Two-dimensional Quantum Wells Research Articles

Laser-field-mediated Rashba and Dresselhaus spin–orbit control in GaInAs/AlInAs quantum wells

For more possibilities in exploring the spin–orbit (SO) control in two-dimensional (2D) semiconductor quantum wells, we resort to the laser field by constructing a generic model including both the axial and transverse field components relative to the well-growth direction, with the axial one primarily modifying the structural potential of the well and the transverse one mainly altering the electron density of states and so the Hartree potential. For GaInAs/AlInAs quantum wells with two-subband occupation, we obtain the full scenario of the laser-field response of the Rashba and Dresselhaus SO couplings, including both the intrasubband (αν and βν with ν = 1,2 denoting the subband index) and intersubband (η and Γ) types, by solving the Poisson–Schrödinger equations. Remarkably, for the Dresselhaus coupling, which is insensitive to the electrical manipulation, we demonstrate that it can be substantially tuned by the laser field and unified SO control of different subbands is possible, suggesting that the optical manner is an effective supplement to the electrical means. Moreover, symmetric (α1=−α2) and selective (α1=0, α2≠0) control of the Rashba coupling of different subbands are also observed with the interplay of the two laser components, which are desirable for the realization of skyrmion lattice and the suppression of spin relaxation of given subbands. Besides, for the intersubband coupling, the Rashba term η is optically tunable while the Dresselhaus term Γ is basically immune to the laser field, i.e., selective control of the intersubband coupling by virtue of optical means. These diverse laser-field-mediated SO features offer exciting possibilities in incorporating 2D quantum wells for new applications in the field of spintronics.

Electron energy relaxation in SiGe quantum wells

In this article, we theoretically calculated the hot electron energy relaxation induced by longitudinal acoustic (LA) phonons, longitudinal optical (LO) phonons and importantly two-transverse acoustic (2TA) phonons in two-dimensional SiGe quantum wells, by incorporating dynamic screening effect in LA phonon scattering and hot-phonon effect in LO phonon scattering mechanisms. At the low-temperature regime, the ELR is found to be dominated by LA phonons and at higher temperature ELR is dominated by LO phonons. In the intermediate temperature range, 2TA phonon scattering mechanism is very important and we incorporated in our calculations. The unscreened longitudinal acoustic (LA) phonon due to deformation potential (DP) coupling is dominant over the screened acoustic piezoelectric phonon contribution. At higher temperatures, there is a crossover from ELR due to LA phonons to ELR due to longitudinal optical (LO) phonons with the cross-over temperature being about Te∼40K. The LA phonon screening effect and hot phonon effect is demonstrated to reduce ELR significantly. In the intermediate temperature range (30 K

Universal rotation gauge via quantum anomalous Hall effect

Integer quantum Hall effect allows to gauge the resistance standard up to more than one part in a billion. Combining it with the speed of light, one obtains the fine-structure constant α ≈ 1/137, a dimensionless reference number that can be extracted from a physical experiment. Most exact notion of this value and especially its possible variation on the cosmological time scales is of enormous relevance for fundamental science. In an optical experiment, the fine-structure constant can be directly obtained as purely geometrical angle by measuring the quantized rotation of light polarization in two-dimensional quantum wells. In realistic conditions, high external magnetic fields have to be applied, which strongly affects possible attainable accuracy. An elegant solution of this problem is provided by quantum anomalous Hall effect where a universal quantized value can be obtained in zero magnetic field. Here, we measure the fine-structure constant in a direct optical experiment that requires no material adjustments or technical calibrations. By investigating the Faraday rotation at the interference maxima of the dielectric substrate, the angle close to one α is obtained at liquid helium temperatures without using a dilution refrigerator. Such calibration and parameter-free experiment provides a system-of-unit-independent access to universal quantum of rotation.

Simulations of Trions and Biexcitons in Layered Hybrid Organic-Inorganic Lead Halide Perovskites.

