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Zhang M.,Xi'an Jiaotong University | Yun F.,Xi'an Jiaotong University | Li Y.,Xi'an Jiaotong University | Ding W.,Xi'an Jiaotong University | And 7 more authors.
Physica Status Solidi (A) Applications and Materials Science | Year: 2015

Optimized dual-wavelength InGaN-based vertical light-emitting diode (LEDs) structures were investigated by numerical simulations. The results show that different quantum-well arrangements in the active region play an important role in obtaining dual-wavelength emission. It is a better way to obtain the dual-wavelength with uniform intensity by arranging quantum wells (QW) with low indium content near the p-side and the QW with high indium near the n-side. This is because the QWs with lower indium near the p-side layer have higher hole-injection efficiency. On the other hand, arranging QW with high indium content near the p-side leads to poor hole-injection efficiency due to the high polarization fields. The physical and optical mechanisms of these phenomena were explained by the intensity of electrostatic fields, energy-band diagrams, and carrier-concentration distribution in the active region of LEDs. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Li Y.,Xi'an Jiaotong University | Yun F.,Xi'an Jiaotong University | Su X.,Shaanxi Supernova Lighting Technology Co. | Liu S.,Shaanxi Supernova Lighting Technology Co. | And 2 more authors.
Journal of Applied Physics | Year: 2014

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 200-330nm 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. © 2014 AIP Publishing LLC.


Li Y.,Xi'an Jiaotong University | Yun F.,Xi'an Jiaotong University | Su X.,Shaanxi Supernova Lighting Technology Co. | Liu S.,Shaanxi Supernova Lighting Technology Co. | And 2 more authors.
Japanese Journal of Applied Physics | Year: 2014

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. © 2014 The Japan Society of Applied Physics.


Huang Y.,Shaanxi University of Technology | Huang Y.,Xi'an Jiaotong University | Yun F.,Shaanxi University of Technology | Yun F.,Xi'an Jiaotong University | And 12 more authors.
Applied Physics Express | Year: 2014

We have demonstrated defect-induced color-tunable monolithic GaN-based vertical light-emitting diodes (VLEDs). With Ag nanorod arrays embedded in p-GaN, large numbers of Ga vacancies (VGa) were produced during the thermal bonding process in VLED fabrication. VGa-related donor-acceptor pair (DAP) transitions in p-GaN resulted in red emission in photoluminescence (PL) measurements as well as a broad electroluminescence (EL) emission spectrum extending from green to red. In combination with high-emission-efficiency blue InGaN/GaN multiple quantum wells (MQWs), the emission color of VLEDs can be changed from red to white by increasing the injection current. © 2014 The Japan Society of Applied Physics.


Zhao Y.-K.,Shaanxi University of Technology | Zhao Y.-K.,Xi'an Jiaotong University | Li Y.-F.,Shaanxi University of Technology | Li Y.-F.,Xi'an Jiaotong University | And 10 more authors.
Chinese Physics B | Year: 2015

Gallium nitride (GaN) based light-emitting diodes (LEDs) with chirped multiple quantum well (MQW) structures have been investigated experimentally and numerically in this paper. Compared to conventional LEDs with uniform quantum wells (QWs), LEDs with chirped MQW structures have better internal quantum efficiency (IQE) and carrier injection efficiency. The droop ratios of LEDs with chirped MQW structures show a remarkable improvement at 600 mA/mm2, reduced down from 28.6% (conventional uniform LEDs) to 23.7% (chirped MQWs-a) and 18.6% (chirped MQWs-b), respectively. Meanwhile, the peak IQE increases from 76.9% (uniform LEDs) to 83.7% (chirped MQWs-a) and 88.6% (chirped MQWs-b). The reservoir effect of chirped MQW structures is the significant reason as it could increase hole injection efficiency and radiative recombination. The leakage current and Auger recombination of chirped MQW structures can also be suppressed. Furthermore, the chirped MQWs-b structure with lower potential barriers can enhance the reservoir effect and obtain further improvement of the carrier injection efficiency and radiative recombination, as well as further suppressing efficiency droop. © 2015 Chinese Physical Society and IOP Publishing Ltd.


