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Nanoscience
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Introduction
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It has been predicted that the four most influential industries in the 21st century will be electronics and IT, biomedical, energy, and transportation. It has also been predicted that nanoscience will have an immense impact on all of these industries due to the revolutionary nano materials that will be used within these industries. Carbon nanotubes, discovered in 1991, have incredible mechanical strength, a long quantum coherence range, and are uncompromised in structural symmetry. Potential applications for their use have already emerged in the fields of atomic force microscopy, field emission devices, and nano electronic elements. In the 21st century, nano technology will, in fact, create a new relationship between man and nature. During 2005, the NCHC's Nanoscience Research Team was involved in the following theoretical and computational projects:
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Quantum Many-Body Study of Nano Structures and Their Optoelectric Characteristics
Utilizing the Ab initio approach, our theoretical studies on nanoclusters focused on the quantum many-body effect and their optical and electronic characteristics. We applied theoretical principles to the material design and analysis of carbon nanotube production, seawater desalination, and drug discovery. We also developed the software to numerically simulate it.
Important Areas of Research:
- The Self-Consistent Density Functional Theory (SCDFT)
The new DFT is derived from a cluster expansion by truncating the higher-order correlations in one term kinetic energy. The formulation allows self-consistent calculation of the exchange correlation effect without imposing additional assumptions to generalize the local density approximation. The pair correlation is described as a two-body collision of bound-state electrons that modifies the electron-electron interaction energy as well as the kinetic energy. The theory admits excited states and has no self-interaction energy.
- Analyzing the Chemical Bonding of Nanoclusters
The study of how nanoclusters bond chemically is currently a major field of focus. In particular, the transitioning of helium clusters to superfluids has received much attention recently. In this study, we analyzed the density degeneracy necessary to yield the ground state energy spectrum. We then calculated the excited state energy crucial for the onset of the superfluid transition.
- Investigating the Quantum Transport of Electrons, Atoms, and Molecules within a Nanoscaled Structure
The topics included in this study include the nucleation of carbon nanotubes using nickel as the catalyst, water osmotic pressure through nanotubes, molecular based quantum dots, and fractional conductance of hydrogen molecules.
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Quantum Information Science (QIS)
QIS is a new area of interdisciplinary research. This frontier science is built upon the quantum physics theory and will be realized through the employment of novel technologies in computing, communication, and nanotechnology. Quantum information has brought about a new understanding of quantum physics. It has also had a great impact on the progress of computing and communication. The technologies enabled via quantum information will deliver a wide range of applications in commodity use and, in particular, space adventure.
The study of quantum information began in the 1980s. A few physicists, namely Richard Feynman, postulated that, due to the nature of wave mechanics, certain quantum mechanics- based computing models will surpass traditional tools. A short time later, David Deutsch of Oxford developed the mathematical prototype. Several additional breakthroughs came about in the early 90s as well. Charles Bennett and his collaborators at IBM's Watson Laboratory proposed the Quantum Teleportation and Superdense Coding protocols. Peter Shor at Bell Lab invented a quantum method, now known as Shor's algorithm, that enabled the very quick prime factorization for any large integer and, thus, lead to the crack of the RSA cryptosystem. The success of these algorithms and protocols are mainly attributed to two unique features - quantum parallelism and quantum entanglement. Preliminary realization of these proposals has been achieved in the laboratory and more are expected.
According to the Moore's law, the complexity of integrated circuits is increasing exponentially. Within ten years, the size of integrated circuits will be pushed to the classical limit. Once these limits are passed, the classical dynamics rules obeyed by electrons and photons that carry information within the system will no longer be valid. In such a regime, quantum mechanics will inevitably come into play.
In this study, we focused our attention on the entanglement aspect of quantum physics in order to gain a better understanding of its basics. We are in the process of constructing a mathematical scheme that may help clarify the qualification and quantification of entangled states. We are interested in the application of entangled states as well.
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Simulation of Carbon Nanotube Field Emission Devices
The discovery of carbon nanotubes (CNTs) has been the focus of much attention recently due to their unique physical properties and potential applications. CNTs exhibit excellent field emissions characteristics due to their high aspect ratio and small radii of curvature. A high field emission current density of 10 mA/cm2 and low turn-on electric field of 0.75 ~ 0.8 V/μm have been reported. These specifications are very advantageous for applications such as flat panel displays.
The emission characteristics and electric field electron dynamics of CNTs are crucial in the design of field emission displays (FED). These characteristics are important because they affect the display's brightness and resolution. It is very difficult, though, to study these two processes at the same time due to their divergence in the spatial dimension. The CNT's field emissions are often studied using quantum mechanics. The moving electrons, however, are viewed as classically charged particles and studied under classical mechanics. The Fowler-Nordheim (F-N) equation is used to study the connection between the characteristics of nanoscale field emissions and the macroscopic field emission model (used to study electron dynamics). Using the F-N equation, the study of Carbon Nanotube Field Emission Displays (CNTFED) can be divided into two parts. In the fist part, the workfunction can be obtained from experiments or from quantum mechanical calculations such as the density function theory. The equivalent workfunction is then used in the F-N equation to model the field emission while the classical electrodynamics is used to study the electron's movements.
