Novel NEGF algorithms : The Non-Equilibrium Green’s Function (NEGF) approach is considered as the state-of-the-art modeling tool for predicting performance and designing emerging nanoscale devices. However, accurate and reliable modeling of the future nanoscale devices requires huge computational efforts, yet the current NEGF algorithms are prohibitively expensive. Collaborated with Stanford University, NASA Ames, University of Alabama, and Purdue University for the development and implementation of novel algorithms and methodologies for the calculations of non-equilibrium Green’s functions and associated charge densities from large sparse matrices describing the underlying nanoscale systems. Different parallel computing methodologies using MPI and OpenMP were studied and implemented in these application software for achieving optimum speed/memory performance.  

[S. Li, S. Ahmed, E. Darve, and G. Klimeck, "Compute the Diagonal of Sparse Matrix Inverse using FIND Algorithm", Journal of Computational Physics, Vol. 227, pp. 9408–9427, 2008]

Full-Band Particle-Based Monte-Carlo Simulation with Anharmonic Corrections for Phonon Transport: Monte Carlo based statistical approach to solve Boltzmann Transport Equation (BTE) has become a norm to investigate heat transport in semiconductors at sub-micron regime, owing to its ability to characterize realistically sized device geometries qualitatively. One weakness of this technique is that the approach predominantly uses empirically fitted phonon dispersion relation as input to determine the properties of phonons and predict the thermal conductivity for a specified material geometry. The empirically fitted dispersion relations assume harmonic approximation, thereby failing to account for thermal expansion, effects of strain on spring stiffness, and accurate phonon-phonon interactions. To account for the anharmonic contributions in the calculation of thermal conductivity, in this work, we employ a coupled molecular mechanics-Monte Carlo (MM-MC) approach. The atomistically-resolved non-deterministic approach adopted in this work is found to produce satisfactory results on heat transport and thermal conductivity in both ballistic and diffusive regimes for III-N nanostructures. Figure on right shows the lattice thermal conductivity of Bi2Te3 thin film, inset illustrating how temperature distribution in the device active region evolves with time. (Supported by the U.S. National Science Foundation)

Quantum potential : Worked in the development of a parameter-free quantum field approach for use in conjunction with particle-based simulations. The method is based on a perturbation theory around thermodynamic equilibrium and leads to a quantum field formalism in which the size of an electron depends upon its energy. The approach when used in the simulations of a conventional nanoscale 25 nm n-channel MOSFET device is found to produce correct experimentally verified threshold voltage shifts of about 220 mV and drain current degradation of about 30%. To further test the applicability, the quantum field formalism is used to calculate the threshold voltage and output characteristics of recently proposed single-gated (SG) and dual-gated (DG) fully-depleted silicon-on-insulator (FDSOI) devices. It is observed that the method quite correctly retrieves the trend in the threshold voltage shift with the variation of silicon film thickness. The simulation results are verified with the available experimental and/or theoretical data.

[Shaikh Ahmed, Christian Ringhofer, and Dragica Vasileska, “Parameter-Free Effective Potential Method for Use in Particle-Based Device Simulations”, IEEE Transactions on Nanotechnology, Vol. 4, 4, pp. 465–471, 2005. ]

Quantum barrier potential in an FET device

Fast Multipole Method (FMM) : In a typical particle-based device simulation experiment, a drawback is that the 3-D Poisson equation must be solved repeatedly to properly describe the self-consistent fields, which consumes over 80% of the total simulation time. To further speed up simulations, we used a 3-D Fast Multi-Pole Method (FMM) instead. The FMM allows calculation of the field and the potential in a system of n particles connected by a central force within operations given certain prescribed accuracy. The FMM is based on the idea of condensing the information of the potential generated by point sources in truncated series expansions. After calculating suitable expansions, the long range part of the potential is obtained by evaluating the truncated series at the point in question and the short range part is calculated by direct summation. The field due to the applied boundary biases is obtained at the beginning of the simulation by solving the Poisson equation. Hence the total field acting on each electron is the sum of this constant field and the contribution from the electron-electron and electron-impurity interactions handled by the FMM calculations. The image charges, which arrive because of the dielectric discontinuity, are handled by the method of images. The correctness of the approach is verified via the simulations of the doping dependence of the low-field electron mobility in a 3-D resistor and its comparison with available theoretical and experimental data.

