High-power GaN-based Light-emitting Diodes Research Articles

Alleviation of efficiency droop in InGaN/GaN multiple quantum well light-emitting diodes with trapezoidal quantum barriers

We numerically investigated the effects of trapezoidal quantum barriers (QBs) on efficiency droop in InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs). Simulations showed that the electrostatic field in QWs of LEDs with trapezoidal barriers is reduced because of the reduced sheet charge density at the QW-QB interface caused by the thin GaN layer in trapezoidal QBs. Additionally, the InGaN grading region in trapezoidal QBs suppresses hot carrier transport and this enhances efficient carrier injection into the QWs. The electroluminescence intensity of an LED with trapezoidal QBs is increased by 10.2% and 6.7% at 245 A cm−2 when compared with the intensities of LEDs with square-type GaN barriers and multilayer barriers, respectively. The internal quantum efficiency (IQE) droop of an LED with trapezoidal QBs is 16% at 300 A cm−2, while LEDs with square-type GaN barriers and multilayer barriers have IQE droop of 31% and 24%, respectively. This IQE droop alleviation in LEDs with trapezoidal QBs is attributed to the reduced energy band bending, efficient hole injection, and more uniform hole distribution in the MQWs that results from reduction of the piezoelectric field by the trapezoidal QBs. These results indicate that the trapezoidal QB in MQWs is promising for enhanced efficiency in high-power GaN-based LEDs.

Implementation of light extraction improvements of GaN-based light-emitting diodes with specific textured sidewalls

Textured-sidewall GaN-based light-emitting diodes (LEDs) with various sidewall angles (15–90°) and convex or concave sidewalls prepared using an inductively-coupled-plasma approach are comprehensively fabricated and studied. The device with 45° sidewalls (Device F) and that with convex sidewalls (Device B) show significant improvements in optical properties. Experiments show that, at an injection current of 350 mA, the light output power, external quantum efficiency, wall-plug efficiency, and luminous flux of Device F (Device B) are greatly improved by 18.3% (18.2%), 18.2% (18.2%), 17.3% (19.8%), and 16.6% (18.4%), respectively, compared to those of a conventional LED with flat sidewalls. In addition, negligible degradation in electrical properties is found. The enhanced optical performance is mainly attributed to increased light extraction in the horizontal direction due to a significant reduction in total internal reflection at the textured sidewalls. Therefore, the reported specific textured-sidewall structures (Devices B and F) are promising for high-power GaN-based LED applications.

Light Extraction Enhancement of GaN-Based Light-Emitting Diodes With Textured Sidewalls and ICP-Transferred Nanohemispherical Backside Reflector

Textured-sidewall GaN-based light-emitting diodes (LEDs) with convex and 45° patterns and an inductively coupled plasma (ICP)-transferred nanohemispherical backside reflector, formed using an ICP etching process, are fabricated and studied. For devices with textured sidewalls, the scattering probability of photons at the GaN/air interface is increased and the light extraction efficiency is enhanced since photons are allowed to find escape cones in the horizontal direction. With the ICP-transferred nanohemispherical backside reflector, reflected photons can be easily scattered and redirected in arbitrary directions for light extraction and thus have more opportunities to escape the devices. The LED with 45° sidewalls and a backside reflector exhibited the significant improvements of 55.8%, 49.3%, 47.2%, and 55.4% in light output power, luminous flux, external quantum efficiency, and wall-plug efficiency, respectively, compared to those of a conventional LED without these specific designs (Device A). In addition, the higher intensities in a light emission mapping image and improved far-field patterns are obtained for the studied device. The enhanced optical performance is mainly attributed to the increased light extraction in all directions due to a significant reduction in the total internal reflection by the textured sidewalls and a backside reflector. Therefore, textured-sidewall structures and an ICP-transferred nanohemispherical backside reflector are promising for high-power GaN-based LED applications.

