Since the advent of nanoscale material based electronic devices, there has been a considerable interest in exploring carbon nanotubes from fundamental science and technological perspectives. In carbon nanotubes, the atoms form a cylindrical structure with a diameter of the order 1nm. The length of the nanotubes can extend up to several hundred micrometers. Carbon nanotubes exhibit a variety of intriguing electronic properties such as semiconducting and metallic behaviour, due to the quantum confinement of the electrons in the circumferential direction. Much of the study dedicated to describe the behaviour of carbon nanotube-based devices assumes for simplicity the nanotube to be a ballistic material. However, in reality the phonon scattering mechanism exists also in nanotubes, of course, and can generally not be neglected, except in very short nanotubes. In this work, we focus attention on exploring the steady-state electron transport properties of semiconducting single-walled carbon nanotubes, including both phonon scattering and defect (vacancy) scattering, using the semi-classical bulk single electron Monte Carlo method.   The electron energy dispersion relations are obtained by applying the zone folding technique to the dispersion relations of graphene, which are calculated using the tight-binding description. The vibrational modes in the carbon nanotubes are studied using a fourth nearest-neighbour force constant model. Both the electron-phonon and the electron-defect interactions are formulated within the tight-binding framework, and their corresponding scattering rates are computed and analyzed. In particular, the dependence of the phonon scattering rate and the defect scattering rate on the diameter of the nanotube, on temperature and on electron energy is studied. It is shown that the differences observed in the scattering rate between different nanotubes mainly stem from the differences in their band structure.   A bulk single electron Monte Carlo simulator was developed to study the electron transport in semiconducting zigzag carbon nanotubes. As a first step, we included only electron-phonon scattering, neglecting all other possible scattering mechanisms. With this scattering mechanism, the steady-state drift velocity and the mobility for the nanotubes (8,0), (10,0), (11,0), (13,0) and (25,0) were calculated as functions of the electric-field strength and lattice temperature, and the results are presented and analysed here. The dependence of the mobility on the lattice temperature can be clearly seen at low electric-field strengths. At such electric-field strengths, the scattering is almost entirely due to acoustic phonons, whereas at high electric-field strengths optical phonon emission processes dominate. It is shown that the saturation of the steady-state drift velocity at high electric-field strengths is due to the emission of high-energy optical phonons. The results indicate the presence of Negative differential resistance for some of the nanotubes considered in this work. The discrepancy found in the literature concerning the physical reason for the appearance of negative differential resistance is clarified, and a new explanation is proposed. It is also observed that the backward scattering is dominant over the forward scattering at high electric-field strengths.                                                                                   We then included also defect scattering, actually electron-vacancy scattering, for the nanotubes (10,0) and (13,0). The steady-state drift velocities for these nanotubes are calculated as functions of the density of vacancies, electric-field strength and the lattice temperature, using three different vacancy concentrations. The results indicate the presence of Negative differential resistance at very low concentration of defects, and how this feature may depend on the concentration of defects. The dependence of the steady-state drift velocity on the concentration of defect and the lattice temperature is discussed. The electron distribution functions for different temperatures and electric field strengths are also calculated and investigated for all the semiconducting nanotubes considered here. In particular, a steep barrier found in the electron distribution function is attributed to the emission of high energy optical phonons.

