Thick Quantum Wells Research Articles

Reduced-Dimensional Perovskites: Quantum Well Thickness Distribution and Optoelectronic Properties.

Reduced-dimensional perovskites (RDPs), a large category of metal halide perovskites, have attracted considerable attention and shown high potential in the fields of solid-state displays and lighting. RDPs feature a quantum-well-based structure and energy funneling effects. The multiple quantum well (QW) structure endows RDPs with superior energy transfer and high luminescence efficiency. The effect of QW confinement directly depends on the number of inorganic octahedral layers (QW thickness, i.e., n value), so the distribution of n values determines the optoelectronic properties of RDPs. Here, it is focused on the QW thickness distribution of RDPs, detailing its effect on the structural characteristics, carrier recombination dynamics, optoelectronic properties, and applications in light-emitting diodes. The reported distribution control strategies is also summarized and discuss the current challenges and future trends of RDPs. This review aims to provide deep insight into RDPs, with the hope of advancing their further development and applications.

Impact of the Interruption Duration on Photoluminescence Properties of MOCVD-Grown GaAsP/InAlGaAs Quantum Well Structures.

The growth interruption technology is introduced to the growth of GaAsP/InAlGaAs quantum well (QW) structure using metal-organic chemical vapor deposition (MOCVD). The effect of growth interruption time (GIT) on the crystalline quality and optical properties are investigated. The two distinctive emission peaks are the transition recombination between the electron level of conduction band and the light and heavy hole level of valence band in the photoluminescence (PL) at room temperature. The PL peaks present a redshift and merge together with the increasing GIT, which is attributed to the QW energy level shift caused by the increase in arsenic concentrations in GaAsP QW, the diversified thickness of QW and the variations of indium components in the InAlGaAs barrier layer. The Gaussian deconvolution parameters in temperature-dependent PL (TDPL) show that the GaAsP/InAlGaAs QW with a GIT of 6 s has a 565.74 meV activation energy, enhancing the carrier confinement in QW and the PL intensity, while the 6 s-GIT GaAsP QW has the increasing interface roughness and the non-radiative centers at the InGaAsP intermediate layer, leading to a spectral broadening. The QW with 10 s-GIT exhibits a small full width at half maximum (FWHM) with the various temperature, indicating reduced interface roughness and excellent crystal quality. An increase in GIT may be suitable for optimizing the optical properties of GaAsP/InAlGaAs QW.

Band alignment engineering of 2D/3D halide perovskite lateral heterostructures.

Two-dimensional (2D)/three-dimensional (3D) halide perovskite heterostructures have been extensively studied for their ability to combine the outstanding long-term stability of 2D perovskites with the superb optoelectronic properties of 3D perovskites. While current studies mostly focus on vertically stacked 2D/3D perovskite heterostructures, a theoretical understanding regarding the optoelectronic properties of 2D/3D perovskite lateral heterostructures is still lacking. Herein, we construct a series of 2D/3D perovskite lateral heterostructures to study their optoelectronic properties and interfacial charge transfer using density functional theory (DFT) calculations. We find that the band alignments of 2D/3D heterostructures can be regulated by varying the quantum-well thickness of 2D perovskites. Moreover, decreasing the 2D component ratio in 2D/3D heterostructures can be favorable to form type-I band alignment, whereas a large component ratio of 2D perovskites tends to form type-II band alignment. We can improve the amount of charge transfer at the 2D/3D perovskite interfaces and the light absorption of 2D perovskites by increasing quantum-well thickness. These present findings can provide a clear designing principle for achieving 3D/2D perovskite lateral heterostructures with tunable optoelectronic properties.

An Approach Toward Broader Emission Bands in Semipolar InGaN Quantum Wells on Convex Lens‐Shaped GaN Microstructures via Lower‐Temperature Growth

InGaN quantum wells (QWs) are grown at different temperatures on convex lens‐shaped GaN microstructures formed on the () plane. Microlens QWs grown at different temperatures have centrosymmetric convex lens shapes, and the wavelength spatial distributions within the structures exhibit similar tendencies. However, lowering the growth temperature broadens the wavelength range of room‐temperature cathodoluminescence spectra from the microlens QWs as ≈70 nm for ≈700 °C, ≈100 nm for ≈660 °C, and ≈150 nm for ≈650 °C. Peak wavelength profiles of the two orthogonal lines along [] and [] indicate that the broader emission bands are mainly due to the significant spatial distribution of the emission wavelength along the [] direction. Because the QW thickness variations are nearly the same along [] and [], the observed difference in the emission wavelength distributions along those directions is attributed to the difference in In incorporation, which is determined by the competition among the growth rate, crystal orientation, and growth temperature.

