Browsing by Author "Yen, Hsi-Hsuan"

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Characteristics of Single-Chip GaN-Based Alternating Current Light-Emitting Diode
(2008-12-01)
Yen, Hsi-Hsuan
;
Kuo, Hao-Chung
;
Yeh, Wen-Yung
;
光電工程學系
;
Department of Photonics
In this study, a GaN-based alternating current light-emitting diode (AC-LED) with 34 numbers of microchips illuminated in each bias direction was fabricated. After calibrating the integration duration, the light output powers of the AC-LED driven by AC and DC were 388.1 and 312.8 mW when the input power was about I W, respectively. The flickering illumination mode of the AC-LED driven by AC decreased the heat accumulation and revealed a higher energy utilization efficiency than that of the AC-LED driven by DC. The larger blue shift and smaller full width at half maximum of the AC-LED driven by AC than those of the AC-LED driven by DC were also observed. [DOI: 10.1143/JJAP.47.8808]
Far-Field and Near-Field Distribution of GaN-Based Photonic Crystal LEDs With Guided Mode Extraction
(2009-07-01)
Lai, Chun-Feng
;
Chi, Jim-Yong
;
Kuo, Hao-Chung
;
Yen, Hsi-Hsuan
;
Lee, Chia-En
The near-field and far-field distribution of GaN LEDs with square photonic crystal (PhC) lattice are experimentally investigated. The optical images of the near-field pattern are obtained from the guided electroluminescent light generated at the center of a special annular structure. For increasing lattice constant, symmetric patterns with varying number of petals according to the symmetry of the PhC are observed. The far-field distribution is studied with GaN-based film-transferred PhC LEDs with the thickness about 1500 nm. Angular-resolved measurements under electrical injection revealed guided modes extraction behavior and polarization based on the Bragg's diffraction with the PhC lattice. Good agreement with the two-dimension free photon band structure is obtained.
Far-field of GaN film-transferred green light-emitting diodes with two-dimensional photonic crystals
(2009-05-25)
Lai, Chun-Feng
;
Chi, Jim-Yong
;
Kuo, Hao-Chung
;
Yen, Hsi-Hsuan
;
Lee, Chia-En
Far-field distributions of GaN-based photonic crystal (PhC) film-transferred light-emitting diodes (FT-LEDs) were investigated. The thickness of the device is about 840 nm. The emission wavelength is around 520 nm. The PhC region is a square lattice with a/lambda around 0.5. Angular-resolved measurements in the Gamma-X and Gamma-M directions were made in the polarized-resolved manner. Guided mode extraction behavior in agreement with the two-dimensional free-photon band calculation was observed. In addition, the pseudo-TM behavior for the non-collinearly coupled modes was observed. The azimuthal-mapping of the angular-resolved spectra revealed the evolution of the collinearly and the non-collinearly coupled modes. Furthermore, the light enhancement of similar to 2.7x and the collimation angle about 102.3 degrees were achieved. (c) 2009 Optical Society of America
GaN alternating current light-emitting device
(2007-06-01)
Yen, Hsi-Hsuan
;
Yeh, Wen-Yung
;
Kuo, Hao-Chung
;
光電工程學系
;
Department of Photonics
We report a new design of the light-emitting device which can be operated under alternating current source directly. The new type alternating current driven light-emitting device (AC LED) can increase the radition area in each bias direction to improve the device efficiency by the Wheatstone Bridge (WB) circuit design. WB-AC LEDs with different designs were fabricated and the electrical and optical characteristics were measured. It is found that the efficiency of the WB-AC LED is influenced by the varied area ratio of the rectified microchip to the central one because of the different current density and forward bias of each microchip. Additionally, the relationship between the area ratio of microchips and the wall plug efficiency of the WB-AC LED has also been discussed. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Highly-directional emission patterns based on near single guided mode extraction from GaN-based ultrathin microcavity light-emitting diodes with photonic crystals
(2010-07-05)
Lai, Chun-Feng
;
Kuo, Hao-Chung
;
Yu, Peichen
;
Lu, Tien-Chang
;
Chao, Chia-Hsin
