Reference Light-emitting Diodes Research Articles

The beneficial effects of a p-type GaInN spacer layer on the efficiency of GaInN/GaN light-emitting diodes

Light-emitting diodes (LEDs) with a Mg-doped p-type Ga1−xInxN (0 ≤ x ≤ 0.07) spacer layer located between an undoped GaN spacer layer and the electron blocking layer are investigated. The LEDs are found to have comparable peak efficiency but less efficiency droop when the crystal quality of the p-type Ga1−xInxN spacer layer is well-controlled by lowering the growth temperature and by using a suitable In composition and Mg doping concentration. All LED samples with the p-type spacer layer show a smaller efficiency droop compared to a reference LED having an undoped GaN spacer. Among the sample sets investigated, an optical power enhancement of 12% at 111 A/cm2 is obtained when inserting a 5 nm-thick p-type Ga0.97In0.03N spacer layer. The results support that carrier transport is the key factor in the efficiency droop observed in GaN-based LEDs.

Enhanced performance characteristics of n-ZnO/p-GaN heterojunction light-emitting diodes by forming excellent Ohmic contact to p-GaN

We report significantly improved performance characteristics of n-ZnO/p-GaN heterojunction light-emitting diodes (LEDs) by forming excellent Ohmic contact to p-GaN. An elaborately designed fabrication procedure to suppress ZnO sputtering damage on p-GaN, i.e., a selective SiO2 passivation of the p-GaN surface, was shown to yield excellent Ohmic contact to p-GaN with a specific contact resistance of 9.8×10−3Ωcm2, while the reference contact formed on non-protected p-GaN showed non-Ohmic behavior. Heterojunction LEDs fabricated with the improved p-Ohmic contact had much better performance characteristics than did the reference LEDs, i.e., the forward voltages at 2mA were 5.6 and 16.2V, the series resistances were 404 and 1693Ω, and the optical output powers at 3mA were 56.4 and 19.9μW for the developed and reference LEDs, respectively.

Improvement of carrier injection symmetry and quantum efficiency in InGaN light-emitting diodes with Mg delta-doped barriers

The effect of δ-doping of In0.06Ga0.94N barriers with Mg on the quantum efficiency of blue light-emitting-diodes (LEDs) with active regions composed of 6 (hex) 3-nm In0.15Ga0.85N is investigated. Compared to the reference sample, δ-doping of the first barrier on the n-side of the LED structure improves the peak external quantum efficiency (EQE) by 20%, owing to the increased hole concentration in the wells adjacent to the n-side, as confirmed by numerical simulations of carrier distributions across the active region. Doping the second barrier, in addition to the first one, did not further enhance the EQE, which likely indicates compensation of improved hole injection by degradation of the active region quality due to Mg doping. Both LEDs with Mg δ-doped barriers effectively suppress the drop of efficiency at high injection when compared to the reference sample, and the onset of EQE peak roll-off shifts from ∼80 A/cm2 in the reference LED to ∼120 A/cm2 in the LEDs with Mg δ-doped barriers.

Design analysis of phosphor-free monolithic white light-emitting-diodes with InGaN/ InGaN multiple quantum wells on ternary InGaN substrates

Phosphor-free monolithic white light emitting diodes (LEDs) based on InGaN/ InGaN multiple quantum wells (MQWs) on ternary InGaN substrates are proposed and analyzed in this study. Simulation studies show that LED devices composed of multi-color-emitting InGaN/ InGaN quantum wells (QWs) employing ternary InGaN substrate with engineered active region exhibit stable white color illumination with large output power (∼ 170 mW) and high external quantum efficiency (EQE) (∼ 50%). The chromaticity coordinate for the investigated monolithic white LED devices are located at (0.30, 0.28) with correlated color temperature (CCT) of ∼ 8200 K at J = 50 A/cm2. A reference LED device without any nanostructure engineering exhibits green color emission shows that proper engineered structure is essential to achieve white color illumination. This proof-of-concept study demonstrates that high-efficiency and cost-effective phosphor-free monolithic white LED is feasible by the use of InGaN/ InGaN MQWs on ternary InGaN substrate combined with nanostructure engineering, which would be of great impact for solid state lighting.

