The generation of hot electrons characterizing the two-plasmon decay (TPD) instability is investigated experimentally both in and out of the plane of polarization of a CO₂ laser incident on an underdense gas target. The results presented here show that, for high intensities (I > ~ 3.5 x 10¹³ W/cm² for a helium target, 1 > ~ 5.5 x 10¹³ W/cm² for a nitrogen target), the electron plasma waves (EPW's) generated by the TPD instability are modified by the electron decay instability (EDI). The relatively short scale lengths at the onset of TPD for these high intensities (<~0.125mm) cause the EPW's propagating towards the higher density regions of the plasma to undergo the EDI resulting in EPW's which contain a vector component perpendicular to the plane of polarization and accelerate electrons by nonlinear Landau damping up to 55° outside the plane of polarization. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

In this report, the optical characteristics of ZnS:Tb AC driven thin-film electroluminescent devices are evaluated. Luminescence at low and room temperature under a constant phosphor field is recorded in order to probe the hot electron energy distribution. Samples fabricated by atomic layer epitaxy and by sputter deposition are investigated and their differences and similarities evaluated. The ALE sample exhibits a drastic increase in luminescence at low temperature, while the sputtered sample displays a decrease in luminescence at low temperature. The reduction of optical phonon scattering and nonradiative transition rates at low temperature are believed to be responsible for these changes. Differences between ALE and sputtered samples are due to the different fabrication methods. The approach suggested by Krupka in 1972 of determining the hot electron energy distribution by taking the ratio intensities of two luminescent transitions from different upper states in Tb�����, is shown to be inaccurate due to the possible nonradiative transitions taking place. The Tb����� impact excitation quantum yield at a wavelength of 489 nm is measured and is shown to agree with Monte Carlo simulations. Saturation occurs due to the increase in band-to-band impact ionization at high phosphor fields. / Graduation date: 1995

Thesis (M.E.E.)--University of Delaware, 2007. / Principal faculty advisor: Ian Appelbaum, Dept. of Electrical and Computer Engineering. Includes bibliographical references.

Graduation date: 1994

We explore the effects of surface plasmon hybridization in graphene nanostructures and silver nanoparticles as applied to novel plasmonic phenomena. The analysis is based on the theory of surface plasmon hybridization under the boundary charges method. This method, which is based in the electrostatic approximation, has been largely used to predict the resonant frequencies in strongly coupled nanoparticle clusters. Here, we extend this formalism to analyze novel plasmonic phenomena such as the blueshift of modes in graphene plasmonics, near-field radiation, thermal transport and plasmon-induced hot carrier generation in silver nanoparticles. Furthermore, we develop analytical solutions for graphene nanodisks and metallic spheres that allow for fast and accurate modeling. The analytic models provide the basis to derive a large number of results, including prediction of hybrid eigenmodes and bandstructures, far-field response, and near-field response under thermally induced fluctuations. We predict that the strong near-filed coupling in graphene nanodisk stacks can induce a blueshift in the resonant frequencies up to the near-infrared part of the spectrum. We find that the strong near-filed coupling between disks can also lead to large values of radiative thermal conductance when thermally induced fluctuations are included. In this regard, an enhancement over the blackbody limit of up to two and four orders of magnitude was observed for co-planar and co-axial disk configurations. The strong coupling between coplanar disks was also explored for the development of plasmonic waveguides by considering long co-planar disk arrays. It was observed that the array posseses great potential for plasmonic waveguiding, with a strong degree of confinement for disks smaller than 200 nm. Thermal activation of the guided modes showed a thermal conductivity of up to 4.5 W/m K and thermal diffusivity of up to 1:4 x 10-3 m2/s. The large values of thermal diffusivity suggest the potential of graphene disk waveguides for thermotronic interconnects. The plasmon-induced hot carrier generation in silver nanosphere dimers was also studied. The modeling considered analytical solution for metallic nanospheres, from which the electrostatic potential of each sphere was obtained. Using these results, the hot carrier generation was explored under the basis of the Fermi golden rule. The results show a large number of hot carriers at the low frequency modes. This values exceed the number of generated hot carriers on a single sphere. The energy distribution of photogenerated electrons and holes showed a large energy gap that can be explored in photocatalysis and photovoltaic energy conversion.

