Quantum-well Structures Research Articles

The laser field controlling on the nonlinear optical specifications of the electric field-triggered Rosen-Morse quantum well

We consider, for the first time, optical specifications such as the nonlinear optical rectification (NOR), the second harmonic generation (SHG) and the third harmonic generation (THG) of quantum well (QW) that has the Rosen-Morse potential confinement and is generated by GaAs/GaAlAs heterostructure. To do this, QW structure is firstly asymmetrized by applying an external electric field, as 10 kV/cm. Therefore, the electric field-triggered (F=10 kV/cm) Rosen-Morse QW (eftRMQW) is taken into consideration throughout the study. In this study, it is elucidated that how to control NOR, SHG and THG optical responses through monochromatic linearly polarized intense laser field (ILF). In order to be able to investigate the ILF effects, the time-dependent of ILF is removed to potential energy term of the wave equation by using Kramers-Henneberger (KH) and the dipole approximations, building the laser-dressed potential. Then, the bound state energies and the wave functions of eftRMQW are obtained using the diagonalization method within the effective mass approach and the parabolic approximation. The influences of the ILF and structural parameters of eftRMQW on NOR, SHG and THG coefficients are discussed in detail. The optimum of structure is ferreted out for devices design and applications that will consider eftRMQW, and for this optimum the alternative parameter analysis is carried out.

High wall-plug efficiency 808-nm laser diodes with a power up to 30.1 W

A very highly efficient InGaAlAs/AlGaAs quantum-well structure was designed for 808 nm emission, and laser diode chips 390-μm-wide aperture and 2-mm-long cavity length were fabricated. Special pretreatment and passivation for the chip facets were performed to achieve improved reliability performance. The laser chips were p-side-down mounted on the AlN sub-mount, and then tested at continuous wave (CW) operation with the heat-sink temperature setting to 25 °C using a thermoelectric cooler (TEC). As high as 60.5% of the wall-plug efficiency (WPE) was achieved at the injection current of 11 A. The maximum output power of 30.1 W was obtained at 29.5 A when the TEC temperature was set to 12 °C. Accelerated life-time test showed that the laser diodes had lifetimes of over 62 111 h operating at rated power of 10 W.

Study of current-voltage characteristics of InAsSb-based LED heterostructures in 4.2 - 300 K temperature range

Current-voltage characteristics of light-emitting diode heterostructures based on InAsSb epitaxial films and a multi quantum-well structure were studied in the temperature range T = 4.2–300 K. It is shown that transport through the heterostructures is governed by the diffusion and recombination mechanisms at temperatures close to 300 K. The tunnelling effect appears in the temperature range 4.2–77 K. The presence of quantum wells in the active layer results in increased diode leakage currents due to tunnelling.

Optical studies of wide-bandgap HgCdTe material used in potential-and quantum-well structures

Optical transmission and photoluminescence were used for the study of wide-bandgap (0.8-1.1 eV) HgCdTe (MCT) material grown by molecular-beam epitaxy. The material, including layers used as spacers and barriers in potential-and quantum-well structures, showed a considerable degree of alloy disorder similar to narrow-gap MCT grown by the same method. In some samples, defect states in the bandgap were found. Optimization of the growth technology for wide-bandgap material should help improving the quality of MCT-based potential-and quantum-well structures designed for various applications.

Luminescence study in InGaAs/AlGaAs multi-quantum-well light emitting diode with p–n junction engineering

Overlapping between the quantum-well structure and p–n junction plays an important role in the development of high-efficiency GaAs-based light-emitting diodes (LEDs). In this work, we apply p–n junction engineering to study electro-photoluminescence spectra under open-circuit, forward voltage, and reverse voltage conditions. Through modifying the doping of p-layer or n-layer, luminescent characteristics are apparently improved as a result of the more effective overlapping of the quantum-well structure and p–n junction caused by energy band modification, with the more balanced carrier and stronger confinement in quantum wells. Furthermore, the electroluminescence spectra of all LEDs studied herein are consistent with the photoluminescence results. This work provides an explicit way to tune p- and n-doping to increase emission efficiency, which is important for research and industrial applications.

