Semiconductor Quantum Dot Structures Research Articles

A kinetic model of a quantum-dot microlaser

The results of a qualitative analysis of a semiclassical model of light generation in low-dimensional solidstate lasers, including quantum-dot microlasers and, on its basis, a numerical modelling of the regular pulsation regime that occurs under conditions of nonlinear shift and broadening of the resonant spectral gain line due to the influence of dipoledipoles interaction and absorption in quasi-resonant transitions on the dielectric susceptibility of the active medium are herein presented. The modelling of lasing was carried out for the parameters of semiconductor quantum-dot structures.

A Quantum-Computing-Based Method for Solving Quantum Confinement Problem in Semiconductor

Technology computer-aided design (TCAD) of semiconductor devices relies on the numerical solution of differential equations in devices. Recent advances in quantum computing provide a new opportunity for TCAD simulations to be performed on a quantum computer. Based on a variational quantum algorithm, we develop a quantum-computing-based method to solve quantum confinement problems in semiconductor nanostructures. As the number of numerical discretization grid points for solving the Schrödinger equation increases, the number of qubits needed scales only logarithmically, ~ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${O}[\log(N)]$ </tex-math></inline-formula> . The method is applied to solve quantum confinement problems at all dimensions, which are related to confinement in a quantum well, semiconductor nanowire, and semiconductor quantum dot structures. The method can treat an anisotropic band structure and electrostatic potential in semiconductor nanostructures. We further show that the design of ansatz plays an important role in the performance of the method in terms of solution accuracy. The quantum-computing-based method can compute the energies and wave functions of both the ground and excited states with high accuracy.

Enhanced determination of emission fine structure and orientation of individual quantum dots based on correction algorithm for spectral diffusion

A robust algorithm based on cross-correlations and lucky imaging reliably allows the correction of spectrally diffused datasets. This step enables the resolution-limited analysis of the emission fine structure of semiconductor quantum dots (QDs). Bright and dark excitonic transitions are resolved with optimum signal-to-noise ratio, allowing for a precise determination of the angular direction of linear polarization of the different lines. The angular phases between polarization directions are intrinsically connected to the orientations of emission dipoles. This fact provides a tool for accurate numerical computation of the azimuth and polar angle of the QD with respect to the optical axis. Our in-situ characterization of QD fine structure and orientation represents a precise and non-invasive method without requiring specialized equipment beyond a standard luminescence setup. In this way, important information is provided whenever efficient coupling of a quantum emitter to the electromagnetic field is targeted by various nano- and micro-optic strategies.

Mechanical Nano-Patterning: Toward Highly-Aligned Ge Self-Assembly on Low Lattice Mismatched GaAs Substrate

Low-dimensional semiconductor structurers formed on a substrate surface at pre-defined locations and with nano-precision placement is of vital interest. The potential of tailoring their electrical and optical properties will revolutionize the next generation of optoelectronic devices. Traditionally, highly aligned self-assembly of semiconductors relies on Stranski- Krastanov growth mode. In this work, we demonstrate a pathway towards ordered configuration of Ge islands on low lattice mismatch GaAs (110) substrate patterned using depth-controlled nanoindentation. Diamond probe tips with different geometries are used to nano-mechanically stamp the surface of GaAs (110). This creates nanoscale volumes of dislocation-mediated deformation which acts to bias nucleation. Results show that nanostamped GaAs exhibits selective-nucleation of Ge at the indent sites. Ge islands formed on a surface patterned using cube corner tip have height of ~10 nm and lateral size of ~225 nm. Larger islands are formed by using Vickers and Berkovich diamond tips (~400 nm). The strain state of the patterned structures is characterized by micro-Raman spectroscopy. A strain value up to 2% for all tip geometries has been obtained. Additionally, strong room temperature photoluminescence (PL) emission is observed around 1.9 µm (650 meV). The observed strain-induced enhancement in the light-emission efficiency is attributed to direct conduction to heavy-hole (cΓ-HH) and conduction to light-hole (cΓ-LH) transitions. The inherent simplicity of the proposed method offers an attractive technique to manufacture semiconductor quantum dot structures for future electronic and photonic applications.