Triode structures are more desirable in the design of FEDs because they consume a smaller amount of electricity and they have higher light efficiency. Not long ago, a new CNT-based FED structure called the planar-gate was introduced. In this type of structure, the lateral gate electrode generates a transverse electric field that "pulls" electrons from neighboring emitters. It is still not understood, though, how the current density is distributed on the anode plate or how the display's resolution is affected by the bias conditions of the emitter and the gate.
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Nano Engineering
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The primary goal of nanoengineering is to find new applications for nanotechnology at the engineering level. This is different from the field of nanoscience which focuses largely on the basic physical and chemical disciplines. Though the material characteristic length is nanoscale, the size of nano devices is actually in micro- or mini-meter scale. Academic groups that perform fundamental research are mostly interested in the study of nanoscale whereas engineers are primarily interested in researching micro- or mini-meter sized device engineering. Nanoscale research focuses on the transportation of electrons between two atoms and there interaction. Conversely, micro- or mini- engineering research does not need to consider electron effects. Basically, researchers need only consider the atomic force interaction between two atoms.
In the future, nanoengineering will attempt to integrate the current classical molecular dynamics theory-based nanoscale simulation with the micro- or mini- scale finite element-based method simulation so that practical engineering applications can be achieved. This is also called multi-scale computation which extends the computation directly from nanoscale to micro- or mini- meter scale simulation.
Recently, we have developed several nanoscale MD simulations on semiconductor fabrication processes such as ionized physical vapor deposition (IPVD), the dual damascene process, and the etched process. These studies are in the forefront of the next generation semiconductor fabrication process at the nanoscale level. Additionally, we studied the mechanical behaviors of carbon nanotubes and C60s under tensile/compression or impact conditions. This research is helpful in discovering new applications for nanomaterials. Finally, our Tainan Science park business unit is cooperating with the Nano Device Laboratory (NDL) on the project "Comparison Between MD Simulation and Deposition Experimental." The purpose of this project is to compare MD simulations with experimental results. We have completed the MD deposition computation algorithm. In the very near future, it should provide us with high accuracy thin film morphology results.
We have two projects planned for 2006. Both projects will focus on the implementation of parallel computing techniques. In particular, they will focus on the microstructure of alloy at high pressure and the diffusion properties of water molecules in a gold nanotube respectively. These two projects will be a collaborative effort between the NCHC, National Cheng Kung University (NCKU), and National Sun Yat-Sen University (NSYSU).
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Simulation of Nanoscale Semiconductor Devices
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Continuing advances in integrated circuit technology impose new challenges for semiconductor devices. Advances in device scaling as well as new structures and materials have generated great interests in recent years. Computer aided design (CAD) for semiconductors provides a software driven approach to explore new physics. Device characteristics play a crucial role in the development of semiconductors.
The oxide thickness of gate dielectric must be scaled down to less than 1.5 nm in order to be able to span both 100 nm and 70 nm technology nodes. Due to gate leakage currents and reliability problems caused by tunneling phenomena and appearing at even low voltages, these values cannot be obtained in a classical MOS/MOSFET structures with SiO2 as the gate oxide. For this reason, the development of advanced devices like silicon on insulator (SOI), double-gate SOI MOSFET (DG SOI MOSFET), and high-k material insulator devices is very important.
In this research, we focused on the structure and operational characteristics of DG SOI MOSFET and high-k devices. We analyzed the device structure's oxide thickness, doping profile, channel length, silicon film thickness, and high-k materials in an attempt to discover the device design rules. We also studied the classical transport, wave property, and ballistic transport phenomenas in nanoscale devices. An analysis of nanoscale semiconductor device modeling was included in this study as well.
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Fundamental Nanoscale Semiconductor Device R&D Topics:
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- The influence of DG SOI MOSFET profile on current-voltage characteristics (i.e. oxide thickness, doping concentration, channel length, and silicon film thickness).
- The influence of DG SOI MOSFET profile on threshold voltage (i.e. oxide thickness, doping concentration, channel length, and silicon film thickness).
- A numerical investigation of various models such as classical transport models, quantum correction models, and quantum transport models. This study is included in the National Science Council's (NSC) working project under contract number NSC 92-2215-E-492-010. The name of the study is "Distributed Computing of the Transport Models with Quantum Corrections in a Two-Dimensional Nanoscale Double Gate MOSFET."
- Optimal design of DG SOI MOSFET.
- The influence of high-k device profiles on capacitance-voltage characteristics (i.e. oxide thickness and high-k materials).
- The influence of high-k device profiles on gate leakage current (i.e. oxide thickness and high-k materials).
- Discussion of the influence on oxidation with high-k materials using different manufacturing processes.
- Modeling and numerical simulation.
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Results:
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This study is extended from previous studies dealing with classical MOSFETs. Our results are as follows:
- The influence of oxide thickness on current-voltage and gate leakage current for MOSFET devices.
- Semiconductor device simulation and parallel computing studies,
- The influence of DG SOI MOSFET profiles on current-voltage characteristics and gate leakage current (i.e. oxide thickness, doping concentration, channel length, and silicon film thickness), and
- The influence of high-k device profiles on capacitance-voltage characteristics and gate leakage current (i.e. s oxide thickness and high-k materials).
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