Browsing by Author "Wu, Yue-Han"

Now showing 1 - 20 of 27

2H-Silicon Carbide Epitaxial Growth on c-Plane Sapphire Substrate Using an AlN Buffer Layer and Effects of Surface Pre-Treatments
(2015-05-01)
Luong, Tien-Tung
;
Binh Tinh Tran
;
Ho, Yen-Teng
;
Wei, Ting-Wei
;
Wu, Yue-Han
The effects of surface pre-treatments and the role of an AN buffer layer for 2H-SiC growth on c-plane sapphire substrates by thermal CVD are investigated. While the crystallinity of SiC directly grown on sapphire substrate always degrades with a hydrogen pre-treatment but improves by optimizing carbonization, the crystallinity of SiC grown on sapphire substrate using an AN buffer grown by MOCVD improves with sufficient time of exposure to the H pre-treatment but always deteriorates with carbonization. Detailed microstructural analysis by phi-scan x-ray diffraction reveals that SiC film grown on sapphire substrate consists of crystalline domains with two different crystallographic orientations which are rotated relative to each other along the [111] axis by 60 degrees. A highly oriented hexagonal 2H-SiC film is obtained on low-cost c-plane sapphire substrate by using an AN buffer. 2H-SiC is unambiguously determined not only by phi-scan x-ray diffraction but also by high-resolution transmission electron microscopy. The growth relationship between 2H-SiC and 2H-AlN are coherent due to the favorable bonding of C and Al between SiC and AlN.
A novel dilute antimony channel In0.2Ga0.8AsSb/GaAs HEMT
(2007-02-01)
Su, Ke-Hua
;
Hsu, Wei-Chou
;
Lee, Ching-Sung
;
Wu, Tsung-Yeh
;
Wu, Yue-Han
This letter reports, for the first time, a high-electron mobility. transistor (HEMT) using A dilute antimony In0.2Ga0.8AsSb channel, which is grown by a molecular-beam epitaxy system. The interfacial quality within the InGaAsSb/GaAs quantum well of the HEMT device was effectively improved by introducing the surfactantlike Sb atoms during the growth of the InGaAs layer. The improved heterostructural quality and electron transport properties have also been verified by various surface characterization techniques. In comparison, the proposed HEMT with (without) the incorporation of Sb atoms has demonstrated the maximum extrinsic transconductance g(m,max) of 227 (180) mS/mm, a drain saturation current density I-DSS of 218 (170) mA/mm, a gate-voltage swing of 1.215 (1.15) V, a cutoff frequency f(T) of 25 (20.6) GHz, and the maximum oscillation frequency f(max) of 28.3 (25.6) GHz at 300 K with gate dimensions of 1.2 x 200 mu m(2).
Analysis of InAsN quantum dots by transmission electron microscopy and photoluminescence
(2008-10-01)
Hsu, Chiung-Chih
;
Hsu, Ray-Quen
;
Wu, Yue-Han
;
Chi, Tung-Wei
;
Chiang, Chen-Hao
Quantum dots (QDs) have great potential in optical fiber communication applications were widely recognized. The structure of molecular beam epitaxy (MBE) grew InAsN QDs were investigated by transmission electron microscopy (TEM) and measured their optical properties by photoluminescence (PL). TEM images show that the InAsN QDs are irregular or oval shaped. Some of the InAsN QDs are observed to have defects, such as dislocations at or near the surface in contrast to InAs QDs, which appear to be defect free. PL results for InAsN QDs showed a red-shifted emission peak. In addition, the InAsN emission peak is broader than InAs QDs, which supports the TEM observation that the size distribution of the InAsN QDs is more random than InAs QDs. The results show that the addition of nitrogen to InAs QDs leads to a decrease in the average size of the QDs, bring changes in the QD's shape, compositional distribution, and optical properties. (C) 2008 Published by Elsevier B.V.
