InGaN-based Green Light-emitting Diodes Research Articles

Ultra-dense Green InGaN/GaN Nanoscale Pixels with High Luminescence Stability and Uniformity for Near-Eye Displays.

Ultra-dense (>4,000 pixels per inch) and highly stable full-color III-nitride nanoscale pixels are crucial for near-eye display technologies like virtual and augmented-reality glasses. In this context, InGaN-based long wavelength green microscale light-emitting diodes face major bottlenecks, such as low efficiency and inadequate wavelength stability. These challenges are associated with the presence of both nonradiative surface defects and the strain induced quantum-confined Stark effect. Herein, we report nanoscale pixelation of green InGaN/GaN LEDs incorporating strain-engineered ultra-dense nanowire (NW) arrays, corresponding to ∼36,000 pixels per inch. The NW pixel arrays exhibit a stable peak wavelength emission at ∼500 nm for over 3 orders of magnitude of injection current densities (from ∼4 A/cm2 to ∼1 kA/cm2). The observed wavelength stability enhancement (a reduced blue-shift of just ∼4 nm) directly results from the suppressed built-in electric field achieved by strain relaxation of the axial multi quantum wells in the NWs. Finite difference time domain simulations show that emission of the pixel array is significantly increased owing to the enhanced spontaneous emission rate (characterized by a high Purcell factor of ≈2) of the ultra-dense NWs. We have demonstrated top-down NWs, where each NW (diameter ranging down to 200 nm) shows excellent uniformity and light output characteristics in direct contrast to bottom-up grown NW heterostructures. The results of this study establish a viable route for realizing nanoscale pixels with high luminescence stability and wafer-scale uniformity with high (>20%) indium composition InGaN/GaN LED heterostructures, for next-generation near-eye displays.

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.

Optoelectronic Simulations of InGaN-Based Green Micro-Resonant Cavity Light-Emitting Diodes with Staggered Multiple Quantum Wells

In this research, we compared the performance of commercial μ-LEDs and three-layered staggered QW μ-LED arrays. We also investigated the self-heating effect. We proposed a green micro-resonant cavity light-emitting diode (µ-RCLED) that consists of a three-layer staggered InGaN with multiple quantum wells (MQWs), a bottom layer of nanoporous n-GaN distributed Bragg reflectors (DBRs), and a top layer of Ta2O5/SiO2 DBRs. We systematically performed simulations of the proposed µ-RCLEDs. For the InGaN MQWs with an input current of 300 mA, the calculated wavefunction overlaps are 8.8% and 18.1% for the regular and staggered structures, respectively. Furthermore, the staggered MQWs can reduce the blue-shift of electroluminescence from 10.25 nm, obtained with regular MQWs, to 2.25 nm. Due to less blue-shift, the output power can be maintained even at a high input current. Conversely, by employing 6.5 pairs of Ta2O5/SiO2 DBRs stacks, the full width at half maximum (FWHM) can be significantly reduced from 40 nm, obtained with ordinary µ-LEDs, to 0.3 nm, and a divergence angle smaller than 60° can be obtained. Our simulation results suggest that the µ-RCLEDs can effectively resolve the wavelength instability and color purity issues of conventional µ-LEDs.

Stacked GaN/AlN last quantum barrier for high-efficiency InGaN-based green light-emitting diodes.

Realization of efficient InGaN-based green light-emitting diodes (LEDs) is highly desirable in solid-state lighting industry. Here, we propose a stacked last quantum barrier (SLQB) GaN/AlN layer for green (∼550nm) LEDs grown on patterned sapphire substrate to improve the device performance. A green LED with a SLQB achieves a light output power (LOP) of 13 mW at 15 mA, which is 35% higher than that of LEDs with a conventional last quantum barrier (CLQB) GaN layer. In addition, the forward voltage of the green LED with the SLQB is reduced by 0.16 V compared with green LEDs with the CLQB. Simulation results demonstrate that the AlN layer in the SLQB increases the effective barrier height for electrons and alleviates the electron leakage effect. It also enables an intraband tunneling process for holes to inject into the active region, promoting the hole injection efficiency. As a result, we successfully obtain InGaN-based green LEDs with remarkably improved carrier injection efficiency by adopting the SLQB structure.

