Electron-hole Wavefunction Overlap Research Articles

(Invited) Ultrahigh Efficiency Excitonic Micro-LEDs

To date, it has remained challenging to achieve high efficiency micro-LEDs, which are desired for next generation displays and virtual/augmented reality. The performance degradation with reducing dimensions for conventional LEDs has been largely attributed to the plasma damage induced on the mesa sidewalls. Here, we show such a fundamental challenge can be overcome by developing nanowire excitonic LEDs. In this study, InGaN/GaN nanowire LED heterostructures were grown on N-polar GaN-on-sapphire substrates utilizing selective area epitaxy. Studies have shown that the exciton binding energy in strain-relaxed InGaN nanostructures can be dramatically enhanced. Due to the strong Coulombic interaction between electrons and holes, the impact of nonradiative SRH recombination on the performance of an excitonic LED is significantly reduced. By exploiting the large exciton oscillator strength of quantum-confined nanostructures, we have achieved an external quantum efficiency (EQE) exceeding 25% for a green-emitting device with lateral dimensions less than 1 micrometer, which is the highest value reported for any LEDs of this size to the best of our knowledge. It is further observed that the EQE and output power of the excitonic LED show a significantly faster rising trend with current injection, compared to conventional devices. We have identified several important factors for achieving high efficiency excitonic LEDs, including the epitaxy of nanostructures to achieve strain relaxation and reduced dislocation densities and the formation of three-dimensional quantum-confinement to enhance electron-hole wavefunction overlap. We have further investigated the effect of Mg incorporation in p-GaN layer on the device performance. We found out that an optimized Mg dopant incorporation in nanowire structures can contribute significantly to the efficiency of micro-LEDs. We have measured significantly enhanced performance by optimizing Mg dopant incorporation, which leads to reduced leakage current and improved EQE. For example, for a red-emitting micro-LED, a peak external quantum efficiency of >8% was measured for a device with a lateral dimension less than one micrometer by optimizing Mg-dopant incorporation. Through these studies, we show that by harnessing the exciton oscillator strength of nearly dislocation-free nanostructures, together with a careful optimization of p-doping in the contact layer, the efficiency bottleneck of micro-LEDs can be addressed.Conflict of interest: Some IP related to this work has been licensed to NS Nanotech, Inc., which was co-founded by Z. Mi. The University of Michigan and Mi have a financial interest in the company.

Enhancing the performance of InGaN-based near-infrared light-emitting diodes under a weakly polarized electric field

This work presents a simulation analysis of nitride multi-quantum-well near-infrared light-emitting diodes (LEDs) with an emission wavelength of 1300 nm using Crosslight-APSYS software. The polarization electric field at the quantum wells was effectively tuned by adopting an Al0.02Ga0.19In0.79N quaternaries compound as the barrier for the In0.78GaN quantum wells, resulting in a near-zero polarization electric field. This approach significantly reduced band bending and increased the overlap of electron-hole wave functions within the quantum wells. Consequently, the internal quantum efficiency (IQE) in the nitride near-infrared LED improved, and the droop effect was significantly reduced. The Auger recombination mechanism that influences the droop effect was further analyzed. This provides an efficient approach for the development of high-performance InGaN-based near-infrared light-emitting diodes with high indium content.

Integrative approach of machine learning and symbolic regression for stability prediction of multicomponent perovskite oxides and high-throughput screening

