Graduation date: 1997

No description available.

There are two principal techniques for performing Monte Carlo electron transport computations. The first, and least common, is the full track-structure method. This method individually models all physical electron interactions including elastic scatter, electron impact ionization, radiative losses and excitations. However, because of the near infinite size of electron interaction cross-sections and highly anisotropic scattering behavior, this method requires an enormous amount of computation time. Alternatively, the Condensed History (CH) method for electron transport lumps the average effects of multiple energy loss and scattering events into one single pseudo-event, or step. Because of this approximation, the CH method can be orders of magnitude faster than the trackstructure method. While the CH method is reasonably accurate in many situations, it can be inaccurate for simulations involving microscopic site sizes such as those often found in radiation biology. For radiation biology and other microdosimetry applications, a computational device called a Field Programmable Gate Array (FPGA) is capable of executing track-structure Monte Carlo electron transport simulations as fast as, or faster than a standard computer performing transport via the CH method—and, it does so with the additional accuracy and level of detail provided by the track-structure method. In this dissertation, data from FPGA based track-structure electron transport computations are presented for five test cases, ranging in complexity from simple slab-style geometries to radiation biology applications involving electrons incident on endosteal bone surface cells. Even for the most complex test case presented, an FPGA is capable of evaluating track-structure electron transport problems more than 500 times faster than a standard computer can perform the same track-structure simulation, and with comparable accuracy.

Spintronics is an emerging technology that arises from the interplay between spin of the charge carrier and the magnetic property of the materials. The miniaturization of spintronic devices requires a deep understanding of ferromagnetic materials at the nanometer scale. This thesis studies the properties of ferromagnetic particles (2-5nm in diameter) using electron transport measurements. A technique to fabricate nanoparticle devices and incorporate microwave in the electron tunneling measurement of the particles is presented. Repeated microwave pulses can directly excite the magnetization of the particles without heating the electrons. Results of the transport measurements on Co particles will be discussed, which demonstrate that electron tunneling through a ferromagnetic particle can induce magnetization excitations in that particle. A physical model regarding the mesoscopic fluctuations is presented to address the current driven magnetization noise. Numerical simulations based on that model are performed to explain the experimental data and validate the model. Electron transport measurements on Ni, Fe, and Ni??Fe?? are conducted. The hysteretic behaviors of the particles in presence of electron tunneling have strong material dependence, which is mainly due to the magnetic shape anisotropy. Electron tunneling is a main source of magnetization noise, while other sources still need to be identified. Some data we collected from literature will be included in this thesis as an appendix.

No description available.

No description available.

The total transmission probability of an asymmetric Aharonov-Bohm ring with an embedded scattering center in both branches and a magnetic flux passing through its center has been investigated. The transmission is calculated from the overall scattering matrix for the ring structure determined by cascading scattering matrices without the symmetric conditions. In this model, by imposing scattering centers with different strengths to the upper and lower arms of the ring, the effects of coupling between the ring and the leads and the magnetic flux in the total transmission probability as a function of transmission phase have been observed. We find the following results from our model study: 1) in the strong coupling limit, there is no zero transmission; 2) with a constant coupling and scatterer in each branch, the transmission peaks get closer together and the transmission becomes smoother as the magnetic flux increases; and 3) in the weak coupling limit with a constant magnetic flux and elastic scattering in each arm, transmission resonances become sharper due to the localized states in the ring and the magnitude of transmission is enhanced at smaller transmission phases.For the case of a single or double barrier in one arm of the ring, the discrete energy levels of the double barrier well in one branch are controlled by the width of the well and continuous levels in the other branch are controlled by a single scattering barrier. We find a peculiar quantum transport through this system such as a symmetric Breit-Wigner (BW) and an asymmetric Fano transmission resonance. The transition from BW to Fano resonance occurs by tuning either the transmission phase of the electrons through a single barrier or the magnetic AB flux threading through the AB ring. The characteristics of the Fano line-shape resonance will be examined by the asymmetric parameter that is a measure of the degree of coupling between the discrete state and the continuum. / Department of Physics and Astronomy