[L. Greengard and V. Rokhlin, “A fast algorithm for particle simulations,” J. Comp. Phys., Vol. 135, no. 2, pp. 280–292, 1997]

[H. Khan, S. Ahmed, and D. Vasileska, C. Heitzinger, C. Ringhofer, “Modeling of FinFET: 3D MC Simulation Using FMM and Unintentional Doping Effects on Device Operation”, Journal of Computational Electronics, Vol. 3, pp. 337–340, 2004]

[D. Vasileska and S. Ahmed, “Narrow-Width SOI Devices: The Role of Quantum Mechanical Size Quantization Effect and the Unintentional Doping on the Device Operation”, IEEE Transactions on Electron Devices, Vol. 52, Issue 2, pp. 227–236, 2005]

Event-biasing for statistical enhancement in the Monte-Carlo device simulations : Enhancement algorithms are especially useful when the device behavior is governed by rare events in the carrier transport process. It is shown that the weight of the particles, as obtained by biasing the Boltzmann equation, survives between the successive steps of solving the Poisson equation. Particular biasing techniques are applied to the simulations of subthreshold conduction in a 15 nm n-channel MOSFET and the convergence of both the terminal current and the channel current is analyzed.  It is found that event-biasing experiments recover precisely the physical averages and the self-consistent field and reduces the time necessary for computation of the desired device characteristics.

[Shaikh Ahmed, Mihail Nedjalkov, and Dragica Vasileska, “Comparative Study of Various Self-Consistent Event Biasing Schemes for Monte Carlo Simulations of Nanoscale MOSFETs,” In Theory and Applications of Monte Carlo Simulations, book edited by Victor (Wai Kin) Chan, ISBN 978-953-51-1012-5, Published: March 6, 2013.]

[M. Nedjalkov, S. Ahmed, and D. Vasileska, “A self-consistent event biasing scheme for statistical enhancement”, Journal of Computational Electronics, Vol. 3, pp. 305–309, 2004]

       Enhancement of Statistics: Reducing Standard Deviation

Parallel computation. Parallel programming and algorithms are parts and parcels in our group and routinely used. Parallel applications like LAMPS and NEMO 3-D are the core components in the software developed by the group. LAMMPS is a classical molecular dynamics code that models an ensemble of particles in a liquid, solid, or gaseous state. It can model atomic, polymeric, biological, metallic, granular, and coarse-grained systems using a variety of force fields and boundary conditions. LAMMPS is designed to be easy to modify or extend with new capabilities, such as new force fields, atom types, boundary conditions, or diagnostics. Excellent parallel scaling exceeding 120,000 cores has recently been performed by our group on ORNL Jaguar XT5, maximum number of atoms simulated being ~3.84 billion! Also, In the self consistent quantum simulations, the most computationally expensive part is the Green’s function calculation at every energy point. For example, the size of the Hamiltonian can be up to 7000×7000, block-tri-diagonal. Depending on the required energy resolution, the equations may need to be solved 1000 times for every Poisson iteration. In order to reduce the computational burden, the MPI (message passing interface) parallelization scheme has been implemented in most of the quantum transport solvers in the independent computation of Green’s function at each point along the energy spectrum.

