Number Of Quantum Wells Research Articles

Dimensional downscaling and quantum engineering: A path to high-performance micro-LEDs

The investigation of the optoelectronic performance of AlGaN-based ultraviolet-C (UV-C) micro light-emitting diodes (μLEDs) emitting at 273 nm is carried out numerically by reducing the chip area from large LED (300 × 300 μm2) to μLED (25 × 25 μm2). However, due to the high surface to volume ratio of μLED, surface recombination becomes dominant that is generated due to robust sidewall defects. The enhanced current spreading in μLED further affects the carrier injection in the active region as the electrons and holes are captured by sidewall defects. These effects are more dominant at low current density in μLED while at high current density, the sidewall defects get saturated, and the surface recombination weakens. Various optimization strategies, such as quantum wells (QWs) width, quantum barriers (QBs) width, and QW number are carried out to study the effect on the performance of 25 × 25 μm2 UV-C μLED. These optimization strategies at low current density (0.1 A/cm2) further improved the electrical/optical properties of AlGaN-based UV-C μLEDs.

Well Number Dependence of Internal Quantum Efficiency in AlGaN Quantum Wells on Low‐Dislocation Sputtered AlN Templates

The internal quantum efficiency (IQE) and cathodoluminescence intensity line profile of AlGaN multiple quantum well (MQW) structures on low‐dislocation face‐to‐face annealed sputtered AlN (FFA Sp‐AlN) and on conventional metalorganic vapor‐phase epitaxy‐grown AlN (MOVPE‐AlN) templates are evaluated and the effect of the number of quantum wells (QWs) on the IQE is discussed. The higher IQE in the FFA samples is probably due to the lower threading dislocation (TD) density; however, the IQE also increases with the number of QWs although the TD density remains constant. Effective diffusion length increases with the number of QWs, indicating that the AlGaN MQW layer helps to suppress point defect diffusion, resulting in IQE increase. Furthermore, the segregation of point defects into the TDs and point defect diffusion via the TDs may explain the difference in the IQE improvement rate between the MQWs on the FFA Sp‐AlN and MOVPE‐AlN templates.

Optimizing performance and energy consumption in GaN(n)/InxGa1-xN/GaN/AlGaN/GaN(p) light-emitting diodes by quantum-well number and mole fraction

High performance and safe light-emitting devices (LEDs) are needed. Highly efficient IIIV nitride semiconductors are known for short-wavelength LEDs. Multiple-quantum well (MQW) are considered in LEDs. Influence of MQW and indium concentration on LED performance are studied here in GaN(n)/InxGa1-xN(i)/GaN(i)/AlGaN(p)/GaN(p) LEDs, where GaN(n) and GaN(p) have different dopants to formulate junctions, InxGa1-xN(i) is a 3 nm-thick intrinsic QW, GaN(i) is barrier intrinsic layer and AlGaN(p) is a 15 nm-thick electron blocking layer (EBL). Simulation is performed by Tcad-Silvaco. Current versus voltage (I-V) plots, luminosity power, band diagram, spectrum response, radiative recombination rate and electric field effect, are investigated to rationalize effects of InxGa1- xN(i) QW number and x. Increasing (x) improves radiative recombination rate, spectral power and band gap at less current. Devices with 6 quantum wells and x= 0.16 or 0.18 exhibit best performance. Minimizing x at 0.16, at high performance, is described.

Effects of parabolic barrier design for multiple GaAsBi/AlGaAs quantum well structures

The results of a comparative study on how the design of multiple quantum structures containing a parabolic barrier profile affects optical properties are presented. All quantum well (QW) structures were grown by molecular beam epitaxy (MBE) on semi-insulating GaAs substrates. The investigated samples consisted of (i) double parabolic quantum wells (type A) or (ii) multiple (two or three) rectangular quantum wells surrounded by parabolic barriers (type B). The optical quality of samples was characterized performing room-temperature (RT-PL) and temperaturedependent photoluminescence (TD-PL) measurements. The investigation aimed at the optimization of a multiple quantum well (MQW) structure design for application in the gain region of near infrared (NIR) laser diodes (LDs) revealed benefits of both double parabolic quantum wells and a mixed design (rectangular MQW with parabolic barriers). The PL band position for all samples was registered in the vicinity around 1.19 eV, which corresponds to the Bi content in QW of ~4.4%. It was shown that all structures of type A exhibit an intense emission, while the intensity of photoluminescence measured for the samples of type B depends on the number of QWs. The weaker intensity of the PL signal from two QWs inserted between parabolic barriers was explained by a larger point defect density at low temperature grown inner GaAs barriers. The room-temperature PL intensity of the structure with three GaAsBi QWs embedded in one parabolic AlGaAs barrier was the highest one. The shift of PL peak position to lower energies (1.16 eV) was attributed to the slightly higher bismuth concentration, 4.9%.

