Browsing by Subject "Dielectric"
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Item Dielectric-graphene integration and electron transport in graphene hybrid structures(2015-05) Fallahazad, Babak; Tutuc, Emanuel, 1974-; Banerjee, Sanjay K.; MacDonald, Allan H.; Register, Leonard F.; Bank, SethDielectrics have been an integral part of the electron devices and will likely resume playing a significant role in the future of nanoelectronics. An important step in assessing graphene potential as an alternative channel material for future electron devices is to benchmark its transport characteristics when integrated with dielectrics. Using back-gated and dual gated graphene field-effect transistors with top high-k metal-oxide dielectric, we study the dielectric thickness dependence of the carrier mobility. We show the carrier mobility decreases after deposition of metal-oxide dielectrics by atomic layer deposition (ALD) thanks to the Coulomb scattering by charged point defects in the dielectric. We investigate a novel method for the ALD of metal-oxide dielectrics on graphene, using an ultrathin nucleation layer that enables the realization of graphene field-effect transistors with aggressively scaled gate dielectric thickness. We show the nucleation layer significantly affects the quality of the subsequently deposited dielectric. In addition, we study transport characteristics of double layer systems. We demonstrate heterostructures consisting of two rotationally aligned bilayer graphene with an ultra-thin hexagonal boron nitride dielectric in between fabricated using advanced layer-by-layer transfer as well as layer pickup techniques. We show that double bilayer graphene devices possess negative differential resistance and resonant tunneling in their interlayer current-voltage characteristics in a wide range of temperatures. We show the resonant tunneling occurs either when the charge neutrality points of the two bilayer graphene are energetically aligned or when the lower conduction sub-band of one layer is aligned with the upper conduction sub-band of the opposite layer. Finally, we study the Raman spectra and the magneto-transport characteristics of A-B stacked and rotationally misaligned bilayer graphene deposited by chemical-vapor-deposition (CVD) on Cu. We show that the quantum Hall states (QHSs) sequence of the CVD grown A-B stacked bilayer graphene is consistent with that of natural bilayer graphene, while the sequence of the QHSs in the CVD grown rotationally misaligned bilayer graphene is a superposition of monolayer graphene QHSs. From the magnetotransport measurements in rotationally misaligned CVD-grown bilayer we determine the layer densities and the interlayer capacitance.Item Plasma damaging process of porous ultra-low-k dielectrics and dielectric repair(2012-08) Huang, Huai, Ph. D.; Ho, P. S.; Tsoi, Maxim; Shih, Chih-Kang; Huang, Rui; Yao, ZhenThe Ultra-low-k material is required to reduce the RC time delay in the integrated circuits. However, the integration of the porous low-k material into the on-chip interconnects was impeded by the plasma induced damage during etching and photoresist stripping processes. This dissertation aims to study the mechanism of plasma damage to porous ultra-low-k dielectrics with the objective to minimize the damage and to develop methods and processes to restore the low-k dielectric after the plasma damage. First, the plasma etching induced surface roughening was studied on blanket porous SiCOH films in the fluorocarbon based plasma. Substantial surface roughening was found in the low polymerization region, where the surface roughening process was initiated by the unevenly distribution of surface fluorocarbon polymers in the pore structure and enhanced by ion induced surface densification. With oxygen addition, the surface densification layer increased the radial diffusion rate difference between the top and the bottom of the pits, resulting in further increase of the surface roughness. The best process optimization was found at a "threshold point" where the surface polymerization level is just high enough to suppress the roughness initiation. The second part of this dissertation investigates the mechanism of the oxygen plasma damaging process. The roles of plasma constituents (i.e. ions, radicals and photons with different wavelengths) were differentiated by an on-wafer filter system. Oxygen radical was identified as the most critical and its damage effect was enhanced by photons with wavelength smaller than 185nm. The oxygen radical kinetics in the porous structure of low-k, including diffusion, reaction and recombination, was described analytically with a plasma altered layer model and then simulated with a Monte Carlo computational method, which give guidelines to minimize the damage. The analytical model of oxygen radical kinetic process is also used to investigate the oxygen plasma damage to patterned low-k structure, which is confirmed by experiments. Finally, the dielectric recovery was studied using silylation and UV broadband thermal treatment, both