Improving output power performance of InGaN-based light-emitting diodes by employing step-down indium contents

We investigated the effect of step-down indium content in InGaN quantum wells (QWs) on the output efficiency of fully packaged InGaN-based light-emitting diodes (LEDs). Both the reference and step-down LED chips give maximum external quantum efficiencies (EQE) of 54.65 and 54.99% at a current density of 4.17 A/cm2, respectively. Step-down LEDs show a lower efficiency droop than reference LEDs. The step-down LEDs exhibit a 5.3% higher EQE at 83.3 A/cm2 than the reference LEDs. As the current density increases from 1.39 to 9.03 A/cm2, the electroluminescence (EL) intensity peaks of the step-down LEDs are slightly more shifted towards the larger energy side than those of the reference LEDs. The polarization field is estimated to be 1.34 and 1.41 MV/cm for the reference and step-down LEDS, respectively. The simulated internal quantum efficiency results of the reference and step-down LEDs are in agreement with the experimental results. The simulation results show that the step-down LEDs have higher hole injection efficiency than the reference LEDs. On the basis of the simulation results, the blue-shift behavior of the reference and step-down LEDs is described and discussed.

Polarization-Engineered High-Efficiency GaInN Light-Emitting Diodes Optimized by Genetic Algorithm

A genetic algorithm is employed to find an optimum epitaxial structure of multiple quantum wells (MQWs) and electron-blocking layer (EBL) for a GaInN-based light-emitting diode (LED). The optimized LED is composed of locally Si-doped quantum barriers (QBs) in the MQWs and a quaternary heterostructured AlGaInN EBL having a polarization-induced electric field directed oppositely to that of a conventional AlGaN EBL. The optimized LED shows 15.6% higher internal quantum efficiency, 24.6% smaller efficiency droop, and 0.21 V lower forward voltage at 200 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> comparing to the reference LED, which has fully Si-doped QB and 20-nm-thick Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.19</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.81</sub> N EBL. We find that local Si doping near the QB/QW interface compensates the negative polarization-induced sheet charge at the interface and reduces electric field in the QWs, thereby enhancing electron-hole wave function overlap. In addition, the inverted polarization field in the quaternary EBL provides a high barrier for electrons but a low barrier for holes, resulting in enhanced electron-blocking and hole-injection characteristics.

Promotion of electron confinement and hole injection in GaN-based green light-emitting diodes with a hybrid electron blocking layer

The InGaN/GaN green light-emitting diode (LED) with a hybrid electron blocking layer (HEBL) formed by partially incorporating a small amount of indium in the AlGaN conventional electron blocking layer is proposed. Influence of the HEBL is investigated both experimentally and theoretically. By fitting the quantum efficiency with the modified ABCD model, these coefficients’ values are obtained and the reduced value of D suggests that the HEBL can weaken the carrier leakage. Subsequent simulation shows that the proposed design builds up two higher potential barriers for electrons and one lower potential barrier for holes, which could effectively enhance electron confinement and hole injection. Therefore more carriers could be restricted in multiple quantum wells and participate in radiative recombination. As a result, the measured light output power of the proposed LED is 80% higher than that of the reference LED at a current of 150 mA and its external quantum efficiency droop ratio is also smaller.

Highly reflective Ti/Ag/Pt contacts to p-GaN for high-efficiency GaN-based light-emitting diodes

The authors report highly efficient GaN-based light-emitting diodes (LEDs) fabricated with Ti/Ag/Pt reflective p-type contact. Compared with the reference Ni/Ag/Pt contact, Ti/Ag/Pt contact annealed at optimized thermal annealing condition produced low specific contact resistances of 5.7 × 10−4 Ω cm2, and good optical reflectivity of 88.4% at 450 nm. This is due to the generated interfacial Ti–O layer upon thermal annealing in N2 ambient, which suppress the excessive inter-diffusion between Ag and GaN layers. Consequently, the LEDs fabricated with Ti/Ag/Pt contacts showed 32.6% brighter light output power and nearly the same forward voltages as compared to the reference LEDs fabricated with Ni/Ag/Pt contacts.

Enhanced light extraction in GaN-based light-emitting diodes by evanescent wave coupling effect

Subwavelength-sized truncated cones satisfying the requirement for evanescent wave coupling effect at the emission wavelength were fabricated on the p-side of a GaN-based blue light-emitting diode (LED). The light-extraction efficiency increased by a factor of approximately 2.2 relative to that of a reference LED with a flat light-extraction surface. This light-extraction enhancement ratio is much larger than that realized by using conventional light-extraction techniques. The evanescent wave coupling effect appearing on the surface of the truncated-cone structure is believed to be the mechanism responsible for the large enhancement of light extraction.

