arXiv subjects

Shaikh Ahmed

Publications and source records attributed to Shaikh Ahmed.

3 recordsLinked to original sources

arXiv2010

Influence of Internal Fields on the Electronic Structure in Self-Assembled InAs/GaAs Quantum Dots

Built-in electrostatic fields in Zincblende quantum dots originate mainly from - (1) the fundamental crystal atomicity and the interfaces between two dissimilar materials, (2) the strain relaxation, and (3) the piezoelectric polarization. In this paper, using the atomistic NEMO 3-D simulator, we study the origin and nature of the internal fields in InAs/GaAs quantum dots with three different geometries, namely, box, dome, and pyramid. We then calculate and delineate the impact of the internal fields in the one-particle electronic states in terms of shift in the conduction band energy states, anisotropy and non-degeneracy in the P level, and formation of mixed excited bound states. Models and approaches used in this study are as follow: (1) Valence force field (VFF) with strain-dependent Keating potentials for atomistic strain relaxation; (2) 20-band nearest-neighbor sp3d5s* tight-binding model for the calculation of single-particle energy states; and (3) For piezoelectricity, for the first time within the framework of sp3d5s* tight-binding theory, four different recently-proposed polarization models (linear and non-linear) have been considered in conjunction with an atomistic 3-D Poisson solver that also takes into account the image charge effects. Specifically, in contrast to recent studies on similar quantum dots, our calculations yield a non-vanishing net piezoelectric contribution to the built-in electrostatic field. Demonstrated also is the importance of full three-dimensional (3-D) atomistic material representation and the need for using realistically-extended substrate and cap layers (systems containing ~2 million atoms) in the numerical modeling of these reduced-dimensional quantum dots.

physics.comp-ph↗

arXiv2009

On the Self-Consistent Event Biasing Schemes for Monte Carlo Simulations of Nanoscale MOSFETs

Different techniques of event biasing have been implemented in the particle-based Monte Carlo simulations of a 15nm n-channel MOSFET. The primary goal is to achieve enhancement in the channel statistics and faster convergence in the calculation of terminal current. Enhancement algorithms are especially useful when the device behavior is governed by rare events in the carrier transport process. After presenting a brief overview on the Monte Carlo technique for solving the Boltzmann transport equation, the basic steps of deriving the approach in presence of both the initial and the boundary conditions have been discussed. In the derivation, the linearity of the transport problem has been utilized first, where Coulomb forces between the carriers are initially neglected. The generalization of the approach for Hartree carriers has been established in the iterative procedure of coupling with the Poisson equation. It is shown that the weight of the particles, as obtained by biasing of the Boltzmann equation, survives between the successive steps of solving the Poisson equation.

physics.comp-ph↗

arXiv2009

Multimillion Atom Simulations with NEMO 3-D

The rapid progress in nanofabrication technologies has led to the emergence of new classes of nanodevices and structures. At the atomic scale of novel nanostructured semiconductors the distinction between new device and new material is blurred and device physics and material science meet. The quantum mechanical effects in the electronic states of the device and the granular, atomistic representation of the underlying material become important. The variety of geometries, materials, and doping configurations in semiconductor devices at the nanoscale suggests that a general nanoelectronic modeling tool is needed. The Nanoelectronic Modeling tool (NEMO 3-D) has been developed to address these needs. Based on the atomistic valence-force field (VFF) method and a variety of nearest-neighbor tight-binding models, NEMO 3-D enables the computation of strain for over 64 million atoms and of electronic structure for over 52 million atoms, corresponding to volumes of (110nmx110nmx110nm) and (101nmx101nmx101nm), respectively. This article discusses the theoretical models, essential algorithmic and computational components, and optimization methods that have been used in the development and the deployment of NEMO 3-D. Also, successful applications of NEMO 3-D are demonstrated in the atomistic calculation of single-particle electronic states of (1) self-assembled quantum dots including long-range strain and piezoelectricity; (2) stacked quantum dots ; (3) Phosphorus impurities in Silicon used in quantum computation; (4) Si on SiGe quantum wells (QWs); and (5) SiGe nanowires.

physics.comp-ph↗