Behaving like atomically precise two-dimensional quantum wells with non-negligible dielectric contrast, the layered hybrid organic-inorganic lead halide perovskites (HOIPs) have strong electronic interactions leading to tightly bound excitons with binding energies on the order of 500meV. These strong interactions suggest the possibility of larger excitonic complexes like trions and biexcitons, which are hard to study numerically due to the complexity of the layered HOIPs. Here, we propose and parametrize a model Hamiltonian for excitonic complexes in layered HOIPs and we study the correlated eigenfunctions of trions and biexcitons using a combination of diffusion MonteCarlo and very large variational calculations with explicitly correlated Gaussian basis functions. Binding energies and spatial structures of these complexes are presented as a function of the layer thickness. The trion and biexciton of the thinnest layered HOIP have binding energies of 35 and 44meV, respectively, whereas a single exfoliated layer is predicted to have trions and biexcitons with equal binding energies of 48meV. We compare our findings to available experimental data and to that of other quasi-two-dimensional materials.

Tailoring Color-Tunable Dual Emissions of Mn2+-Alloyed Two-Dimensional Perovskite Quantum Wells

While manganese (Mn2+)-doped chalcogenide and perovskite CsPbX3 nanocrystals (NCs) have been extensively investigated, the research of color-tunable dual-emissive Mn2+-incorporated two-dimensional (2D) quantum wells (QWs) is limited. Here, we present alloyed Mn:(PEA)2PbBr4 2D perovskite QWs, achieving efficient dual-color emissions with high energy transfer efficiency (ΦET = 56%) and solid-state photoluminescence quantum yield (PLQY = 54%). Importantly, excitation and temperature are demonstrated as two efficient approaches to precisely modulate color-tunable dual emissions with excellent reversibility. The results indicate possible applications of such materials in photo-/thermal switchable markers and efficient thermometers for biomedical imaging and diagnostic investigations.

Magneto-optics of layered two-dimensional semiconductors and heterostructures: Progress and prospects

Beginning with the “conventional” two-dimensional (2D) quantum wells based on III–V and II–VI semiconductors in the 1970s, to the recent atomically thin sheets of van der Waals materials such as 2D semiconducting transition metal dichalcogenides (TMDCs) and 2D magnets, the research in 2D materials is continuously evolving and providing new challenges. Magneto-optical spectroscopy has played a significant role in this area of research, both from fundamental physics and technological perspectives. A major challenge in 2D semiconductors such as TMDCs is to understand their spin-valley-resolved physics and their implications in quantum computation and information research. Since the discovery of valley Zeeman effects, deep insights into the spin-valley physics of TMDCs and their heterostructures have emerged through magneto-optical spectroscopy. In this Perspective, we highlight the role of magneto-optics in many milestones such as the discovery of interlayer excitons, phase control between coherently excited valleys, determination of exciton-reduced masses, Bohr radii and binding energies, physics of the optically bright and dark excitons, trions, and other many-body species such as biexcitons and their phonon replicas in TMDC monolayers. The discussion accompanies open questions, challenges, and future prospects in the field including comments on the magneto-optics of van der Waals heterostructures involving TMDCs and 2D magnets.

Kinetic Molecular Cationic Control of Defect-Induced Broadband Light Emission in 2D Hybrid Lead Iodide Perovskites.

In this study, we examine the effects of changing organic cation concentrations on the efficiency and photophysical implications of exciton trapping in two-dimensional hybrid lead iodide self-assembled quantum wells (SAQWs). We show that increasing the concentration of alkyl and aryl ammonium cations causes the formation of SAQWs at a liquid-liquid interface to possess intense, broadband subgap photoluminescence (PL) spectra. Electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopic studies suggest that materials formed under these cation concentrations possess morphologies consistent with inhibited crystallization kinetics but exhibit qualitatively similar bulk chemical bonding to nonluminescent materials stabilized in the same structure from precursor solutions containing lower cation concentrations. Temperature- and power-dependent PL spectra suggest that the broadband subgap light emission stems from excitons self-trapped at defect sites, which we assign as edge-like, collective I vacancies using a simple model of the chemical equilibrium driving material self-assembly. These results suggest that changes to the availability of molecular cations can suitably control the light emission properties of self-assembled hybrid organic-inorganic materials in ways central to their applicability in lighting technologies.

