Reduced Efficiency Droop Research Articles

Spontaneous Emission Studies for Blue and Green InGaN-Based Light-Emitting Diodes and Laser Diodes

We investigated the efficiency droop phenomenon in blue and green GaN-based light-emitting diodes (LEDs) and laser diodes (LDs), which poses a significant challenge in high-power LEDs and is characterized by a reduction in external quantum efficiency at higher injection currents. Utilizing identical epi-structures for blue and green LEDs and LDs, with variations only in indium composition, our experiments revealed a gradual blue shift in the emission wavelengths as the injection current increased. Notably, the blue LED demonstrated a smaller shift compared to the green LED. In addition, the full width at half maximum of emission spectra increased with increasing injection current density, indicative of efficiency droop. Significantly, LDs consistently exhibited lower junction temperatures despite operating at higher current densities. This is attributed to the enhanced heat dissipation capability of the ridge waveguide LD structure, which results in a narrower emission spectrum and reduced efficiency droop compared to mesa LED structures. These outcomes highlight the efficiency of the ridge waveguide LD structure in heat dissipation from the active layer, offering crucial insights for the advancement of high-power light-emitting devices.

Designing p-region of AlGaN ultraviolet light-emitting diodes for the improved performance

We have used aluminum indium gallium nitride (AlInGaN) graded layers instead of the conventional combination of three p-type layers i.e., AlGaN final quantum barrier (FQB), AlGaN electron blocking layer (EBL), and top p-AlGaN hole injecting layer to increase the performance of ultraviolet light-emitting diodes (UV LEDs). In this study, the effects of graded quaternary layers (GQLs) on efficiency droop have also been investigated. These GQLs improve the injection of holes into the quantum wells. Our simulated results show that our proposed structure (LED K2) has a high peak efficiency of ∼ 68% and a significantly reduced efficiency droop of ∼41% than LED K1. The radiative recombination rate in the multiquantum wells (MQWs) of LED K2 is improved by ∼90% which can be attributed to enhanced carrier overlap or recombination in the multiquantum wells. The concentration of electrons in the active region of LED K2 is increased by ∼ 56% as compared to LED K1. This is due to the high aluminum (Al) content in GQLs of LED K2 which minimizes the electron leakage.

Carrier Confinement in Ultraviolet-C AlGaN Light-Emitting Diodes

We have numerically analyzed three different light-emitting diodes (LEDs) namely LED S1, LED S2, and LED S3 in the ultraviolet (UV) region. The band diagram, radiative recombination, and carrier distribution have been studied for LED S2 and LED S3 in comparison to LED S1. Our simulation study has shown that both the proposed structures i.e., LED S2 and LED S3 prove to be better strategies for impeding electron overflow and improving hole inflow. LED S2 and LED S3 have been found to have a reduced efficiency droop at 200 A cm−2 than reference LED S1. Increase in carrier concentration and radiative recombination rate have directly affected the internal quantum efficiency (IQE) of LED S2 and LED S3. The emission spectra of proposed LEDs have increased markedly as compared to LED S1.

Improving Charge Carrier Transport Properties in AlGaN Deep Ultraviolet Light Emitters Using Al-Content Engineered Superlattice Electron Blocking Layer

In this study, we investigate a unique Al-content engineered superlattice electron blocking layer (AESL-EBL) for improving the charge carrier transport properties of AlGaN quantum well (QW) deep ultraviolet (DUV) light-emitting diode (LED) structures. LED structures without EBL, with conventional bulk EBL (BEBL), and superlattice EBL (SL-EBL) are used for comparison. It is found that the LED structure with the AESL-EBL can exhibit superior electron blocking and hole injection, leading to reduced efficiency droop and improved light output power, compared to LED structures without EBL and with BEBL. Notably, the LED structure with AESL-EBL also outperforms the LED structure with SL-EBL, benefitting from the Al-content engineered SL. In the end, such an ASEL-EBL is applied to a DUV laser diode structure, and by optimizing the device structure low Mg-induced internal loss of around 1 cm−1 can be obtained.

Design and analysis of novel high-performance III-nitride MQW-based nanowire white-LED using HfO2/SiO2 encapsulation

A novel, white light-emitting diode structure with improved thermal characteristics is designed for providing efficient light which may be used especially in the underground mining environment. It describes a new nano hafnium oxide-silica doped silicone layer as LED encapsulation material that promises enhanced efficiency by 30.1% and reduced efficiency droop of 0.491%. The enhanced power and efficiency of the LED with HfO2/SiO2 doped bi-layer are attributed to the significant minimization of overflow of electrons which is fundamentally responsible for efficiency degradation through p-GaN region. In this article encapsulant material based on nano HfO2/SiO2 not only enhances light extraction but opens a broad new range of encapsulant engineering capabilities composites. Our designed LED is generated from a monolithic composition of blue and yellow spectrum which eventually creates the white light. This minimizes the problems related to multiple numbers of LEDs, green gap and phosphor color rendering.

