K. F. Brennan

Bio: K. F. Brennan is an academic researcher from Georgia Tech Research Institute. The author has contributed to research in topics: Semiconductor & Quantum well. The author has an hindex of 12, co-authored 19 publications receiving 720 citations.

Monte Carlo simulation of electron transport in the III-nitride wurtzite phase materials system: binaries and ternaries

Abstract: We present a comprehensive study of the transport dynamics of electrons in the ternary compounds, Al/sub x/Ga/sub 1-x/N and In/sub x/Ga/sub 1-x/N. Calculations are made using a nonparabolic effective mass energy band model. Monte Carlo simulation that includes all of the major scattering mechanisms. The band parameters used in the simulation are extracted from optimized pseudopotential band calculations to ensure excellent agreement with experimental information and ab initio band models. The effects of alloy scattering on the electron transport physics are examined. The steady state velocity field curves and low field mobilities are calculated for representative compositions of these alloys at different temperatures and ionized impurity concentrations. A field dependent mobility model is provided for both ternary compounds AlGaN and InGaN. The parameters for the low and high field mobility models for these ternary compounds are extracted and presented. The mobility models can be employed in simulations of devices that incorporate the ternary III-nitrides.

Electron wave optics in semiconductors

Abstract: Starting from fundamental principles, quantitative analogies between quantum mechanical electron waves in semiconductor materials and electromagnetic optical waves in dielectrics are presented. This, in turn, suggests many new classes of electron wave optical devices such as narrow‐band superlattice interference filters. Phase effects associated with an electron wave are incorporated using an ‘‘electron wave phase refractive index’’ that is proportional to the square root of the product of the electron effective mass and the electron kinetic energy. It is shown that the amplitude of an electron wave is analogous to the electric field of a TE polarized electromagnetic wave (or to the magnetic field of a TM polarized electromagnetic wave) in a dielectric. Amplitude effects associated with an electron wave are incorporated using an ‘‘electron wave amplitude refractive index’’ that is proportional to the square root of the ratio of the kinetic energy to the effective mass. A simple expression for the critical...

Monte Carlo study of high-field carrier transport in 4H-SiC including band-to-band tunneling

Abstract: A full-band ensemble Monte Carlo simulation has been used to study the high-field carrier transport properties of 4H-SiC. The complicated band structure of 4H-SiC requires the consideration of band-to-band tunneling at high electric fields. We have used two models for the band-to-band tunneling; one is based on the overlap test and the other on the solution of the multiband Schrodinger equations. The latter simulations have only been performed for holes in the c-axis direction, since the computer capacity requirement are exceedingly high. Impact-ionization transition rates and phonon scattering rates have been calculated numerically directly from the full band structure. Coupling constants for the phonon interaction have been deduced by fitting of the simulated low-field mobility as a function of lattice temperature to experimental data. Secondary hot electrons generated as a consequence of hole-initiated impact ionization are considered in the study for both models of band-to-band tunneling. When the mul...

Semiconductor quantum wells as electron wave slab waveguides

Abstract: A quantum well in a semiconductor can act as a slab waveguide for electron waves in a manner analogous to the way a layered dielectric can act as a slab waveguide for electromagnetic waves (e.g., as commonly employed in integrated optics). In this work, the case of a general electron asymmetric slab waveguide (a quantum well comprised of three materials each with a different potential energy and a different effective mass) is analyzed and the conditions for electron waveguiding are quantified. Electron waveguide modes exist for electron energies in the well and for electron energies above one or both of the potential energy barriers. Furthermore, due to dispersion, each electron waveguide mode has an upper‐energy cutoff as well as a lower‐energy cutoff. This is in contrast to electromagnetic guided modes which typically have only lower‐energy (low‐frequency) cutoffs. At the upper‐energy cutoff the electron wave is refracted into the substrate and/or cover. An example quantum well waveguide consisting of Ga0.80 Al0.20 As (substrate), GaAs (film), Ga0.55 Al0.45 As (cover) is analyzed. This structure is a single‐mode electron waveguide for GaAs thicknesses of from 5 (1.413 nm) to 26 monolayers (7.349 nm).

Semiconductor superlattice interference filter design

Abstract: The quantitative analogies that have been previously established [J. Appl. Phys. 65, 814 (1989)] between electron wave propagation in semiconductors and optical wave propagation in dielectrics may be used to translate thin‐film optical device designs into semiconductor superlattice device designs. The procedure for this direct mapping is also described in the above reference. The resulting designs, however, have compositions that are not constrained to be within a usable compositional range and they have layer thicknesses that are not constrained to be integer multiples of a monolayer thickness. In the present work, a systematic design procedure is presented that includes these required practical constraints. This procedure is then applied to the design of Ga1−xAlxAs superlattice narrow interference filters. For pass kinetic energies in the range of 0.14–0.20 eV, compositions (values of x) and numbers of monolayer thicknesses needed to produce quarter‐wavelength layers are calculated. The detailed design ...

