Height Of Quantum Dots Research Articles

Size Control and Photoluminescence Characteristics of InSb Quantum Dots on GaSb Substrates Grown by Molecular Beam Epitaxy

InSb self‐assembled quantum dots (QDs) have the possibility to increase the luminescence intensity of light‐emitting diodes operating in the midinfrared region. As a first step toward the midinfrared luminescence, the single‐layer InSb QDs are grown on GaSb (100) substrates by molecular beam epitaxy and the effects of V/III ratio and InSb coverage on size and density of InSb QDs and photoluminescence (PL) characteristics are investigated. As the V/III ratio increases, the distribution of QD height broadens and goes from monomodal to multimodal. The increase in the number of large QDs and their multimodal distribution are thought to be due to the ripening process. The PL intensity is maximum at a V/III ratio of 2 with PL wavelength of 1.76 μm. As the InSb coverage increases at a V/III ratio of 2, QD size and density increase while maintaining a monomodal distribution in QD size. However, the PL peak intensity decreases with increasing InSb coverage due to the increase of nonradiative QDs. The highest PL peak intensity at an InSb coverage of 2.0 monolayer and a V/III ratio of 2 with PL wavelength of 1.69 μm are obtained. The present single‐layer InSb QD shows the emission in the near‐infrared region.

Modeling of light absorption in self-assembled truncated conical quantum dot structures

Quantum Dots have shown a significant potential as a top candidate for infrared photodetection at higher temperatures. In the presented work, a theoretical model for estimating the coefficient of optical absorption of self-assembled truncated conical quantum dot is developed. This model considers both bound-to-continuum and bound-to-bound absorption mechanisms that increase the accuracy of the absorption coefficient estimation. The developed model is based on estimating the bound states by diagonalizing the Hamiltonian matrix, where the density of states is computed using the Non-Equilibrium Greens function and the effective mass theory to obtain the unbound states. The kinetic equation of Green’s function is solved numerically by finite difference method. Besides, the effects of quantum dot size, height, aspect ratio, and density on the coefficient of the optical absorption are investigated. The results of the developed model are contrasted with those of other alternative QD structures where the truncated conical QD structure results in a higher absorption coefficient in infrared range than semispherical and conical QD structures.

Optical gain of CdxZn1−xTe quantum dot structures

AbstractGain spectra of undoped and doped quantum dot (QD) structures are studied under the inhomogeneous broadening assumption at four mole fractions . For the QD structure, two peaks appear due to the excited state (ES) and ground state (GS) transitions. The gain for the doped structures doubles the undoped ones. The gain increases while the wavelength is reduced with increasing Cd content due to the broader band discontinuity between the QD states. The discontinuity of the bands for each structure (mole fraction) is calculated, which is one of the merits of this work. While the structure offers the peak wavelengths 470 and 630 nm, other mole fractions offer the wavelengths between them. These visible bands are essential in different applications. The effect of QD size effect is also examined. The wavelength is extended by 20 nm for each 1 nm QD height increment.

Annealing-Modulated Surface Reconstruction for Self-Assembly of High-Density Uniform InAs/GaAs Quantum Dots on Large Wafers Substrate.

In this work, we developed pre-grown annealing to form β2 reconstruction sites among β or α (2 × 4) reconstruction phase to promote nucleation for high-density, size/wafer-uniform, photoluminescence (PL)-optimal InAs quantum dot (QD) growth on a large GaAs wafer. Using this, the QD density reached 580 (860) μm-2 at a room-temperature (T) spectral FWHM of 34 (41) meV at the wafer center (and surrounding) (high-rate low-T growth). The smallest FWHM reached 23.6 (24.9) meV at a density of 190 (260) μm-2 (low-rate high-T). The mediate rate formed uniform QDs in the traditional β phase, at a density of 320 (400) μm-2 and a spectral FWHM of 28 (34) meV, while size-diverse QDs formed in β2 at a spectral FWHM of 92 (68) meV and a density of 370 (440) μm-2. From atomic-force-microscope QD height distribution and T-dependent PL spectroscopy, it is found that compared to the dense QDs grown in β phase (mediate rate, 320 μm-2) with the most large dots (240 μm-2), the dense QDs grown in β2 phase (580 μm-2) show many small dots with inter-dot coupling in favor of unsaturated filling and high injection to large dots for PL. The controllable annealing (T, duration) forms β2 or β2-mixed α or β phase in favor of a wafer-uniform dot island and the faster T change enables optimal T for QD growth.