This work unfolds a robust and interpretable strategy for evaluating the stability and potential photovoltaic application of 6526 multicomponent perovskite oxides, employing a synergetic methodology that intertwines advanced machine learning (ML) algorithms and symbolic regression based on genetic programming. Initially, ML algorithms, namely XGBoost, LightGBM, and random forest, were harnessed, with elemental oxidation state and electronegativity serving as input features, achieving R2 values of 0.98, 0.98, and 0.74, respectively, on the test set for predicting the formation enthalpy, a criterion for perovskite stability. Despite the amplified interpretability offered by SHAP analysis, the inherent “black-box” nature of ML obfuscates a transparent understanding of intrinsic relations between input features and performance. To surmount this, symbolic regression introduced not only elucidates a clear functional relationship between input features and perovskite stability but also achieves a commendable R2 of 0.79 on the test set. Subsequent high-throughput screening, based on perovskite stability ranking, designated the top 500 stable perovskites for band gap calculation using the PBE functional, wherein DyNdHf2O6, CeEuAl2O6, and CeSrAl2O6 emerged as potential candidates for photovoltaic applications and were subjected to further electronic structure simulations employing the HSE06 functional, encompassing density of states, band structure, charge density, and optical absorption spectra. Ultimately, CeEuAl2O6, boasting an optical direct bandgap of 2.31 eV and minimal electron-hole wavefunction overlap, stands out as the prime choice for photovoltaic materials. This research not only pioneers the exploration of enhancing the interpretability of ML but also propels theoretical guidance for the evolution of photovoltaic cells by bridging predictive modeling with high-throughput screening.

Theoretical Analysis of GeSn Quantum Dots for Photodetection Applications.

GeSn alloys have recently emerged as complementary metal-oxide-semiconductor (CMOS)-compatible materials for optoelectronic applications. Although various photonic devices based on GeSn thin films have been developed, low-dimensional GeSn quantum structures with improved efficiencies hold great promise for optoelectronic applications. This study theoretically analyses Ge-capped GeSn pyramid quantum dots (QDs) on Ge substrates to explore their potential for such applications. Theoretical models are presented to calculate the effects of the Sn content and the sizes of the GeSn QDs on the strain distributions caused by lattice mismatch, the band structures, transition energies, wavefunctions of confined electrons and holes, and transition probabilities. The bandgap energies of the GeSn QDs decrease with the increasing Sn content, leading to higher band offsets and improved carrier confinement, in addition to electron-hole wavefunction overlap. The GeSn QDs on the Ge substrate provide crucial type-I alignment, but with a limited band offset, thereby decreasing carrier confinement. However, the GeSn QDs on the Ge substrate show a direct bandgap at higher Sn compositions and exhibit a ground-state transition energy of ~0.8 eV, rendering this system suitable for applications in the telecommunication window (1550 nm). These results provide important insights into the practical feasibility of GeSn QD systems for optoelectronic applications.

The effective prolongation of the excited-state carrier lifetime of CsPbI2Br with applying strain.

In recent years, all-inorganic perovskites CsPbX3 (X = Cl, Br, I) have emerged as excellent candidates for solar cells due to their remarkable thermal stability and suitable bandgaps. Among them, CsPbI2Br is a hotspot in perovskite material research currently. Non-radiative electron-hole recombination often leads to significant energy losses, impacting the efficiency of solar cells, so a thorough understanding of carrier recombination mechanisms is crucial. Our work investigated the carrier recombination dynamics in detail and proved that strains can effectively reduce nonradiative recombination. In this study, using first-principles calculations combined with nonadiabatic (NA) molecular dynamics (MD), we demonstrate that applying 2% tensile and 2% compressive strains to CsPbI2Br can modify the bandgap, induce moderate disorder, reduce the overlap of electron-hole wavefunctions, decrease NA coupling, and shorten decoherence time, thereby minimizing non-radiative recombination and extending the carrier lifetime. Especially the 2% tensile strain exhibits more effective control performance, significantly reducing non-radiative electron-hole recombination and extending the charge carrier lifetime to 14.59 ns, nearly five times that of the pristine CsPbI2Br system (3.12 ns). This study reveals the impact mechanism of strain on carrier behavior in perovskite solar cells, providing a new non-chemical strategy for modulating the lifetime of photo-generated carriers and enhancing the efficiency of all-inorganic perovskite solar cells.