Gain Characteristics of Optically Pumped UVC Lasers with Wide AlGaN Single‐Quantum‐Well Active Regions

Herein, the lasing threshold and gain characteristics of ultraviolet‐C optically pumped edge‐emitting lasers with thick single‐quantum‐well (SQW) active regions are investigated by the variable‐stripe length method. Positive net modal gain is observed in lasers with AlGaN‐based SQWs with thicknesses up to 12 nm. The lasers show a reduction of the threshold power density with increasing SQW thickness, with the lowest threshold of 1.3 MW cm−2 achieved for a 9 nm SQW laser. The high gain and low threshold in lasers with thick quantum wells are attributed to lasing from excited states, where the polarization fields are screened by the carriers in the fundamental state and the barriers, thus recovering larger electron and hole wavefunctions overlap. These findings are supported by k · p simulations, and for a 9 nm SQW, the calculation predicts a contribution of the fundamental transition to the gain of 75% for non‐resonant optical excitation and below 1% for resonant optical or electrical excitation.

Effects of structural defects on optical properties of InxGa1−xN layers and quantum wells

This review concentrates on the microstructure of InxGa1−xN layers and quantum wells (QWs) in relation to their optical properties. The microstructure of InxGa1−xN, with a constant In(x) concentration, shifts with layer thickness. Only layers below 100 nm for x = 0.1 are nearly defect-free. A photoluminescence peak is observed at 405 nm, in line with ∼10% In, suggesting band-edge luminescence. Layers with greater thickness and In content present a corrugated surface with numerous structural defects, including V-defects, causing redshifts and multi-peaks in photoluminescence up to 490 nm. These defects, resembling those in GaN, lead to a corrugated sample surface. Atomic force microscopy shows a 3.7-fold larger corrugation in samples with 20 QWs compared to those with 5 QWs measured on 2 × 2 μm2 areas. Like in GaN, dual growth on different crystallographic planes results in varied QW thicknesses, influencing optical traits of devices made from InxGa1−xN layers. The purpose of this review and the chosen subject is to highlight the significant contribution of Wladek Walukiewicz and his group to the current research on the properties of InxGa1−xN, which are crucial alloys in the field of optoelectronics.

Effect of dilute nitride GaNAs quantum well thickness on spin amplification dynamics of tunnel-coupled InAs quantum dots

Electron spin dynamics of dilute nitride GaNAs quantum well (QW)-InAs quantum dots (QDs) tunnel-coupled structures having different QW thicknesses were studied via circularly polarized time-resolved photoluminescence. The rate equation fitting considering a capture of QD electron spins by the GaNAs localized states via tunnel transfer revealed that the spin amplification dynamics of the QDs depended on the QW thickness. For the QW thickness of 5 nm, although the temporal amplification of QD electron spin polarization was slow owing to the weak wavefunction coupling between the QW and QD, the long duration of high electron spin polarization was observed because of the suppressed capture of QD majority spins relative to the efficient removal of QD minority spins. When the QW thickness increased from 5 to 20 nm, the strong spin filtering in GaNAs and strong wavefunction coupling led to the fast amplification of QD electron spin polarization with high initial spin polarization. However, the spin polarization rapidly decays after the amplification owing to the removal of both QD majority and minority spins, originating from many effective localized states. These results indicate that the time-dependent QD electron spin polarization and the resultant spin amplification can be widely tuned by changing the thickness of the tunnel-coupled GaNAs QW.

Modeling polarization effects on internal quantum efficiency of micro-LEDs at low current density

This study investigates the optoelectronic properties of blue micro-light-emitting diodes (µ-LEDs) by modeling the semipolar single quantum well (QW) at low current density. Through simulation analyses, the influences of the eight selected crystal orientations and different QW thicknesses on the internal quantum efficiency (IQE) and forward voltage are investigated for low current density LEDs. The simulation results of the energy band diagram show that reversed polarization increases the band tilt in the same direction as the built-in electric field, resulting in IQE poorer than that of LEDs whose polarization is of the same magnitude but positive. However, reversed polarization also reduces the hole barrier height, resulting in lower forward voltage at the same current density. Finally, it is found that weakly positive-polarized (101̄2) LEDs have the highest IQE and maximum allowed thickness at low current density, which is due to the flat energy band with a reduced band tilt caused by weakly positive polarization.