individually and in combination. After both vapor and supercritical CO₂ silylation, surface carbon and hydrophobicity were partially recovered. However, the recovery effect was limited to the surface. In comparison, UV treatment can effectively remove water from the bulk of the damaged film and consolidate the silanol bonds with the help of thermal activation. The combination of UV and silylation treatments is more effectively for dielectric recovery than UV or silylation alone. The "UV first" treatment provided a better recovery in sequential processes. Under the same conditions, simultaneous treatments by silylation and UV irradiation achieved better bulk and surface recovery than the sequential process.Item Preliminary non-destructive assessment of moisture content, hydration and dielectric properties of Portland cement concrete(Texas A&M University, 2007-04-25) Avelar Lezama, IvanMoisture availability is a focal point in the structural development of young concrete. Under low humidity and hot weather conditions, concrete loses moisture rapidly as it hardens, and it is very difficult, if not impossible, to minimize this loss even though proper curing procedures are used. Early losses in moisture content jumpstart premature surface self-dissecation, increase surface paste porosity, prevent concrete from achieving the mechanical properties for which it was originally designed, and facilitate the development of surface distresses such as spalling. Curing effectiveness and structural assessment of young concrete is generally done through conventional destructive or invasive testing. However, there is no fully established non-destructive testing protocol to assess moisture content and its effects on concrete properties quantitatively in an on-site, fast, and non-invasive way. The possibility and feasibility of establishing a testing protocol with such attributes is explored. Previous research on pavement bases has used dielectric measurements to relate moisture content to their structural performance. Due to the high dielectric value of water as compared to any other material used in construction, it is possible to relate high volumetric water content to high dielectric readings. In this study, compressive strength tests combined with dielectric and mass measurements are used to investigate how dielectric properties change with hydration. The results of this study suggest that it may be possible to approximate the volumetric moisture content in concrete by measuring the dielectric value of concrete as it hardens.Item Spectroscopic studies of boron carbo-nitride(2010-12) Ahearn, Wesley James; Ekerdt, John G.; Hwang, Gyeong S.BCxNy films were characterized as a potential pore sealing layer for low κ dielectrics. The changes in chemical bonding were studied as a function of growth temperature to aid in understanding the variation in electrical performance of these films. Thermal chemical vapor deposition of BCxNy using dimethylamine borane and ethylene were deposited on porous methylsilsesquioxane substrates at 335 °C and 1 Torr. BCxNy was able to seal the porous interface with a thickness of 3.9 nm for both blanket and patterned substrates. BCxNy films deposited over a temperature range of 300-400 °C with dimethylamine borane and either ethylene or ammonia coreactant gas were characterized. Films deposited with ethylene became more concentrated in B at the expense of C with increasing temperature. These films favored C-B intermixing over C-C and B-B bonding at higher temperature. H was detected in the form of B-H and C-H bonds. Films deposited with ammonia became concentrated in N at the expense of B, and favored B-N viii bonding at higher temperatures. H was found in the films as B-H, C-H, and N-H bonds. The amount of H in the films decreased with increasing growth temperature for both ethylene and ammonia coreacted films. The valence band offset of C-rich films increased from 0.17 ± 0.22 eV to 0.32 ± 0.22 eV when deposited at 300°C and 400 °C. For the Nrich films, the valence band offset shifted from 0.26 ± 0.28 at 300 °C to -0.15 ± 0.24 eV at the same deposition conditions. High temperature annealing from 400-800 °C in forming gas caused all BCxNy films to decrease in thickness up to 30%. At the same time, the index of refraction increased, and likely, the dielectric constant. X-ray photoelectron spectroscopy revealed little change in the constituent bonding, suggesting that volatile –H containing species were removed during the annealing process.Item The impact of field enhancements and charge injection on the pulsed breakdown strength of water(2006-05) Wetz, David A.; Mankowski, John J.; Kristiansen, Magne; Dickens, James C.In any high voltage pulsed power system there is a need for a dielectric material to serve as a charge storage medium, switching medium, or insulator. Water, with its high dielectric constant, εr = 81, and favorable physical properties is an ideal candidate for use in compact pulsed power systems. Several research efforts have been conducted over the last several decades to