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Dependence of operating conditions on lifetime of Phosphor Quantum dots-based white LEDs
(2019-01-01)
Singh, Preetpal
;
Tan, Cher Ming
;
Kuo, Hao-Chung
;
Wang, Kai
;
Yang, Hongchen
Lifetime of packaged Phosphor- Quantum dot (QDs) based white LEDs is studied. Various stress parameters such as blue light luminance, temperature and moisture are considered and their effect on the performance of the Phosphor- QD based LED is investigated. The lumen degradation rate and degradation trend for LEDs is observed to be different under varying stress conditions. Lumen recovery process is also observed under light stress test.
High-Bandwidth Green Semipolar (20-21) InGaN/GaN Micro Light-Emitting Diodes for Visible Light Communication
(2020-08-19)
Chen, Sung-Wen Huang
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Huang, Yu-Ming
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Chang, Yun-Han
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Lin, Yue
;
Liou, Fang-Jyun
The light-emitting diode (LED) is among promising candidates of light sources in visible light communication (VLC); however, strong internal polarization fields in common c-plane LEDs, especially green LEDs, result in low frequency and limited transmission performance. This study aims to overcome the limited 3-dB bandwidth of long-wavelength InGaN/GaN LEDs. Thus, semipolar (20-21) micro-LEDs (mu LEDs) were fabricated through several improved approaches on epitaxy and chip processes. The mu LED exhibits a 525 nm peak wavelength and good polarization performance. The highest 3-dB bandwidth up to 756 MHz and 1.5 Gbit/s data rate was achieved under a current density of 2.0 kA/cm(2). These results suggest a good transmission capacity of green semipolar (20-21) mu LEDs in VLC applications.
High Circular Polarized Nanolaser with Chiral Gammadion Metal Cavity
(2020-05-12)
Yu, Cheng-Li
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Hsiao, Yu-Hao
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Chang, Chiao-Yun
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Cheng, Pi-Ju
;
Lin, Hsiang-Ting
We demonstrate a circularly polarized laser with the metal-gallium-nitride gammadion nanocavities. The ultraviolet lasing signal was observed with the high circular dichroism at room temperature under pulsed optical pump conditions. Without external magnetism which breaks the time-reversal symmetry to favor optical transitions of a chosen handedness, the coherent outputs of these chiral nanolasers show a dissymmetry factor as high as 1.1. The small footprint of these lasers are advantageous for applications related to circularly polarized photons in future integrated systems, in contrast to the bulky setup of linearly-polarized lasers and quarter-wave plates.
Alternative Strategy to Reduce Surface Recombination for InGaN/GaN Micro-light-Emitting Diodes-Thinning the Quantum Barriers to Manage the Current Spreading
(2020-08-06)
Chang, Le
;
Yeh, Yen-Wei
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Hang, Sheng
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Tian, Kangkai
;
Kou, Jianquan
Owing to high surface-to-volume ratio, InGaN-based micro-light-emitting diodes (mu LEDs) strongly suffer from surface recombination that is induced by sidewall defects. Moreover, as the chip size decreases, the current spreading will be correspondingly enhanced, which therefore further limits the carrier injection and the external quantum efficiency (EQE). In this work, we suggest reducing the nonradiative recombination rate at sidewall defects by managing the current spreading effect. For that purpose, we properly reduce the vertical resistivity by decreasing the quantum barrier thickness so that the current is less horizontally spreaded to sidewall defects. As a result, much fewer carriers are consumed in the way of surface nonradiative recombination. Our calculated results demonstrate that the suppressed surface nonradiative recombination can better favor the hole injection efficiency. We also fabricate the mu LEDs that are grown on Si substrates, and the measured results are consistent with the numerical calculations, such that the EQE for the proposed mu LEDs with properly thin quantum barriers can be enhanced, thanks to the less current spreading effect and the decreased surface nonradiative recombination.