Plasmon-induced hot carrier science and technology

Abstract: The discovery of the photoelectric effect by Heinrich Hertz in 1887 set the foundation for over 125 years of hot carrier science and technology. In the early 1900s it played a critical role in the development of quantum mechanics, but even today the unique properties of these energetic, hot carriers offer new and exciting opportunities for fundamental research and applications. Measurement of the kinetic energy and momentum of photoejected hot electrons can provide valuable information on the electronic structure of materials. The heat generated by hot carriers can be harvested to drive a wide range of physical and chemical processes. Their kinetic energy can be used to harvest solar energy or create sensitive photodetectors and spectrometers. Photoejected charges can also be used to electrically dope two-dimensional materials. Plasmon excitations in metallic nanostructures can be engineered to enhance and provide valuable control over the emission of hot carriers. This Review discusses recent advances in the understanding and application of plasmon-induced hot carrier generation and highlights some of the exciting new directions for the field.

Integrated optics

Abstract: In order to enable optical systems to operate with a high degree of compactness and reliability it is necessary to combine large number of optical functions in small monolithic structures. A development, somewhat reminiscent of that that took place in Integrated Electronics, is now beginning to take place in optics. The initial challenge in this emerging field, known appropriately as "Integrated Optics", is to demonstrate the possibility of performing basic optical functions such as light generation, coupling, modulation, and guiding in Integrated Optical configurations. The talk will review the main theoretical and experimental developments to date in Integrated Optics. Specific topics to be discussed include: Material considerations, guiding mechanisms, modulation, coupling and mode losses. The fabrication and applications of periodic thin film structures will be discussed.

Hot-Electron Photodetection with a Plasmonic Nanostripe Antenna

Abstract: Planar metal–oxide–metal structures can serve as photodetectors that do not rely on the usual electron–hole pair generation in a semiconductor. Instead, absorbed light in one of the metals can produce a current of hot electrons when the incident photon energy exceeds the oxide barrier energy. Despite the desirable traits of convenient fabrication and room-temperature operation at zero bias of this type of device, the low power conversion efficiency has limited its use. Here, we demonstrate the benefits of reshaping one of the metallic contacts into a plasmonic stripe antenna. We use measurements of the voltage-dependence, spectral-dependence, stripe-width dependence, and polarization-dependence of the photocurrent to show that surface plasmon excitations can result in a favorable redistribution in the electric fields in the stripe that enhances the photocurrent. We also provide a theoretical model that quantifies the spectral photocurrent in terms of the electrical and optical properties of the junction. ...

Observation of an electronic bound state above a potential well

Abstract: SHORTLY after the birth of quantum mechanics, von Neumann and Wigner made the remarkable proposal1 that certain spatially oscillating attractive potentials could support bound states at energies above the potential barriers (that is, spatially confined states within the continuum) by means of diffractive interference. Because of their unusual geometry, such potentials were regarded as mathematical curiosities2,3, although more recently it has been suggested that they might be found in certain atomic and molecular systems4,5. Following the observation of discrete electronic states in ultra-thin semiconductor layered structures6,7 (for example, in quantum wells), Stillinger8 and Herrick9 proposed that super-lattices might be used to construct potentials supporting these 'positive energy' bound states. Here we report direct evidence of such states in semiconductor heterostructures grown by molecular-beam epitaxy10. Infrared absorption measurements reveal a narrow, isolated transition from a bound state within a quantum well to a bound state at an energy greater than the barrier height; this state is spatially localized by Bragg reflections.

Valence band splittings and band offsets of AlN, GaN and InN.

Abstract: First‐principles electronic structure calculations on wurtzite AlN, GaN, and InN reveal crystal‐field splitting parameters ΔCF of −217, 42, and 41 meV, respectively, and spin–orbit splitting parameters Δ0 of 19, 13, and 1 meV, respectively. In the zinc blende structure ΔCF≡0 and Δ0 are 19, 15, and 6 meV, respectively. The unstrained AlN/GaN, GaN/InN, and AlN/InN valence band offsets for the wurtzite (zinc blende) materials are 0.81 (0.84), 0.48 (0.26), and 1.25 (1.04) eV, respectively. The trends in these spectroscopic quantities are discussed and recent experimental findings are analyzed in light of these predictions.