Matching Items (4)

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Ion transport in surface modified cylindrical silicon-on-insulator nanopore with field effect modulation

Description

Solid-state nanopore research, used in the field of biomolecule detection and separation, has developed rapidly during the last decade. An electric field generated from the nanopore membrane to the aperture

Solid-state nanopore research, used in the field of biomolecule detection and separation, has developed rapidly during the last decade. An electric field generated from the nanopore membrane to the aperture surface by a bias voltage can be used to electrostatically control the transport of charges. This results in ionic current rectification that can be used for applications such as biomolecule filtration and DNA sequencing.

In this doctoral research, a voltage bias was applied on the device silicon layer of Silicon-on-Insulator (SOI) cylindrical single nanopore to analyze how the perpendicular gate electrical field affected the ionic current through the pore. The nanopore was fabricated using electron beam lithography (EBL) and reactive ion etching (RIE) which are standard CMOS processes and can be integrated into any electronic circuit with massive production. The long cylindrical pore shape provides a larger surface area inside the aperture compared to other nanopores whose surface charge is of vital importance to ion transport.

Ionic transport through the nanopore was characterized by measuring the ionic conductance of the nanopore in aqueous hydrochloric acid and potassium chloride solutions under field effect modulation. The nanopores were separately coated with negatively charged thermal silicon oxide and positively charged aluminum oxide using Atomic Layer Deposition. Both layers worked as electrical insulation layers preventing leakage current once the substrate bias was applied. Different surface charges also provided different counterion-coion configurations. The transverse conductance of the nanopore at low electrolyte concentrations (<10-4 M) changed with voltage bias when the Debye length was comparable to the dimensions of the nanopore.

Ionic transport through nanopores coated with polyelectrolyte (PE) brushes were also investigated in ionic solutions with various pH values using Electrochemical Impedance spectroscopy (EIS). The pH sensitive poly[2–(dimethylamino) ethyl methacrylate] (PDMAEMA) PE brushes were integrated on the inner walls as well as the surface of the thermal oxidized SOI cylindrical nanopore using surface-initiated atom transfer radical polymerization (SI-ATRP). An equivalent circuit model was developed to extract conductive and resistive values of the nanopore in ionic solutions. The ionic conductance of PE coated nanopore was effectively rectified by varying the pH and gate bias.

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Created

Date Created
  • 2015

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TEM characterization of electrically stressed high electron mobility transistors

Description

High electron mobility transistors (HEMTs) based on Group III-nitride heterostructures have been characterized by advanced electron microscopy methods including off-axis electron holography, nanoscale chemical analysis, and electrical measurements, as well

High electron mobility transistors (HEMTs) based on Group III-nitride heterostructures have been characterized by advanced electron microscopy methods including off-axis electron holography, nanoscale chemical analysis, and electrical measurements, as well as other techniques. The dissertation was organized primarily into three topical areas: (1) characterization of near-gate defects in electrically stressed AlGaN/GaN HEMTs, (2) microstructural and chemical analysis of the gate/buffer interface of AlN/GaN HEMTs, and (3) studies of the impact of laser-liftoff processing on AlGaN/GaN HEMTs. The electrical performance of stressed AlGaN/GaN HEMTs was measured and the devices binned accordingly. Source- and drain-side degraded, undegraded, and unstressed devices were then prepared via focused-ion-beam milling for examination. Defects in the near-gate region were identified and their correlation to electrical measurements analyzed. Increased gate leakage after electrical stressing is typically attributed to "V"-shaped defects at the gate edge. However, strong evidence was found for gate metal diffusion into the barrier layer as another contributing factor. AlN/GaN HEMTs grown on sapphire substrates were found to have high electrical performance which is attributed to the AlN barrier layer, and robust ohmic and gate contact processes. TEM analysis identified oxidation at the gate metal/AlN buffer layer interface. This thin a-oxide gate insulator was further characterized by energy-dispersive x-ray spectroscopy and energy-filtered TEM. Attributed to this previously unidentified layer, high reverse gate bias up to −30 V was demonstrated and drain-induced gate leakage was suppressed to values of less than 10−6 A/mm. In addition, extrinsic gm and ft * LG were improved to the highest reported values for AlN/GaN HEMTs fabricated on sapphire substrates. Laser-liftoff (LLO) processing was used to separate the active layers from sapphire substrates for several GaN-based HEMT devices, including AlGaN/GaN and InAlN/GaN heterostructures. Warpage of the LLO samples resulted from relaxation of the as-grown strain and strain arising from dielectric and metal depositions, and this strain was quantified by both Newton's rings and Raman spectroscopy methods. TEM analysis demonstrated that the LLO processing produced no detrimental effects on the quality of the epitaxial layers. TEM micrographs showed no evidence of either damage to the ~2 μm GaN epilayer generated threading defects.