Effect of Washing Solvents on the Properties of Air-Synthesized Perovskite CsPbBr3 Quantum Dots for Quantum Dot-Based Light-Emitting Devices
(2020-01-01)
Chen, Lung-Chien
;
Tien, Ching-Ho
;
Tseng, Po-Wei
;
Tseng, Zong-Liang
;
Huang, Wei-Lun
In order to improve the stability and cost of perovskite CsPbBr3 and establish a preparation process for quantum dot materials with excellent luminescence properties, CsPbBr3 quantum dots were synthesized by the hot injection method and the ice bath method in air ambient. Then, a proprietary purification system was used to improve the properties of the quantum dots and prepare high-quality perovskite quantum dot films. Optimal material and light characteristics were exhibited with ethyl acetate:hexane as the purification solvent. In the solution form, the photoluminescence wavelength could be maintained at 508 nm with almost no shift compared with other conditions, and the quantum yield could reach 99%; however, superior performance was obtained in the thin-film form, which exhibited little to no shift in the photoluminescence wavelength and, surprisingly, achieved a quantum yield of 91.6% with a narrow PL emission with a full width at half-maximum of 28 nm. In addition, exceptional thin-film uniformity was observed from scanning electron microscopy image analyses. The CsPbBr3 quantum dot light-emitting diode purified with ethyl acetate:hexane demonstrated an open-circuit voltage of 4 V, a maximum luminous brightness of 488 cd m(-2), and a maximum external quantum efficiency of 0.14%.
CSP-LEDs Combined With Light Guide Without Reflective Matrix for Antiglare Design
(2020-01-01)
Ye, Zhi Ting
;
Ruan, Moujay
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Kuo, Hao-Chung
;
光電工程學系
;
光電工程研究所
This study proposed a model with different luminous intensity profiles without reflective matrix bar for light fixtures that used asymmetric luminous intensity(ALI) CSP-LEDs (Chip Scale Package-Light emitting diode) combined with light guide as light source module, and analyzed illumination uniformity and unified glare rating (UGR) performance by using a classroom as an example. The study constructed a model of light fixture use in classrooms. Four important parameters affect the comfort of human eye that include UGR, illumination, uniformity of spatial and light distribution of light fixture, in particular, light distribution of light fixture was major affect factor for direct glare. Therefore, light fixture designers previously used many diverse structures to prevent glare, but which will result in a loss of light fixture efficiency about 25%-35% that caused by material absorption and ray loss. This study proposed used ALI CSP-LEDs combined with light guide as light source to optimize different light distribution angles without attached reflective matrix bar or second optical lens on light fixture to simulate a light fixture in a classroom and to evaluate UGR, illumination and uniformity values. We also consider the light fixture tilt angle caused effect of UGR and uniformity. On the basis of the simulation and experimental results, ALI CSP-LEDs beam angle relative to vertical and horizontal axes were 150 degrees and 120 degrees, respectively, the light distribution profile for a beam angle 90 degrees provided the highest illumination uniformity and a UGR of less than 19. Thus, light fixtures with a light distribution angle of beam angle 90 degrees supply human-friendly for anti-glare lighting.
The Study of High Breakdown Voltage Vertical GaN-on-GaNp-i-nDiode with Modified Mesa Structure
(2020-08-01)
Ho, Wen-Chieh
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Liu, Yao-Hsing
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Wu, Wen-Hsuan
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Huang Chen, Sung-Wen
;
Tzou, Jerry
In this paper, we fabricated Gallium Nitride (GaN) verticalp-i-ndiodes grown on free-standing GaN (FS-GaN) substrates. This homogeneous epitaxy led to thicker GaN epi-layers grown on the FS-GaN substrate, but a high crystalline quality was maintained. The vertical GaNp-i-ndiode showed a low specific on-resistance of 0.85 m ohm-cm(2)and high breakdown voltage (BV) of 2.98 kV. The high breakdown voltage can be attributed to the thick GaN epi-layer and corresponds to the mesa structure. Improvement of the device characteristics by the mesa structure was investigated using device simulations. We proved that a deeper mesa depth is able to decrease the electric field at the bottom of the mesa structure. Furthermore, a smaller mesa bevel angle will assist the BV up to 2.98 kV at a 60 degrees bevel angle. Our approach demonstrates structural optimization of GaN verticalp-i-ndiodes is useful to improve the device performance.
