Optimizing n-type contact design and chip size for high-performance indium gallium nitride/gallium nitride-based thin-film vertical light-emitting diode

The effects of the n-contact design and chip size on the electrical, optical and thermal characteristics of thin-film vertical light-emitting diodes (VLEDs) were investigated to optimize GaN-based LED performance for solid-state lighting applications. For the small (chip size: 1000×1000 µm²) and large (1450×1450 µm²) VLEDs, the forward bias voltages are decreased from 3.22 to 3.12 V at 350 mA and from 3.44 to 3.16 V at 700  mA, respectively, as the number of n-contact via holes is increased. The small LEDs give maximum output powers of 651.0–675.4 mW at a drive current of 350 mA, while the large VLEDs show the light output powers in the range 1356.7–1380.2 mW, 700 mA, With increasing drive current, the small chips go through more severe degradation in the wall-plug efficiency than the large chips. The small chips give the junction temperatures in the range 51.1–57.2 °C at 350  mA, while the large chips show the junction temperatures of 83.1–93.0 °C at 700  mA, The small LED chips exhibit lower junction temperatures when equipped with more n-contact via holes.     

Investigation of n-ohmic contact of vertical GaN-based light-emitting diodes on graphite substrate with Ag-In bonding

Vertical light-emitting diodes (VLEDs) were successfully transferred from a GaN-based sapphire substrate to a graphite substrate by using low-temperature and cost-effective Ag-In bonding, followed by the removal of the sapphire substrate using a laser lift-off (LLO) technique. One reason for the high thermal stability of the AgIn bonding compounds is that both the bonding metals and Cr/Au n-ohmic contact metal are capable of surviving annealing temperatures in excess of 600 °C. Therefore, the annealing of n-ohmic contact was performed at temperatures of 400 °C and 500 °C for 1 min in ambient air by using the rapid thermal annealing (RTA) process. The performance of the n-ohmic contact metal in VLEDs on a graphite substrate was investigated in this study. As a result, the final fabricated VLEDs (chip size: 1000 µm×1000 µm) demonstrated excellent performance with an average output power of 538.64 mW and a low operating voltage of 3.21 V at 350 mA, which corresponds to an enhancement of 9.3% in the light output power and a reduction of 1.8% in the forward voltage compared to that without any n-ohmic contact treatment. This points to a high level of thermal stability and cost-effective Ag-In bonding, which is promising for application to VLED fabrication.     

Nanofabrication of gallium nitride photonic crystal light-emitting diodes

We describe a comparison of nanofabrication technologies for the fabrication of 2D photonic crystal structures on GaN/InGaN blue LEDs. Such devices exhibit enhanced brightness and the possibility of controlling the angular emission profile of emitted light. This paper describes three nano lithography techniques for patterning photonic crystal structures on the emitting faces of LEDs: direct-write electron beam lithography, hard stamp nanoimprint lithography and soft-stamp nanoimprint lithography with disposable embossing masters. In each case we describe variations on the technique as well as its advantages and disadvantages. Complete process details have been given for all three techniques. In addition, we show how high performance GaN dry etch techniques, coupled with optical process monitoring can transfer resist patterns into underlying GaN material with high fidelity.     

Investigation of light output performance for gallium nitride-based light-emitting diodes grown on different shapes of patterned sapphire substrate

GaN-based blue light-emitting diodes (LEDs) on various patterned sapphire substrates (PSSs) are investigated in detail. Hemispherical and triangular pyramidal PSSs have been applied to improve the performance of LEDs compared with conventional LEDs grown on planar sapphire substrate. The structural, electrical, and optical properties of these LEDs are investigated. The leakage current is related to the crystalline quality of epitaxial GaN films, and it is improved by using the PSS technique. The light output power and emission efficiency of the LED grown on triangular pyramidal PSS with optimized fill factor show the best performance in all the samples, which indicates that the pattern structure and fill factor of the PSS are related to the capability of light extraction.     

n-GaN表面Ti/Al/Ti/Au电极的电学特性

实验研究了淀积在GaN上的Ti/Al/Ti/Au电极的电学和热学特性,绘制了不同退火温度下的I-V曲线,得到了最低的欧姆接触电阻率(ρs=1.2×10-4 Ω·cm2),并通过X射线衍射谱分析了GaN与Ti/Al/Ti/Au电极接触表面在退火过程中的固相反应.实验结果表明,在Ti/Al表面增加Ti/Au保护层能够保证Al层在高温时不发生球化和氧化,电极更稳定可靠能够进一步提高欧姆接触特性.     

