Browsing by Author "Gregory N. Parsons, Committee Member"

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    Circuit and Integration Technologies for Molecular Electronics
    (2005-06-07) Nackashi, David Peter; John F. Muth, Committee Member; Veena Misra, Committee Member; Paul D. Franzon, Committee Chair; Gregory N. Parsons, Committee Member
    Methods for fabricating a 2D array of gold nanoparticles were investigated for the purpose of creating a cross-linked molecular network. A controllable process for quickly and easily depositing and patterning regions of gold nanoparticles was developed. This process involves first patterning gold electrodes used for electrical measurement on the wafers. Regions are then defined in photoresist where the dense gold nanoparticles are desired. Finally, the nanoparticles are deposited using a short evaporation, resulting in island formation through the Volmer-Weber growth mechanism. The resist is then stripped in a process known as liftoff, and the result is a wafer-scale substrate with well-defined regions for molecular interconnect. Before assembly, these structures conduct less than 110pA of current at submicron electrode gap distances, and often less than 20pA. As determined from SEM image analysis, it is possible to quickly and easily deposit and pattern regions on silicon dioxide containing over 4,100 per um2, each with an average area of approximately 80nm2. The number of particles, average area and fill density can be controlled to allow for a number of applications and at a variety of scales. The smaller, more numerous particles integrate into sub-500nm gaps, and the larger, meandering lines integrate into micron-scale structures.
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    Computer Simulation of Chemical Reactions in Porous Materials
    (2002-08-21) Turner, Christoffer Heath; Gregory N. Parsons, Committee Member; Keith E. Gubbins, Committee Chair; Carol K. Hall, Committee Member; George W. Roberts, Committee Member
    Understanding reactions in nanoporous materials from a purely experimental perspective is a difficult task. Measuring the chemical composition of a reacting system within a catalytic material is usually only accomplished through indirect methods, and it is usually impossible to distinguish between true chemical equilibrium and metastable states. In addition, measuring molecular orientation or distribution profiles within porous systems is not easily accomplished. However, molecular simulation techniques are well-suited to these challenges. With appropriate simulation techniques and realistic molecular models, it is possible to validate the dominant physical and chemical forces controlling nanoscale reactivity. Novel nanostructured catalysts and supports can be designed, optimized, and tested using high-performance computing and advanced modeling techniques in order to guide the search for next-generation catalysts - setting new targets for the materials synthesis community. We have simulated the conversion of several different equilibrium-limited reactions within microporous carbons and we find that the pore size, pore geometry, and surface chemistry are important factors for determining the reaction yield. The equilibrium-limited reactions that we have modeled include nitric oxide dimerization, ammonia synthesis, and the esterification of acetic acid, all of which show yield enhancements within microporous carbons. In conjunction with a yield enhancement of the esterification reaction, selective adsorption of ethyl acetate within carbon micropores demonstrates an efficient method for product recovery. Additionally, a new method has been developed for simulating reaction kinetics within porous materials and other heterogeneous environments. The validity of this technique is first demonstrated by reproducing the kinetics of hydrogen iodide decomposition in the gas phase, and then predictions are made within slit-shaped carbon pores and carbon nanotubes. The rate constant is found to increase by a factor of 47 in carbon nanotubes, as compared to the same reaction in the bulk gas phase. Overall, the results of these simulation studies demonstrate improvements in chemical reaction yield and chemical kinetics that are possible by understanding the nature of confined reactions, and applying this knowledge to catalyst design.
