Browsing by Author "Wang, Chun-Wen"

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Direct observation of melting behaviors at the nanoscale under electron beam and heat to form hollow nanostructures
(2012)
Huang, Chun-Wei
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Hsin, Cheng-Lun
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Wang, Chun-Wen
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Chu, Fu-Hsuan
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Kao, Chen-Yen
"We report the melting behaviours of ZnO nanowire by heating ZnO-Al2O3 core-shell heterostructures to form Al2O3 nanotubes in an in situ ultrahigh vacuum transmission electron microscope (UHV-TEM). When the ZnO-Al2O3 core-shell nanowire heterostructures were annealed at 600 degrees C under electron irradiation, the amorphous Al2O3 shell became single crystalline and then the ZnO core melted. The average vanishing rate of the ZnO core was measured to be 4.2 nm s(-1). The thickness of the Al2O3 nanotubes can be precisely controlled by the deposition process. Additionally, the inner geometry of nanotubes can be defined by the initial ZnO core. The result shows a promising method to obtain the biocompatible Al2O3 nanotubes, which may be applied in drug delivery, biochemistry and resistive switching random access memory (ReRAM)."
Growth and properties of single-crystalline Ge nanowires and germanide/Ge nano-heterostructures
(2012)
Tsai, Cheng-Yu
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Yu, Shih-Ying
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Hsin, Cheng-Lun
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Huang, Chun-Wei
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Wang, Chun-Wen
Single-crystalline Ge nanowires have been synthesized on Au-coated Si substrates through a thermal evaporation, condensation method and vapor-liquid-solidmechanism. The [111] growth direction of the Ge nanowires was analyzed using HRTEM and fast Fourier transform diffraction patterns. Global back-gated Ge nanowire field-effect transistors (FETs) on the Si3N4 dielectrics were fabricated and studied, showing p-type behavior and a field effect hole mobility of 44.3 cm(2) V-1 s(-1). The Ge channel length could be well controlled through the annealing process. After a rapid thermal annealing (RTA) process, Ni2Ge/Ge/Ni2Ge nano-heterostructures were formed. The electrical transport properties were effectively improved by the heterojunction rather than the metal contact. The epitaxial relationship between Ge and orthorhombic Ni2Ge was Ge [110]//Ni2Ge[110] and Ge(-11-1)//Ni2Ge(1-1-2). From electrical transport properties, the measured resistivity of the Ge nanowires was much lower than intrinsic bulk Ge material. A room temperature photoluminescence spectrum of the Ge nanowires possessed a broad blue emission with a peak at 462 nm in wavelength, which was attributed to the oxide-related defect states. Due to the existence of the defects, a Ge nanowire FET was able to detect visible light and serve as a nanowire photodetector.
High-yield synthesis of ZnO nanowire arrays and their opto-electrical properties
(2012)
Kao, Chen-Yen
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Hsin, Cheng-Lun
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Huang, Chun-Wei
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Yu, Shih-Ying
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Wang, Chun-Wen
In this article, ZnO nanostructures were synthesized via the hydrothermal method which used ZnCl2 and HMTA mixed solution as the precursor. A multistep growth was adopted to improve the growth restriction of a closed system, not only the length but also the aspect ratio were increased with steps of growth, and the shape of nanorods maintained integrity. Furthermore, photoluminescence spectra which have the near-band-edge-emission (similar to 3.37 eV) and defect-related emission show the optical properties of ZnO nanostructures. The defect-related emission intensity was greatly enhanced with the increasing surface area of ZnO nanowires. The level of the OH group was attributed to the yellow-light emission (similar to 580 nm) and the red-shift phenomenon. In addition, we fabricated two types of ultraviolet photodetectors: a single nanowire device and a nanowire-array device, operating at a low bias (less than 5 mV). With the lower energy consumption and the weaker persistent photoconductive effect, our ultraviolet photodetectors have better performance, exhibiting a short response time and higher sensitivity.
Improved Performance of ZnO-Based Resistive Memory by Internal Diffusion of Ag Atoms
(2012-08-01)
Peng, Chung-Nan
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Wang, Chun-Wen
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Huang, Jian-Shiou
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Chang, Wen-Yuan
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Wu, Wen-Wei
An Ag/ZnO/Pt memory device, which has much better resistive switching behaviour than Pt/ZnO/Pt device was demonstrated. The detailed resistive mechanisms for the Pt/ZnO/Pt and the Ag/ZnO/Pt systems are proposed and investigated. Microstructures are observed by transmission electron microscope (TEM), indicating that the formation of conducting path for both systems is different. For the Pt/ZnO/Pt device, the conductive filament path is constructed by the oxygen vacancies from top to bottom electrodes under a larger enough bias at a forming process. For the Ag/ZnO/Pt device, the filament path was grown by oxygen vacancies combined with an internal diffusion of Ag atoms under a large bias and can provide the lowest energy barrier for electrons transported between two electrodes during set and reset processes, which reduces formation of other conducting paths after each switching. Accordingly, the stable switching performance of the Ag/ZnO/Pt device can be achieved over 100 cycles even the thickness of ZnO film <25 nm.
