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Improved performance of 375 nm InGaN/AlGaN light-emitting diodes by incorporating a heavily Si-doped transition layer
(2012-01-01)
Huang, Shih-Cheng
;
Shen, Kun-Ching
;
Tu, Po-Min
;
Wuu, Dong-Sing
;
Kuo, Hao-Chung
High performance 375-nm ultraviolet (UV) InGaN/AlGaN light-emitting diodes (LEDs) was developed using a heavy Si-doping technique with metalorganic chemical vapor deposition (MOCVD). From the transmission electron microcopy (TEM) image, the dislocation density was reduced after inserting a heavily Si-doping growth mode transition layer (GMTL) between un-doped GaN layer and Si-doped Al0.02Ga0.98N contact layer. The internal quantum efficiency (IQE) of the sample with GMTL measured by power-dependent photoluminescence shows 39.4% improved compared with the sample without GMTL. When the vertical type LED chips (size: 1mmx1mm) driving by a 350-mA current, the output powers of the LEDs with and without GMTL were measured to be 286.7 mW and 204.2 mW, respectively. As much as 40.4% increased light output power was achieved. Therefore, using the GMTL to reduce dislocation defects would be a promising prospective for InGaN/AlGaN UV LEDs to achieve high internal quantum efficiency.
Enhanced Light Extraction of High-Voltage Light Emitting Diodes Using a Sidewall Chamfer Structure
(2017-06-01)
Wu, Ping-Chen
;
Ou, Sin-Liang
;
Horng, Ray-Hua
;
Wuu, Dong-Sing
;
電子工程學系及電子研究所
High-voltage light-emitting diodes (HV-LEDs) were prepared with 4 x 2 microcells. A novel technique for enhancing the light extraction of HV-LEDs by using wet-etched chamfer structures in the sidewalls of each cell is proposed. The thicknesses of the u-GaN layers used were in the range of 3-7 mu m. Simulations revealed that the light extraction of the HV-LEDs with chamfer structures would be enhanced by increasing the u-GaN thickness. The output power (@ 80 mA) of the HV-LEDs without chamfer structures was 538.0-539.1 mW. However, the output powers (@ 80 mA) of the HV-LEDs with chamfer structures and u-GaN thicknesses of 3, 5, and 7 mu m were increased to 555.3, 573.1, and 561.6 mW, respectively. The 7 mu m thick u-GaN layer led to an extremely large thickness of the HV-LED epitaxial structure, which caused difficulties during the wet etching process. Because of the apparent decrease in the wet etching rate of GaN, the damages to the epitaxial structures of the HV-LED with the 7 mu m thick u-GaN was more obvious. Therefore, the device's light extraction was slightly reduced. The results confirm that chamfer structures are highly useful for improving the performance of HV-LEDs, especially for the u-GaN thickness of 5 mu m.
Characterizations of Metal-Oxide-Semiconductor Field-Effect Transistors of ZnGaO Grown on Sapphire Substrate
(2017-03-01)
Shen, Yi-Siang
;
Wang, Wei-Kai
;
Horng, Ray-Hua
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
Zinc gallate (ZnGaO) epilayers were grown on a c-plane sapphire substrate by metalorganic chemical vapor deposition and fabricated into metal-oxide-semiconductor field-effect transistors (MOSFETs). The ZnGaO MOSFETs exhibited a complete channel pinch-off of the drain current for VGS < -4.43 V, high off-state breakdown voltage of 378 V, high I-ON/I-OFF ratio of 10(6), and low gate leakage current.
