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

High-performance GaN-based light-emitting diodes on patterned sapphire substrate with a novel hybrid Ag mirror and atomic layer deposition-TiO2/Al2O3 distributed Bragg reflector backside reflector

GaN-based light-emitting diodes (LED) on a patterned sapphire substrate with a novel hybrid atomic layer deposition (ALD)-TiO 2 Al 2 O 3 distributed Bragg reflector (DBR) and Ag mirror have been proposed and fabricated. Due to the excellent thickness uniformity of ALD for the proposed reflector, high reflectivity over 99.3% at an incident angle of 5 deg has been achieved. It was also found that the reflectivity of a backside reflector with an Ag mirror slightly depends on incident light wavelength and incident angle. Moreover, because of the good adhesion between TiO 2 /Al 2 O 3 DBR and the Ag mirror, the fabrication process was simplified and reliable. With a 60 mA current injection, an enhancement of 5.2%, 8.9%, and 47.1% in light output power (LOP) at the 460 nm wavelength was realized for the proposed LED with Ag mirror and 3-pair ALD-TiO 2 Al 2 O 3 DBR as compared with a LED with a traditional Ag mirror and 3-pair TiO 2 /SiO 2 DBR, with Al mirror and 3-pair ALD-TiO 2 Al 2 O 3 DBR, and without backside reflector, respectively. This result shows that the ALD-TiO 2 /Al 2 O 3 DBR can be used to enhance the LOP greatly and improve adhesion between the sapphire substrate and the metallic mirror, and thus is very promising for fabricating high performance GaN-based LEDs.

Light Extraction Enhancement of GaN-Based Light-Emitting Diodes Using Crown-Shaped Patterned Sapphire Substrates

In this letter, we report the high performance GaN-based light-emitting diodes (LEDs) with embedded air void array grown by metal-organic chemical vapor deposition. The donut-shaped air void was formed at the interface between crown-shaped patterned sapphire substrates (CPSS) and the GaN epilayer by conventional photolithography. The transmission electron microscopy images demonstrate that the threading dislocations were significantly suppressed by epitaxial lateral overgrowth (ELOG). The Monte Carlo ray-tracing simulation reveals that the light extraction of the air-voids embedded LED was dramatically increased due to a strong light reflection and redirection by the air voids.

High efficiency GaN-based light-emitting diodes with embedded air voids/SiO2 nanomasks

In this paper, the high performance GaN-based light-emitting diodes (LEDs) with embedded microscale air voids and an SiO2 nanomask by metal–organic chemical vapor deposition (MOCVD) were demonstrated. Microscale air voids and an SiO2 nanomask were clearly observed at the interface between GaN nanorods (NRs) and the overgrown GaN layer by scanning electron microscopy (SEM). From the reflectance spectra we show strong reflectance differences due to the different refractive index gradient between the GaN grown on the nanotemplate and sapphire. It can increase the light extraction efficiency due to additional light scattering. The transmission electron microscopy (TEM) images show the threading dislocations were suppressed by nanoscale epitaxial lateral overgrowth (NELOG). The LEDs with embedded microscale air voids and an SiO2 nanomask exhibit smaller reverse-bias current and large enhancement of the light output (65% at 20 mA) compared with conventional LEDs.

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 cm 2 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.

Formation of Thermally Stable AgCu-Based Reflectors by a Two Step Alloy Method for Vertical Light-Emitting Diodes

Two-step alloyed indium–tin-oxide (ITO)/Ni/AgCu/Pt reflectors for high-performance GaN-based vertical light-emitting diodes (VLEDs) were investigated. The ITO layer was first annealed at 650 °C for 1 min in air to make an Ohmic contact and then the Ni/AgCu/Pt reflectors were deposited and subsequently annealed at 400 °C for 1 min in air to improve their reflectance and mechanical adhesion with the ITO layer. It was shown that the reflectance of the ITO/Ni/AgCu/Pt reflectors at 460 nm was slightly increased from 82 to 87% after two-step annealing. Based on the secondary ion mass spectrometry depth profiles, this improvement was attributed to the formation of a transparent Ni-oxide and the existence of Cu atoms near ITO/AgCu/Pt interface regions suppressing the inter and out-diffusion of Ag. The VLEDs fabricated with the ITO/Ni/AgCu/Pt reflectors showed an approximately 4.4% higher output power and much better current–voltage characteristics than those with the Ag-based reflectors.

Enhanced Light Output Power of GaN-Based Vertical Light-Emitting Diodes by Using Highly Reflective ITO–Ag–Pt Reflectors

Highly reflective and thermally stable indium-tin-oxide (ITO)-Ag-Pt p-type reflectors for use in high-performance GaN-based light-emitting diodes (LEDs) have been investigated. The specific contact resistance of the ITO-Ag-Pt contacts was found to be 7.2 t10-5Omegamiddotcm2. The ITO-Ag-Pt contacts showed a higher reflectance after thermal annealing (82% at 460 nm), while the reflectance of the ITO-Ag contacts was reduced from 81% to 65%. In addition, surface agglomeration was drastically decreased, indicating that the Pt layer efficiently prevents the surface agglomeration of the Ag layer. The vertical LEDs (VLEDs) fabricated with the ITO-Ag-Pt contacts had a 17% higher output power (at 20 mA) than the VLEDs fabricated with the ITO-Ag contacts.

High Performance GaN-based Light-emitting Diodes with Geometric GaN Shaping Structure

InGaN-GaN light emitting diodes (LEDs) with p-roughened surface and n-bowl structure were fabricated by low temperature growing, ICP etching, wafer-bonding, and laser lift-off technologies. Ni/Ag/Ni mirror was introduced on the n-bowl surface. It was found the light intensity of PRUB-LEDs with different bowl size are all higher than PR-LED. The intensity of PRUB-LED with 3 μm bowls was 2.33 times higher than that of the PR-LED.

High Performance GaN-based Light-emitting Diodes with Geometric GaN Shaping Structure

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High output power GaN-based light-emitting diodes using an electrically reverse-connected p-Schottky diode and p-InGaN–GaN superlattice

We demonstrate the high-performance and excellent reliability characteristics of InGaN–GaN light-emitting diodes (LEDs) using an electrically reverse-connected p-Schottky diode and p-InGaN–GaN superlattice (SL). Measurements show that the LED with the Schottky diode and SL yields lower series resistance and higher output power compared to normal LEDs. In addition, the device degradation rate of the proposed LED is 33 times as low as that of the normal LED at high electrical stress of 410 A/cm2, indicating excellent reliability behavior. These results mean that the use of p-Schottky diode and p-SL is very promising for the realization of high-performance GaN-based LEDs.

High transparency low resistance oxidized Ni/Au–ZnO contacts to p-GaN for high performance LED applications

This paper presents high-transparency low-resistance contact on p-GaN using oxidized Ni/Au-ZnO:Al 2 O 3 (2 wt%) for high-performance GaN-based light-emitting diodes (LEDs) applications. It is shown that oxidized Ni/Au-ZnO contact to p-GaN yields an operating voltage of around 4.7 V at 20 mA compared to 4.1 V of a reference oxidized Ni/Au(5 nm/5 nm)-Ni/Au(20 nm/120 nm) sample without the ZnO: Al 2 O 3 (2 wt%) layer. Enhancement of transmittance of about 60% at the wavelength of 450 nm over a conventional N 2 -annealed semi-transparent Ni/Au(5 nm/5 nm) contact is measured on glass substrates.