Browsing by Author "Liu, Che-Yu"

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A Study of Mechanical Lift-Off Technology for High-Efficiency Vertical LEDs Using Micro-Porous GaN Template
(2013-01-01)
Lee, Chia-Yu
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Lin, Da-Wei
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Liu, Che-Yu
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Hsu, Shih-Chieh
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Kuo, Hao-Chung
The high efficiency vertical light emitting diodes (V-LEDs) using mechanical lift-off with Micro-Porous GaN template formed by high temperature molten KOH wet etching process were demonstrated. The average threading dislocation density (TDD) of u-GaN after regrowth was estimated by transmission electron microscopy (TEM) reduced from 2x10(9) to 1x10(8) cm(2). The sapphire substrate was easily removed by mechanical lift-off during wafer bonding process. The light output of V-LEDs are greatly enhanced by 100% compared with Conventional-LEDs at an operating current of 20mA due to the high quality thin GaN and excellent heating dissipation.
Bridging the "green gap" of LEDs: giant light output enhancement and directional control of LEDs via embedded nano-void photonic crystals
(2016-01-01)
Tsai, Yu-Lin
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Liu, Che-Yu
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Krishnan, Chirenjeevi
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Lin, Da-Wei
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Chu, You-Chen
Green LEDs do not show the same level of performance as their blue and red cousins, greatly hindering the solid-state lighting development, which is the so-called "green gap". In this work, nano-void photonic crystals (NVPCs) were fabricated to embed within the GaN/InGaN green LEDs by using epitaxial lateral overgrowth (ELO) and nano-sphere lithography techniques. The NVPCs act as an efficient scattering back-reflector to outcouple the guided and downward photons, which not only boost the light extraction efficiency of LEDs with an enhancement of 78% but also collimate the view angle of LEDs from 131.5 degrees to 114.0 degrees. This could be because of the highly scattering nature of NVPCs which reduce the interference giving rise to Fabry-Perot resonance. Moreover, due to the threading dislocation suppression and strain relief by the NVPCs, the internal quantum efficiency was increased by 25% and droop behavior was reduced from 37.4% to 25.9%. The enhancement of light output power can be achieved as high as 151% at a driving current of 350 mA. Giant light output enhancement and directional control via NVPCs point the way towards a promising avenue of solid-state lighting.
Collective Lasing Behavior of Monolithic GaN-InGaN Core-Shell Nanorod Lattice under Room Temperature
(2017-10-01)
Huang, Chia-Yen
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Lin, Jing-Jie
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Chang, Tsu-Chi
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Liu, Che-Yu
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Tai, Tzu-Ying
We demonstrated a monolithic GaN-InGaN core-shell nanorod lattice lasing under room temperature. The threshold pumping density was as low as 140 kW/cm(2) with a quality factor as high as 1940. The narrow mode spacing between lasing peaks suggested a strong coupling between adjacent whisper gallery modes (WGM), which was confirmed with the far field patterns. Excitation area dependent photoluminescence revealed that the long-wavelength lasing modes dominated the collective lasing behavior under a large excitation area. The excitation-area-dependent lasing behavior resulted from the prominent optical coupling among rods. According to the optical mode simulations and truncated-rod experiments, we confirmed that the fine-splitting of lasing peaks originated from the coupled supermodes existing in the periodic nanorod lattices. With wavelength-tunable active materials and a wafer-level scalable processing, patterning optically coupled GaN-InGaN core-shell nanorods is a highly practical approach for building various on-chip optical components including emitters and coupled resonator waveguides in visible and ultraviolet spectral range.
Color-conversion efficiency enhancement of quantum dots via selective area nano-rods light-emitting diodes
(2016-08-22)
Liu, Che-Yu
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Chen, Tzu-Pei
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Kao, Tsung Sheng
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Huang, Jhih-Kai
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Kuo, Hao-Chung
A large enhancement of color-conversion efficiency of colloidal quantum dots in light-emitting diodes (LEDs) with novel structures of nanorods embedded in microholes has been demonstrated. Via the integration of nano-imprint and photolithography technologies, nanorods structures can be fabricated at specific locations, generating functional nanostructured LEDs for high-efficiency performance. With the novel structured LED, the color-conversion efficiency of the existing quantum dots can be enhanced by up to 32.4%. The underlying mechanisms can be attributed to the enhanced light extraction and nonradiative energy transfer, characterized by conducting a series of electroluminescence and time-resolved photoluminescence measurements. This hybrid nanostructured device therefore exhibits a great potential for the application of multi-color lighting sources. (C) 2016 Optical Society of America
Effect of InGaN/GaN Multiple Quantum Wells with p-n Quantum Barriers on Efficiency Droop in Blue Light-emitting Diodes
(2013-01-01)
Wang, Sheng-Wen
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Lin, Da-Wei
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Lee, Chia-Yu
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Liu, Che-Yu
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Lan, Yu-Pin
In this study, the structures of InGaN/GaN multiple quantum wells (MQWs) with p-n quantum barriers in various positions were proposed to investigate the efficiency droop behavior for blue light-emitting diodes (LEDs). The simulated electric field diagrams showed that the quantum well (QW) sandwiched by the p-n quantum barriers had a less electric field than the other QWs due to the original polarization-related electric field was partially balanced off by the built-in electric field of the p-n quantum barriers. In addition, the simulation results demonstrated that by selecting suitable position of p-n quantum barriers, the distribution of carriers could be effectively improved; hence the droop behavior could also be suppressed.