Wang H.,Xi'an Physical Education University | Wang H.,Xi'an Jiaotong University | Yun F.,Xi'an Physical Education University | Yun F.,Xi'an Jiaotong University | And 13 more authors.
Wuli Xuebao/Acta Physica Sinica | Year: 2015

Residual stress conditions in GaN-based LEDs will have a significant influence on device performance and reliability. In this paper, GaN-based vertical LEDs under different stress conditions are fabricated by bonding with three types of submounts (Al2O3 submount, CuW submount and Si submount), changing the soak temperature (290℃, 320℃, 350℃ and 380℃) and using different laser energy densities (875, 945 and 1015 mJ · cm-2). The warpage and Raman scattering spectra of those GaN-based LEDs are measured. The experimental results show that the residual stress conditions in GaN-based vertical LEDs are a consequence of the bonded submounts and bonded metal, and the soak temperature is the primary factor that determines the degree of residual stress in LED chips. In the laser lift-off process, changing laser energy density in an appropriate range has little influence on residual strain of LED chips, and the micro-cracks in GaN layer caused by LLO process will play a role in releasing the residual stress. The warpage of epitaxial sapphire substrate becomes large after boding with Si submount, the residual stress in GaN-based vertical LEDs is tensile stress and becomes larger with the soak temperature rising. When GaN epi wafer bonds with Al2O3 submount and CuW submount, the warpages becomes small and large respectively and the residual stress in chips is compressive stress. Because of the mismatch of coefficient of thermal expansion, the compressive stress in GaN-based LED chips increases for Al2O3 submount and drops for CuW submount with the soak temperature rising. © 2015 Chinese Physical Society.


Wang S.,Shaanxi University of Technology | Wang S.,Xi'an Jiaotong University | Li Y.,Shaanxi University of Technology | Li Y.,Xi'an Jiaotong University | And 9 more authors.
Optics Express | Year: 2016

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. © 2016 Optical Society of America.


Liu H.,Shaanxi University of Technology | Liu H.,Xi'an Jiaotong University | Li Y.,Shaanxi University of Technology | Li Y.,Xi'an Jiaotong University | And 9 more authors.
AIP Advances | Year: 2016

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. © 2016 Author(s).


Huang Y.-P.,Shaanxi University of Technology | Huang Y.-P.,Xi'an Jiaotong University | Yun F.,Shaanxi University of Technology | Yun F.,Xi'an Jiaotong University | And 11 more authors.
Wuli Xuebao/Acta Physica Sinica | Year: 2014

The ohmic reflectivity of Ni/Ag/Ti/Au in contact with p-GaN is studied. It is found that under different thickness values of Ni, different annealing temperatures and different annealing atmospheres, the performances of Ni/Ag/Ti/Au electrode are greatly changed. The contact resistivity is measured using the transmission line model. The reflectivity of the electrode is investigated by using a spectrophotometer. The results reveal that the thinner the Ni metal layer, the higher its reflectivity is, in addition, the thickness value of Ni metal has a little influence on contact resistivity. There appears an abrupt decrease in reflectivity of electrode after annealing at a temperature higher than 400°C. It is noticed that the reflectivity decreases more sharply after annealing in oxygen atmosphere than in nitrogen atmosphere. However, annealing in oxygen atmosphere is more helpful to reduce the contact resistivity. The comprehensive evaluations of the contact resistivity and reflectivity indicate that the better performances of Ni (1 nm)/Ag/Ti/Au electrode after rapid annealing in oxygen atmosphere at 400°C are achieved: its contact resistance reaches 5.5 × 103 Ω·cm2 and reflectivity rises up to 85% at 450 nm. Light emitting diode (LED) of vertical structure is fabricated with an optimal electrode. The LED under an injection current of 350 mA can achieve the following working parameters: the working voltage is 3.2 V, the optical output power is 270 mW, and the electro-optical conversion efficiency is 24%. © 2014 Chinese Physical Society.


Li Q.,Shaanxi University of Technology | Li Q.,Xi'an Jiaotong University | Gong Z.,Xi'an Jiaotong University | Li Y.,Shaanxi University of Technology | And 6 more authors.
Nanoscale Research Letters | Year: 2016

Polystyrene sphere was chosen as a catalyst to fabricate indium-tin-oxide (ITO) nanowires (NWs) with a low-temperature (280–300 °C) electron-beam deposition process, bearing high purity. The ITO NWs with diameter of 20–50 nm and length of ~2 um were obtained. X-ray diffraction and high-resolution transmission electron microscope show high crystal quality. The transmittance is above 90 % at a wavelength 400 nm or more, superior to the ITO bulk film. Owing to the unique morphology gradient of the ITO NWs, the effective refractive index of ITO NWs film is naturally graded from the bottom to the top. The ITO NWs have been used on LED devices (λ = 450 nm), which improved the light output power by 31 % at the current of 150 mA comparing to the one without NWs and did not deteriorate the electrical properties. Such ITO NWs open opportunity in LED devices to further improve light extraction efficiency. © 2016, Li et al.

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