Browsing by Author "Chen, Wei-Kuo"

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Cathodoluminescence studies of GaAs nano-wires grown on shallow-trench-patterned Si
(2010-11-19)
Lee, Ling
;
Fan, Wen-Chung
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Ku, Jui-Tai
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Chang, Wen-Hao
;
Chen, Wei-Kuo
The optical properties of GaAs nano-wires grown on shallow-trench-patterned Si(001) substrates were investigated by cathodoluminescence. The results showed that when the trench width ranges from 80 to 100 nm, the emission efficiency of GaAs can be enhanced and is stronger than that of a homogeneously grown epilayer. The suppression of non-radiative centers is attributed to the trapping of both threading dislocations and planar defects at the trench sidewalls. This approach demonstrates the feasibility of growing nano-scaled GaAs-based optoelectronic devices on Si substrates.
Cathodoluminescence studies of GaAs nano-wires grown on shallow-trench-patterned Si
(2010-11-19)
Lee, Ling
;
Fan, Wen-Chung
;
Ku, Jui-Tai
;
Chang, Wen-Hao
;
Chen, Wei-Kuo
The optical properties of GaAs nano-wires grown on shallow-trench-patterned Si(001) substrates were investigated by cathodoluminescence. The results showed that when the trench width ranges from 80 to 100 nm, the emission efficiency of GaAs can be enhanced and is stronger than that of a homogeneously grown epilayer. The suppression of non-radiative centers is attributed to the trapping of both threading dislocations and planar defects at the trench sidewalls. This approach demonstrates the feasibility of growing nano-scaled GaAs-based optoelectronic devices on Si substrates.
Current properties of GaNV-defect using conductive atomic force microscopy
(2006-08-01)
Lee, Lino
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Ku, Ching-Shun
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Ke, Wen-Cheng
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Ho, Chih-Wei
;
Huang, Huai-Ying
Current conduction behavior on GaN V-defect was studied comprehensively using conductive atomic force microscopy. Experimental results indicate that the forward current in the V-defect region is at least three-order higher than that at surrounding area. On the other hand, a snowflake-like leakage current pattern was observed in the V-defect owing to the ease of current breakdown at the crest lines and perimeters. Further static current-voltage measurement suggests that the current flow is governed by Schottky emission and Fowler-Nordheim tunneling for V-defect region and surrounding area, respectively.
Disorder Chaos in the Spherical Mean-Field Model
(2015-07-01)
Chen, Wei-Kuo
;
Hsieh, Hsi-Wei
;
Hwang, Chii-Ruey
;
Sheu, Yuan-Chung
;
應用數學系
We study the problem of disorder chaos in the spherical mean-field model. It concerns the behavior of the overlap between two independently sampled spin configurations from two Gibbs measures with the same external parameters. The prediction states that if the disorders in the Hamiltonians are slightly decoupled, then the overlap will be concentrated near a constant value. Following Guerra\'s replica symmetry breaking scheme, we establish this at the levels of the free energy and the Gibbs measure.
Effects of growth temperature on InN/GaN nanodots grown by metal organic chemical vapor deposition
(2008-05-15)
Chang, Wen-Hao
;
Ke, Wen-Cheng
;
Yu, Shu-Hung
;
Lee, Lin
;
Chen, Ching-Yu
InN nanodots grown on GaN by metal organic chemical vapor deposition using conventional growth mode as well as flow-rate modulation epitaxy at various growth temperatures (550-730 degrees C) were investigated. We found that different precursor injection schemes together with the effect of growth temperatures greatly influenced the surface morphology of InN nanodots and their photoluminescence (PL) properties. The sample grown at around 650 degrees C showed the highest growth rate. For samples grown at higher temperatures, the residual carrier concentration was reduced and the PL efficiency was improved. Furthermore, we found that the growth of InN nanodots is still sustainable even at a temperature higher than 700 degrees C while retaining their optical quality. (c) 2008 American Institute of Physics.