Reduction of Defects in AlGaN Grown on Nanoscale-Patterned Sapphire Substrates by Hydride Vapor Phase Epitaxy
(2017-06-01)
Tasi, Chi-Tsung
;
Wang, Wei-Kai
;
Tsai, Tsung-Yen
;
Huang, Shih-Yung
;
Horng, Ray-Hua
In this study, a 3-m-thick AlGaN film with an Al mole fraction of 10% was grown on a nanoscale-patterned sapphire substrate (NPSS) using hydride vapor phase epitaxy (HVPE). The growth mechanism, crystallization, and surface morphology of the epilayers were examined using X-ray diffraction, transmission electron microscopy (TEM), and scanning electron microscopy at various times in the growth process. The screw threading dislocation (TD) density of AlGaN-on-NPSS can improve to 1-2 x 10(9) cm(-2), which is significantly lower than that of the sample grown on a conventional planar sapphire substrate (7 x 10(9) cm(-2)). TEM analysis indicated that these TDs do not subsequently propagate to the surface of the overgrown AlGaN layer, but bend or change directions in the region above the voids within the side faces of the patterned substrates, possibly because of the internal stress-relaxed morphologies of the AlGaN film. Hence, the laterally overgrown AlGaN films were obtained by HVPE, which can serve as a template for the growth of ultraviolet III-nitride optoelectronic devices.
Improved Performance of High-Voltage Vertical GaN LEDs via Modification of Micro-Cell Geometry
(2017-06-01)
Wu, Ping-Chen
;
Ou, Sin-Liang
;
Horng, Ray-Hua
;
Wuu, Dong-Sing
;
電子工程學系及電子研究所
Vertical-type high-voltage light-emitting diodes (HV-LEDs) with 2 x 2 micro-cells were fabricated on Cu substrates, and the micro-cell geometry was modified to enhance the optoelectronic performance. The current spreading in micro-cell is most dominantly affected by the distance between electrode and edge of chip. When square cells were combined in a HV-LED, the device performance was poor due to an obvious current-crowding phenomenon that occurred near the electrodes. This was attributed that the electrodes in these four square micro-cells were all far away from the edges, resulting in the severe current-crowding phenomenon. On the contrary, as the HV-LED was prepared with four rectangle, triangle, or L-shaped micro-cells, the electrodes were close to the edges of micro-cells and the current spreading effect can be easily improved. Although a HV-LED connected with L-shaped cells possessed a better current spreading effect and a lower surface temperature, the light extraction was relatively low because of an electrode-shading loss effect. When triangular cells were used to prepare the HV-LED, the device achieved a superior optoelectronic performance compared with that of other cells because of a lower current-crowding effect and a more uniform light emission. After an epoxy package process, a lower forward voltage of 14.9 V and a higher output power of 353.2 mW were obtained using this HV-LED at an injection current of 80 mA. Additionally, the wall-plug efficiencies of this device at 20 and 80 mA were 41.1% and 29.7%, respectively. The results confirm that the design of triangular cell is beneficial for enhancing the optoelectronic performance of HV-LEDs. Furthermore, the fabrication processes of vertical LEDs have high potential for HV-LED applications.
Performance comparison of p-side-up thin-film AlGaInP light emitting diodes with aluminum-doped zinc oxide and indium tin oxide transparent conductive layers
(2016-04-01)
Tseng, Ming-Chun
;
Wuu, Dong-Sing
;
Chen, Chi-Lu
;
Lee, Hsin-Ying
;
Lin, Yu-Chang
Transparent conductive layers (TCLs) deposited on a GaP window layer were used to fabricate high-brightness p-side-up thin-film AlGaInP light-emitting diodes (LEDs) by the twice wafer-transfer technique. Indium tin oxide (ITO) and aluminum-doped zinc oxide (AZO) were used as TCLs for comparison. The TCLs improved droop of external quantum efficiencies (EQE) of LEDs and junction temperature, which result in increasing the light output power and thermal stability of the LEDs. The droop efficiency of Ref-LED, ITO-LED and AZO-LED were 64%, 27% and 15%, respectively. The junction temperature of ITO-LED and AZO-LED reduced to 49.3 and 39.6 degrees C at an injection current of 700 mA compared with that (80.8 degrees C) of Ref-LED. The LEDs with AZO layers exhibited the most excellent LED performance. The emission wavelength shifts of LEDs without a TCL, with an ITO layer, and with an AZO layer were 17, 8, and 3 nm, respectively, when the injection current was increased from 20 to 1000 mA. The above results are promising for the development of AZO thin films to replace ITO thin films for AlGaInP LED applications. (C) 2016 Optical Society of America
