GaAs Quantum Research Articles

Electronic states and optical characteristics of GaAs Spherical quantum dot based on Konwent-like confining potential: Role of the hydrogenic impurity and structure parameters

In this study, we investigate for the first time the electronic states and optical characteristics of GaAs quantum dot with an on-center hydrogenic impurity using a Konwent-like spherical potential well. We numerically solve the radial component of Schrodinger equation in the framework of the effective-mass approximation (EMA) to determine the energy level structure and wavefunctions of the system. We then calculate the optical absorption coefficient (OAC) of the system using Fermi's golden rule and discuss the influence of the energy separation (E1p-E1 s), the dipole matrix element and the parameters of the potential on the OAC. We calculate and compare radial distributions for the lowest four electronic states (i.e., 1 s, 1p, 2 s, and 2p) of the system with and without the on-center impurity for different values of the parameters of the Konwent-like potential. Our results show how vary the parameters of the potential to tune the OAC towards red or blue shift and also to control its amplitude.

Beyond the Four-Level Model: Dark and Hot States in Quantum Dots Degrade Photonic Entanglement.

Entangled photon pairs are essential for a multitude of quantum photonic applications. To date, the best performing solid-state quantum emitters of entangled photons are semiconductor quantum dots operated around liquid-helium temperatures. To favor the widespread deployment of these sources, it is important to explore and understand their behavior at temperatures accessible with compact Stirling coolers. Here we study the polarization entanglement among photon pairs from the biexciton-exciton cascade in GaAs quantum dots at temperatures up to ∼65 K. We observe entanglement degradation accompanied by changes in decay dynamics, which we ascribe to thermal population and depopulation of hot and dark states in addition to the four levels relevant for photon pair generation. Detailed calculations considering the presence and characteristics of the additional states and phonon-assisted transitions support the interpretation. We expect these results to guide the optimization of quantum dots as sources of highly entangled photons at elevated temperatures.

Enhancing intermediate band solar cell performances through quantum engineering of dot states by droplet epitaxy

AbstractWe report the effect of the quantum dot aspect ratio on the sub‐gap absorption properties of GaAs/AlGaAs quantum dot intermediate band solar cells. We have grown AlGaAs solar cells containing GaAs quantum dots made by droplet epitaxy. This technique allows the realization of strain‐free nanostructures with lattice matched materials, enabling the possibility to tune the size, shape, and aspect ratio to engineer the optical and electrical properties of devices. Intermediate band solar cells have been grown with different dot aspect ratio, thus tuning the energy levels of the intermediate band. Here, we show how it is possible to tune the sub‐gap absorption spectrum and the extraction of charge carriers from the intermediate band states by simply changing the aspect ratio of the dots. The tradeoff between thermal and optical extraction is in fact fundamental for the correct functioning of the intermediate band solar cells. The combination of the two effects makes the photonic extraction mechanism from the quantum dots increasingly dominant at room temperature, allowing for a reduction of the open circuit voltage of only 14 mV, compared to the reference cell.

Population Transfer Among the Quantum Dot Eigenstates Driven by Time‐dependent Anharmonic Potential: Role of Noise

AbstractThe study meticulously inspects the time‐average excitation rate (TAER)of lowest energy state electronic probability of GaAs quantum dot (QD)to the higher energy states. The excitation occurs as the anharmonic potential(a part of QD confinement) starts varying with time either periodicallyor randomly. The anharmonic potential is characterized by either evenor oddparity. Gaussian white noise (GWN)has also been introduced to the QD either via additiveor multiplicativemode. The study announces that the coupled impact of the given physical quantity, the symmetry of the anharmonicity, presence/absence of noise, the route of entrance of noise, and the nature of time progress of anharmonicity constant, ultimately regulates and fine‐tunes the attributes of the TAER diagrams. The TAER diagrams comprise of regular rise, regular fall, maximization (pertinent to production of high value of nonlinear optical (NLO) properties), minimization and saturation (pertinent to dynamic freezing). The outcomes of the study seem to be recognizable in various applications of devices containing QD.

External Field‐induced Transitions in Quantum Dot: Role of Noise‐anharmonicity Interplay

AbstractThis work scrutinizes the time‐average excitation rate (TAER) among the GaAs quantum dot (QD) eigenstates under the aegis of Gaussian white noise (GWN) and the parity of the anharmonic potential (odd/even). GWN connects with the system by additive or multiplicative mode. The said excitation of the ground state population has been triggered by an external field which may be a polychromatic radiation field (PRF), or pulsed field (PF) or chirped pulsed field (CPF). The study reveals the subtle nuances of the interplay between noise (additive or multiplicative), anharmonicity (odd or even) and the external field (PRF, PF or CPF) that finally govern the attributes of the TAER diagrams. The TAER profiles exhibit persistent growth, persistent decline, maximization (important in view of production of prominent nonlinear optical properties), minimization and saturation (relevant to the dynamic freezing). The findings appear useful for regulating the TAER among the GaAs QD eigenstates which have substantial technological relevance.