Enhanced Light Extraction of a High-Power GaN-Based Light-Emitting Diode With a Nanohemispherical Hybrid Backside Reflector

A high-power GaN-based light-emitting diode (LED) with an inductively coupled plasma (ICP)-transferred nanohemispherical hybrid backside reflector is studied. A self-assembled 100 ± 5 nm SiO2 nanosphere monolayer is drop-coated on the backside of a sapphire substrate as a mask to transfer nanohemispherical patterns onto the backside of the sapphire substrate by ICP. Nanohemispherical patterns could be transferred to the deposited backside reflector. Thus, reflected photons could be redirected and scattered into arbitrary directions for light extraction. As compared with a conventional LED without a backside reflector, at 350 mA, the studied device exhibits a 118.2% enhancement in light output power without the degradation of electrical properties. Note that the adhesion between an ICP-transferred sapphire substrate and the hybrid backside reflector is better than when directly inserting an SiO2 nanosphere monolayer in the device. Thus, the process yield could be enhanced for applying in the solid-state lighting.

Effects of pre-annealed ITO film on the electrical characteristics of high-reflectance Ni/Ag/Ni/Au contacts to p-type GaN

In this study, a Ni/Ag/Ni/Au multilayer with first Ni layer of 0.5nm was first optimized for high reflectivity (92.3%), low specific contact resistance (2.1×10−3Ωcm2) and good attachment strength to p-type GaN. To further decrease the contact resistance, the p-type GaN surface was previously treated with pre-annealed indium-tin-oxide (ITO) film before deposition of the Ni/Ag/Ni/Au multilayer, and resulted in a lower specific contact resistance of 1.9×10−4Ωcm2. The X-ray photoelectron spectroscopy results indicated that Ga 2p core level of the p-type GaN surface with the pre-annealed ITO film had a lower binding energy, leading to a reduction in the contact resistance. Furthermore, GaN-based flip-chip light-emitting diodes (LEDs) with and without the pre-annealed ITO film were fabricated. The average forward voltage of the flip-chip LEDs fabricated with the pre-annealed ITO film is 3.22V at an injection current density of 35A/cm2, which is much lower than that (3.49V) of flip-chip LEDs without the pre-annealed ITO film. These results reveal that the proposed approach is effectively to fabricate high quality p-type contacts toward high power GaN-based LEDs.

Effect of the multiarray chip structure on the optical performance of GaN-based light emitting diodes

Multiarray chip (MAC) light-emitting diodes (LEDs) are introduced as an alternative approach to decrease the junction temperature, leading to reduced efficiency droop in high-power GaN-based LEDs. Despite using the same LED epiwafer, the authors found that the efficiency droop decreased on reducing the chip size from 1 × 1 to 8 × 8 MAC-LED cells. Although electroluminescence (EL) intensities of all MAC-LEDs demonstrated almost comparable results below an injection current of 100 mA, the efficiency droop of MAC-LEDs reduced when the current density was 10 A/cm2 owing to decrease in the satellite chip size. By using the forward voltage method, the junction temperature of MAC-LED gradually decreased on increasing the number of satellite chips in the MAC-LED. Based on these results, the authors suggest that the MAC-LED would be effective in improving the heat-dissipating rate, resulting in the reduction of efficiency droop without significantly decreasing the EL intensity.

A superior design for high power GaN-based light-emitting diode packages

The radiation performance of light-emitting diodes with hemispherical encapsulated lenses at the packaging level is analyzed by a Monte Carlo ray-tracing simulation. Based on optical and thermal considerations, a superior design for integrated LED packages is proposed and consistent trends are obtained in both simulated and experimental results. For integrated LED packages conforming to our design rules, a 7.1% enhancement of light output intensity is demonstrated and a 5.1°C reduction of junction temperature given a 700mA injection current is measured. This improvement may be of particular significance for an LED module system with higher power and/or larger arrays.

