Xilin Su

Deep hole injection assisted by large V-shape pits in InGaN/GaN multiple-quantum-wells blue light-emitting diodes

We investigated the hole injection mechanism in InGaN/GaN blue light-emitting diodes by growing monolithic dual-wavelength multiple-quantum-wells and measuring the electroluminescence spectra at different current densities under room temperature. By analyzing the spectral competition from quantum wells at different vertical locations, the hole injection depth was quantitatively measured. During the epitaxial growth, large size V-shape pits with 200–330 nm diameter were intentionally formed in the active region by controlling the growth condition. It was found that such defect has a significant influence on the hole injection depth. With large V-shape pits and reduced quantum barrier thickness, the hole can be injected beyond 8 pairs of quantum well/quantum barrier. And less “droop” effect at large current density were observed. A carrier transport model with the presence of large V-shape pits is established.

Laser patterning of Y 3 Al 5 O 12 :Ce 3+ ceramic phosphor platelets for enhanced forward light extraction and angular color uniformity of white LEDs.

We present a facile fabrication process to directly fabricate cone-shaped microwells arrays on single crystal Y3Al5O12:Ce3+ (YAG:Ce) ceramic phosphor platelets (CPPs) by short-pulse laser direct patterning. Compared to unpatterned YAG:Ce CPP with smooth surface, the forward-to-total ratio of emission photons of patterned YAG:Ce CPPs was enhanced from 53.2% up to 78.2%, and the total emission within 4-π degree is 6% higher. The fabricated patterns are also beneficial in increasing the color conversion efficiency of YAG:Ce CPPs by 7.6%. The patterned YAG:Ce CPPs display much better correlated color temperature (CCT) uniformity under varied currents. The angular correlated color temperature uniformity (ACU) of patterned YAG:Ce CPPs reaches as high as 0.933 compared to 0.730 of the unpatterned one. These results suggest that laser patterning of YAG:Ce CPP could effectively manipulate its luminance, chromaticity and illumination pattern, which may lead to further technological advancements for diversified applications of film-type CPPs in highly efficient white LEDs.

Mechanism of hole injection enhancement in light-emitting diodes by inserting multiple hole-reservoir layers in electron blocking layer

In this study, gallium nitride (GaN) based light-emitting diodes(LEDs) with single and multiple hole-reservoir layers (HRLs) inserted in the electron-blocking layer (EBL) have been investigated numerically and experimentally. According to simulation results, a better electron confinement and a higher hole injection level can be achieved by the multiple HRLs inserted in the EBL region. To further reveal the underlying mechanism of hole injection enhancement experimentally, the active regions were intentionally designed to emit photons with three different wavelengths of 440 nm, 460 nm, and 480 nm, respectively. Based on the experimental results of photoluminescence(PL) and time-resolved PL(TRPL) measurements conducted at 298 K, the remarkable enhancement (148%) of PL intensities and significant increase in the decay times of the quantum wells close to p-GaN can be obtained. Therefore, the mechanism is proposed that carriers are able to reserve in the EBL region with multiple HRLs for a much longer time. Meanwhile, carriers could diffuse into the active region by tunnelling and/or thermo-electronic effect and then recombine efficiently, leading to the better carrier reservoir effect and higher hole injection in LEDs. As a result, by inserting multiple HRLs in the EBL region instead of single HRL, the experimental external quantum efficiency is enhanced by 19.8%, while the serious droop ratio is markedly suppressed from 37.0% to 27.6% at the high current injection of 100 A/cm2.

Modulating dual-wavelength multiple quantum wells in white light emitting diodes to suppress efficiency droop and improve color rendering index

In this paper, gallium nitride (GaN) based white light-emitting diodes (WLEDs) with modulated quantities of blue (In0.15Ga0.85N) quantum wells (QWs) and cyan QWs (In0.18Ga0.82N) in multiple QW (MQW) structures have been investigated numerically and experimentally. It is demonstrated that the optical performance of LEDs is sensitive to the quantities of cyan QWs in dual-wavelength MQW structures. Compared to the LEDs with respective 0, 4, and 8 cyan QWs (12 QWs in total), the optical performance of the sample with 6 cyan QWs is the best. The deterioration of the optical performance in the sample with less (4 pairs) cyan QWs or more (8 pairs) cyan QWs than 6 cyan QWs may be ascribed to weakened reservoir effect or more defects induced. Compared to conventional blue LEDs (12 blue QWs), the sample with 6 cyan QWs could effectively suppress the efficiency droop (the experimental droop ratio decreases from 50.3% to 39.5% at 80 A/cm2) and significantly improve the color rendering index (CRI, increases from 66.4 to 77.0) simultaneously. We attribute the droop suppression to the strengthened reservoir effect and carrier confinement of deeper QWs (higher indium composition) incorporated in the dual-wavelength MQW structures, which lead to the better hole spreading and enhanced radiative recombination. Meanwhile, the remarkable experimental CRI improvement may result from the wider full-width at half-maximum of electroluminescence spectra and higher cyan intensity in WLED chips with dual-wavelength MQW structures.

