GaN RF switches are widely used in today’s communication systems. With digital communications getting more and more popular nowadays, the need for improving the performance of involved RF switches is inevitable. Designing low ON-state resistance GaN switches are exceedingly important to improve the switch insertion loss, isolation and power loss. Moreover, considerations need to be taken into account to improve the switching speed of the involved GaN HEMTs. In this dissertation, a new GaN HEMT structure called “Tunnel MOS Heterostructure FET (TMOSHFET)” is introduced which has lower ON-state resistance and faster switching speed compared to conventional AlGaN/GaN HEMTs. In the switch ON process, the channel of this device is charged up by electron tunneling from a layer underneath the channel as opposed to typical AlGaN/GaN HEMTs in which electron injection from the source is charging up the channel. The tunneling nature of this process together with the shorter travel distance of electrons in TMOSHFET provide for a faster switching speed. In order to understand the tunneling mechanisms in TMOSHFET, the fabrication of AlGaN/GaN Schottky Barrier Diodes (SBDs) with various AlGaN thicknesses is demonstrated on Si (111) substrate. The impacts of SF6 dry etching on the trap density and trap state energy of AlGaN surface are investigated using the GP/w- w method. Various tunneling mechanisms at different biases are then characterized in samples and compared with each other. To improve the source and drain resistances in TMOSHFET, a model is generated to optimize the 2DEG density and electric field in AlGaN/GaN heterostructure based on Al mole fraction, AlGaN thickness and the thickness of SiN passivation layer and it is experimentally verified by non-contact Hall 2DEG density measurements. The spontaneous and piezoelectric polarizations together with strain relaxation have been implemented into the model, taking into account the annealing effects. From the experimental data on obtained parameters, the operation and device parameterization of the TMOSHFET is outlined and design considerations to improve the device R_(ON)-V_(BR) figure of merit are discussed.

A Pan-style scanning tunneling microscope (STM), with a vertical coarse approach mechanism, was designed, built and tested. The microscope will be operated in ultra-high vacuum and also at cryogenic temperatures (8 K) inside a continuous flow cryostat. Fundamental differences in operating principle exist between the new microscope and the beetle-type inertial sliders [1] that have been the mainstay of the group for the last eight years. While Pan-style microscopes do already exist [2], they remain challenging to build, and an active area of research [3]. This system represents a bold departure from well-trodden paths, and will greatly expand the range of experiments that our group can perform. The operating principles of inertial piezoelectric motors are detailed. Design guidelines for a piezoelectric motor are given, and used in the design of the vertical coarse approach motor. A simple, inexpensive implementation for creating waveforms with an extremely fast fall time is discussed. Motor performance is tested, and a minimum step size of 20nm is found for frequencies ranging from 0 Hz to 3 kHz. The motor operates with high dynamic range: individual 20nm steps can be taken, as well as being able to move at a velocity of 0.4mm s−1. Little is known about the vibrational properties of Pan-style microscopes. Vibrational testing of the microscope revealed the expected scanner bending mode at 1.6 kHz (above the scanner bending mode of our beetles at 1.2 kHz), and a complicated response signal above this frequency. Custom extension springs for an eddy-current damping system are built and tested. A low resonant frequency of 1.8 Hz is found, which is ideal for the application. Initial testing of the STM in ambient conditions is performed on two different surfaces. A moir´e supermesh [4] with periodicity 3nm is observed on a highly-oriented pyrolytic graphite (HOPG) surface, and agrees well with previously published results. Using a flame-annealed Gold on mica surface, a low drift rate of 0.6nm s−1 is observed over a period of 13 minutes. Single-height atomic steps are observed on both surfaces. Additionally, the microscope is shown to be capable of zooming into different features on a surface, and scanning at different length scales. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2009-06-24 13:06:16.683

No description available.

Non-statistical recrossing in ketene cycloadditions with alkenes, heavy-atom tunneling and the mechanism of the decarboxylation of Mandelylthiamin is investigated in this dissertation. A combination of experimental kinetic isotope effects and theoretical models and kinetic isotope effects is utilized for this endeavor. This dissertation also describes how the use of quasiclassical dynamic trajectories, microcanonical RRKM calculations, and canonical variational transition state theory in combination with small-curvature tunneling approximations is utilized to help advance our research methodology to better understand mechanism. In the cycloaddition of dichloroketene with cis-2-butene, significant amounts of recrossing is observed using quasiclassical dynamic trajectories. An unusual inverse 13C intramolecular KIE lead us to investigate the role that heavy atoms play in non-statistical recrossing. More importantly, this discovery has uncovered a new phenomena of entropic intermediates that not only applies to ketene cycloadditions, but can also be applicable to other "concerted" reactions such as Diels-Alder reactions. The ring-opening of cyclopropylcarbinyl radical has revealed that heavy-atom tunneling plays a major role. The intramolecular 13C kinetic isotope effects for the ring-opening of cyclopropylcarbinyl radical were unprecedentedly large and in combination with theoretical predictions and multidimensional tunneling corrections, the role of tunneling in this reaction can be better understood. The mechanism decarboxylation of mandelylthiamin has been extensively studied in the literature. However, until the use of theoretically predicted KIEs and theoretical binding motifs the rate-limiting step of this reaction has been hotly debated. In this dissertation, a discussion of how the theoretical KIEs indicate the initial C-C bond as the rate-limiting step and chelating binding motifs of pyridinium and mandelylthiamin to explain the observed catalysis is given.