Chemical polishing method of GaAs specimens for transmission electron microscopy
(2010-01-01)
Wu, Yue-Han
;
Chang, Li
;
材料科學與工程學系
;
Department of Materials Science and Engineering
A practical method for transmission electron microscopy specimen preparation of GaAs-based materials with quantum dot structures is presented to show that high-quality image observations in high-resolution transmission electron microscopy (HRTEM) can be effectively obtained. Specimens were prepared in plan-view and cross-section using ion milling, followed by two-steps chemical fine polishing with an ammonia solution (NH(4)OH) and a dilute H(2)SO(4) solution. Measurements of electron energy loss spectroscopy (EELS) and atomic force microscopy (AFM) proved that clean and flat specimens can be obtained without chemical residues. HRTEM images show that the amorphous regions of carbon and GaAs can be significantly reduced to enhance the contrast of lattice images of GaAs-based quantum structure. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.
Dealloyed Pt2Os nanoparticles for enhanced oxygen reduction reaction in acidic electrolytes
(2014-05-05)
Lee, Yi-Juei
;
Hsieh, Yu-Chi
;
Tsai, Ho-Cheng
;
Lu, I-Te
;
Wu, Yue-Han
Carbon-supported Pt2Os (Pt2Os/C) nanoparticles in 3.55 nm sizes are synthesized from a wet chemical reflux process. Subsequently, the Pt2Os/C undergoes a dealloying treatment in which multiple cyclic voltammetric scans are imposed to dissolve the Os atoms selectively from the surface of the Pt2Os nanoparticles. X-ray diffraction signals from the dealloyed sample (DA-Pt2Os/C) indicate a fcc phase and composition analysis suggests Pt4Os. Line scans from the scanning transmission electron microscope confirm that the surface of Pt4Os is depleted with the Os atoms. This agrees with our quantum mechanics (Density Funtional theory) calculations, which predict for the Pt3Os composition that the surface skin layer is pure Pt. The DA-Pt2Os/C shows impressive electrocatalytic behaviors (0.29 mA mu g(Pt)(-1) in mass activity and 1.03 mA cm(Pt)(-2) in specific activity) for the oxygen reduction reaction (ORR) in oxygen-saturated 0.1 M aqueous HClO4 solution, as compared to those of commercially available Pt/C and as-synthesized Pt2Os/C. In stability test, the DA-Pt2Os/C demonstrates a better retention of ORR activities and a smaller loss of electrochemical active surface area. We verify experimentally that a four-electron step is responsible for the ORR process occurring on the DA-Pt2Os/C. (C) 2014 Elsevier B.V. All rights reserved.
Defects in semipolar (11(2)over-bar(2)over-bar) ZnO grown on (112) LaAlO3/(La, Sr) (Al, Ta)O-3 substrate by pulsed laser deposition
(2013-03-27)
Tian, Jr-Sheng
;
Wu, Yue-Han
;
Peng, Chun-Yen
;
Chiu, Kun-An
;
Shih, Yi-Sen
The microstructure of semipolar (11 (2) over bar(2) over bar) ZnO deposited on (112) LaAlO3/(La, Sr) (Al, Ta)O-3 was investigated by transmission electron microscopy. The ZnO shows an in-plane epitaxial relationship of [11 (2) over bar3](ZnO) parallel to [11 (1) over bar](LAO/LSAT) with oxygen-face sense polarity. The misfit strain along [11 (2) over bar3](ZnO) and [1 (1) over bar 00](ZnO) is relieved through the formation of misfit dislocations with the Burgers vectors b = 1/6[11 (2) over bar3](ZnO) and b = 1/3 < 1 (2) over bar 10 >(ZnO), respectively. The line defects in the semipolar ZnO are predominantly perfect dislocations, and the dislocation density decreases with increasing ZnO thickness as a result of dislocation reactions. Planar defects were observed to lie in the M-plane and extend along < 0001 >, whereas basal stacking faults were rarely found.
Dislocation reduction in GaN film using Ga-lean GaN buffer layer and migration enhanced epitaxy
(2011-07-29)
Wong, Yuen-Yee
;
Chang, Edward Yi
;
Wu, Yue-Han
;
Hudait, Mantu K.
