for strong multiexciton emission; and (3) the emission of MXs can become stronger with the increase of multiexciton order. We also exploited the strong enhancement of MX emission to perform second-order photon correlation and cross-correlation experiments using very low pump fluences and observed a strong photon bunching that decays with increasing pump fluence.

Narrowing the length distribution of Ge nanowires

Synthesis of nanostructures of uniform size is fundamental because the size distribution directly affects their physical properties. We present experimental data demonstrating a narrowing effect on the length distribution of Ge nanowires synthesized by the Au-catalyzed molecular beam epitaxy on Si substrates. A theoretical model is developed that is capable of describing this puzzling behavior. It is demonstrated that the direction of the diffusion flux of sidewall adatoms is size dependent and has a major effect on the growth rate of differently sized nanowires. We also show that there exists a fundamental limitation on the maximum nanowire length that can be achieved by molecular beam epitaxy where the direction of the beam is close to the growth axis.

CHEMICAL DESIGN OF COMPLEX NANOSTRUCTURED METAL OXIDES IN SOLUTION

Nanostructured materials with controlled architectures are desirable for many applications, among which, metal oxides are especially important in optics, electronics, biology, catalysis, and energy conversions. Various chemical routes have been widely investigated for the synthesis of nanostructured metal oxide particles and films. More recently, deliberately designed chemical strategies have been used to produce particles and films composed of more complex crystal structures. In this paper, we discuss some recent progresses in the design of complex nanostructures through chemical routes, emphasize particularly on metal oxides. We first review some basic concepts involved in the fabrication of complex nanostructures, including crystal nucleation and growth, shape controlling and ripening process. We then describe more recent work on the use of different methods to synthesize a wide range of complex nanostructures, including hierarchical structures, heterostructures, as well as oriented nanowires and nanotubes. Such purposely built materials are designed, and engineered to match the physical, chemical, and structural requirements of their applications.

MAGNETOPOLARON IN A CYLINDRICAL QUANTUM DOT

We examine the magnetopolaron state in a cylindrical quantum dot with a transverse parabolic potential and a high rectangular potential well in the longitudinal direction. The quadratic dependence of the magnetopolaron energy versus Fröhlich electron–phonon coupling constant for different cyclotron radii and constant structure radius is modulated by a logarithmic function seems to depend on the Fröhlich coupling constant. The same law is seen in the case of magnetopolaron energy versus Fröhlich electron–phonon coupling constant for different structure radii and constant cyclotron radius. The energies are seen to be lifted in different fashions in the case of the structure and cyclotron radii. The high degrees of confinement (or high magnetic field) lead to an enhancement in the effective electron–phonon coupling that in turn brings about the possibility that in spite of weak polar coupling as in GaAS say, the polaron problem may also have strong-coupling counterparts arising from confinement or magnetic field effects.

The polaron mass increases with increasing Fröhlich electron–phonon coupling constant. The dependence seems to be fourth-order law of the Fröhlich coupling constant modulated by a logarithmic function.

Surface x-ray diffraction results on the III-V droplet heteroepitaxy growth process for quantum dots: recent understanding and open questions

In recent years, epitaxial growth of self-assembled quantum dots has offered a way to incorporate new properties into existing solid state devices. Although the droplet heteroepitaxy method is relatively complex, it is quite relaxed with respect to the material combinations that can be used. This offers great flexibility in the systems that can be achieved. In this paper we review the structure and composition of a number of quantum dot systems grown by the droplet heteroepitaxy method, emphasizing the insights that these experiments provide with respect to the growth process. Detailed structural and composition information has been obtained using surface X-ray diffraction analyzed by the COBRA phase retrieval method. A number of interesting phenomena have been observed: penetration of the dots into the substrate (“nano-drilling”) is often encountered; interdiffusion and intermixing already start when the group III droplets are deposited, and structure and composition may be very different from the one initially intended.

Lasing in ultra-narrow emission from GaAs quantum dots coupled with a two-dimensional layer

We report electrically injected lasing in GaAs quantum dots (QDs) grown on GaAs(001) by droplet epitaxy. High-quality GaAs QDs with superior uniformity are formed using improved growth techniques involving the insertion of a two-dimensional layer, control of the As flux for GaAs crystallization, and thin AlGaAs layer capping with high-temperature annealing. The QDs show ultra-narrow luminescence with a linewidth of 20 meV. Ground-state lasing from a laser diode containing fivefold-stacked QD layers is observed at low temperature under pulsed operation.

Helical quantum states in HgTe quantum dots with inverted band structures

We investigate theoretically the electron states in HgTe quantum dots (QDs) with inverted band structures. In sharp contrast to conventional semiconductor quantum dots, the quantum states in the gap of the HgTe QD are fully spin-polarized and show ringlike density distributions near the boundary of the QD and spin-angular momentum locking. The persistent charge currents and magnetic moments, i.e., the Aharonov-Bohm effect, can be observed in such a QD structure. This feature offers us a practical way to detect these exotic ringlike edge states by using the SQUID technique.

