InGaN-based Light-emitting Diodes Research Articles

A quantitative method for determination of carrier escape efficiency in GaN-based light-emitting diodes: A comparison of open- and short-circuit photoluminescence

We propose a method to quantitatively analyze the internal quantum efficiency (IQE) as well as the efficiencies of non-radiative recombination in the active region (NRA) and carrier escape out of the active region (ESC) by comparing open-circuit (OC) to short-circuit (SC) conditions of InGaN-based light-emitting diodes (LEDs). First, the IQE was extracted from excitation-power dependent photoluminescence at low temperature, and the electron-hole wavefunction overlaps were calculated under OC and SC conditions. Then, the NRA and ESC efficiencies were quantitatively deduced and also compared with photocurrent data. The proposed method would be useful for assessing and designing quantum barriers and analyzing leakage current in LEDs.

Correction: Corrigendum: A novel wavelength-adjusting method in InGaN-based light-emitting diodes

The pursuit of high internal quantum efficiency (IQE) for green emission spectral regime is referred as “green gap” challenge. Now researchers place their hope on the InGaN-based materials to develop high-brightness green light-emitting diodes. However, IQE drops fast when emission wavelength of InGaN LED increases by changing growth temperature or well thickness. In this paper, a new wavelength-adjusting method is proposed and the optical properties of LED are investigated. By additional process of indium pre-deposition before InGaN well layer growth, the indium distribution along growth direction becomes more uniform, which leads to the increase of average indium content in InGaN well layer and results in a redshift of peak-wavelength. We also find that the IQE of LED with indium pre-deposition increases with the wavelength redshift. Such dependence is opposite to the IQE-wavelength behavior in conventional InGaN LEDs. The relations among the IQE, wavelength and the indium pre-deposition process are discussed.

Improvement of light power and efficiency droop in GaN-based LEDs using graded InGaN hole reservoir layer

InGaN-based light-emitting diodes with graded indium composition p-type InGaN hole reservoir layer (HRL) are numerically investigated using the APSYS simulation software. It is found that by gradient increasing indium composition in growth direction of the p-InGaN HRL can improve light output power, lower current leakage and efficiency droop. Based on numerical simulation and analysis, these improvements on the electrical and optical characteristics are attributed mainly to tailoring energy band in p–n junction vicinal region, and finally enhanced the hole injection efficiency and electron blocking efficiency.

A GaN–AlGaN–InGaN last quantum barrier in an InGaN/GaN multiple-quantum-well blue LED

The advantages of a GaN–AlGaN–InGaN last quantum barrier (LQB) in an InGaN-based blue light-emitting diode are analyzed via numerical simulation. We found an improved light output power, lower current leakage, higher recombination rate, and less efficiency droop compared with conventional GaN LQBs. These improvements in the electrical and optical characteristics are attributed mainly to the specially designed GaN–AlGaN–InGaN LQB, which enhances electron confinement and improves hole injection efficiency.

High brightness InGaN-based yellow light-emitting diodes with strain modulation layers grown on Si substrate

InGaN-based multiple quantum wells (MQWs) yellow light-emitting diodes (LEDs) were grown on Si substrate by metal organic vapor deposition. Blue MQWs were introduced as strain modulation layers for yellow MQWs. The LED chips emitted 72-mW yellow light with 566-nm dominant wavelength and 9.4 % external quantum efficiency (EQE) at 350 mA under room temperature, and it reached a peak EQE of 22.2 % at 0.7 mA. A comparison sample without strain modulation layers exhibited much weaker performance. The results reveal that long-wavelength emission of InGaN system is reliable if the strain of MQWs has been properly modulated.

Dramatic reduction in process temperature of InGaN-based light-emitting diodes by pulsed sputtering growth technique

P-type doping of GaN by pulsed sputtering deposition (PSD) at a low growth temperature of 480 °C and dramatic reduction in the growth process temperature for InGaN-based light-emitting diodes (LEDs) were achieved. Mg-doped GaN layers grown on semi-insulating GaN at 480 °C exhibited clear p-type conductivity with a hole concentration and mobility of 3.0 × 1017 cm−3 and 3.1 cm2 V−1 s−1, respectively. GaN/In0.33Ga0.67N/GaN LEDs fabricated at 480 °C showed clear rectifying characteristics and a bright electroluminescence emission near 640 nm. These results indicate that this low temperature PSD growth technique is quite promising for the production of nitride-based light-emitting devices on large-area glass substrates.