Long-lived quantum coherence and nonlinear properties of a two-dimensional semiconductor quantum well

In this paper, we present an analytical solution to the Maxwell–Bloch equations of the two-level semiconductor quantum well, G a A s / A l G a A s . In addition, we discuss the effects of coherent Rabi oscillations Ω ( t ) and the frequency of the semiconductor system ν ( t ) on atomic occupation probabilities, ρ 11 ( t ) and ρ 22 ( t ) , population inversion, ρ z ( t ) , and information entropies, H ( σ x ) , H ( σ y ) , and H ( σ z ) . We observe clearly the emergence of long-lived quantum coherence and the decay in curves for some special cases of Ω ( t ) and ν ( t ) . Also, we show that the dynamic nonlinear properties of the system can be controlled by changing the values of the coherent Rabi oscillations Ω ( t ) and the frequency of the semiconductor system ν ( t ) . Due to the lack of mathematical treatment of such systems, our study promises significant advantages for a large number of nonlinear dynamic systems, opening up a wide range of applications for semiconductor quantum wells.

Dispersion of exciton-polariton based on ZnO/MgZnO quantum wells at room temperature**Project supported by the National Natural Science Foundation of China (Grant Nos. 11974433, 91833301, and 11974122), the Guangdong Natural Science Fund for Distinguished Young Scholars, China (Grant No. 2016A030306044), and the Science and Technology Program of Guangzhou, China (Grant No. 201707020014).

We report observation of dispersion for coupled exciton-polariton in a plate microcavity combining with ZnO/MgZnO multi-quantum well (QW) at room temperature. Benefited from the large exciton binding energy and giant oscillator strength, the room-temperature Rabi splitting energy can be enhanced to be as large as 60 meV. The results of excitonic polariton dispersion can be well described using the coupling wave model. It is demonstrated that mode modification between polariton branches allowing, just by controlling the pumping location, to tune the photonic fraction in the different detuning can be investigated comprehensively. Our results present a direct observation of the exciton-polariton dispersions based on two-dimensional oxide semiconductor quantum wells, thus provide a feasible road for coupling of exciton with photon and pave the way for realizing novel polariton-type optoelectronic devices.

Excitons in Cu2O : From quantum dots to bulk crystals and additional boundary conditions for Rydberg exciton-polaritons

We propose schemes for calculation of optical functions of a semiconductor with Rydberg excitons for a wide interval of dimensions. We have started with a zero-dimensional structure (quantum dot), then going to one-dimensional (quantum wire), two-dimensional (quantum wells and wide quantum wells), and finally three-dimensional bulk crystals; our analytical findings are illustrated numerically, showing an agreement with available experimental data. Calculations including exciton-polaritons are performed; the case of a large number of polariton branches is discussed, and obtained theoretical absorption spectra show good agreement with experimental data.

Experimental and Theoretical Examination of the Photosensitivity Spectra of Structures with In0.4Ga0.6As Quantum Well-Dots of the Optical Range (900–1050 nm)

Solar cells based on a new type of nanostructure (quantum well-dots) have been studied. These quantum well-dots (QWDs) are In0.4Ga0.6As layers with marked composition and thickness modulations. The energies of the observed optical transitions in QWDs turn out to be close to the energies of transitions involving a light hole and a heavy one in a two-dimensional quantum well of the same nominal thickness and composition. The ground-state peak is characterized by strong TE polarization (>70%), while the shorter-wavelength peak is almost unpolarized (<10%).

Epitaxial Stabilization of Tetragonal Cesium Tin Iodide.