Influence of different aspect ratios on the structural and electrical properties of GaN thin films grown on nanoscale-patterned sapphire substrates
(2016-07-01)
Lee, Fang-Wei
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Ke, Wen-Cheng
;
Cheng, Chun-Hong
;
Liao, Bo-Wei
;
Chen, Wei-Kuo
This study presents GaN thin films grown on nanoscale-patterned sapphire substrates (NPSSs) with different aspect ratios (ARs) using a homemade metal-organic chemical vapor deposition system. The anodic aluminum oxide (AAO) technique is used to prepare the dry etching mask. The cross-sectional view of the scanning electron microscope image shows that voids exist between the interface of the GaN thin film and the high-AR (i.e. similar to 2) NPSS. In contrast, patterns on the low-AR (similar to 0.7) NPSS are filled full of GaN. The formation of voids on the high-AR NPSS is believed to be due to the enhancement of the lateral growth in the initial growth stage, and the quick-merging GaN thin film blocks the precursors from continuing to supply the bottom of the pattern. The atomic force microscopy images of GaN on bare sapphire show a layer-by-layer surface morphology, which becomes a step-flow surface morphology for GaN on a high-AR NPSS. The edge-type threading dislocation density can be reduced from 7.1 x 10(8) cm(-2) for GaN on bare sapphire to 4.9 x 10(8) cm(-2) for GaN on a high-AR NPSS. In addition, the carrier mobility increases from 85 cm(2)/Vs for GaN on bare sapphire to 199 cm(2)/Vs for GaN on a high-AR NPSS. However, the increased screw-type threading dislocation density for GaN on a low-AR NPSS is due to the competition of lateral growth on the flat-top patterns and vertical growth on the bottom of the patterns that causes the material quality of the GaN thin film to degenerate. Thus, the experimental results indicate that the AR of the particular patterning of a NPSS plays a crucial role in achieving GaN thin film with a high crystalline quality. (C) 2016 Elsevier B.V. All rights reserved.
Influence of Mg-containing precursor flow rate on the structural, electrical and mechanical properties of Mg-doped GaN thin films
(2012-10-15)
Ke, Wen-Cheng
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Jian, Sheng-Rui
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Chen, I-Chen
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Jang, Jason S. -C.
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Chen, Wei-Kuo
The effects of Mg-containing precursor flow rate on the characteristics of the Mg-doped GaN (GaN:Mg) were systematically studied in this study. The GaN:Mg films were deposited on sapphire substrates by metal-organic chemical-vapor deposition (MOCVD) with various flow rates of 25, 50, 75 and 100 sccm of bis-(cyclopentadienyl)-magnesium (Cp2Mg) precursor. The structural, electrical and nanomechanical properties of GaN:Mg thin films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM). Hall measurement and nanoindentation techniques, respectively. Results indicated that GaN:Mg films obtained with 25 sccm Cp2Mg possess the highest hole concentration of 3.1 x 10(17) cm(-3). Moreover, the hardness and Young's modulus of GaN:Mg films measured by a Berkovich nanoindenter operated with the continuous contact stiffness measurements (CSM) option showed positive dependence with increasing flow rate of Cp2Mg precursor, presumably due to the solution hardening effect of Mg-doping. (C) 2012 Elsevier B.V. All rights reserved.
InGaN-Based Light-Emitting Diodes Grown on a Micro/Nanoscale Hybrid Patterned Sapphire Substrate
(2016-12-21)
Ke, Wen-Cheng
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Lee, Fang-Wei
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Chiang, Chih-Yung
;
Liang, Zhong-Yi
;
Chen, Wei-Kuo
A hybrid patterned sapphire substrate (hybrid-PSS) was prepared using an anodic aluminum oxide etching mask to transfer nanopatterns onto a conventional patterned sapphire substrate with microscale patterns (bare-PSS). The threading dislocation (TD) suppression of light-emitting diodes (LEDs) grown on a hybrid-PSS (HP-LED) exhibits a smaller reverse leakage current compared with that of LEDs grown on a bare-PSS (BP-LED). The strain-free GaN buffer layer and fully strained InGaN active layer were evidenced by cross-sectional Raman spectra and reciprocal space mapping of the X-ray diffraction intensity for both samples. The calculated piezoelectric fields for both samples are close, implying that the quantum-confined Stark effect was not a dominant mechanism influencing the electroluminescence (EL) peak wavelength under a high injection current. The bandgap shrinkage effect of the InGaN well layer was considered to explain the large red-shifted EL peak wavelength under high injection currents. The estimated LED chip temperatures rise from room temperature to 150 degrees C and 75 degrees C for BP-LED and HP-LED, respectively, at a 600-mA injection current. This smaller temperature rise of the LED chip is attributed to the increased contact area between the sapphire and the LED structural layer because of the embedded nanopattern. Although the chip generates more heat at high injection currents, the accumulated heat can be removed to outside the chip effectively. The high diffuse reflection (DR) rate of hybrid-PSS increases the escape probability of photons, resulting in an increase in the viewing angle of the LEDs from 130 degrees to 145 degrees. The efficiency droop was reduced from 46% to 35%, effects which can be attributed to the elimination of TDs and strain relaxation by embedded nanopatterns. In addition, the light output power of HP-LED at 360-mA injection currents exhibits a similar to 22.3% enhancement, demonstrating that hybrid-PSSs are beneficial to apply in high-power LEDs.