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    Directed Assembly and Manipulation of Anisotropic Colloidal Particles by External Fields
    (2010-01-12) Gangwal, Sumit; Gregory N. Parsons, Committee Member; John F. Muth, Committee Member; Saad A. Khan, Committee Member; Orlin D. Velev, Committee Chair
    The application of external fields to anisotropic particles can be an efficient means of programmed assembly of novel materials and is a rapidly expanding research field. We report a series of studies on the assembly and manipulation of surface patterned anisotropic colloidal particles (whose surfaces are physically or chemically different) by external alternating current (AC) electric and magnetic fields. The fundamental results include the first experimental observation of induced-charge electrophoretic (ICEP) motion of asymmetric metallodielectric microspheres and the formation of novel assembled structures of these particles by dielectrophoresis (particle interaction with external AC electric ﬠeld gradients) and by magnetophoresis (migration and interaction of particles in an inhomogeneous magnetic field). The experimental and modeling techniques developed and fundamental principles uncovered could be used to engineer the processes of directed and/or programmed assembly of other types of anisotropic particles. “Janus†particles were prepared by coating dielectric, polystyrene latex microspheres with a conductive metal layer on one hemisphere. The phase space for AC electric field intensity and frequency was explored for these particles on a glass surface between two electrodes. A rich variety of metallodielectric structures and dynamics were uncovered, which are very different from those obtained from directed dielectrophoretic assembly of plain dielectric or plain conductive particles. The application of low frequency AC ﬠelds to aqueous suspensions of the Janus particles leads to unbalanced liquid flows around each half of the particle causing nonlinear, ICEP particle motion (perpendicular to the ﬠeld direction). Above 10 kHz field frequency, the metallodielectric particles assemble into new types of chain structures, where the metallized halves of neighboring particles align into lanes along the ﬠeld direction. These staggered chains were confined together to form two-dimensional metallodielectric crystals. The experimental results of the orientation of Janus particles in the electric field and the formation of staggered chains were interpreted by means of numerical simulations of the electric energy of the system. The assembly of Janus metallodielectric particles may ﬠnd applications in liquid-borne microcircuits and materials with directional electric and heat transfer. The electrokinetic motion of the particles may ﬠnd applications in microactuators and microfluidic devices. The assembly of magnetic Janus colloids (having 50% surface coating of iron on polystyrene microspheres) under the combined (and sometimes competing) dielectrophoretic and magnetophoretic forces was investigated. The structures formed by magnetic fields have the advantage that the particle interactions are bistable. They can result in permanent structures, which could be disassembled on demand by remote demagnetization and then reassembled into new stable structures, thus recycling the building blocks. The assembly of magnetic anisotropic particles may find numerous potential applications, among which are bifunctional drug delivery agents and novel flexible displays. We found that even more unusual types of new structures are formed when high frequency (> 50 kHz) AC electric fields are applied to suspensions of “patchy†particles. The microspheres, produced by glancing angle metal deposition, have either a single patch that is less than 50% of the total latex particle surface or two metallic patches on each pole of the particle. These patchy particles assemble in electric fields by interacting with each other in two or more directions, pre-programmed by the patch size and orientation. The multi-directional chains were confined together to form a percolated network of particles and lattices of unusual symmetry. Simulation results indicate that the assembly pattern of these particles into multi-directional chains is guided by quadrupolar and multipolar interactions, which allow for the future development of new strategies for highly controlled “programmed†assembly by external fields.
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    Germanosilicide Contacts to Ultra-shallow P+N Junctions of Nanoscale CMOS Integrated Circuits by Selective Deposition of In-situ Doped Silicon-Germanium Alloys
    (2003-04-21) Liu, Jing; Mehmet C. Ozturk, Committee Chair; John R. Hauser, Committee Member; Gregory N. Parsons, Committee Member; Carlton Osburn, Committee Member