Pressure-dependent photoluminescence of CdSe/ZnS quantum dots: Critical point of different pressure regimes

High pressure condition can alter and control the electronic structure of semiconductor quantum dots, and therefore provides novel insight for designing optoelectronic devices. Here we report the pressure-dependent photoluminescence (PL) of CdSe/ZnS core/shell quantum dots (QD) in different pressure regimes. The center of the PL spectrum has blue-shift when hydrostatic pressure increases, due to the increasing bulk modulus. However, the shifting rate becomes lower in the higher pressure regime, i.e. above the critical pressure point, mainly due to the interaction between QDs. Accordingly, the lifetimes of PL drop in the low pressure regime, due to the increased pressure-induced trapping states, then increase above the critical pressure point, indicating the interaction between dots likewise. The observed critical pressures for both cases are consistent with each other, and strongly depend on the QD concentration, which forcefully support the aforementioned interaction model.

Phase transition and charge transport through a triple dot device beyond the Kondo regime**Project supported by the National Natural Science Foundation of China (Grant No. 11504102), the Scientific Research Items Foundation of Hubei Educational Committee (Grant Nos. Q20161803 and D20171803), the Doctoral Scientific Research Foundation of Hubei University of Automotive Technology (Grant No. BK201407).

Semiconductor quantum dot structure provides a promising basis for quantum information processing, within which to reveal the quantum phase and charge transport is one of the most important issues. In this paper, by means of the numerical renormalization group technique, we study the quantum phase transition and the charge transport for a parallel triple dot device in the strongly correlated limit, focusing on the effect of inter-dot hopping t beyond the Kondo regime. We find the quantum behaviors depend closely on the initial electron number on the dots, and the present model may map to single, double, and side-coupled impurity models in different parameter spaces. An orbital spin-1/2 Kondo effect between the conduction leads and the bonding orbital, and several magnetic-frustration phases are demonstrated when t is adjusted to different regimes. To understand these phenomena, a canonical transformation of the energy levels is given, and important physical quantities with respect to increasing t and necessary theoretical discussions are shown.

Nature of the Positron State in CdSe Quantum Dots.

Previous studies have shown that positron-annihilation spectroscopy is a highly sensitive probe of the electronic structure and surface composition of ligand-capped semiconductor quantum dots (QDs) embedded in thin films. The nature of the associated positron state, however, whether the positron is confined inside the QDs or localized at their surfaces, has so far remained unresolved. Our positron-annihilation lifetime spectroscopy studies of CdSe QDs reveal the presence of a strong lifetime component in the narrow range of 358-371ps, indicating abundant trapping and annihilation of positrons at the surfaces of the QDs. Furthermore, our abinitio calculations of the positron wave function and lifetime employing a recent formulation of the weighted density approximation demonstrate the presence of a positron surface state and predict positron lifetimes close to experimental values. Our study thus resolves the long-standing question regarding the nature of the positron state in semiconductor QDs and opens the way to extract quantitative information on surface composition and ligand-surface interactions of colloidal semiconductor QDs through highly sensitive positron-annihilation techniques.

Enhanced luminescence and optical performance through strain minimization in self-assembled InAs QDs using dual quaternary-ternary/ternary-quaternary capping

Theoretical simulations are necessary to scientifically progress, envisage and gain the first hand knowledge of the properties of semiconductor quantum dot (QD) structure. The impact of dual ternary-quaternary/quaternary-ternary capping layers on the square based truncated pyramidal InAs QD and their influence on the energy band structure as well as the strain profile has been investigated in this study. A comparative analysis has been demonstrated with binary, ternary and quaternary capping layer counterparts. The capping layers are: GaAs (binary), InGaAs/GaAs (ternary), InAlGaAs/GaAs (quaternary), InGaAs/InAlGaAs/GaAs (dual ternary-quaternary) and InAlGaAs/InGaAs/GaAs (dual quaternary-ternary). The hydrostatic and biaxial strain for all the heterostructures have also been computed and compared. The biaxial strain was highest for the QD capped with InAlGaAs/GaAs and InAlGaAs/InGaAs/GaAs, leading to the red shift of the ground state emission in the photoluminescence (PL) spectrum. The possible absorption peaks were also calculated for all QDs from the electron (EL) and hole (HL) eigenstates. The lowest energy transition (EL1-HL1) at 300 K is near to the communication wavelength for the optimum (InAlGaAs/InGaAs/GaAs capped) QD, which is at 1.35 µm. The optimized QD heterostructure claimed the highest electrostatic potential of 0.4986 V. This study will be helpful in predicting few optical and electrical characteristics based on the strain distribution inside the structure. The optimized QD structure having a consistent strain profile inside it would render superior luminescence characteristics. Finally, to verify the accuracy of the simulation, the simulated transition peaks at 19 K and 300 K were validated with the experimental PL data, and the percentage error was within 5%.

Aptamer-Modified Semiconductor Quantum Dots for Biosensing Applications.