Investigation of type-II Ga(As)Sb/GaAs quantum dots embedded in an InGaAs quantum well for solar cell applications

We theoretically investigate the electronic and optical properties of charge carriers in type-II Ga(As)Sb/GaAs quantum dots (QDs) embedded in an InGaAs quantum well (QW), to determine their viability for use in intermediate band solar cell (IBSC) devices. This hybrid structure has the advantage of allowing a large flexibility to alter wave function distribution of the electron ground state, which is often considered as an intermediate band (IB) in QD-IBSC. The polarized transitions dipole moment (TDM), the polarized absorption coefficients and the radiative lifetime were studied as a function of QD height. Our theoretical results show an enhancement of inter-band absorption spectrum with a redshift of the peak corresponding to h0→IB transition and a blueshift of the continuum→IB peak, as the height of QD increases. The intra-band light absorption spectrum exhibits a redshift and its magnitude remains almost unchanged, as the height of QD increases. Furthermore this structure provides strong intra-band light absorption comparable to that of type-I QDs and equivalent to h0→IB transition strength. A long carrier lifetime in IB of about 30ns was found, but remains more than an order of magnitude less than the inter-band radiative time due to the low confinement potentials of the electron along the ρ-axis in this type-II dot-in-well (DWELL) structure.

Polarization-dependent intervalence band absorption in quantum dots

In this work, we compute the electronic band structure of ZnCdSe/ZnS semiconductor quantum dots (QDs) using k. p method, and studied the dependence of QDs parameters on the in-plane polarization anisotropy and degree of polarization (DOP) of intersubband absorption spectra of QDs. Higher energy levels in the valence band show nonlinear dependence on the parabolic confinement strength and exhibit strong anti-crossing. We find that the optical transitions for specific polarization directions are defined by the size, strain, and shape anisotropy of the QDs. We observe that increasing the shape anisotropy, strain, and QDs radius enhanced the in-plane polarization anisotropy, whereas increasing QDs height reduces the in-plane polarization anisotropy. The DOP is found to increase with increasing QDs radius and strain and reducing the shape anisotropy and QDs height. Furthermore, the QDs parameters play a key role in deciding the absorption peak of the specific polarization. The present study suggests that the combination of shape, size, and strain could provide an optimal approach to tune the intraband polarization response from QDs.

Design and simulation of type-I graphene/Si quantum dot superlattice for intermediate-band solar cell applications

Recent experiments suggest graphene-based materials as candidates for use in future electronic and optoelectronic devices. In this study, we propose a new multilayer quantum dot (QD) superlattice (SL) structure with graphene as the core and silicon (Si) as the shell of QD. The Slater–Koster tight-binding method based on Bloch theory is exploited to investigate the band structure and energy states of the graphene/Si QD. Results reveal that the graphene/Si QD is a type-I QD and the ground state is 0.6 eV above the valance band. The results also suggest that the graphene/Si QD can be potentially used to create a sub-bandgap in all Si-based intermediate-band solar cells (IBSC). The energy level hybridization in a SL of graphene/Si QDs is investigated and it is observed that the mini-band formation is under the influence of inter-dot spacing among QDs. To evaluate the impact of the graphene/Si QD SL on the performance of Si-based solar cells, we design an IBSC based on the graphene/Si QD (QDIBSC) and calculate its short-circuit current density (Jsc) and carrier generation rate (G) using the 2D finite difference time domain (FDTD) method. In comparison with the standard Si-based solar cell which records Jsc = 16.9067 mA/cm2 and G = 1.48943 × 1028 m−3⋅s−1, the graphene/Si QD IBSC with 2 layers of QDs presents Jsc = 36.4193 mA/cm2 and G = 7.94192 × 1028 m−3⋅s−1, offering considerable improvement. Finally, the effects of the number of QD layers (L) and the height of QD (H) on the performance of the graphene/Si QD IBSC are discussed.Graphical abstract

