InGaN-based Light-emitting Diodes Research Articles

Enhanced Hole Transport of Nonpolar InGaN-based Light-emitting Diodes with Lateral p-type Superlattice Doping Structure

Enhanced Hole Transport of Nonpolar InGaN-based Light-emitting Diodes with Lateral p-type Superlattice Doping Structure

Impact of Localization on the Optical Properties of InGaN-Based Red Light-Emitting Diodes Grown on a Silicon Substrate

Impact of Localization on the Optical Properties of InGaN-Based Red Light-Emitting Diodes Grown on a Silicon Substrate

Demonstration of 651 nm InGaN-based red light-emitting diode with an external quantum efficiency over 6% by InGaN/AlN strain release interlayer.

This work reports a high-performance InGaN-based red-emitting LED with a strain-release interlayer (SRI) consisting of an InGaN stress-release layer (SRL) and an AlN dislocation confinement layer (DCL) in unintentionally doped GaN (u-GaN). The SRL introduces a tensile strain which could decrease the in-plane compressive stress of the u-GaN layer, while the DCL could reduce the dislocation density and thus improve the crystal quality of the u-GaN layer. Consequently, a high-efficiency InGaN-based red-emitting LED with a peak wavelength of 651 nm and an external quantum efficiency of 6.04% is realized. In addition, the room-temperature photoluminescence (PL) mapping emission wavelength is uniform across a 4-inch wafer with a standard deviation of 3.3 nm. Therefore, the proposed SRI offers good potential for mass-producing high-performance and long-wavelength InGaN-based red-emitting LEDs.

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.

Phosphor-free micro-pyramid InGaN-based white light-emitting diode with a high color rendering index on a β-Ga2O3 substrate.

We demonstrate the InGaN/GaN-based monolithic micro-pyramid white (MPW) vertical LED (VLED) grown on (-201)-oriented β-Ga2O3 substrate by selective area growth. The transmission electron microscopy (TEM) reveals an almost defect-free GaN pyramid structure on (10-11) sidewalls, including stacked dual-wavelength multi-quantum wells (MQWs). From the electroluminescence (EL) spectra of the fabricated MPW VLED, a white light emission with a high color rendering index (CRI) of 97.4 is achieved. Furthermore, the simulation shows that the light extraction efficiency (LEE) of the MPW VLED is at least 4 times higher compared with the conventional planar LED. These results show that the MPW VLED grown on β-Ga2O3 has great potential for highly efficient phosphor-free white light emission.

Achieving InGaN-Based Red Light-Emitting Diodes by Increasing the Growth Pressure of Quantum Wells

Achieving InGaN-Based Red Light-Emitting Diodes by Increasing the Growth Pressure of Quantum Wells

Wafer-Scale Transferrable GaN Enabled by Hexagonal Boron Nitride for Flexible Light-Emitting Diode.

Epitaxy growth and mechanical transfer of high-quality III-nitrides using 2D materials, weakly bonded by van der Waals force, becomes an important technology for semiconductor industry. In this work, wafer-scale transferrable GaN epilayer with low dislocation density is successfully achieved through AlN/h-BN composite buffer layer and its application in flexible InGaN-based light-emitting diodes (LEDs) is demonstrated. Guided by first-principles calculations, the nucleation and bonding mechanism of GaN and AlN on h-BN is presented, and it is confirmed that the adsorption energy of Al atoms on O2 -plasma-treated h-BN is over 1eV larger than that of Ga atoms. It is found that the introduced high-temperature AlN buffer layer induces sufficient tensile strain during rapid coalescence to compensate the compressive strain generated by the heteromismatch, and a strain-relaxation model for III-nitrides on h-BN is proposed. Eventually, the mechanical exfoliation of single-crystalline GaN film and LED through weak interaction between multilayer h-BN is realized. The flexible free-standing thin-film LED exhibits ≈66% luminescence enhancement with good reliability compared to that before transfer. This work proposes a new approach for the development of flexible semiconductor devices.

