published_or_final_version / abstract / Physics / Master / Master of Philosophy

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / Applied Geosciences / Master / Master of Science

Charge trapping dynamics in Si/SiO2/Hf(1-x)SixO2 and III-V film stack systems are characterized using spectroscopic ellipsometry (SE) and second harmonic generation (SHG). For the first time, discrete absorption features within the bandgap of the SiO2 interfacial layer are identified using SE, and their relation to both intrinsic and process-induced defects is proposed. Sensitivity of the absorption features to process conditions is demonstrated and evidence that these defects contribute to Vfb roll-off is presented. Defects in the Hf(1-x)SixO2 films are probed with fs laser-induced internal multi-photon photo-excitation (IMPE) and time dependent electrostatic field induced second harmonic (TD-EFISH) generation. For the as deposited HfO2 films, a unique TD-EFISH response is identified and explained by resonant two photon ionization of a specific point defect and subsequent tunneling of the photoelectrons to the Si substrate. Charge trapping kinetics for all Hf(1-x)SixO2 films are investigated. Two characteristic trap cross sections are identified and found to be insensitive to dielectric film and process conditions, and associated with a surface “harpooning” mechanism. EFISH from non-centrosymmetric III-V media, including GaAs and In0.53Ga0.47As, is also studied. The anisotropic and time dependent SHG response from different chemically treated In0.53Ga0.47As surfaces is clearly distinguishable and associated with a process-induced change in the surface depletion field. / text

Neural tube defects (NTDs) are a world health issue, affecting approximately 1 in every 1000 live births. These congenital defects arise from the improper closure of the neural tube during development, resulting in significant, life-threatening malformations of the central nervous system. Although it has been observed that supplementing women of child-bearing age with folates greatly decreases the chances of having an NTD affected baby, unfortunately these defects still occur. It is accepted that these complex disorders arise from a combination of genetic, environmental, and dietary influences. One such dietary influence is the one-carbon metabolism metabolite, homocysteine. Homocysteine is a byproduct of methylation reactions in the cell that exists in an inverse homeostasis with folate. Homocysteine can also undergo a transformation that allows it to then react with exposed lysine or cysteine residues on proteins, in a process known as N-homocysteinylation or S-homocysteinylation respectively. High levels of homocysteine have been long correlated with many disease states, including NTDs. One potential mechanism by which homocysteine confers its negative effects is through protein N-homocysteinylation. Here, a novel and high-throughput assay for N-homocysteinylation determination is described. This assay is shown to be accurate with mass spectrometry then shown to be biologically relevant using known hyperhomocysteinemia mouse models. This assay was then applied to a cohort of neural tube closure staged mouse embryos with two different genetic mutations that have previously been shown to predispose mice to NTDs. The genotypes explored here are mutations to the LRP6 gene and the Folr1 gene, both of which have been described as folate-responsive NTD mouse models. It was seen that maternal diet and embryonic genotype had the largest influence on the developmental outcome of these embryos; however, the inverse relationship between folate and homocysteine seemed to be established at this early time point, emphasizing the importance of the balance in one-carbon metabolism. One of these genes, LRP6, was then explored in a human cohort of spina bifida cases. Four novel mutations to the LRP6 gene were found and compared to the mouse model used in the previous study. One of the mutations found in the human population was seen to mimic that of the LRP6 mouse model, therefore expanding the potential of this NTD model. / text

Recently, Bi₂Se₃ was added to a new class of materials known as topological insulators. While several studies have provided tantalizing hints towards novel physical properties, such as backscatter suppression and spin-polarized transport, several concerns remain in actual materials. In particular, high defect densities, strong surface band bending, and potential fluctuations have been observed. Here, scanning tunneling microscopy and spectroscopy are used to reveal surface effects in Bi₂Se₃ and CuxBi₂Se₃. First, a detailed examination of defects in bulk-grown samples is described. Then, I provide an analysis of molecular beam epitaxy results, done in collaboration with colleague Yuxuan Chen. Following this, I provide a detailed study of individual point defects in Cu-doped Bi₂Se₃ and examine how Cu is incorporated into the Bi₂Se₃ lattice. Finally, through spectroscopic analysis, a novel depth-sensitive measurement of the local band bending field is developed. Furthermore, for the first time, fluctuations of the Dirac point can be correlated to specific near-surface defects, namely Se vacancies. These analyses provide valuable insights into the preparation of future samples for the investigation of topological insulators. / text