Magnetic Semimetals and Quantized Anomalous Hall Effect in EuB_{6}.

Exploration of the novel relationship between magnetic order and topological semimetals has received enormous interest in a wide range of both fundamental and applied research. Here we predict that "soft" ferromagnetic material EuB_{6} can achieve multiple topological semimetal phases by simply tuning the direction of the magnetic moment. Explicitly, EuB_{6} is a topological nodal-line semimetal when the moment is aligned along the [001] direction, and it evolves into a Weyl semimetal with three pairs of Weyl points by rotating the moment to the [111] direction. Interestingly, we identify a composite semimetal phase featuring the coexistence of a nodal line and Weyl points with the moment in the [110] direction. Topological surface states and anomalous Hall conductivity, which are sensitive to the magnetic order, have been computed and are expected to be experimentally observable. Large-Chern-number quantum anomalous Hall effect can be realized in its [111]-oriented quantum-well structures.

Biexcitons in 2D (iso-BA)2PbI4 perovskite crystals

Abstract Two-dimensional (2D) organic-inorganic hybrid perovskites have attracted growing attention recently due to their naturally formed quantum-well structure, unique photoelectric properties and better environmental stability compared to three-dimensional perovskites. The reduced screening and enhanced Coulomb interaction in 2D perovskites result in the formation of excitonic complexes. While the properties of free excitons have been well investigated, studies on biexcitons remain elusive. Here, we report on the biexcitons in 2D (iso-BA)2PbI4 (BA=C4H9NH3) crystals. The biexciton emission can be observed under a very low excitation power density of 6.4 W/cm2 at 78 K. The biexciton exhibits a large biexciton binding energy of 46 meV due to the large exciton binding energy of (iso-BA)2PbI4. Furthermore, the biexcitons exhibit a favorable polarization orientation, resulting in different anisotropy between biexcitons and excitons. Our findings would motivate more studies on biexcitons in 2D perovskites and pave the way for exploiting the many-body physics for biexciton lasing and optical storage devices.

Interface Studies in HgTe/HgCdTe Quantum Wells

Transmission electron microscopy (TEM) is used for the study of interfaces in two HgTe/HgCdTe single quantum‐well (QW) structures grown by molecular beam epitaxy on GaAs substrates. The studies are conducted in bright‐field and scanning/high‐angle annular dark field modes. The effect of the growth mode on the sharpness of interfaces in the QWs is investigated. Effective in situ ellipsometric control over chemical composition and thickness of the layers constituting the QW structures is demonstrated.

Intersubband and intrasubband electron scattering by acoustic-phonons in quantum wells

We study the intersubband and intrasubband electron scattering by acoustic-phonons in quantum wells by comparing the cyclotron-resonance (CR) full-width at half-maximum (FWHM) when without intersubband transitions with that when having intersubband transitions in quantum-well structure due to electrons-acoustic phonons scattering. Using operator projection technique, the expressions for CR magneto-optical absorption powers (MOAPs) are obtained. Using graphical and numerical method, we obtain the dependence of the MOAPs on the photon energy, temperature, quantum-well width, and external magnetic field. We obtain the FWHM as profile of the curve based on the profile technique. The FWHM is calculated for both when without intersubband transitions and when having intersubband transitions. The influence of the temperature, external magnetic field, and well width on the FWHM have been considered and discussed. The results for the CR-peak's FWHM in the case without intersubband transitions are compared with those in the case having inter-subband transitions. In addition, the contribution of acoustic phonons in the range of low- and high-temperatures have also been considered and compared.