;
Yang, Tsung-Hsi
A GaN buffer layer grown under Ga-lean conditions by plasma-assisted molecular beam epitaxy (PAMBE) was used to reduce the dislocation density in a GaN film grown on a sapphire substrate. The Ga-lean buffer, with inclined trench walls on its surface, provided an effective way to bend the propagation direction of dislocations, and it reduced the dislocation density through recombination and annihilation processes. As a result, the edge dislocation density in the GaN film was reduced by approximately two orders of magnitude to 2 x 10(8) cm(-2). The rough surface of the Ga-lean buffer was recovered using migration enhanced epitaxy (MEE), a process of alternating deposition cycle of Ga atoms and N(2) radicals, during the PAMBE growth. By combining these two methods, a GaN film with high-crystalline-quality and atomically-flat surface can be achieved by PAMBE on a lattice mismatch substrate. (C) 2011 Elsevier B.V. All rights reserved.
Dislocation reduction in GaN grown on stripe patterned r-plane sapphire substrates
(2007-07-09)
Chen, Hou-Guang
;
Ko, Tsung-Shine
;
Ling, Shih-Chun
;
Lu, Tien-Chang
;
Kuo, Hao-Chung
Extended defect reduction in GaN can be achieved via direct growth on stripe patterned (1(-)1 02) r-plane sapphire substrates by metal organic chemical vapor deposition. The striped mesa is along [11 (-)2 0] with two etched sides in {0001} and {1 (-)1 01} faces. GaN grown on both etched facets in epitaxy exhibit different crystallographic relationships with sapphire substrate which are (1 (-)1 02)(sapphire)parallel to(11 (-)2 0)(GaN) and [11 (-)2 0](sapphire)parallel to[ (-)1 100](GaN), and (0001)(sapphire)parallel to(0001)(GaN) and [11 (-)2 0](sapphire)parallel to[ (-)1 100](GaN), respectively. The dislocation densities can be significantly reduced through epitaxial growth on the inclined lateral faces of mesas. Dislocation density in the order of similar to 10(7) cm(-2) can be achieved in the tilted GaN. (C) 2007 American Institute of Physics.
Effect of In/Al ratios on structural and optical properties of InAlN films grown on Si(100) by RF-MOMBE
(2014-05-01)
Chen, Wei-Chun
;
Wu, Yue-Han
;
Peng, Chun-Yen
;
Hsiao, Chien-Nan
;
Chang, Li
InxAl1-xN films were deposited on Si(100) substrate using metal-organic molecular beam epitaxy. We investigated the effect of the trimethylindium/trimethylaluminum (TMIn/TMAI) flow ratios on the structural, morphological, and optical properties of InxAl1-xN films. Surface morphologies and microstructure of the InxAl1-xN films were measured by atomic force microscopy, scanning electron microscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM), respectively. Optical properties of all films were evaluated using an ultraviolet/visible/infrared (UV/Vis/IR) reflection spectrophotometer. XRD and TEM results indicated that InxAl1-xN films were preferentially oriented in the c-axis direction. Besides, the growth rates of InxAl1-xN films were measured at around 0.6 mu m/h in average. Reflection spectrum shows that the optical absorption of the InxAl1-xN films redshifts with an increase in the In composition.