Selectively-Grown Quantum Dot Active Region Lasers

Nanopatterns formed by dense nanoscale (20–30nm diameter) block copolymer lithography and subsequent selective area MOVPE were utilized to realize room temperature operation of quantum dot (QD) active region laser diodes on InP substrates, by employing a carrier-injection layer adjacent to the QDs. Initial studies focused on InGaAs QDs grown on GaAs substrates, which exhibit relatively broad photoluminescence (PL) emission linewidths (FWHM~93meV at RT). Atomic force microscopy measurements reveal that a wide distribution of QD thickness variations correlates well to the observed broad PL linewidths, based on the simulated range of QD transitions. The fabricated edge-emitting devices on InP substrate exhibit lasing emission near <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.57\sim 1.67~\mu \text{m}$ </tex-math></inline-formula> at room temperature with a threshold current density as low as 1.6 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for 4mm-long devices. The observed dependency of device characteristics, such as threshold current density and lasing wavelength, on the cavity length and on the heat sink temperature suggests the dominant lasing transition energy corresponds to the QD excited state, except for the longest cavity length devices (L = 4mm) which operate on the QD ground state transition near room temperature. We believe these data represent the only reports of lasers in this wavelength region employing QDs formed by diblock co-polymer lithography and selective area MOVPE growth. These data also suggest that QD height variations need to be further reduced, through MOVPE growth optimization, to achieve higher optical gain from the QD ground state transition.

Electron transport properties for a zigzag graphene nanoribbon embedding multiple rectangular quantum dots under a periodically modulated magnetic field

Based on the tight-binding model and recursive Green-function method, the electronic transport properties for a zigzag graphene nanoribbon (ZGNR) embedded with a finite-length superlattice composing of a single or multiple rectangular quantum dot (s) under a periodically modulated magnetic field are investigated. The numerical results of simulation show that the transmittance coefficient T is not only related to the structure of the finite-length superlattice, but also to the addition of a periodically modulated magnetic field. By increasing the height or width of a single rectangular quantum dot (QD), resonant dip of the transmission coefficient T distinctly shifts toward the lower energy region. For a ZGNR embedding n rectangular QDs, the resonant peak splitting is (n-1)-fold in the most energy regions, while the resonant peak splitting is (n-2)-fold in transmission zero (E ≈ 0.37). Nevertheless, three new resonant peaks appear at E ≈ 0.29, E ≈ 0.61 and E ≈ 0.71 under the addition of a periodically modulated magnetic field. Correspondingly, the resonance peak splitting becomes n-fold in the minigap at the edge of transmittance plateau (high energy side). It is determined that the number of quasibound states in the ZGNR can be controlled under a periodically modulated magnetic field. In addition, the conductance and local density of states (LDOS) of ZGNRs are discussed, respectively. With increasing the width of a single rectangular QD, the conductance gradually decreases in the Fermi energy. An uprush peak of the LDOS appears at the position of the conductance peaks or dips under a magnetic field. Therefore, these interesting quantum phenomena are expected to be used as the building blocks of the future nanoelectronics.

Electronic and Optical Properties of InAs QDs Grown by MBE on InGaAs Metamorphic Buffer.

We present the optical characterization of GaAs-based InAs quantum dots (QDs) grown by molecular beam epitaxy on a digitally alloyed InGaAs metamorphic buffer layer (MBL) with gradual composition ensuring a redshift of the QD emission up to the second telecom window. Based on the photoluminescence (PL) measurements and numerical calculations, we analyzed the factors influencing the energies of optical transitions in QDs, among which the QD height seems to be dominating. In addition, polarization anisotropy of the QD emission was observed, which is a fingerprint of significant valence states mixing enhanced by the QD confinement potential asymmetry, driven by the decreased strain with increasing In content in the MBL. The barrier-related transitions were probed by photoreflectance, which combined with photoluminescence data and the PL temperature dependence, allowed for the determination of the carrier activation energies and the main channels of carrier loss, identified as the carrier escape to the MBL barrier. Eventually, the zero-dimensional character of the emission was confirmed by detecting the photoluminescence from single QDs with identified features of the confined neutral exciton and biexciton complexes via the excitation power and polarization dependences.