MOCVD grown ultraviolet microlight-emitting diodes with an InGaN/GaN/AlGaN active region design

In this work, we present an InGaN/GaN/AlGaN active region design for high-efficiency UV LED grown by MOCVD. The use of the GaN layer allows the AlGaN layer to be grown in optimized conditions which will enhance the overall crystal quality and device performance. The InGaN/GaN/AlGaN structure showed an improvement in the internal quantum efficiency (IQE) simulation in comparison with the conventional InGaN/AlGaN structure, with 15% and 14% at operating current density of 20 and 30 A/cm2, respectively. The IQE advantage remains at more than 10% in the operating current density range from 0 to 100 A/cm2 due to an enhanced electron-hole wavefunction overlap. The InGaN/GaN/AlGaN quantum wells were utilized to grow UV-A structures and micro-LEDs with sizes ranging from 20 × 20 to 100 × 100 μm2 were fabricated. The fabricated micro-LEDs showed a reliable emission at around 372 nm with 9 nm FWHM. Micro-LED devices with smaller sizes showed an improvement in relative external quantum efficiency due to its advantage in current spreading. This manuscript presents the potential to achieve both improved crystal quality and high-efficiency device using InGaN/GaN/AlGaN active region design.

Interface Engineering and Electron-Hole Wave Function Overlap of InAs/AlSb Superlattice Infrared Detectors

InAs/AlSb is a material system that can be used as a low-noise avalanche detector and operates in the short-wave infrared band. The interface parameters determine the wave function overlap (WFO). Maximizing the WFO of InAs/AlSb superlattices improves the quantum efficiency (QE) of infrared avalanche photodetectors (APDs). However, this remains a huge challenge. Here, the 8-band k·p perturbation method based on Bloch wave envelope function approximation was used to calculate the energy level structure of InAs/AlSb superlattices. The results indicate that the WFO is enhanced with increasing InSb interface thickness or when the InSb (or AlAs) interface is far from the intersection of InAs and AlSb. As the AlAs interface thickness increases, the WFO enhances and then reduces. The maximum increase in WFO is 15.7%, 93%, and 156.8%, respectively, with three different models. Based on the stress equilibrium condition, we consider the interface engineering scheme proposed for enhancing WFO with an increase of 16%, 114%, and 159.5%, respectively. Moreover, the absorption wavelength shift is less than ±0.1 μm. Therefore, the interface layer thickness and position can be designed to enhance the WFO without sacrificing other properties, thereby improving the QE of the device. It provides a new idea for the material epitaxy of APDs.

Substrate Ferroelectric Proximity Effects Have a Strong Influence on Charge Carrier Lifetime in Black Phosphorus.

By stacking monolayer black phosphorus (MBP) with nonpolarized and ferroelectric polarized bilayer hexagonal boron nitride (h-BN), we demonstrate that ferroelectric proximity effects have a strong influence on the charge carrier lifetime of MBP using nonadiabatic (NA) molecular dynamics simulations. Through enhancing the motion of phosphorus atoms, ferroelectric polarization enhances the overlap of electron-hole wave functions that improves NA coupling and decreases the bandgap, resulting in a rapid electron-hole recombination completing within a quarter of nanoseconds, which is two times shorter than that in nonpolarized stackings. In addition to the dominant in-plane Ag2 mode in free-standing MBP, the out-of-plane high-frequency Ag1 and low-frequency interlayer breathing modes presented in the heterojunctions drive the recombination. Notably, the resonance between the breathing mode within bilayer h-BN and the B1u mode of MBP provides an additional nonradiative channel in ferroelectric stackings, further accelerating charge recombination. These findings are crucial for charge dynamics manipulation in two-dimensional materials via substrate ferroelectric proximity effects.

Great Influence of Pressure and Isotope Effects on Nonradiative Charge Loss in Hybrid Organic-Inorganic Perovskites.

