Schang-Jing Hon

GaN-Based Light-Emitting Diode With Sputtered AlN Nucleation Layer

The crystal quality, electrical, and optical characteristics of GaN-based light-emitting diodes (LEDs) were improved using a sputtered AlN nucleation layer. Replacing the in situ AlN nucleation layer with the sputtered AlN nucleation layer reduced the (002) and (102) X-ray rocking curve widths of the GaN layer from 318.0 to 201.1 and 412.5 to 225.0 arcsec, respectively. The -20-V reverse leakage current of the LEDs with the sputtered AlN nucleation layer is about three orders less than that of the LEDs with the in situ AlN nucleation layer. In addition, the LEDs with sputtered AlN nucleation layer could sustain more than 60% passing yield on the ESD test of under a -600-V machine mode, whereas the LEDs with the in situ AlN nucleation layer sustained less than 40% passing yield. Moreover, the 20-mA output power of the LEDs with the sputtered AlN nucleation layer also improved by approximately 5.73% compared with that of the LEDs with the in situ AlN nucleation layer.

GaN-Based LEDs With a Chirped Multiquantum Barrier Structure

We report the fabrication of GaN-based blue light-emitting diodes (LEDs), which separately incorporate the three different electron blocking layers (EBLs), namely, a conventional AlGaN, a uniform multiquantum barrier (UMQB), and a chirped multiquantum barrier (CMQB). On the administration of 20 mA injection current, the corresponding LED output powers measured were 27.5, 27.2, and 25.4 mW for CMQB LED, UMQB LED, and LED, respectively, with a conventional AlGaN EBL. It was also found that the LED with CMQB EBL exhibited a significantly lower drooping effect and a smaller forward bias as compared with LEDs with a conventional AlGaN EBL and UMQB EBL.

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.

High-power GaN LED chip with low thermal resistance

The recent breakthrough in high power GaN LEDs efficiency makes the adoption of these tiny solid state light emitting devices into general lighting application earlier than expected before. However, heat management is still an important issue for these white high power GaN LEDs. So far, the most popular driving current for 1mm square die is about 350mA but there is a trend to increase the driving current up to 1A or even higher. In order not to degrade the LED performance at such a high current operation, it is very important to reduce the thermal resistance and keep the junction temperature below 60 degree centigrade. In the past, GaN flip chip, thin GaN LED, or GaN on SiC or GaN substrate are some typical structures used to make high power LEDs with low thermal resistance. However, all of these methods need very complicated chip process or using very expensive substrates and are difficult to meet general lighting dollar per lumen target. In this study, we proposed a cheaper way to make a high power LED die with lower thermal resistance. We will report how we can achieve the thermal resistance of high power GaN LED die less than 1°C/W.

Numerical Simulation of GaN-Based LEDs With Chirped Multiquantum Barrier Structure

The authors report the numerical simulation of GaN-based light-emitting diodes (LEDs) with either a conventional AlGaN electron blocking layer (EBL), uniform multiquantum barrier (UMQB) structure, or chirped multiquantum barrier (CMQB) structure. It is found that the 102-meV effective barrier height simulated from the LED with CMQB structure is larger than those simulated from the LEDs with a UMQB structure (90 meV) and with conventional AlGaN EBL (60 meV). With the large effective barrier height, it is found that LEDs with a CMQB structure exhibit smaller leakage current. It is also found that the maximum internal quantum efficiencies are 0.703, 0.842, and 0.887, for the LEDs with conventional EBL, UMQB structure, and CMQB structure, respectively. In addition, it is found that forward voltages simulated from the LEDs with CMQB structure and with UMQB structure are both smaller than that simulated from the LED with conventional AlGaN EBL. These results also agree well with the experimental data.

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.

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)

Enhanced Current Spreading for GaN-Based Side-View LEDs by Adding an Metallic Stripe Across the Long Side of the Chip

The authors propose a simple method to enhance current spreading of GaN-based side-view light-emitting diodes (LEDs) by adding a metallic stripe across the long side of the chip. It was found that 20 mA output power of the LED could be enhanced from 8.54 to 9.2 mW by adding the metallic stripe. It was also found that further the LED output power could be enhanced to 9.68 mW by partially thinning down the metallic stripe chemically. These improvements could be attributed to the more uniform current distribution across the LED chip.

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)

GaN-Based LEDs With Rough Surface and Selective KOH Etching

The authors propose a simple method to enhance the performances of GaN-based light-emitting diodes (LEDs) with rough surface. By using KOH to selectively smooth the area beneath the p-contact pad, it was found that we could reduce the forward voltage and enhance output power of the GaN-based LEDs. It was also found that wall-plug efficiency (WPE) of the proposed LEDs was 10.3% larger than that of the conventional LEDs with rough surface and 33.9% larger than that of the conventional LEDs with flat surface. Furthermore, it was found that the use of selective KOH etching will not degrade drooping effect of the LEDs.