Piezo-phototronic enhanced dual-wavelength InGaN/GaN multiple quantum wells Micro-LED arrays

As a promising technology for next-generation displays due to their high brightness and low power consumption, InGaN-based micro light-emitting diodes (Micro-LEDs) have attracted great attention in recent years. In this work, we detailedly investigate the carrier recombination and transportation process in dual-wavelength InGaN/GaN multiple quantum wells (MQWs) Micro-LED arrays under piezo-phototronic effects using electroluminescence (EL) and time-resolution photoluminescence measurements. With the increase of external strain, the violet EL intensity of dual-wavelength Micro-LED arrays first increases obtaining a maximum enhancement of ∼12% and then decreases, whereas blue EL emission almost maintains constant. Additionally, as the size of Micro-LED decreases, the enhancement obtained via piezo-phototronic effect will reduce, which is attributed to their inherently weaker piezoelectric polarization effect. Combining with dynamic analysis of carriers in the blue quantum well (QW), it is concluded that strain-induced interface polarized charges promote the wave function overlap of electron–hole pair, but reduce the injection of hole carriers in blue QW. Superposition of the above two factors enables the blue EL intensity stable under piezoelectric coupling. These results present a promising potential of piezo-phototronic effects to improve the Micro-LEDs devices.

Flexible topographical design of light-emitting diodes realizing electrically controllable multi-wavelength spectra

Multi-wavelength visible light emitters play a crucial role in current solid-state lighting. Although they can be realized by combining semiconductor light-emitting diodes (LEDs) and phosphors or by assembling multiple LED chips with different wavelengths, these design approaches suffer from phosphor-related issues or complex assembly processes. These challenges are significant drawbacks for emerging applications such as visible light communication and micro-LED displays. Herein we present a platform for tailored emission wavelength integration on a single chip utilizing epitaxial growth on flexibly-designed three-dimensional topographies. This approach spontaneously arranges the local emission wavelengths of InGaN-based LED structures through the local In composition variations. As a result, we demonstrate monolithic integration of three different emission colors (violet, blue, and green) on a single chip. Furthermore, we achieve flexible spectral control via independent electrical control of each component. Our integration scheme opens the possibility for tailored spectral control in an arbitrary spectral range through monolithic multi-wavelength LEDs.

Performance Improvement of InGaN-Based Red Light-Emitting Diodes via Ultrathin InN Insertion Layer

The serious separation of electron–hole wavefunctions, which is caused by the built-in electric field, prevents electron–hole radiative recombination in quantum wells (QWs) in high-In-content InGaN-based red light-emitting diodes (LEDs). Here, we propose a staggered structure that inserts an ultrathin InN layer in the single quantum well (SQW) to reduce the piezoelectric polarization and suppress the quantum confined Stark effect (QCSE). We have numerically simulated the effects of SQW with the InN insertion layer (IL) on the energy band structure and electron–hole wavefunctions of the red LED. Owing to alleviated piezoelectric polarization and improved overlaps of electron–hole wavefunctions, the simulation results have revealed that the internal quantum well (IQE) of the red LED with InN IL exhibits 42% higher than that of the red LED with a square-shaped QW (SSQW) at 60 A/cm2, and the efficiency droop ratio of red LED with InN IL is 48% lower than that of red LEDs with SSQW. Furthermore, we have found that the position of InN IL can affect the energy states of carriers, which has a great influence on the IQE and peak emission wavelength of red LEDs.

Effect of hydrostatic pressure on the Auger coefficient of InGaN/GaN multiple-quantum-well laser diode

A numerical model was used to analyze the Auger coefficient in a c-plane InGaN/GaN multiple-quantum-well laser diode (MQWLD) under hydrostatic pressure. Finite difference techniques were employed to acquire energy Eigenvalues and their corresponding Eigenfunctions of InGaN/GaN MQWLD, and the hole Eigenstates were calculated via a 6 × 6 k.p method under applied hydrostatic pressure. It was found that a change in pressure up to 10 GPa increases the carrier density in the quantum well and barriers and the effective band gap. Based on the result, the exaction binding energy decreased, the electric field rate increased up to 0.77 MV / cm, and the Auger coefficient decreased down to 2.1 × 10 − 31 and 0.6 × 10 − 31 cm6 s − 1 in the MQW and barrier regions, respectively. Also, the calculations demonstrated that the hole-hole-electron (CHHS) and electron-electron-hole (CCCH) Auger coefficients had the largest contribution to the Auger coefficient. Our study provides more detailed insight into the origin of the Auger recombination rate drop under hydrostatic pressure in InGaN-based light-emitting diodes.

