Browsing by Author "Ke, Wen-Cheng"

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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
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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.
Effects of growth temperature on InN/GaN nanodots grown by metal organic chemical vapor deposition
(2008-05-15)
Chang, Wen-Hao
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Ke, Wen-Cheng
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Yu, Shu-Hung
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Lee, Lin
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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.
Improved light reflectance and thermal stability of Ag-based ohmic contacts on p-type GaN with La additive
(2013-05-01)
Chen, I-Chen
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Cheng, Bo-Yuan
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Ke, Wen-Cheng
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Kuo, Cheng-Huang
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Chang, Li-Chuan
We investigated the effect of La additive on the improvement of light reflectance and thermal stability of Ag contacts on p-GaN. A high reflectance of over 90% at 460 nm wavelength and low specific contact resistivity of 5.5 x 10(-5) Omega cm(2) were obtained from La-containing Ag contacts annealed at 300 degrees C for 1 min, which also show better thermal stability than Ag contacts after annealing in air ambient. The experimental results reveal that the addition of La could effectively slow down Ag migration in < 1 1 1 > direction during annealing, and thus suppress the Ag agglomeration at elevated temperature, leading to a good ohmic contact with improved high reflectance and thermal stability. (C) 2013 Elsevier Ltd. All rights reserved.
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
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Cheng, Chun-Hong
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Liao, Bo-Wei
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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.
Influence of the GaN capping thickness on the strain and photoluminescence properties of InN/GaN quantum dots
(2013-02-01)
Ke, Wen-Cheng
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Wu, Yue-Han
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Houng, Wei-Chung
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Wei, Chih-An
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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.
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
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Liang, Zhong-Yi
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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.
InGaN-based light-emitting diodes grown on various aspect ratios of concave nanopattern sapphire substrate
(2018-10-31)
Ke, Wen-Cheng
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Chiang, Chih-Yung
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Son, Widi
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Lee, Fang-Wei
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電子物理學系
This study presented the optoelectronic properties of InGaN-based light-emitting diodes (LEDs) grown on nanoscale-patterned sapphire substrates (NPSSs) with various aspect ratio (AR) concave nanopatterns. Concave nanopatterns with low ARs of similar to 0.5 (LAR-NPSS) and high ARs of 2, 3 and 4 (HAR-NPSS) were achieved using an anodic aluminum oxide template as the dry etching mask. The diffuse transmission (DT) rate of the incident light wavelength at 450 nm increased linearly, from 2.2% for LAR-NPSS to 12.7% for HAR-NPSS with an AR of 4. However, the diffuse reflection (DR) rate for HAR-NPSS did not increase linearly with the increasing AR values. The limit of the DR rate for HAR-NPSS could be attributed to the guiding effect when the incident light entered into the high AR concave nanopattern; most of the incident light could not be reflected (returned to the air), therefore there was no significant contribution to the DR rate. The luminous intensities of LEDs grown on bare sapphire (B-LED), HAR-NPSS (HAR-LED) and LAR-NPSS (LAR-LED) were 39, 38 and 74 mcd, respectively. Interestingly, the lowest brightness of any LED was the HAR-LED, which had higher DT and DR rates. This significant discrepancy could be attributed to a large number of voids occurring at the initial growth stage of the LED epitaxial layer on the HAR-NPSS that caused the emitted photons to remain trapped inside the LED. In addition, increasing the LED epilayer coverage area (i.e. 1.6-fold) with the inclined facets of low AR concave nanopatterns effectively scattered the emitted photons, leading to an increased light emitting intensity of the LED. The experimental results indicated that the specific AR concave nanopattern of an NPSS could increase the LED epitaxial layer coverage area and prevent void formation, which plays a crucial role in achieving high-brightness in InGaN-based LEDs.
Mechanical Characteristics of Mg-Doped GaN Thin Films by Nanoindentation
(2012-06-01)
Jian, Sheng-Rui
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Ke, Wen-Cheng
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Juang, Jenh-Yih
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電子物理學系
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Department of Electrophysics
"The nanomechanical characteristics of Mg-doped GaN (GaN:Mg) thin films deposited on sapphire substrates were investigated by nanoindentation evaluation. The concentration of Mg in the GaN:Mg thin films was controlled by varying the flow rate of the bis (cyclopentadienyl) magnesium (Cp2Mg) precursor during the metal-organic chemical vapor deposition (MOCVD) process. Apparent discontinuity in the load-displacement curve (the so-called ""pop-in"" event) featuring the homogeneous nucleation of dislocations was observed during the loading cycle of nanoindentation tests. Moreover, the loading at which the pop-in takes place appears to increase with increasing Mg-doping. The hardness and Young's modulus of the GaN:Mg thin films obtained by operating the Berkovich nanoindenter with the continuous contact stiffness measurements (CSM) mode also respectively increased from 20.4 GPa and 317.6 GPa to 26.2 GPa and 377.5 GPa for films obtained by increasing the flow rate of Cp2Mg from 25 to 100 sccm, corresponding to Mg concentration of 4.54 x 10(18) atom/cm(3) to 1.14 x 10(20) atom/cm(3) as revealed by secondary ion mass spectroscopy."
Nanoparticle structure and manufacturing process of multi-wavelength light emitting device
(2007-05-17)
Chen, Wei-Kuo
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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 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.
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.
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.