Diversity Analysis of Defect Distribution and Carriers Quantization in Cu 2 ZnSnS 4/Cu2ZnSn(S,Se) 4 Kesterite Quantum Well Solar Cell

Kesterite materials, with a major advantage of direct bandgap and earth abundant material,are popular for low cost and thin film solar cell applications. Despite the popularity, the material fails to achieve significant efficiency due to the emergence of defect states during the growth process. In this paper multiple approaches have been investigated focusing on implementation of Quantum Wells(QW) to enhance the performance of solar cell. QW structure are created using CZTS x Se 1−x and Cu 2 ZnSnS 4 as QW and barrier material layers respectively. The performance evaluation of the QW solar cell is also carried out with the presence of most popular donor-like defect states in a gaussian and tail distribution way. The behavior of the device is investigated in presence of broad range of QWs from 2 to 100QWs to ensure the effect of defects on rise in QWs. The worst case performance of the solar cell is obtained to be efficiency of 9.6% under a high level of defect states. A remarkable observation on the effect of increase in number of QWs and defect states on solar cell parameters solar cell are concluded.

Analytical Model of InP QWs for Efficiency Improvement in GaInP/Si Dual Junction Solar Cell

Quantum wells (QWs) are proven to enhance the short circuit current and voltage preservation measures help in efficiency enhancement with little degradation of open circuit voltage . Herein, an integrated GaInP/Si dual junction solar cell is proposed by incorporating the QWs in the top cell and carrier‐selective passivated contact at the bottom cell of the design. The QW increases the photon absorption of sub‐bandgap energies, buffer layer reduces the misfits between the lattice mismatched layers, and passivation mechanism improves the overall efficiency. Increasing the number of QWs increases the depletion width of QW region. This reduces the carrier lifetime () and increases the recombinations in the QW region. It results in the reduction in passivation quality and, therefore, reduces the open circuit voltage (). QW strain balancing and measures to reduce the threading dislocations has been considered. Also, the wide bandgap GaInP tunnel junction along with highly doped GaAs tunnel junction has been analyzed to observe the parasitic effect. The GaInP/Si‐integrated model achieves efficiency of 33.39% when the QWs are incorporated in the top cell. The effect of wide bandgap tunnel junction is also evaluated using GaInP tunnel junction.

Balancing the Number of Quantum Wells in HgCdTe/CdHgTe Heterostructures for Mid-Infrared Lasing.

HgCdTe-based heterostructures with quantum wells (QWs) are a promising material for semiconductor lasers in the atmospheric transparency window (3-5 μm) thanks to the possibility of suppressing Auger recombination due to the no-parabolic law of carrier dispersion. In this work, we analyze the thresholds of stimulated emission (SE) under optical pumping from heterostructures with a different number of QWs in the active region of the structure. Total losses in structures are determined from the comparison of thresholds for the different number of QWs in the active region. It is shown that, thanks to the increased modal gain, a higher number of QWs results in lower threshold pumping intensity and, consequently, higher temperature of SE. These results indicate that improvements to the modal gain can result in a moderate uplift in the temperature of SE from mid-infrared HgCdTe-based heterostructures. On the other hand, at a high enough QW count threshold, the intensity no longer depends on the number of the QWs and is determined by the transparency concentration of a single QW.

Plasmon gain in HgTe/CdHgTe multi-quantum-well heterostructures

The work is devoted to the theoretical study of plasmon gain in HgTe/CdHgTe multi-quantum-well heterostructures. The spectra of plasmons and plasmon gain are found in structures with 2–8 quantum wells (QWs) under the condition of inverse band population. A nonmonotonic increase in the plasmon gain with an increase in the number of QWs is shown. The dependence of the threshold concentration of nonequilibrium carriers for stimulated plasmon emission on the number of QWs in structures with 1–8 QWs has been studied.