Efficiency and droop improvement in green InGaN/GaN light-emitting diodes on GaN nanorods template with SiO2 nanomasks
(2012-12-03)
Lin, Da-Wei
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Lee, Chia-Yu
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Liu, Che-Yu
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Han, Hau-Vei
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Lan, Yu-Pin
This study presents the green InGaN/GaN multiple quantum wells light-emitting diodes (LEDs) grown on a GaN nanorods template with SiO2 nanomasks by metal-organic chemical vapor deposition. By nanoscale epitaxial lateral overgrowth, microscale air voids were formed between nanorods and the threading dislocations were efficiently suppressed. The electroluminescence measurement reveals that the LEDs on nanorods template with SiO2 nanomasks suffer less quantum-confined Stark effect and exhibit higher light output power and lower efficiency droop at a high injection current as compared with conventional LEDs. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4768950]
Enhanced Color-Conversion Efficiency of Hybrid Nanostructured-Cavities InGaN/GaN Light-Emitting Diodes Consisting of Nontoxic InP Quantum Dots
(2017-09-01)
Liu, Che-Yu
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Chen, Tzu-Pei
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Huang, Jhih-Kai
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Lin, Tzu-Neng
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Huang, Chia-Yen
Color-conversion efficiency enhancement of hybrid light-emitting diodes (LEDs) by cadmium-free colloidal quantum dots (QDs) and a novel selective area nanocavities structure has been demonstrated. Combining nanoimprinting and photolithography techniques, nanocavities array can be fabricated at designated locations on the LEDs. The color-conversion efficiency of selective area nanocavities LED can be enhanced by up to 13%. The significant color-conversion efficiency enhancement is attributed to resonance of InP QDs emission in nanocavities and nonradiative energy transfer from LED active layers to InPQDs, which has been investigated and characterized by finite domain time-domain simulation, electroluminescence, and time-resolved photoluminescence measurements. This hybrid nanostructured device, therefore, exhibits a great potential for the applications of multicolor lighting sources and micro-LED.
Enhanced Light Extraction Efficiency of GaN-Based Hybrid Nanorods Light-Emitting Diodes
(2015-07-01)
Huang, Jhih-Kai
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Liu, Che-Yu
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Chen, Tzi-Pei
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Huang, Hung-Wen
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Lai, Fang-I
High light extraction GaN-based light-emitting diodes (LEDs) with a hybrid structure of straight nanorods located in an array of microholes have been successfully demonstrated. Via the nanoimprint lithography and photolithography techniques, high aspect-ratio light-guiding InGaN/GaN nanorods can be fabricated and regularly arranged in microholes, resulting in a great improvement of the light extraction for the GaN-based LED device. The light output power of the hybrid nanorods LED is 22.04 mW at the driving current standard of 25.4 A/cm(2), an enhancement of 38.7% to the conventional GaN-based LEDs. Furthermore, with a modification of the hybrid structures\' dimensions and locations, the emitted optical energy can be redistributed to obtain light-emitting devices with homogenueous optical field distributions.
GaN Micromechanical Resonators with Meshed Metal Bottom Electrode
(2015-03-01)
Ansari, Azadeh
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Liu, Che-Yu
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Lin, Chien-Chung
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Kuo, Hao-Chung
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Ku, Pei-Cheng
This work describes a novel architecture to realize high-performance gallium nitride (GaN) bulk acoustic wave (BAW) resonators. The method is based on the growth of a thick GaN layer on a metal electrode grid. The fabrication process starts with the growth of a thin GaN buffer layer on a Si (111) substrate. The GaN buffer layer is patterned and trenches are made and refilled with sputtered tungsten (W)/silicon dioxide (SiO2) forming passivated metal electrode grids. GaN is then regrown, nucleating from the exposed GaN seed layer and coalescing to form a thick GaN device layer. A metal electrode can be deposited and patterned on top of the GaN layer. This method enables vertical piezoelectric actuation of the GaN layer using its largest piezoelectric coefficient (d(33)) for thickness-mode resonance. Having a bottom electrode also results in a higher coupling coefficient, useful for the implementation of acoustic filters. Growth of GaN on Si enables releasing the device from the frontside using isotropic xenon difluoride (XeF2) etch and therefore eliminating the need for backside lithography and etching.