Solid-State Lighting with High Brightness, High Efficiency, and Low Cost
(2014-01-01)
Horng, Ray-Hua
;
Lau, Kei May
;
Kuo, Hao-Chung
;
Tansu, Nelson
;
光電工程學系
Effect of Top-Region Area of Flat-Top Pyramid Patterned Sapphire Substrate on the Optoelectronic Performance of GaN-Based Light-Emitting Diodes
(2016-01-01)
Hsueh, Hsu-Hung
;
Ou, Sin-Liang
;
Peng, Yu-Che
;
Cheng, Chiao-Yang
;
Wuu, Dong-Sing
The flat-top pyramid patterned sapphire substrates (FTP-PSSs) have been prepared for the growth of GaN epilayers and the fabrication of lateral-type light-emitting diodes (LEDs) with an emission wavelength of approximately 470 nm. Three kinds of FTP-PSSs, which were denoted as FTP-PSS-A, FTP-PSS-B, and FTP-PSS-C, respectively, were formed through the sequential wet etching processes. The diameters of circle areas on the top regions of these three FTP-PSSs were 1, 2, and 3 mu m, respectively. Based on the X-ray diffraction results, the full-width at half-maximum values of rocking curves at (002) plane for theGaN epilayers grown on conventional sapphire substrate (CSS), FTP-PSS-A, FTP-PSS-B, and FTP-PSS-C were 412, 238, 346, and 357 arcsec, while these values at (102) plane were 593, 327, 352, and 372 arcsec, respectively. The SpeCLED-Ratro simulation results reveal that the LED prepared on FTP-PSS-A has the highest light extraction efficiency than that of the other devices. At an injection current of 350 mA, the output powers of LEDs fabricated on CSS, FTP-PSS-A, FTP-PSS-B, and FTP-PSS-C were 157, 254, 241, and 233 mW, respectively. The results indicate that both the crystal quality of GaN epilayer and the light extraction of LED can be improved via the use of FTP-PSS, especially for the FTP-PSS-A.
Enhanced light extraction in wafer-bonded p-side-up thin-film AlGaInP light emitting diodes via zinc oxide nanorods
(2016-10-01)
Tseng, Ming-Chun
;
Wuu, Dong-Sing
;
Chen, Chi-Lu
;
Lee, Hsin-Ying
;
Lin, Yu-Chang
ZnO nanorods grown via hydrothermal method on the aluminum-doped zinc oxide (AZO) thin film were used to fabricate high-brightness p-side-up thin-film AlGaInP light-emitting diodes (LEDs). The AZO thin film was not only used as current spreading layers but also as a seed layer of ZnO nanorods. The AZO thin film was prepared using atomic layer deposition. The ZnO nanorods improved light extraction, thus increasing the light output power of the LEDs. The output powers of LEDs with optimum ZnO nanorod structures were increased by 32% at an injection current of 700 mA, as compared with that of an LED with AZO thin film. The emission wavelength shifts of LEDs with an AZO thin film and optimum ZnO nanorod structure were 18 and 11 nm, respectively, when the injection current was increased from 20 to 1000 mA. The ZnO nanorods not only provide more light extraction but also keep the thermal stability of the LED device without any degradation. The results are promising for the developed high brightness AlGaInP LED applications with low fabrication cost through ZnO nanorods grown by hydrothermal method to enhance the light extraction efficiency. (C) 2016 Optical Society of America
Large-area, uniform white light LED source on a flexible substrate
(2015-09-21)
Sher, Chin-Wei
;
Chen, Kuo-Ju
;
Lin, Chien-Chung
;
Han, Hau-Vei
;
Lin, Huang-Yu
This study demonstrates the flexible white LED structure with high lumen efficiency and uniform optical performance for neutral white and warm white CCT. Flip-chip LEDs were attached on a polyimide substrate with copper strips as electrical and thermal conduction paths. Yellow phosphors are mixed with polydimenthysiloxane (PDMS) to provide mechanical support and flexibility. The light efficiency of this device can reach 120 lm/W and 85% of light output uniformity of the emission area can be achieved. Moreover, the optical simulation is employed to evaluate various designs of this flexible film in order to obtain uniform output. Both the pitch between the individual devices and the thickness of the phosphor film are calculated for optimization purpose. This flexible white LED with high lumen efficiency and good reliability is suitable for the large area fixture in the general lighting applications. (C) 2015 Optical Society of America

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