<p>Timing-related defects are becoming increasingly important in nanometer-technology integrated circuits (ICs). Small delay variations induced by crosstalk, process variations, power-supply noise, as well as resistive opens and shorts can potentially cause timing failures in a design, thereby leading to quality and reliability concerns. All these effects are noticeable in today's technologies and they are likely to become more prominent in the next-generation process technologies~\cite{itrs2007}.</p><p>The detection of small-delay defects (SDDs) is difficult because of the small size of the introduced delay. Although the delay introduced by each SDD is small, the overall impact can be significant if the target path is critical, has low slack, or includes many SDDs. The overall delay of the path may become larger than the clock period, causing circuit failure or temporarily incorrect results. As a result, the detection of SDDs typically requires fault excitation through least-slack paths. However, widely-used automatic test-pattern generation (ATPG) techniques are not effective at exciting small delay defects. On the other hand, the usage of commercially available timing-aware tools is expensive in terms of pattern count inflation and very high test-generation times. Furthermore, these tools do not target real physical defects.</p><p>SDDs are induced not only by physical defects, but also by run-time variations such as crosstalk and power-supply noise. These variations are ignored by today's commercial ATPG tools. As a result, new methods are required for comprehensive coverage of SDDs.</p><p>Test data volume and test application time are also major concerns for large industrial circuits. In recent years, many compression techniques have been proposed and evaluated using industrial designs. However, these methods do not target sequence- or timing-dependent failures while compressing the test patterns. Since timing-related failures in high-performance integrated circuits are now increasingly dominated by SDDs, it is necessary to develop timing-aware compression techniques.</p><p>This thesis addresses the problem of selecting the most effective test patterns for detecting SDDs. A new gate and interconnect delay-defect probability measure is defined to model delay variations for nanometer technologies. The proposed technique intelligently selects the best set of patterns for SDD detection from a large pattern set generated using timing-unaware ATPG. It offers significantly lower computational complexity and it excites a larger number of long paths compared to previously proposed timing-aware ATPG methods. It is shown that, for the same pattern count, the selected patterns are more effective than timing-aware ATPG for detecting small delay defects caused by resistive shorts, resistive opens, process variations, and crosstalk. The proposed technique also serves as the basis for an efficient SDD-aware test compression scheme. The effectiveness of the proposed technique is highlighted for industrial circuits.</p><p>In summary, this research is targeted at the testing of SDDs caused by various underlying reasons. The proposed techniques are expected to generate high-quality and compact test patterns for various types of defects in nanometer ICs. The results of this research are expected to provide low-cost and effective test methods for nanometer devices, and they will lead to higher shipped-product quality.</p> / Dissertation

<p>Quantum Dot Infrared Photodetectors (QDIPs) are important alternatives to conventional infrared photodetectors with high potential to provide required detector performance, such as higher temperature operation and multispectral response, due to the 3-D quantum confinement of electrons, discrete energy levels, and intrinsic response to perpendicular incident light due to selection rules. However, excessive dark current density, which causes QDIPs to underperform theoretical predictions, is a limiting factor for the advancement of QDIP technologies. The purpose of this dissertation research is to achieve a better understanding of dopant incorporation into the active region of QDIPs, which is directly related to dark current control and spectral response. From this dissertation research, doping related dipole fields are found to be responsible for excessive dark current in QDIPs. </p><p>InAs/GaAs QDIPs were grown using solid source molecular beam epitaxy (MBE) with different doping conditions. The QDIPs were optically characterized using photoluminescence and Fourier transform infrared (FT-IR) spectroscopy. Devices were fabricated using standard cleanroom fabrication procedures. Dark current and capacitance measurements were performed under different temperature to reveal electronic properties of the materials and devices. A novel scanning capacitance microscopy (SCM) technique was used to study the band structure and carrier concentration on the cross section of a quantum dot (QD) heterostructure. In addition, dark current modeling and bandstructure calculations were performed to verify and better understand experimental results.