Browsing by Author "Yu, Hung-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]
Broadband antireflection sub-wavelength structure of InGaP/InGaAs/Ge triple junction solar cell with composition-graded SiNx
(2015-05)
Chung, Chen-Chen
;
Lo, Hsiao-Chieh
;
Lin, Yen-Ku
;
Yu, Hung-Wei
;
Binh Tinh Tran
This work reports a fabrication strategy to improve the antireflective ability of a InGaP/GaAs/Ge triple-junction solar cell, by combining a nano-templating technique and a chemical-synthesis approach. SiH4 and N-2 were used as ammonia-free reaction gases in a plasma-enhanced chemical vapor deposition (PECVD) to prepare Si3N4 as an original antireflective coating (ARC) layer with better chemical stability. Composition-graded SiNx was successfully integrated with sub-wavelength structure by modulating SiH4/N-2 ratio during PECVD deposition, and followed by a controllable gold-nanoparticle masking technique on top of the solar cell. Finite-difference time-domain solution was employed to simulate and optimize the aspect-ratio of the ARC, under the condition of variable refractive index over a broad wavelength window, and followed by the masking technique to obtain the desired ARC dimension. This enabled a low light reflectance (<10%) over a broad spectral bandwidth (300-1800 nm) for the solar cell with excellent stability, because of the triple advantages of structural optimization, better chemical stability and graded refractive index of the ARC. The solar cell\'s performance was tested and showed great competitiveness to those of forefront studies, suggesting the feasibility of the proposed technology.
Characterizations of GaN Films Grown on Si (111) Substrates with Various Growth Temperatures of Multiple AlN Buffer Layers
(2012)
Binh-Tinh Tran
;
Chang, Edward Yi
;
Lin, Kung-Liang
;
Luong, Tien-Tung
;
Yu, Hung-Wei
We present the effect of multiple AlN buffer layers on characterizations of GaN film quality, which includes a thin high-low-high-temperature (HLHT) AlN buffer layers. The study is based on two different thicknesses of the GaN films on the buffers and found that the HLHT AlN buffer layers could significantly affect on the GaN films qualities. The buffer plays a very important role for the growth of GaN film on Si (111) substrate. The GaN film with an uniformly faceted surface and very high-quality has been obtained at the optimized multiple HLHT AlN buffer layers of 50-nm-thick at 1010-800-1010 degrees C.
Direct growth of a 40 nm InAs thin film on a GaAs/Ge heterostructure by metalorganic chemical vapor deposition
(2014-09-01)
Yu, Hung-Wei
;
Wang, Tsun-Ming
;
Nguyen, Hong-Quan
;
Wong, Yuen-Yee
;
Tu, Yung-Yi
In this paper, the authors directly grew an InAs thin film (40 nm) by metalorganic chemical vapor deposition on GaAs/Ge substrates by using flow-rate modulation epitaxy with an appropriate V/III ratio. The growth of a high-quality InAs thin film with periodic 90 degrees misfit dislocations was related to a uniform monolayer In atom distribution at the InAs/GaAs interface. The In monolayer effectively minimized the difference between surface energy and strain energy, producing a stable interface during material growth. The authors also found that a tightly controlled V/III ratio can improve the quality of the InAs islands on the GaAs/Ge heterostructures, though it is not the key factor in InAs thin-film growth. (C) 2014 American Vacuum Society.
Effect of Different Tunnel diodes on The Efficiency of Multi-junction III-V Solar cells
(2014-01-01)
Yu, Hung-Wei
;
Hong-Quan Nguyen
;
Chung, Chen-Chen
;
Hsu, Ching-Hsiang
;
Hsiao, Chih-Jen
InGaP/GaAs dual-junction solar cells with different tunnel diodes (TDs) grown on misoriented GaAs substrates are investigated. It is found that the solar cells with P++-AlGaAs/N++-GaAs TDs grown on 10 degrees off GaAs substrates show a higher external quantum efficiency (EQE) but also generate a higher peak current density (J(peak)) than the solar cells with P++-GaAs/N++-InGaP TDs grown on 10 degrees off GaAs substrates. Furthermore, smooth surface (rms roughness: 1.54 angstrom) and sharp interface for the GaAs/Al0.3Ga0.7As TDs were obtained when the (100) tilted 10 degrees off toward [111] GaAs substrate was used. The conversion efficiency of InGaP/GaAs dual-junction solar cell with N++-GaAs/P++-AlGaAs TD grown on the (100) tilted 10 degrees off toward (111) GaAs substrate is close to 20%.