Redistribution of carrier localization in InGaN-based light-emitting diodes for alleviating efficiency droop

We propose an efficient method for reducing efficiency droop in InGaN QWs by redistributing carrier localization via thermal annealing, which results in increased internal quantum efficiency (IQE) in this study. Entire structures of InGaN-based light-emitting diode (LED) were grown using metal-organic chemical vapor deposition. After thermal annealing, the photoluminescence (PL) intensity of the InGaN LED was gradually increased up to 900 °C, as confirmed by room-temperature macro-PL spectroscopy. The temperature-dependent PL spectra indicated that the In-rich InGaN cluster's local energy band was flattened by the thermal annealing treatment. Moreover, excitation power-dependent PL spectroscopy confirmed that redistribution of carrier localization in InGaN QWs alleviated the efficiency droop phenomenon. Furthermore, the flattened local energy band of the In-rich InGaN cluster and the increased carrier localization site implied improved IQE.

Comparative investigation into polarization field-dependent internal quantum efficiency of semipolar InGaN green light-emitting diodes: A strategy to mitigate green gap phenomenon

Semipolar InGaN-made green light-emitting diodes (LEDs) have sparked tremendous interest within the photonics community in recent past as advantageous replacements for already operable polar (0001)-oriented LEDs due to reduction in efficiency droop and related green gap phenomenon. Nevertheless, any theoretical investigation in order to compare internal quantum efficiency (IQE); output power and 3 dB bandwidth (BW) within non-c-plane-oriented InGaN green LEDs is innately unavailable. This study intends to describe the consequence of strain-induced polarization field on aforesaid major optical and electronic characteristics of In0.29Ga0.71N/GaN green LED along semipolar (101̅2), (112̅2), (101̅1), (202̅1), (303̅1̅), (202̅1̅) and (101̅1̅) crystal orientations by introducing modified ABC model which involves the contribution from phase-space filling (PSF) and carrier drift leakage. Our numerical investigation illustrates that best IQE, droop ratio, light emission spectra, output power and I-V profile can be obtained from (112̅2)-oriented InGaN green LED due to inconsiderable appearance of intrinsic plus lattice-mismatch induced piezoelectric polarization charge. 3 dB BW and differential carrier lifetime (DLT) is also shown to be superior for this green LED structure. In addition with these, the evidence noticed from verification of external quantum efficiency (EQE) with previous experimental reports on semipolar (112̅2) InGaN green LEDs indicates reasonable accuracy of our theoretical approach.

InGaN quantum well with gradually varying indium content for high-efficiency GaN-based green light-emitting diodes.

High-efficiency GaN-based green LEDs are of paramount importance to the development of the monolithic integration of multicolor emitters and full-color high-resolution displays. Here, the InGaN quantum well with gradually varying indium (In) content was proposed for improving the performance of GaN-based green LEDs. The InGaN quantum well with gradually varying In content not only alleviates the quantum-confined Stark effect (QCSE), but also yields a low Auger recombination rate. Consequently, the gradual In content green LEDs exhibited increased light output power (LOP) and reduced efficiency droop as compared to constant In content green LEDs. At 60 A/cm2, the LOPs of the constant In content green LEDs and the gradual In content green LEDs were 33.9 mW and 55.2 mW, respectively. At 150 A/cm2, the efficiency droops for the constant In content green LEDs and the gradual In content green LEDs were 61% and 37.6%, respectively. This work demonstrates the potential for the gradual In content InGaN to replace constant In content InGaN as quantum wells in LED devices in a technologically and commercially effective manner.

Performance enhancement of an N-polar nitride deep-ultraviolet light-emitting diode with compositionally graded p-AlGaN.

Highly efficient hole injection into a AlGaN quantum well is desirable in nitride deep-ultraviolet light-emitting diodes (DUV LEDs) for high optical performance. In this work, we report the observation of enhanced hole injection in the N-polar AlGaN-based DUV LEDs with compositionally graded p-AlxGa1-xN (x = 0.65-0.75) by simulation and show that the enhanced hole injection leads to the increase of the peak internal quantum efficiency (IQE) and the significant reduction of efficiency droop at high current density. This work might activate researchers to realize the efficient polarization p-type doping of N-polar AlGaN with high Al content and thus to achieve high performance DUV LEDs experimentally.

