Browsing by Author "Chao, Chu-Li"

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Crystal quality improvement of a-plane GaN using epitaxial lateral overgrowth on nanorods
(2010-04-01)
Ling, Shih-Chun
;
Chao, Chu-Li
;
Chen, Jun-Rong
;
Liu, Po-Chun
;
Ko, Tsung-Shine
The crystal quality of a-plane GaN films was improved by using epitaxial lateral overgrowth on nanorod GaN template. The scanning electron microscope images showed the fully coalesced regrowth process completed within only 2 mu m thickness. The a-plane GaN films grown on nano-rod template showed superior surface quality and less surface pits. The on-axis and off-axis FWHMs of X-ray rocking curves decreased with the increase in the etching depth of the nano-rod template. TEM analysis revealed that the dislocation density was reduced to around 1.2 x 10(9) cm(-2) by the nano-rod epitaxial lateral overgrowth. In addition, the photoluminescence intensity of the a-plane GaN was enhanced significantly. These results demonstrated the opportunity of achieving a-plane GaN films with high crystal quality via nano-rod epitaxial lateral overgrowth. (C) 2009 Elsevier B.V. All rights reserved.
High-Quality Free-standing GaN Thick-films Prepared by Hydride Vapor Phase Epitaxy using Stress Reducing Techniques
(2009)
Huang, Hsin-Hsiung
;
Lee, Wei-I
;
Chen, Kuei-Ming
;
Chu, Ting-Li
;
Wu, Pei-Lun
As one of the most mature techniques for manufacturing free-standing GaN substrates, hydride vapor phase epitaxy (HVPE) always encounters problems associated with residue thermal stress, such as GaN bending and cracking during and after growth. This work presents a patterning approach and a non-patterning approach to reduce stress in thick GaN films grown on sapphires by HVPE. The patterning approach, forming dot air-bridged structures, adopted standard photolithography to fabricate hexagonally aligned patterns of dots on GaN templates. Following HVPE growth, regular voids were formed and buried in the GaN thick-films. These voids helped to relax the stress in the GaN thick-films. In the non-patterning approach, thick GaN films were simply grown at a specially set sequence of ramping temperatures during HVPE growth without any patterned structure. This temperature-ramping technique, gives crack-free high-quality 2"-diameter GaN films, thicker than 250 mu m, on sapphires in high yields. These thick GaN films can be separated from sapphire using conventional laser-induced lift-off processes, which can be followed by subsequent HVPE regrowths. A 600 mu m-thick free-standing GaN films has a typical dislocation density of around 4x10(6) cm(-2) with a full width at half maximum (FWHM) in the high resolution X-ray diffraction (HRXRD) spectrum of GaN (002) of around 150 arcsec. The residual stress in the thick GaN films was analyzed by micro-Raman spectroscopy. The effectiveness of the patterning and the non-patterning techniques in reducing the strain in GaN films is discussed. The advantages and weaknesses of the patterning and the non-patterning techniques will be elucidated.
Improvement in Crystalline Quality of InGaN-Based Epilayer on Sapphire via Nanoscaled Epitaxial Lateral Overgrowth
(2010)
Chiu, Ching-Hsueh
;
Lin, Da-Wei
;
Li, Zhen-Yu
;
Chiu, Chin-Hua
;
Chao, Chu-Li
In this study, a high-performance GaN-based light-emitting diode (LED) was achieved using a nanocolumn patterned sapphire substrate (NCPSS) with low-pressure metal-organic chemical vapor deposition (LP-MOCVD). The surface roughness was evaluated by atomic force microscopy (AFM). The mechanisms of carrier localization in the GaN-based LED fabricated on NCPSS were discussed referring to the results obtained from the power-dependent photoluminescence measurements. Moreover, from the transmission electron microscopy (TEM) image, the threading dislocation densities (TDDs) through the GaN-based LED fabricated on NCPSS were found to be about 10 times lower than those fabricated on planar substrates. Finally, the internal quantum efficiency (IQE) of the GaN-based LED fabricated on NCPSS was as high as 48% at 30 mW, corresponding to a current of 20 mA, which is higher than that of a GaN-based LED fabricated on a planar sapphire substrate by 8%. The use of NCPSS is suggested to be effective for elevating the emission efficiency of the GaN-based LED owing to an improvement in crystal quality. (C) 2010 The Japan Society of Applied Physics
Method for modulating the wafer bow of free-standing GaN substrates via inductively coupled plasma etching
(2010-12-01)
Chen, Kuei-Ming
;
Yeh, Yen-Hsien