The electronic transport properties in low dimensional systems have several important features which have attracted a wide range of both experimental and theoretical interests. The quantum ballistic transport regime in these small systems is achieved when dimensions of systems are less than the elastic mean free path and the phase coherent length. We have studied electron transport in a nanochannel, containing repulsive delta impurities. To find the conductance, a recursive Green's function method is used. The study includes finding the transverse eigenvaiues, eigenfunctions, and computing hopping integrals to determine the Green's propagators.The effects of the number, and position of the repulsive delta impurities with various potential strengths on the conductance in a nanostructure have been presented. A FORTRAN program has been used and developed for the numerical calculations. The general practical applications for nanostructures include the ability to make electronic devices smaller, denser and operate at very low voltages. The future electronic devices will utilize the developments of conductance through components having dimensions on the nanometer scale. / Department of Physics and Astronomy

Multiple longitudinal potential barriers affect electron transport in a nanostructure. In addition to a symmetric, aperiodic quantum wire with five potential barriers, a quantum wire containing thirteen potential barriers was modeled based on a quasiperiodic sequence known as the Fibonacci sequence. Conductance of the wires and transmission coefficients of the corresponding one-dimensional systems were calculated and analyzed in this work.Conductance calculations were performed using a FORTRAN computer program, tbgf, which performs the tight-binding, recursive Green's function method. Also Program Tran Coeffwas developed in order to calculate one-dimensional transmission coefficients using the method known as wavefunction matching. This method matches electronic wavefunctions in neighboring regions to determine the one-dimensional transmission coefficients at the electron energy normalized to the barrier strengths. The program uses the data file generated by Program Data Input. / Department of Physics and Astronomy

Self-consistent Monte Carlo simulation studies of n-channel Si/SiGe modulation doped field effect transistors (MODFETs) and silicon-on-insulator lateral bipolar junction transistors (SOI- LBJTs) are reported in this thesis. As a preliminary to the device studies Monte Carlo simulations of electron transport in bulk Si strained as if grown on Si(_0.77)Ge(_0.23) and Si(_0.55)Ge(_0.45) substrates have been carried out at 300 K, for field strengths varied from 10(^4) to 2 x 10(^7) Vm(^-1). The calculations indicate an enhancement of the average electron drift velocity when Si is tensilely strained in the growth plane. The enhancement of electron velocity is more marked at low and intermediate electric fields, while at very high fields the velocity saturates at about the same value as unstrained Si. In addition the ensemble Monte Carlo method has been used to study the transient response to a stepped electric field of electrons in strained and unstrained Si. The calculations suggest that significant velocity overshoots occurs in strained material. Simulations of n-channel Si/Si(_1=z)Ge(_z) MODFETs with Ge fractions of 0.23, 0.25, and 0.45 have been performed. Five depletion mode devices with x = 0.23 and 0.25 were studied. The simulations provide information on the microscopic details of carrier behaviour, including carrier velocity, kinetic energy and carrier density, as a function of position in the device. Detailed time-dependent voltage signal analysis has been carried out to test device response and derive the frequency bandwidth. The simulations predict a current gain cut-off frequency of 60 ± 10 GHz for a device with a gate length of 0.07 /nm and a channel length of 0.25 um. Similar studies of depletion and enhancement mode n-channel Si/Sio.55Geo.45 MODFETs with a gate length of 0.18 /im have been carried out. Cut-off frequencies of 60 ±10 GHz and 90± 10 GHz are predicted for the depletion and enhancement mode devices respectively. A Monte Carlo model has also been devised and used to simulate steady state and transient electron and hole transport in SOI-LBJTs. Four devices have been studied and the effects of junction depth and silicon layer thickness have been investigated. The advantage of the silicon-on-insulator technology SOI device is apparent in terms of higher collector current, current gain, and cut-off frequency obtained in comparison with an all-silicon structure. The simulations suggest that the common-emitter current gain of the most promising SOI-LBJT structure considered could have a cut-off frequency approaching 35 ± 5 GHz.