Theoretical Study of Excitonic Complexes in GaAs/AlGaAs Quantum Dots Grown by Filling of Nanoholes

In this work, a theoretical study of the electronic and the optical properties of a new family of strain-free GaAs/AlGaAs quantum dots (QDs) obtained by AlGaAs nanohole filling is presented. The considered model consists of solving the three-dimensional effective-mass Schrödinger equation, thus providing a complete description of the neutral and charged complex excitons’ fine structure. The QD size effect on carrier confinement energies, wave functions, and s-p splitting is studied. The direct Coulomb interaction impact on the calculated s and p states’ transition energies is investigated. The behaviour of the binding energy of neutral and charged excitons (X− and X+) and biexciton XX versus QD height is studied. The addition of the correlation effect allows to explain the nature of biexcitons often observed experimentally.

Atomic-Scale Characterization of Droplet Epitaxy Quantum Dots.

The fundamental understanding of quantum dot (QD) growth mechanism is essential to improve QD based optoelectronic devices. The size, shape, composition, and density of the QDs strongly influence the optoelectronic properties of the QDs. In this article, we present a detailed review on atomic-scale characterization of droplet epitaxy quantum dots by cross-sectional scanning tunneling microscopy (X-STM) and atom probe tomography (APT). We will discuss both strain-free GaAs/AlGaAs QDs and strained InAs/InP QDs grown by droplet epitaxy. The effects of various growth conditions on morphology and composition are presented. The efficiency of methods such as flushing technique is shown by comparing with conventional droplet epitaxy QDs to further gain control over QD height. A detailed characterization of etch pits in both QD systems is provided by X-STM and APT. This review presents an overview of detailed structural and compositional analysis that have assisted in improving the fabrication of QD based optoelectronic devices grown by droplet epitaxy.

Stark shift and exciton binding energy in parabolic quantum dots: hydrostatic pressure, temperature, and electric field effects

ABSTRACT The temperature, hydrostatic pressure, and external electric field effects on the confined exciton in cylindrical quantum dots by considering a parabolic confining potential are investigated. The effects of these external perturbations on the binding energy and interband emission energy are calculated numerically by adopting the variational method within the effective mass approximation. Our findings indicate that the exciton binding energy and interband emission energy depend significantly on decreasing electric field, diminishing temperature, and enhancing the hydrostatic pressure. The contribution of the electric field on the binding energy becomes more important for wide axial parabolic quantum well. We have also shown that the Stark effect of exciton diminishes almost linearly with increasing hydrostatic pressure. Furthermore, the electric field and the quantum dot height lead to enhancing the Stark shift. The behaviour of the excitonic Stark shift as a function of the applied electric field proves the existence of a dipole moment. This physical parameter becomes important for the weak confinement regime.

Optical and Electronic Properties of Symmetric InAs/(In,Al,Ga)As/InP Quantum Dots Formed by Ripening in Molecular Beam Epitaxy: A Potential System for Broad-Range Single-Photon Telecom Emitters

