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Invited Talk-2

Title of the talk (tentative):

Shaikh Shahid Ahmed 
Southern Illinois University at Carbondale
1230 Lincoln Drive, Carbondale, IL 62901, USA


As semiconductor devices shrink into the nanoscale regime and new classes of nanodevices emerge, performance and reliability of nanosystems engineered from these devices are fundamentally governed by an intricate interplay of long-range structural and built-in electrostatic fields (strain, polarization, applied and environmental biases), atomistic/granular (defects, disorder, surface relaxation) effects on the bandstructure, quantum mechanical size-quantization, and nonlinear, highly stochastic and dynamically-coupled charge and phonon transport processes. While numerical simulation is playing a key role in semiconductor R&D today, for low-dimensionality devices, modeling approaches based on a continuum representation of the underlying material become invalid. On the other hand, a correct quantum description is well achieved by means of ab initio methods. A manifold of flavors and codes exist (such as ABINIT, SIESTA, PWscf) each of which is adapted to the specific needs of their users. Unfortunately, while offering intellectual appeal, these ab initio materials science methods can only model very small and closed systems having ~100 atoms near or at equilibrium. This fundamentally originates from the complexity of the quantum representation of electrons, possessing several internal degrees of freedom, compared to those of the simple particles used in classical calculations. The requirement of self-consistency further slows down such methodology. Note that, today’s nanoscale devices exploit physical processes at a scale where the number of atoms in the active regionis on the order of 10,000 to more than 100 million[1]! Therefore, modeling of nanodevices, to be adequately accurate yet computationally efficient, must employ a multiscale and modular approach. Also, the plethora of geometry, material, and doping configurations in semiconductor devices at the nanometer scale suggests that a general nanoelectronic modeling tool is needed.

This work describes our on-going efforts to develop a multiscale Quantum Atomistic Device Simulator (QuADS 3-D) where: a) material parameters are obtained atomistically using first-principles, b) structural relaxation and phonon dispersions are studied via molecular mechanics/dynamics, c) a variety of tight-binding models (s, sp3s*, sp3d5s*) are used for the calculation of electronic bandstructure and interband transition rates, and d) coupled charge-phonon transport is simulated using a combined Monte Carlo-NEGF framework[2][3][4]. The atom-by-atom simulation capability in QuADS 3-D exposes new degrees-of-freedom at nanoscale (such as engineering the stress, hybrid crystal cuts, composition, surface polarization, and electrostatics) and creates transformative design routes for boosting performance and reliability of novel nanoelectronic devices. Successful applications of QuADS 3-D will be demonstrated by four examples: (1) quantum and coulomb effects in nanoscale FETs (nanowires, 2D MoS2, CNTs, and junctionless FETs); (2) correlation of structuralmodifications and reliability of nanoscale HEMTs; (3) efficiency droop in nanostructured III-N LEDs; and (4) effects of contact resistances in determining the efficiency of nanostructured thermoelectric coolers. QuADS 3-D uses several novel, memory-miserly, parallel and fast algorithms[5], and incorporates state-of-the-art fault-tolerant software design approaches, which enables the simulator to assess the reliability of available petaflop computing platforms (TeraGrid, NCCS, NICS). A web-based online inter¬active version of the simulator for educational use willalso be available on http://www.nanoHUB.org.