;
Wu, Yin-Hao
;
Chiang, Chen-Hao
;
Yang, Din-Ru
The bowing curvature of the free-standing GaN substrate significantly decreased almost linearly from 067 to 0 056 m(-1) (i e the bowing radius increased from 1 5 to 178 m) with increase in inductively coupled plasma (ICP) etching time at the N-polar face and eventually changed the bowing direction from convex to concave Furthermore the Influences of the bowing curvature on the measured full width at half maximum (FWEIM) of high-resolution X-ray diffraction (HRXRD) in (0 0 2) reflection were also deduced which reduced from 176 8 to 888 arcsec with increase in ICP etching time Decrease in the nonhomogeneous distribution of threading dislocations and point defects as well as V(Ga)-O(N) complex defects on removing the GaN layer from N-polar face which removed large amount of defects was one of the reasons that improved the bowing of the free-standing GaN substrate Another reason was the high aspect ratio of needle-like GaN that appeared at the N-polar face after ICP etching which released the compressive strain of the free-standing GaN substrate By doing so crack-free and extremely flat free-standing GaN substrates with a bowing radius of 178 m could be obtained (C) 2010 Elsevier B V All rights reserved
Nanorod epitaxial lateral overgrowth of a-plane GaN with low dislocation density
(2009-06-22)
Ling, Shih-Chun
;
Chao, Chu-Li
;
Chen, Jun-Rong
;
Liu, Po-Chun
;
Ko, Tsung-Shine
The crystal quality of a-plane GaN films was improved by using epitaxial lateral overgrowth on a nanorod GaN template. The investigation of x-ray diffraction showed that the strain in a-plane GaN grown on r-plane sapphire could be mitigated. The average threading dislocation density estimated by transmission electron microscopy was reduced from 3x10(10) to 3.5x10(8) cm(-2). From the temperature-dependent photoluminescence, the quantum efficiency of the a-plane GaN was enhanced by the nanorod epitaxial lateral overgrowth (NRELOG). These results demonstrated the opportunity of achieving a-plane GaN films with low dislocation density and high crystal quality via NRELOG.
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.
Strain-reduced GaN thick-film grown by hydride vapor phase epitaxy utilizing dot air-bridged structure
(2009-05-01)
Huang, Hsin-Hsiung
;
Chao, Chu-Li
;
Chi, Tung-Wei
;
Chang, Yu-Lin
;
Liu, Po-Chun
A 300 pm GaN thick-film, in diameter 1.5 in, was demonstrated without any crack by hydride vapor phase epitaxy (HVPE) growth. The technique used in relaxing the residual stress caused by differences of thermal expansion coefficients (TEC) and lattice constants between GaN and sapphire substrate to prevent GaN film from crack is called a dot air-bridged structure. After the laser lift-off process, 300-mu m-thick freestanding GaN wafer, in diameter 1.5 in, could be fabricated. The compressive stress in the dot air-bridged structure was measured by micro-Raman spectroscopy with the E(2)(high) phonon mode. The compressive stress could be reduced to as small as 0.04 GPa, which could prevent the crack during the epitaxial process for GaN growth by HVPE. It is important to obtain a large-area crack-free GaN thick-film, which can be used for fabricating freestanding GaN wafer. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.
Stress and Defect Distribution of Thick GaN Film Homoepitaxially Regrown on Free-Standing GaN by Hydride Vapor Phase Epitaxy
(2010)
Chen, Kuei-Ming
;
Yeh, Yen-Hsien
;
Wu, Yin-Hao
;
Chiang, Chen-Hao
;
Yang, Din-Ru
A 220-mu m-thick Gallium nitride (GaN) layer was homoepitaxially regrown on the Ga-polar face of a 200-mu m-thick free-standing c-plane GaN by hydride vapor-phase epitaxy (HVPE). The boundary of the biaxial stress distribution in the GaN substrate after regrowth was clearly distinguished. One half part, the regrown GaN, was found to be more compressive than the other half part, the free-standing GaN. Additionally, the densities of the screw and mixed dislocations reduced from 2.4 x 10(7) to 6 x 10(6) cm(-2) after regrowth. Furthermore, the yellow band emission almost disappeared, accompanied by a peak emission at approximately 380 nm related to the edge dislocation was under slightly improved in regrown GaN. We conclude that the reduction of the dislocation defects and Ga vacancies and/or O impurities are the two main reasons for the higher compressive stress in the regrown GaN than in the free-standing GaN, causing the curvature of the GaN substrate to be twice concave after regrowth. (C) 2010 The Japan Society of Applied Physics