Effect of Stair-Case Electron Blocking Layer on the Performance of Blue InGaN Based LEDs

Staircase electron blocking layer (EBL) is incorporated in InGaN-based blue light-emitting diodes to numerically investigate the efficiency droop mechanism by using the APSYS simulation software. It is found that gradually reducing aluminum (Al) composition in the growth direction of the AlGaN staircase EBL can improve light output power, lower current leakage, and efficiency droop. To the contrary, increasing the Al composition in the staircase EBL along the growth direction will aggravate the electron leakage and efficiency droop. These distinct features are attributed mainly to discrepancy energy band tailoring in the EBL region, and finally different electron blocking efficiency.

Orangish-yellow-emitting Ca3Si2O7:Eu2+ phosphor for application in blue-light based warm-white LEDs

A Eu(2+)-activated Ca3Si2O7:Eu(2+) orangish-yellow-emitting phosphor with strong luminescence was synthesized and its crystal structure was determined on the basis of XRD profiles using synchrotron radiation. The crystal structure was refined by the Rietveld refinement method. The excitation and emission spectra of the Ca3Si2O7:Eu(2+) phosphor show broad excitation bands in the range of 240-550 nm and a broad yellow emission band centered at 603 nm, depending on the concentration of Eu(2+). The optimized concentration of Eu(2+) in the Ca3Si2O7:Eu(2+) phosphor was determined to be 0.015 mol. The critical distance and average decay time were found to be short and fast, respectively, ranging from 19.74 Å to 13.69 Å and from 2.56 μs to 2.34 μs on increasing the Eu(2+) doping content. Warm-white light-emitting diodes (LEDs) fabricated using an InGaN-based blue LED chip combined with the Ca3Si2O7:0.015Eu(2+) phosphor gave color rendering indices between 76.0 and 38.9, correlated color temperatures between 1924 K and 4992 K, and tuned CIE chromaticity coordinates in the range from orangish-yellow (0.543, 0.389) to reddish purple (0.333, 0.219). The color coordinates and emission intensity of a Ca3Si2O7:0.015Eu(2+)-based white LED display were slightly yellow-shifted and the intensity increased on increasing the forward-bias current. These results indicate that orangish-yellow-emitting Ca3Si2O7:0.015Eu(2+) can serve as a promising candidate for applications in warm-white LEDs.

Improved light output from InGaN LEDs by laser-induced dumbbell-like air-voids

We report inducing an array of dumbbell-like air-voids inside the sapphire substrate in InGaN-based light-emitting diodes (LEDs) to improve the light extraction from LED device by a picosecond (Ps) pulse laser. At an injection current of 100 mA, the light output power (LOP) of packaged LEDs with laser-induced air-voids can be improved by 24.7% compared with conventional LEDs. Far-field radiation pattern has verified that this great improvement in LOP is due to the light scattering occurred at the interface of sapphire/air-voids. Current-Voltage curves show that the laser processing of air-voids will not degrade the LED electrical properties. Furthermore, leakage current at a level of ~5 nA at -10V has demonstrated an enhancement in the LED electrical performance with laser-induced air-voids. Second focusing mechanism, which is originated in the local heating effect around the laser focus, has been proposed to explain the formation of dumbbell-like air-voids.

A novel wavelength-adjusting method in InGaN-based light-emitting diodes

The pursuit of high internal quantum efficiency (IQE) for green emission spectral regime is referred as “green gap” challenge. Now researchers place their hope on the InGaN-based materials to develop high-brightness green light-emitting diodes. However, IQE drops fast when emission wavelength of InGaN LED increases by changing growth temperature or well thickness. In this paper, a new wavelength-adjusting method is proposed and the optical properties of LED are investigated. By additional process of indium pre-deposition before InGaN well layer growth, the indium distribution along growth direction becomes more uniform, which leads to the increase of average indium content in InGaN well layer and results in a redshift of peak-wavelength. We also find that the IQE of LED with indium pre-deposition increases with the wavelength redshift. Such dependence is opposite to the IQE-wavelength behavior in conventional InGaN LEDs. The relations among the IQE, wavelength and the indium pre-deposition process are discussed.