[1] Shaikh Ahmed et al., "Multimillion Atom Simulations with NEMO 3-D", In Meyers, Robert (Ed.) Encyclopedia of Complexity and Systems Science, vol. 6, pp 5745–5783. Springer New York 2009.
[2] Shaikh Ahmedet al., "Quantum Atomistic Simulations of Nanoelectronic Devices using QuADS," in Nano-Electronic Devices: Semiclassical and Quantum Transport Modeling, book edited by D. Vasileska and S. M. Goodnick, Springer, pp. 405–441, 2011.
[3] Shaikh Ahmedet al., "Electronic Structure of InN/GaN Quantum Dots: Multimillion Atom Tight-Binding Simulations," IEEE Transactions on Electron Devices, vol. 57, issue 1, pp. 164–173, 2010.
[4] Shaikh Ahmedet al., "Parameter-Free Effective Potential Method for Use in Particle-Based Device Simulations," IEEE Transactions on Nanotechnology, vol. 4, pp. 465–471, July 2005.
[5] Song Li et al., "Compute the Diagonal of Sparse Matrix Inverse using FIND Algorithm," Journal of Computational Physics, vol. 227, pp. 9408–9427, 2008.
[6] AfsanaSharmin, et al., "Multiscale Design of Nanostructured Thermoelectric Coolers: Effects of Contact Resistances," IEEE/TMS Journal of Electronic Materials, vol. 44, no. 6, pp. 1697–1703, 2015.
[7] Shaikh Ahmed, et al., "Multimillion-Atom Modeling of InAs/GaAs Quantum Dots: Interplay of Geometry, Quantization, Atomicity, Strain, and Linear and Quadratic Polarization Fields," Journal of Computational Electronics, vol. 14, pp. 543–556, 2015.


invited2Shaikh Shahid Ahmed received the B.S. degree in electrical and electronic engineering from Bangladesh University of Engineering and Technology (BUET), Dhaka, Bangladesh, in 1998, and the M.S. and Ph.D. degrees in electrical engineering from Arizona State University, Tempe, in 2003 and 2005, respectively. From 2005 to 2007, he was a Postdoctoral Research Associate in the School of Electrical and Computer Engineering and at the National Science Foundation (NSF) Network for Computational Nanotechnology (NCN), Purdue University, West Lafayette, Indiana, USA. In August 2007, Shaikh Shahid Ahmed joined Southern Illinois University, Carbondale, Illinois, USA, where he is currently a Full Professor in the Department of Electrical and Computer Engineering.

Research activities in Professor Ahmed’s Group focus mainly in the field of computational nanoelectronics and involves multiscale modeling of electronic structure and transport in nanoscale devices including novel transistors, semiconducting 2-D structures and nanowires, quantum dots and nanocrystals, solid-state lighting sources and their reliability, nanoscale thermoelectric and piezoelectric energy-harvesting devices, and nanoelectronic devices for applications in harsh environments. The goal is three-fold: a) better understand the underlying physical processes; b) explore and exploit the enhanced degrees-of-freedom available at nanoscale for device optimization; and c) develop cyber-enabled community nanoelectronics simulation software. Research and computational efforts in Professor Ahmed’s group make extensive use of advanced algorithms and state-of-the-art high-performance cluster and CPU/GPGPU distributed computing platforms.

Shaikh Shahid Ahmed is the recipient of 2009 Oak Ridge National Lab/ORAU High-Performance Computing Award and his group has successfully scaled their scientific software on more than 130,000 cores in then Jaguar XT5 fastest supercomputing platform in the world. Professor Ahmed has authored/co-authored 11 nanoelectronics software tools (nanoFET, CNTFET, QuaMC 2D, Schred, nanoMOS, FETtoy, MOSFET, MOScap, QPC, nanoSSL, and multiscaleTEC), access to which are freely available on NSF’s nanoHUB.org. As of July 2016, these software tools have served around 14,100 individual users worldwide, running more than 638,000 simulations [https://nanohub.org/members/9293/usage]. Shaikh Shahid Ahmed is the Principal Investigator of the NSF funded Southern Illinois High Performance Computing Research Infrastructure (SIHPCI). He is the recipient of 2013 Dean JuhWah Chen Outstanding Faculty Award from SIU College of Engineering. He has published over 70 papers/articles in refereed journals and proceedings, authored six book chapters, and delivered more than 80 technical talks. Professor Ahmed is a senior member of the IEEE and member of the American Physical Society.