Effect of the Circle-Grid Electrodes on Concentrated GaAs Solar Cell Efficiency
(2014-09-01)
Chung, Chen-Chen
;
Binh Tinh Tran
;
Han, Ming-Hung
;
Lin, Kung-Liang
;
Yu, Hung-Wei
In this study, we investigate the effect of the shading factor of the front grid pattern on concentrated solar cell efficiency, taking the trade-off between the series resistance of the electrodes and the amount of incident light into consideration. We examine the thermal effect with regard to five different circle-grid electrode patterns of the front contact. The front contacts with different grid patterns affect the characteristics of light-concentrated-type GaAs single-junction solar cells. The device parameters analyzed include the open-circuit voltage (V-oc), short-circuit current (I-sc), fill factor (FF) and conversion efficiency (eta). The results of our study show that for a concentration ratio greater than 60x with AM1.5G, the device with a shading factor of 7.1% has the best cell efficiency of 27.05%, due to the smaller current crowding at the center spot. The results indicate that the conversion efficiency of solar cells can be improved by establishing a compromise between the shading effect and the series resistance effect.
Effect of V/III Ratios on Surface Morphology in a GaSb Thin Film Grown on GaAs Substrate by MOCVD
(2014-01-01)
Hsiao, Chih-Jen
;
Liu, Chun-Kuan
;
Huynh, Sa-Hoang
;
Minh, Thien-Huu Ha
;
Yu, Hung-Wei
The epitaxial growth of GaSb thin film with different V/III ratios on high-lattice-mismatched GaAs (001) substrates by metal organic chemical vapor deposition (MOCVD) was investigated. Under optimal V/III ratio of 2.5, we found that there are many periodic 90 degrees interfacial misfit dislocation (IMF) arrays existing at the GaAs/GaSb interface. The surface roughness is about 3.6nm, 2.2nm, 3.8nm, respectively while different V/III ratios (1.25, 2.5, 5) were adopted. These results demonstrated that the hill-and valley structure on the surface of GaSb/GaAs heterostructure can be effectively improved, and formed smooth surface morphology.
Effective reduction of bowing in free-standing GaN by N-face regrowth with hydride vapor-phase epitaxy
(2009-05-01)
Chen, Kuei-Ming
;
Huang, Hsin-Hsiung
;
Kuo, Yi-Lin
;
Wu, Pei-Lun
;
Chu, Ting-Li
Free-standing GaN films prepared with hydride vapor-phase epitaxy (HVPE) technique usually show bowing resulting from the high densities of defects near the N-polar face after separation from the original substrates. To solve the problem, a simple technique has been developed. A GaN layer was regrown on the N-polar face of the free-standing GaN by HVPE. High-resolution X-ray diffraction (HRXRD) measurements were performed to compare the bowings among GaN films before laser lift-off (LLO), after LLO, and after regrowth. The apparent reductions of XRD full-width at half-maximum (FWHM), along with the increase of XRD peak intensity, after regrowth clearly demonstrate the effectiveness of this method to eliminate bowings of the free-standing GaN films. (C) 2009 Elsevier B.V. All rights reserved.
Effects of initial GaN growth mode on the material and electrical properties of AlGaN/GaN high-electron-mobility transistors
(2014-09-01)
Wong, Yuen-Yee
;
Chang, Edward Yi
;
Huang, Wei-Ching
;
Lin, Yueh-Chin
;
Tu, Yung-Yi
AlGaN/GaN high-electron-mobility transistors (HEMTs) with different initial GaN growth modes were prepared on AIN/sapphire substrates. Secondary ion mass spectroscopy and Hall effect measurements confirmed that a highly conductive n-type region was generated at the GaN/AIN buffer interface during the three-dimensional to two-dimensional (3D-2D) growth mode transition. This n-type region was created by oxygen impurity incorporation during the 3D-2D growth mode transition, and its thickness increased with the transition time. The existence of this n-type conduction path not only changed the material properties but also degraded the device performance of HEMTs. (C) 2014 The Japan Society of Applied Physics
Effects of NH3 Flow Rate During AlGaN Barrier Layer Growth on the Material Properties of AlGaN/GaN HEMT Heterostructure
(2017-10-01)
Lumbantoruan, Franky J.