 LAMMPS scaling in Cray XT5 machine at ORNL                     

Electronic structure of realistically-sized quantum dots : The theoretical knowledge of the electronic structure of nanoscale semiconductor devices is the first and most essential step towards the interpretation and the understanding of the experimental data and reliable device design at the nanometer scale. Electronic band structure of a solid originates from the wave nature of particles and depicts the allowed and forbidden energy states of electrons in the material. Recently we have studied symmetry breaking and energy level splitting in self-assembled Zincblende and Wurtzite quantum dots through atomistic simulations. The symmetry in quantum dots realized from III-V materials is lowered due to two fundamental symmetry breaking mechanisms: (a) the underlying crystal, which lacks inversion symmetry, (2) the presence of strain, and (3) strain induced piezoelectric potential. In III-N materials, in addition to piezoelectric field, there exists a pyroelectric contribution as well. Results show a significant dependence of the dot states and optical polarization on the geometry (Box/Dome/Pyramid) and size of the QDs. NEMO 3-D, a versatile and open source electronic structure code that can handle device domains relevant for realistic large devices, is used in this work. Realistic devices containing millions of atoms can be computed with reasonably, easily available cluster computers. NEMO 3-D employs a VFF Keating model for strain and the 20-band sp³d⁵s^* empirical tight-binding model for the electronic structure computation. It is released under an open source license and maintained by the NCN at Purdue University, West Lafayette under the supervision of Professor Gerhard Klimeck.

Quantum dots on a thin (one atomic layer) InAs wetting layer. Two major computational domains are also shown. Delec: central smaller domain for electronic structure (quantum) calculation, and Dstrain: outer domain for strain calculation. In the figure: s is the substrate height, c is the cap layer thickness, h is the dot height, and d is the dot diameter/base length as appropriate.

Conduction band ground states in box (B), dome (D), and pyramid (P) shaped InN/GaN quantum dots including interface effects (w/out strain), strain, piezoelectricity, and pyroelectricity.

Conduction band ground states in box (B), dome (D), and pyramid (P) shaped InAs/GaAs quantum dots including interface effects (w/out strain), strain, and piezoelectricity.

Topmost valence band and first 4 Conduction band wavefunctions in dome shaped InN/GaN quantum dots including (1) interface effects, (2) strain, (3) piezoelectricity, and (4) pyroelectricity.

[Shaikh Ahmed, Sharnali Islam, and Shareef Mohammed, “Electronic Structure of InN/GaN Quantum Dots: Multimillion Atom Tight-Binding Simulations”, special issue of IEEE Transactions on Electron Devices on LEDs, vol. 57, 1, pp. 164–173, January 2010]

[Shaikh Ahmed, Neerav Kharche, Rajib Rahman, Muhammad Usman, Sunhee Lee, Hoon Ryu, Hansang Bae, Steve Clark, Benjamin Haley, Maxim Naumov, Faisal Saied, Marek Korkusinski, Rick Kennel, Michael Mclennan, Timothy B. Boykin, and Gerhard Klimeck, “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]

[Gerhard Klimeck, Shaikh Ahmed, Neerav Kharche, Hansang Bae, Steve Clark, Benjamin Haley, Sunhee Lee, Maxim Naumov, Hoon Ryu, Faisal Saied, Marta Prada, Marek Korkusinski, and Timothy B. Boykin, “Atomistic Simulation of Realistically Sized Nanodevices Using NEMO 3-D: Part I–Models and Benchmarks”, IEEE Transactions on Electron Devices, Vol. 54, 9, pp. 2079–89, 2007]

[Gerhard Klimeck, Shaikh Ahmed, Neerav Kharche, Marek Korkusinski, Muhammad Usman, Marta Prada, and Timothy Boykin, “Atomistic Simulation of Realistically Sized Nanodevices Using NEMO 3-D: Part II–Applications”, IEEE Transactions on Electron Devices, Vol. 54, 9, pp. 2090–99, 2007]

Atomistic electronic structure of ZB and WZ nanowires : In the last decade, nanowires (NWs) made from a wide variety of materials have drawn considerable interest because of their potential applications in various optoelectronic and high-mobility electronic devices. Using NWs in these devices, the performance is enhanced due to increased charge localization and reduction in the defect density. In this work, electronic bandstructure of [0001]-oriented Wurtzite nanowires with square cross sections is calculated using sp3s*d5 and sp3s* tight-binding models and then used to parameterize the bandgap and Gamma-valley effective masses. The materials used include: group III-V, group III-nitrides, group II-VI (CdSe, ZnSe, CdS and ZnS) and 2H-SiC.