Enhanced terahertz absorption of quantum wells sandwiched between heavily doped contacts based on micro-cavity resonance

A novel high-efficiency microcavity structure of quantum wells sandwiched between periodic heavily Si-doped GaAs top contact gratings and bottom contact film has been proposed as the optical coupler of a terahertz quantum well photodetector (THz QWP). Similar to metal at visible light, highly doped semiconductors exhibit plasma frequencies at mid- and far-infrared wavelengths. The intersubband absorption spectra and electric field distribution of the microcavity THz QWP are calculated with the finite difference time-domain method. Our results indicate that the frequency of the surface plasmon polariton can be tuned to the microcavity resonant mode under an optimized structure and the intersubband absorption is efficiently enhanced by the microcavity structure. When the doping concentration of the contact exceeds 1018 cm−3, the intersubband absorption of the microcavity THz QWP at the response wavelength is over one order of magnitude higher than that of the standard 45° device. In addition, the angle of the incident light only influences the intensity of the absorptivity, indicating that the designed device was independent of the periodic surface structure.

The strain, energy band and photoluminescence of GaAs0.92Sb0·08/Al0.3Ga0.7As multiple quantum wells grown on GaAs substrate

GaAsSb based materials have become the promising system for infrared semiconductor lasers and detectors. In this article, the strain, energy band structures and photoluminescence (PL) of GaAs0.92Sb0·08/Al0.3Ga0.7As strained quantum wells (QWs) grown with molecular beam epitaxy are systematically analyzed both theoretically and experimentally. The theoretical results are derived by Kane's model and k⋅p method, and the optical properties of a high-quality GaAs0.92Sb0·08/Al0.3Ga0.7As strained QWs sample are thoroughly investigated by excitation- and temperature-dependent PL measurements. The theoretical results show the strain has significant influence on the band structure of QWs. In experimental part, it is found that the light-hole exciton emission coexists with the heavy-hole exciton line in the temperature range of 50 K–300 K. However, the emission of localized excitons, which is caused by the nonuniformity of component in the GaAsSb well layer, takes over the light-hole exciton emission at lower temperatures (<50 K).

Electronic states and interband tunneling conditions in type-II quantum well heterostructures

Type-II quantum well (QW) heterostructures based on the InAs/GaSb/AlSb material system have important applications in interband tunneling and midinfrared optoelectronic devices. From the perspective of interband tunneling conditions in widely used type-II QWs such as N-QW, W-QW, and M-QW structures, this work studies how energy levels for electronic and light-hole states are varied with various combinations of semiconductor material layers. Using a two-band model, analytical expressions are derived for the transition well widths that correspond to the critical layer thicknesses for bound- or quasibound states to appear in the interband tunneling region in type-II heterostructures, which provide meaningful connections to semiconductor material parameters. Calculations of transition well widths are carried out for several InAs/GaSb/AlSb-based type-II QW structures. The results and understanding gained from the study may be helpful in designing and developing midinfrared optoelectronic devices such as interband cascade lasers, photodetectors, and thermophotovoltaic cells.

Heat-driven acoustic phonons in lamellar nanoplatelet assemblies.

Colloidal CdSe nanoplatelets, with the electronic structure of quantum wells, self-assemble into lamellar stacks due to large co-facial van der Waals attractions. These lamellar stacks are shown to display coherent acoustic phonons that are detected from oscillatory changes in the absorption spectrum observed in infrared pump, electronic probe measurements. Rather than direct electronic excitation of the nanocrystals using a femtosecond laser, impulsive transfer of heat from the organic ligand shell, excited at C-H stretching vibrational resonances, to the inorganic core of individual nanoplatelets occurs on a time-scale of <100 ps. This heat transfer drives in-phase longitudinal acoustic phonons of the nanoplatelet lamellae, which are accompanied by subtle deformations along the nanoplatelet short axes. The frequencies of the oscillations vary from 0.7 to 2 GHz (3-8 μeV and 0.5-1 ns oscillation period) depending on the thickness of the nanoplatelets-but not their lateral areas-and the temperature of the sample. Temperature-dependence of the acoustic phonon frequency conveys a substantial stiffening of the organic ligand bonds between nanoplatelets with reduced temperature. These results demonstrate a potential for acoustic modulation of the excitonic structure of nanocrystal assemblies in self-assembled anisotropic semiconductor systems at temperatures at or above 300 K.