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Wong, Yuen-Yee
;
Huang, Wei-Ching
;
Yu, Hung-Wei
;
Chang, Edward-Yi
NH3 flow rate during AlGaN barrier layer growth not only affects the growth efficiency and surface morphology as a result of parasitic reactions but also influences the concentration of carbon impurity in the AlGaN barrier. Carbon, which decomposes from metal precursors, plays a role in electron compensation for AlGaN/GaN HEMT. No 2-dimensional electron gas (2-DEG) was detected in the AlGaN/GaN structure if grown with 0.5 slm of NH3 due to the presence of higher carbon impurity (2.6 x 10(19) cm(-2)). When the NH3 flow rate increased to 6.0 slm, the carbon impurity reduced to 2.10 x 10(18) atom cm(-3) and the 2 DEG electron density recovered to 9.57 x 10(12) cm(-2).
Efficiency improvement of InGaP/GaAs/Ge solar cells by hydrothermal-deposited ZnO nanotube structure
(2014-07-05)
Chung, Chen-Chen
;
Binh Tinh Tran
;
Lin, Kung-Liang
;
Ho, Yen-Teng
;
Yu, Hung-Wei
In this paper, a zinc oxide (ZnO) nanotube, fabricated by the hydrothermal growth method on triple-junction (T-J) solar cell devices to enhance efficiency, is investigated. Compared to those of bare T-J solar cells (without antireflection (AR) coating) and solar cells with Si3N4 AR coatings, the experimental results show that the T-J solar cells, which use a ZnO nanotube as an AR coating, have the lowest reflectance in the short wavelength spectrum. The ZnO nanotube has the lowest light reflection among all experimental samples, especially in the range of 350 to 500 nm from ultraviolet (UV) to visible light. It was found that a ZnO nanotube can enhance the conversion efficiency by 4.9%, compared with a conventional T-J solar cell. The Si3N4 AR coatings also enhance the conversion efficiency by 3.2%.The results show that a cell with ZnO nanotube coating could greatly improve solar cell performances.
Fabrication and characterization of n-In0.4Ga0.6N/p-Si solar cell
(2012-07-01)
Tran, Binh-Tinh
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Chang, Edward-Yi
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Trinh, Hai-Dang
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Lee, Ching-Ting
;
Sahoo, Kartika Chandra
Electro-optic characteristics of a fabricated n-In0.4Ga0.6N/p-Si hetero-structure solar cell on Si substrate with Al and ITO (or Ti/Al/Ni/Au) materials for p and n-type contacts were investigated in this letter. The solar cell devices with ITO as n-type contacts were also compared to the solar cell using Ti/Al/Ni/Au as n-type contact in this study. High short-circuit current density observed for solar cell with ITO as n-type contacts due to the increased amount of light reaching the solar cell. The device with ITO contact exhibited an open-circuit voltage (V-oc) of 1.52 V and a short-circuit current density (J(sc)) of 8.68 mA/cm(2) with 54% fill factor. The conversion and external quantum efficiency (EQE) of the solar cell were 7.12 and 20.8%, respectively. Besides, a relationship between V-oc and In content in the InxGa1-xN alloys for this type of solar cell was also derived. (C) 2012 Elsevier B.V. All rights reserved.
