Properties Of GaN-based Light-emitting Diodes Research Articles

Modulation and optoelectronic properties of GaN-based light-emitting diodes on GaN template

We investigated the modulation responses and optoelectronic properties of light-emitting diodes (LEDs) grown on free-standing (0001) GaN. These LEDs have a larger modulation bandwidth than those grown on sapphire at a higher current density, and a maximum of −3 dB modulation bandwidth of 510 MHz was achieved, which is 1.7 times larger compared to LEDs grown on sapphire. In addition, due to the lower substrate temperature, there may be an increase in the indium that is incorporated into the active region as well as the point defects of the LEDs grown on GaN, which will influence the optoelectronic properties.

Modelling of microcavity effect in InGaN/GaN heterostructures for interfacial study

Photoluminescence (PL) spectrum provides the most conventional measurement for emission properties of GaN-based light-emitting diodes (LEDs), in which Fabry–Perot oscillations are often observed modulating the emission peaks. A fitting model for PL intensity accounting the microcavity between air/GaN and GaN/sapphire heterostructure was proposed to treat the PL spectra of different GaN-based LEDs, extracting key factors that implied the interfacial optical properties. This approach was successfully verified by measurements of spectroscopic ellipsometry, then was applied to quantitatively analyse the interfacial-defect-related distortion of dielectric properties. The extracted oscillation coefficient is sensitive to the change of interface qualities and reveals the optical properties of internal interfaces. The new method may also be applied to the other heterojunction LEDs.

Surface morphological, structural, electrical and optical properties of GaN-based light-emitting diodes using submicron-scaled Ag islands and ITO thin films

An ITO-Ag islands complex as a new transparent conducting electrode (TCE) structure (on the 5 nm-thick p-InGaN/90 nm-thick p-GaN) for achieving high-performance and more reliable GaN-based LEDs were fabricated. A normal LED with a conventional ITO TCE was also compared. The surface morphological, structural, electrical and optical properties of fabricated GaN-based light-emitting diodes using a complex electrode of submicron-scaled Ag islands and ITO thin films are explored by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), current-voltage (I-V) and output power-current (L-I) techniques. Surface morphology investigations revealed Ag islands formed uniformly on the p-InGaN/p-GaN surface during rapid thermal annealing at 400 °C for 1 min in N2 ambient. The ohmic properties and overall device-performance of the suggested contact and device structures were superior to those in the conventional ITO contact and normal ITO LED structures. Based on the results of XRD and XPS measurements, the formation of the intermetallic gallide phases (AgGa) is responsible for better performance characteristics of the ITO-Ag islands device. The significant improvements are described in terms of the conducting bridge influence, highly effective micro-mirror effect, and wider photon window via the roughened structure.

Effects of p-type GaN thickness on optical properties of GaN-based light-emitting diodes

The influence of p-type GaN (pGaN) thickness on the light output power (LOP) and internal quantum efficiency (IQE) of light emitting diode (LED) was studied by experiments and simulations. The LOP of GaN-based LED increases as the thickness of pGaN layer decreases from 300 nm to 100 nm, and then decreases as the thickness decreases to 50 nm. The LOP of LED with 100-nm-thick pGaN increases by 30.9% compared with that of the conventional LED with 300-nm-thick pGaN. The variation trend of IQE is similar to that of LOP as the decrease of GaN thickness. The simulation results demonstrate that the higher light efficiency of LED with 100-nm-thick pGaN is ascribed to the improvements of the carrier concentrations and recombination rates.

Performance Analysis of GaN-Based Light-Emitting Diodes With Lattice-Matched InGaN/AlInN/InGaN Quantum-Well Barriers

In this paper, the properties of GaN-based light-emitting diodes (LEDs) with lattice-matched InGaN/AlInN/InGaN (LM-IAI) quantum-well (QW) barriers are investigated numerically. Distributions of electrostatic field, carrier current density, carrier concentration and radiative recombination rate are simulated, and internal quantum efficiency (IQE) and emission power are calculated. The results show that the LEDs with LM-IAI barriers have higher IQE and emission power over their conventional counterparts with GaN barriers due to the mitigation of the quantum-confined Stark effect and the suppression of electron leakage. Furthermore, the performances of the nitride-based LEDs with the proposed barriers can be further improved by dismissing the electron-blocking layer, which is attributed to the improvement of hole injection efficiency and the decrease of overall Auger recombination.

Fabrication and characteristics of GaN-based light-emitting diodes with a reduced graphene oxide current-spreading layer.

A reduced graphene oxide (GO) layer was produced on undoped and n-type GaN, and its effect on the current- and heat-spreading properties of GaN-based light-emitting diodes (LEDs) was studied. The reduced GO inserted between metal electrode and GaN semiconductor acted as a conducting layer and enhanced lateral current flow in the device. Especially, introduction of the reduced GO layer on the n-type GaN improved the electrical performance of the device, relative to that of conventional LEDs, due to a decrease in the series resistance of the device. The enhanced current-spreading was further of benefit, giving the device a higher light output power and a lower junction temperature at high injection currents. These results therefore indicate that reduced GO can be a suitable current and heat-spreading layer for GaN-based LEDs.