Effect of Pt barrier on thermal stability of Ti/Al/Pt/Au in ohmic contact with Si-implanted n-type GaN layers

We report the effect of the Pt barrier on the thermal stability of Ti/Al/Pt/Au in ohmic contact with Si-implanted n-type GaN layers. Ti/Al/Au (25/100/200 nm) and Ti/Al/Pt/Au (25/100/50/200 nm) multilayers were, respectively, deposited on as-implanted and recovered Si-implanted n-type GaN samples. The associated dependence of the specific contact resistance on the annealing time at various temperatures was compared. The long-term ohmic stability of a Ti/Al/Pt/Au multilayer in contact with a Si-implanted n-type GaN layer was much better than that of the Ti/Al/Au multilayer. This superior stability is attributed to the barrier function of the Pt interlayer. The Pt/Au bilayer can also passivate the propensity of oxidation for the conventional Ti/Al bilayer in contact with n-type GaN layers at elevated temperatures.     

低温生长n型氮化镓插入层提高LED抗静电能力

在这篇论文里,我们通过在InGaN/GaN 多量子阱和n型氮化镓层中间插入一层低温生长的n型氮化镓显著提高了LED的抗静电能力。通过引入低温生长的氮化镓插入层使得LED抗击穿电压超过4000V的良品率从9.9%提升到74.7%。低温生长的氮化镓插入层作为后续生长的多量子阱的缓冲层,释放了量子阱中的应力并且改善了量子阱的界面质量。另外,我们证明了在氮气气氛下生长低温氮化镓插入层对于LED抗静电能力的改善要强于氢气气氛,同时也进一步证明低温插入层对量子阱中应力的释放有利于提高LED的抗静电能力。光电测试结果表明,在引入低温nGaN缓冲层后,LED的电学特性并没有衰退,并且LED的光输出功率提高了13.9%。     

Design of vertical fin arrays with heat pipes used for high-power light-emitting diodes

As Light-Emitting Diodes (LEDs) are negatively affected by high temperature, the thermal design for them is critical for better light quality, reliability and lifetime. In this work, a thermal design of vertical fin arrays with heat pipes as passive cooling was applied. The heat pipes can supply high thermal conductivity with much less weight and volume compared to copper or aluminum base and consequently less obstruction to air flow with enhanced natural convection. As the natural convection and radiation dominate heat transfer in this case, the optimum vertical fin spacing was calculated by the most used empirical correlations. Then, the design was numerical investigated by Computational Fluid Dynamics (CFD) to obtain best thermal performance. As the fin spacing was both optimized by correlations and modelling, the optimum thermal design achieved. Finally, we manufactured and tested the design experimentally which consistently approved the thermal design compared to correlations and simulation.     

Electrical characterization and DLTS analysis of a gold/n-type gallium nitride Schottky diode

This work describes a comparison of current density–voltage (J–V) and capacitance–voltage (C–V) properties measured as a function of temperature; deep trap properties are measured by deep level transient spectroscopy (DLTS) of Schottky diodes fabricated on n-type gallium nitride (GaN grown by metal organic vapor phase epitaxy (MOVPE). Unexpected behavior in the standard Richardson plot was observed in the temperature range 165–480 K, reflecting a range of Schottky barrier heights and a variation of ideality factor. This was explained by applying a Gaussian spatial distribution of barrier heights across the Schottky diode. C–V measurements were carried out in the temperature range 165–480 K to compare the temperature dependence of the barrier height with those obtained by the Gaussian distribution method. DLTS and high-resolution Laplace DLTS (LDLTS) show a majority carrier peak centered at 450 K.     