Using the noise-equivalent temperature difference to compare superlarge photoreceivers based on quantum-well multilayer structures

A method has been developed for analyzing the temperature resolution of far-IR and mid-IR photoreceivers. The features of creating silicon multiplexers for such photoreceivers are discussed. The noise-equivalent temperature difference is analyzed for IR photoreceivers based on silicon multiplexers with framewise accumulation from photodetectors based on quantum-well multilayer structures. The silicon multiplexers thus developed are compared in order to use photosensitive chips, including those with increased dark currents, to create IR photoreceivers with temperature resolution at the response level of similar photoreceivers from leading companies. The structural–technological principles of the creation of mosaic photoreceivers are developed for the case of superhigh size. The technological level thus achieved is discussed for high-precision microassembly of submodule chips into mosaic photoreceivers. Methods are proposed for forming the multispectral photosensitivity response of the mosaic photoreceivers. A comparative analysis is carried out of the size of the “blind zone” of different defining materials.

Unprecedented random lasing in 2D organolead halide single-crystalline perovskite microrods.

Three-dimensional organic-inorganic hybrid halide perovskites have been demonstrated as great materials for applications in optoelectronics and photonics. However, their inherent instabilities in the presence of moisture, light, and heat may hinder their commercialization. Alternatively, emerging two-dimensional (2D) organic-inorganic hybrid perovskites have recently attracted increasing attention owing to their great environmental stability and inherent natural quantum-well structure. In this work, we have synthesized a high-quality long-chain organic diammonium spacer assisted 2D hybrid perovskite FA-(N-MPDA)PbBr4 (FA = formamidinium and N-MPDA = N-methylpropane-1,3-diammonium) by the slow evaporation at constant temperature method. The millimeter-sized single-crystalline microrods demonstrate low threshold random lasing behavior at room temperature. The single-crystalline 2D hybrid perovskite random laser achieved a very narrow linewidth (∼0.1 nm) with a low threshold (∼0.5 μJ cm-2) and a high quality factor (∼5350). Furthermore, the 2D hybrid microrod laser shows stable lasing emission with no measurable degradation after at least 2 h under continuous illumination, which substantially proves the stability of 2D perovskites. Our results demonstrate the promise of 2D organic-inorganic microrod-shaped perovskites and provide an important step toward the realization of high-performance optoelectronic devices.

A Quantum-Well Charge-Trap Synaptic Transistor With Highly Linear Weight Tunability

In this work, a novel synaptic transistor has been proposed and analyzed through technology computer-aided design (TCAD) simulation. The proposed device has merits of full-Si processing compatibility, short- and long-term plasticity, high energy efficiency, and linear and symmetric conductance adjustability. The proposed device consists of a quantum-well structure and a charge-trap unit for realizing both short- and long-term memories, respectively. The quantum-well charge-trap synaptic transistor (QW CTS) employs two independent gates to separate inference and weight adjustment operation. An optimally designed and validated QW CTS has demonstrated a highly linear and symmetric weight tunability, with an ultra-low energy consumption of ~1.5 fJ per synaptic event. The QW CTS can be a core element in the hardware-driven Si neuromorphic system.

Structure-Dependent Photoluminescence in Low-Dimensional Ethylammonium, Propylammonium, and Butylammonium Lead Iodide Perovskites.

