Browsing by Subject "Density-functional theory"
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Item Advanced modeling for end-of-the-roadmap CMOS and potential beyond-CMOS applications(2016-05) Crum, Dax Michael; Register, Leonard F.; Banerjee, Sanjay K; Tutuc, Emanuel; Lee, Jack C; MacDonald, Allan HEnd-of-the-roadmap CMOS devices are explored via particle-based ensemble semi-classical Monte Carlo (MC) methods employing quantum corrections (QCs) to address quantum confinement and degenerate carrier populations. The significance of such QCs is illustrated through simulation of n-channel III-V and Si FinFETs. Original contributions include our treatment of far-from-equilibrium degenerate statistics and QC-based modeling of surface-roughness scattering, as well as considering quantum-confined phonon and ionized-impurity scattering in 3D. Typical MC simulations approximate degenerate carrier populations as Fermi distributions to model the Pauli-blocking (PB) of scattering to occupied final states. To allow for increasingly far-from-equilibrium non-Fermi carrier distributions in ultra-scaled and III-V devices, we instead generate the final-state occupation probabilities used for PB by sampling the local carrier populations as a function of energy and energy valley. This process is aided by the use of fractional carriers or sub-carriers, which minimizes classical carrier-carrier scattering. Quantum confinement effects are addressed through quantum-correction potentials (QCPs) generated from coupled Schrödinger-Poisson solvers, as commonly done. However, we use our valley- and orientation-dependent QCPs not just to redistribute carriers in real space, or even among energy valleys, but also to calculate confinement-dependent phonon, ionized-impurity, and surface-roughness scattering rates. Collectively, these quantum effects can substantially reduce and even eliminate otherwise expected benefits of considered InGaAs FinFETs over otherwise identical Si FinFETs, despite higher thermal velocities in InGaAs. Beyond-CMOS device concepts are also being considered for future applications. Thin-film sub-5 nm magnetic skyrmions constitute an ultimate scaling alternative for beyond-CMOS data storage technologies. These robust non-collinear spin-textures can be moved and manipulated by spin-polarized or non-spin-polarized electrical currents, which is extremely attractive for integration with current memory technologies. An innovative technique to detect isolated nano-skyrmions with a current-perpendicular-to-plane is shown, which has immediate implications for device concepts. Such a mechanism is explored by studying the atomistic electronic structure of the magnetic quasiparticles. The tunneling conductance is quite sensitive to spatial variations in the electronic structure, as a large atomistic conductance anisotropy up to 20 is found for magnetic skyrmions in Pd/Fe/Ir(111) magnetic thin-films. This spin-mixing magnetoresistance effect possibly could be incorporated in future magnetic storage technologies.Item First-principles atomistic modeling for property prediction in silicon-based materials(2010-12) Bondi, Robert James; Hwang, Gyeong S.; Mullins, C. B.; Ekerdt, John G.; Chelikowsky, James R.; Banerjee, Sanjay K.The power of parallel supercomputing resources has progressed to the point where first-principles calculations involving systems up to 10³ atoms are feasible, allowing ab initio exploration of increasingly complex systems such as amorphous networks, nanostructures, and large defect clusters. Expansion of our fundamental understanding of modified Si-based materials is paramount, as these materials will likely flourish in the foreseeable cost-driven future in diverse micro- and nanotechnologies. Here, density-functional theory calculations within the generalized gradient approximation are applied to refine configurations of Si-based materials generated from Metropolis Monte Carlo simulations and study their resultant structural properties. Particular emphasis is given to the contributions of strain and disorder on the mechanical, optical, and electronic properties of modified Si-based materials in which aspects of compositional variation, phase, strain scheme, morphology, native defect incorporation, and quantum confinement are considered. The simulation strategies discussed are easily extendable to other semiconductor systems.