</p><p>Two widely used QDIP doping methods with different doping concentrations have been studied in this dissertation research, namely direct doping in InAs QD layer, and modulation doping in the GaAs barrier above InAs QD layer. In the SCM experiment, electron redistribution has been observed due to band-bending in the modulation-doping region, while there is no band-bending observed in directly doped samples. A good agreement between the calculated bandstructure and experimental results leads to better understanding of doping in QD structures. The charge filling process in QDs has been observed by an innovative polarization-dependent FT-IR spectroscopy. The red-shift of QD absorbance peaks with increasing electron occupation supports a miniband electronic configuration for high-density QD ensembles. In addition, the FT-IR measurement indicates the existence of donor-complex (DX) defect centers in Si-doped QDIPs. The existence of DX centers and related dipole fields have been confirmed by dark current measurements to extract activation energies and by photocapacitance quenching measurements. </p><p>With the understanding achieved from experimental results, a further improved dark current model has been developed based on the previous model originally established by Ryzhii and improved by Stiff-Roberts. In the model described in this dissertation, two new factors have been considered. The inclusion of background drift current originating from Si shallow donors in the low bias region results in excellent agreement between calculated and measured dark currents at different temperatures, which has not been achieved by previous models. A very significant effect has been observed in that dark current leakage occurs due to the dipole field caused by doping induced charge distribution and impact-ionized DX centers. </p><p>Last but not least, QDIPs featuring the dipole interface doping (DID) method have been designed to reduce the dark current density without changing the activation energy (thus detection wavelength) of QDIPs. The DID samples involve an InAs QD layer directly-doped by Si, as well as Be doping in the GaAs barrier on both sides of the QD layer. The experimental result shows the dark current density has been significantly reduced by 104 times without any significant change to the corresponding activation energy. However, the high p-type doping in the GaAs barrier poses a challenge in that the Fermi level is reduced to be well below the QD energy states. High p-type doping is reported to reduce the dark current, photocurrent and the responsivity of the devices. </p><p>To conclude, it is significant to identify to effect of Si-induced defect centers on QDIP dark currents. The subsequent study reveals doping induced dipole fields can have significant effects on QDIP device performance, for example, causing charge leakage from QDs and reducing activation energy, thereby increasing dark current density. The DID approach developed in this work is a promising approach that could help address these issues by using controlled dipole fields to reduce dark current density without changing the minimum detectable energy of QDIPs.</p> / Dissertation

Photovoltaics offers a unique solution to energy and environmental problems simultaneously. However, widespread application of photovoltaics will not be realized until costs are reduced by about a factor of four without sacrificing performance. Silicon crystallization and wafering account for about 55% of the photovoltaic module manufacturing cost, but can be reduced significantly if a ribbon silicon material, such as String Ribbon Si, is used as an alternative to cast Si. However, the growth of String Ribbon leads to a high density of electrically active bulk defects that limit the minority carrier lifetime and solar cell performance. The research tasks of this thesis focus on the understanding, development, and implementation of defect passivation techniques to increase the bulk carrier lifetime in String Ribbon Si in order to enhance solar cell efficiency. Hydrogen passivation of defects in Si can be performed during solar cell processing by utilizing the hydrogen available during plasma-enhanced chemical vapor deposition (PECVD) of SiNx:H films. It is shown in this thesis that hydrogen passivation of defects during the simultaneous anneal of a screen-printed Al layer on the back and a PECVD SiNx:H film increases the bulk lifetime in String Ribbon by more than 30 ?A three step physical model is proposed to explain the hydrogen defect passivation. Appropriate implementation of the Al-enhanced defect passivation treatment leads to String Ribbon solar cell efficiencies as high as 14.7%. Further enhancement of bulk lifetime up to 92 ?s achieved through in-situ NH3 plasma pretreatment and low-frequency (LF) plasma excitation during SiNx:H deposition followed by a rapid thermal anneal (RTA). Development of an optimized two-step RTA firing cycle for hydrogen passivation, the formation of an Al-doped back surface field, and screen-printed contact firing results in solar cell efficiencies as high as 15.6%. In the final task of this thesis, a rapid thermal treatment performed in a conveyer belt furnace is developed to achieve a peak efficiency of 15.9% with a bulk lifetime of 140 ?Simulations of further solar cell efficiency enhancement up to 17-18% are presented to provide guidance for future research.

Wave Number Selection and Defect Dynamics in Patterns with Hexagonal Symmetry Denis B. Semwogerere 108 Pages Directed by Dr. Michael F. Schatz We report quantitative measurements of wave number selection, secondary instability and defect dynamics in hexagonal patterns. A novel optical technique ("thermal laser writing") is used to imprint initial patterns with selected characteristics in a B뮡rd-Marangoni convection experiment. Initial patterns of ideal hexagons are imposed to determine the band of stable-pattern wave numbers. For small values of control parameter epsilon the measured stable band is found to agree quantitatively with theoretical predictions at the low-wave-number side of the band, and qualitatively at the high-wave-number side. Long-wavelength perturbations of ideal hexagonal patterns suggested by theory are imposed for epsilon=0.46 and their growth rates are measured to investigate the mechanisms of secondary instability. Our results suggest a transverse-phase instability limits stable hexagons at low wave number while a longitudinal-phase instability limits high-wave-number hexagons. Initial patterns containing an isolated penta-hepta defect are imprinted to study defect propagation directions and velocities. The experimental results agree well with theoretical predictions. The experimental investigations are discussed in the context of patterns with hexagonal symmetry formed under nonequilibrium external driving conditions.