Nano-Scale SiO&lt;SUB&gt;2&lt;/SUB&gt; Patterned &lt;I&gt;n&lt;/I&gt;-Type GaN Substrate for 380 nm Ultra Violet Light Emitting Diodes

380 nm Ultraviolet (UV) light emitting diodes (LEDs) were grown on patterned n-type GaN substrate (PNS). Wet etched self-assembled indium tin oxide (ITO) nano clusters serves as dry etching mask for converting the SiO2 layer grown on n-GaN template into SiO2 nano dots by inductively coupled plasma etching. In the pre-experiment, crystal quality and optical properties of n-GaN were greatly improved by applying PNS process. In this work, etch-pits density (EPD) method confirmed that PNS with SiO2 nano dots have superior crystalline properties. Thus Reference LED without PNS, 1-step PNS LEDs with SiO2 nano dots size were 200 nm, 250 nm, 300 nm and 300 nm 2-step PNS LED were fabricated. LEDs show almost the same operating voltage of about 3.4 V at an injection current of 50 mA. Light intensity was enhanced by ~2.1 times and 3.2 times for 300 nm 1-step and 300 nm 2-step PNS, respectively. FDTD simulation results show a similar tendency. As a result, PNS promotes epitaxial lateral overgrowth (ELOG) for defect reduction as well as act as a light scattering point.

Vertical-Injection GaN-Based Light-Emitting Diodes Fabricated With Schottky-Contact Current Blocking Layer

Vertical-injection GaN-based light-emitting diodes (LEDs) fabricated with a Schottky-contact current blocking layer (ScCBL) are reported. The Ar plasma treatment to the heavily Mg-doped p-GaN led to excellent rectifying Schottky behavior, which was found to occur as a result of the suppressed hopping conduction at the contact/p-GaN interface. This could be due to the preferential removal of the Mg dopants by the Ar plasma treatment, as verified by secondary ion mass spectroscopy. Compared with the reference LEDs fabricated with SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> CBL, the LEDs fabricated with ScCBL showed an identifying current-voltage curve, while the output power increased 5%, indicating that the ScCBL fabricated with the Ar plasma treatment could be used practically to improve process yields and lower the cost of vertical LEDs.

Utilizing Two-Dimensional Photonic Crystals in Different Arrangement to Investigate the Correlation Between the Air Duty Cycle and the Light Extraction Enhancement of InGaN-Based Light-Emitting Diodes

This paper demonstrates that the optimal light extraction enhancement of the 2-D photonic crystals (PhCs) light-emitting diodes (LEDs) among different air duty cycles (ADADCsCs) is independent of the geometry and the shape of the 2-D PhCs. Moreover, it also discusses that the side-directional emission property of the 2-D PhCs LEDs in hexagonal lattice arrangement (HLA) is better than that in square lattice arrangement (SLA). Finally, the light output power of the 2-D PhCs LEDs in SLA and in HLA with the ADC of 51% are significantly improved by 60.4% and 81.9%, respectively, as compared with the reference LED at an injection current of 350 mA.

Strong Enhancement in Light Output of GaN-Based LEDs With Graded-Refractive-Index ITO Deposited on Textured V-Shaped Pits

An interesting structure of GaN-based light-emitting diodes (LEDs) with different indium-tin-oxide (ITO) films deposited on naturally textured V-shaped pits (V-pits) is fabricated and studied. The sputtered ITO assists the textured surface to get a better contact and thus reduces the forward voltage. Moreover, the ITO films prepared by different methods can serve as graded-refractive-index antireflective coatings, which can enhance the performance of the V-pits LED further. The results show that the V-pits LED with two layers ITO films have a stronger light output power than the reference LED by 52.1% at 20 mA, and the forward voltage is only a little higher than the standard LED.

Effects of the electrode metal structure and the current blocking layer on the characteristics of blue GaN-based light-emitting diodes

The influence of the electrode metal structure and the current blocking layer on the characteristics of blue GaN-based light-emitting diodes (LEDs) was investigated. The changes in the electrode metal structure, along with the current blocking layer (CBL) were found to enable control of the optical and the electrical properties of the fabricated LEDs. Thus, the light output power at an injection current of 90 mA was increased by about 8.5% for the LED with a Cr/Al/Ni/Au electrode metal and a SiO2 CBL and by about 9.0% for the LED with a Ti/Al/Ni/Au electrode metal and without a SiO2 CBL over the reference LED with only Cr/Ni/Au electrode metal. This was due to the reduction of the current-crowding effect and to the absorption of photons near the P/N pad by the metal electrode. Furthermore, the operating voltage under the same injection current of 90 mA was changed from 3.30 V for the reference LED to 3.37 V and 3.28 V for the LED with a SiO2 CBL and for the LED with Ti/Al/Ni/Au electrode metal, respectively. The results confirmed that the electrode metal structure and the CBL played critical roles in the improvement of the emission characteristics in GaN-based LEDs.