Graphene nanoribbon :

Carbon nanotubes. The electronic behavior of metallic carbon nanotubes under the influence of atomistic vacancy defects present in the channel is theoretically investigated using the Non-Equilibrium Green’s function (NEGF) method self-consistently coupled with three-dimensional (3D) electrostatics. A nearest neighbor tight binding model based on a single pz orbital is used for the device Hamiltonian. A single vacancy defect in the channel of a small diameter metallic carbon nanotube can decrease its conductance by a factor of two. More than one vacancy in the channel can further drastically decrease the conductance. Larger diameter nanotubes suffer less from the presence of vacancy defects. The presence of a single vacancy locally modulates the LDOS significantly in the device. More importantly, regardless of the chirality of the nanotube, the transmission is reduced throughout the entire energy spectrum (by one quantum unit in some regions). The work is done in collaboration with Neophytos Neophytou and Gerhard Klimeck at Purdue University.

[N. Neophytou, S. Ahmed, G. Klimeck, “Influence of vacancies on metallic nanotube transport performance”, Applied Physics Letter, 90, 182119, 2007]

Quantum simulations of nanoscale dual-gate MOSFETs. There is a virtual consensus that the most practically scalable variety of all novel MOSFETs, that are in the focus of many researchers’ study today, are the double-gate SOI MOSFETs with a sub-10 nm gate length, ultra-thin, intrinsic channels and highly doped (degenerate) bulk electrodes. In such transistors, short channel effects typical for their bulk counterparts are minimized, while the absence of dopants in the channel maximizes the mobility and hence drive current density. Such advanced MOSFETs may be practically implemented in several ways including planar, vertical, and FinFET geometries. However, several design challenges have been identified such as a process tolerance requirement of within 10% of the body thickness and an extremely sharp doping profile with a doping gradient of 1 nm/decade. The SIA forecasts that this new device architecture may extend MOSFETs to the 22 nm node (9-nm physical gate length) by 2016. Intrinsic device speed may exceed 1 THz and integration densities will be more than 1 billion transistors/cm².This work focused mainly on the modeling and simulations of the size-quantization effect within a fully quantum mechanical Non-equilibrium Green’s Function (NEGF) approach and a quantum-corrected Monte Carlo transport framework for 2-D MOSFET structures and presents benchmark results of three software packages namely nanoFET, nanoMOS and QuaMC 2-D.

[Shaikh Ahmed, Gerhard Klimeck, Derrick Kearney, Michael McLennan, MP Anantram, “Quantum Simulations of Dual Gate MOSFET Devices: Building and Deploying Community Nanotechnology Software Tools on NanoHUB.org”, Journal of High Speed Electronics, Vol. 17, 3, pp 485–494, 2007]

Unintentional /discrete dopant effects in Si nanowires : Numerical simulations are performed to study the single-charge-induced ON-current fluctuations (random telegraphic noise) in silicon nanowire field-effect transistors. A 3-D fully atomistic quantum-corrected particle-based Monte Carlo device simulator (MCDS 3-D) has been used in this work. Our study confirms that the presence of single channel charges modifies the electrostatics (carrier density) and dynamics (mobility) of the device, both of which play important roles in determining the magnitude of the current fluctuations. The relative impact (percentage change in the ON-current) depends on an intricate interplay of device size, geometry, channel (crystal) orientation, gate bias, and energetics and spatial location of the charge.