Kinetic Competition Model and Size-Dependent Phase Selection in 1-D Nanostructures
(2012-06-01)
Chen, Yu
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Lin, Yung-Chen
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Huang, Chun-Wei
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Wang, Chun-Wen
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Chen, Lih-Juann
The first phase selection and the phase formation sequence between metal and silicon (Si) couples are indispensably significant to microelectronics. With increasing scaling of device dimension to nano regime, established thermodynamic and kinetic models in bulk and thin film fail to apply in 1-D nanostructures. Herein, we present an unique size-dependent first phase formation sequence in 1-D nanostructures, with Ni-Si as the model system. Interfacial-limited phase which forms the last in thin film, NiSi2, appears as the dominant first phase at 300-800 degrees C due to the elimination of continuous grain boundaries in 1-D suicides. On the other hand, theta-Ni2Si, the most competitive diffusion-limited phase takes over NiSi2 and wins out as the first phase in small diameter nanowires at 800 degrees C. Kinetic parameters extracted from in situ transmission electron microscope studies and a modified kinetic growth competition model quantitatively explain this observation. An estimated critical diameter from the model agrees reasonably well with observations.
Optimization of the nanotwin-induced zigzag surface of copper by electromigration
(2016)
Chen, Hsin-Ping
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Huang, Chun-Wei
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Wang, Chun-Wen
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Wu, Wen-Wei
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Liao, Chien-Neng
By adding nanotwins to Cu, the surface electromigration (EM) slows down. The atomic mobility of the surface step-edges is retarded by the triple points where a twin meets a free surface to form a zigzag-type surface. We observed that EM can alter the zigzag surface structure to optimize the reduction of EM, according to Le Chatelier\'s principle. Statistically, the optimal alternation is to change an arbitrary (111)/(hkl) zigzag pair to a pair having a very low index (hkl) plane, especially the (200) plane. Using in situ ultrahigh vacuum and high-resolution transmission electron microscopy, we examined the effects of different zigzag surfaces on the rate of EM. The calculated rate of surface EM can be decreased by a factor of ten.
Real Time Observation of the Formation of Hollow Nanostructures through Solid State Reactions
(2014-05-06)
Lai, Huang-Yen
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Huang, Chun-Wei
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Chiu, Chung-Hua
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Wang, Chun-Wen
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Chen, Jui-Yuan
We demonstrate the formation of hollow nickel germanide nanostructures of Ni-Ge core-shell nanoparticles by solid state reactions. The structural evolutions of nickel germanide hollow nanostructures have been investigated in real-time ultrahigh vacuum transmission electron microscopy (UHV-TEM). Annealed above 450 degrees C, the nonequilibrium interdiffusion of core and shell species occurred at the interface; thus, Ni germanide hollow nanostructures were formed by solid state reactions involving the Kirkendall effect. In addition, the different hollow nanostructures formed from different core diameters of Ni-Ge core-shell nanoparticles have been studied. Also, we propose the mechanism with effects of the size and annealing duration on the solid state reactions based on the Kirkendall effect.
Resistive switching of Au/ZnO/Au resistive memory: an in situ observation of conductive bridge formation
(2012-10-08)
Peng, Chung-Nan
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Wang, Chun-Wen
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Chan, Tsung-Cheng
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Chang, Wen-Yuan
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Wang, Yi-Chung
A special chip for direct and real-time observation of resistive changes, including set and reset processes based on Au/ZnO/Au system inside a transmission electron microscope (TEM), was designed. A clear conducting bridge associated with the migration of Au nanoparticles (NPs) inside a defective ZnO film from anode to cathode could be clearly observed by taking a series of TEM images, enabling a dynamic observation of switching behaviors. A discontinuous region (broken region) nearby the cathode after reset process was observed, which limits the flow of current, thus a high resistance state, while it will be reconnected to switch the device from high to low resistance states through the migration of Au NPs after set process. Interestingly, the formed morphology of the conducting bridge, which is different from the typical formation of a conducting bridge, was observed. The difference can be attributed to the different diffusivities of cations transported inside the dielectric layer, thereby significantly influencing the morphology of the conducting path. The current TEM technique is quite unique and informative, which can be used to elucidate the dynamic processes in other devices in the future.