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Created

Date Created
  • 2012

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Numerical simulations of electrically induced chloride ion transport and moisture permeability through cracked concrete

Description

The main objective of this study is to numerically investigate: (i) the ionic transport, especially chloride ion penetration into cementitious materials under imposed electric fields, and (ii) moisture transport through

The main objective of this study is to numerically investigate: (i) the ionic transport, especially chloride ion penetration into cementitious materials under imposed electric fields, and (ii) moisture transport through cracked concretes as a function of the crack geometry. Numerical methods were implemented to simulate the ionic transport process, based on coupling the Nernst-Planck equation and Poisson's equation to account for transport dominated by electromigration. This mathematical model was also modified to account for the chloride binding mechanism (physical and chemical trapping of chlorides by the cement hydrates) and the concentration dependence of the diffusion coefficient of each ion in the transport process. To validate the numerical model, experimental data from a companion work was used in this study. The non-steady state migration test, which is one of the common accelerated chloride ion transport test, is numerically simulated. The simulation provides a linear relationship between ionic concentration and ionic flux, which indicates that the diffusion part is negligible under a strong external voltage environment. The numerical models along with adjustments for the concentration-dependent diffusion coefficients, a pore structure factor (from electrical measurements) and chloride binding considerations are found to be successful in predicting the chloride penetration depth into plain and modified concretes under imposed electrical potentials. Moisture transport through cracked concrete was examined in the second part of this thesis. To better understand the crack's influence on the permeability, modified Louis' equation was chosen to relate the permeability with crack characteristics. 3D concrete crack models were developed using a MATLAB program with distinct crack tortuosities, roughnesses and sizes. As a comparison, Navier-Stokes equation and the Lattice Boltzmann method were also applied on the 3D model of the cracked concrete to evaluate their permeability. The methodology developed here is expected to be useful in understanding the influence of cracking on moisture transport, and when properly coupled with an ionic transport model that will be further developed, helps comprehensively understand the coupling effects of moisture and ionic transport on deterioration in concrete structures.

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Created

Date Created
  • 2014

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Field effect modulation of ion transport in silicon-on-insulator nanopores and their application as nanoscale coulter counters

Description

In the last few years, significant advances in nanofabrication have allowed tailoring of structures and materials at a molecular level enabling nanofabrication with precise control of dimensions and organization at

In the last few years, significant advances in nanofabrication have allowed tailoring of structures and materials at a molecular level enabling nanofabrication with precise control of dimensions and organization at molecular length scales, a development leading to significant advances in nanoscale systems. Although, the direction of progress seems to follow the path of microelectronics, the fundamental physics in a nanoscale system changes more rapidly compared to microelectronics, as the size scale is decreased. The changes in length, area, and volume ratios due to reduction in size alter the relative influence of various physical effects determining the overall operation of a system in unexpected ways. One such category of nanofluidic structures demonstrating unique ionic and molecular transport characteristics are nanopores. Nanopores derive their unique transport characteristics from the electrostatic interaction of nanopore surface charge with aqueous ionic solutions. In this doctoral research cylindrical nanopores, in single and array configuration, were fabricated in silicon-on-insulator (SOI) using a combination of electron beam lithography (EBL) and reactive ion etching (RIE). The fabrication method presented is compatible with standard semiconductor foundries and allows fabrication of nanopores with desired geometries and precise dimensional control, providing near ideal and isolated physical modeling systems to study ion transport at the nanometer level. Ion transport through nanopores was characterized by measuring ionic conductances of arrays of nanopores of various diameters for a wide range of concentration of aqueous hydrochloric acid (HCl) ionic solutions. Measured ionic conductances demonstrated two distinct regimes based on surface charge interactions at low ionic concentrations and nanopore geometry at high ionic concentrations. Field effect modulation of ion transport through nanopore arrays, in a fashion similar to semiconductor transistors, was also studied. Using ionic conductance measurements, it was shown that the concentration of ions in the nanopore volume was significantly changed when a gate voltage on nanopore arrays was applied, hence controlling their transport. Based on the ion transport results, single nanopores were used to demonstrate their application as nanoscale particle counters by using polystyrene nanobeads, monodispersed in aqueous HCl solutions of different molarities. Effects of field effect modulation on particle transition events were also demonstrated.

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Created

Date Created
  • 2011