Der-Ming Kuo

Nitride-Based LEDs With Phosphoric Acid Etched Undercut Sidewalls

We propose a simple defect-selective wet etching method to form oblique sidewalls for GaN-based epitaxial layers with phosphoric acid. Using the same defect-selective wet etching, we also prepared GaN-based light-emitting diodes (LEDs) with undercut sidewalls. Compared with conventional LEDs with vertical sidewalls, it was found that output intensity of the LEDs prepared by defect-selective wet etching was 30% higher.

Enhanced Light Output of GaN-Based Vertical-Structured Light-Emitting Diodes With Two-Step Surface Roughening Using KrF Laser and Chemical Wet Etching

A two-step roughening process that uses a KrF excimer laser and KOH chemical etching for the n-GaN layer surface of vertically structured GaN-based light-emitting diodes (VLEDs) to yield circular protrusions with hexagonal cones atop for light extraction enhancement is demonstrated. A possible mechanism of the formation of the circular protrusions commenced by laser irradiation with nonuniform etching rates at sites with various dislocation densities was investigated. An improvement in light output power of about 95% at 350-750 mA compared to that of flat VLEDs was obtained for the two-step roughened VLEDs, which is attributed to the increase in surface emission area and dimensions of roughness, and, in particular, the decrease in the n-GaN layer thickness.

GaN-Based LEDs With Sapphire Debris Removed by Phosphoric Etching

The authors proposed a simple hot phosphoric etching method to enhance output power of GaN-based light-emitting diodes (LEDs) by 34%. By immersing the sample in H3PO4 at 220°C for 40 min, it was found that debris contaminants induced by nanosecond laser scribing could be effectively removed. It was also found that the hot phosphoric etching method will not degrade electrical characteristics of the fabricated LEDs.

Improved light output of GaN-based vertical light emitting diodes using SiO2 nanotube arrays and transparent metal oxide current conduction layer

The use of a refractive index matching (RIM) structure with indium zinc oxide (IZO) transparent conduction layer and SiO2 nanotube (SiO2-NT) arrays to improve light extraction of vertical structure KOH-etched GaN-based light emitting diodes (VLEDs) is demonstrated. Compared to regular VLED with KOH-roughened surface, it shows considerable gains in light emitted critical angle and light output power by 21.3° and 103% at 350 mA, respectively. These improvements could be attributed to the effectiveness of the IZO/SiO2-NT RIM scheme in ameliorating current crowding and significantly minimizing the total internal reflection effect.

Nitride-based LEDs with oblique sidewalls and a light guiding structure

The authors propose a simple method to further improve light extraction efficiency of GaN-based phosphoric acid etched light-emitting diodes (LED) by forming a light guiding structure on a sapphire substrate. Compared with conventional LEDs, it was found that the output intensity of the phosphoric acid etched LEDs with a light guiding structure was 40% higher. It was also found that the light guiding structure can effectively enhance LED output intensity in the vertical directions. (Some figures in this article are in colour only in the electronic version)

The reliability analysis of GaN-based light-emitting diodes with different current-blocking layers

This study employed Ar plasma treatment to selectively damage the p-GaN surface under the p-pad electrode as a current-blocking layer (CBL) on nitride-based light-emitting diodes (LEDs). Increasing the resistivity of the p-GaN region under the p-pad electrode can reduce the current flowing vertically downward from the p-pad electrode. At an injection current of 20 mA, the light output power of LEDs with Ar plasma treatment was 13% larger than that of conventional LEDs. At an injection current of 100 mA, the temperature of the p-pad metal on LEDs with Ar plasma treatment is 13 ◦ C lower than that of the LEDs with a SiO2 CBL. However, the electrostatic discharge endurance of LEDs with Ar plasma treatment is the worst due to the surface damage of p-GaN under the p-pad electrode. (Some figures in this article are in colour only in the electronic version)

Preparation of SiO2 Nanotubes with Controllable Inner/Outer Diameter and Length Using Hydrothermally Grown ZnO Nanowires as Templates