Single-crystalline Ge nanowires and Cu3Ge/Ge nano-heterostructures
(2012)
Hsu, Shan-Chun
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Hsin, Cheng-Lun
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Huang, Chun-Wei
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Yu, Shih-Ying
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Wang, Chun-Wen
Single-crystalline germanium nanowires were synthesized via vapor-liquid-solid mechanism. The characteristics of the Ge nanowires were investigated by a transmission electron microscope to identify the [111] growth direction. The Ge nanowire-based field-effect-transistors on Si3N4 dielectrics were fabricated, showing a p-type semiconducting behavior with hole mobility of 47.03 cm(2) V-1 s(-1). The formation of Cu3Ge/Ge/Cu3Ge nanoheterostructures was demonstrated with the reaction between copper contacts and Ge nanowires by rapid thermal annealing. The diameter-dependent electrical transport property of Ge nanowires indicates that with diameters of more than 80 nm, the resistivity of Ge nanowires decreased with diameter decrease, while with diameters of less than 80 nm, it increased. With multiple annealing processes, the channel length of the Ge nanowire transistors can be successfully controlled. From electrical measurements of each annealing step, the electrical transport property was significantly improved by sequential formation of Cu3Ge contacts. The gradual formation of the germanide structure reduces Fermi level pinning effect and increases the Ohmic behavior of electrical transportation.
The Influence of Surface Oxide on the Growth of Metal/Semiconductor Nanowires
(2011-07-01)
Lu, Kuo-Chang
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Wu, Wen-Wei
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Ouyang, Hao
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Lin, Yung-Chen
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Huang, Yu
We report the critical effects of oxide on the growth of nanostructures through silicide formation. Under an in situ ultrahigh vacuum transmission electron microscope, it is observed from the conversion of Si nanowires into the metallic PtSi grains epitaxially through controlled reactions between lithographically defined Pt pads and Si nanowires. With oxide, instead of contact area, single crystal PtSi grains start forming either near the center between two adjacent pads or from the ends of Si nanowires, resulting in the heterostructure formation of Si/PtSi/Si. Without oxide, transformation from Si into PtSi begins at the contact area between them, resulting in the heterostructure formation of PtSi/Si/PtSi. The nanowire heterostructures have an atomically sharp interface with epitaxial relationships of Si(20-2)//PtSi(10-1) and Si[111]//PtSi[111]. Additionally, it has been observed that; the existence of oxide significantly affects not only the growth position but also the growth behavior and the growth rate by two orders of magnitude. Molecular dynamics simulations have been performed to support our experimental results and the proposed growth mechanisms. In addition to fundamental science, the significance of the study matters for future processing techniques in nanotechnology and related applications as well.
Thermal Stability and Performance of NbSiTaTiZr High-Entropy Alloy Barrier for Copper Metallization
(2011)
Tsai, Ming-Hung
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Wang, Chun-Wen
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Tsai, Che-Wei
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Shen, Wan-Jui
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Yeh, Jien-Wei
Development of better diffusion barriers for Cu metallization is one of the key issues for the microelectronics industry. Although metallic diffusion barriers offer many advantages, their application is hindered due to their inferior thermal stability relative to ceramic barriers. Here we report on a metallic diffusion barrier, NbSiTaTiZr, which shows thermal stability comparable to ceramic barriers. The outstanding performance of NbSiTaTiZr is due to its better structural and chemical stability at high temperatures. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.056111jes] All rights reserved.
Well-aligned ZnO nanowires with excellent field emission and photocatalytic properties
(2012)
Chu, Fu-Hsuan
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Huang, Chun-Wei
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Hsin, Cheng-Lun
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Wang, Chun-Wen
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Yu, Shih-Ying
Well-aligned ZnO nanowires (NWs) were successfully synthesized on Si(100) by the process of carbothermal reduction and vapor-liquid-solid method. Scanning electron microscopy and transmission electron microscopy results confirmed that ZnO NWs were single crystalline wurtzite structures and grew along the [0001] direction. The influences of substrate temperature and total pressure on the growth were discussed. The well-aligned ZnO NWs show good field emission properties, and the emitter constructed of pencil-like ZnO NWs exhibited a low turn-on field (3.82 V mu m(-1)) and a high field enhancement factor (beta 2303). Finally, we demonstrated that the as-prepared ZnO NWs with small diameter on the substrate have good photocatalytic activity toward degradation of methylene blue. Using ZnO NWs with Au nanoparticles (NPs) would decrease the recombination rate of hole-electron pairs due to the great shift of the Fermi level to the conduction band. Hence, adding Au NPs was a promising method to enhance the photocatalytic performance of ZnO NWs. It is significant that photocatalyst fabricated by ZnO NWs can apply to the degradation of organic pollution, and solve the environmental issues.