The intrinsic softness of hybrid organic-inorganic perovskites (HOIPs) allows their lattice and optoelectronic performance to be tunable to external pressure. Using nonadiabatic (NA) molecular dynamics, we demonstrate that a mild pressure accelerates hot electron relaxation and suppresses nonradiative electron-hole recombination in CH3NH3PbI3. Both processes are governed by NA coupling, which is enhanced between the electronic states of the quasi-continuous bands while is decreased between the band-edge states by reducing the electron-hole wave function overlap. Hydrogen/deuterium isotope exchange alleviates the pressure-induced NA coupling by increasing lattice rigidity and decreasing wave function overlap, slowing down both the hot electron relaxation and electron-hole recombination processes. The simulated time scales of sub-3 ps for hot electron relaxation and half nanoseconds for recombination agree well with the experiments. The study suggests that the isotope exchange can mitigate the pressure-caused fast losses of hot electrons and further prolong the charge carrier lifetime in HOIPs.

Improvement in quantum efficiency of green GaN-based micro-LED by trapezoidal quantum well

The main challenges in the application of GaN-based full-color micro-light-emitting diodes (micro-LEDs) are size-dependent effects and low quantum efficiency, especially for green micro-LEDs. In this study, we propose to use a trapezoidal quantum well (QW) structure in GaN-based green micro-LEDs. The trapezoidal QW structure is able to improve the electron hole wave function overlap, thus increasing the external quantum efficiency (EQE). Green micro-LEDs with active region size of 75 × 64 μm2 are fabricated by metal organic chemical vapor deposition. The peak EQE of the green trapezoidal QW micro-LED reaches 15.8% at a low injection current density of 0.6A/cm2, which is 1.96 times higher than that of conventional rectangular QW. The test results indicate that the trapezoidal QW structure greatly improves the quantum efficiency of green micro-LED, inhibits the blue shift of peak wavelength and improves color purity, which paves the way to single substrate full-color micro-LED display with high-performance. The technology proposed in this paper does not change the production line equipment, but improves the traditional process, so it is expected to significantly improve the external quantum efficiency of the existing green micro-LED under the existing equipment. The production technology of existing InGaN QW micro-LED is directly adopted, and the EQE can be further improved if some new dry etching and passivation methods are applied.

Significant Quantum Efficiency Enhancement of InGaN Red Micro-Light-Emitting Diodes with a Peak External Quantum Efficiency of up to 6%

We demonstrate a significant quantum efficiency enhancement of InGaN red micro-light-emitting diodes (μLEDs). The peak external quantum efficiency (EQE) of the packaged 80 × 80 μm2 InGaN red μLEDs was largely increased to 6.0% at 12 A/cm2, representing the significant process in exploring the efficiency of InGaN red μLEDs. The improvement of the EQE is attributed to the significant enhancement of the quantum efficiency, which is confirmed by the electron–hole wavefunction overlap in the InGaN quantum well from the band gap simulation and the photoluminescence intensity ratio at room temperature/low temperature. Ultrasmall 5 × 5 μm2 InGaN red μLEDs were also obtained, which show a high peak EQE of 4.5%. This work demonstrates a simple approach to achieving highly efficient InGaN red μLEDs, which are very promising candidates for ultrasmall red μLEDs required by AR/VR displays.

Design of InGaN-ZnSnGa2N4 quantum wells for high-efficiency amber light emitting diodes

A novel type-II InGaN-ZnSnGa2N4 quantum well (QW) structure is proposed based on recent experimental achievements for the successful epitaxy of ZnSnN2-GaN alloys and the determination of their band offsets with GaN. The simulation results indicate that this structure is promising as the active region for high-efficiency InGaN-based amber (λ ∼ 590 nm) light-emitting diodes (LEDs). The hole wavefunction in the valence band is better confined with the insertion of a monolayer scale of ZnSnGa2N4 into the InGaN QW while the electron wavefunction in the conduction band is better confined with the incorporation of an AlGaN layer in the GaN quantum barrier. The band structure of the InGaN-ZnSnGa2N4 QW is numerically simulated based on the experimentally measured band offsets between ZnSnGa2N4 and GaN. With the InGaN-ZnSnGa2N4 QW design, a low In content (20%) is required in the InGaN layer to reach a peak emission wavelength of ∼590 nm, yet an In composition of 25% is needed to reach the same emission wavelength for a conventional InGaN QW with the same layer thicknesses. Moreover, the electron-hole wavefunction overlap (Гe1−hh1) for the InGaN-ZnSnGa2N4 QW design reaches 18% for an emission wavelength at ∼590 nm. This result is much improved over the conventional InGaN QW overlap of 5% emitting at the same wavelength. The increase in electron-hole wavefunction overlap results in an approximately 14 times enhancement in the predicted spontaneous emission radiative recombination rate of the InGaN-ZnSnGa2N4 QW as compared to that of the conventional InGaN QW. This InGaN-ZnSnGa2N4 QW structure design can be promising to pave a new way to achieve high efficiency amber LEDs.

