Electron Emission from Microwave-Plasma Chemically Vapor Deposited Carbon Nanotubes
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ELECTRON EMISSION FROM MICROWAVE-PLASMA CHEMICALLY VAPOR DEPOSITED CARBON NANOTUBES YONHUA TZENG*, CHAO LIU*, CALVIN CUTSHAW*, AND ZHENG CHEN** * Alabama Microelectronics Science and Technology Center, Department of Electrical and Computer Engineering, Auburn University, AL 36849 ** Space Power Institute, Auburn University, AL 36849 ABSTRACT A microwave plasma CVD reactor was used for the deposition of carbon nanotubes on substrates. Hydrocarbon or oxyhydrocarbon mixtures were used as the carbon source. Hot electrons in the microwave plasma at temperatures exceeding 10,000C provided a means of dissociating the vapor or gas feedstock, heating the substrate, and allowing gas species to react in the gas phase as well as on the surface of the substrate leading to the deposition of desired carbon coatings. A high vacuum chamber was used to characterize the electron emission properties of these carbon nanotube coatings using a one-millimeter diameter tungsten rod with a hemispherical tip as the anode while the carbon nanotube coatings served as the cathode. The current-voltage characteristics of the carbon nanotube coatings were measured and used for calculating the electric field at which electron emission turned on as well as calculating the field enhancement factor of the carbon nanotubes. Field emission of electrons from carbon nanotubes starting from an electric field lower than 1 volt per micrometer has been achieved. INTRODUCTION Micro-engineering in the nano-meter scale using carbon atoms as the building block has resulted in amazingly excellent characteristics of cabon-based structures that finds broad applications ranging from microelectronics to cutting tools. One of these amazing carbon materials is CVD diamond that has been intensively studied in the past fifteen years or so. More recently, the science and practical applications of carbon nanotubes became popular because of its equally great commercial and technological potentials. Carbon, when properly arranged in a 3-dimensional order, forms the hardest materials on earth, diamond. In addition to being an attractive gemstone, diamond is the champion in almost every aspect of materials properties. Its thermal conductivity is five times better than copper at room temperature making it an excellent material as the heat spreader for high power density integrated circuits. Its bandgap energy is higher than SiC and most of compound semiconductors making it desirable for fabricating high power density electronic devices or sensors and actuators for operation in high-temperature and radiation hostile environments. Its super hardness and chemical inertness make it an economic material for cutting tools or simply used as protective coatings. By building carbon structures in the nano-meter scale, it is possible to make carbon tubes that are as small as a few nanometers in diameters and are formed by layers of graphite-like carbon structures. When these carbon nano-tubes are coated on a smooth or porous substrate with a conductive surface layer, they emit electrons with
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