High Color-Conversion Efficiency CdSe Quantum Dots Hybrid Light Emitted Diode
(2016)
Liu, Che-Yu
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Kao, Tsung Sheng
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Chen, Tzu-Pei
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Chi, Gou-Chung
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Kuo, Hao-Chung
In this work, we have successfully demonstrated that the color-conversion efficiency in colloidal quantum dots can be greatly enhanced by light-emitting diodes (LEDs) with novel structures of nanorods embedded in microholes. Via an integrated technology of the nano-imprint and photolithography, nanorods structures can be fabricated at specific locations, generating functional nanostructured LEDs for high-efficiency light-emitting performance. With such a structured LED, the color-conversion efficiency of the existing quantum dots can be enhanced up to 12.5 %.
High Efficiency GaN-based Light Emitting Diodes with Embedded Air Voids/SiO2 Nanomasks
(2012-01-01)
Chiu, Ching-Hsueh
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Lin, Chien-Chung
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Han, Hau-Vei
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Lin, Da-Wei
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Chen, Yan-Hao
We fabricated high efficiency LEDs with embedded micro-scale air voids and SiO2 nanomask exhibit smaller reverse-bias current and great enhancement of the light output (65% at 20mA) compared with the conventional LEDs.
High efficiency GaN-based light-emitting diodes with embedded air voids/SiO(2) nanomasks
(2012-02-03)
Chiu, Ching-Hsueh
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Lin, Chien-Chung
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Han, Hau-Vei
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Liu, Che-Yu
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Chen, Yan-Hao
In this paper, the high performance GaN-based light-emitting diodes (LEDs) with embedded microscale air voids and an SiO(2) nanomask by metal-organic chemical vapor deposition (MOCVD) were demonstrated. Microscale air voids and an SiO(2) nanomask were clearly observed at the interface between GaN nanorods (NRs) and the overgrown GaN layer by scanning electron microscopy (SEM). From the reflectance spectra we show strong reflectance differences due to the different refractive index gradient between the GaN grown on the nanotemplate and sapphire. It can increase the light extraction efficiency due to additional light scattering. The transmission electron microscopy (TEM) images show the threading dislocations were suppressed by nanoscale epitaxial lateral overgrowth (NELOG). The LEDs with embedded microscale air voids and an SiO(2) nanomask exhibit smaller reverse-bias current and large enhancement of the light output (65% at 20 mA) compared with conventional LEDs.
High efficiency GaN-based light-emitting diodes with embedded air voids/SiO2 nanomasks
(2012-02-03)
Chiu, Ching-Hsueh
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Lin, Chien-Chung
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Han, Hau-Vei
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Liu, Che-Yu
;
Chen, Yan-Hao
In this paper, the high performance GaN-based light-emitting diodes (LEDs) with embedded microscale air voids and an SiO2 nanomask by metal-organic chemical vapor deposition (MOCVD) were demonstrated. Microscale air voids and an SiO2 nanomask were clearly observed at the interface between GaN nanorods (NRs) and the overgrown GaN layer by scanning electron microscopy (SEM). From the reflectance spectra we show strong reflectance differences due to the different refractive index gradient between the GaN grown on the nanotemplate and sapphire. It can increase the light extraction efficiency due to additional light scattering. The transmission electron microscopy (TEM) images show the threading dislocations were suppressed by nanoscale epitaxial lateral overgrowth (NELOG). The LEDs with embedded microscale air voids and an SiO2 nanomask exhibit smaller reverse-bias current and large enhancement of the light output (65% at 20 mA) compared with conventional LEDs.
High-Performance Ultraviolet 385-nm GaN-Based LEDs With Embedded Nanoscale Air Voids Produced Through Atomic Layer Deposition and Al2O3 Passivation
(2016-04)
Liu, Che-Yu
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Huang, Chia-Yen
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Wu, Pei-Yu
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Huang, Jhih-Kai
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Kao, Tsung Sheng
This letter developed the high-performance GaN-based light-emitting diodes (LEDs) for ultraviolet (UV) light emission at 385 nm with the light output power enhanced by up to 52% compared with that of the conventional UV-LEDs. The high-power-efficiency UV-LEDs include sidewall-passivated Al2O3 dielectric nanoscale air voids, which were created using the atomic layer deposition method and act as intermediate media for improving the internal quantum efficiency, reducing threading dislocation, and relaxing strain. The fabricated air void nanostructure also enhanced the light extraction efficiency by similar to 32.7%, resulting in a high-power UV light-emitting device.