Gold-Free Cu-Metallized III-V Solar Cell
(2014-01-01)
Hsu, Ching-Hsiang
;
Chang, Hsun-Jui
;
Yu, Hung-Wei
;
Hong-Quan Nguyen
;
Ma, Jer-Shen
-Au-free, fully Cu-metallized InGaP/InGaAs/Ge triple-junction solar cells using Pd/Ge/Cu as front contact and Pt/Ti/Pt/Cu/Cr as back contact were fabricated and the results are reported for the first time. From the specific contact resistance measurement, these Cu-metallized ohmic contacts have low contact resistance in the order of 10(-6) Omega-cm(2). AES and TEM results clearly show the formation mechanisms of the Cu-metallization ohmic structures, for Pd/Ge/Cu contact, it was due to the formation of Ge diffusion into the GaAs layer, and for the Pt/Ti/Pt/Cu/Cr contact, it was due to high work function of Pt layer, these copper metallized ohmic contacts were quite stable even after 310 degrees C annealing. The I-V curves of the Cu-metallized InGaP/InGaAs/Ge triple-junction solar cells showed similar electrical characteristics to the solar cells with Au-metallized triple junction solar cell. Overall, the Pd/Ge/Cu and Pt/Ti/Pt/Cu ohmic contacts have been successfully applied to the InGaP/InGaAs/Ge triple-junction solar cells and demonstrated excellent performance.
Gold-Free Fully Cu-Metallized InGaP/InGaAs/Ge Triple-Junction Solar Cells
(2014-12-01)
Hsu, Ching-Hsiang
;
Chang, Edward Yi
;
Chang, Hsun-Jui
;
Yu, Hung-Wei
;
Hong Quan Nguyen
Copper contacts and interconnects were developed for GaAs and Ge for low-cost solar cell application. In addition, thermally annealed Pd/Ge and Pt/Ti/Pt metallizations were created for ohmic contacts to n-GaAs and p-Ge with contact resistance of 4.4 x 10(-6) and 6.9 x 10(-6) Omega cm(2), respectively. Utilizing such metallization structure for InGaP/InGaAs/Ge triple-junction device structure solar cells were fabricated that delivered conversion efficiency of 23.11%, which is average efficiency for the above device structure.
High Quality Ge Epitaxial Films Grown on In0.51Ga0.49P/GaAs and GaAs Substrates by Ultra High Vacuum Chemical Deposition
(2014-01-01)
Su, Yung-Hsuan
;
Tang, Shih-Hsuan
;
Nguyen, Chi Lang
;
Kuan, Ching-Wen
;
Yu, Hung-Wei
The epitaxial growth of high quality Ge thin films on different materials of In0.51Ga0.49P and GaAs by ultra high vacuum chemical vapor deposition (UHVCVD) system was studied. The crystallinity of high quality Ge layers on In0.51Ga0.49P and GaAs layers can be proved by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The comparison of surface morphology between Ge grown on In0.51Ga0.49P and GaAs was also analyzed by atomic force microscopy (AFM). The roughness of Ge on GaAs shows better than that of In0.51Ga0.49P. Both of these structures were designed for fabricating p-channel metal-oxide-semiconductor field-effect transistor (MOSFET) for the integration of Ge p-channel device with III-V n-channel electronic device.
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.
Hydrothermal growth of ZnO nanotubes on InGaP/GaAs/Ge Solar Cells
(2014-01-01)
Chung, Chen-Chen
;
Lin, Kung-Liang
;
Yu, Hung-Wei
;
Quan, Nguyen-Hong
;
Dee, Chang-Fu
A new design where ZnO nanotubes were grown on the antireflection (AR) layer coated on triple-junction (T-J) solar cell devices to enhance the light conversion efficiency. Compared to the bare T-J solar cells (without an AR layer), the performance of Si3N4 AR coated solar cell showed improvement. The sample with a layer of ZnO nanotubes grown in top of AR layer showed the lowest light reflection compared with the bare and solely AR coated T-J solar cell especially in the spectrum range of 350-500 nm. The use of ZnO nanotubes have increased the conversion efficiency by 4.9% compared with the conventional T-J solar cell. While the Si3N4 AR coated sample only increased the conversion efficiency by 3.2%. This result is quite encouraging as further refinement and variation in the experiment procedures could possibly bring more exciting performance in the future.