Suppressed quantum-confined Stark effect in InGaN-based LEDs with nano-sized patterned sapphire substrates

This paper demonstrates that quantum-confined Stark effect (QCSE) within the multiple quantum wells (MQWs) can be suppressed by the growths of InGaN-based light-emitting diodes (LEDs) on the nano-sized patterned c-plane sapphire substrates (PCSSs) with reducing the space. The efficiency droop is also determined by QCSE. As verified by the experimentally measured data and the ray-tracing simulation results, the suppressed efficiency droop for the InGaN-based LED having the nano-sized PCSS with a smaller space of 200 nm can be acquired due to the weaker function of the QCSE within the MQWs as a result of the smaller polarization fields coming from the lower compressive strain in the corresponding epitaxial layers.

Advantage of InGaN-based light-emitting diodes with trapezoidal electron blocking layer

In this study, a trapezoidal-shaped electron blocking layer is proposed to improve efficiency droop of InGaN/GaN multiple quantum well light-emitting diodes. The energy band diagram, carrier distribution profile, electrostatic field, and electron current leakage are systematically investigated between two light-emitting diodes with different electron blocking layer structures. The simulation results show that, when traditional AlGaN electron blocking layer is replaced by trapezoidal-shaped electron blocking layer, the electron current leakage is dramatically reduced and the hole injection efficiency in markedly enhanced due to the better polarization match, the quantum-confined Stark effect is mitigated and the radiative recombination rate is increased in the active region subsequently, which are responsible for the alleviation of efficiency droop. The optical performance of light-emitting diodes with trapezoidal-shaped electron blocking layer is significantly improved when compared with its counterpart with traditional AlGaN electron blocking layer.

InGaN-based resonant-cavity light-emitting diodes with dielectric-distributed Bragg reflectors

An InGaN-based resonant-cavity light-emitting diode (RC-LED) chip with a large chip area of 1 × 1 mm2 was fabricated using ZrO2/SiO2 dielectric-distributed Bragg reflectors (DBRs) on both sides of the cavity. After the growth of InGaN-based n- and p-epitaxial layers on a sapphire substrate, ZrO2/SiO2 layers for one DBR were deposited on it; then, holes with ∼10-µm diameters were patterned through the DBR layers. The DBR layer’s side of the whole sample was thermally bonded to a silicon substrate by using Au bonding metal; then, the sapphire substrate of the sample was removed using a laser lift-off technique. ZrO2/SiO2 layers for the other DBR were deposited on n-GaN for complete formation of the cavity. Through the electrical and the optical characterizations, we showed that our RC-LED has the improved optical output power and forward directionality in comparison with a conventional LED.

InGaN-Based Light-Emitting Diodes Fabricated on Nano Patterned Sapphire Substrates with Pillar Height of More than 600 nm by Nanoimprint Lithography

GaN-based light-emitting diodes (LEDs) were fabricated on nano patterned sapphire substrates (nano-PSSs) by nanoimprint (NIP) lithography. A nano-PSS with a pillar height of more than 600 nm was achieved. The surface emission of the LEDs was strongly affected by pillar height, and the surface emission intensity was highest at a pillar height of 250 nm. In contrast, the external quantum efficiency of the LEDs on the nano-PSSs with diameters of 100 and 450 nm was approximately 30% higher than that on a flat sapphire substrate, which is similar to that on a conventional PSS.

Improved light emission of GaN-based light-emitting diodes by efficient localized surface plasmon coupling with silver nanoparticles

Abstract An approach for fabricating localized surface plasmon (LSP)-enhanced light-emitting diodes (LEDs) with Ag nanoparticles (NPs) in close proximity to the active layers is demonstrated. The blue light emission from InGaN/GaN multiple quantum wells (MQWs) is increased by 4.4-fold at a wavelength of 470 nm. A faster decay time and a ∼1.5-fold higher spontaneous emission rate confirm the fast recombination channel provided by the LSP coupling mode of the Ag NPs. The internal quantum efficiency is improved by ∼53%. The proposed approach provides a low-cost, large-area, and simple platform for the development of high-efficiency InGaN-based plasmonic LEDs.