New top-down approach for fabricating high-aspect-ratio complex nanostructures with 10 nm scale features

We describe a new patterning technique, named "secondary sputtering lithography" that enables fabrication of ultrahigh-resolution (ca. 10 nm) and high aspect ratio (ca. 15) patterns of three-dimensional various shapes. In this methodology, target materials are etched and deposited onto the side surface of a prepatterned polymer by using low Ar ion bombarding energies, based on the angular distribution of target particles by ion-beam bombardment. After removal of the prepatterned polymer, high aspect ratios and high-resolution patterns of target materials are created.

Carrier transport investigation in short-wavelength InAs/AlGaAs quantum dots

Spatially resolved photoluminescence has been used to investigate the details of the carrier capture and recombination dynamics in InAs/AlGaAs self-assembled quantum dots. The spatial PL distribution displays a Gaussian-like profile, whose width depends upon the temperature and detection energy being analyzed. The results give evidence of carrier thermalization between dots with different sizes. The effects of carrier transport in the quantum dot (QD) structure and carrier capture cannot be separated. The results can be modeled by assuming a carrier hopping process.

Optimal capping layer thickness for stacked quantum dots

We study the effect of strain on the vertical and lateral self-organization of nanoscale patterns and stacked quantum dots during epitaxial growth. The computational approach is based on the level set method in combination with an atomistic strain code. Strain changes the energetics of microscopic parameters during growth, and thus determines the nucleation sites and the growth of islands and dots. Our results show that strain can lead to vertical alignment as well as lateral organization. Moreover, our simulations suggest that there is an optimal thickness of the capping layer to get the best alignment and most uniform size distribution of stacked quantum dots, and that its variation can be used to control the formation of interesting structures.

Tailoring of morphology and emission wavelength of AlGaInAs quantum dots

We present a study of the growth, morphology and optical properties of Al(x)Ga(1-x-y)In(y)As quantum dots (QDs) for a wide range of Al and In concentrations (0≤x≤0.34 and 0.43≤y≤0.60). Short emission wavelengths between 660 and 940 nm and QD surface densities up to 1.1 × 10(11) cm(-2) have been achieved. Our results show that by varying both the Al concentration and the In concentration an independent adjustment of strain and QD band gap is possible. This additional degree of freedom can be employed for tailoring AlGaInAs QDs with the desired emission wavelength, surface density and average size. AlGaInAs QDs thus offer new possibilities for future QD device design.

Fringing field directed assembly of nanomaterials

This letter reports on a new gas-phase printing approach to deposit nanomaterials into addressable areas on a surface with 50 nm lateral accuracy. Localized fringing fields that form around conventional resist patterns (PMMA and SiO2) with openings to a silicon substrate are used to direct the assembly of nanomaterials into the openings. Directed assembly was observed due to a naturally occurring inbuilt charge differential at the material interface that was further enhanced by corona charging to yield a field strength exceeding 1 MV/m in Kelvin probe force microscopy (KFM) measurements. The assembly process is independent of the nanomaterial source and type: an evaporative, plasma, and electrospray source have been tested to deposit silicon and metallic nanoparticles. The results suggest a potential route to form nanolenses on the basis of charged resist structures; a 3-fold size reduction has been observed between the structures and the assembled particles. Applications range from the integration of functional nanomaterial building blocks to the elimination of lift-off steps in semiconductor processing.

Highly Efficient Luminescent Organic Clusters with Quantum Dot-Like Properties

Organic clusters with uniform structure and size have been synthesized and studied. The quantum confinements on organic arms [chains shorter than exciton Bohr radii (aB)] have been confirmed by the discrete energy levels from PLE, narrow PL lines, size effect from UV, and spherical shape from TEM, as well as Raman peaks red-shifting and broadening. The results indicate the materials have QD-like properties.

Hidden symmetries in the energy levels of excitonic 'artificial atoms'

Quantum dots or 'artificial atoms' are of fundamental and technological interest--for example, quantum dots may form the basis of new generations of lasers. The emission in quantum-dot lasers originates from the recombination of excitonic complexes, so it is important to understand the dot's internal electronic structure (and of fundamental interest to compare this to real atomic structure). Here we investigate artificial electronic structure by injecting optically a controlled number of electrons and holes into an isolated single quantum dot. The charge carriers form complexes that are artificial analogues of hydrogen, helium, lithium, beryllium, boron and carbon excitonic atoms. We observe that electrons and holes occupy the confined electronic shells in characteristic numbers according to the Pauli exclusion principle. In each degenerate shell, collective condensation of the electrons and holes into coherent many-exciton ground states takes place; this phenomenon results from hidden symmetries (the analogue of Hund's rules for real atoms) in the energy function that describes the multi-particle system. Breaking of the hidden symmetries leads to unusual quantum interferences in emission involving excited states.