The thesis develops two characterizations of spatio-temporal complex patterns. While these are developed for the patterns of fluid flow in experiments on Rayleigh-Benard Convection(RBC), they are adaptable to a wide range of spatially extended systems. The characterizations may be especially useful in cases where one does not have good models describing the dynamics, making numerical and analytic studies difficult. In Spiral Defect Chaos(SDC), a weakly turbulent regime of RBC, the convective rolls exhibit complex spatial and temporal dynamics. We study the dynamics of SDC through local defect formations between convective rolls as well as the topological rearrangements of these rolls at a global scale. A laser based thermal actuation system is developed to reproducibly impose initial states for the fluid flow and construct ensembles of trajectories in the neighborhood of defect nucleation. This is used to extract the modes and their growth rates, characterizing the linear manifold corresponding to defect nucleation. The linear manifold corresponding to instabilities resulting in defect formation is key to building efficient schemes to control the dynamics exhibited. We also develop the use of computational homology as a tool to study spatially extended dynamical systems. A quantitative measure of the topological features of patterns is shown to provide insights into the underlying dynamics not easily uncovered otherwise. In the case of RBC, the homology of the patterns is seen to indicate asymmetries between hot and cold regions of the flow, stochastic evolution at a global scale as well as bifurcations occurring well into the turbulent regime of the flow.

It is usually to see a large number of pipelines separating around the refineries, chemical and petro-chemical plants. The corrosion and erosion defects are unavoidable to occur in transporting pipe line. Especially, the maintain stuff usually find out breakage pipe or leaking liquid at elbowing pipe line because of the corrosion and erosion defects. So it is essential to examine these pipelines with an efficient method. The use of guided waves method is very attractive to solve this problem since guided wave could be excited at one circle on the pipeline and propagate over considerable distance. To choose guided wave torsion mode T (0, 1) as excitation mode because its group velocity doesn¡¦t change with frequencies. And the research analyzes the mode conversion that occurred when T (0, 1) mode propagated after the elbow pipe. The research also discusses the signal difference in different depth, circumferential distribution and axial length defects on the elbow pipe. The erosion defect usually occurs in the elbow pipe line and it would change with fluid velocity, causticity of fluid and flow direction. Therefore, the research designs the defects according to the character of erosion defect by finite element method software and simulates T (0, 1) mode propagating in the pipe line. Then this research extracts and analyzes the reflection signals from defects. In this guided wave experiment, the research manufactures the defect on elbow pipe. Because the erosion defect could be usually found at outer side of elbow pipe, artificial defect would be set there. And the elbow pipe is manufactured with different depth, circumferential distribution and axial length defect. The research would discuss the relationship between change of defect and reflection signal. By elbow pipe defect signals of simulation and experiment consequence, the different depth, circumferential distribution and axial length defect signals could be still distinguished. The signals with different axial length defect that received from straight pipe and elbow pipe are similar and are affected by signal constructive and destructive interference. So the research could get maximum and minimum defect signal amplitudes from one-fourth wavelength axial defect and half wavelength axial defect. Therefore, the axial length defect of elbow pipe could be estimated from defect signals and this consequence could help judge the level of damaged elbow pipe. T (0, 1) mode has better sensitivity to outside of the pipe than inside of the pipe. So the bigger signal amplitude could be received from the notch at outside of the pipe. In the process of wave propagation simulation, there are overlapping waveforms and mode conversions occur at elbow pipe. This situation causes the defect signals were amplified at elbow pipe. In practical detection, the misjudgments of amplified defect signals should be attended to.