We present a detailed experimental optical study supported by theoretical modeling of $\mathrm{In}\mathrm{As}$ quantum dots (QDs) embedded in an $(\mathrm{In},\phantom{\rule{-1.5pt}{0ex}}\mathrm{Al},\phantom{\rule{-1.5pt}{0ex}}\mathrm{Ga})\mathrm{As}$ barrier lattice matched to $\mathrm{In}\mathrm{P}$(001) grown with the use of a ripening step in molecular beam epitaxy. The method leads to the growth of in-plane symmetric QDs of low surface density, characterized by a multimodal size distribution resulting in a spectrally broad emission in the range of $1.4$--$2.0\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$, essential for many near-infrared photonic applications. We find that, in contrast to the $\mathrm{In}\mathrm{As}$/$\mathrm{In}\mathrm{P}$ system, the multimodal distribution results here from a two-monolayer difference in QD height between consecutive families of dots. This may stem from the long-range ordering in the quaternary barrier alloy that stabilizes QD nucleation. Measuring the photoluminescence (PL) lifetime of the spectrally broad emission, we find a nearly dispersionless value of $1.3\ifmmode\pm\else\textpm\fi{}0.3$ ns. Finally, we examine the temperature dependence of emission characteristics. We underline the impact of localized states in the wetting layer playing the role of carrier reservoir during thermal carrier redistribution. We determine the hole escape to the $(\mathrm{In},\phantom{\rule{-1.5pt}{0ex}}\mathrm{Al},\phantom{\rule{-1.5pt}{0ex}}\mathrm{Ga})\mathrm{As}$ barrier to be a primary PL quenching mechanism in these QDs.

Light group velocity in quantum dots under electromagnetically induced transparency by using second quantization formalism

This work aims to investigate the light group velocity in cylindrical quantum dots (QDs) under electromagnetically induced transparency phenomenon. The second quantization formalism is used to solve the problem because this system is considered as a many body system. By using annihilation and creation operators in fully quantum approach and density matrix method the effects of different parameters such as the photons number of control and probe laser fields, height and radius of QD on the group velocity are investigated. The results show that the slow light frequency ranges can be created with a small number of photons. Also, it is shown that the width of slow light frequency ranges depends on the photons number of the control laser fields and the polarization of light. Moreover, we compare group velocity for z- and x-polarization.

Excitonic complexes in InAs/InP nanowire quantum dots

InAs quantum dots embedded in InP nanowires form an important platform for basic research studies, as well as for quantum dot applications. Notably, understanding of nanowire quantum dot spectral properties is essential in both cases. Therefore, in this work we use atomistic theory to study spectra of the single exciton $(X)$, the biexciton $(XX)$, the triexciton $(XXX)$, and the positively and negatively charged trions $({X}^{+}$ and ${X}^{\ensuremath{-}})$ confined in these nanostructures. We focus on the role of vertical and lateral confinement, therefore, we systematically study a large family of quantum dots with different heights and diameters, and find the important role of correlations due to presence of higher states. We find that the order of excitonic binding energies is a characteristic feature of InAs/InP nanowire quantum dots being (ordered from negative to positive values): ${X}^{\ensuremath{-}},XX$, and ${X}^{+}$, with strongly bound ${X}^{\ensuremath{-}}$, rather weakly bound $XX$, and typically unbound ${X}^{+}$. Next, we determine the key role of alloy randomness due to intermixing, which turns out to especially important for larger quantum dot heights and phosphorous contents over 40%. In selected cases, the alloying can lead to an unbound biexciton, and can even reverse ordering of excitonic lines.

Self-assembled InAs/InGaAsP/InP quantum dots: Intraband relaxation impacted by ultrathin GaP sublayer

The influence of an ultrathin GaP (or GaAs) sublayer on the nonradiative intraband relaxation in InAs/InGaAsP/InP quantum dots (QDs) is investigated. It is found that, based on our studies, the QDs with some heights (e.g., 1.5 nm) and GaP sublayer thicknesses (e.g., 1.03 monolayers) present the first excited state (ES) with higher state degeneracy with respect to ground state (GS), which suggests that the Auger relaxation is triggered more easily. We also find that the energy difference of the ES and GS decreases with increasing sublayer thickness, which suggests that the electron–phonon interaction is affected. This work further presents a study of intraband relaxation for an InAs/InP QD with a GaP or GaAs sublayer. It is found that there is a critical thickness of the GaP sublayer: When the sublayer is less than the critical thickness, the intraband relaxation is only determined by one-longitudinal optical (LO) phonon or two-LO phonons, which is dependent on QD heights. However, with the GaAs sublayer, QDs do not have the above feature. This finding may be helpful for designing and optimizing high-speed QD devices.

