Semiconductor Quantum Dot Structures Research Articles

ENTANGLEMENT CREATION IN SEMICONDUCTOR QUANTUM DOT CHARGE QUBIT

We study theoretically the appearance of quantum correlations in two- and three-electron scattering in single and double dots. The key role played by transport resonances into entanglement formation between the single-particle states is shown. Both reflected and transmitted components of the scattered particle wavefunction are used to evaluate the quantum correlations between the incident carrier and the bound particle(s) in the dots. Our investigation provides a guideline for the analysis of decoherence effects due to the Coulomb scattering in semiconductor quantum dots structures.

Internal Structure of InP/ZnS Nanocrystals Unraveled by High-Resolution Soft X-ray Photoelectron Spectroscopy

High-energy resolution photoelectron spectroscopy (DeltaE < 200 meV) is used to investigate the internal structure of semiconductor quantum dots containing low Z-contrast elements. In InP/ZnS core/shell nanocrystals synthesized using a single-step procedure (core and shell precursors added at the same time), a homogeneously alloyed InPZnS core structure is evidenced by quantitative analysis of their In3d(5/2) spectra recorded at variable excitation energy. When using a two-step method (core InP nanocrystal synthesis followed by subsequent ZnS shell growth), XPS analysis reveals a graded core/shell interface. We demonstrate the existence of In-S and S(x)-In-P(1-x) bonding states in both types of InP/ZnS nanocrystals, which allows a refined view on the underlying reaction mechanisms.

A numerical method for exact diagonalization of semiconductor quantum dot model

An approach to the exact diagonalization of many-electron Hamiltonian in semiconductor quantum dot (QD) structures is proposed. The QD model is based on 3D finite hard-wall confinement potential and nonparabolic effective-mass approximation (EMA) that render analytical basis functions such as Laguerre polynomials inaccessible for the numerical treatment of this kind of models. In this approach, the many-electron wave function is expanded in a basis of Slater determinants constructed from numerical wave functions of the single-electron Hamiltonian with the nonparabolic EMA which results in a cubic eigenvalue problem from a finite difference discretization. The nonlinear eigenvalue problem is solved by using the Jacobi–Davidson method. The Coulomb matrix elements in the many-electron Hamiltonian are obtained by solving Poisson's problems via GMRES. Numerical results reveal that a good convergence can be achieved by means of a few single-electron basis states.

Structural and optical properties of ultrananocrystalline diamond / InGaAs/GaAs quantum dot structures

The combination of the unique properties of ultrananocrystalline diamond (UNCD) films and of semiconductor quantum dot (QD) structures could significantly improve the performance of different electronic and optoelectronic devices, where e.g. good thermal management and advanced mechanical parameters are required. In the current work quantum dot InGaAs/GaAs heterostructures have been grown by molecular beam epitaxy (MBE) with different densities between 1.6×1010cm−2 and 1.6×1011cm−2 controlled by the substrate temperature in the range between 490 and 515°C. These structures were overgrown with UNCD by microwave plasma chemical vapor deposition (MWCVD) using methane/nitrogen mixtures at 570°C. Scanning electron microscopy (SEM) reveals that without ultrasonic pretreatment the diamond nucleation density on QD structures is low and only separate islands of UNCD are deposited, while after pretreatment thin closed films are formed. From the cross-section SEM images a growth rate of ca. 3nm/min is estimated which is very close to that on silicon at the same deposition conditions. The UNCD coatings exhibit a morphology consisting of two types of structures as shown by atomic force microscopy (AFM). The first one includes nodules with diameters between 180 and 350nm varying with the density of the underlying QDs; the second is formed by a kind of granular substructure of these nodules with diameters of about 40nm for all QD densities. The optical properties were investigated by photoluminescence (PL) spectroscopy before and after the deposition of UNCD. The PL signals of QD structures overgrown with UNCD, although with decreased intensity, remain almost unchanged with respect to the peak positions and widths, revealing that the UNCD/QD structures retain the optical properties of uncoated InGaAs/GaAs quantum dots.

Comparison of electromagnetically induced transparency schemes in semiconductor quantum dot structures: Impact of many-body interactions

We investigate the impact of many-body interactions on group-velocity slowdown achieved via electromagnetically induced transparency in quantum dots using three different coupling-probe schemes (ladder, $V$, and $\ensuremath{\Lambda}$, respectively). We find that for all schemes many-body interactions have an important impact on the slow light properties. In the case of the $\ensuremath{\Lambda}$ and $V$ schemes, the minimum required coupling power to achieve slow light is significantly reduced by many-body interactions. $V$ type schemes are found to be generally preferable due to a favorable redistribution of carriers in energy space.