A full range of optoelectronic devices has been demonstrated incorporating hybrid organic-inorganic halide perovskites including high-performance photovoltaics, light emitting diodes, and lasers. Tin-based inorganic halide perovskites, such as CsSnX3 (X = Cl, Br, I), have been studied as promising candidates that avoid toxic lead halide compositions. One of the main obstacles for improving the properties of all-inorganic perovskites and transitioning their use to high-end electronic applications is obtaining crystalline thin films with minimal crystal defects, despite their reputation for defect tolerance in photovoltaic applications. In this study, the single-domain epitaxial growth of cesium tin iodide (CsSnI3) on closely lattice matched single-crystal potassium chloride (KCl) substrates is demonstrated. Using in situ real-time diffraction techniques, we find a new epitaxially-stabilized tetragonal phase at room temperature that expands the possibility for controlling electronic properties. We also exploit controllable epitaxy to grow multilayer two-dimensional quantum wells and demonstrate epitaxial films in a lateral photodetector architecture. This work provides insight into the phase control during halide perovskite epitaxy and expands the selection of epitaxially accessible materials from this exciting class of compounds.

Flow dynamics of the resonances of a two-dimensional circular quantum well

Flow dynamics of the resonances of a two-dimensional circular quantum well

Plasmonically-powered hot carrier induced modulation of light emission in a two-dimensional GaAs semiconductor quantum well.

A hot-electron-enabled route to controlling light with dissipative loss compensation in semiconductor quantum light emitters has been realized for tunable quantum optoelectronic devices via a two-species plasmon system. The dual species nano-plasmonic system is achieved by combining UV-plasmonic gallium metal nanoparticles (GaNPs) with visible-plasmonic gold metal nanoparticles (AuNPs) on a near-infrared two-dimensional GaAs/AlGaAs quantum well emitter. It has been demonstrated that while hot carrier-powered charge-transfer processes can result in semiconductor doping and increased optical absorption, photo-generated carrier density in the quantum well can also be modulated by off-resonant plasmonic interaction without thermal dissipation. Merging these essential emitter-friendly optical characteristics in the two-species plasmon system, we effectively modulate the frequency of the emitted light. The wavelength of the emitted light is tuned by the plasmonically powered hot electron process induced by the AuNPs with a 10-fold emission enhancement induced by the GaNPs. The additional plasmonic element provides functionality to achieving an active plasmonic light emitter that is otherwise far from reach with conventional single plasmonic material-based semiconductors.

Temperature effects on state-energy levels and transition frequency of magnetopolaron in asymmetric two-dimensional Gaussian quantum wells

Influence of the magnetic field on the energy levels and the excitation energy (EE) between the two states of the magnetopolaron strongly coupled in asymmetric Gaussian quantum wells is examined by the variational method of Pekar type. Then, we discover the temperature and magnetic field effects on the energy levels and EE with quantum statistics theory. According to the results of the article, the energy levels and the EE are a function relationship with the temperature rising and magnetic field increasing.

Concept of Lattice Mismatch and Emergence of Surface States in Two-dimensional Hybrid Perovskite Quantum Wells

Surface states are ubiquitous to semiconductors and significantly impact the physical properties and, consequently, the performance of optoelectronic devices. Moreover, surface effects are strongly amplified in lower dimensional systems such as quantum wells and nanostructures. Layered halide perovskites (LHPs) are two-dimensional solution-processed natural quantum wells where optoelectronic properties can be tuned by varying the perovskite layer thickness n, i.e., the number of octahedra spanning the layer. They are efficient semiconductors with technologically relevant stability. Here, a generic elastic model and electronic structure modeling are applied to LHPs heterostructures with various layer thickness. We show that the relaxation of the interface strain is triggered by perovskite layers above a critical thickness. This leads to the release of the mechanical energy arising from the lattice mismatch, which nucleates the surface reorganization and may potentially induce the formation of previously observed lower energy edge states. These states, which are absent in three-dimensional perovskites are anticipated to play a crucial role in the design of LHPs for optoelectronic systems.