Impact of Interfaces on Photoluminescence Efficiency of High-Indium-Content (In,Ga)N Quantum Wells

$(\mathrm{In},\mathrm{Ga})\mathrm{N}$-based light-emitting diodes (LEDs) are known to suffer from low electron-hole wave-function overlap due to a high piezoelectric field. Staggered $(\mathrm{In},\mathrm{Ga})\mathrm{N}$ quantum wells (QWs) are proposed to increase the wave-function overlap and improve the efficiency of LEDs, especially for long-wavelength emitters. In this work, we show evidence that the growth of staggered QWs by plasma-assisted molecular beam epitaxy has another beneficial effect, as it allows a reduction in the formation of defects, responsible for nonradiative Shockley-Read-Hall recombination, at the bottom interface of the QW. Staggered QWs comprised an $(\mathrm{In},\mathrm{Ga})\mathrm{N}$ layer of an intermediate $\mathrm{In}$ content between the barrier and the QW. We show that insertion of such a layer results in a significant increase of the luminescence intensity, even if the calculated wave-function overlap drops. We study the dependence of the thickness of such an intermediate-$\mathrm{In}$-content layer on photoluminescence intensity behavior. Staggered QWs exhibit increased cathodoluminescence homogeneity that is a fingerprint of a lower density of defects, in contrast to standard QWs for which a high density of dark spots is observed in QW-emission mapping. Transmission electron microscopy of standard QWs reveals the formation of basal-plane stacking faults and voids that can result from vacancy aggregation. A stepwise increase of the $\mathrm{In}$ content in staggered QWs prevents the formation of point defects and results in an increased luminescence efficiency. The $\mathrm{In}$-composition difference between the barrier and the well is, therefore, a key parameter to control the formation of point defects in the high-$\mathrm{In}$-content QWs, influencing the luminescence efficiency. Characteristics of the cyan laser diode (LD) utilizing staggered QWs are presented.

Study on Modulation Bandwidth of GaN-Based Micro-Light-Emitting Diodes by Adjusting Quantum Well Structure.

GaN-based blue micro-light-emitting diodes (μ-LEDs) with different structures were designed, of which the effect of quantum well (QW) structure on modulation bandwidth was numerically explored. By using trapezoidal QWs, the quantum-confined Stark effect (QCSE) can be reduced, leading to an enhanced electron-hole wave function overlap, thereby increasing the recombination rate and reducing the differential carrier lifetime. In addition, the improved hole transport also creates favorable conditions for shortening the differential carrier lifetime. Furthermore, by comparing with traditional μ-LEDs with different thicknesses of QW, the modulation bandwidth of μ-LEDs with trapezoidal QWs exhibits a large advantage at lower current densities of below 2 kA/cm2.

(Invited) Molecular Beam Epitaxy and Dislocation Dynamics of Metamorphic Inassb for Long-Wavelength Infrared Applications