Polarity dependence of the electrical characteristics of Ag reflectors for high-power GaN-based light emitting diodes

We report on the polarity dependence of the electrical properties of Ag reflectors for high-power GaN-based light-emitting diodes. The (0001) c-plane samples become ohmic after annealing in air. However, the (11–22) semi-polar samples are non-ohmic after annealing, although the 300 °C-annealed sample shows the lowest contact resistivity. The X-ray photoemission spectroscopy (XPS) results show that the Ga 2p core level for the c-plane samples experiences larger shift toward the valence band than that for the semi-polar samples. The XPS depth profile results show that unlike the c-plane samples, the semi-polar samples contain some amounts of oxygen at the Ag/GaN interface regions. The outdiffusion of Ga atoms is far more significant in the c-plane samples than in the semi-polar samples, whereas the outdiffusion of N atoms is relatively less significant in the c-plane samples. On the basis of the electrical and XPS results, the polarity dependence of the electrical properties is described and discussed

Implementation of High-Power GaN-Based LEDs With a Textured 3-D Backside Reflector Formed by Inserting a Self-Assembled ${\rm SiO}_{2}$ Nanosphere Monolayer

Enhanced light extraction efficiency (LEE) of high-power GaN-based light-emitting diodes (LEDs) is achieved by inserting a self-assembled SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanosphere monolayer between the substrate and backside reflectors. Due to the presence of concave surfaces and photonic crystal-like air voids, downward photons emitted from multiple quantum well toward 3-D backside reflectors, could be reflected, scattered, and redirected into arbitrary directions for light extraction. These textured 3-D backside reflectors with an SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanosphere monolayer could also extract the lateral light inside device into the normal direction and improve LEE. As compared with a conventional LED without a backside reflector and an LED with a planar hybrid backside reflector, at 350 mA, the studied device with a 3-D hybrid backside reflector exhibits 136.4% (165%) and 23.6% (27.4%) enhancements in light output power (luminous flux) without the degradation of electrical properties. Higher light intensities in light emission mapping image and far-field pattern are also obtained. These results show that a textured 3-D backside reflector could be easily formed by inserting an SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanosphere monolayer to significantly enhance the performance of high-power GaN-based LEDs.

High power GaN-based LEDs with low optical loss electrode structure

Abstract GaN-based light-emitting diodes (LEDs) with discontinuous n-electrode and reflective p-electrode structure was fabricated and investigated. At 350-mA injection current, the LED with low optical loss n-electrode structure exhibited a 2.5% higher light output power than that of the LED with conventional n-electrode structure. This enhancement in light output power was attributed to the reduced loss of active region area. The LED with reflective p-electrode exhibited a 4.1% higher light output power than that of the LED with conventional p-electrode structure, which is attributed to the reduced light absorption by the opaque p-electrode. It was also found that the 350-mA forward voltage only increased slightly for the LED with discontinuous n-electrode and reflective p-electrode structure compared to the LED with conventional electrode structure.

Improved Light Extraction Efficiency of a High-Power GaN-Based Light-Emitting Diode With a Three-Dimensional-Photonic Crystal (3-D-PhC) Backside Reflector

An interesting approach to improving the light extraction efficiency of high-power GaN-based light-emitting diodes (LEDs) by the use of a 3-D-photonic crystal (3-D-PhC) backside reflector is studied. A 3-D-PhC backside reflector is formed by coating a self-assembled SiO2 nanosphere monolayer between the hybrid reflector and backside of a sapphire substrate. The 3-D-PhC structure is used to enhance light scattering. At 350 mA, as compared with a conventional LED (with a hybrid reflector while only without the 3-D-PhC structure), the studied device exhibits 23.6% enhancement in the light output power without the degradation of electrical properties. Therefore, the performance of high-power GaN-based LEDs could be further improved by using a 3-D-PhC backside reflector.

Enhanced thermal stability of Ag ohmic reflector for InGaN/GaN light-emitting diode using a Ru capping layer

A Ru capping layer is employed to improve the thermal stability of Ag ohmic reflectors for high-power GaN-based light-emitting diodes (LEDs). The 20-nm-thick Ru capping layer is shown to be fairly effective in suppressing agglomeration by forming RuO2. The Ag/Ru contacts exhibit specific contact resistance of 8.1×10−5Ωcm2 and reflectance of ∼89% at a wavelength of 450nm when annealed at 500°C for 1min, which are much better than that of Ag only contacts. Blue LEDs fabricated with the 500°C-annealed Ag/Ru contacts give a forward voltage of 2.98V at an injection current of 20mA, which is lower than that (3.02V) of LEDs with the 500°C-annealed Ag only contacts. LEDs with the 500°C-annealed Ag/Ru contacts show 25% higher output power (at 20mA) than LEDs with the 500°C-annealed Ag only contacts. X-ray photoemission spectroscopy examinations are performed to describe the improved electrical performance of the Ag/Ru contacts.