Electrical characteristics of In/ITO p-type ohmic contacts for high-performance GaN-based light-emitting diodes

We have investigated the annealing-induced improved electrical properties of In(10 nm)/ITO(200 nm) contacts with p-type GaN. The contacts become ohmic with a specific contact resistance of 2.75×10^–3 Ω cm² upon annealing at 650 °C in air. X-ray photoemission spectroscopy (XPS) Ga 2p core levels obtained from the interface regions before and after annealing indicate a large band-bending of p-GaN, resulting in an increase in the Schottky barrier height. STEM/energy dispersive X-ray (EDX) profiling results exhibit the formation of interfacial In-Ga-Sn-oxide. Based on the STEM and XPS results, the ohmic formation mechanisms are described and discussed. It is also shown that patterning by nano-imprint lithography improves the light output power of blue LEDs by 18–28% as compared to that of LEDs fabricated with unpatterned In/ITO contacts.     

InGaN/GaN light-emitting diodes with a reflector at the backside of sapphire substrates

Nitride-based light-emitting diodes (LEDs) with a reflector at the backside of the sapphire substrates have been demonstrated. It was found that an SiO₂/TiO₂ distributed-Bragg reflector (DBR) structure could reflect more downward-emitting photons than an Al-mirror layer. It was also found that the 20-mA output power was 2.76 mW, 2.65 mW, and 2.45 mW for the DBR LED, Al-reflector LED, and conventional LED, respectively. With the same 50-mA current injection, the integrated-electroluminescence (EL) intensity of a DBR LED and an Al-reflector LED was 19% and 15% larger than that observed from a conventional LED.     

InGaN/GaN multiple quantum well green light-emitting diodes prepared by temperature ramping

High-quality InGaN/GaN multiple-quantum well (MQW) light-emitting diode (LED) structures were prepared by a temperature-ramping method during metal-organic chemical-vapor deposition (MOCVD) growth. Two photoluminescence (PL) peaks, one originating from well-sensitive emission and one originating from an InGaN quasi-wetting layer on the GaN-barrier surface, were observed at room temperature (RT). The observation of high-order double-crystal x-ray diffraction (DCXRD) satellite peaks indicates that the interfaces between InGaN-well layers and GaN-barrier layers were not degraded as we increased the growth temperature of the GaN-barrier layers. With a 20-mA and 160-mA current injection, it was found that the output power could reach 2.2 mW and 8.9 mW, respectively. Furthermore, it was found that the reliability of the fabricated green LEDs prepared by temperature ramping was also reasonably good.     

Enhancement of light output power of GaN-based vertical light emitting diodes by optimizing n-GaN thickness

The light output and electrical characteristics of GaN-based vertical light emitting diodes were investigated as a function of n-GaN thickness. The forward voltage increases from 3.34 to 3.42 V at an injection current of 350 mA as the n-GaN thickness decreases from 5.0 to 2.0 μm. Even at a high injection current of 2.0 A, LEDs with 2.0 μm-thick n-GaN reveal stable forward characteristics which are comparable to those of LEDs with 5.0 μm-thick n-GaN. All the samples exhibit almost the same reverse current up to approximately −8 V. The output power increases with decreasing n-GaN layer thickness. For example, LEDs with 2.0 μm-thick n-GaN yield about 12% higher light output power as compared to LEDs with 5.0 μm-thick n-GaN. Their light output power continuously increases without saturation as the injection current increases up to 1 A. The n-GaN thickness dependence of the electrical characteristics is described and discussed.     

Improving the performance of organic light-emitting diodes containing BCP/LiF/Al by thermal annealing

In this paper, we examined the effect of post-packaging annealing on the performance of organic light-emitting diodes containing tris-(8-hydroxyquinoline) aluminum (Alq,) or 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) in direct contact with a LiF-Al bilayer cathode. The detailed electroluminescent (EL) characteristics were compared before and after annealing at 70 /spl deg/C for 5 hrs. It was found that better luminous efficiency as well as greater power efficiency could be achieved for devices with BCP/LiF/Al structure. However, other devices consisting of Alq/sub 3//LiF/Al were less affected. It is believed that the thermal treatment helps to enhance the electron injection for the former, and less helpful for the latter.     