Nanoparticle structure and manufacturing process of multi-wavelength light emitting device
(2007-05-17)
Chen, Wei-Kuo
;
Ke, Wen-Cheng
A structure of multi-wavelength light emitting device comprises multi-stacked active layer structure. Each stacked layer comprises lower energy bandgap well 4 and higher energy bandgap barrier layer 3 wherein at least one stacked layer in the device contains nanoparticles. As a result, the emitting wavelengths of the multi-stacked active layer structure consist parts (or all) of the emitting wavelengths come from the stack layers containing nanoparticles, and parts (or all) of the emitting wavelengths come from the stack layers not containing nanoparticles. In another embodiment, parts (or all) of the emitting wavelengths of the multi-stacked active layer structure can be also used to trigger one or more phosphorescences from the phosphors, thus the emitting wavelengths of such a phosphors converted light emitting device may come partially from the multi-stacked active layer itself and partially (or all) from the phosphors.
Optical properties associated with strain relaxations in thick InGaN epitaxial films
(2014-03-10)
Tsai, Wen-Che
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Hsu, Chia-He
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Fu, Shao-Fu
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Lee, Fang-Wei
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Chen, Chin-Yu
Structural and optical properties of thick InGaN layers with strain and composition inhomogeneities are investigated. High resolution x-ray diffractions (XRD) and reciprocal space mapping (RSM) along an asymmetric axis reveal that the In composition inhomogeneity is accompanied by strain relaxations during the growth of thick InGaN layers. According to the structural analysis, the commonly observed double photoluminescence (PL) peaks have been confirmed to be associated with the strain relaxation in thick InGaN films. Temperature-dependent PL measurements further indicate that the relaxed phase in InGaN films exhibits better emission efficiency than the strained phase. Recombination dynamics reveal that the carrier localization effect is more pronounced in the relaxed phase due to the compositional pulling effect. The correlations between emission efficiency and localization effect in thick InGaN films are discussed. (C)2014 Optical Society of America
Optical Properties of Uncapped InN Nanodots Grown at Various Temperatures
(2009-03-01)
Chen, Ching-Yu
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Lee, Ling
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Tai, Shin-Kai
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Fu, Shao-Fu
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Ke, Wen-Cheng
Photoluminescence (PL) measurements were employed to investigate the emission properties of uncapped InN nanodot samples grown at temperature from 550 to 725 degrees C. Our results indicate that once the In droplets are formed, the optical properties of InN dots deteriorate markedly. As for those droplet-free samples grown at temperatures >= 600 degrees C, good luminescence properties are obtained. The corresponding 20 K PL peak energy is found to be almost constant at approximately 0.77 eV with a slow increase in its linewidth from 71 to 74 meV. A strong temperature-induced energy blue shift at approximately 20 to 280 K is considered to be partially connected to the band-filling effects of thermally stimulated surface electrons. (C) 2009 The Japan Society of Applied Physics
Process for manufacturing self-assembled nanoparticles
(2006-02-09)
Chen, Wei-Kuo
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Lee, Ming-Chih
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Chou, Wu-Ching
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Chen, Wen-Hsiung
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Ke, Wen-Cheng
Process for fabricating self-assembled nanoparticles on buffer layers without mask making and allowing for any degree of lattice mismatch; that is, binary, ternary or quaternary nanoparticles comprising Groups III-V, II-VI or IV-VI. The process includes a first step of applying a buffer layer, a second step of turning on the purge gas to modulate the first reactant to the lower first flow rate, then the second reactant is supplied to the buffer layer to form a metal-rich island on the buffer layer, and a third step of turning on purge gas again to modulate the first reactant to the higher second flow rate onto the buffer layer. On the metal-rich island is formed the nanoparticles of the binary, ternary or quaternary III-V, II-VI and IV-IV semiconductor material. This is then recrystallized under the first reactant flow at high temperature forming high quality nanoparticles.