Modeling of Masked Droplet Deposition for Site-Controlled Ga Droplets.

Site-controlled Ga droplets on AlGaAs substrates are fabricated using area-selective deposition of Ga through apertures in a mask during molecular beam epitaxy (MBE). The Ga droplets can be crystallized into GaAs quantum dots using a crystallization step under As flux. In order to model the complex process, including the masked deposition of the droplets and a reduction of their number during a thermal annealing step, a multiscale kinetic Monte Carlo (mkMC) simulation of self-assembled Ga droplet formation on AlGaAs is expanded for area-selective deposition. The simulation has only two free model parameters: the activation energy for surface diffusion and the activation energy for thermal escape of adatoms from a droplet. Simulated droplet numbers within the opening of the aperture agree quantitatively with the experimental results down to the perfect site-control, with one droplet per aperture. However, the model parameters are different compared to those of the self-assembled droplet growth. We attribute this to the presence of the mask in close proximity to the surface, which modifies the local process temperature and the As background. This approach also explains the dependence of the model parameters on the size of the aperture.

Polarization-independent enhancement of optical absorption in a GaAs quantum well embedded in an air-bridge bull’s-eye cavity with metal electrodes

Electron spins in gate-defined quantum dots (QDs) formed in semiconductor quantum wells (QWs) are promising stationary qubits for implementing large-scale quantum networks in a scalable manner. One key ingredient for such a network is an efficient photon–spin interface that converts any polarization state of a flying photonic qubit to the corresponding spins state of the electron in gate-defined QDs. A bull’s-eye cavity is an optical cavity structure that can enhance the photon absorption of an embedded gate-defined QD without polarization dependence. In this paper, we report the successful fabrication of air-bridge bull’s-eye cavities with metal electrodes and demonstrate the nearly polarization-independent optical absorption of a GaAs QW embedded in the cavities. This work marks an important step toward realizing an efficient photon–spin interface using gate-defined QDs.

Experimental progress of topological exciton insulators

Under Coulomb interactions, electrons and holes have a phase transition from a semi-metal state into an excitonic ground state, with a gap spontaneously opening at the Fermi level. The excitonic ground state is called excitonic insulator. The excitonic insulator has attracted extensive attention in condensed matter physics, but the experimental evidence of such a quantum state is still under search. Until recent years, optical spectral and transport evidence of the excitonic insulator has first been observed in shallowly inverted InAs/GaSb quantum wells, which confirms the existence of the exciton insulator in a two-dimensional system. Surprisingly, one-dimensional helical-like topological edge states, which are not sensitive to temperature nor magnetic field, have been observed in the excitonic insulator state by transport measurements. This new quantum phase cannot be well explained by existing single-particle theories, which is called a topological excitonic insulator. In this paper, we systematically review the experimental studies on the topological excitonic insulator in the InAs quantum well and GaAs quantum well, including magneto transport, terahertz transmission, capacitance, and Coulomb drag measurements. These experimental results comprehensively characterize the bulk properties and edge properties of the topological excitonic insulator. Furthermore, as a ground state consisting of bosons, the topological excitonic insulator is expected to have a quantum phase transition into Bose-Einstein condensate with macroscopic coherence under extreme conditions, which provides a new platform for studying the quantum phenomena of Coulomb interactions in low dimensions.

Impact of the p-cubic Dresselhaus term on the spin Hall effect

It is well known that the Dresselhaus spin-orbit coupling (SOC) in semiconductor two dimensional electron gases (2DEGs) possesses both linear and cubic in momentum contributions. Nevertheless, the latter is usually neglected in most theoretical studies. However, recent Kerr rotation experiments have revealed a significant enhancement of the cubic Dresselhaus interaction by increasing the drift velocities in 2DEGs hosted in GaAs quantum wells. Here, we present a study of the optical spin Hall conductivity in 2DEGs under the simultaneous presence of Rashba and (linear plus cubic) Dresselhaus SOC. The work was done within the Kubo formalism in linear response. We show that the coexistence of the Rashba and cubic Dresselhaus SOC in 2DEGs promotes a strong anisotropy of the band spin splitting which in turn leads to a very characteristic frequency dependence of the spin Hall conductivity. We find that the spin Hall conductivity response could be very sensible to sizeable cubic-Dresselhaus coupling strength. This may be of relevance for the optical control of spin currents in 2DEGs with non-negligible cubic-Dresselhaus SOC.