Electron Emission Properties of Nitrogen-Induced Localized Defects in InAsN/GaAs Quantum Dots
(2011-11-01)
Yang, Cheng-Hong
;
Hsieh, Meng-Chien
;
Wu, Chia-Wei
;
Chang, Yen-Ting
;
Wu, Yue-Han
The electron-emission properties of nitrogen-induced (N-induced) localized defects in InAsN/GaAs quantum dots (QDs) are investigated in detail by capacitance-voltage (C-V) profiling and bias-dependent deep-level transient spectroscopy (DLTS). The incorporation of nitrogen (N) into InAs QDs is shown to produce localized defects near QDs and threading-dislocation-related defects in the top GaAs layer. The threading dislocation is associated with an electron-emission energy of approximately 0.648 eV and emission from the sample surface toward the QDs. The electron-emission energy from the QDs associated with the localized defects increases from 0.19 to 0.36 eV, indicating that both types of defect near QDs have low electron emission energies. Therefore, the change in emission energy is attributable to the defects across the QD interface where a band offset exists. The C-V profile at 300 K shows extended carrier depletion near the QDs. As ac frequency increases, an electron-emission peak emerges at the QDs; this peak is followed by another prominent peak, suggesting that the localized defect that is responsible for this latter peak has an energy below the QD electron ground state. On the basis of a C-V profile simulation, this defect is located at the QD at the observed emission energy below the GaAs conduction band, 360 meV. A comparison with InAsN QD and strain relaxation InAs QD samples reveals that the localized defect arises from a nitrogen alloy fluctuation in the QD. The energy location of this defect reveals a possibility that incorporated N is associated with a particular mode. (C) 2011 The Japan Society of Applied Physics
Epitaxial growth of semipolar InN(10(1)over-tilde3) on LaAlO3 substrate: Epitaxial relationship analysis
(2017-05-01)
Chen, Wei-Chun
;
Kuo, Shou-Yi
;
Tian, Jr. -Sheng
;
Wang, Wei-Lin
;
Lai, Fang-I
Epitaxial semipolar InN(10 (1) over tilde3) crystals were grown on a LaAlO3(112) substrate using radio frequency plasma-assisted molecular beam epitaxy without any interlayer. The lattice mismatch between InN and LaAlO3 was estimated to be % 7.75% along the [1 (2) over tilde 10](InN) direction and 0.2% along the [(3) over tilde 032](InN) direction. The InN film is epitaxic with the LaAlO3 substrate, with orientation relationships between InN(10 (1) over tilde3) parallel to LaAlO3(112) and [1 (2) over tilde 10](InN) parallel to [11 (1) over tilde](LAO). InN grown on LaAlO3(112) appears (10 (1) over tilde3) plane orientated, with two types of domains. Semipolar InN(10 (1) over tilde3) layers can be grown at 510 degrees C and show X-ray rocking curve FWHMs of 1830 and 1408 arcsec for InN(0002) and InN(10 (1) over tilde3), respectively. However, InN grown at 510 degrees C has the highest peak intensity and the narrowest FWHM of the prepared samples, indicating high-quality crystal growth. (C) 2017 The Japan Society of Applied Physics
Epitaxial growth of titanium oxycarbide on MgO (001) substrates by pulsed laser deposition
(2011-01-15)
Do, Hien
;
Yen, Tzu-Chun
;
Tian, Chih-Sheng
;
Wu, Yue-Han
;
Chang, Li
Epitaxial TiC(x)O(y) thin films were grown on MgO (0 0 1) substrates by using pulsed laser deposition method. High-resolution X-ray diffraction and transmission electron microscopy were used to examine crystallinity and microstructure of epitaxial TiC(x)O(y) film on MgO. The chemical composition of the film is determined to be x similar to 0.47 and y similar to 0.69 by X-ray photoelectron spectroscopy. Atomic force microscopy revealed that the surface of TiC(x)O(y) film is very smooth with roughness of 0.18 nm. The resistivity of the TiC(x)O(y) film measured by four-point-probe method was about 137 mu Omega cm. (C) 2010 Elsevier B.V. All rights reserved.
Growth of epitaxial ZnO thin film on yttria-stabilized zirconia single-crystal substrate
(2007-01-01)
Chao, Yen-Cheng
;
Lin, Chih-Wei
;
Ke, Dong-Jie
;
Wu, Yue-Han
;
Chen, Hou-Guang
ZnO growth on yttria-stabilized zirconia (YSZ) (I 1 1) single-crystal substrate has been carried out by metalorganic chemical vapor deposition (MOCVD). High-quality epitaxial ZnO has been evidenced by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Cross-sectional TEM from all deposited films reveals that the interface between ZnO and YSZ is atomically flat, and orientation relationship is deduced to be [1 (1) over bar 100](ZnO//)[11 (2) over bar](YSZ,) (2 (1) over bar(1) over bar0)(ZnO)//[001](YSZ) and (0 0 0 2)(ZnO)//(111)(YSZ). It has been found that surface roughness increases with the substrate temperature in the range of 500-700 degrees C. The growth rate also varies with the temperature. (c) 2006 Elsevier B.V. All rights reserved.