Performance of InGaN green light-emitting diodes with on-chip photodetectors based on wire-bonding and flip-chip configurations

In this work, we report the performance study of InGaN-based green light-emitting diodes (LEDs) with on-chip photodetectors (PDs) based on wire-bonding and flip-chip configurations. Compared with a conventional wire-bonding design, the LED-PD device, which incorporates a flip-chip design, can offer superior optical and thermal performances and, under an LED current of 200 mA, its light output and detected photocurrent increase by 37.7% and 14.7%, respectively. The different extents of enhancement in both light output and photocurrent are also studied by analyzing their optical, electrical, and thermal properties under varying LED currents. The results provide important guidance for the design of LED-PD integrated systems operating at different current densities.

Increasing the internal quantum efficiency of green GaN-based light-emitting diodes by employing graded quantum well and electron blocking layer

By numerical investigation of various structures, we have proposed a promising single quantum well device for efficient green InGaN-based light-emitting diodes. The peak internal quantum efficiency of the proposed structure, at lower current density, is enhanced and shows significant droop reduction at high current density. The efficiency droop ratio of the proposed structure is ~ 31% at 100 A cm−2. Similarly, the light output power is also doubled as compared to conventional light-emitting diode. In the proposed structure, the simultaneously graded quantum well and electron blocking layer are employed. Therefore, the injection of holes into the active region becomes easier due to graded electron blocking layer and radiative recombination is boosted due to graded quantum well. By implement the proposed structure, both the problem of injection of holes and separation of electron–hole wavefunctions are greatly reduced.

Augmenting the internal quantum efficiency of GaN-based green light-emitting diodes by sandwiching active region with p-AlGaN layers

In this work, InGaN-based green light-emitting diodes (LEDs) are studied theoretically. We have minimized the problem of asymmetrical carrier distribution in the active region. We have sandwiched the active region between p-AlGaN layers and observed improvement in the device performance. The hole injection, as well as the distribution, is improved remarkably in all the QWs. The asymmetry between the concentrations of electrons and holes in the emission region is reduced in contrast to the conventional structure. The radiative recombination is also enhanced because all the QWs contribute greatly in radiative recombination.

Systematical and Numerical Investigations on InGaN-Based Green Light-Emitting Diodes: Si-Doped Quantum Barriers, Engineered p-Electron Blocking Layer and AlGaN/GaN Structured p-Type Region

Abstract Here, we numerically prove that the severe polarization-induced electric field in the active region for [0001]-oriented InGaN-based green light-emitting diodes (LEDs) is reduced when heavily Si-doped GaN quantum barriers are adopted. However, the electron injection is accordingly sacrificed for the insufficient confinement capability of the p-type electron blocking layer (p-EBL). Hence, p-EBL structures with/without gradient AlN alloys are discussed to reduce the electron leakage, and the importance of the positive sheet polarization charges at the interface between the last quantum barrier (LQB) and the p-EBL on affecting the blocking barrier height for electrons is especially highlighted. Moreover, we also suggest utilizing specially designed p-AlGaN/p-GaN structure instead of p-GaN structure to serve as the p-type hole supplier, which is able to increase the kinetic energy of holes, thus assisting holes to overcome the energy band barrier height in p-EBL and increasing the hole concentration within the quantum wells.

Effect of low-temperature GaN cap layer thickness on the optoelectronic performance of InGaN green LEDs with V-shape pits

The low-temperature GaN cap layer was designed after quantum wells for the V-pits contained InGaN-based green light-emitting diodes (LEDs). It is found that the LEDs’ performances are greatly improved with a thicker cap layer. The indium distribution in InGaN/GaN multiple quantum wells (MQWs) becomes more homogeneous. The quality of interface between the quantum well and quantum barrier is promoted, and the carrier confinement effect is enhanced. Thereby the external quantum efficiency (EQE) increases at high current density. However, the sample with a relative thicker cap layer exhibits the lower EQE at low current density, which is attributed to there being more defects at the upper interface of QWs, and the increased probability of carriers meeting at these non-radiative recombination centers. Meanwhile, only the major emission peak emitted from c-plane MQWs can be observed in the electroluminescence spectrum of 100 K, and other emission peaks are hardly visible, which indicates that the sample with the thicker cap has better carrier transport performance.