The purpose of this work is to develop a new model for LDD n-MOSFET degradation in drain current under long-term AC use conditions for lifetime projection which includes a self-limiting effect in the hot-electron induced device degradation. Experimental results on LDD n-channel MOSFETs shows that the maximum drain current degradation is a function of the AC average substrate current under the various AC stress conditions but not a function of frequency or waveforms or different measurement configurations. An empirical model is constructed for circuit applications. It is verified that the self-limiting in drain current is due to the thermal re-emission of a trapped-hot-electron in the oxide. Results show that self-heating during AC stress releases trapped electrons, which in turn limits the maximum amount of drain current degradation. Moreover, tunneling to and from traps model is employed to visualize the internal mechanism of thermal recovery of electrons under different bias conditions. Although the LDD device structure can reduce the hot electron effect, various processing technologies can also affect the device reliability. A carbon doped LDD device with the first and the second level metal and passivation layer but without any final anneal shows that a significant reduction in the shifts of the threshold voltage of MOSFETs with time can be achieved. However, the long-term reliability projection of nMOSFETs based on DC stress tests alone is shown to be overly pessimistic. / Graduation date: 1992

A series of experiments have been performed to understand fast electron generation from ultra intense laser-solid interaction, and their transports through a cold material. Using Micro-Electro-Mechanical Systems (MEMS), we contrived various shape of cone and wedge targets. The first set of experiment was for investigating hot electron generations by measuring x-ray production in different energy ranges. K[alpha] and hard x-ray yields were compared when the laser was focused into pyramidal shaped cone targets and wedge shaped targets. Hot electron production is highest in the wedge targets irradiated with transverse polarization, though K[alpha] is maximized with wedge targets and parallel polarization. These results are explained with particle-in-cell (PIC) simulations utilizing PICLS and OOPIC codes. We also investigate hot electron transport in foil, wedge, and cone targets by observing the transition radiation emitted from the targets rear side along with bremsstrahlung x-ray measurement. Twodimensional images and spectra of 800 nm coherent transition radiation (CTR) along with ballistic electron transport analysis have revealed the spatial, temporal, and temperature characteristics of hot electron micro-pulses. Various patterns from different target-laser configurations suggest that hot electrons were guided by the strong static electromagnetic fields at the target boundary. Evidence about fast electron guiding in the cone is also observed. CTR at 400 nm showed that two distinct beams of MeV electrons are emitted from the target rear side at the same time. This measurement indicates that two different mechanisms, namely resonance absorption and j x B heating, create two populations of electrons at the targets front side and drive them to different directions, with distinct temperatures and temporal characteristics. This interpretation is consistent with the results from 3D-PIC code Virtual Laser Plasma Laboratory (VLPL). / text

When energy relaxation between electrons and the lattice is slow, an elevated electronic temperature different from that of the lattice persists. In this regime, hot charge carriers control the energy transport in a material. In this thesis, I show how hot carriers can dominate graphene's response enabling it to exhibit novel properties. First, I examine how light is converted to electrical currents in graphene and show that hot carriers play an integral role in this multi-stage process. I show that photocurrent in graphene p-n junctions is dominated by a Photo-thermoelectric effect in which a light-induced elevated hot carrier temperature drives a thermoelectric current. Furthermore, I show that the generation and cooling of hot carriers in graphene during photoexcitation proceeds in an unusual way. In the former, carrier-carrier scattering dominates the initial photoexcitation cascade enabling efficient hot carrier generation. In the latter, a new cooling mechanism - disorder-assisted scattering (supercollisions) - dominates electron-lattice cooling over a wide range of temperatures (including room temperature). Second, I examine the transport characteristics of double layer graphene heterostructures (specifically, G/h-BN/G heterostructures). I show that Coulomb coupling results in vertical (out-of-plane) energy transfer between electrons in proximal (but electrically insulated) graphene layers. This couples lateral (in-plane) charge and energy transport of electrons in the two layers to give rise to a new energy-driven Coulomb drag (inter-layer transresistance) that dominates when the two layers are at charge neutrality. Third, I examine energy transport in charge neutral graphene. I show that the combination of fast carrier-carrier scattering, high electronic quality, and slow electron-lattice cooling (hot carriers) gives rise to a regime of ballistic heat transport. This manifest as electronic energy waves with velocity on the order of graphene's Fermi velocity. The new phenomena enabled by hot carriers and the ideas/approaches described in this thesis provide a basis with which to exploit hot carrier effects in graphene and opens new vistas for controlling and harnessing energy flows on the nanoscale. / Engineering and Applied Sciences

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

Hot carrier effects in sub-micron lightly doped drain (LDD) n-channel MOSFETs under static (DC) stress are studied in order to establish the degradation mechanisms of such devices. Degradation is monitored as a function of time at various gate voltages. Under accelerated aging conditions (i.e. large drain voltages) the gate voltage for maximum degradation is found to be different than the gate voltage for which the substrate current is maximum; this is in contrast to the results of previous workers who found degradation and substrate current to be strongly correlated. However, under normal operating conditions, degradation and substrate current are found to be correlated. Furthermore, through the use of charge pumping measurements it is shown that two primary mechanisms are accountable for the degradation of these devices at small and large gate voltages. First, at large gate voltages there is an increase in the degradation which is predominantly due to electron injection and trapping in the oxide. An alternating static injection experiment shows that this type of electron trapping degradation is recoverable. Second, at small gate voltages degradation is mainly related to interface state generation near the drain LDD region. Floating gate measurements demonstrate that electron and hole injection occurs at large and small gate voltages, respectively. It is also shown that maximum interface state creation occurs when electron and hole injection happens simultaneously. / Graduation date: 1994