Properties of GaN-based light-emitting diodes on patterned sapphire substrate coated with silver nanoparticles prepared by mask-free chemical etching

This study reports on the use of a template that is made of silver nanoparticles (ANPs) that are dispersed on a patterned sapphire substrate (PSS) to improve the light output power of GaN-based light-emitting diodes (LEDs). The dipping of a sapphire substrate in hot H2SO4 solution generates white reaction products that are identified as a mixture of polycrystalline aluminum sulfates. These white reaction products can act as a natural etching mask in the preparation of an ANP-coated PSS (PSS-ANP) template. The optimal annealing temperature and time, surface morphology, and optical characteristics of the PSS-ANP template were investigated. The light output power of an LED that is bonded to the PSS-ANP template is approximately double than that of an LED that is not.

Enhanced Optical and Electrical Properties of GaN-Based Light-Emitting Diodes with Interconnecting Indium Tin Oxide Nanowires Using Self-Assembled NaCl Particles

Enhanced Optical and Electrical Properties of GaN-Based Light-Emitting Diodes with Interconnecting Indium Tin Oxide Nanowires Using Self-Assembled NaCl Particles

Junction temperature and optoelectronic properties of GaN-based light-emitting diodes with through-hole prepared by laser drill

This study demonstrates the feasibility of a large-area (1 mm × 1 mm) GaN-based light-emitting diode (LED) with a through-hole prepared using a laser drill and a back side Au thermally conductive layer. At 350 mA, the light output powers for GaN LEDs with and without (control sample) the through-hole were 45.2 and 39 mW, respectively. The improved light output power of the LED with the through-hole is caused by a thermal conduction effect. The junction temperature of GaN-based LEDs with the through-hole increased slightly from 19 to 29 °C as the injection current increased from 20 to 400 mA.

Effect of Varied Undoped GaN Thickness on ESD and Optical Properties of GaN-Based LEDs

The optical property and electrostatic discharge (ESD) endurance of GaN-based light-emitting diodes (LEDs) with varied undoped GaN thickness are studied and demonstrated. As the undoped GaN thickness increased, the generation of V-shaped defects in the active region was suppressed. Therefore, the output power for a 6-μ.m-thick undoped GaN layer would be enhanced remarkably due to the reduction of nonradiative recombination ratio within the active region. On the other hand, under a reverse ESD pulse voltage of 5.5 kV, the survival rate of the LEDs with an undoped GaN layer thickness of 1.5, 4, and 6 μm were 75%, 65%, and 55%, respectively. It was found that the ESD endurance for a 1.5-μ.m-thick undoped GaN layer with higher internal capacitance was obviously better than others. This could be related to the built-in electric field of LEDs induced by spontaneous polarization field.

Improved light output power of GaN-based light-emitting diodes by using Ag grids

We investigate the effect of 1-(1-D) and 2-dimensional (2-D) patterned Ag grids combined with Al-doped ZnO contacts on the electrical and optical properties of GaN-based light-emitting diodes (LEDs). The ratio of the grid width to the gap between the grids varies from 2.8 for the 1-D to 38.7 for the 2-D grids. All of the patterned Ag grid/AZO contacts show transmittances in the range of 86.4-94.0%. The LEDs fabricated with the differently patterned Ag grid/AZO contacts exhibit forward-bias voltages ranging from 3.67 to 4.62V at an injection current of 20mA, which are much lower than those (7.44V) of the LEDs with the AZO only contacts. It is further shown that the LEDs with the patterned Ag grid/AZO contacts produce a 78.0-108.7% higher output power (at 20mA) than those with the AZO only contacts. The reason for the improved output performance of the LEDs with the Ag grid/AZO contacts is briefly described in terms of the contact areas and plasma-induced damage of p-GaN.

Comparison of GaN-Based Light-Emitting Diodes by Using the AlGaN Electron-Blocking Layer and InAlN Electron-Blocking Layer

Optical properties of GaN-based light-emitting diodes (LEDs) are studied numerically by using AlGaN and InAlN electron-blocking layers (EBLs). Through the simulations of emission spectra, carrier concentration distribution, energy band, electrostatic field, internal quantum efficiency and output power, the results show that the LEDs with design of the InAlN EBL structure have a better performance over the original LEDs using an AlGaN EBL. The spectrum intensity and output power are enhanced significantly, and the efficiency droop of internal quantum efficiency is improved effectively with this design of InAlN EBL structure. It is proved that the strengths of carrier confinement and electron leakage current play a critical role in the performance of luminescence in LEDs.

