R. Chris Bowen

Single and multiband modeling of quantum electron transport through layered semiconductor devices

Non-equilibrium Green function theory is formulated to meet the three main challenges of high bias quantum device modeling: self-consistent charging, incoherent and inelastic scattering, and band structure. The theory is written in a general localized orbital basis using the example of the zinc blende lattice. A Dyson equation treatment of the open system boundaries results in a tunneling formula with a generalized Fisher-Lee form for the transmission coefficient that treats injection from emitter continuum states and emitter quasi-bound states on an equal footing. Scattering is then included. Self-energies which include the effects of polar optical phonons, acoustic phonons, alloy fluctuations, interface roughness, and ionized dopants are derived. Interface roughness is modeled as a layer of alloy in which the cations of a given type cluster into islands. Two different treatments of scattering; self-consistent Born and multiple sequential scattering are formulated, described, and analyzed for numerical t...

Quantitative simulation of a resonant tunneling diode

Quantitative simulation of an InGaAs/InAlAs resonant tunneling diode is obtained by relaxing three of the most widely employed assumptions in the simulation of quantum devices. These are the single band effective mass model (parabolic bands), Thomas-Fermi charge screening, and the Esaki-Tsu 1D integral approximation for current density. The breakdown of each of these assumptions is examined by comparing to the full quantum mechanical calculations of self-consistent quantum charge in a multiband basis explicitly including the transverse momentum.

QUANTUM DEVICE SIMULATION WITH A GENERALIZED TUNNELING FORMULA

We present device simulations based on a generalized tunneling theory. The theory is compatible with standard coherent tunneling approaches and significantly increases the variety of devices that can be simulated. Quasi‐bound and continuum states in the leads are treated on the same footing. Quantum charge self‐consistency is included in the leads and the central device region. We compare the simulated I–V characteristics with the experimental I–V characteristics for two complex quantum device structures and find good agreement.

RESONANT-TUNNELING DIODES WITH EMITTER PREWELLS

Resonant-tunneling diodes (RTDs) incorporating an emitter prewell structure are studied both theoretically and experimentally in order to investigate the utility of the emitter region as a device design parameter. The experiments show a tendency for peak bias, current, and the peak-to-valley ratio to increase for wider prewells, behavior likewise seen in both very simple and detailed calculations. Both the simple and more complete models point to interactions between states associated with the prewell and the main quantum well as the reasons for the increase in peak current. These results suggest design guidelines to affect peak bias, current, or the peak-to-valley ratio of RTDs.

Resolution of Resonances in a General Purpose Quantum Device Simulator (NEMO)

Electron transport in quantum devices is governed by discrete quantum states due to electron confinement. A crucial requirement for the modeling of quantum devices is the the numerical identification and resolution of these quantum states. We present an algorithm utilized in our general purpose quantum device simulator (NEMO), where we locate the resonances of the system first and then generate the optimized grid used to integrate over the resonances. We find this algorithm important in the modeling of coherent transport involving ultrafine resonances and crucial for the modeling of incoherent transport.

Lower limits of line resistance in nanocrystalline back end of line Cu interconnects

The strong non-linear increase in the Cu interconnect line resistance with decreasing linewidth presents a significant obstacle to their continued downscaling. In this letter we use the first principles density functional theory based electronic structure of Cu interconnects to find the lower limits of their line resistance for metal linewidths corresponding to future technology nodes. We find that even in the absence of scattering due to grain boundaries, edge roughness or interfaces, quantum confinement causes a severe increase in the line resistance of Cu. We also find that when the simplest scattering mechanism in the grain boundary scattering dominated limit is added to otherwise coherent electronic transmission in monocrystalline nanowires, the lower limit of line resistance is significantly higher than projected roadmap requirements in the International Technology Roadmap for Semiconductors.

Effect of realistic metal electronic structure on the lower limit of contact resistivity of epitaxial metal-semiconductor contacts

The effect of realistic metal electronic structure on the lower limit of resistivity in [100] oriented n-Si is investigated using full band Density Functional Theory and Semi-Empirical Tight Binding calculations. It is shown that the “ideal metal” assumption may fail in some situations and, consequently, underestimate the lower limit of contact resistivity in n-Si by at least an order of magnitude at high doping concentrations. The mismatch in transverse momentum space in the metal and the semiconductor, the so-called “valley filtering effect,” is shown to be sensitive to the details of the transverse boundary conditions for the unit cells used. The results emphasize the need for explicit inclusion of the metal atomic and electronic structure in the atomistic modeling of transport across metal-semiconductor contacts.

Atomistic Electronic Structure Calculations of Unstrained Alloyed Systems Consisting of a Million Atoms

The broadening of the conduction and valence band edges due to compositional disorder in alloyed materials of finite extent is studied using an sp3s* tight binding model. Two sources of broadening due to configuration and concentration disorder are identified. The concentrational disorder dominates for systems up to at least one million atoms and depends on problem size through an inverse square root law. Significant differences (over 12 meV) in band edge energies are seen depending on choice of granularity of alloy clusters.

On the feasibility of ab initio electronic structure calculations for Cu using a single s orbital basis

The accuracy of a single s-orbital representation of Cu towards enabling multi-thousand atom ab initio calculations of electronic structure is evaluated in this work. If an electrostatic compensation charge of 0.3 electron per atom is used in this basis representation, the electronic transmission in bulk and nanocrystalline Cu can be made to compare accurately to that obtained with a Double Zeta Polarized basis set. The use of this representation is analogous to the use of single band effective mass representation for semiconductor electronic structure. With a basis of just one s-orbital per Cu atom, the representation is extremely computationally efficient and can be used to provide much needed ab initio insight into electronic transport in nanocrystalline Cu interconnects at realistic dimensions of several thousand atoms.

Writing Research Software in a Large Group for the NEMO Project

The nanoelectronic modeling (NEMO) program is the result of a three-year development effort involving four universities and the former Corporate Research and Development Laboratory of Texas Instruments, now Applied Research Laboratory, Raytheon TI Systems, to create a comprehensive quantum device modeling tool for layered semiconductor structures. Based on the non-equilibrium Green function formalism, it includes the effects of quantum charging, bandstructure and incoherent scattering from alloy disorder, interface roughness, acoustic phonons, and polar optical phonons. NEMO addresses the diverse needs of two different types of users: (i) the engineer/experimentalist who desires a black-box design tool and (ii) the theorist who is interested in a detailed investigation of the physics. A collection of models trade off physical content with speed and memory requirements. Access to this comprehensive theoretical framework is accommodated by a Graphical User Interface (GUI) that facilitates device prototyping and in situ data analysis. We describe a hierarchical software design that allows rapid incorporation of theory enhancements while maintaining a user-friendly GUI, thus satisfying the conflicting criteria of ease of use and ease of development. The theory and GUI modules share data structures that define the device structure, material parameters, and simulation parameters. These data structures may contain general data such as integer and real numbers, option lists, vectors, matrices and the labels for both batch and GUI operation. NEMO generates the corresponding GUI elements at run-time for display and entry of these data structures.