investigate possible ways of increasing water’s dielectric strength [Joshi, Kun]. In the research documented here, experiments have been conducted in order gain further insight into the mechanisms that initiate the electrical breakdown of water. With the application of a large enough pulsed electric field, any metal conductor will begin to emit electrons from its surface, especially from field enhancements on the electrode surface, into the dielectric region. This emission initiates the breakdown of water, though various theories of exactly what happens in the water prior to breakdown have been developed. Some suggest that the breakdown event is purely electronic where the emission enters the gap until a conductive channel is formed [1]. Others have suggested that the emission causes rapid heating within the water causing it to vaporize and a bubble to form [2,3,4]. Neither theory has been conclusively confirmed as the primary mechanism. Several factors have been found to either increase or decrease the electric holdoff strength of water including the electrode surface roughness, the electrode material, the electrode surface area, and the water conductivity. Experiments have been conducted to test the effect each of these factors has on water’s dielectric strength and in each experiment, a water gap was tested under pulsed conditions with pulse widths of roughly 2 μs. Peak electric fields over 1 MV/cm and peak currents over 3.5 kA have been recorded across the gap. In all of the tests, electrodes machined with a Bruce profile were used on both the anode and cathode sides of the gap. Random and known surface roughness patterns were applied to the electrodes through mechanical sanding and etching processes. Surface roughnesses ranging from 0.26 ìm to 1.96 ìm and electrode surface areas ranging from 0.44 cm2 to 75 cm2 were tested. Electrodes constructed of various materials including Aluminum, Molybdenum, Copper, Tungsten, Nickel, Stainless Steel, and Zinc Oxide, all of which had a constant surface area of 5 cm2, were also tested. The conductivity of the water was varied from 1 µS/cm to 38.5 µS/cm. Additionally, shadowgraph images of a point plane geometry were taken to further understand the breakdown processes that occur.Item The impact of field enhancements and charge injection on the pulsed breakdown strength of water(Texas Tech University, 2006-05) Wetz, David A.; Mankowski, John J.; Dickens, James C.; Kristiansen, MagneIn any high voltage pulsed power system there is a need for a dielectric material to serve as a charge storage medium, switching medium, or insulator. Water, with its high dielectric constant, εr = 81, and favorable physical properties is an ideal candidate for use in compact pulsed power systems. Several research efforts have been conducted over the last several decades to investigate possible ways of increasing water’s dielectric strength [Joshi, Kun]. In the research documented here, experiments have been conducted in order gain further insight into the mechanisms that initiate the electrical breakdown of water. With the application of a large enough pulsed electric field, any metal conductor will begin to emit electrons from its surface, especially from field enhancements on the electrode surface, into the dielectric region. This emission initiates the breakdown of water, though various theories of exactly what happens in the water prior to breakdown have been developed. Some suggest that the breakdown event is purely electronic where the emission enters the gap until a conductive channel is formed [1]. Others have suggested that the emission causes rapid heating within the water causing it to vaporize and a bubble to form [2,3,4]. Neither theory has been conclusively confirmed as the primary mechanism. Several factors have been found to either increase or decrease the electric holdoff strength of water including the electrode surface roughness, the electrode material, the electrode surface area, and the water conductivity. Experiments have been conducted to test the effect each of these factors has on water’s dielectric strength and in each experiment, a water gap was tested under pulsed conditions with pulse widths of roughly 2 μs. Peak electric fields over 1 MV/cm and peak currents over 3.5 kA have been recorded across the gap. In all of the tests, electrodes machined with a Bruce profile were used on both the anode and cathode sides of the gap. Random and known surface roughness patterns were applied to the electrodes through mechanical sanding and etching processes. Surface roughnesses ranging from 0.26 ìm to 1.96 ìm and electrode surface areas ranging from 0.44 cm2 to 75 cm2 were tested. Electrodes constructed of various materials including Aluminum, Molybdenum, Copper, Tungsten, Nickel, Stainless Steel, and Zinc Oxide, all of which had a constant surface area of 5 cm2, were also tested. The conductivity of the water was varied from 1 µS/cm to 38.5 µS/cm. Additionally, shadowgraph images of a point plane geometry were taken to further understand the breakdown processes that occur.