Although the pairing mechanism in MgB2 is thought to be phonon mediated, there are still many experimental results that lack appropriate explanation. For example, there is no consensus about the magnitude of the energy gap, its temperature dependence, and whether it has only one-gap or not. Many techniques have been used to investigate this, like Raman spectroscopy, farinfrared transmission, specific heat, high-resolution photoemission and tunneling. Most tunneling data on MgB2 are obtained from mechanical junctions. Measurements of energy gap by these junctions have many disadvantages like the instability to temperature and field changes. On the other hand, sandwich-like planar junctions offer a stable and reliable measurement for temperature dependence of the energy gap, where any variation in the tunneling spectra can be interpreted as a direct result from the sample under study. To the best of our knowledge, we report the first energy gap temperature- and magnetic field-dependence of MgB2/Pb planar junctions. Study of the temperature-dependence shows that the small gap value (reported by many groups and explained as a result of surface degradation) is a real bulk property of MgB2. Moreover, our data is in favor of the two-gap model rather than the onegap, multi-gap, or single anisotropic gap models. The study of magnetic field effect on the junctions gave an estimation of the upper critical field of about 5.6 T. The dependence of energy gap on the field has been studied as well. Our junctions show stability against temperature changes, but "collapsed" when the magnetic field (applied normal to the junction barrier) is higher than 3.2 T. The irreversible structural change switched the tunnling mechanism from quisiparticle tunneling into Josephson tunneling. Josephson I-V curves at different temperatures have been studied and the characteristic voltages are calculated. The estimated MgB2 energy gap from supercurrent tunneling in weak link junctions agrees very well with that from quasiparticle tunneling. Reported properties on polycrystalline, single crystal and thin film MgB2 samples are widely varied, depending on the details of preparation procedure. MgB2 single crystals are synthesized mainly by heat treatment at high temperature and pressure. Single crystals prepared by this way have the disadvantages of Mg deficiency and shape irregularity. On the other hand, improving the coupling of grain boundaries in polycrystalline MgB2 (has the lowest normal state resistivity in comparison to many other practical superconductors) will be of practical interest. Consequently, we have been motivated to look for a new heat treatment to prepare high quality polycrystalline and single crystal MgB2 in the same process. The importance of our new method is its simplicity in preparing single crystals (neither high pressure cells nor very high sintering temperatures are required to prepare single crystals) and the quality of the obtained single crystal and polycrystalline MgB2. This method gives high quality and dense polycrystalline MgB2 with very low normal state resistivity (σ(40 ) = 0.28 cm). Single crystals have an average diagonal of 50 m and 10 m thickness with a unique shape that resembles the hexagonal crystal structure. Furthermore, preparing both forms in same process gives a great opportunity to study inconsistencies in their properties. On the other hand, magnesium diboride thin films have also been prepared by magnetron sputtering under new preparation conditions. The prepared thin films have a transition temperature of about 35.2 K and they are promising in fabricating tunnel junctions.

In this thesis, photo luminescent diodes with the device structure of ITO/PEI/(CdTe/PDDA)*n/Al were fabricated using the method of layer-by-layer self assembly. The film thicknesses were varied from 150 nm to 380 nm. The films were characterized through X-ray diffraction (XRD), optical absorption and photoluminescence (PL) measurements. The XRD results on the film indicated a cubic crystalline structure (111) for the nano-CdTe particles. The band gap of the nano-particles were evaluated to be 2.1 eV in solution and 2 eV in films, which was further confirmed by the PL measurements as the solution exhibited a yellow luminescence while the film exhibited orange luminescence. The J vs. V curves revealed that the diodes exhibited rectifying behavior in both the forward and reverse biasing. Two models of current transport, one based on a Schottky mechanism and the other based on a tunneling mechanism were developed and were compared with the experimental values. The tunneling model developed could simulate the experimental currents up to four orders of magnitude. The tunneling mechanism of charge transport was further proved by the capacitance vs. voltage curves, which were identical to that of ITO/MEH-PPV/Al devices, where tunneling mechanism was the dominant method of charge transport.

No description available.

Due to the complex characteristics of tunneling problems, tunneling engineering is perhaps one of the areas in which numerical methods are more frequently adopted in practice. One important application is to examine the influence of an excavation and construction procedures on the stress distribution and deformation in the ground surrounding the opening and on the tunnel lining. Another important aspect of tunneling problems that can be accounted for in a numerical analysis is the complex geometry associated with typical applications. This is not only related to the shape of the opening, but also to the presence of non-homogenous or non-isotropic soil strata. / A constitutive model that is suitable for the analysis of structured clays has been developed in this study based on the multilaminate framework. The model takes into account the elastic unloading-reloading, inherent and induced anisotropy, destructuration and bonding effects. The model is implemented into Plaxis finite element program, successfully calibrated and used to investigate the response of structured sensitive clay to the construction of the Gatineau tunnel in Gatineau, Quebec and the Ottawa sewer tunnel in Ottawa, Ontario. Numerical results were compared to the field measurements taken during the tunnel construction. To improve the performance of the numerical model, an implicit integration algorithm is implemented and proven to be very effective as compared to the conventional explicit integration methods. The effect of different soil parameters including bonding and anisotropy on the tunneling induced displacements and lining stresses is also examined using a comprehensive parametric study. The results indicated that soil bonding and anisotropy have significant effects on the shape of the settlement trough as well as the magnitudes of surface displacements and lining stresses induced by tunneling.

The scanning tunneling microscope (STM) based on quantum tunneling can attain atomic-scale spatial resolution and help elucidate a wealth of phenomena in the microscopic world. However a limitation in scanning tunneling microscopy is the low temporal resolution due to readout circuit frequency rolloff at a few kHz. This limitation can be overcome by using downmixing directly in the tunneling junction. With this technology we measure the high frequency vibrational modes (~ 1 MHz) of MEMS doubly-clamped beams and explore the implication of STM downmixing for nanomechanics.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.