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

Fabrication of high-brightness GaN-based light-emitting diodes via thermal nanoimprinting of ZnO-nanoparticle-dispersed resin

We fabricated high-brightness GaN-based light-emitting diodes (LEDs) with highly refractive patterned structures by using a thermal nanoimprint lithography (NIL). A highly refractive ZnO-nanoparticle-dispersed resin (ZNDR) was used in NIL, and a submicron hole, a submicron high-aspect-ratio pillar, and microconvex arrays were fabricated on the indium tin oxide (ITO) top electrode of GaN-based LED devices. We analyzed the light extraction mechanism for each of the three types of patterns by using a finite element method simulation, and found that the high-aspect-ratio pillar had a great ability to improve light extraction owing to its waveguide effect and prominent scattering effect. As a result, the light output power, which was measured in an integrating sphere, of the LED device was enhanced by up to 19.6% when the high-aspect-ratio pillar array was formed on the top ITO electrode of the device. Further, the electrical properties of none of the patterned LED devices fabricated using ZNDR degraded in comparison to those of bare LED devices.

Transient voltage suppressor diode designed for the protection of high-brightness GaN-based LEDs from various electrostatic discharge shocks

Transient voltage suppressor (TVS) diodes were fabricated using low-temperature epitaxy technology and were employed to improve the electrostatic discharge (ESD) strength of GaN light emitting diodes (LEDs). The ESD performance and the protection capability of the TVS diodes were investigated using various ESD simulators of the human body model (HBM), the IEC (International Electrotechnical Commission) 61000-4-2 (IEC), and a transmission line pulse (TLP) analysis. According to the IEC, the manufactured TVS diode could withstand in excess of ±30 kV without any degradation in the I-V characteristics; meanwhile, the GaN LED itself exhibited catastrophic degradation caused by weak ESD power. The GaN LED assembled with the TVS diode had improved ESD robustness from ±3.8 kV to ±8 kV according to the HBM, from ±1.2 kV to > ±30 kV according to the IEC, and from 4.3 A to > ±30 A according to the TLP analysis. Furthermore, its performance was maintained perfect I–V manner with negligible changes in radiant power, leakage current and breakdown voltage up to the limit of the ESD simulators. Namely, the manufactured TVS diodes were effective in the protection of sensitive GaN LEDs from very strong ESD shocks.

Light Extraction Enhancement of GaN LEDs by Hybrid ZnO Micro-Cylinders and Nanorods Array

A hybrid patterned ZnO micro-cylinders and nanorods array (MCNR) was fabricated to improve the light extraction efficiency for high brightness GaN-based light-emitting diodes (LEDs). The light extraction efficiency of the LEDs with the ZnO MCNR is increased by 86.4% compared with conventional planar LEDs. LEDs with ZnO MCNR have greater light extraction efficiency than those only with ZnO micro-cylinders or ZnO nanorods array. The optical-electrical characteristics and far-field radiation patterns are measured and analyzed. The local optical field distribution patterns of the LEDs with ZnO micro-cylinders, ZnO nanorods, and ZnO MCNR are investigated using confocal scanning electroluminescence microscopy. The higher light extraction enhancement effect of ZnO MCNR is attributed to that it combines the light outputting effect from side walls of ZnO micro-cylinders and the light scattering effect of ZnO nanorods array.

Formation of low-resistance and transparent indium tin oxide ohmic contact for high-brightness GaN-based light-emitting diodes using a Sn–Ag interlayer

We report on the formation of low-resistance and highly transparent indium tin oxide (ITO) ohmic contacts to p-GaN using a Sn–Ag alloy interlayer. Although the as-deposited Sn–Ag(6 nm)/ITO(200 nm) contacts show non-ohmic behaviors, the scheme becomes ohmic with specific contact resistance of 4.72×10 −4 Ω cm 2 and produce transmittance of ∼91% at wavelength of 460 nm when annealed at 530 °C. Blue light-emitting diodes (LEDs) fabricated with the Sn–Ag/ITO contacts give forward-bias voltage of 3.31 V at injection current of 20 mA. LEDs with the Sn–Ag/ITO contacts show the improvement of the output power by 62% (at 20 mA) compared with LEDs with Ni/Au contacts.