Unconventional Fractional Quantum Hall States in a Wide Quantum Well

A bilayer electron system that is formed in a 60-nm-wide GaAs quantum well and has a large difference of the electron densities in the layers has been studied. It has been found that, when a magnetic field is tilted from the normal to the plane of the system, integer quantum Hall effect states at the filling factors of Landau levels of 1 and 2 disappear; instead, fractional quantum Hall effect states in the interval between these filling factors appear at the filling factors νF = 4/3, 10/7, and 6/5 with odd denominators and at the filling factor νF = 5/4. Several different states can be observed under the variation of the magnetic field. The detected fractional quantum Hall effect states are interpreted as combined states with the same filling factor 1 in the layer with the higher density and with the filling factors νF – 1 in the layer with the lower density. These states are formed because of the redistribution of electrons between the layers, which occurs under the variation of the magnetic field. The appearance of the state with the filling factor νF = 5/4 with the even denominator is presumably attributed to the dominance of the interlayer electron–electron interaction over the intralayer one for electrons in the layer with the lower density.

Impact of illumination on quantum lifetime in selectively doped GaAs single quantum wells with short-period AlAs/GaAs superlattice barriers

Impact of illumination on high-mobility dense 2D electron gas in selectively doped single GaAs quantum well with short-period AlAs/GaAs superlattice barriers at T=4.2 K in magnetic fields B<2 T has been studied. It was demonstrated that illumination at low temperatures gives rise to enhancement of electron density, mobility and quantum lifetime in studied heterostructures. The enhancement of quantum lifetime after illumination for single GaAs quantum well with modulated superlattice doping had been explained as consequence of decrease in effective concentration of remote ionized donors. Keywords: persistent photoconductivity, quantum lifetime, anisotropic mobility, superlattice barriers.

Strain and magnetic field effect on thermodynamic properties of a two dimensional GaAs quantum dot

The strain and magnetic field effect on thermodynamic properties of a two dimensional GaAs quantum dot (QD) is investigated within the effective mass approximation. To this end, first we have obtained the wave functions and energy levels of the system in the presence of Bychkov-Rashba and Dressalhaus terms. Then using the obtained energy levels, we have computed the partition function by the Poisson summation formalism. Afterward, we have deduced some thermodynamic properties such as mean energy, entropy, specific heat and free energy using the canonical ensemble approach. Finally, the thermodynamic properties for a two dimensional GaAs QD have been discussed in detail.

Влияние подсветки на квантовое время жизни в селективно-легированных одиночных GaAs квантовых ямах с короткопериодными AlAs/GaAs-сверхрешеточными барьерами

Impact of illumination on high-mobility dense 2D electron gas in selectively doped single GaAs quantum well with short-period AlAs/GaAs superlattice barriers at T = 4.2 K in magnetic fields B < 2 T has been studied. It was demonstrated that illumination at low temperatures gives rise to enhancement of electron density, mobility and quantum lifetime in studied heterostructures. The enhancement of quantum lifetime after illumination for single GaAs quantum well with modulated superlattice doping had been explained as consequence of decrease in effective concentration of remote ionized donors.

Elliptical Polarization of Localized States in an Anisotropic Single GaAs Quantum Ring

Localized states in an anisotropic single GaAs quantum ring were investigated in terms of polarization dependence of micro-photoluminescence spectrum at 5K. Given four Stokes parameters measured with a pair of linear polarizers and waveplates, the elliptical polarization states of two different vertical confinement states (k=1 and k=2) were compared with phase, rotation, and ellipticity angles. While the polarized emission intensity of the k=2 states becomes enhanced along [1,1,0] compared to that along [1,1¯,0], the polarization asymmetry of the k=1 states shows the opposite result. We conclude the polarization state is determined by the shape of the lateral wavefunctions. In the k=2 state, crescent-like wavefunctions are strongly localized, but the k=1 state consists of two crescent-like wavefunctions, which are connected weakly through quantum tunneling.

Spin-orbit interaction effects on a [formula omitted] complex in a GaAs quantum dot in a magnetic field

The spin-orbit interaction effects on the energetics of a negative donor centre localized in a 3D Gaussian GaAs quantum dot placed in a magnetic field are studied variationally by using a unitary transformation and a Jastrow function to incorporate the role of Coulomb interaction. The Dresselhaus interaction is shown to increase the binding of the negative donor. The diamagnetic susceptibility and the dipole moment of the system are also calculated.