Effect of unevenly-distributed V pits on the optical and electrical characteristics of green micro-light emitting diode

In this study, we investigated the effect of different-size V pits on the opto-electrical performance of different-size InGaN-based green (520 nm) light-emitting diodes (LEDs). The size of V pits varied from 57 to 250 nm. Two different-size LEDs (10 and 300 µm) were fabricated with flip-chip structures. It was shown that at 4 A cm−2, the forward voltages of 300 µm-size and 10 µm-size LEDs were in the range of 2.32–2.82 V and 2.23–2.53 V, respectively. Compared with the LEDs with small V pits, the LEDs with large V pits produced higher output power at the high current region (>50 A cm−2), but lower output power at the low current region (<20 A cm−2). The 300 µm-size LEDs displayed maximum external quantum efficiency (EQE) at 2 A cm−2, whereas the 10 µm-size LEDs exhibited peak EQE at current densities ⩾13 A cm−2. For all samples, the electroluminescence (EL) wavelengths were blue shifted with increasing current density. Unlike the 300 µm-size LEDs, the 10 µm-size LEDs exhibited a large deviation in the light output, EQE, and EL wavelength. The 10 µm-size LEDs revealed larger S than the 300 µm-size LEDs. The backscattered electron (BSE) and monochromatic cathodoluminescence (CL) results showed that for all samples, V pits were unevenly distributed across the whole wafer. Based on the CL and BSE results, the effect of different V pit sizes on the performance of green micro-LEDs at the low current region is described and discussed.

Amelioration of internal quantum efficiency of green GaN-based light-emitting diodes by employing variable active region

Abstract We have designed and investigated green InGaN-based light-emitting diodes numerically. We have proposed an efficient device structure with improved carrier transport as well as distribution of holes in the active region. In our proposed structure, we employ a variable active region to improve the device performance. The variable active region comprises of quantum wells with increasing thickness and quantum barriers with decreasing thickness. Both quantum wells and quantum barriers are varied from n-side to p-side of the device. The transport and distribution of holes have significantly improved in our proposed structure exhibiting excellent performance in comparison with other structures.

Combined effects of V pits and chip size on the electrical and optical properties of green InGaN-based light-emitting diodes

The fabrication of ultra-high-resolution micro-displays with low power consumption is essential for applications in virtual reality and augmented reality systems. In this regard, GaN-based light-emitting diodes (LEDs) have been regarded as a promising candidate for self-emissive micro-displays. However, micro-LEDs suffer from sidewall defects and low quantum efficiency at a low current. Thus, we investigated the electrical and optical properties of InGaN-based green (520 nm) LEDs as functions of V pits and chip size. With decreasing chip size, the forward voltage of all samples increased at the same injection current. The samples with V pits exhibited markedly higher current density than those without V pits. With decreasing chip size, the current densities of all the samples increased, whereas the output power at the same current density decreased. As the chip size decreased, the peak external quantum efficiency of all the samples decreased, which was attained at high current densities. Furthermore, the samples with V pits displayed more efficient current spreading behavior, smaller ideality factors, and a smaller S parameter than the ones without V pits. The emission wavelength of the small samples (<300 μm) was blue-shifted with increasing current density. Based on the cathodoluminescence results, the effects of chip size, current density, and V pits on the electroluminescence spectral shift of green LEDs are described and discussed.

