Xuejiao Sun

Surface plasmon-enhanced nanoporous GaN-based green light-emitting diodes with Al2O3 passivation layer.

A surface plasmon (SP)-enhanced nanoporous GaN-based green LED based on top-down processing technology has been successfully fabricated. This SP-enhanced LED consists of nanopores passing through the multiple quantum wells (MQWs) region, with Ag nanorod array filled in the nanopores for SP-MQWs coupling and thin Al(2)O(3) passivation layer for electrical protection. Compared with nanoporous LED without Ag nanorods, the electroluminescence (EL) peak intensity for the SP-enhanced LED was greatly enhanced by 380% and 220% at an injection current density of 1 and 20A/cm(2), respectively. Our results show that the increased EL intensity is mainly attributed to the improved internal quantum efficiency of LED due to the SP coupling between Ag nanorods and MQWs.

Degradation and corresponding failure mechanism for GaN-based LEDs

The degradation behaviors of high power GaN-based vertical blue LEDs on Si substrates were measured using in-situ accelerated life test. The results show that the dominant failure mechanism would be different during the operation. Besides that, the corresponding associated failure mechanisms were investigated systematically by using different analysis technologies, such as Scan Electron Microscopy, Reflectivity spectroscopy, Transient Thermal Analysis, Raman Spectra, etc. It is shown that initially, the failure modes were mainly originated from the semiconductor die and interconnect, while afterwards, the following serious deterioration of the radiant fluxes was attributed to the package. The interface material and quality, such as die attach and frame, play an important role in determining the thermal performance and reliability. In addition, the heating effect during the operation will also release the compressive strain in the chip. These findings will help to improve the reliability of GaN-based LEDs,...

Comparative analysis of thermal and optical properties for the LEDs with different circuit boards

We have fabricated three types of high power LEDs with different circuit boards: Al2O3 ceramic board, epoxy PCB with Cu-deposited on the wall of holes and epoxy PCB with Cu-filled through holes. The different contribution of the package have been separated and investigated by using the derivative of temperature rise in the time domain and thermal transient method. The results show that although the thermal conductivity coefficient of epoxy PCB is much smaller than that of ceramic, PCB integrated with Cu can improve the heat-dissipation significantly and decrease the thermal resistance. In addition, this design can also improve the EQE droop effect.

Degradation behavior of GaN-based LEDs encapsulated with different phosphors

The degradation behaviors of GaN-based LEDs encapsulated with different phosphors were studied. The results of the aging experiments show that the yellow phosphor is the most stable phosphor, although the green and red phosphors can help to improve the color quality, they strongly influence the reliability. Comparing the accelerating factors, such as the aging temperature and relatively humidity, although the aging temperature 25 °C is less than 55 °C, but because of a higher relative humidity, the luminous flux and color temperature of white LEDs were still strongly influenced, especially for the phosphors. The optical power and luminous flux of blue LEDs, LEDs with yellow phosphor and LEDs with the yellow/red mixed phosphors aged at 25 °C (RH 21%) and 55 °C (RH 10%) increased during the first 400 hours and then decreased after that, but LEDs with green/red mixed phosphors decreased continually with the aging time.

Composite degradation model and corresponding failure mechanism for mid-power GaN-based white LEDs

The degradation mechanism of mid-power GaN-based white LEDs were investigated by using the in-situ multi-functional accelerated aging tests. The changes of the luminous flux and the chromaticity shift during the stress time show some correlations. To quantitatively analyze the degradation behavior, a composite model considering the luminous flux increasing and decreasing mechanisms was proposed and the results agree well with the experiments in the entire aging time. Furthermore, different analytical technologies have been used to understand the cause of luminous flux degradation and chromaticity shift. The results show that the chromaticity shift was mainly due to the phosphors deterioration, while the serious degradation of luminous flux was the overall effects from the package, including the phosphors deterioration and oxidation of silicone encapsulant.The degradation mechanism of mid-power GaN-based white LEDs were investigated by using the in-situ multi-functional accelerated aging tests. The changes of the luminous flux and the chromaticity shift during the stress time show some correlations. To quantitatively analyze the degradation behavior, a composite model considering the luminous flux increasing and decreasing mechanisms was proposed and the results agree well with the experiments in the entire aging time. Furthermore, different analytical technologies have been used to understand the cause of luminous flux degradation and chromaticity shift. The results show that the chromaticity shift was mainly due to the phosphors deterioration, while the serious degradation of luminous flux was the overall effects from the package, including the phosphors deterioration and oxidation of silicone encapsulant.

Surface plasmon enhanced GaN based light-emitting diodes by Ag/SiO 2 nanoparticles

The QW-SP coupled nanorod LED with Ag/SiO2 nanoparticals (Nps) was fabricated. From both the light emission and the transmission spectrum, the energy coupling effects have been verified. The PL intensity of SP coupled LED increased by 2.1 times compared with the reference sample, combined with a blue shift of 2 nm from 456 to 454 nm. Furthermore, the time resolved photoluminescence (TRPL) results also confirm that the QW-SP coupling can enhance the carrier spontaneous emission rate dramatically. Our finding gives an alternative solution to fabricate QW-SP coupled LEDs to realize both high-power and high-speed.

Analysis of GaN based LED performance before and after the ESD shock

GaN-based LEDs are fast replacing the traditional lighting in numerous applications because of many advantages, such as energy saving and green environments. But as for the high level LED applications, one of the greatest concerns and key requirements is the reliability. In general, GaN epilayers are usually grown on an insulating sapphire substrate since no suitable substrates could be utilized. In this case, except for the degradation of light output power, electrostatic discharge (ESD) is one of the main reliability concerns and draws lots of attention. There are many reported methods[1~3] to overcome the ESD-induced damage, such as combining with a Si-based Zener diode via the flip-chip process, building an internal GaN Schottky diode inside the chip, inserting a high-temperature grown p-type cap layer into an epitaxial structure. But to characterize the ESD endurance, it is normally necessary to operate the ESD shock, which is destructive and irreversible procedure. Therefore, the aim of this study is to compare the electrical and optical properties before and after ESD test, and find out the relationship between the intrinsic structure and ESD characteristic. Three type of LEDs with different structure were prepared and performed using HBM model ESD shock ranging from 0~8KV. I-V, C-V, luminous flux measurements were carried out before and after the ESD test, respectively. The results demonstrate that the ESD endurance can be improved by the inner stress modulation.