Electronic structure of Mn-doped III-V semiconductor quantum dots

The electronic structure of GaAs and InAs quantum dots (QDs) containing a single substitutional Mn impurity is investigated in the envelope-function formalism. The Mn impurity in these compounds is known to be a shallow acceptor in the configuration ${d}^{5}+h$ and characterized by a strong antiferromagnetic $sp\text{\ensuremath{-}}d$ exchange interaction between the hole $(h)$ and the Mn ion. Our model for the hole states is based on the Luttinger Hamiltonian and the Coulomb potential with a central-cell correction that accounts for the observed binding energy and the effective $g$ factor in the bulk. The binding energy as well as the exchange contribution is found to increase with decreasing QD size. However, in contrast with the case of spherical nanocrystals (NCs), the binding energy in lens-shaped self-assembled QDs in the low-confinement limit is lower than that in the bulk because of their highly anisotropic shape. With an on-center impurity, NCs retain the bulk ${T}_{d}$ symmetry and the ground state is a $j=3/2$-like ${\ensuremath{\Gamma}}_{8}({T}_{d})$ level. In self-assembled QDs it splits into two doublets: ${\ensuremath{\Gamma}}_{6}$ $(|{j}_{z}|=1/2)$ and ${\ensuremath{\Gamma}}_{7}$ $(|{j}_{z}|=3/2)$ of ${D}_{2d}$, which mix in the presence of in-plane asymmetry, both belonging to ${\ensuremath{\Gamma}}_{5}$ of the reduced symmetry ${C}_{2v}$. The order and the splitting between the doublets depend on the degree of confinement and the strain-induced separation between the light- and heavy-hole valence bands. In lattice-matched GaAs/(Ga,Al)As QDs the ground-state doublet is $|{j}_{z}|=3/2$-like in the low-confinement limit. As the lateral size decreases there is a rapid crossover to a $|{j}_{z}|=1/2$-like ground state in QDs of typical sizes. On the other hand, in strained InAs/GaAs QDs the ground state is always $|{j}_{z}|=3/2$-like and the splitting relatively large. The $sp\text{\ensuremath{-}}d$ coupling with the Mn spin $S=5/2$ finally leads to a splitting of the ground-state doublet into six doubly-degenerate levels. The components are close to one another as the effective exchange parameters are an order of magnitude smaller than in the bulk. Our results thoroughly contradict the previously adopted picture based on treating the confinement potential as a small perturbation to the bulk impurity levels. We also consider the lowest two-hole states: the ground state in InAs/GaAs QDs is a singlet almost uncoupled to the Mn spin. We deduce the zero-field fine structure of the excitonic transitions and compare the results with the recently reported photoluminescence spectra.

Linewidth of resonances in scanning tunneling spectroscopy

Cryogenic scanning tunneling spectroscopy is increasingly used to study the electronic structure of adatoms, molecules, and semiconductor quantum dots. However, the width of the conductance resonances that indicate the energy levels is much larger than the thermal energy, and this is not well understood. Here, we present a comprehensive study of the line shape and width of the conductance resonances observed with small colloidal semiconductor quantum dots. Experimentally, the line shape and width are studied for CdSe quantum dots of different sizes, with nanocrystals being chemically or physically attached to the substrate. The influence of the temperature is studied from 5 K up to room temperature. We have also varied the set-point current via the tip-to-dot distance to study the effects of dissipative heating of the quantum dot. We present basic calculations of the effects of electron-phonon coupling, charge and dipole fluctuations in the close environment of the quantum dot, mechanical oscillations of the quantum dot in the tunneling junction, and internal heating by nonresonant electron transport. A comparison with the experimental results shows that electron-phonon coupling forms the main contribution to the line broadening for the lowest resonance. Fluctuations of the charge landscape around the quantum dot are most likely involved in an additional broadening. More importantly, these potential fluctuations wash out the vibronic structure of the line shape that should arise from electron-phonon coupling. Our results show that in the case of semiconductor quantum dots, internal heating due to nonresonant electron transport is not important.

Mode-locked quantum-dot lasers

Semiconductor lasers are convenient and compact sources of light, offering highly efficient operation, direct electrical control and integration opportunities. In this review we describe how semiconductor quantum-dot structures can provide an efficient means of amplifying and generating ultrafast (of the order of 100 fs), high-power and low-noise optical pulses, with the potential to boost the repetition rate of the pulses to beyond 1 THz. Such device designs are opening up new possibilities in ultrafast science and technology.