Enhanced output power of (indium) gallium nitride light emitting diodes by a transparent current spreading-film composed of a disordered network of indium tin oxide nanorods

A thin film consisting of a disordered nanorod network of indium tin oxide (ITO) and conventional ITO films are fabricated on gallium nitride (GaN) based-light emitting diodes (LEDs) by electron beam evaporation. The surface morphologies are observed by scanning electron microscopy (SEM). The disordered nanorod network of ITO is grown in vacuum without oxygen. It can be applied directly on the LED as the current spreading film unlike other nanorods which require growth on a conductive layer. The transmittance, current–voltage characteristic, and the dependence of light output power on current are measured for disordered nanorod network ITO LEDs and conventional ITO LEDs, respectively. The measurement results indicate that the nanorod network provides a significant improvement in the light output power of GaN-based LEDs. The influence of the structure of ITO films on the light output power of GaN-based LEDs is discussed.     

Edge-emitting electroluminescence polarization investigation of InGaN/GaN light-emitting diodes grown by metal-organic chemical vapor deposition on sapphire (0001)

The edge-emitting electroluminescence (FL) state of polarization of blue and green InGaN/GaN light-emitting diodes (LEDs) grown in EMCORE’s commercial reactors was studied and compared to theoretical evaluations. Blue (∼475 nm) LEDs exhibit strong EL polarization, up to a 3:1 distinction ratio. Green (∼530 nm) LEDs exhibit smaller ratios of about 1.5:1. Theoretical evaluations for similar InGaN/GaN superlattices predicted a 3:1 ratio between light polarized perpendicular (E⊥c) and light polarized parallel (E‖c) to the c axis. For the blue LEDs, a quantum well-like behavior is suggested because the E⊥c mode dominates the E‖c mode 3:1. In contrast, for the green LEDs, a mixed quantum well (QW)-quantum dot (QD) behavior is proposed, as the ratio of E⊥c to E‖c modes drops to 1.5:1. The EL polarization fringes were also observed, and their occurrence may be attributed to a symmetric waveguide-like behavior of the InGaN/GaN LED structure. A large 40%/50% drop in the surface root mean square (RMS) from atomic force microscopy (AFM) scans on blue/green LEDs with and without EL fringes points out that better surfaces were achieved for the samples exhibiting fringing. At the same time, a 25%/10% increase in the blue/green LED photoluminescence (PL) intensity signal was found for samples displaying EL interference fringes, indicating superior material quality and improved LED structures.     

Improved performance of deep-blue polymer light-emitting diodes by one-step coating self-assembly hole injection/transport nanocomposites with both the optical and electrical optimization

In this study we demonstrate an easy solution-processed highly efficient deep-blue polymer light-emitting diode (PLED) via a simple one-step coating of self-assembly hole injection/transport nanocomposites to achieve both a finer hole ohmic contact and an increased light outcoupling, which is the first time report about both the optical and electrical optimization without necessitating changes in the design or structure of the wide bandgap deep-blue PLEDs themselves. The contact angle and surface energy measurement results demonstrate that triazine-based hole injection molecules can vertically migrate towards the bottom PEDOT:PSS layer to obtain a stable minimum of free energy, resulting in an optimal top-to-bottom HOMO energy level arrangement and an improved hole mobility in deep-blue PLEDs. The random surface nanostructure was formed on top of the hole bilayer, leading to the enhancement of light outcoupling verified by transmittance, transmittance haze and light extraction efficiency. Furthermore, in order to explore the reasons of the hole light scattering formation process, a transient drying monitoring technique is applied to track the drying process of the nanostructure films, revealing this approach effectiveness by easily modifying mixing ratios for obtaining different light outcoupling abilities.