Long-wavelength infrared (IR) III-V based devices have long been of interest for potential applications such as chemical sensing and large format IR imaging. Within the III-V family, only the InAs1-xSbx bulk alloy in composition range 0.45 ≤ x ≤ 0.8, offers the required bandgap energy (Eg ) from 100 to 125 meV at an operating temperature of 80K and below [1]. Over this composition range however, the InAs1-xSbx lattice constant varies from 6.24 to 6.39 Å, where a lack of conventional substrates has restricted progress on the growth and study of this material system. To bridge this lattice-constant gap while maintaining relatively low defect densities, we employed a metamorphic step-graded InAs1-xSbx buffer on GaSb, enabling the study of low-Eg InAs1-xSbx as a function of growth conditions. Using this method, we investigated the effect of substrate temperature (Tsub ) and group V to group III flux ratio (beam equivalent pressure, V/III) on Sb incorporation of the lowest-Eg cap layer [2]. We also used x-ray reciprocal space mapping (RSM) to examine the effect of growth conditions on strain and dislocation dynamics. Following these growth studies, we employed the metamorphic InAs1-xSbx in an InAs/InAsSb superlattice designed with a cutoff wavelength of 9 µm which leads to improved absorption compared with the lattice-matched counterpart.We first grew, via molecular beam epitaxy, several InAs1-xSbx step-graded structures in which the Sb/(As+Sb) flux ratio was varied from 0.05 to 0.50 in 0.05 increments (see figure), under various Tsub and V/III. Nomarski imaging revealed smoother surfaces under a V/III=10, the highest ratio we attempted. At this higher V/III, we observed the cross-hatch morphology expected for metamorphic materials and found that the cross-hatch spacing changes, implying a change in dislocation dynamics, with Tsub . We then used photoluminescence (PL) to measure the Sb-content in the cap layer as well as compare intensities between samples. We found the highest Sb-incorporation to occur when Tsub =415 C and V/III=10, while the most intense samples used Tsub =415-430 C and V/III=10 [2].Using RSM along [110] with (004) and (115) reflections, we identified the Sb composition in each layer. This allowed comparison of Sb-content as a function of Sb/(As+Sb) for various Tsub and V/III. The results suggest that V/III has little effect on Sb incorporation, in direct conflict with our previous PL results [2]. To understand the discrepancy between PL and RSM, we measured (004) RSM of the same three samples with the x-ray beam incident along [1-10], revealing extremely different strain relaxation compared to the [110] case (see figure). Asymmetric strain relaxation has been observed in other III-V graded buffer systems and has been explained by different dislocation formation energies and glide velocities along each direction resulting from the core structure of the dislocation being terminated with either a group-III or a group-V element [3]. Transmission electron microscopy is ongoing to further understand the dislocation dynamics in these samples.Taking this all together allowed us to investigate the effect of substrate lattice-constant on strain-balanced InAs/InAsSb superlattices designed for 9 µm cutoff wavelength [4]. Theoretically, by using a larger substrate lattice-constant the superlattice design results in larger electron-hole wavefunction overlap, ultimately increasing photon absorption. Our experimental results confirm this theory, even in the presence of increased threading dislocations inherent to the required lattice-mismatch.[1] I. Vurgaftman et al. JAP 89, 5815-5875 (2001).[2] Tomasulo et al. J. Vac. Sci. and Technol. B 36, 02D108 (2018).[3] France et al. J. Appl. Phys. 107, 103530 (2010); Gelczuk et al., J. Cryst. Growth 310, 3014 (2008).[4] Affouda, Tomasulo et al., Appl. Phys. Lett. 110, 181107 (2017). Figure 1

The trilayer exciton emission in WSe2/WS2/MoS2 van der Waals heterostructures

In this Letter, we present the experimental observation of trilayer exciton emission in the WSe2/WS2/MoS2 heterostructures by photoluminescence spectroscopy at a low temperature of ∼10 K. It is found that the trilayer exciton has a lower exciton binding energy than that in the WSe2/MoS2 heterobilayer due to the insertion of monolayer WS2, which is manifested by the blueshift of this trilayer exciton peak relative to the bilayer exciton. Through the measurements of the exciton peak position as a function of excitation power, the trilayer excitons exhibited the density-dependent repulsive interaction among them, and the reduction of binding energy ∼8 meV is deduced. Moreover, the trilayer exciton has a longer lifetime than the bilayer exciton, resulting from the reduction of electron–hole wavefunction overlap.