Air-void embedded GaN-based light-emitting diodes grown on laser drilling patterned sapphire substrates

Air-void structure was introduced in GaN-based blue light-emitting diodes (LED) with one-step growth on periodic laser drilling patterned sapphire substrate, which free of any photolithography or wet/dry etching process. The influence of filling factors (FF) of air-void on crystal quality and optical performance were investigate. Transmission electron microscopy images and micro-Raman spectroscopy indicated that the dislocation was bended and the partially compressed strain was released. When FF was 55.43%, compared with the LED structure grown on flat sapphire substrate, the incorporation of air-void was observed to reduce the compressed stress of ∼20% and the luminance intensity has improved by 128%. Together with the simulated reflection intensity enhancement by finite difference time-domain (FDTD) method, we attribute the enhanced optical performance to the combined contribution of strong back-side light reflection of air-void and better GaN epitaxial quality. This approach provides a simple replacement to the conventional air-void embedded LED process.

Surface plasmon enhanced green light emitting diodes with silver nanorod arrays embedded in p-GaN

We demonstrated surface-plasmon (SP) enhanced green light-emitting diodes (LEDs). Three types of Ag nanorod arrays with a minimum distance between the quantum well (QW) and Ag of 20, 40, and 55 nm respectively were fabricated on p-GaN layer. Photoluminescence measurements showed ?175% emission enhancement for the 20 nm spacing while almost no enhancement for the 55 nm spacing. Simulation result showed that a localized surface plasmon resonance (LSPR) at a wavelength of ?500 nm generated by Ag nanorod arrays induced InGaN/GaN QW and SP coupling. However, the electrical field of the LSPR generated by Ag nanorods only spread ?40 nm in the vertical direction in GaN. This simulation result well explains the observation of SP-QW coupling emission enhancement for 20 nm spacing between Ag and QW, and the lack of enhancement for the 55 nm spacing samples.

Whispering gallery mode lasing from InGaN/GaN quantum well microtube

In this work, we have successfully fabricated microtubes by strain-induced self-rolling of a InGaN/GaN quantum wells nanomembrane. Freestanding quantum wells microtubes, with a diameter of 6 µm and wall thickness of 50 nm, are formed when the coherently strained InGaN/GaN quantum wells heterostructure is selectively released from the hosting substrate. Periodic oscillations due to whispering-gallery modes resonance were found superimposed on photoluminescence spectra even at low optical excitation power. With increasing pumping power density, the microtube is characterized by a stimulated emission with a threshold as low as 415 kW/cm2. Such emission shows predominant TM polarization parallel to the microtube axis.

Improving Peak-Wavelength Method to Measure Junction Temperature by Dual-Wavelength LEDs

In this paper, the improvement of the method measuring the junction temperature of light-emitting diodes (LEDs) has been studied experimentally. A practical method is proposed with only three measurement procedures. With the consideration of indium (In) composition and blue shift, the method has a high applicability, which is practical for the LED chips vary from blue to green chips under different currents, including the packaged chips. On the other hand, according to the experimental and derived results, the junction-temperature difference and peak-wavelength shift in both blue-shift and red-shift fields show similar parabolic-like relations. To simplify the experimental processes, dual-wavelength LEDs were fabricated and measured instead of conventional single-wavelength LEDs.

Growth, Characterization, and Application of Well-defined Separated Gan-based Pyramid Array on Micropatterned Sapphire Substrate

We tried to obtain microstructures on a three-dimensional (3D) micropatterned substrate by laser drilling. The influences of the dimensions of the drilling holes on the morphology and the material quality of the grown structures were studied. Uniform micropyramid arrays with relatively low dislocation density can be achieved by adjusting the laser drilling parameters. The internal quantum efficiency was estimated to be improved by a factor of 3 for a pyramid structure compared with that of planar LEDs. We fabricated 5 × 7 mm2 flexible LEDs employing the pyramid structure and the devices exhibited good flexibility without performance reduction after bending.

Carrier injection modulated by V-defects in InGaN/GaN multiple-quantum-well blue LEDs

We investigated the carrier injection mechanism in InGaN/GaN blue light-emitting diodes by growing monolithic dual-wavelength multiple quantum wells and measuring electroluminescence spectra at different current densities at room temperature. During the epitaxial growth, V-defects of different sizes were intentionally formed in the active region area by controlling the growth conditions. We found that the size of the V-defects has a significant effect on the spectral competition of dual-wavelength emissions. With small V-defects, light emitted from quantum wells near p-GaN is dominant. In a sample with large V-defects, quantum wells near n-GaN contribute more to carrier recombination. The hole injection depth of eight pairs of quantum wells far from p-GaN is quantitatively estimated. We attribute the different behaviors to the modulation of carrier injection depth by the formed V-defects.