Hybrid organic-inorganic perovskites have attracted great attention as the next generation materials for photovoltaic and light-emitting devices. However, their environment instability issue remains as the largest challenge for practical applications. Recently emerging two-dimensional (2D) perovskites with Ruddlesden-Popper structures are found to greatly improve the stability and aging problems. Furthermore, strong confinement of excitons in these natural quantum-well structures results in the distinct and narrow light emission in the visible spectral range, enabling the development of spectrally tunable light sources. Besides the strong quasi-monochromatic emission, some 2D perovskites composed of the specific organic cations and inorganic layer structures emit a pronounced broadband emission. Herein, we report the light-emitting properties and the degradation of low-dimensional perovskites consisting of the three shortest alkylammonium spacers, mono-ethylammonium (EA), n-propylammonium (PA), and n-butylammonium (BA). While (BA)2PbI4 is known to form well-oriented 2D thin films consisting of layers of corner-sharing PbI6 octahedra separated by a bilayer of BA cations, EA with shorter alkyl chains tends to form other types of lower-dimensional structures. Nevertheless, optical absorption edges of as-prepared fresh EAPbI3, (PA)2PbI4, and (BA)2PbI4 are obviously blue-shifted to 2.4-2.5 eV compared to their 3D counterpart, methylammonium lead iodide (MAPbI3) perovskite, and they all emit narrow excitonic photoluminescence. Furthermore, by carefully optimizing deposition conditions, we have achieved a predominantly 2D structure for (PA)2PbI4. However, unlike (BA)2PbI4, upon exposure to ambient environment, (PA)2PbI4 readily transforms to a different crystal structure, exhibiting a prominently broadband light from ∼500 to 800 nm and a gradual increase in intensity as structural transformation proceeds.

Luminescence and scintillation properties of (C6H5(CH2)2NH3)2(Ba,Pb)Br4 with self-organized bi-dimensional quantum-well structures

(C6H5(CH2)4NH3)2BaxPb1−xBr4 (x = 0, 0.1, 0.25 and 0.5) crystals were prepared by the solvent diffusion method to evaluate the scintillation properties. The samples exhibit photoluminescence and scintillation with a sharp emission peaking around 440 nm due to excitons, and the decay time is dominantly several nanoseconds. The pulse height spectrum under 137Cs gamma-ray irradiation demonstrates that the scintillation light yields of (C6H5(CH2)2NH3)2Ba0.25Pb0.75Br4 and (C6H5(CH2)2NH3)2Ba0.5Pb0.5Br4 crystals are approximately 19 000 ph MeV−1.

Electronic and Optical Properties of Staggered ZnO/ZnO1−xSx-ZnO1−ySy/ZnO Quantum Wells for Bluish-Green Light-Emitting Diodes

The optical and electronic properties of staggered ZnO/ZnO1−xSx-ZnO1−ySy/ZnO (for x > y) bluish-green light-emitting diodes with quantum well (QW) structures are investigated using the multiband effective mass theory. The obtained results are compared with those of conventional ZnO1−xSx/ZnO QW structures, and the staggered QW structures are found to have a comparatively larger optical matrix, showing a substantial, yet gradual, reduction in the internal field effect. Thus, the staggered ZnO/ZnO1−xSx-ZnO1−ySy/ZnO QW structure design leads to a significantly higher spontaneous emission compared to that of conventional ZnO1−xSx/ZnO QW structures. This analysis indicates the potential of using a staggered QW structure for high-efficiency ZnO-based bluish-green LEDs.

The overlap of electron and hole wavefunctions in the InxGa1-xN/GaN graded quantum well LED is much superior to the symmetrically staggered: Even to that of a trapezoidal quantum well

The strong polarization fields in the InxGa1-xN/GaN quantum well (QW) LEDs, separates the electrons and holes spatially, which decreases the optical performance. Various non-square indium profiles of such QWs are studied to improve the overlap of electron and hole wavefunctions. We have investigated the overlap of electron and hole wave functions, band structures, field distributions and hole concentration of the symmetrically staggered, trapezoidal and graded QW LEDs, in the emission of the blue spectral region. The striking new results are presented with suitable depictions and discussions. In the trapezoidal and graded QW structures, the overlap of electron and hole wavefunctions is significantly increased as compared to the symmetrically staggered QW. The overlap of electron and hole wavefunctions of the graded structure is even higher than the trapezoidal QW. The best results are obtained for the graded structure, where the overlap function and hole concentration are significantly increased as compared to other QW structures. An important feature emerges from the graded structure that the change of the peak emission energy with increase in the current density is minimized.