Origins of the high-energy electroluminescence peaks in long-wavelength (∼495–685 nm) InGaN light-emitting diodes

We investigate the unexpected high-energy electroluminescence (EL) peaks observed in long-wavelength InGaN light-emitting diodes (LEDs) with ground state emission peaks between ∼495 and 685 nm by studying the EL spectra of LEDs with varying quantum well (QW) thicknesses and indium compositions. In addition to the ground state emission, two high-energy emission peaks were observed in the LEDs with thick QWs and high indium compositions. The less energetic high-energy emission peak (2.4–2.6 eV) is attributed to the optical transitions involving excited states. Factors influencing the excited state transitions, such as the QW thickness and indium compositions, were also examined by simulations to better understand the occurrence of these transitions. The more energetic high-energy emission peak (2.8–3.1 eV) originates from V-defect sidewalls and was verified through micro-photoluminescence measurements. Identification of the high-energy emission peaks is essential as it enables targeted epitaxial or growth optimizations to minimize or eliminate these undesirable emission peaks. This work demonstrates the importance of using thin QWs to suppress the unwanted high-energy emissions due to excited state transitions and V-defect sidewalls for long-wavelength InGaN LEDs.

Energetics of Interfaces and Strain Partition in GaN/AlN Pseudomorphic Superlattices

We present the results of a twofold experimental and computational study of (0001) GaN/AlN multilayers forming pseudomorphic superlattices. High-Resolution Transmission Electron Microscopy (HRTEM) shows that heterostructures with four c-lattice parameters thick GaN Quantum Wells (QW) are misfit-dislocation free. Accurate structural data are extracted from HRTEM images via a new methodology optimizing the residual elastic energy stored in the samples. Total energy calculations are performed with several models analogous to the experimental QWs with increasing thicknesses of GaN, whereas this of the AlN barrier is kept fixed at n = 8 c-lattice parameters. With vanishing external stresses, minimum energy configurations of the studied systems correspond to different strain states. Linear elasticity accurately yields the corresponding lattice parameters, suppressing the need for on-purpose total energy calculations. Theoretically justified parabolic fits of the excess interfacial energy yield the values of interfacial stress and elastic stiffness as functions of the GaN QW thickness. Total species-projected densities of states and gap values extracted from there allow deciphering the effect of the evolving strain on the electronic structure of the superlattice. It is found that the gap energy decreases linearly with increasing the strain of the QW. These results are briefly discussed in the light shed by previous works from the literature.

Phase-pure 2D tin halide perovskite thin flakes for stable lasing.

Ruddlesden-Popper tin halide perovskites are a class of two-dimensional (2D) semiconductors with exceptional optoelectronic properties, high carrier mobility, and low toxicity. However, the synthesis of phase-pure 2D tin perovskites is still challenging, and the fundamental understanding of their optoelectronic properties is deficient compared to their lead counterparts. Here, we report the synthesis of a series of 2D tin perovskite bulk crystals with high phase purity via a mixed-solvent strategy. By engineering the quantum-well thickness (related to n value) and organic ligands, the optoelectronic properties, including photoluminescence emission, exciton-phonon coupling strength, and exciton binding energy, exhibit a wide tunability. In addition, these 2D tin perovskites exhibited excellent lasing performance. Both high-n value tin perovskite (n > 1) and n = 1 tin perovskite thin flakes were successfully optically pumped to lase. Furthermore, the lasing from 2D tin perovskites could be maintained up to room temperature. Our findings highlight the tremendous potential of 2D tin perovskites as promising candidates for high-performance lasers.

Theoretical study of the electronic and optical properties of ZnO/MgO rock salt superlattices

An analysis of some electronic and optical properties of rock salt (RS) ZnO/MgO superlattices related to potential applications in optoelectronics and electronics is carried out.In order to find possible light emission mechanisms, the electronic band structure, projected and k-resolved projected density of states and optical parameters such as the refractive index, reflection and absorption coefficients are calculated.We show that the bandgap of the RS ZnO/MgO superlattice is indirect for all considered thicknesses of quantum wells and barriers. Despite the indirect character of the bandgap, other features of the studied superlattice, such as the flat character of the top of the valence band and the bottom of the conduction band, combined with strong electron-phonon coupling and high exciton binding energy, may indicate the possibility of light emission similar to that in hexagonal BN.