Atomistic theory of electronic and optical properties of InAsP/InP nanowire quantum dots

We present here an atomistic theory of the electronic and optical properties of hexagonal InAsP quantum dots in InP nanowires in the wurtzite phase. These self-assembled quantum dots are unique in that their heights, shapes, and diameters are well known. Using a combined valence-force-field, tight-binding, and configuration-interaction approach we perform atomistic calculations of single-particle states and excitonic, biexcitonic, and trion complexes as well as emission spectra as a function of the quantum dot height, diameter, and As versus P concentration. The atomistic tight-binding parameters for InAs and InP in the wurtzite crystal phase were obtained by ab initio methods corrected by empirical band gaps. The low energy electron and hole states form electronic shells similar to parabolic or cylindrical quantum confinement, only weakly affected by hexagonal symmetry and As fluctuations. The relative alignment of the emission lines from excitons, trions, and biexcitons agrees with that for InAs/InP dots in the zincblende phase in that biexcitons and positive trions are only weakly bound. The random distribution of As atoms leads to dot-to-dot fluctuations of a few meV for the single-particle states and the spectral lines. Due to the high symmetry of hexagonal InAsP nanowire quantum dots the exciton fine structure splitting is found to be small, of the order a few $\ensuremath{\mu}\mathrm{eV}$ with significant random fluctuations in accordance with experiments.

Exciton–phonon properties in cylindrical quantum dot with parabolic confinement potential under electric field

Abstract The mixed confinement potential effect on exciton in cylindrical quantum dot (QD) is calculated in the presence of an electric field with and without confined longitudinal optical (LO) phonon mode contribution. The variational approach within the effective mass approximation is used to describe the exciton–phonon interaction with three variational parameters of the trial wave function. The ground state binding energy of exciton has been calculated numerically for the parabolic (axial direction) and square (lateral direction) finite confinement potentials under the electric field applied along the growth direction of the QD. The system both with and without LO-phonon contribution is investigated. The combined effects of applied electric field and parabolic confinement on the ground state binding energy, the polaronic correction and the Stark shift are examined in detail. The findings indicate that the electric field strongly reduces the binding energy in both ignoring and considering LO-phonon interaction especially for wider thickness of QD. We have shown that the contribution of LO-phonon leads to an increase in the binding energy and to decrease in the Stark shift energy. Furthermore, our numerical results illustrate a strong dependence of Stark shift on the QD height, electric field strength and LO-phonon contribution.

Internal quantum efficiencies of AlGaN quantum dots grown by molecular beam epitaxy and emitting in the UVA to UVC ranges

AlyGa1−yN quantum dots (QDs) have been grown by molecular beam epitaxy on AlxGa1−xN (0001) using a 2-dimensional–3-dimensional growth mode transition that leads to the formation of QDs. QDs have been grown for Al compositions y varying between 10% and 40%. The influence of the active region design [composition y, QD height, and bandgap difference (ΔEg) between the AlxGa1−xN cladding layer and the AlyGa1−yN QDs] is discussed based on microscopy, continuous wave photoluminescence (PL), and time-resolved PL (TRPL) measurements. In particular, increasing y leads to a shift of the QD emission toward shorter wavelengths, allowing covering a spectral range in the UV from 332 nm (UVA) to 276 nm (UVC) at room temperature (RT). The low-temperature (LT) internal quantum efficiency of the QD ensembles was estimated from TRPL experiments at 8 K and values between 11% and 66% were deduced. The highest internal quantum efficiency (IQE)-LT is found for the QDs with higher Al content y. Then, the PL spectrally integrated intensity ratios between RT and LT were measured to estimate the IQE of the samples at RT. The PL ratio is higher for larger ΔEg, for QDs with y of 0.1 or 0.2, and high PL intensity ratios up to 30% were also measured for QDs with larger y of 0.3 and 0.4. RT IQE values between 5% and 20% are deduced for AlyGa1−yN QDs emitting in the 276–308 nm range.