Improved wavelength stability and heat dissipation of InGaN-based light-emitting diodes using a graphene interlayer on patterned sapphire substrate

This study presents a straightforward strategy that embeds a graphene interlayer between InGaN-based light-emitting diodes (InGaN LEDs) and patterned sapphire substrate (PSS substrate) for substantial improving device performances of wavelength stability and heat dissipation. The InGaN LEDs on the graphene interlayer/PSS substrate (Gr-LED) have lower piezoelectric fields of 1.16 MV/cm than 1.60 MV/cm for InGaN LEDs on the PSS substrate (Ref-LED). The low piezoelectric field diminishes the screen of the polarization field resulting in a stable electroluminescence peak wavelength. At 100 mA driving current, the chip temperature of Gr-LED shows a decrease in around 24.4 °C relative to Ref-LED. The thermal resistances measured in a vacuum environment are 351 and 128 °C/W for Ref-LED and Gr-LED, respectively. The low thermal resistance of Gr-LED is believed to be due to a low misfit dislocation density of the aluminum nitride nucleation layer that increases the vertical direction of heat transport to PSS. This work demonstrates that the graphene/PSS substrate is a promising substrate for high-power InGaN LEDs.

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

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

Recombination Rate Analysis of InGaN-Based Red-Emitting Light-Emitting Diodes

The recombination rates are measured and analyzed for red-emitting InGaN light-emitting diodes (LEDs) to better understand the factors that limit their efficiency. InGaN/AlGaN/GaN multiple quantum well (MQWs) are grown with x≥0.28 in the In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> N quantum well. The Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-y</sub> N interlayers (ILs) with high Al-content (y>0.8) are employed because they result in smoother surfaces with smaller V-pits and higher photoluminescence efficiency. The IL-MQWs are formed on GaN and In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-z</sub> N/GaN superlattice (SL) underlayers (ULs) with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</i> = 0.015, 0.025, and 0.065. Differences in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</i> coefficients (radiative recombination) within this set result from changes in wavefunction overlap caused by differences in layer thickness and composition in the IL-MQW. IL-MQWs grown on SL-ULs have <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</i> coefficients (Shockley-Reed-Hall recombination) that are lower than expected, indicating that the SL-ULs help reduce defect formation. Compared to shorter wavelength InGaN-based LEDs, the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</i> coefficients are ~100 times lower due to lower wavefunction overlap. A and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> coefficients are higher because of a higher number of defects.

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.

Low temperature absolute internal quantum efficiency of InGaN-based light-emitting diodes

A method to reveal low temperature absolute internal quantum efficiency of an InGaN-based light-emitting diode (LED) is reported. Structural (via scanning electron and atomic force microscopies, x-ray diffraction, and Raman spectroscopy) and optical (via temperature- and power-dependent photoluminescence) properties of InGaN-based blue LEDs are coupled with a channel-based recombination model, which reveals the low temperature absolute internal quantum efficiency of the LED on traditional sapphire and emerging Si (111) substrates as 27.5% and 71.1%, respectively. High low-temperature absolute internal quantum efficiency of the former is attributed to its lower defect density and less strain. Our results show assuming perfect (i.e., 100%) internal quantum efficiency in an InGaN-based LED at low temperatures could be misleading, and using negative thermal quenching properties of defect luminesce bands with a channel-based recombination model can be a method to quantify LED's less than ideal low temperature internal quantum efficiencies.

Monolithic Multicolor Emissions of InGaN-Based Hybrid Light-Emitting Diodes Using CsPbBr3 Green Quantum Dots.