This study investigates the distribution of highly-directional far-field emission on GaN-based ultrathin microcavity light-emitting diodes (uMCLEDs) with photonic crystals (PhCs). The ultrathin 550 nm cavity, PhC lattice constant of 370 nm, and hole depth of 250 nm in the GaN PhC uMCLED provide near single guided mode extraction and a pattern of high directionality radiation. Angular-spectral-resolved electroluminescence measurements reveal photon-band structure agreement with the fundamental mode effective refractive index dispersion curve. In addition, GaN PhC uMCLED increase the output power extraction efficiency by 145.9% (similar to 2.46x) compared with GaN non-PhC uMCLED, and a directional far-field emission pattern at half intensity of nearly +/- 15 degrees. (C) 2010 American Institute of Physics. [doi:10.1063/1.3459970]
Particular Failure Mechanism of GaN-Based Alternating Current Light-Emitting Diode Induced by GaO(x) Oxidation
(2010-08-01)
Yen, Hsi-Hsuan
;
Kuo, Hao-Chung
;
Yeh, Wen-Yung
;
光電工程學系
;
Department of Photonics
This investigation describes the unique failure mechanism of the Wheatstone bridge circuit-type alternating current light-emitting diode (WB AC-LED). The micro-LEDs in WB AC-LED rectifying branches were reverse biased, and therefore, positive charges (holes) accumulated in the n-type GaN layer under the active region and combined with the GaN material and OH(-) ions to generate GaO(x) oxidation grains. The GaO(x) generation speed was fast with high reverse voltage applied to micro-LEDs, and the expansion of GaO(x) dimensions degraded the opto-electrical characteristics and eventually caused failure of the WB AC-LED. The root-mean-square reverse voltage dropped across each micro-LED from -13.1 to -6.7 V with different micro-LEDs array arrangements extended the WB AC-LED life-time from being less than 650 h to more than 1600 h, respectively.
Particular Failure Mechanism of GaN-Based Alternating Current Light-Emitting Diode Induced by GaOx Oxidation
(2010-08-01)
Yen, Hsi-Hsuan
;
Kuo, Hao-Chung
;
Yeh, Wen-Yung
;
光電工程學系
;
光電工程研究所
This investigation describes the unique failure mechanism of the Wheatstone bridge circuit-type alternating current light-emitting diode (WB AC-LED). The micro-LEDs in WB AC-LED rectifying branches were reverse biased, and therefore, positive charges (holes) accumulated in the n-type GaN layer under the active region and combined with the GaN material and OH- ions to generate GaOx oxidation grains. The GaOx generation speed was fast with high reverse voltage applied to micro-LEDs, and the expansion of GaOx dimensions degraded the opto-electrical characteristics and eventually caused failure of the WB AC-LED. The root-mean-square reverse voltage dropped across each micro-LED from -13.1 to -6.7 V with different micro-LEDs array arrangements extended the WB AC-LED life-time from being less than 650 h to more than 1600 h, respectively.
Polarized light emission from photonic crystal light-emitting diodes
(2008-06-16)
Lai, Chun-Feng
;
Chi, Jim-Yong
;
Yen, Hsi-Hsuan
;
Kuo, Hao-Chung
;
Chao, Chia-Hsin
We have experimentally studied polarization characteristics of the two-dimensional photonic crystal (PhC) light-emitting diodes (LEDs) using an annular structure with square lattice and observed a strong polarization dependence of the lattice constant and orientation of the PhC. The extracted light from the GaN PhC LEDs has P/S ratios of 5.5 (similar to 85% polarization light) for light propagating in the Gamma X direction and 2.1 (similar to 68% polarization light) for the Gamma M direction, respectively. Based on the couple mode theory, the dependence of polarization behaviors on different lattice constant and orientation was found to be in good agreement with theoretical discussion.
Reduction of Efficiency Droop in InGaN Light-Emitting Diode Grown on Self-Separated Freestanding GaN Substrates
(2011-06-15)
Chao, Chu-Li
;
Xuan, Rong
;
Yen, Hsi-Hsuan
;
Chiu, Ching-Hsueh
;
Fang, Yen-Hsiang
Using a GaN nanorod template in a hydride vapor phase epitaxy (HVPE) system can manufacture a freestanding GaN (FS-GaN) substrate with threading dislocation densities down to similar to 10(7) cm(-2). In this letter, we report InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) grown on this FS-GaN substrate. The defect densities in the homoepitaxially grown LEDs were substantially reduced, leading to improved light emission efficiency. Compared with the LED grown on sapphire, we obtained a lower forward voltage, smaller diode ideality factor, and higher light-output power in the same structure grown on FS-GaN. The external quantum efficiency (EQE) of LEDs grown on FS-GaN were improved especially at high injection current, which brought the efficiency droop phenomenon greatly reduced at high current density.