Browsing by Author "Tu, Li-Wei"

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A Novel Technique for Growing Crack-Free GaN Thick Film by Hydride Vapor Phase Epitaxy
(2008-11-01)
Huang, Hsin-Hsiung
;
Chen, Kuei-Ming
;
Tu, Li-Wei
;
Chu, Ting-Li
;
Wu, Pei-Lun
To prevent the cracking of GaN thick films grown on a sapphire substrate by hydride vapor phase epitaxy (HVPE), a novel technique without complex processes is developed. By adding a temperature ramping step in the HVPE GaN epitaxy process, more than 300-mu m-thick high-quality crack-free GaN thick films on sapphire substrate can be obtained by this technique. After separation by a conventional laser-induced lift-off process, a 1.5 in. 300 mu m freestanding GaN wafer with a dislocation density of approximately 1 x 10(7) cm(-2) could be fabricated without any cracks. No additional designed-patterned or stress-reduced structures were applied in these samples to reduce the dislocation density and thermal stress. [DOI: 10.1143/JJAP.47.8394]
Fabrication and characterization of back-side illuminated InGaN/GaN solar cells with periodic via-holes etching and Bragg mirror processes
(2014-08-25)
Chang, Yi-An
;
Chen, Fang-Ming
;
Tsai, Yu-Lin
;
Chang, Ching-Wen
;
Chen, Kuo-Ju
In this study, the design and fabrication schemes of back-side illuminated InGaN/GaN solar cells with periodic via-holes etching and Bragg mirror processes are presented. Compared to typical front-side illuminated solar cells, the improvements of open-circuit voltage (V-oc) from 1.88 to 1.94 V and short-circuit current density (J(sc)) from 0.84 to 1.02 mA/cm(2) are observed. Most significantly, the back-side illuminated InGaN/GaN solar cells exhibit an extremely high fill factor up to 85.5%, leading to a conversion efficiency of 1.69% from 0.66% of typical front-side illuminated solar cells under air mass 1.5 global illuminations. Moreover, the effects of bottom Bragg mirrors on the photovoltaic characteristics of back-side illuminated solar cells are studied by an advanced simulation program. The results show that the J(sc) could further be improved with a factor of 10% from the original back-side illuminated solar cell by the structure optimization of bottom Bragg mirrors. (C) 2014 Optical Society of America
Growth and Characteristics of a-Plane GaN on ZnO Heterostructure
(2012)
Huang, Huei-Min
;
Kuo, Chin-Chia
;
Chang, Chiao-Yun
;
Lin, Yuan-Ting
;
Lu, Tien-Chang
We report on growth and characteristics of a-plane GaN/ZnO/GaN epitaxial structures, which can be applied to various optoelectronic devices. The unique optical transitions intrinsic to heterovalent interfaces were found and analyzed. Clear carrier localization effect in the GaN/ZnO heterointerface is observed from the S-shaped energy shift with increasing temperature in the temperature-dependent photoluminescence measurement. The carrier localization also results in strong luminescent intensity and dominates the emission spectrum at room temperature. In addition, the acceptor level originated from the Zn out-diffusion from the ZnO layer is observed in the low temperature photoluminescence spectra, and its binding energy is estimated to be about 0.311 eV. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.080203jes] All rights reserved.
Growth and Characteristics of a-Plane GaN on ZnO Heterostructure
(2013)
Huang, Huei-Min
;
Kuo, Chin-Chi
;
Chang, Chiao-Yun
;
Lin, Yuan-Ting
;
Lu, Tien-Chang
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.
Robustness of a Topologically Protected Surface State in a Sb2Te2Se Single Crystal
(2016-11-18)
Lee, Chao-Kuei
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Cheng, Cheng-Maw
;
Weng, Shih-Chang
;
Chen, Wei-Chuan
;
Tsuei, Ku-Ding
A topological insulator (TI) is a quantum material in a new class with attractive properties for physical and technological applications. Here we derive the electronic structure of highly crystalline Sb2Te2Se single crystals studied with angle-resolved photoemission spectra. The result of band mapping reveals that the Sb2Te2Se compound behaves as a p-type semiconductor and has an isolated Dirac cone of a topological surface state, which is highly favored for spintronic and thermoelectric devices because of the dissipation-less surface state and the decreased scattering from bulk bands. More importantly, the topological surface state and doping level in Sb2Te2Se are difficult to alter for a cleaved surface exposed to air; the robustness of the topological surface state defined in our data indicates that this Sb2Te2Se compound has a great potential for future atmospheric applications.
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.
Study of Nonpolar GaN/ZnO Heterostructures Grown by Molecular Beam Epitaxy
(2013-07-01)
Chang, Chiao-Yun
;
Huang, Huei-Min
;
Lan, Yu-Pin
;
Lu, Tien-Chang
;
Tu, Li-Wei
The growth mechanism and characteristics of the nonpolar GaN/ZnO heterostructure grown on the r-plane sapphire substrate by using molecular beam epitaxy were studied. The crystal interaction between GaN and ZnO epitaxial layers was clarified by using transmission electron microscopy and X-ray diffraction. A new epitaxial relationship of ZnGa2O4 (220)//GaN (10 (13) over bar) in the normal surface direction was obtained in the GaN/ZnO heterostructure. It was believed that the formation of ZnGa2O4 (220) was due to the recrystallizadon of the ZnO layer with Ga atoms, which in turn resulted in the formation of semipolar-oriented GaN. In addition, the main optical transition in the GaN/ZnO heterostructure was attributed to the existence of the interface states and new ZnO:(Ga,N) alloys.
Study of Nonpolar GaN/ZnO Heterostructures Grown by Molecular Beam Epitaxy
(2013-07-01)
Chang, Chiao-Yun
;
Huang, Huei-Min
;
Lan, Yu-Pin
;
Lu, Tien-Chang
;
Tu, Li-Wei
The growth mechanism and characteristics of the nonpolar GaN/ZnO heterostructure grown on the r-plane sapphire substrate by using molecular beam epitaxy were studied. The crystal interaction between GaN and ZnO epitaxial layers was clarified by using transmission electron microscopy and X-ray diffraction. A new epitaxial relationship of ZnGa2O4 (220)//GaN (10 (13) over bar) in the normal surface direction was obtained in the GaN/ZnO heterostructure. It was believed that the formation of ZnGa2O4 (220) was due to the recrystallizadon of the ZnO layer with Ga atoms, which in turn resulted in the formation of semipolar-oriented GaN. In addition, the main optical transition in the GaN/ZnO heterostructure was attributed to the existence of the interface states and new ZnO:(Ga,N) alloys.
Type II GaSb quantum ring solar cells under concentrated sunlight
(2014-03-10)
Tsai, Che-Pin
;
Hsu, Shun-Chieh
;
Lin, Shih-Yen
;
Chang, Ching-Wen
;
Tu, Li-Wei
A type II GaSb quantum ring solar cell is fabricated and measured under the concentrated sunlight. The external quantum efficiency confirms the extended absorption from the quantum rings at long wavelength coinciding with the photoluminescence results. The short-circuit current of the quantum ring devices is 5.1% to 9.9% more than the GaAs reference's under various concentrations. While the quantum ring solar cell does not exceed its GaAs counterpart in efficiency under one-sun, the recovery of the open-circuit voltages at higher concentration helps to reverse the situation. A slightly higher efficiency (10.31% vs. 10.29%) is reported for the quantum ring device against the GaAs one. (C)2014 Optical Society of America