Recombination dynamics and carrier lifetimes in highly mismatched ZnTeO alloys
(2013-12-23)
Lin, Yan-Cheng
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Tasi, Ming-Jui
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Chou, Wu-Ching
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Chang, Wen-Hao
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Chen, Wei-Kuo
This study investigates the recombination dynamics in highly mismatched ZnTeO alloys using time-resolved photoluminescence (PL) spectroscopy. The large PL energy redshift with increasing O content and the disappearance of the ZnTe emission verify the O-induced conduction band anticrossing effect. The incorporation of O generates electron localization below the E_ conduction subband tail, which provide additional optical transitions and cause complex recombination mechanisms. Photoexcited free electrons in both the E+ and the E- conduction subbands favor rapid relaxation to low energy states. Additionally, temperature-independent long carrier lifetimes (>130.0 ns) that are induced by localized electrons increase with O concentration. (C) 2013 AIP Publishing LLC.
Recombination lifetimes in InN films studied by time-resolved excitation-correlation spectroscopy
(2009-11-01)
Liu, Horng-Chang
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Hsu, Chia-He
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Chou, Wu-Ching
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Chen, Wei-Kuo
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Chang, Wen-Hao
Recombination dynamics in degenerate InN were investigated by means of time-resolved excitation-correlation spectroscopy. The photoluminescence decay times are determined beyond the spectral response and temporal resolution limits of conventional photon-counting detectors. Spectral and temperature dependence of decay times reveal the effects of hole localizations on the recombination mechanisms. At low temperatures, the radiative lifetime tau(r) is insensitive to temperature and significantly longer than that predicted for the radiative band-to-band recombination, indicative of a transition dominated by the free-to-bound recombination without k conservation. Above a certain temperature determined by the electron concentration, we find tau(r)similar to T(3/2), as expected for the band-to-band transition when the k-selection rule holds. We determine a lower limit for the bimolecular recombination coefficient B in InN at 300 K as 5.6x10(-11) cm(3)/s.
Size-dependent strain relaxation in InN islands grown on GaN by metalorganic chemical vapor deposition
(2009-02-09)
Tsai, Wen-Che
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Lin, Feng-Yi
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Ke, Wen-Cheng
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Lu, Shu-Kai
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Cheng, Shun-Jen
We report Raman measurements on InN islands grown on GaN by metalorganic chemical vapor deposition. The Raman frequency of the InN E(2) mode is found to decrease exponentially with the island's aspect ratio, indicating a size dependent strain relaxation during the island formation. Our results suggest that most of the strain at the InN-GaN interface have been released plastically during the initial stage of island formations. After that, the residual strain of only -3.5x10(-3) is further relaxed elastically via surface islanding. The experimental data are in agreement with the strain relaxation predicted from a simplified model analysis as well as three-dimensional finite-element simulations.
Structural and optical properties of In-rich InGaN nanodots grown by metallo-organic chemical vapor deposition
(2007-10-10)
Tsai, Wen-Che
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Lin, Hsuan
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Ke, Wen-Chen
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Chang, Wen-Hao
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Chou, Wu-Ching
The surface morphologies, alloy compositions and emission properties of In- rich InxGa1-xN nanodots ( x >= 0.87) grown by metallo-organic chemical vapor deposition at various growth temperatures (550-750 degrees C) were investigated. We found that the nucleation of InGaN dots was dominated by the surface migration of In adatoms. A higher Ga content can be achieved at lower growth temperatures due to the relatively lower migration ability of Ga adatoms. At higher growth temperatures, the InGaN dots tend to decompose into In- rich islands and a thin Ga-rich layer. These In- rich islands exhibit photoluminescence emission in the near-infrared range. Another visible emission band was also observed for samples grown at higher temperatures. The formation of a thin Ga-rich layer is likely to be responsible for the visible emission.