High Brightness GaN-Based Light-Emitting Diodes

This paper reviews our recent progress of GaN-based high brightness light-emitting diodes (LEDs). Firstly, by adopting chemical wet etching patterned sapphire substrates in GaN-based LEDs, not only could increase the extraction quantum efficiency, but also improve the internal quantum efficiency. Secondly, we present a high light-extraction 465-nm GaN-based vertical light-emitting diode structure with double diffuse surfaces. The external quantum efficiency was demonstrated to be about 40%. The high performance LED was achieved mainly due to the strong guided-light scattering efficiency while employing double diffuse surfaces

Enhanced output of GaN-based light-emitting diodes with stripe-contact electrodes

High brightness GaN-based light-emitting diodes(LEDs) with stripe-contact electrodes have been developed. The p-type Ohmic contact layer is composed of oxidized Ni∕Au stripes and NiO stripes. A Ag (3000Å) omnidirectional reflector covers the p-type contact. The n-type contact is a Ti∕Al planar film with a Ti∕Al stripe. All Ni∕Au, NiO, and Ti∕Al stripes surround the center of the LED mesa. At 20mA current operation, the light output power of GaN-based LEDs with the stripe-contact electrodes is 16.26%–35.37% higher than that of the conventional LEDs.

The silicon oxynitride layer deposited at low temperature for high-brightness GaN-based light-emitting diodes

The deposition process of passivation layer for GaN-LEDs can affect the optical and electric properties of these devices. In this paper, we use an ammonia-free process to deposit the silicon oxynitride film at low temperature (100 °C) through plasma enhanced chemical vapor deposition (PECVD) to serve as the passivation layer of GaN-LEDs, investigating the relationship between the refractive index of silicon oxynitride film and the light output of GaN-LEDs, and furthermore, analyzing the properties of the devices before and after the passivation. It is found that the film with the refractive index of 1.54 possesses good characteristics and the optical and electrical properties of GaN-LEDs improve greatly after the deposition of the layer. Compared with the optical and electrical properties of GaN-LEDs without a passivation layer, the light output can be increased by as high as 17.8% after the deposition of the silicon oxynitride passivation layer; the forward voltage decreased and the reverse leakage current reduced obviously. The deposition process can greatly improve the optical and electric characteristics of GaN-LEDs.

Investigation of indium–tin-oxide ohmic contact to p-GaN and its application to high-brightness GaN-based light-emitting diodes

Indium–tin-oxide (ITO) contacts to p-GaN exhibit ohmic characteristics by inserting a 10-nm-thick p-In 0.1Ga 0.9N layer as an intermediate. The specific contact resistivity of 4.5 × 10 −2 Ω cm 2 was obtained while annealing ITO/p-GaN contacts at 500 °C. Based on the variation of contact resistivity with respect to temperature, the dominant transport mechanism of ITO/p-GaN structure tended from thermionic field emission to thermionic emission when the post-annealing temperature was raised from 400 °C to 600 °C. From the XPS, XRD and SIMS results, the outdiffusion of gallium atoms and the formation of Ga–O bonds could introduce the gallium vacancies and increase the net concentration of holes, which would benefit the carrier tunneling through the interface. The GaN-based light-emitting diodes (LEDs) with 500 °C-annealed ITO contacts exhibited the forward voltage of 3.43 V and output power of 4.30 mW at a 20-mA-current injection. Although the forward voltage showed a little higher than the conventional LEDs by 0.2 V, the external quantum efficiency and power efficiency were enhanced by about 46% and 36%, respectively. As for the life test, LEDs with 500 °C-annealed ITO contacts presented a similar reliability as the conventional LEDs. Therefore, ITO contacts can make GaN-based LEDs highly bright and reliable in practice.