Shichao Zhu

Enhancement of the modulation bandwidth for GaN-based light-emitting diode by surface plasmons

We have fabricated the surface plasmon (SP) coupled GaN-based nanorod LEDs with Ag nanoparticles (Nps), and demonstrate the enhancement of the optical modulation bandwidth by SPs. Compared with the LED without Ag Nps, the optical modulation bandwidth of the LED with Ag Nps increases by a factor of ~2 at 57 A/cm2. The photoluminescence (PL) and electroluminescence (EL) experimental results are consistent with each other, and both suggest the effective coupling between quantum wells (QWs) and SPs. Furthermore, the current dependent modulation frequency characteristics show that the QW-SP coupling can increase the modulation bandwidth, especially for LEDs with high intrinsic internal quantum efficiency (IQE). These findings will help to open a new solution to design the ultrafast LED light source for the application of the visible light communication.

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.

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).

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...

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.

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.