Semiconductor quantum dots have attracted extensive interest in the biosensing area because of their properties, such as narrow and symmetric emission with tunable colors, high quantum yield, high stability and controllable morphology. The introduction of various reactive functional groups on the surface of semiconductor quantum dots allows one to conjugate a spectrum of ligands, antibodies, peptides, or nucleic acids for broader and smarter applications. Among these ligands, aptamers exhibit many advantages including small size, high chemical stability, simple synthesis with high batch-to-batch consistency and convenient modification. More importantly, it is easy to introduce nucleic acid amplification strategies and/or nanomaterials to improve the sensitivity of aptamer-based sensing systems. Therefore, the combination of semiconductor quantum dots and aptamers brings more opportunities in bioanalysis. Here we summarize recent advances on aptamer-functionalized semiconductor quantum dots in biosensing applications. Firstly, we discuss the properties and structure of semiconductor quantum dots and aptamers. Then, the applications of biosensors based on aptamer-modified semiconductor quantum dots by different signal transducing mechanisms, including optical, electrochemical and electrogenerated chemiluminescence approaches, is discussed. Finally, our perspectives on the challenges and opportunities in this promising field are provided.

Spin dynamics in SiGe quantum dot lines and ring molecules

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.

Scanning probe microscopy and spectroscopy of colloidal semiconductor nanocrystals and assembled structures.

Colloidal semiconductor nanocrystals become increasingly important in materials science and technology, due to their optoelectronic properties that are tunable by size. The measurement and understanding of their energy levels is key to scientific and technological progress. Here we review how the confined electronic orbitals and related energy levels of individual semiconductor quantum dots have been measured by means of scanning tunneling microscopy and spectroscopy. These techniques were originally developed for flat conducting surfaces, but they have been adapted to investigate the atomic and electronic structure of semiconductor quantum dots. We compare the results obtained on colloidal quantum dots with those on comparable solid-state ones. We also compare the results obtained with scanning tunneling spectroscopy with those of optical spectroscopy. The first three sections provide an introduction to colloidal quantum dots, and a theoretical basis to be able to understand tunneling spectroscopy on dots attached to a conducting surface. In sections 4 and 5 , we review the work performed on lead-chalcogenide nanocrystals and on colloidal quantum dots and rods of II-VI compounds, respectively. In section 6 , we deal with colloidal III-V nanocrystals and compare the results with their self-assembled counter parts. In section 7 , we review the work on other types of semiconductor quantum dots, especially on Si and Ge nanocrystals.

Understanding and optimizing spin injection in self-assembled InAs/GaAs quantum-dot molecular structures

Semiconductor quantum-dot (QD) structures are promising for spintronic applications owing to their strong quenching of spin relaxation processes that are promoted by carrier and exciton motions. Unfortunately, the spin injection efficiency in such nanostructures is very low and the exact physical mechanism of the spin loss is still not fully understood. Here, we show that exciton spin injection in self-assembled InAs/GaAs QDs and QD molecular structures (QMSs) is dominated by localized excitons confined within the QD-like regions of the wetting layer (WL) and GaAs barrier layer that immediately surround the QDs and QMSs. These localized excitons in fact lack the commonly believed 2D and 3D character with an extended wavefunction. We attribute the microscopic origin of the severe spin loss observed during spin injection to a sizable anisotropic exchange interaction (AEI) of the localized excitons in the WL and GaAs barrier layer, which has so far been overlooked. We determined that the AEI of the injected excitons and, thus, the efficiency of the spin injection processes are correlated with the overall geometric symmetry of the QMSs. This symmetry largely defines the anisotropy of the confinement potential of the localized excitons in the surrounding WL and GaAs barrier. These results pave the way for a better understanding of spin injection processes and the microscopic origin of spin loss in QD structures. Furthermore, they provide a useful guideline to significantly improve spin injection efficiency by optimizing the lateral arrangement of QMSs and overcome a major challenge in spintronic device applications utilizing semiconductor QDs.

High quality InP nanopyramidal frusta on Si

Octagonal nanopyramidal InP frusta grown selectively on silicon.

Effects of probe field intensity in nonlinear optical processes in asymmetric semiconductor quantum dots

We study nonlinear optical absorption and nonlinear optical rectification in an asymmetric semiconductor quantum dot structure under a strong probe field excitation. We apply a form of the rotating wave approximation for asymmetric quantum systems, solve the relevant density matrix equations under steady state conditions, and derive the formulae for nonlinear optical absorption and nonlinear optical rectification under the interaction with a strong probe field. The differences between our formulae and those of a previous study are also presented for the case of an electron confined in an asymmetric double quantum dot nanostructure.