C-Plane Blue Micro-LED With 1.53 GHz Bandwidth for High-Speed Visible Light Communication

A high-speed c-plane InGaN/GaN blue micro-scale light-emitting diodes (Micro-LEDs) with a pixel diameter of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$60~\mu \text{m}$ </tex-math></inline-formula> , and a high −3 dB modulation bandwidth of 1.53 GHz at the current density of 5996 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> were demonstrated. The ultra-thin quantum well (QW) structure with InGaN layer of 1 nm was designed, and optimized variable temperature growth processes were adopted to increase In incorporation and localization effect in the quantum wells, which greatly reduce the polarization field and increase the overlap of electron-hole wave functions. The differential carrier lifetime was effectively shortened, increasing the modulation bandwidth of the Micro-LED. And a data rate of 5.27 Gbps with a bit error rate (BER) of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.1\times 10^{-{3}}$ </tex-math></inline-formula> was demonstrated with orthogonal frequency-division multiplexing (OFDM) signal for a 1 m free space real-time visible light communication (VLC) based on such high-speed micro-LED. Promising results in this work may support a great development of high-speed VLC applications.

Interplay between Auger recombination, carrier leakage, and polarization in InGaAlN multiple-quantum-well light-emitting diodes

In conventional hexagonal InGaAlN multiple-quantum-well (MQW) (h-) light-emitting diodes (LEDs), carrier leakage from QWs is the main source of internal quantum efficiency (IQE) degradation without contributing to the LED efficiency droop. Our analysis based on the newly developed Open Boundary Quantum LED Simulator indicates that radiative recombination is hampered by the poor electron–hole wavefunction overlap induced by strong internal polarization for which QW carriers mostly recombine via Auger scattering rather than by radiative processes. By contrast, in non-polar h-LEDs, the IQE peak doubles its value compared to conventional h-LEDs while quenching the efficiency droop by 70% at current density of 100 A/cm2. Those effects are further enhanced in cubic InGaAlN MQW (c-) LEDs for which the IQE peak increases by an additional 30%, and the efficiency droop is further reduced by 80% compared to non-polar h-LEDs, thanks to the larger optical transition matrix element and the strong electron–hole wavefunction overlap in c-LEDs. Overall, a c-LED with a low efficiency droop of 3% at 100 A/cm2 is anticipated, paving a clear pathway toward ultimate solid-state lighting.

Dilute-As InGaNAs/GaN Quantum Wells for High-Efficiency Red Emitters

The dilute-As InGaNAs-based quantum well (QW) active region and the corresponding large-overlap design is analyzed by using a self-consistent 6-band <inline-formula> <tex-math notation="LaTeX">$k\cdot p$ </tex-math></inline-formula> method. Our study evaluates the optical transition energy, ground state electron-hole wavefunction overlap, and optical gain characteristics of a dilute-As InGaNAs QW consisting of a 3.3 nm thick In_0.19Ga_0.81N_0.94As_0.06 well and 10 nm thick GaN barriers. The calculation suggests that the In_0.19Ga_0.81N_0.94As_0.06 QW may result in a transition wavelength and optical gain comparable to a conventional 3.3 nm thick In_0.35Ga_0.65N QW, while preserving low In-content (&#x007E;19&#x0025;). In addition, a staggered dilute-As InGaNAs QW design with a modulated As-doping profile in the InGaNAs QW is presented, and the calculation results in a significantly enhanced wavefunction overlap and optical gain. This study reveals the excellent potential of the dilute-As InGaNAs semiconductor for application as a high efficiency active region material for future long wavelength emitters.

Delta InN-InGaN Quantum Wells With AlGaN Interlayers for Long Wavelength Emission

An active region design based on InGaN / delta-InN quantum well (QW) with AlGaN interlayer (IL) and GaN barriers (delta-structure) is investigated for potential high-efficiency visible light emitters. Numerical simulations demonstrate a large wavelength redshift with a simultaneous increase of the electron-hole wavefunction overlap for the delta-structure as compared to the conventional InGaN QW with AlGaN IL and GaN barriers. Proof of concept experimental growths via the metalorganic chemical vapor deposition demonstrate the effect of the delta-InN insertion into the conventional InGaN-based QW.