Bandgap engineering, monolithic growth, and operation parameters of GaSb-based SESAMs in the 2–2.4 µm range

We present the detailed growth and characterization of novel GaSb-based semiconductor saturable absorber mirrors (SESAMs) operating in the 2–2.4 µm spectral range. These SESAMs at different wavelengths are bandgap engineered using ternary material compositions and without strain compensation. We observe that even when the thickness of quantum wells (QWs) exceeds the critical thickness we obtain strain relaxed SESAMs that do not substantially increase nonsaturable losses. SESAMs have been fabricated using molecular beam epitaxy with a AlAs0.08Sb0.92/GaSb distributed Bragg reflector (DBR) and strained type-I InxGa1-xSb or type-II W-like AlSb/InAs/GaSb QWs in the absorber region. All the type-I SESAMs show excellent performance, which is suitable for modelocking of diode-pumped semiconductor, ion-doped solid-state, and thin-disk lasers. The recovery time of the type-II SESAM is too long which can be interesting for laser applications. The dependence of the SESAM design, based on its QW number, barrier material, and operation wavelength are investigated. A detailed characterization is conducted to draw conclusions from macroscopic nonlinear and transient absorption properties at different wavelengths in the 2–2.4 µm range for the corresponding devices.

Vertical hole transport through unipolar InGaN quantum wells and double heterostructures

We report unipolar hole transport through unintentionally doped (UID) c-plane Ga-polar InGaN heterostructures to investigate the impact of alloy disorder on vertical transport. Simulations and experimental investigations were conducted on unipolar InGaN double heterostructures (DHs) and quantum well (QW) test structures by varying thicknesses and the number of QWs. The structures were simulated using one-dimensional (1D) and three-dimensional (3D) algorithms, incorporating the effects of random alloy disorder in the 3D model using the newly developed Localization Landscape theory. The electrical polarization discontinuity between GaN and InGaN results in a significant barrier for vertical carrier transport. Band-diagram and current density-voltage simulations indicate asymmetric polarization barriers to the vertical hole transport for the InGaN DHs. For the QW structures, however, the simulations indicate a symmetric barrier to the hole transport in forward and reverse bias. In the case of the InGaN DH layers, the 3D simulation results indicate a smaller barrier to hole transport compared to 1D simulations. For QW simulations, the barriers were found to be the same in both 1D and 3D simulations. The simulation results are experimentally verified using unipolar p-type vertical transport structures, enabled by n-to-p tunnel junctions to facilitate the current spreading within the bulk material for the mesa structures grown by ammonia-molecular beam epitaxy. The results indicate that increasing the UID ${\mathrm{In}}_{0.1}{\mathrm{Ga}}_{0.9}\mathrm{N}$ DH layer thickness from 15 to 30 nm increases the forward bias voltage drop (\ensuremath{\sim}2 V at $500\phantom{\rule{0.16em}{0ex}}\mathrm{A}/\mathrm{c}{\mathrm{m}}^{2}$) more than the reverse bias voltage drop (\ensuremath{\sim}0.2 V at $500\phantom{\rule{0.16em}{0ex}}\mathrm{A}/\mathrm{c}{\mathrm{m}}^{2}$). For the QW structures, increasing the number of QWs from one to three increases the voltage penalty similarly in forward and reverse directions (\ensuremath{\sim}0.25 V per QW at $500\phantom{\rule{0.16em}{0ex}}\mathrm{A}/\mathrm{c}{\mathrm{m}}^{2}$). The results are beneficial in understanding the impact of alloy disorder on the transport properties of the III-nitride heterostructures.

Electro-Optical Numerical Modeling for the Design of UVA Nitride-Based Vertical-Cavity Surface-Emitting Laser Diodes

Design rules are proposed for UVA vertical-cavity surface-emitting laser diodes, and the modal gain and cavity properties are analyzed using an electro-optical numerical model. To achieve a low threshold current, it is important to align the gain-peak and resonance wavelengths, but the quantum-confined Stark effect makes that difficult because the former wavelength shifts with changing current density. The relationship between the gain peak and the injection current is discussed. Also, the quantum-well design affects the gain characteristics, and the gain properties with different quantum-well number, composition, and width are discussed.