Through the deposition of a thin SiO2 film to sheathe hydrothermally grown (HTG) ZnO nanowires (ZnO-NWs), unveiling their top portion, and then selectively removing ZnO-NWs by wet chemical etching, SiO2 nanotubes (SiO2-NTs) with controllable inner/outer diameters and lengths were fabricated. The prepared SiO2-NTs with average inner/outer diameters and lengths of approximately 200/300 nm and 1.5 µm, respectively, exhibited a superior transmittance of 92% in the visible light spectrum. The surface roughened process using SiO2-NTs on vertical-structure GaN light-emitting diodes (VLEDs) showed additional light output improvement of about 11.6% at 350 mA and 10% at 750 mA, compared with those of VLEDs with ZnO-NWs, suggesting the effectiveness and promising applications of the proposed SiO2-NTs in optics and optoelectronics devices.

Enhanced Light Output of Vertical-Structured GaN-Based Light-Emitting Diode with Surface Roughening Using KrF Laser and ZnO Nanorods

To further improve the performance of vertical-structured GaN-based light-emitting diodes (V-LEDs), surface roughening using a KrF laser and KOH wet chemical etching, followed by hydrothermal growth of vertically aligned ZnO nanorods on top of the n-GaN surface were investigated and discussed. Compared with that of the V-LEDs (300×300 µm2 in chip size) with only surface KOH wet etching, the formation of curved protrusions and ZnO nanorods on the n-GaN surface typically enables an increase in light output power (Lop) by 29% at 20 mA and 41% at 100 mA with a decrease in forward voltage (Vf) from 3.24 to 3.06 V at 20 mA and 3.9 to 3.7 V at 100 mA, respectively. The cumulative effect of the curved protrusions, hexagonal cones, and vertically aligned ZnO nanorods formed as a result of effectively reducing the effective thickness of the n-GaN layer, improving the ohmic contact to n-GaN, increasing the surface emission area, and enhancing the escape probability of photons was responsible for these improvements.

A Screen-Printed Sn-Based Substrate Technology for the Fabrication of Vertical-Structured GaN-Based Light-Emitting Diodes

A dicing-free substrate technology was proposed and demonstrated to simplify the fabrication of vertical-structured metal substrate GaN-based light-emitting diodes (VM-LEDs) using a Sn-based solder screen printing technique with patterned laser lift-off technology. As compared with conventional sapphire substrate GaN-based LEDs, VM-LEDs with an effective emission area of 1000×1000 µm2 were found to have a 0.38 (0.87) V reduction in forward voltage at 350 (700) mA. In addition, their enhancement in light output power in the current range of 350–700 mA was found to successively increase from 55 to 76%. By considering these results, the power conversion efficiency of VM-LEDs was found to be 2.14 times that of regular LEDs at 700 mA.

Use of Highly Reflective Ohmic Contact and Surface KrF Laser Roughening to Improve Light Output of Vertical GaN-Based Light-Emitting Diodes

A highly reflective ohmic contact and surface roughening by KrF excimer laser technique to improve the optoelectronic properties of high-power vertical metallic-substrate GaN-based light-emitting diodes (VM-LEDs) were proposed and investigated. A metal system comprising of Ni/Ag/Ni was employed to serve as a reflector and ohmic contact to p-GaN, which exhibits a good ohmic contact (3.05times10-4 Omega cm2) and high reflectivity (89% at the wavelength of 465 nm) after thermal annealing at 500degC in N2 ambient for 10 min. After the removal of sapphire using laser lift-off process (LLO), KrF excimer laser irradiation was adopted to etch the u-GaN layer and then roughen the surface of the exposed n-GaN layer. As compared to regular lateral-structure GaN-based LEDs with ITO transparent conduction layer (TCL), the fabricated VM-LEDs with a chip size of 1000 mumtimes1000 mum demonstrated a typical increase in light output power (Lop) (i. e., DeltaLop/Lop) by 325.5% at 350 mA with a decrease in forward voltage (Vf) from 3.67 V down to 3.41 V. As compared to the VM-LEDs with u-GaN etching employing inductively coupled plasma, about 125% enhancement in Lop has been obtained at 350 mA from the proposed samples.