Effects of V-pits covering layer position on the optoelectronic performance of InGaN green LEDs

The impact of the V-pits covering layer (VCL) position on the optoelectronic performance of InGaN-based green light-emitting diodes (LEDs) was investigated. It is found that earlier covering of V-pits will hinder the hole injection via the sidewall of V-pits, and then result in less quantum wells (QWs) participating in radioluminescence. The current-voltage characteristics show that the LEDs with earlier covering of V-pits have higher operating voltage at room temperature, and a more dramatic voltage rise with the reduction of temperature. Meanwhile, more manifested emission peaks for sidewall QWs and deeper QWs near to n-type layer was observed in the sample with earlier coveing of V-pits at cryogenic temperatures, for the reason that the holes being injected via V-pits sidewall have higher kinetic energy and could transport to deeper QWs.

Highly efficient InGaN green mini-size flip-chip light-emitting diodes with AlGaN insertion layer

We demonstrate highly efficient InGaN-based mini-size green light-emitting diodes (mLEDs) with AlGaN insertion layer in InGaN/GaN multiple quantum wells (MQWs) using metal organic chemical vapor deposition (MOCVD). High resolution transmission electron microscopy (HRTEM) results reveal that ‘V’ defects within active region can be effectively reduced by AlGaN insertion layer. Photoluminescence (PL) and time resolved photoluminescence (TRPL) results indicate an increase of radiative recombination efficiency. Very high performance 523 nm InGaN green flip-chip mLEDs (0.025 mm2) with distributed Bragg reflector (DBR) show a high external quantum efficiency (EQE) of 38.0%, a high wall-plug efficiency (WPE) of 32.1% and a low forward voltage of 2.8 V at a working current density of 20 A cm−2, which are very promising for display application.

Very high external quantum efficiency and wall-plug efficiency 527 nm InGaN green LEDs by MOCVD.

We demonstrate very high luminous efficacy InGaN-based green light-emitting diodes (LEDs) grown on c-plane patterned sapphire substrates (PSS) using metal organic chemical vapor deposition (MOCVD). The 527 nm green LEDs show a peak external quantum efficiency (EQE) of 53.3%, a peak wall-plug efficiency (WPE) of 54.1% and a peak luminous efficacy of 329 lm/W, respectively. A high EQE of 38.4%, a WPE of 32.1% and a very low forward voltage of 2.86 V were obtained at a typical working current density of 20 A/cm2. By operating low cost green LEDs at a low current density, our devices (0.5 mm2) demonstrating an EQE and a WPE higher than 50% and an efficacy of 259 lm/W at 4 A/cm2 with an output power of 24 mW. High crystal quality of the InGaN/GaN MQWs was characterized by X-ray diffraction (XRD) and the advantage of the epitaxy design was investigated by APSYS software simulation. These results provide a simple way to achieve very high efficiency InGaN green LEDs.

Effect of Green Quantum Well Number on Properties of Green GaN-Based Light-Emitting Diodes**Supported by the National Key Research and Development Program of China under Grant No 2016YFB0400601, and the National Natural Science Foundation of China under Grant Nos 61704069, 11674147, 61604066, 51602141 and 11604137.

InGaN-based green light-emitting diodes (LEDs) with different green quantum well numbers grown on Si (111) substrates by metal organic chemical vapor deposition are investigated. It is observed that V-shaped pits appear in the AFM images with the green quantum well number increasing from 5 to 9, and results in larger reverse-bias leakage current. Meanwhile, in the case of the sample with the number from 5 to 7 then to 9, the external quantum efficiency increases firstly, and then decreases. These phenomena may be related to the size of V-shaped pits in the active area and the distribution of electrons and holes in the active area caused by V-shaped pits. The optimal number of green quantum wells is determined to be 7.