Performance Improvement of Quantum Well Infrared Photodetectors Through Dark Current Reduction Factor

A device model for quantum well-infrared photodetectors (QWIPs) is studied. The developed model accounts for the self-consistent potential distribution. An analytical model for the calculation of dark current reduction factor (DCRF) and the signal to noise ratio (SNR) is presented using the Matlab environment. The aim is to reduce the dark current and improve the SNR by adjusting the device parameters such as spacing between the wells, bias voltage, and the operating temperature of these detectors. This model takes into account the donor concentration in the barriers and the concentration of electrons above the barriers that have little attention in the literature. In these devices, decreasing the amount of DCRF and increasing the SNR represents the main goal for the device improvements. From the proposed model, we concluded that the contact layer and space-charge slightly affect the dark current in QWIP with a large number of quantum wells (QWs). These effects are significant in the case of a moderate number of QWs.

Lateral carrier injection for the uniform pumping of several quantum wells in InGaN/GaN light-emitting diodes.

Most optoelectronic devices share the same basic epitaxial structure - a stack of quantum wells (QWs) sandwiched between p- and n-doped layers. In nitride semiconductors, where holes have 20-times lower mobility than electrons, the holes are able to populate only the topmost 1-2 QWs. The inability to distribute the holes in a large-enough number of QWs is a cause of high Auger recombination in nitride LEDs. Lateral carrier injection is an alternative design, in which the doped regions are situated at the sides of the QW stack and the carriers diffuse horizontally into the QWs. Given that the carriers are injected into all available QWs, it finally makes sense to grow structures with a large number of QWs. We report the results of our computer simulations, which explore the advantages of LCI-based LEDs in terms of energy efficiency.

High Efficiency Inverted GaAs and GaInP/GaAs Solar Cells With Strain‐Balanced GaInAs/GaAsP Quantum Wells

AbstractHigh‐efficiency solar cells are essential for high‐density terrestrial applications, as well as space and potentially vehicle applications. The optimum bandgap for the terrestrial spectrum lies beyond the absorption range of a traditional dual junction GaInP/GaAs cell, with the bottom GaAs cell having higher bandgap energy than necessary. Lower energy bandgaps can be achieved with multiple quantum wells (QWs), but such a pathway requires advanced management of the epitaxial growth conditions in order to be useful. Strain‐balanced GaAsP/GaInAs QWs are incorporated into a single junction GaAs solar cell and a dual junction GaInP/GaAs solar cell, leading to 27.2% efficiency in the single junction device and a one‐sun record 32.9% efficiency in the tandem device. Good carrier collection and low non‐radiative recombination are observed in the cells with up to 80 QWs. The GaAs cells employ a rear‐heterojunction architecture to boost the open‐circuit voltage to over 1.04 V in the quantum well device, despite the large number of QWs.

AlGaInAs/InP Based Five & Three Quantum Wells Mode Locked Laser Diodes: A Comparative Study

Comparison of performance of semiconductor mode-locked laser diodes fabricated using AlGaInAs/InP material containing 5 and 3 quantum wells (QWs) inside the active region is reported. The simulations and experimental results show that lasers containing five QWs materials produce larger beam divergence and temporally broader optical pulses. For improvement in the mode-locking of lasers and reducing the far-field pattern, the number of QWs inside the active region was decreased from five to three and a far-field decreasing layer along with a thick spacer layer were introduced in the n-cladding region of epitaxial material. Before growing the material, simulations were carried out to optimise the design. The lower optical confinement factor and higher gain saturation energy of three QWs based mode-locked lasers provide higher average and peak output power, reduced and symmetric far-field pattern, better radio frequency (RF) spectra, shorter optical pulses, and stable optimal mode-locking for a wide range of gain current and saturable absorber reverse voltage.