Singlet-triplet filtering and entanglement in a quantum dot structure

We consider two interacting electrons in a semiconductor quantum dot structure, which consists of a small dot within a larger dot, and demonstrate a singlet-triplet filtering mechanism which involves spin-dependent resonances and can generate entanglement. By studying the exact time evolution of singlet and triplet states we show how the degree of both filtering and spin entanglement can be tuned using a time-dependent gate voltage.

A three-dimensional model for artificial atoms and molecules: influence of substrate orientation and magnetic field dependence

A full three-dimensional model for the calculation of the electronic structure of semiconductor quantum dots (QD) and molecules (QDM) grown on high index surfaces and/or in the presence of an external magnetic field is presented. The strain distribution of the dots is calculated using continuum elasticity and singe-particle states are extracted from the nonsymmetrized eight-band k·p theory. The model properly takes into account the effects of different substrate orientation by rotation of the coordinate system in such a way that one coordinate coincides with the growth direction, whereas the effects of a tilted external magnetic field are taken into account through the Zeeman effect and employing a gauge invariant scheme based on Wilson's formulation of lattice gauge theory. We point out the role of piezoelectricity for InAs/GaAs QDs grown on [11k], where k = 1,2,3,4,5,7,9 and for QDMs containing eight InAs/GaAs QDs grown on [11l], where l = 1,2,3. We predict the variation of the transition energies of the QDM as a function of substrate orientation and interdot distances in the molecule. We address the magnetic field direction dependent variation of the electronic properties of QD and QDM.

Study of the evanescent wave coupled semiconductor quantum dot amplifying fiber

Based on the character of semiconductors and the structure of optical fiber coupler, a new amplifying fiber, coupled semiconductor quantum dot amplifying fiber (CSQDAF), has been presented. A simplified model of PbS quantum dot amplifying fiber is built on the energy band structure of semiconductor quantum dots, and a simple expression deduced from the two-level rate equations and light propagation equations is shown in this paper, by which the gain of quantum dot amplifying fiber can be calculated. A gain of approximately 4.5 dB has been measured in this coupled semiconductor quantum dot amplifying fiber at a wavelength of 1310 nm, when the fiber is pumped by a laser operating at a wavelength of 980 nm with power of 30 mW.

Electron energy levels in ZnSe quantum dots

The size dependence of electron energy levels of ZnSe quantum dots were studied by size selective photoluminescence excitation spectroscopy at low temperature. ZnSe quantum dots of different sizes were synthesized by a high temperature wet chemical route. Cubic zinc-blende crystallites with sizes ranging from 1.5to4.5nm showing only band edge luminescence were obtained. Four excited state transitions have been observed in photoluminescence excitation spectroscopy. This study establishes a connection between the electron energy levels of ZnSe quantum dots and their corresponding size. The experimentally observed excited states for ZnSe quantum dots have been analyzed on the basis of “effective mass approximation” calculations. The observed transitions for wide band gap ZnSe are compared with the well studied material, viz., CdSe. The present work enables one to gain further insight into the size dependence of the electronic structure of semiconductor quantum dots.

Coulomb corrections to the slowdown factor in quantum-dot quantum coherence

Quantum-coherence induced group-velocity slowdown in a semiconductor quantum-dot structure is investigated using a many-body theory. The predictions are found to be noticeably different from those obtained in the independent-particle treatment typically used for describing atomic quantum coherence. In particular, Hartree-Fock renormalizations can lead to over two orders of magnitude reduction in the predicted pump intensity requirement for group-velocity slowdown to occur. Results are presented for the slowdown factor and slowdown-bandwidth product in a pulsed InAs–GaAs quantum-dot Λ scheme.

Interrelation of structural and electronic properties inInxGa1−xN∕GaNquantum dots using an eight-bandk∙pmodel

We present an eight-band $\mathbf{k}∙\mathbf{p}$-model for the calculation of the electronic structure of wurtzite semiconductor quantum dots (QDs) and its application to indium gallium nitride $({\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N})$ QDs formed by composition fluctuations in ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ layers. The eight-band $\mathbf{k}∙\mathbf{p}$-model accounts for strain effects, piezoelectricity and pyroelectricity, and spin-orbit and crystal-field splitting. Exciton binding energies are calculated using the self-consistent Hartree method. Using this model, we studied the electronic properties of ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ QDs and their dependence on structural properties, i.e., their chemical composition, height, and lateral diameter. We found a dominant influence of the built-in piezoelectric and pyroelectric fields, causing a spatial separation of the bound electron and hole states and a redshift of the exciton transition energies. The single-particle energies as well as the exciton energies depend heavily on the composition and geometry of the QDs.