Structural and optical properties of indium-rich InGaN islands
(2008)
Tsai, Wen-Che
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Lin, Hsuan
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Ke, Wen-Chen
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Chang, Wen-Hao
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Chou, Wu-Ching
Indium-rich InxGa1-xN islands (x >= 0.87) grown by metalor-ganic chemical vapor deposition at growth temperatures ranging from 550-750 degrees C were investigated. With the increasing growth temperature, the InGaN dot size increases while the dot density decreases due to the enhanced surface migration of In/Ga adatoms at elevated temperatures. In addition, the compositiom of InGaN-islands was also found to be varied with the growth temperature. At lower growth temperatures, a higher Ga content can be achieved due to the relatively lower migration ability of Ga adatoms. At higher growth temperatures, the deposited InGaN material tend to decompose into In-rich islands and a thin Ga-rich layer, Photoluminescence investigations show that these In-rich islands exhibit a near-infrared emission band . For samples grown at higher temperatures a visible emission band was also observed. The visible emission band may arise from defect hands introduced during the formation of Ga-rich InGaN layer. (c) 2008 WILEY-VCH Veriag GmbH & Co. KGaA, Weinheim.
Structural and optical properties of InN/GaN nanodots grown by metalorganic chemical vapor deposition
(2008)
Chang, Wen-Hao
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Lee, Lin
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Chen, Ching-Yu
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Tsai, Wen-Che
;
Lin, Hsuan
InN nanodots grown on GaN by metalorganic chemical vapor deposition (MOCVD) using conventional growth mode as well as flow-rate modulation epitaxy (FME) at various growth temperatures (550-730 degrees C) were investigated. We found that different precursor injection schemes together with the effect of growth temperatures greatly influenced the surface morphology of InN dots and their photoluminescence (PL) properties. The best growth efficiency of InN was achieved by FME at around 650 degrees C. The residual carrier concentration and PL efficiency was also be improved when a high growth temperature was used. Our results indicated that InN nanodots can be grown at a temperature even higher than 700 degrees C while maintain their optical quality. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Temperature-dependent decay dynamics in highly mismatched ZnSe1-xTex alloy
(2012-02-13)
Lin, Yan-Cheng
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Jiang, Wei-Shi
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Chou, Wu-Ching
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Chen, Wei-Kuo
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Chang, Wen-Hao
This study investigates the temperature-dependent decay dynamics in highly mismatched ZnSe0.950Te0.050 alloy using photoluminescence (PL) and time-resolved PL spectroscopy. The PL peak energy exhibits a V-shaped dependence on temperature (10-300 K), indicating strong carrier localization. Kohlrausch's stretched exponential law, in which the deduced stretching exponent beta is highly consistent with the V-shaped PL peak shift, closely corresponds to the complex decay curves over a wide temperature range. Additionally, the PL lifetime tau initially increases and then monotonically declines as the temperature increases. These findings agree excellently with the low electron-hole binding energy upon thermal ionization of weakly bound electrons. (C) 2012 American Institute of Physics. [doi:10.1063/1.3687187]
Trap-assisted tunneling in aluminum-doped ZnO/indium oxynitride nanodot interlayer Ohmic contacts on p-GaN
(2015-10-21)
Ke, Wen-Cheng
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Lee, Fang-Wei
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Yang, Cheng-Yi
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Chen, Wei-Kuo
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Huang, Hao-Ping
This study developed an Ohmic contact formation method for a ZnO:Al (AZO) transparent conductive layer on p-GaN films involving the introduction of an indium oxynitride (InON) nanodot interlayer. An antisurfactant pretreatment was used to grow InON nanodots on p-GaN films in a RF magnetron sputtering system. A low specific contact resistance of 1.12 x 10(-4) Omega cm(2) was achieved for a sample annealed at 500 degrees C for 30 s in nitrogen ambient and embedded with an InON nanodot interlayer with a nanodot density of 6.5 x 10(8) cm(-2). By contrast, a sample annealed in oxygen ambient exhibited non-Ohmic behavior. X-ray photoemission spectroscopy results showed that the oxygen vacancy (V-o) in the InON nanodots played a crucial role in carrier transport. The fitting I-V characteristic curves indicated that the hopping mechanism with an activation energy of 31.6 meV and trap site spacing of 1.1 nm dominated the carrier transport in the AZO/InON nanodot/p-GaN sample. Because of the high density of donor-like oxygen vacancy defects at the InON nanodot/p-GaN interface, positive charges from the underlying p-GaN films were absorbed at the interface. This led to positive charge accumulation, creating a narrow depletion layer; therefore, carriers from the AZO layer passed through InON nanodots by hopping transport, and subsequently tunneling through the interface to enter the p-GaN films. Thus, AZO Ohmic contact can be formed on p-GaN films by embedding an InON nanodot interlayer to facilitate trap-assisted tunneling. (C) 2015 AIP Publishing LLC.