Low-temperature bonding technique for fabrication of high-power GaN-based blue vertical light-emitting diodes

Low-temperature Sn fusion bonding technique was proposed to fabricate GaN-based blue vertical light-emitting diodes (LEDs) on Si substrate. First, by studying photoluminescence (PL) spectra of GaN LED epilayers/Ag-based mirror/Si structures fabricated at different bonding temperatures, it was confirmed that the quality of Ag-based mirror was not degraded when the bonding temperature was 250°C. Then GaN-based blue vertical LEDs were fabricated at this bonding temperature. As compared with conventional GaN-based LEDs, the vertical LEDs revealed improved forward current–voltage characteristic, especially, the reverse current of the vertical LEDs was as low as 39nA at the reverse bias of −10V. In the mean time, vertical LEDs showed an increase in light output of about 127% at 200mA, and no saturation was observed as the driving current increased to 500mA. Further measurement revealed that vertical LEDs had a much lower junction temperature. These results indicate that the Sn fusion bonding technique is an effective way for fabrication of high-power GaN-based LEDs.

High-Power GaN-Based Light-Emitting Diodes Using Thermally Stable and Highly Reflective Nano-Scaled Ni–Ag–Ni–Au Mirror

In this study, we have fabricated the high-power GaN based vertical light-emitting diodes (VLEDs) by exploiting a thermally stable nano-scaled Ni-Ag-Ni-Au mirror. After being treated at 600 °C for 1 min in air ambient, the nano-scaled Ni-Ag-Ni-Au (5/2000/1000/2000 A) mirror shows the specific contact resistance of 6.0 × 10-4 Ω·cm2 and its reflectivity has increased from 89.5% to 93.0%. The increment in the reflectivity is due to the diffusion of Ni contact layer into Ag layer. Moreover, the reflectivity of the mirror has hardly deteriorated even after the wafer was thermally bonded with graphite substrate at 320 °C for 5 min, meaning that the proposed mirror is thermally reliable. The light output power of the nano-scaled VLED (Chip size: 1 × 1 mm2) with thermal treatment at 600 °C for 1 min is enhanced by 8% as compared to one without thermal treatment at an injection current of 350 mA. To our knowledge, our nano-scaled mirror has the highest reflec tivity among other Ag-based multilayered reflectors, and thus it is a promising candidate for high-performance VLED.

Improvement of thermal management of high-power GaN-based light-emitting diodes

The thermal management of high-power light-emitting-diode (LED) devices employing various die-attach materials is analyzed. Three types of die-attach materials are tested, including silver paste, Sn–3wt.% Ag–0.5wt.% Cu (SAC305) solder, and SAC305 solder added with a small amount of carbon nanotubes (CNTs). The analysis of thermal management is performed by comparing the temperatures of the LED chips in use and the total thermal resistances of the LED devices obtained respectively from the thermal infrared images and thermal transient analysis. Due to the high thermal conductivity of CNT, the addition of CNTs into the SAC305 solder reduces the total thermal resistance and chip temperature of the LED device, and the thermal management of the LED devices is improved accordingly.

Failure modes and effects analysis for high-power GaN-based light-emitting diodes package technology

In this study, nondestructive test is developed to analyze the structure failure of LED package. The relationship between thermal resistance analysis and LED package failure structure is build with T3Ster thermal transient tester and scanning electron microscope (SEM). The failure LED device with defect in the attaching layer and gap between LED chip and copper are designed advisedly. The failure factors of LED package have been measured with thermal resistance analysis and SEM cross-section images. The thermal dissipation performance of LED with defect in the attaching layer is indicated by thermal resistance analysis combined with SEM cross-section images. The blister in attaching layer results in 4.4K/W additional thermal resistance. The gap between LED chip and copper also makes high additional thermal resistance with 8.6K/W. Different failures of LED packages are indicated obviously using thermal transfer analysis. The LED package failure structure such as interface defect between solder and cup-shaped copper is able to forecast without destructive measurement.