Mesa-Free III-V Nitride Light-Emitting Diodes with Flat Surface

A mesa-free pn-junction GaInN light-emitting diode (LED) structure is proposed and demonstrated. Some of the p-type top surface area of the LED epitaxial wafer is converted from p-type to n-type by means of Si-ion implantation. The resulting pn-junction LED wafer has cathode and anode contact points at the same surface height so that no dry-etching process is needed to access the n-type region. The measured specific contact resistance on the type-converted n-type layer and the light-output power of the mesa-free pn-junction LED are very comparable to those obtained by a reference LED.

Enhancement of light-extraction efficiency in AlGaInP light-emitting diodes using evanescent wave coupling effect

Sub-wavelength-sized ridge structures that satisfy evanescent wave coupling effect requirements at the emission wavelength were fabricated through the photolithography and wet chemical etching of the light-extraction surface of a thin-film type AlGaInP light-emitting diode (LED). By doing this, the light output power relative to a reference LED with a flat light-extraction surface was increased by a factor of approximately 3.8, corresponding to an equivalent enhancement in light-extraction efficiency that is possibly attributable to an evanescent wave coupling effect occurring on the surfaces of the minute ridge structures.

Size-controllable nanopyramids photonic crystal selectively grown on p-GaN for enhanced light-extraction of light-emitting diodes

Size-controllable p-GaN hexagonal nanopyramids (HnPs)-photonic crystal (PhC) structures were selectively grown on flat p-GaN layer for the elimination of total internal reflection of light-emitting diodes (LEDs). The LEDs with HnPs-PhC of 46.3% bottom fill factor (PhC lattice constant is 730 nm) showed an improved light output power by 99.9% at forward current of 350 mA compared to the reference LEDs with flat p-GaN layer. We confirmed the effect of HnPs-PhC with different bottom fill factors and the effect of nanopyramid-shaped and nanocolumn-shaped PhC on the light-extraction of LEDs was also investigated by using three-dimensional finite-difference time-domain simulations.

Nano-patterned dual-layer ITO electrode of high brightness blue light emitting diodes using maskless wet etching

We propose a dual-layer transparent Indium Tin Oxide (ITO) top electrode scheme and demonstrate the enhancement of the optical output power of GaN-based light emitting diodes (LEDs). The proposed dual-layer structure is composed of a layer with randomly distributed sphere-like nano-patterns obtained solely by a maskless wet etching process and a pre-annealed bottom layer to maintain current spreading of the electrode. It was observed that the surface morphologies and optoelectronic properties are dependent on etching duration. This electrode significantly improves the optical output power of GaN-based LEDs with an enhancement factor of 2.18 at 100 mA without degradation in electrical property when compared to a reference LED.

Efficiency Enhancement of White Light-Emitting Diodes via Nano-Textured Silicone Encapsulant

We textured the surface of a silicone encapsulant to increase the extraction efficiency of white light-emitting diodes (LEDs) by using a plasma treatment. Here, the surface morphology could be controlled by changing the plasma condition and texturing morphology of the silicone encapsulant were proportion to the increased ratio of white LEDs. The luminous efficacy of the surface textured LEDs were increased 9.70% relative to the reference LED. Furthermore, the Fourier transform infrared spectroscopy spectrum showed that the chemical bonds of the silicone encapsulant were not changed by the argon-nitrogen plasma treatment, thereby reducing degradation of the optical characteristics and improving the reliability of LEDs.

High-efficiency GaN-based light-emitting diodes fabricated with identical Ag contact formed on both n- and p-layers

The authors report high-efficiency GaN-based light-emitting diodes (LEDs) fabricated with identical Ag contact formed on both n- and p-layers. Ag contacts thermally annealed at optimized conditions yielded low specific contact resistances of 4.5 × 10(-4) and 9.4 × 10(-4) Ωcm(2), and high optical reflectivity (at 450 nm) of 88.1 and 85.3% for n- and p-contact, respectively. LEDs fabricated with identical Ag contacts formed on both layers showed 31% brighter light output power and nearly the same forward voltages as compared to reference LEDs. This indicates that Ag contact can be used as a reflective electrode for both n- and p-layers, leading to enhanced extraction efficiency and fewer process steps.