The fundamental physics and techniques employed in Laplace transform deep level transient spectroscopy (L-DLTS) are reviewed. A Laplace-DLTS system has been constructed. The high resolving power of this system has been demonstrated experimentally. The L-DLTS system was applied to characterize the defects in undoped n-type ZnO thin film grown by the pulsed laser deposition (PLD) method. A 0.3 eV deep trap has been identified. The formations of Ec-0.39eV and Ec-0.20eVcan be enhanced when the sample surface is seriously damaged by high temperature annealing.AnEc-0.25eV trap is identified in the freshly grown samples, but would disappear after the storage of 3 months. Copper doped n-type ZnO thin film samples with low carrier concentration (n~〖10〗^16 〖cm〗^(-3)) were investigated by using both conventional and Laplace DLTS techniques. Positive DLTS signal peaks were detected that are suspected to be contributed by the minority carrier (hole carrier) emission. A physics model involving the inversion layer of a metal-insulator-semiconductor contact has been invoked to interpret the hole carrier concentration existing near the metal-semiconductor interface. Expression for the defect concentration is determined as a function of the temperature of DLTS peaks. AnEv+0.6eV defect with high concentration (N_T~〖10〗^17 〖cm〗^(-3)) was detected. The concentration of Ev+0.6eVcan be enhanced when the annealing temperature was increased from 750 to 900 degree C. / published_or_final_version / Physics / Master / Master of Philosophy

Among a number of fundamental issues, the origin of inertial mass remains one of the major open problems in particle physics. Furthermore, topological effects related to non perturbative field configurations are poorly understood in those gauge theories of direct relevance to our physical universe. Motivated by such issues, this Thesis provides a deeper understanding for the appearance of topological effects in abelian gauge field theories, also in relation to the existence of a mass gap for the gauge interactions. These effects are not accounted for when proceeding through gauge fixings as is customary in the literature. The original Topological-Physical factorisation put forth in this work enables to properly identify in topologically massive gauge theories (TMGT) a topological sector which appears under formal limits within the Lagrangian formulation. Our factorisation then allows for a straightforward quantisation of TMGT, accounting for all the topological features inherent to such dynamics. Moreover dual actions are constructed while preserving the gauge symmetry also in the presence of dielectric couplings. All the celebrated mass generation mechanisms preserving the gauge symmetry are then recovered but now find their rightful place through a network of dualities, modulo the presence of topological terms generating topological effects. In particular a dual formulation of the famous Nielsen-Olesen vortices is constructed from TMGT. Within a novel physically equivalent picture, these topological defects are interpreted as dielectric monopoles.

This dissertation describes a new simulation technique for an automatic test generation system, SCIRTSS version 4.0 (Sequential Circuit Test Sequence System). This test generation system is driven by the hardware compiler AHPL, a Hardware Programming Language, and an intelligent heuristic-based search for test vector generation. Using a fault-injection gate-level simulator and the generated test vector, all the faulty states of the circuit are simulated in parallel and the simulator is thus able to find all detected faults by a particular input sequence. The major objective of this research was to develop a faster replacement for the existing simulation process. The philosophy of divide and conquer is used in the development of the new simulation technique. Sequential networks are divided into combinational sub-networks, and, if necessary, the combinational sub-networks are further reduced into fan-out free regions. Thus, the problem is reduced to a relatively simple combinational one. In addition to the classical faults, the new simulator attempts to detect CMOS stuck-open faults. Several circuits were tested under SCIRTSS 4.0 using both the existing and the new simulation techniques. The results are listed in this paper to verify superiority of the new simulation technique.

Microdefects in wafers sliced from selected positions along Czochralski (CZ)-grown, silicon single crystal ingots were investigated by means of transmission electron microscopy (TEM). Specimens taken from the central regions of these wafers, previously subjected to specific thermal treatments, were prepared either by ultrasonic cutting and jet thinning or by an anisotropic thinning method. Ultrasonic cutting was found to generate microdefects in the thin surface regions of the TEM specimen discs. The density of ultrasonically generated defects (USD's) was found to vary directly with the ultrasonic energy input from the cutter. Ultrasonic waves transmitted through abrasive slurry into the discs, causing lattice vibrations, are believed to be responsible for the microdefect generation. Anisotropic thinning for the preparation of TEM specimens was carried out in an agitated bath of KOH-Isopropyl Alcohol (IPA)-H₂O at 80°C and 60°C. A great number of high-surface-quality, self-supporting thin films can be produced with large (about 30 mils square) electron-transparent areas. Edges of the thin films are in <110> directions and can be used as quick reference for defect orientation during electron microscopy. Specimens from heat-treated wafers disclosed the presence of precipitates measuring some 100-1500 nm on one side, surrounded by prismatic dislocations punched out in <110> directions in the crystal. The precipitates appear to be thin platelets (less than 40 Å in thickness), lying on {100} planes and are viewed either as flat squares or rectangles, or as edge-on rods inclined 45° to the <110> directions. The edges of the platelets are in <110> directions. Prismatic punched-out dislocation loops are formed in rows, the axes of which are in <110> directions. A row of loops seen edge-on is similar in size if its axis is in the surface <110> directions. When loop axes are in the oblique <110> directions from the surface, they appear as closed rhombus loops with line senses in <112> directions. Their size increases with distance from the precipitate. The observed dislocation loops were found to be of interstitial type with a Burger's vector of a/2 <110>. The total defect density (precipitates and dislocation loops) of a specimen depend strongly on the thermal history of the wafer and on the wafer position in the ingot.