Analysis of light extraction efficiency of GaN-based light-emitting diodes

GaN-based light emitting diodes (LEDs) as one of the most important light source in next-generation solid-state lighting have been extensively studied and remarkable progress has been obtained. However, the light extraction efficiency (LEE) is not sufficient to satisfy application requirements. Most of the photons generated in multiple quantum well (MQW) always are trapped inside the semiconductor because both the reflection index of GaN and InGaN are higher than that of air, which results in total internal reflection (TIR). Great efforts were made in enhancing light extraction of LEDs experimentally in previous investigation. However, detailed theoretical studies in predicting the LEE of different types of LEDs are not available. In this paper the light extraction efficiency(LEE) of conventional chip (CC), flip chip (FC) and vertical chip (VC) is investigated using Monte Carlo ray tracing method is presented in conventional chip (CC). Monte Carlo ray tracing simulation based on statistics is known to be one of the most suitable methods to analyze the dependence of the light extraction efficiency of LEDs on the variety of the device parameters. Diffused bottom surface was found to be better for improvement of LEE than the perfect mirror surface. When there is a material with higher refraction index on the top surface the VC structure has the highest LEE no matter the bottom surface is perfect mirror or diffuse surface, which is above 80%. It is indicated that VC is potential in high-power GaN-based LEDs from the results.

Thermal Management and Interfacial Properties in High-Power GaN-Based Light-Emitting Diodes Employing Diamond-Added Sn-3 wt.%Ag-0.5 wt.%Cu Solder as a Die-Attach Material

The thermal management of high-power GaN-based light-emitting diodes (LEDs) soldered with Sn-3 wt.%Ag-0.5 wt.%Cu (SAC305) solder and diamond-added SAC305 solder was evaluated. Diamond addition was found to significantly reduce the surface temperature and total thermal resistance of the LEDs, revealing that diamond-added SAC305 solder is a promising die-attach material for high-power LED packaging. Interfacial reactions in the LED solder joints were also investigated. The thin Au wetting layer in the chip’s backside metallization was rapidly consumed in the initial stage of reflow, forming an AuSn4 phase at the interface. Subsequently, the AuSn4 phase detached from the interface, leading to dewetting of the SAC305 solder from the LED chip. To avoid dewetting, a new backside metallization of LED chips should be developed for SAC305 solder.

Optimized Thermal Management From a Chip to a Heat Sink for High-Power GaN-Based Light-Emitting Diodes

To improve heat dissipation of sapphire-based LEDs, we develop a new LED package with a dual heat spreader design. The first heat spreader is a cup-shaped copper sheet, which was directly contacted with sapphire to enhance heat dissipation of the chip itself. The second heat spreader is the die-bonding material of diamond-added AgSnCu solder and a high thermal conductive metal-core printed circuit board (MCPCB), where the conventional dielectric layer was replaced with a thin diamond-like layer. Characterization results demonstrate that the diamond-added composite solder is useful in reducing LED thermal resistance, thus avoiding the thermal accumulation phenomenon. In addition, a LED packaged on the new MCPCB exhibits smaller total thermal resistance and larger light output power.

A chip-level electrothermal-coupled design model for high-power light-emitting diodes

An advanced three-dimensional electrothermal-coupled simulation model basing on finite-element method numerical simulation is developed to study the electrical and thermal properties of chip-level high-power GaN-based light-emitting diodes (LEDs). The current spreading, heat generation, and transfer in the device are comprehensively considered in this model. The current-spreading effect of the transparent current-spreading layer and the thermal performance of LEDs with interdigitated-electrodes are investigated. The simulation results prove that the temperature distribution in the active layer is strongly affected by the electrode pattern. The obvious heat accumulation in LEDs with conventional interdigitated-electrode patterns can be seen both in the simulated results and the infrared measured results. The heat transfer efficiency can be improved by using a symmetry electrode pattern design. The thermal management of the bump configurations in flip-chip LEDs is also studied. A more reasonable and thermal effective bump configuration is presented, and the simulated results show that a lower average temperature and more uniform heat distribution in the chips can be obtained.