Design principles for electronic charge transport in solution-processed vertically stacked 2D perovskite quantum wells

State-of-the-art quantum-well-based devices such as photovoltaics, photodetectors, and light-emission devices are enabled by understanding the nature and the exact mechanism of electronic charge transport. Ruddlesden–Popper phase halide perovskites are two-dimensional solution-processed quantum wells and have recently emerged as highly efficient semiconductors for solar cell approaching 14% in power conversion efficiency. However, further improvements will require an understanding of the charge transport mechanisms, which are currently unknown and further complicated by the presence of strongly bound excitons. Here, we unambiguously determine that dominant photocurrent collection is through electric field-assisted electron–hole pair separation and transport across the potential barriers. This is revealed by in-depth device characterization, coupled with comprehensive device modeling, which can self-consistently reproduce our experimental findings. These findings establish the fundamental guidelines for the molecular and device design for layered 2D perovskite-based photovoltaics and optoelectronic devices, and are relevant for other similar quantum-confined systems.

Coulomb Drag of Dipole Excitons in a Hybrid Exciton–Electron System

The effect of Coulomb drag on a gas of dipole excitons in spatially separated two-dimensional quantum wells containing electron and exciton gases is studied theoretically. The Coulomb drag of excitons can be used to control exciton transport in transistor structures whose active element is a two-dimensional gas of dipole excitons. Expressions for the exciton cross conductivity as a function of temperature are obtained for the diffusion and ballistic transport regimes. For each regime, the limiting cases in terms of the ratio of the Coulomb interaction screening length to the distance between the gases are analyzed. It is shown that, at temperatures exceeding considerably the exciton-gas degeneracy temperature, the cross conductivity is independent of the temperature, while in the opposite case it vanishes exponentially.

Energy transfer mechanisms in layered 2D perovskites.

Two-dimensional (2D) perovskite quantum wells are generating broad scientific interest because of their potential for use in optoelectronic devices. Recently, it has been shown that layers of 2D perovskites can be grown in which the average thicknesses of the quantum wells increase from the back to the front of the film. This geometry carries implications for light harvesting applications because the bandgap of a quantum well decreases as its thickness increases. The general structural formula for the 2D perovskite systems under investigation in this work is (PEA)2(MA)n-1[PbnI3n+1] (PEA = phenethyl ammonium, MA = methyl ammonium). Here, we examine two layered 2D perovskites with different distributions of quantum well thicknesses. Spectroscopic measurements and model calculations suggest that both systems funnel electronic excitations from the back to the front of the film through energy transfer mechanisms on the time scales of 100's of ps (i.e., energy transfer from thinner to thicker quantum wells). In addition, the model calculations demonstrate that the transient absorption spectra are composed of a progression of single exciton and biexciton resonances associated with the individual quantum wells. We find that exciton dissociation and/or charge transport dynamics make only minor contributions to the transient absorption spectra within the first 1 ns after photo-excitation. An analysis of the energy transfer kinetics indicates that the transitions occur primarily between quantum wells with values of n that differ by 1 because of the spectral overlap factor that governs the energy transfer rate. Two-dimensional transient absorption spectra reveal a pattern of resonances consistent with the dominance of sequential energy transfer dynamics.

Compositional and strain analysis of In(Ga)N/GaN short period superlattices

Extensive high resolution transmission and scanning transmission electron microscopy observations were performed in In(Ga)N/GaN multi-quantum well short period superlattices comprising two-dimensional quantum wells (QWs) of nominal thicknesses 1, 2, and 4 monolayers (MLs) in order to obtain a correlation between their average composition, geometry, and strain. The high angle annular dark field Z-contrast observations were quantified for such layers, regarding the indium content of the QWs, and were correlated to their strain state using peak finding and geometrical phase analysis. Image simulations taking into thorough account the experimental imaging conditions were employed in order to associate the observed Z-contrast to the indium content. Energetically relaxed supercells calculated with a Tersoff empirical interatomic potential were used as the input for such simulations. We found a deviation from the tetragonal distortion prescribed by continuum elasticity for thin films, i.e., the strain in the relaxed cells was lower than expected for the case of 1 ML QWs. In all samples, the QW thickness and strain were confined in up to 2 ML with possible indium enrichment of the immediately abutting MLs. The average composition of the QWs was quantified in the form of alloy content.