Abstract: Background: At present, device closure of interatrial communication has become a well established technique in order to adequately treat severe left-to-right shunt associated with ASDs. During the traditional procedure, fluoroscopy with the waist of a compliant balloon is used to determine the appropriate size of the closure device and defect sizing. Choice of adequate closure device using transthoracic echocardiography (TTE) has been hitherto unreported. Methods & Materials: Between December 2002 and October 2004, 40 patients (15 males, 25 females, mean age; 11.7 ¡Ó 7.8 years ) with secundum ASDs underwent transcatheter closure at our institution. In group 1, 30 patients had the procedure by balloon sizing and TTE sizing. In 10 patients (group 2), TTE sizing was used as the sole too l for selecting device size and the device size was chosen to be based on the Amplatzer septal occluder ( ASO ) size and TTE size ratio in group 1. The procedure was performed under continuous transoesophageal echocardiographic monitor with general anesthesia. Results: The correlation was found between TTE and stretched balloon sizing diameter SBD ( y= 1.2645x-1.4465; R&#x00B2;=0.9861 ), and between TTE size and ASO size ( y = 1.3412x-1.2864; R&#x00B2;=0.9929 ) in group 1. In group 2, statistical correlation between TTE and ASO ( y=1.3419x-0.1172; R&#x00B2;=0.9934 ) was also found. Good linear regression between TTE size and ASO chosen size was noted in group 1 and group 2 (R&#x00B2;=0.99).In group 2, successful device implantation was accomplished in all patients whose device size was chosen to be based on the ASO and TTE ratio in group 1. Conclusions: TTE sizing is a safe and ideal method to measure interatrial defect and choose the occluding device respectively. With our experience, the sizing based on the TTE is generally easier than measurement from the balloon sizing.

This thesis is about the kinetics of anisotropic acidic/hydrothermal dissolution of apatite bulk single crystal vs. nanorods, and the kinetic phase change of dense nanocondensates of SnO2 vs. Ni-dissolved SnO2 prepared by laser ablation condensation technique. In the first regard, directional dissolution of a natural (OH,F,Cl)-bearing apatite has been studied at various solution pH values (0~3) and 30 oC. This apatite showed abnormally high O-H stretching frequencies due to the substitution of Cl for OH. The advance of dissolution front indicated that steady-state directional dissolution for pH = 0-2 followed an apparent rate law of rate(mole / m2h)¡×kaH+n, where the rate constants (k) are 2.15 and 1.61; and the rate orders (n) are 1.44 and 1.30 for [0001] and <11 0> directions, respectively. Previous study, however, indicated a smaller n value (n = 0.55~0.70) for fluorapatite powders at higher pHs. A nonlinear pH dependence of logarithmic dissolution rate at a wide pH range implied that the surface active sites and/or rate-determining steps have changed when the acidity of solution and/or the composition of the apatite were changed. The opening of etch pits on basal planes further indicated that the dissolution rates along the three principal directions have the following relationship: [0001] > <11-20> > <10-10> for pH=0-1, but the order was reversed for pH > 3. As a comparison, static immersion of needle-like hydroxyapatite nanoparticles in neutral hydrothermal solution at 100oC caused preferential dissolution along the crystallographic c-axis to form nanorods with a lower aspect ratio. The anisotropic dissolution behavior is due to diffusion-controlled rapid dissolution at the sharp tip, and interface-controlled dissolution at side surfaces in terms of active sites. Extensive dissolution was accompanied with amorphization via explosive generation of dislocations, forming corrugated surface with both negative and positive curvature regions. The amorphous residue was significantly Ca and OH depleted when treated in the hydrothermal solution at pH=3. The BET specific surface area of the apatite nanoparticles remained 45¡Ó1 m2/g after immersion in neutral solution at 100oC for 36 h, but drastically decreased to 24.5 m2/g in acidic (pH =3) solution at 100oC for 8 h due to coalescence of the partially amorphized apatite powders. The specific surface area and average pore size also remained nearly unchanged for the dry pressed powders subject to firing at 100oC, but decreased and increased, respectively when sintered shortly at 600oC in air. BJH measurements at 77 K indicated the N2 adsorption/desorption hysteresis loops shift toward high relative pressure for sintered/hydrothermally etched powders indicating a higher activation energy of forming overlain liquid-like