Indium-rich InAlN films on GaN/sapphire by molecular beam epitaxy
(2014-03)
Wu, Yue-Han
;
Wong, Yuen-Yee
;
Chen, Wei-Chun
;
Tsai, Dung-Sheng
;
Peng, Chun-Yen
Here, we report the results of characterization of indium (In)-rich InAlN films on GaN/Sapphire (0001) substrates grown by molecular beam epitaxy. The highquality similar to 123 nm thick InAlN films with 85% In content without phase separation were assessed with x-ray diffraction and transmission electron microscopy (TEM) with x-ray energy dispersive spectroscopy. High-resolution TEM analysis reveals the relaxation at InAlN/GaN interface with misfit dislocations of 1.59 nm spacing. Finally, optical and electrical properties of the InAlN films are presented from absorption spectroscopy and Hall measurements.
Influence of the GaN capping thickness on the strain and photoluminescence properties of InN/GaN quantum dots
(2013-02-01)
Ke, Wen-Cheng
;
Wu, Yue-Han
;
Houng, Wei-Chung
;
Wei, Chih-An
;
材料科學與工程學系
InN quantum dots (QDs) were grown on 1 mu m thick GaN/(0001) sapphire substrates by low pressure metal organic chemical vapor deposition. A single crystalline 10-nm thick GaN capping layer was achieved on the InN QDs by the flow-rate modulation epitaxy method at 650 degrees C. The (002) omega/2 theta scans of the X-ray diffraction measurements show that the reduction of the lattice constant with a capping thickness indicate that the GaN capping process exerts a compressive strain on the InN QDs. The residual strain was reduced from 0.245% to -0.245% as the GaN cap thickness increases from 0 to 20 nm. In addition, the analysis of the photoluminescence peak energy estimated that the free electron concentration (i.e. density of indium (In) vacancy) decreased from 1.62x10(18) cm(-3) to 1.24x10(18) cm(-3). The suggestion here is that the increase of the compressive strain on InN QDs due to the increased GaN capping layer thickness provides the high driving force for the interdiffusion of the In atom and gallium (Ga) atoms between the interface of InN QDs and the GaN capping layer. Thus, we believe that more Ga atoms can diffuse from the GaN capping layer and substitute the high density of In vacancy in the InN QDs, resulting in a decrease of the free electron concentration in the InN QDs with the increase in the GaN capping layer thickness. (C) 2012 Elsevier B.V. All rights reserved.
Influence of V/III Flow Ratio on Growth of InN on GaN by PA-MOMBE
(2013)
Chen, Wei-Chun
;
Tian, Jr-Sheng
;
Wu, Yue-Han
;
Wang, Wei-Lin
;
Kuo, Shou-Yi
InN epilayers were prepared on c-GaN/sapphire substrates by plasma-assisted metal-organic molecular beam epitaxy using N-2 and trimethylindium precursor as the V/III sources. We studied the influence of the V/III flow ratio on the film structure, surface morphology, film compositions, and optical and electrical properties using scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray diffraction, X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), photoluminescence (PL) measurement and Hall effect. The results show that epitaxial InN films can be obtained with the V/III ratio in the range between 1.81 and 4 at 500 degrees C. The InN growth rate decreases from 1.9 to 1.4 mu m/h when the ratio increases from 1.81 to 4. The surfaces of the InN films are not smooth in the V/III range used. Cross-sectional TEM revealed that the planar defect density in InN is as high as similar to 1.5 x 10(6) cm(-1) at a V/III ratio of similar to 1.81. XPS and SIMS results show that the film surface contains oxygen, which is found to affect the measured carrier mobility and concentration. (C) 2013 The Electrochemical Society. All rights reserved.
Investigation of TMAl preflow to the properties of AlN and GaN film grown on Si(111) by MOCVD
(2014-01-01)
Lumbantoruan, Franky
;
Wong, Yuan-Yee
;
Wu, Yue-Han
;
Huang, Wei-Ching
;
Shrestra, Niraj Man
The influence of TMAl preflow to the AlN buffer layer and GaN thin film was studied by Optical Microscope, Atomic Force Microscope, X-ray diffraction and Transmission Electron Microscope. Different duration of TMAl preflow lead to substantially differences of the AlN buffer layer and GaN film properties in terms of surface morphology and crystal quality. It was found without TMAl preflow the crystal quality of AlN buffer layer and GaN deteriorated due to the formation of amorphous interlayer between Si and AlN. Meltback etching and cracks was observed on the surface of GaN grown on AlN without TMAl preflow. However, overlong duration of TMAl preflow degraded the properties of AlN buffer layer and the subsequent GaN layer. GaN grown with longer TMAl preflow suffer of poor crystal quality, high density cracks and rough surface morphology. With the optimum duration of TMAl preflow, crystal quality and surface roughness of GaN can be improved.