Reduction of efficiency droop in c-plane InGaN/GaN light-emitting diodes using a thick single quantum well with doped barriers

We report on c-plane InGaN/GaN single quantum well (QW) light-emitting diodes (LEDs) of different well widths (3 or 9 nm) with and without doped barriers. QW barriers were doped with the aim of reducing the internal electric field (FQW) in the QW to increase the electron-hole overlap, therefore increasing the recombination rates and resulting in the reduction of the efficiency droop. We, indeed, observed, through biased photocurrent spectroscopy, a reduction in FQW with doped barriers, with FQW being in the same direction of the p-n junction field at zero bias as opposed to the junction field for LEDs without doped barriers. Even with the improvement in the ground state wavefunction overlap, the ground state transition rate remains low for thick QWs. Transitions through excited states were observed for both thick QW LEDs with and without doped barriers. The thick QW LED without doped barriers displayed low external quantum efficiency (EQE), likely as a result of the carrier overflow due to the poor confinement of carriers in the excited states. On the other hand, for LEDs with doped barriers, the flatter band in the QW resulting from the lower FQW reduces the energy separation between the eigenstates, leading to better confinement of carriers in the excited states. With doped barriers, we demonstrated a low efficiency droop 9-nm-thick single QW LED with a peak EQE of 42% at 40 A/cm2 and an EQE of 36% at 400 A/cm2.

High-efficiency green micro-LEDs with GaN tunnel junctions grown hybrid by PA-MBE and MOCVD

We fabricated p-i-n tunnel junction (TJ) contacts for hole injection on c-plane green micro-light-emitting diodes (micro-LEDs) by a hybrid growth approach using plasma-assisted molecular beam epitaxy (PA-MBE) and metal–organic chemical vapor deposition (MOCVD). The TJ was formed by an MBE-grown ultra-thin unintentionally doped InGaN polarization layer and an n ++ / n + - GaN layer on the activated p ++ - GaN layer prepared by MOCVD. This hybrid growth approach allowed for the realization of a steep doping interface and ultrathin depletion width for efficient inter-band tunneling. Compared to standard micro-LEDs, the TJ micro-LEDs showed a reduced device resistance, enhanced electroluminescence intensity, and a reduced efficiency droop. The size-independent J-V characteristics indicate that TJ could serve as an excellent current spreading layer. All these results demonstrated that hybrid TJ contacts contributed to the realization of high-performance micro-LEDs with long emission wavelengths.

Reduction of efficiency droop by inserting superlattice quaternary-ternary electron blocking layer in GaN-based light-emitting diodes

We report a reduced the unwanted hole blocking of the electron blocking layer (EBL) by replacing the conventional EBL with a quaternary-ternary superlattice electron blocking layer. The effective barrier height for holes is reduced from ~ 348 meV to ~ 261 meV, thus, improving the concentration of holes by ~ 50% with respect to LED with conventional EBL. Similarly, the radiative recombination rate is also improved by ~ 79%. As a result, the overall droop ratio of internal quantum efficiency (IQE) of the proposed LED is reduced to ~ 23% at 100 A/cm2 in comparison to conventional LED.

Hole Transport in Ultraviolet Light-Emitting Diode via p-Type Injection Channel

In order to enable lateral hole injection, a unique structure with a p-type AlGaN injection channel is proposed to replace the conventional deep-ultraviolet light-emitting diode (DUV LED) structure. This type of modification results in the probability of obtaining a 1.65-fold higher light output power when the channel band gap is between the bandgap of quantum-well (QW) and p-AlGaN, which is demonstrated at the current density of 100 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . In comparison, the maximum internal quantum efficiency is increased by 23.43% relative to the traditional AlGaN DUV LED structure with a 10.27% reduced efficiency droop at 100 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The effective injection of holes would provide greater confinement of electrons. The ultimate effect would result in superior radiative recombination of the proposed DUV LED structure inside the QW.

Designing anti-trapezoidal electron blocking layer for the amelioration of AlGaN-based deep ultraviolet light-emitting diodes internal quantum efficiency

We have proposed and investigated a DUV LED device with anti-trapezoidal quaternary electron blocking layer (EBL) numerically. The simulation results show that the anti-trapezoidal quaternary EBL is a better approach for diminishing the electron overflow and boosting the hole into the active region. This improves radiative recombination, which is responsible for the significant enhancement in IQE and remarkable reduction in efficiency droop. The results reveal that the electron and hole concentration is increased by 19 % and 29 % respectively.

Reduction in efficiency droop/decline of green GaN-based light-emitting diodes by employing heterostructure cap layer

In this study, we theoretically studied the enhancement of carrier injection by employing heterostructure layers before p-GaN in green light-emitting diodes. It is evident from the simulation results that improved carrier injection results in high internal quantum efficiency and light output power in the structures using heterostructures on the p-side. In comparison to conventional structure, reduced efficiency droop is observed suggesting promising high-performance green light-emitting diodes.