Growth of κ-([Al,In]xGa1-x)2O3 Quantum Wells and Their Potential for Quantum-Well Infrared Photodetectors.

The wide band gap semiconductor κ-Ga2O3 and its aluminum and indium alloys have been proposed as promising materials for many applications. One of them is the use of inter-sub-band transitions in quantum-well (QW) systems for infrared detectors. Our simulations show that the detection wavelength range of nowadays state of the art GaAs/AlxGa1-xAs quantum-well infrared photodetectors (QWIPs) could be substantially excelled with about 1-100 μm using κ-([Al,In]xGa1-x)2O3, while at the same time being transparent to visible light and therefore insensitive to photon noise due to its wide band gap, demonstrating the application potential of this material system. Our simulations further show that the QWIPs efficiency critically depends on the QW thickness, making a precise control over the thickness during growth and a reliable thickness determination essential. We demonstrate that pulsed laser deposition yields the needed accuracy, by analyzing a series of (InxGa1-x)2O3 QWs with (AlyGa1-y)2O3 barriers with high-resolution X-ray diffraction, X-ray photoelectron spectroscopy (XPS) depth profiling, and transmission electron microscopy (TEM). While the superlattice fringes of high-resolution X-ray diffraction only yield an average combined thickness of the QWs and the barrier and X-ray spectroscopy depth profiling requires elaborated modeling of the XPS signal to accurately determine the thickness of such QWs, TEM is the method of choice when it comes to the determination of QW thicknesses.

Bright Emission at Reverse Bias After Trailing Edge of Driving Pulse in Wide InGaN Quantum Wells

In group‐III‐nitride quantum wells (QWs), a strong piezoelectric field is formed. A built‐in potential from the p–n junction is working in the opposite direction depending on an externally applied voltage. Furthermore, the electric field in the QW can be partially screened by a high charge carrier density. Herein, the influence of these effects on recombination and spectrum for over 10 nm wide InGaN QWs is investigated. The very low overlap of electrons and holes would suggest inefficient devices. However, it is shown that thick QWs can be more effective and reach high optical gain. This can be explained by the screening of the electric field, resulting in a high overlap of excited electron and hole states that enable lasing. Herein, a pulsed electrical excitation scheme is used, where carrier injection at forward bias and predominant recombination at zero or reverse bias are separated in time. The interplay between the piezoelectric field and the built‐in potential on carrier recombination in dependence on an external bias voltage is observed. In particular, a strong increase of the radiative recombination rate after the trailing edge of the driving pulse is observed.

Highly Sensitive Hydrogen Sulfide Sensor Based on GaN/GaInN Heterostructure

Accurate detection of gases such as hydrogen sulfide in the exhaled human breath is of great interest for medical professionals as it can possibly help in the early detection of organ malfunction and other diseases. GaInN heterostructure sensors are sensitive to the changes in the surface potential caused by the adsorption of gas molecules. A quantum well (QW) placed close to the surface experiences a change in the quantum‐confined Stark effect and as a result shifts its photoluminescence signal. Several parameters of the GaInN sensors grown by metal organic vapor phase epitaxy are optimized such as the GaN cap layer, QW thickness, and doping concentration. Moreover, how various metal functionalization layers can improve its sensitivity and selectivity is investigated. Gold (Au) and Silver (Ag) shows sensitivity to hydrogen sulfide in the 10–100 parts per billion (ppb) range. Ammonia gas is also detected in the 5–10 range (ppm) with a sensor structure covered with a thin gold layer.

Optical gain calculation and structure optimization of type-II GaAsSb/InAs/InGaAs quantum well heterostructures

This paper reports an optimization scheme of type-II GaAsSb/InAs/InGaAs quantum wells using the six bands k · p method. By calculating the optical gain, the optimal structures of the quantum wells are obtained. We observed that without an electric field, the thickness values of symmetrical quantum wells designed by the AdaGrad algorithm are 2.97, 2.81, and 6.21 nm, respectively, and the gain intensity is 6489 cm−1 that is about 1600 cm−1 higher than the original empirical design scheme. The thickness of asymmetric quantum wells designed by the Adam algorithm is 5.56, 2.72, 1.88, 3.16, and 2.46 nm under the external electric field of 75 kV/cm, respectively, and the gain intensity is 5729 cm−1 that is about 2300 cm−1 higher than the original empirical design scheme. The optimized scheme effectively suppresses the gain attenuation caused by the electric field and is helpful to improve the performance of interband cascade lasers.