To address the increasing demand for multicolor light-emitting diodes (LEDs), a monolithic multicolor LED with a simple process and high reliability is desirable. In this study, organic-inorganic hybrid LEDs with violet and green wavelengths were fabricated by depositing CsPbBr3 perovskite green quantum dots (QDs) as the light-converting material on InGaN-based violet LEDs. As the injection current was increased, the total electroluminescence (EL) intensities of the hybrid LEDs increased, whereas the light-converted green emission efficiency of the CsPbBr3 QDs decreased. The maximum green-to-violet EL spectral intensity ratio of the hybrid LEDs with CsPbBr3 QDs was achieved with the injection current of <10 mA. Moreover, the EL spectral ratio of the green-to-violet emission decreased at an injection current of 100 mA. The light-conversion intensity of the CsPbBr3 QDs decreased linearly as the junction temperature of the hybrid LEDs was increased with increasing injection current, similar to the temperature-dependent photoluminescence degradation of CsPbBr3 QDs. In addition, the junction temperature of the hybrid LED was minimized by pulse injection to suppress the thermal degradation of QDs and increase the light conversion efficiency to green emission. Therefore, the overall emission spectrum color coordinates of the hybrid LEDs exhibited a red shift from violet to blue in the low-current region and a blue shift toward violet as the green emission of the QDs was decreased above 10 mA.

Analysis of Current Transport Layer Localized Resistivity Increase After High Stress on InGaN LEDs

This article reports on the effects of high current stress on InGaN-based light-emitting diodes (LEDs.) The analyzed devices are high power blue LEDs with an emitting area of 1 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> subjected to a constant current stress at increasing current until the catastrophic device failure. By means of optical, electrical and photoluminescence analysis we hypothesize that the optical power decrease is related to a degradation in the current transport layers of the devices. The effect has been modelized as a gradual and localized increase in the current transport layer resistance; the model has been validated by means of SPICE electrical simulations. The results clearly indicate that, to extend the operating range of high-power devices, it is crucial to control the current crowding effect, which is here considered to play a significant role in the degradation in the current transport layer.

Optimal design of heat dissipation modules for high-power LED based on the Taguchi method

The semiconductor component of InGaN-based blue light-emitting diodes (LED) emits white light when combined with a yellow phosphor mixture. However, owing to the lattice dislocations and defect points in GaN, it exhibits a high thermal resistance, which leads to heat accumulation and an increase in temperature. This is problematic as overheating causes LED to produce dark spots and lines and reduces the luminous flux and optical power of high-power LED. In this study, we propose a variety of optimal structures for heat-transfer modules and apply the proposed architectures in the assembly of high-power LED. First, the high-power LED substrate was coated with a film of aluminum nitride. Then, copper fins were connected to the vacant spaces in the circuit boards to increase the surface area of the heat-transfer region. The Taguchi method was used to identify the optimal substrate thickness, fin arrangement, and fin depth for the effective heat dissipation in a 12 W high-power LED. A dielectric layer was grown on the surface of the aluminum nitride film to serve as a passivation layer to insulate the patient. The passivation layer reduces the physical damage caused by thermal stress, thereby improving the service life and characteristics of heat-transfer modules. The proposed design not only yields a stable LED substrate (with low thermal stress) but also induces reliable heat transfer.

In-Composition Graded Quantum Barriers for Polarization Manipulation in InGaN-Based Yellow Light-Emitting Diodes

Highly efficient indium gallium nitride (InGaN)-based yellow light-emitting diodes (LEDs) with low efficiency droop have always been pursued for next-generation displays and lighting products. In this work, we report an InGaN quantum barrier (QB) with linear-increase In-composition along [0001] direction for InGaN-based yellow LEDs. With the In-composition in QBs systematically engineered, three QB structures including linear-increase QB (LIQB), linear-decrease QB (LDQB) and commonly used flat QB (FQB) were investigated by simulation. The results show that the LIQB not only yields enhanced electron confinement, but also contributes to suppressed polarization field. Consequently, the yellow LED incorporated with LIQBs demonstrates improved radiative recombination rates and the efficiency droop is alleviated. Under a current density of 100 A/cm2, the efficiency droop ratios of LEDs with FQBs, LDQBs and LIQBs are 58.7%, 62.2% and 51.5%, respectively. When current density varies from 1 A/cm2 to 60 A/cm2, the blueshift values of peak emission wavelength for LEDs with FQBs, LDQBs and LIQBs are 14.4 nm, 16.5 nm and 13.0 nm, respectively. This work is believed to provide a feasible solution for high-performance InGaN-based LEDs in long-wavelength spectral region.