Haicheng Cao

Enhancing the spontaneous emission rate by modulating carrier distribution in GaN-based surface plasmon light-emitting diodes

Based on the nanorod structure, we have fabricated GaN-based surface plasmon light-emitting diodes with Ag nanoparticles deposited laterally proximity to the multiple quantum wells (MQWs) region, which allows us to investigate the quantum well - surface plasmon (QW-SP) coupling effect. Our results show that the QW-SP coupling effect increases significantly when the SP resonant wavelength of Ag nanoparticles is close to the QW emission wavelength, especially by using a shorter wavelength light source, which will further enhance the spontaneous emission rate. Combined with the simulations, we find that the enhancement is due to the decreased excitation light penetration depth into the active region, which can modulate the carrier distribution and increase the proportion of SP-coupled carriers in the MQWs of LEDs. To increase the spontaneous emission rate for the electrical QW-SP coupled LEDs, we can use single QW or MQW structure to confine the carriers in the topmost QW, which will effectively increase the proportion of SP-coupled carriers. Our findings pave a way to design the ultrafast LED light source for the application of visible light communication (VLC).

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,...

GaN-based flip-chip parallel micro LED array for visible light communication

In this study, GaN-based flip-chip parallel micro light-emitting diode (μLED) arrays have been fabricated. Compared to a single LED with the same active region area, flip-chip parallel μLED arrays are superior on both modulation bandwidth and light output because of the uniform current spreading, improved heat dissipation, and higher light extraction efficiency. With this structure, an injected current density up to 7900 A/cm2 has been achieved with a modulation bandwidth of ∼227 MHz. Meanwhile, the optical power is above 30 mW, which is more suitable for visible light communication in free space. The influence of resistance-capacitance (RC) time constant and carrier lifetime on the modulation bandwidth of parallel μLED arrays has also been investigated in details. This study will help the design of GaN-based LEDs to both enhance the modulation bandwidth and optical power.

Influence of quantum confined Stark effect and carrier localization effect on modulation bandwidth for GaN-based LEDs

We have fabricated GaN-based light-emitting diodes (LEDs) with different quantum well (QW) thicknesses to investigate the influence of the quantum confined Stark effect (QCSE) and carrier localization effect on the carrier recombination processes under both direct current (DC) and alternating current (AC) biases. At low current density, QCSE dominates the carrier recombination and decreases the radiative recombination rate. With increasing the current density, QCSE will be screened by injected carriers, and both optical power and modulation bandwidth can be increased. When the polarization field is completely compensated, the carrier localization effect starts to dominate. By reducing the influence of the QCSE and carrier localization effect, a high modulation bandwidth of ∼700 MHz was achieved at a low current density of 425 A/cm2 for the LED with 5 nm QW. Our findings will pave an alternative solution for co-optimization of the modulation bandwidth and efficiency for LEDs at a relatively low current den...

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.

Light Modulation and Water Splitting Enhancement Using a Composite Porous GaN Structure

On the basis of the laterally porous GaN, we designed and fabricated a composite porous GaN structure with both well-ordered lateral and vertical holes. Compared to the plane GaN, the composite porous GaN structure with the combination of the vertical holes can help to reduce UV reflectance and increase the saturation photocurrent during water splitting by a factor of ∼4.5. Furthermore, we investigated the underlying mechanism for the enhancement of the water splitting performance using a finite-difference time-domain method. The results show that the well-ordered vertical holes can not only help to open the embedded pore channels to the electrolyte at both sides and reduce the migration distance of the gas bubbles during the water splitting reactions but also help to modulate the light field. Using this composite porous GaN structure, most of the incident light can be modulated and trapped into the nanoholes, and thus the electric fields localized in the lateral pores can increase dramatically as a result of the strong optical coupling. Our findings pave a new way to develop GaN photoelectrodes for highly efficient solar water splitting.

Controlled Synthesis of ZnO Nanostructures by Electrodeposition without Any Pretreatment and Additive Regent

ZnO nanostructures have been fabricated using electrodeposition method without any additive reagent and nucleation-layer. The influences of the applied voltage, temperature, electrolyte concentration, and time on the nanostructures of ZnO have been investigated using cyclic voltammety (CV), X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). The result shows that the 1-dimensional (1D) nanostructures tend to be formed at lower voltage and electrolyte concentration, while 2-dimentional (2D) nanostructures can be easily obtained at higher voltage and concentration. Although increasing temperature is helpful to grow 1D nanostructures, but excessive high temperature will destroy the ZnO nanostructures because of the high solubility of ZnO. Furthermore, we reveal the mechanism of the formation of ZnO nanostructures mainly depends on the competition between the hydroxylation and dehydration reaction. Our work is helpful for developing the photocatalytic and photodetection applications using different ZnO nanostructures.