Effect of Mg-Preflow for p-AlGaN Electron Blocking Layer on the Electroluminescence of Green LEDs with V-Shaped Pits**Supported by the National Key R&D Program of China under Grant Nos 2016YFB0400600 and 2016YFB0400601, the State Key Program of the National Natural Science of China under Grant No 61334001, the Key R&D Program of Jiangxi Province under Grant No 20165ABC28007, the Natural Science

InGaN-based green light-emitting diodes (LEDs) with and without Mg-preflow before the growth of p-AlGaN electron blocking layer (EBL) are investigated experimentally. A higher Mg doping concentration is achieved in the EBL after Mg-preflow treatment, effectively alleviating the commonly observed efficiency collapse and electrons overflowing at cryogenic temperatures. However, unexpected decline in quantum efficiency is observed after Mg-preflow treatment at room temperature. Our conclusions are drawn such that the efficiency decline is probably the result of different emission positions. Higher Mg doping concentration in the EBL after Mg-preflow treatment will make it easier for a hole to be injected into multiple quantum wells with emission closer to p- GaN side through the c-plane rather than the V-shape pits, which is not favorable to luminous efficiency due to the preferred occurrence of accumulated strain relaxation and structural defects in upper QWs closer to p-GaN. Within this framework, apparently disparate experimental observations regarding electroluminescence properties, in this work, are well reconciled.

Monolithic integration of AlGaInP-based red and InGaN-based green LEDs via adhesive bonding for multicolor emission

In general, to realize full color, inorganic light-emitting diodes (LEDs) are diced from respective red-green-blue (RGB) wafers consisting of inorganic crystalline semiconductors. Although this conventional method can realize full color, it is limited when applied to microdisplays requiring high resolution. Designing a structure emitting various colors by integrating both AlGaInP-based and InGaN-based LEDs onto one substrate could be a solution to achieve full color with high resolution. Herein, we introduce adhesive bonding and a chemical wet etching process to monolithically integrate two materials with different bandgap energies for green and red light emission. We successfully transferred AlGaInP-based red LED film onto InGaN-based green LEDs without any cracks or void areas and then separated the green and red subpixel LEDs in a lateral direction; the dual color LEDs integrated by the bonding technique were tunable from the green to red color regions (530–630 nm) as intended. In addition, we studied vertically stacked subpixel LEDs by deeply analyzing their light absorption and the interaction between the top and bottom pixels to achieve ultra-high resolution.

Improvement of green InGaN-based LEDs efficiency using a novel quantum well structure**Project supported by the National Key Research and Development Program of China (Grant Nos. 2016YFB0400300 and 2016YFB0400600), the National Natural Science Foundation of China (Grant Nos. 11574362, 61210014, and 11374340), and the Innovative Clean-Energy Research and Application Program of Beijing Municipal Science and Technology Commission (Grant No. Z151100003515001).

The green light emitting diodes (LEDs) have lower quantum efficiency than LEDs with other emission wavelengths in the visible spectrum. In this research, a novel quantum well structure was designed to improve the electroluminescence (EL) of green InGaN-based LEDs. Compared with the conventional quantum well structure, the novel structure LED gained 2.14 times light out power (LOP) at 20-mA current injection, narrower FWHM and lower blue-shift at different current injection conditions.

The Efficiency Droop of InGaN-Based Green LEDs with Different Superlattice Growth Temperatures on Si Substrates via Temperature-Dependent Electroluminescence**Supported by the National Natural Science Foundation of China under Grant No 61334001, the National Key Research and Development Program of China under Grant Nos 2016YFB0400600, 2016YFB0400601 and 2016YFB0400100, the National Science Foundation for Young Scientists of China under Grant No 21405076, and

InGaN-based green light-emitting diodes (LEDs) with different growth temperatures of superlattice grown on Si (111) substrates are investigated by temperature-dependent electroluminescence between 100 K and 350 K. It is observed that with the decrease of the growth temperature of the superlattice from 895°C to 855°C, the forward voltage decreases, especially at low temperature. We presume that this is due to the existence of the larger average size of V-shaped pits, which is determined by secondary ion mass spectrometer measurements. Meanwhile, the sample with higher growth temperature of superlattice shows a severer efficiency droop at cryogenic temperatures (about 100 K–150 K). Electron overflow into p-GaN is considered to be the cause of such phenomena, which is relevant to the poorer hole injection into multiple quantum wells and the more reduced effective active volume in the active region.