A gallium nitride single-photon source operating at 200 K

Fundamentally secure quantum cryptography has still not seen widespread application owing to the difficulty of generating single photons on demand. Semiconductor quantum-dot structures have recently shown great promise as practical single-photon sources, and devices with integrated optical cavities and electrical-carrier injection have already been demonstrated. However, a significant obstacle for the application of commonly used III-V quantum dots to quantum-information-processing schemes is the requirement of liquid-helium cryogenic temperatures. Epitaxially grown gallium nitride quantum dots embedded in aluminium nitride have the potential for operation at much higher temperatures. Here, we report triggered single-photon emission from gallium nitride quantum dots at temperatures up to 200 K, a temperature easily reachable with thermo-electric cooling. Gallium nitride quantum dots also open a new wavelength region in the blue and near-ultraviolet portions of the spectrum for single-photon sources.

Characterization of hydrogen passivated defects in strain-engineered semiconductor quantum dot structures

The effects of hydrogen incorporation on carrier relaxation and recombination efficiencies in a large series of InAs self-assembled quantum dot structures deposited on InGaAs lower confining layers with different thicknesses and compositions have been addressed. With increasing H dose we observe an improvement in the radiative efficiency. By comparing steady state and time resolved photoluminescence measurements, it is established that the H passivation does not enhance the relaxation and capture efficiencies, but instead directly improves the emission yield from carriers in the dots. We therefore conclude that the H-passivated defects are located nearby, or even inside, the dots.

Synthesis and characterisation of electropolymerised polyaniline/Q–ZnSe composite films

This letter reports our results on the fabrication of dispersed polyaniline (PANI)/semiconductor quantum dot structures via an electrochemical route for possible application as light emitting devices. High-resolution atomic force microscopy and grazing angle X-ray diffraction techniques have been used to characterize the films. It is shown that the polyaniline films grow as ordered bundles of closely packed polymeric strands in which the ZnSe quantum dots can be uniformly dispersed. The dispersed PANI/Q–ZnSe films have been shown to yield significantly enhanced luminescence.

Parallel multi-bandk·pcode for electronic structure of zinc blend semiconductor quantum dots

We present a parallel implementation of the multi-bank k·p code () for calculation of the electronic structure and optical properties of zinc blend structure semiconductor quantum dots. The electronic wave-functions are expanded in a plane wave basis set in a similar way to ab initio calculations. This approach allows one to express the strain tensor components, the piezoelectric field and the arbitrary shape of the embedded quantum dot in the form of coefficients in the Fourier transform, significantly simplifying the implementation. Most of the strain elements can be given in an analytical form, while very complicated quantum dot shapes can be modelled as a linear combination of the Fourier transform of several characteristic shapes: box, cylinder, cone etc. We show that the parallel implementation of the code scales very well up to 512 processors, giving us the memory and processor power to either include more bands, as in the dilute nitrogen quantum dot structures, or to perform calculations on bigger quantum dots/supercells structures keeping the same “cut-off” energy. The program performance is demonstrated on the pyramidal shape InAs/GaAs, dilute nitrogen InGaAsN, and recently emerged volcano-like InAs/GaAs quantum dot systems.

Variational Monte Carlo for Interacting Electrons in Quantum Dots

We use a variational Monte Carlo algorithm to solve the electronic structure of two-dimensional semiconductor quantum dots in external magnetic field. We present accurate many-body wave functions for the system in various magnetic field regimes. We show the importance of symmetry, and demonstrate how it can be used to simplify the variational wave functions. We present in detail the algorithm for efficient wave function optimization. We also present a Monte Carlo -based diagonalization technique to solve the quantum dot problem in the strong magnetic field limit where the system is of a multiconfiguration nature.

Structure, Composition and Optoelectronic Properties of Small Pyramidal Semiconductor Quantum Dots of Ga and In Atoms With As

ABSTRACTElectronic properties of small pyramidal clusters of Ga, In and As atoms are investigated using a multi-configuration self-consistent field approximation (MCSCF). The structure and composition of some of these (pre-designed) clusters have been derived from those of the symmetry elements of the corresponding bulk fcc lattices, to reflect the cluster formation in confinement or on surfaces. The counterpart (vacuum) clusters obtained without any spatial constraints applied to the atomic positions have also being synthesized computationally. It has been shown that manipulations with the cluster structure, composition and spatial constraints permit to engineer virtual (i.e., fundamental theory-based, computational) clusters with pre-designed electronic properties that can serve as prototypes for nanoscale heterostructure (NHS) units. Computational data so obtained are in agreement with existing experimental evidence concerning such pyramidal clusters.