Investigation on microstructure in GaN epitaxial growth on the stripe-patterned r-plane sapphire substrates
(2008-04-01)
Chen, Hou-Guang
;
Ko, Tsung-Shine
;
Chang, Li
;
Wu, Yue-Han
;
Lu, Tien-Chang
This study reports on the reduction of dislocations in the GaN grown on the stripe-patterned r-plane sapphire substrates via metal-organic chemical vapor deposition (MOCVD). The stripes oriented along the sapphire [1 1 (2) over bar 0] direction with asymmetrical side faces were fabricated by lithographic and wet-etching processes. The two etching sides of sapphire-striped mesa are {0 0 0 1} and {1 (1) over bar 0 1} faces. GaN grown on both etching facets exhibits different epitaxial relationships with the sapphire substrate. The GaN grown from the {0 0 0 1} side face of the sapphire mesa contains a low-dislocation density in the order of 10(7) cm(-2). The interfacial regions between the GaN and patterned sapphire substrate are also studied to clarify the behavior of GaN epitaxial growth on the inclined sapphire faces and defect-reduction mechanism. (C) 2007 Elsevier B.V. All rights reserved.
Investigations on highly stable thermal characteristics of a dilute In(0.2)Ga(0.8)AsSb/GaAs doped-channel field-effect transistor
(2008-04-01)
Su, Ke-Hua
;
Hsu, Wei-Chou
;
Lee, Ching-Sung
;
Hu, Po-Jung
;
Wu, Yue-Han
This work reports for the first time a novel In(0.2)Ga(0.8)AsSb/GaAs heterostructure doped-channel field-effect transistor (DCFET) grown by the molecular beam epitaxy system. The interfacial quality within the InGaAsSb/GaAs quantum well of the DCFET device has been effectively improved by introducing surfactant-like Sb atoms during the growth of the Si-doped InGaAs channel layer. The improved device characteristics include the peak extrinsic transconductance (g(m, max)) of 161.5 mS mm(-1), the peak drain-source saturation current density (IDSS, max) of 230 mA mm(-1), the gate-voltage swing (GVS) of 1.65 V, the cutoff frequency (f(T)) of 12.5 GHz and the maximum oscillation frequency (f(max)) of 25 GHz at 300 K with the gate dimensions of 1.2 x 200 mu m(2). The proposed design has also shown a stable thermal threshold coefficient (partial derivative V(th)/partial derivative T) of -0.7 mV K(-1).
Morphology and optical properties of single- and multi-layer InAs quantum dots
(2010-08-01)
Hsu, Chiung-Chih
;
Hsu, Ray-Quen
;
Wu, Yue-Han
;
機械工程學系
;
材料科學與工程學系
An understanding of the structural and optical properties of quantum dots (QDs) is critical for their use in optical communication devices. In this study, single- and multi-layer self-organized InAs QDs grown on (001) GaAs substrates by molecular beam epitaxy (MBE) were investigated. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images show that the lateral size of multi-layer InAs QDs are larger and flatter than single-layer InAs QDs, which are oval-shaped. The change in shape and size may be attributed to the presence of InGaAs spacer layers in multi-layer InAs QDs. Reciprocal spacer mapping and fast Fourier transformation images clearly show that InGaAs spacer layers present in the multi-layer InAs QDs structures help to release the strain originally existing in the QDs. In addition, the photoluminescence peak of the multi-layer InAs QDs is broader than QD in the single-layer one, which implies that the multi-layer InAs QDs size variation is more random than the single-layer one and this corresponds with the HAADF-STEM images. These results prove that spacer layers release strain influencing the morphology and optical properties of the QDs.