Hybrid Composites of Quantum Dots, Monolayer WSe2, and Ag Nanodisks for White Light-Emitting Diodes
(2020-07-24)
Chang, Chiao-Yun
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Yu, Cheng-Li
;
Lin, Chen-An
;
Lin, Hsiang-Ting
;
Lee, Andrew Boyi
We investigated hybrid zero-dimensional core-shell CdSe/ZnS quantum dot (QD)/two-dimensional monolayer WSe2 semiconductors with an Ag nanodisk (ND) for manipulating plasmonic-enhanced photoluminescence (PL) and color conversion efficiency. The absorption spectrum of the local surface plasmon resonance (LSPR) effectively overlaps with that of QDs or monolayer WSe2 to considerably enhance PL. The broad absorption spectrum of the LSPR simultaneously overlapped with the emission spectrum of QDs and the absorption spectrum of excitons in WSe2 to enhance the color conversion efficiency. The highest efficiency of color conversion from QDs to WSe2 with Ag ND was 53%. In the future, hybrid QD/transition metal dichalcogenide light emitters could be further integrated with GaN-based white light-emitting diodes to manipulate the color temperature and expand the color gamut to develop a miniature white light-emitting diode.
Fabrication of highly efficient hybrid device structure based white light emitting diodes
(2020-07-10)
Verma, Akta
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Sharma, S. K.
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Lin, Chih-Hao
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Manikandan, Arumugam
;
Kuo, Hao-Chung
In this study, white light is generated from the combination of BaMgAl10O17:Mn2+ (BAM) green phosphor and colloidal red CdSe/ZnS quantum dots (QDs) via two different solid and hybrid type geometries. Problem of aggregation in solid type device is discussed in details and motivates us to move towards the fabrication of hybrid type device structure. The photoluminescence (PL) characterization was used to record the excitation and emission spectra of BAM and QDs materials. The size of CdSe/ZnS QDs was confirmed by transmission electron microscopy technique. The CdSe/ZnS QDs owned high quantum yield (>50%) and contributed to fabrication of high luminous efficiency of white light emitting diodes (WLEDs). The hybrid-type WLED showed 98% lm/W with outstanding color gamut values with a National Television System Committee (NTSC) value of 138% and Rec.2020 value of 105%. Moreover, the hybrid type device structure shows only 5% attenuation in luminous efficiency after 500 h of storage which maintains a great optical property.
Dielectric metasurfaces: From wavefront shaping to quantum platforms
(2020-05-01)
Li, Chuanlin
;
Yu, Peng
;
Huang, Yongjun
;
Zhou, Qiang
;
Wu, Jiang
Metasurfaces are nanopatterned structures of sub-wavelength thickness. Their effective refractive index and spectral characteristic can be tailored by material composition, intrinsic and extrinsic resonances, structure size, and ambient conditions. Consequently, they allow for phase, amplitude, polarisation, and spatial control of an optical field beyond what natural materials can offer. Dielectric metasurfaces with lower loss have opened a wide range of new applications such as enhanced imaging, structural colour, holography, and planar sensors. In particular, beam steering and control measures such as nonlinear optics, ultrafast optics, and quantum optics are of increasing importance for quantum communication, computation, and information processing. In this review, the recent progress on dielectric metasurfaces is summarised, including advanced fabrication technologies and novel applications from advanced wavefront shaping to quantum platforms. In addition, a perspective for the future development of the field is presented.

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