Effect of Growth Pressure of Undoped GaN Layer on the ESD Characteristics of GaN-Based LEDs Grown on Patterned Sapphire

The effect of growth pressure of underlying undoped GaN(u-GaN) layer on the electrical properties of GaN-based light-emitting diodes (LEDs) grown on patterned sapphire substrates (PSS) is evaluated. The electrostatic discharge (ESD) endurance voltages could increase from 4000 to 7000 V when the growth pressure of u-GaN layers is increased from 100 to 500 torr, while the forward voltages and light output powers remain almost the same. Poor ESD endurance ability could be attributed to the underlying GaN layer grown under relative low pressure, which leads to significant surface pits. This could be further attributed to the imperfect coalescence of crystal planes above the convex sapphire patterns. The pits are associated with TDs behaving as a leakage path to degrade electrical performance.

Enhanced ESD properties of GaN-based light-emitting diodes with various MOS capacitor designs

High electrostatic discharge (ESD) protection of GaN-based light-emitting diodes (LEDs) has been developed using a metal–oxide semiconductor (MOS) capacitor. This structure is realized by adopting various metal electrode patterns. The MOS capacitor can be implemented by extending the metal line directly from the p-type electrode to the top surface of an SiO 2-capped n-GaN layer near the vicinity of the n-type electrode. By connecting a MOS capacitor in parallel with the GaN-based LED, the negative ESD strike could be significantly increased from 385 to 1075 V of human body mode (HBM).

Effects of electrode shape on the properties of GaN-based light-emitting diode

Using Crosslight APSYS software, we study the effect of the electrode shapes on the electrical and optical properties for the chip structures of p-GaN, InGaN/InGaN multiple quantum well, n-GaN and sapphire. Six kinds of optimized electrodes are presented. By optimzing the electrode shape, the current density distribution are more uniform, and the current crowding effects are reduced. And for the chips with optimized electrodes their electrical and optical properties are improved, and the light emission efficiencies and transferring efficiencies have been also improved.

Improvement of Electrostatic Discharge Characteristics and Optical Properties of GaN-Based Light-Emitting Diodes

To improve the positive- and negative-voltage electrostatic discharge (ESD) characteristics of GaN light-emitting diodes (LEDs), an air gap was introduced as an ESD protection structure in an Al film on the bottom side of a sapphire substrate. The negative-voltage ESD characteristic of GaN LEDs with an air gap was remarkably improved from -0.3 to -4 kV. The degradation of electroluminescent intensity of GaN LEDs, which was caused by ESD stress, was also suppressed by an air gap in GaN LEDs. An ESD-stress-induced current is believed to flow in an air gap to protect the multiquantum well of GaN LEDs.

Growth and Characterizations of GaN-Based LEDs Grown on Wet-Etched Stripe-Patterned Sapphire Substrates

The material and electrical properties of GaN-based light-emitting diodes (LEDs) grown on wet-etched stripe-patterned substrates were investigated. Footprint-like patterns, located directly above the inclined groove sidewalls, were found on the as-grown LED surface. Cross-sectional transmission electron microscopy (TEM) showed that ‘tumor’-like structures with poor crystal quality were initiated on the inclined sidewalls, seeding dislocation bundles in the subsequently grown crystal. The high dislocation density slowed down the growth above the inclined sidewall, resulting in the uneven morphology. The fabricated devices showed over 30% enhancement in light output power as a result of improvements in both internal and extraction efficiencies.

Temperature Effect on the Optoelectronic Properties of GaN-based Light-Emitting Diodes with ITO p-Contacts

In this study, indium-tin-oxide (ITO) films were used as p-contacts in GaN-based light-emitting diodes (LEDs). The dependence of the optoelectronic performance of LEDs on various annealing temperatures of ITO films was studied. At an annealing temperature of the transmittance of ITO films can reach 98.6% at a wavelength of . The specific contact resistance characterized by the circular transmission line model is as low as . Furthermore, GaN-based LEDs employing an ITO p-contact annealed at have high luminescence intensities of 243.2 mcd at a injection current. Under the same injection current, the proposed LED also has the largest electroluminescence intensity with the ITO p-contact annealed at .

Effects of the passivation layer deposition temperature on the electrical and optical properties of GaN-based light-emitting diodes

In this paper, silicon oxynitride is deposited through plasma-enhanced chemical vapor deposition (PECVD) to serve as an antireflection passivation layer. We have studied the effects of the deposition temperature (from 100 to 300 °C) on the electrical and optical performances of GaN-LEDs. It is found that the light output of GaN-LEDs improves greatly after the deposition of SiON antireflection passivation layer at 200 °C and is better than that of GaN-LEDs whose layers are deposited at 100 and 300 °C. The electrical properties of GaN-LED do not degrade at 100 and 200 °C, but degrade significantly at 300 °C.