Prominent Heat Dissipation in Perovskite Light-Emitting Diodes with Reduced Efficiency Droop for Silicon-Based Display.

Solution-processed perovskite light-emitting diodes (LEDs) possess outstanding optoelectronic properties for potential solid-state display applications. However, poor device stability results in significant efficiency droop partly being ascribed to Joule heating when LEDs are operated at high current densities. Herein, we used monocrystal silicon (c-Si) as the substrate and a charge injection layer to alleviate the thermal affection in perovskite LED (PeLED). By incorporating silicon oxide (SiOx) and poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl) (TFB) layers to tune the charge injection balance in a c-Si-based device, a PeLED achieves an external quantum efficiency of 2.12% with a current efficiency of 6.06 cd A-1. Benefiting from excellent heat dissipation of c-Si, the PeLEDs display reduced efficiency droop and extended operational lifetime. Furthermore, both electroluminescent (EL) dynamic information and static pattern displays of a c-Si-based PeLED have been successfully demonstrated. These results reveal the feasibility of potential practical c-Si-based PeLEDs with reduced efficiency droop for EL display applications.

Enhanced red light emission from dense Si quantum dot-based silicon oxynitride light-emitting diodes with reduced efficiency droop

Strong red light emission from silicon oxynitride-based (SiOxNy-based) light-emitting diodes (LEDs) was investigated. The introduction of dense Si quantum dots (Si QDs) into SiOxNy not only increases carrier injection efficiency but also results in a significant enhancement of more than 300% in the light emission efficiency compared with SiOxNy-based LEDs without Si QDs. Moreover, the efficiency droop phenomenon is remarkably suppressed in the SiOxNy-based LED containing dense Si QDs. Analysis of the dominant recombination process indicates that the improved performance of the SiOxNy-based LEDs results from the increased bimolecular radiative recombination probability.

High-Performance Semi-Polar InGaN/GaN Green Micro Light-Emitting Diodes

Semi-polar micro-LEDs have gain increasing interests due to the advantages of polarization control and quantum efficiency improvement. In this work, a novel semi-polar (20–21)-plane micro-LEDs array has been designed and manufactured. In comparison with c-plane micro-LEDs, semi-polar micro-LEDs indicate better electrical and optical performance. The relative EQE of semi-polar micro-LEDs remains at 62% under the injected current density of 775.6 A/cm2, which indicates a reduced efficiency droop due to less polarization in MQWs. It has been further proved by a significant reduction of 55% in emission peak blue-shift under the injected current density from 11.1 A/cm2 to 775.6 A/cm2. In addition, the carrier recombination dynamics and spatial light distribution of semi-polar micro-LEDs with different pixel sizes have been studied. Fast recombination lifetime in smaller size semi-polar micro-LEDs indicates a promising way to be used as a high modulation bandwidth light source. Stable and uniform light distribution in a wider range of spatial azimuths further supports for the semi-polar micro-LEDs as a strong candidate for the applications of high-resolution display and high-speed visible light communication.

Temperature Dependence of Efficiency Droop in GaN-based Blue Light-emitting Diodes from 20 to 80°C

We investigate the temperature dependence of efficiency droop in InGaN/GaN multiple-quantum-well (MQW) blue light-emitting diodes (LEDs) in the temperature range from 20 to 80°C. When the external quantum efficiency (EQE) and the wall-plug efficiency (WPE) of the LED sample were measured as injection current and temperature varied, the droop of EQE and WPE was found to be reduced with increasing temperature. As the temperature increased from 20 to 80°C, the droop ratio of EQE was decreased from 16% to 14%. This reduction in efficiency droop with temperature can be interpreted by a temperature-dependent carrier distribution in the MQWs. When the carrier distribution and radiative recombination rate in MQWs were simulated and compared for different temperatures, the carrier distribution was found to become increasingly homogeneous as the temperature increased, which is believed to partly contribute to the reduction in efficiency droop with increasing temperature.

Impurity resonant state p-doping layer for high-efficiency nitride-based light-emitting diodes

Impurity resonant state p-doping was recently proposed as an alternative to the commonly used p-GaN for high-efficiency nitride-based light-emitting diodes (LEDs) in order to address the issue of electron leakage and efficiency droop. We demonstrated that due to the effective acceptor ionization and reduction of the potential barrier for hole injection, a more symmetrical electron and hole injection and distribution could be achieved. Our experimental results revealed that the LEDs with this alternative p-type layer exhibited a higher quantum efficiency and reduced efficiency droop. We expect this approach to be equally applicable for deep-UV laser diodes and LEDs to replace the commonly used absorptive p-GaN layer.