A Complete Physical Germanium-on-Silicon Quantum Dot Self-Assembly Process

Achieving quantum dot self-assembly at precise pre-defined locations is of vital interest. In this work, a novel physical method for producing germanium quantum dots on silicon using nanoindentation to pre-define nucleation sites is described. Self-assembly of ordered ~10 nm height germanium quantum dot arrays on silicon substrates is achieved. Due to the inherent simplicity and elegance of the proposed method, the results describe an attractive technique to manufacture semiconductor quantum dot structures for future quantum electronic and photonic applications.

MOVPE GROWTH OF THE InP BASED MID-IR EMISSION QUANTUM DOT STRUCTURES

In this paper, semiconductor quantum dot structures for mid-infrared emission were self-assembled on InP substrate by using metal–organic vapor phase epitaxy growth. The InAs quantum dots grown at different conditions have been investigated. To improve the grown quantum dot's shape, the dot density and the dot size uniformity, a two-step growth method has been used and investigated. By changing the composition of the In x Ga 1-x As matrix layer of the InAs / In x Ga 1-x As / InP quantum dot structure, emission wavelength of the InAs quantum dot structure has been extended to the longest &gt; 2.35 μm measured at 77 K. For the narrower bandgap semiconductor InAsSb quantum dots, the emission wavelength was measured at &gt; 2.8 μm.

Probing into hybrid organic-molecule and InAs quantum-dots nanosystem with multistacked dots-in-a-well units

Hybridizing air-stable organic-molecules with advanced III-V semiconductor quantum-dots (QDs) structures can be utilized to create a new generation of biochemical sensing devices. In order to enhance their optical performances, the active regions in these QDs structures commonly consist of multistacked dots-in-a-well (DWELL) units. The effects of grafted molecules on the performances of the QDs structures with multistacked DWELLs, however, still remain unclear. Here, we show the significant improvements in the optical properties of InAs QDs in a hybrid nanosystem obtained by grafting biocompatible diazonium salt compound (amine donor) atop InAs QDs structure. Since its interface between the QDs structure and molecular monolayer retains an uncontaminated and non-oxidized condition, the nanosystem is an ideal platform to study the intrinsic properties of charge-carrier transport inside the system. Because of the complexity of the energy-levels in the QDs structure due to the existing surface QDs and DWELLs, selective excitation wavelengths (400, 633, and 885 nm, respectively) with different photo-energies are used to exactly analyze the complete charging mechanism in these QDs. A clear view of charge-carrier transfer inside the nanosystem is revealed by employing photoluminescence technique under selective-wavelength excitations. The present work provides new quantitative evidences for exploiting inorganic QDs applications in complex biological systems.

The Electronic Structure of Coupled Semiconductor Quantum Dots Arranged as a Graphene Hexagonal Lattice under a Magnetic Field

We study the electronic spectrum of coupled quantum dots (QDs) arranged as a graphene hexagonal lattice in the presence of an external perpendicular magnetic field. In our tight-binding model, the effect of the magnetic field is included in both the Peierls phase of the Hamiltonian and the tight-binding basis Wannier function. The energy of the system is analyzed when the magnetic flux through the lattice unit cell is a rational fraction of the quantum flux. The calculated spectrum has recursive properties, similar to those of the classical Hofstadter butterfly. However, unlike the ideal Hofstadter butterfly structure, our result is asymmetric since the impacts of the specific material and the magnetic field on the wavefunctions are included, making the results more realistic.

Influence of electron-acoustic phonon scattering on off-resonant cavity feeding within a strongly coupled quantum-dot cavity system

We present a medium-dependent quantum optics approach to describe the influence of electron-acoustic phonon coupling on the emission spectra of a strongly coupled quantum-dot cavity system. Using a canonical Hamiltonian for light quantization and a photon Green function formalism, phonons are included to all orders through the dot polarizability function obtained within the independent Boson model. We derive simple user-friendly analytical expressions for the linear quantum light spectrum, including the influence from both exciton and cavity-emission decay channels. In the regime of semiconductor cavity-QED, we study cavity emission for various exciton-cavity detunings and demonstrate rich spectral asymmetries as well as cavity-mode suppression and enhancement effects. Our technique is nonperturbative, and non-Markovian, and can be applied to study photon emission from a wide range of semiconductor quantum dot structures, including waveguides and coupled cavity arrays. We compare our theory directly to recent and apparently puzzling experimental data for a single site-controlled quantum dot in a photonic crystal cavity and show good agreement as a function of cavity-dot detuning and as a function of temperature.

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.