Biaxial strain effects on the band structure and absorption coefficient of GaAs1-x-yNxBiy/GaAs MQWs calculated using k.p method

In this work, we derive the 16 band strain-dependent k·p Hamiltonian using finite difference method for dilute bismide-nitride compound GaNAsBi-based strained quantum wells (QWs). The effect of the biaxial strain (-0.57≤ε∥≤0.50%) on electronic band structure of GaNAsBi/GaAs double QWs (DQWs) operating at the near-infrared spectrum and telecommunications window (1.3 and 1.55 μm) are examined. We use the band anti-crossing model, envelop function formalism and Bir-Pikus theory in conjugation with k·p perturbation method to describe the optoelectronics properties (band offsets, absorption coefficient) behavior of the DQWs structures. Results show that the tensile strained GaNAsBi/GaAs DQWs can be used for the detection of 1, 0.95, and 0.80 eV energy range. This device seem be differentiated by higher carrier confinement and optical absorption compared to compressive strained one. The enhancement of spectral response is analyzed by increasing the QWs number and the coupling between them.

Effects of number of quantum wells and Shockley-Read-Hall recombination in deep-ultraviolet light-emitting diodes.

Effects of the number of quantum wells (QWs) and Shockley-Read-Hall (SRH) recombination in deep-ultraviolet (DUV) light-emitting diodes (LEDs) are investigated theoretically. Simulation results show that, for DUV LEDs with high crystalline quality, light output power increases with an increasing number of QWs. As for the DUV LEDs with poor crystalline quality, light output power may decrease with an increasing number of QWs due to the deteriorated SRH recombination. The injection current density is also an important factor regarding the impact of the number of QWs. When operated at low current density, for the DUV LED with poor crystalline quality, light output power may decrease with an increasing number of QWs.

Optical Properties of InGaN/GaN Multiple Quantum Well Structures Grown on GaN and Sapphire Substrates

A set of samples based on InGaN/GaN multiple quantum well (QW) structures with an extremely thick active region and high QW numbers was prepared on different substrates by metal organic vapor phase epitaxy. Their morphology was studied by SEM images, and their optical properties, including photoluminescence (PL) excitation, PL emission spectra, and PL decay curves, were measured by means of time-resolved luminescence spectroscopy. The obtained spectral features were tentatively explained by using the band scheme, which is commonly used for the description of InGaN/GaN multiple QW (MQW) structures. The application potential in the field of nanoscintillators used for fast timing of studied structures is discussed as well.

Barriers to carrier transport in multiple quantum well nitride-based c -plane green light emitting diodes

The presence of alloy disorder in III-nitride materials has been demonstrated to play a significant role in device performance through effects such as carrier localization and carrier transport. Relative to blue light emitting diodes (LEDs), these effects become more severe at green wavelengths. Because of the potential fluctuations that arise due to alloy disorder, full three-dimensional (3D) simulations are necessary to accurately relate materials properties to device performance. We demonstrate experimentally and through simulation that increased quantum well (QW) number in $c$-plane green LEDs contributes to excess driving voltage, and therefore reduced electrical efficiency. Experimentally, we grew an LED series with the number of QWs varying from one to seven and observed a systematic increase in voltage with the addition of each QW. Trends in LED electrical properties obtained from 3D simulations, which account for the effects of random alloy fluctuations, are in agreement with experimental data. Agreement is achieved without the need for adjusting polarization parameters from their known values. From these results, we propose that the polarization induced barriers at the GaN/InGaN (lower barrier/QW) interfaces and the sequential filling of QWs both contribute significantly to the excess forward voltage in multiple QW $c$-plane green LEDs.

Total-InGaN-thickness dependent Shockley-Read-Hall recombination lifetime in InGaN quantum wells

The mechanism behind the quantum-well-width dependent Shockley-Read-Hall (SRH) recombination lifetime is investigated in the InGaN/GaN quantum wells (QWs). According to the literature, the strong dependence of SRH lifetime on QW width is proposed to originate from the electron-hole separation in c-plane QWs, just as the radiative recombination. However, in this work, by temperature dependent steady-state time-resolved photoluminescence experiment, it is found that besides the QW width, the SRH lifetime also increases significantly with increasing QW number, which cannot be explained by the electron-hole separation. The two kinds of dependences of SRH lifetime can be attributed to the same source, judging from their similar activation energies, which is the existence of indium atoms compensating the SRH recombination centers generated during the epitaxy and thereby prolonging the SRH lifetime. The density of SRH centers deduced from our analysis decreases with the total thickness of the InGaN layer in a consistent manner for both the QW-width dependent and QW-number dependent sets of samples.