Investigation of Light Extraction Efficiency and Internal Quantum Efficiency in High-Power Vertical Blue Light-Emitting Diode with 3.3 W Output Power

Light extraction efficiency (LEE) and internal quantum efficiency (IQE) of InGaN-based vertical blue light-emitting diode (LED) structures are investigated by numerical simulations and experiments. LEE of vertical LEDs is calculated for various structural and material parameters by using three-dimensional finite-difference time-domain (FDTD) simulations, and the optimum textured patterns on the n-GaN surface is found from the FDTD simulation. High-power vertical LED structures are fabricated based on the simulation results. The output power at 3 A injection current is measured to be 3.3 W, and the peak value of the external quantum efficiency (EQE) is found to be 64%. In addition, LEE of the fabricated vertical LED is expected to be 70–80% from the FDTD simulations. Combining the results of EQE and LEE, the peak IQE of the experimented vertical LED can be estimated to be 80–90%.

Efficiency and droop improvement in a blue InGaN-based light emitting diode with a p-InGaN layer inserted in the GaN barriers

The advantages of a blue InGaN-based light-emitting diode with a p-InGaN layer inserted in the GaN barriers is studied. The carrier concentration in the quantum well, radiative recombination rate in the active region, output power, and internal quantum efficiency are investigated. The simulation results show that the InGaN-based light-emitting diode with a p-InGaN layer inserted in the barriers has better performance over its conventional counterpart and the light emitting diode with p-GaN inserted in the barriers. The improvement is due to enhanced Mg acceptor activation and enhanced hole injection into the quantum wells.

Line-of-Sight Visible Light Communications With InGaN-Based Resonant Cavity LEDs

Efficient InGaN-based resonant-cavity light-emitting diodes (RCLEDs) are investigated as potential light sources for visible light communications. Experimentally, InGaN multiple-quantum-well LEDs with a reasonable crystalline quality for blue light emission were grown by metal organic vapor phase epitaxy. A two-pair distributed Bragg reflector, along with a metallic Ag layer, was used to form a structure containing an optical cavity. The resulting InGaN RCLEDs exhibit improved performance over conventional LEDs in terms of light output power, external quantum efficiency, and directionality. In addition, two plano-convex lenses with facing convex surfaces were used to build a directed line-of-sight optical link between the transmitter and the receiver. Data transmission with rates up to 100 Mbit/s is demonstrated over a distance of 100 cm, implying the potential for indoor visible light communications with phosphor-converted white RCLEDs, provided that the slow-responding phosphorescent emission could be effectively filtered.

Fabrication of warm, high CRI white LED using non-cadmium quantum dots

We reported on the synthesis of two bright non-cadmium quantum dots (QDs) of green (512 nm)-emitting InP/ZnS and orange (583 nm)-emitting CuInS2 (CIS)/ZnS core/shell and their application for the fabrication of solid-state lighting device. The spectral overlap between absorptions from both QDs and emission from InGaN-based blue light-emitting diode (LED) chip was excellent. Thus, the efficient down-conversion of blue-to-QD emission for the generation of white light was accomplished by sequentially dispensing InP/ZnS QDs on top of CIS/ZnS ones within epoxy resin. The white QD-LED generated a high-quality white light with a high color rendering index of 90 and a warm color temperature of 3803K under a drive current of 20 mA. Drive current-dependent variations of its primary electroluminescent properties were investigated in details.

Temperature and injection current dependent optical and thermal characteristics of InGaN-based green large-area light-emitting diodes

Optical, spectral, and thermal characteristics of InGaN/GaN multiple quantum well green light-emitting diodes (LEDs) with a large chip size of 0.8 × 1 mm2, operating at λ ∼ 525 nm, on patterned sapphire substrate (PSS) were studied. The temperature-dependent optical and spectral properties of LEDs were measured and analyzed. The junction temperature (Tj) was also determined by the forward voltage method, in comparison with the simulation results which were theoretically calculated by a three-dimensional anisotropic heat dissipation model based on the finite element method. Under an injection current of 350 mA at 298 K, the optical output power and forward voltage were obtained to be 63.5 mW and 3.98 V, respectively. The characteristic temperature was found to be ∼438 K at an injection current of 350 mA. The Tj was increased with increasing the injection current. From the measured Tj values, the thermal resistance (Rth) value was experimentally obtained to be ∼6.31 K W−1, which was well consistent with the simulated result (Rth ∼ 6.24 K W−1).