Hot Electron Transport in Two-dimensional SiGe/Si Quantum Wells

Abstract: The hot carrier energy loss rate in a two-dimensioal electron gas in SiGe/Si quantum well has been theoretically studied and carrier concentration ranging from 1.0x1012 to 5.0x1014 m-2. The energy loss rate in this highly non-parabolic system is dominated by acoustic deformation potential scattering, whereas the acoustic piezoelectric scattering is negligible. We also studied variation of energy loss rate with thickness of various quantum wells

Correlating structural distortions and optical shifts in carboxylate-exchanged CdSe nanoplatelets.

Quasi-2D nanomaterials such as semiconducting nanoplatelets (NPLs) have drawn considerable interest due to their tunable optical properties and large surface to volume ratios. Cadmium selenide (CdSe) NPLs are of particular fundamental interest since their thicknesses can be controlled with atomic precision using well-established solution-phase synthetic techniques. Additionally, their large surface area makes them especially susceptible to changes in the identity of the capping ligands and, therefore, good model systems for understanding surface chemistry. In the current work, we explore the role of these ligands in altering the lattice parameters and optical properties of CdSe NPLs. We build on prior research that has employed varying binding groups, including thiols, phosphonic acids, and halides, to demonstrate ligand-dependent optical bandgap changes and concomitant lattice distortions as determined by powder x-ray diffraction (PXRD). Our work investigates the correlations between ligand-induced optical and structural changes with a series of ligands that maintain a consistent carboxylic acid binding group, thus allowing us to probe secondary ligand effects. We perform ligand exchanges on oleic acid-capped CdSe NPLs with benzoic acids, cinnamic acids, and cyclohexanecarboxylic acid. In all cases, the optical bandgap decreases upon ligand exchange, and a correlated expansion in the thickness of the NPLs is observed via PXRD. We also observe that the benzoic acids produce larger optical and structural distortions than the cinnamic acids. We show that the optical and structural correlation is nearly quantitatively described by quantum confinement effects, with the thicker quantum wells exhibiting smaller energy gaps.

Design tradeoffs between traditional hexagonal and emerging cubic InXGa(1–X)N/GaN-based green light-emitting diodes

Here, we report on the design tradeoffs between traditional hexagonal and emerging cubic InXGa(1−X)N/GaN-based green (520nm≤λ≤550nm) light-emitting diodes with special emphasis on the electron blocking layer, number of quantum wells, and thicknesses of quantum wells and barriers. We identified three crucial design rules for cubic green light-emitting diodes: (1) no need for an electron blocking layer; (2) use of a wide quantum well; and (3) choice of thin quantum barriers in multi-quantum well light-emitting diode designs. These design rules increase the internal quantum efficiency of cubic green light-emitting diodes by ∼30.5% under 100A/cm2 injection with respect to traditional designs. Overall, the design rules of cubic light-emitting diodes and their differentiating nature from the traditional, hexagonal ones are crucial for the advent of next-generation cubic light-emitting diodes.

Low-efficiency-droop c-plane InGaN/GaN light-emitting diodes through the use of thick single quantum wells and doped barriers

Efficiency droop at high current densities is a problem for InGaN-based light-emitting diodes (LEDs), especially for conventional c-plane devices. The large internal electric fields in c-plane quantum wells (QWs) lead to an increase in the active region carrier density (n), causing the electrical efficiency droop onset to occur at low current densities. Here, we present an approach to reduce the internal electric fields (Eint) in c-plane QWs by placing doped p-type and n-type GaN barriers close to the QW. The reduced Eint also allows a thick QW active region design, which helps to lower n to further reduce the droop. The concept of using doped barriers to control Eint is explained using theory and device simulations. Following that, multiple series of thick single QW (SQW) LEDs were grown and characterized. Key parameters in the epitaxial design such as the doping levels and the relative position of the doped barriers were systematically studied and optimized. By using optimized doped barriers and a thick SQW, c-plane LEDs with a low-efficiency droop of 14% at 300 A/cm2 [with respect to the peak external quantum efficiency (EQE)] and a high peak EQE of 49% were demonstrated.