Highly Efficient InGaN-based LED with Embedded Cubic Airvoids
(2014)
Lin, Da-Wei
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Huang, Jhih-Kai
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Liu, Che-Yu
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Chang, Ruey-Wen
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Wang, Sheng-Wen
Highly efficient InGaN-based LEDs with embedded sidewall passivation cubic airvoids made by nanoimprint lithography were demonstrated. The LEDs with embedded airvoids exhibit a 45% enhancement of light output at 20 mA compared with conventional LEDs.
Highly Efficient Light Extraction GaN-based Light Emitting Diode with Nano-rods in Micro-holes Compound Structure
(2014)
Liu, Che-Yu
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Huang, Jhih-Kai
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Lin, Da-Wei
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Huang, Hung-Wen
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Lee, Po-Tsung
We demonstrated a high-power GaN-based light emitting diodes (LEDs) which have micro-hole array and nano-rods compound structure by nanoimprint lithography (NIL). The nanorods structure inside the micro-hole could efficiently guide the trapped light from the GaN epilayer. Therefore, the light output power of LED with micro-hole array and nanorods was as high as 1.27 times, as compared with standard LED.
Improvement of emission uniformity by using micro-cone patterned PDMS film
(2014-02-24)
Liu, Che-Yu
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Chen, Kuo-Ju
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Lin, Da-Wei
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Lee, Chia-Yu
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Lin, Chien-Chung
Micro-patterned PDMS film was fabricated and combined with LED chip on board (COB) package to improve the emission uniformity of LED chip. The micro scale patterned sapphire substrate (PSS) was used as a mold to fabricate micro-cone patterned PDMS (MC-PDMS) film. A strong scattering effect from this MC-PDMS film can be verified by the high haze ratio and the Bi-directional Transmission effect. The angle dependent color temperature measurement system was used to measure the Delta CCT of COB with and without MC-PDMS. The measurement results indicate that the Delta CCT was reduced from 1025K to 428K. This improvement can effectively eliminate the yellow ring effect of LED chip. This technology can be thus considered as a cost-effective way for the next generation of light source packages. (C) 2014 Optical Society of America
Large Area MoS2 van der Waals Epitaxy on III-Ns and the Epitaxial Formation of a n-MoS2/p-InGaN Diode
(2016-01-01)
Jung, Kyooho
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Liu, Che-Yu
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Kim, J. D.
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Choi, Wonsik
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Zhou, Weidong
Epitaxial growth of aligned MoS2 triangles on GaN substrates and their evolution into a continuous sheet of >centimeter size are demonstrated. An n-MoS2/p-InGaN diode with clear rectifying behavior is realized by direct van der Waals epitaxy. Epitaxial growth of aligned MoS2 triangles on GaN substrates and their evolution into a continuous sheet of >centimeter size are demonstrated. An n-MoS2/p-InGaN diode with clear rectifying behavior is realized by direct van der Waals epitaxy.
Light Extraction Enhancement of GaN-Based Light-Emitting Diodes Using Crown-Shaped Patterned Sapphire Substrates
(2012-07-15)
Chiu, Ching-Hsueh
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Hsu, Lung-Hsing
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Lee, Chia-Yu
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Lin, Chien-Chung
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Lin, Bo-Wen
In this letter, we report the high performance GaN-based light-emitting diodes (LEDs) with embedded air void array grown by metal-organic chemical vapor deposition. The donut-shaped air void was formed at the interface between crown-shaped patterned sapphire substrates (CPSS) and the GaN epilayer by conventional photolithography. The transmission electron microscopy images demonstrate that the threading dislocations were significantly suppressed by epitaxial lateral overgrowth (ELOG). The Monte Carlo ray-tracing simulation reveals that the light extraction of the air-voids embedded LED was dramatically increased due to a strong light reflection and redirection by the air voids.
Mode-Hopping Phenomena in the InGaN-Based Core-Shell Nanorod Array Collective Lasing
(2018-07-01)
Huang, Chia-Yen
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Tai, Tzu-Ying
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Lin, Jing-Jie
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Chang, Tsu-Chi
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Liu, Che-Yu
A two-dimensional monolithic InGaN-based core-shell it array was excited under a varying temperature. We observed an abrupt dominant mode hopping from 3.29 to 141 eV in the collective lasing as the temperature decreased from T = 240 to 225 K, Photoluminescence under a near-threshold pumping density revealed the splitting of spontaneous emission. between the core and the shell with a decreasing temperature. The bandgap evolution of GaN and InGaN showed opposite trends due to the interaction of temperature effects and bandgap renormalization effects. Theoretical simulation revealed the differences of gain spectra evolution between room lire and to temperature due to the difference the carrier dynamics, In the optically coupled nanorod array, the dominant lasing;mode was not only determined by it structure, but also strongly influenced by external operating conditions. Rattier than singular microcavity lasers the coupled nanorod array demonstrated a broader parameter space for on-chip wavelength manipulation.