Effect of electron–electron interaction on magnitude of quantum oscillations of dissipative resistance in magnetic fields

Magneto-intersubband resistance oscillations (MISOs) of highly mobile 2D electrons in symmetric GaAs quantum wells with two populated subbands are studied in magnetic fields B=(B⊥,B∥) tilted from the normal to the 2D electron layer at different temperatures T. The in-plane component (B∥) of the field B induces magnetic entanglement between subbands, leading to beating in oscillating density of states (DOS) and to MISO suppression. Model of the MISO suppression is proposed. Within the model, a comparison of MISO amplitude in the entangled and disentangled (B∥=0) 2D systems yields both difference frequency of DOS oscillations, df, and strength of the electron–electron interaction, described by parameter εF∗, in the 2D system. These properties are analyzed using two methods, yielding consistent but not identical results for both df and εF∗. The analysis reveals an additional angular dependent factor of MISO suppression. The factor is related to spin splitting of quantum levels in magnetic fields.

Effects of the confinement potential parameters and optical intensity on the linear and nonlinear optical properties of spherical quantum dots

We study the linear and nonlinear optical properties of a spherical GaAs quantum dot with screened modified Kratzer potential (SMKP) by solving the time-independent Schrödinger wave equation using the diagonalization method. The obtained electronic properties of the system alongside the compact density formalism are used to evaluate the linear, third-order nonlinear and total optical absorption coefficients and change in the relative refractive index of the system. We show that the confinement potential parameters and optical intensity significantly affects the behaviour of absorption coefficients and refractive index changes. It is important to note that the findings of this study will find application in optoelectronics and related areas.

High Efficient Solar Cell Based on Heterostructure Constructed by Graphene and GaAs Quantum Wells.

Despite the fascinating optoelectronic properties of graphene, the power conversion efficiency (PCE) of graphene based solar cells remains to be lifted up. Herein, it is experimentally shown that the graphene/quantum wells/GaAs heterostructure solar cell can reach a PCE of 20.2% and an open-circuit voltage (Voc ) as high as 1.16V at 90K. The high efficiency is a result of carrier multiplication (CM) effect of graphene in the graphene/GaAs heterostructure. Especially, the external quantum efficiency (EQE) in the ultraviolet wavelength can be improved up to 72.2% based on the heterostructure constructed by graphene/In0.15 Ga0.85 As/GaAs0.75 P0.25 quantum wells/GaAs. The EQE increases as the light wavelength decreases, which indicates more carriers can be effectively excited by the higher energy photons through CM effect. Owing to these physical characters, the graphene/GaAs heterostructure solar cell will provide a possible way to exceed Shockley-Queisser (S-Q) limit.

Effects of applied magnetic field and pressure on the diamagnetic susceptibility and binding energy of donor impurity in GaAs quantum dot considering the non-parabolicity model’s influence

ABSTRACT In this article, we investigate the influence of the hydrostatic pressure, magnetic field, and conduction band non-parabolicity on both the diamagnetic susceptibility and the binding energy of shallow donor impurity in a quantum disk made out of GaAs. The Hamiltonian of the investigated problem has been solved within the framework of the effective-mass approximation. The energy minimisation has been performed using a variational approach. Our results reveal that both the diamagnetic susceptibility and binding energy have been reduced with increasing the disk size. Moreover, the diamagnetic susceptibility increases as the impurity moves from the extremity to the centre of the disk. However, both the diamagnetic susceptibility and binding energy have been improved under applied magnetic field, hydrostatic pressure, and by considering the conduction band non-parabolicity model as well. We hope that the reported results will be a modest contribution to further theoretical research in the field of nanostructures.

Robust Quantum Hall Ferromagnetism near a Gate-Tuned ν=1 Landau Level Crossing.

In a low-disorder two-dimensional electron system, when two Landau levels of opposite spin or pseudospin cross at the Fermi level, the dominance of the exchange energy can lead to a ferromagnetic, quantum Hall ground state whose gap is determined by the exchange energy and has skyrmions as its excitations. This is normally achieved via applying either hydrostatic pressure or uniaxial strain. We study here a very high-quality, low-density, two-dimensional hole system, confined to a 30-nm-wide (001) GaAs quantum well, in which the two lowest-energy Landau levels can be gate tuned to cross at and near filling factor ν=1. As we tune the field position of the crossing from one side of ν=1 to the other by changing the hole density, the energy gap for the quantum Hall state at ν=1 remains exceptionally large, and only shows a small dip near the crossing. The gap overall follows a sqrt[B] dependence, expected for the exchange energy. Our data are consistent with a robust quantum Hall ferromagnet as the ground state.