    One of the key challenges for future CMOS technology nodes is to form source/drain junctions with very small parasitic series resistance values. This requires fundamentally new junction and contact formation technologies to produce ultra-shallow junctions with super-abrupt doping profiles, above equilibrium dopant activation and contact resistivity values near 10⁻⁸ ohm-cm². Recently, this laboratory demonstrated a new junction formation technology based on selective deposition of heavily doped Si[subscript 1-x]Ge[subscript x] alloys in source/drain regions isotropically etched to the desired depth. The results to date indicate that the technology has the potential to meet all junction and contact requirements of future CMOS technology nodes. Of particular interest to this thesis is the smaller bandgap of Si[subscript 1-x]Ge[subscript x] resulting in a smaller metal-semiconductor barrier height, which is a key advantage in reducing the contact resistivity of a metal-semiconductor contact. In this work, formation of germanosilicide contacts to heavily boron doped Si[subscript 1-x]Ge[subscript x] alloys was studied with the intention to find a contact solution for future CMOS technology nodes beyond 100 nm. During the early stages of the research project, germanosilicides of Ti, Co, Ni, Pt, W, Ta, Mo and Zr were studied to identify the most promising candidates as source/drain contacts. The first set of experiments showed that Zr, Ni and Pt may have advantages over other candidates. Of the three germanosilicides, zirconium di-germanosilicide, Zr(Si[subscript 1-x]Ge[subscript x])₂ exhibited the best thermal stability but suffered from a high resistivity and excessive substrate consumption. Ni and Pt germanosilicides were considered attractive because they were both mono-germanosilicides and consumed much less Si[subscript 1-x]Ge[subscript x] than Zr(Si[subscript 1-x]Ge[subscript x])₂. Additionally, both had resistivity values lower than that of Zr germanosilicide which could be reached at temperatures as low as 300 °C. Of the three germanosilicides, NiSi[subscript 1-x]Ge[subscript x] was found to be the only one capable of yielding the desired contact resistivity of ˜ 10⁻⁸ ohm-cm² on both p⁺ and n⁺ Si[subscript 1-x]Ge[subscript x]. Unfortunately, NiSi[subscript 1-x]Ge[subscript x] was found to suffer from Ge out-diffusion, which had a direct negative impact on its thermal stability. NiSi[subscript 1-x]Ge[subscript x] formed at temperatures above 450 °C exhibited high sheet resistance and a rough germanosilicide⁄Si[subscript 1-x]Ge[subscript x] interface. Below this temperature, ultra-shallow p⁺-n juntions with self-aligned NiSi[subscript 1-x]Ge[subscript x] contacts were formed with excellent reverse bias junction leakage characteristics. It was also observed that the thermal stability of NiSi[subscript 1-x]Ge[subscript x] formed on heavily boron doped Si[subscript 1-x]Ge[subscript x] was noticeably better. A new approach was proposed to form ultra-thin Ni Si[subscript 1-x]Ge[subscript x] layers with enhanced thermal stability. By inserting a thin Pt interlayer between Ni and Si[subscript 1-x]Ge[subscript x], the thermal stability of NiSi[subscript 1-x]Ge[subscript x] was found to be significantly improved. On boron doped Si[subscript 1-x]Ge[subscript x], the material was found to be stable at least up to 700 °C with a total starting metal thickness of 10 nm. Pt incorporation was also found to result in better surface and interface roughness. This work has shown that high quality boron doped Si[subscript 1-x]Ge[subscript x] source⁄drain junctions with NiSi[subscript 1-x]Ge[subscript x] contacts can be formed. The junctions exhibit contact resistivity values near 10⁻⁸ ohm-cm², which satisfies the requirements of future CMOS technology nodes.
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    A Study on the Optimization of the Recessed Silicon Germanium Junction Parameters of p-channel MOSFETs with Channels under Uniaxial Compressive Strain
    (2007-03-22) Chopra, Saurabh; Veena Misra, Committee Co-Chair; Carlton M. Osburn, Committee Member; Mehmet C. Ozturk, Committee Chair; Gregory N. Parsons, Committee Member
    It has been shown that recessed SiGe source/drain technology enhances the performance of metal oxide semiconductor field effect transistors (MOSFETs), by providing enhanced channel mobility and reduced source/drain contact resistivity. The focus of this dissertation is to study the effect of the recessed SiGe junction parameters on the biaxial compressive strain in SiGe, and its impact on the bandgap and contact resistivity. Due to its smaller size, boron can partially compensate the compressive strain in SiGe. This behavior was modeled using the covalent radii of Si, Ge and B, to calculate the lattice parameter of the ternary SiGeB alloy. It was also shown using micro-Raman spectroscopy that Houghton's kinetic model accurately predicted the SiGeB critical thickness. Formation of NiSiGe on SiGe was also studied, and it was found that NiSiGe induced tensile strain in SiGe, thereby reducing the compressive strain in the junctions. The impact of the NiSiGe thickness on SiGe bandgap was also studied using p⁺(SiGe)-n(Si) diodes, and it was shown that increasing the NiSiGe thickness led to an increase in the bandgap, due to loss in compressive strain. It was also shown that the barrier height followed the SiGe bandgap, and hence increased with NiSiGe thickness. The impact of the SiGe bandgap and the barrier height on contact resistivity was studied using four-terminal Kelvin structures. It was shown that contact resistivity increased with SiGe thickness and NiSiGe thickness, due to reduced biaxial compressive strain. It was shown that with fully strained SiGe junctions, and a germanium concentration of x=0.28, a minimum contact resistivity of 2.5 X 10⁻⁸ Ohm-cm² could be obtained. While the experiments in this dissertation are limited to SiGe and NiSiGe contacts, the fundamental knowledge gained from this work is expected to have a much wider impact. Specifically, this thesis introduces strain as a new parameter in contact engineering because of its impact on the semiconductor band structure and the metal-semiconductor barrier height, regardless of the metal and semiconductor choices.