Low resistance copper-based ohmic contact for AlGaN/GaN high electron mobility transistors
(2013-10-07)
Wong, Yuen-Yee
;
Chen, Yu-Kong
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Maa, Jer-Shen
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Yu, Hung-Wei
;
Tu, Yung-Yi
Ti/Al/Ni/Cu ohmic contact for AlGaN/GaN structure has been fabricated. The Ni layer played an important role in achieving low specific contact resistance (r(c)), smooth morphology, and excellent edge acuity. With a 50-angstrom Ni layer, a r(c) of 1.35 x 10(-6) Omega-cm(2) and a root-mean-square roughness of 7.65 nm have been realized. The characterization results indicated that no evidence of Cu diffusion into the semiconductor layers. The formation of Al-Cu and Ti-Cu alloys might have confined the Cu within the ohmic metal. In the absence of gold, the surface roughening caused by Au-Al alloy in conventional Ti/Al/Ni/Au structure was also prevented. (C) 2013 AIP Publishing LLC.
Performance improvement of highly mismatched GaSb layers on GaAs by interfacial-treatment-assisted chemical vapor deposition
(2017-01)
Hsiao, Chih-Jen
;
Minh-Thien-Huu Ha
;
Liu, Chun-Kuan
;
Hong-Quan Nguyen
;
Yu, Hung-Wei
Strain-relieved GaSb quantum dots on GaAs can be achieved by either periodic interfacial misfit (IMF) or the conventional Stranski-Krastanov (SK) growth modes by changing the growth parameters. In this study, the Sb interfacial treatment was employed to improve the GaSb crystal quality including low defect density, smooth surface morphology, and high hole mobility. This technique yields two-dimensional (2D) islands with a height as low as 1.7 nm and width up to 190 nm in the IMF growth mode. In contrast to the interfacial treatments conventionally employed in the initial strain relaxation of GaSb/GaAs hererostructure, the Sb treatment promotes the formation of strong Ga-Sb bonds on the surface of the grown island, which effectively reduces the interfacial free energy and thus promotes the formation of 2D islands. With the Sb interfacial treatment, a high-relaxation 100-nm GaSb epilayer was grown on the GaAs substrate, the epilayers was strain relaxed and exhibited enhanced electrical properties with a high hole mobility of similar to 667 cm(2) V-1 s(-1) and with superior optical properties as evidenced by the photoluminescence B-line peak. The results of this study demonstrate an effective interfacial-treatment growth technique to relax the initial strain for the highly mismatched GaSb layers grown on a GaAs substrate.
Rapid and homogeneous dispersion of pt catalyst particles on multi-walled carbon nanotubes by temperature-controlled microwave polyol method
(2008-04-01)
Chen, Chieng-Ming
;
Chen, Mi
;
Yu, Hung-Wei
;
Lu, Su-Chen
;
Chen, Chia-Fu
Fuel cells, particularly direct methanol fuel cells (DMFCs), have great potential as mobile power sources. It has been extensively reported that the DMFC performance strongly depends on the electrode catalyst, and many research groups have focused on the preparation of high efficiency cathode catalysts using all types of metal nanoparticle and support. However, most of these methods are either time-consuming with multiple-steps or there is low loading of the nanocatalyst dispersed on the supports. In this study, platinum nanoparticles of uniform diameter (approximately 4.3 nm) were efficiently dispersed on multi-walled carbon nanotubes (MWCNTs) by the temperature-control led, microwave-assisted polyol method with the addition of sodium dodecyl sulfate (SDS) and [poly(vinyl pyrrolidone)] (PVP). The addition of SDS and PVP in chloroplatinic acid/ethylene glycol (EG) solution can cause the efficient and homogeneous dispersion of the catalyst particles on MWCNTs, enhance the adsorption of platinum particles, and reduce the residue of platinum particles. Moreover, temperature was determined to be the most important factor affecting the reduction of Pt in the process, and large amounts of Pt particles were characterized when the temperature ranged from 140 to 160 degrees C. The results indicate that the optimum temperature ranges from 140 to 160 degrees C, and that the loading amount of Pt/CNTs can reach more than 50 wt % in only 90 s of processing time. The significant improvements in dispersion and loading are due to the ability of SDS, to wrap the PVP-stabilized Pt nanoparticles around the surface of the carbon nanotubes. The characterization analyses were conducted by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), and thermal gravinnetric analysis (TGA).