nitrogen layers. This can be attributed to a lower surface energy of the powders due to their shape change and/or partial amorphization. Alternatively, desorption through cavitation via the small voids could occur, in particular for such treated samples with characteristic bimodal pore size distribution. In the second subject, dense SnO2 with fluorite-type related structures were synthesized via very energetic Nd-YAG laser pulse irradiation of oxygen-purged Sn target. Combined effects of rapid heating to very high temperatures, nanophase effect, and dense surfaces account for the condensation of fluorite-type structure which transformed martensitically to baddeleyite-type accompanied with twinning, commensurate shearing and shape change. Alternatively Pa-3-modified fluorite-type hardly survived transformation to a-PbO2 type and rutile type in the dynamic process analogous to the case of static decompression. In addition, the rutile-type SnO2 nanocondensates have {110}, {100} and {101} facets, which are beneficial for {~hkl} vicinal attachment to form edge dislocations, faults and twinned bicrystals. The {011}-interface relaxation, by shearing along <011> directions, accounts for a rather high density of edge dislocations near the twin boundary thus formed. The rutile-type SnO2 could be alternatively transformed from orthorhombic CaCl2-type structure (denoted as o) following parallel crystallographic relationship, (0 1)r//(0 1)o; [111]r//[111]o, and full of commensurate superstructures and twins parallel to (011) of both phases. Furthermore, SnO2-NiO solid solution (ss) condensates were fabricated by laser ablation on Ni-Sn target at 1.1 J/pulse and oxygen flow of 50 L/min. AEM observations indicated that the particles were more or less coalesced/agglomerated as nano chain aggregate or in close packed manner. The Ni-rich condensates have rock salt structure with defect clusters not in paracrystalline distribution as would otherwise develop into the spinel phase. The Sn-rich condensates are predominantly rutile-type with minor baddeleyite-type, which are vulnerable to martensitic transformation/relaxation to form {101} incommensuare faults as well as epitaxial twin variants of rutile upon rapid cooling and/or electron irradiation. Islands of metallic Ni-Sn-NiSn were partially oxidized/solidified when deposited on silica glass.

In this thesis, in order to study the structural, optical and electrical transport properties of Tl2In2S3Se, TlInSeS and Tl2In2SSe3 crystals, X-ray diffraction (XRD), energy dispersive spectroscopic analysis (EDSA), transmission, reflection, photoluminescence (PL), thermally stimulated current (TSC) and photoconductivity decay (PC) measurements were carried out. Lattice parameters and atomic composition of these crystals were determined from XRD and EDSA experiments, respectively. By the help of transmission and reflection experiments, the room temperature absorption data were analyzed and it was revealed the coexistence of indirect and direct band gap energies of the studied crystals. Moreover, the refractive index dispersion parameters - oscillator energies, dispersion energies, oscillator strengths, oscillator wavelengths and zero-frequency refractive indexes were determined. Temperature-dependent transmission measurements made it possible to find the rate of change of indirect band gaps with temperature, absolute zero values of the band gap energies and Debye temperatures of these crystals. From the analysis of the transmission and reflection measurements, it was established that, there is a decrease in the values of indirect and direct band gaps energies and an increase in zero-frequency refractive indexes with increasing of selenium content. PL measurements were carried out to obtain the detailed information about recombination levels in crystals studied. The behavior of PL spectra were investigated as a function of laser excitation intensity and temperature. The variation of the spectra with laser excitation intensity and temperature suggested that the observed emission bands in these crystals were due to the donor-acceptor pair recombination. TSC measurements were carried out with various heating rates at different illumination temperatures to obtain information about trap levels in these crystals. The mean activation energies, attempt-to-escape frequencies, concentrations and capture cross sections of the traps were determined as a result of TSC spectra analysis. The analysis of experimental TSC curves registered at different light illumination temperatures revealed the exponential trap distribution in the studied crystals. From the analysis of PC measurements, carrier lifetimes were obtained.