[Ramya Hindupur, Sharnali Islam, and Shaikh Ahmed, "Atomistic Modeling of Unintentional Single Charge Effects in Silicon Nanowire FETs," Technical Proceedings of IEEE Nanotechnology Materials and Devices Conferences (NMDC) 2010, October 12-15, pp. 282-285, California, USA]

Gamma-valley effective mass as a function of the cross-sectional dimension of silicon nanowire.

 Electron distribution in the active region.

Energy plot for V_G = V_D = 0.8 V when a single impurity is present at the source-end of the channel.

Comparison of percentage change in the threshold volatge due to the presence of a single negative charge in [100], [110], and [111] silicon nanowires.

Modeling Random Dopant Fluctuation Effects in Nanoscale Tri-Gate MOSFETs. The tri-gate FET has been hailed as the biggest breakthrough in transistor technology in the last 20 years. The increase in device performance (faster switching, low power, improved short channel effects, etc.), coupled with the reduction in device size, would allow for huge gains in the electronics industry. In this work, an atomistic quantum-corrected Monte Carlo 3-D device simulator was used to not only investigate the validity of these claims, but also how quantum size quantization and random dopant fluctuation (RDF) affect the tri-gate FET performance and how to curb these issues. The main findings are as follow: 1) carrier scattering leads to ON current degradation of ~30% and hence cannot be ignored; 2) deviations in threshold voltage due to random channel doping are smaller in the tri-gate FET; 3) RDF due to the source/drain discreteness can be engineered by adjusting the source/drain junction depth. With randomness reduced, the overall performance should increase when used in ICs, where consistency in device characteristics is essential.

Self-heating in silicon-on-insulator FETs :

Bandstructure and crystal orientation effects in III-V MOS devices. Nanoscale double-gate n-MOSFETs with silicon and III-V (GaAs and InAs) channels are studied using numerical simulation. The device structures are based on the ITRS 14nm node (year 2020), and are simulated using the program nanoMOS, which utilizes the NEGF technique for treating ballistic electron transport in the channel. The effective masses used are obtained by extraction from the full band structure using the sp³d⁵s^* empirical tight-binding method. This process returns effective mass values for all valleys which are far more accurate than bulk values for the ultra-thin-body MOSFET. The results indicate that for digital logic applications, III-V materials offer little or no performance advantage over silicon for ballistic devices near the channel length scaling limit.

[Himadri S. Pal, Kurtis D. Cantley, Shaikh S. Ahmed, and Mark S. Lundstrom, "Influence of Bandstructure and Channel Structure on the Inversion Layer Capacitance of Silicon and GaAs MOSFETs", IEEE Transactions on Electron Devices, vol. 55, issue 3, pp 904–908, 2008]

Modeling Floating Body Memory Devices. Numerical simulations using the Silvaco TCAD tool have been performed to model dual-gate floating body ZRAM devices. Floating body memories are a new generation memory cells which are being researched as an alternative for DRAM memory in order to get rid of the bulky storage capacitor. The states are written into the device using impact ionization to generate a large number of holes in the substrate, which alter the threshold voltage of the device. Amongst a group of variants, dual gate structures help reduce drain-induced barrier lowering and hence leakage, while having better control of the charge in the substrate. In addition to the bulk DG-ZRAM cell, a recently proposed DG-ZRAM structure with a SiGe quantum well (QW) introduced into the substrate (which acts as a hole storage pocket) was also simulated. Comparisons in terms of noise margin (∆I_D) have been made for both the devices, which show that the structure with the QW in the substrate performs better than the bulk structure. For the QW cell, simulations have been performed taking into consideration gate electrodes with different work functions and it has been observed that while aluminum has a detrimental impact in conventional MOSFETs due to high off-state leakage current, it can be used to obtain low power ZRAM memory cells. Parameters such as QW doping density, composition, channel length, QW thickness and its position from the top gate have been varied to obtain the optimum noise margin for the device.

[Ramya Hindupur and Shaikh Ahmed]

Atomistic Modeling of Degradation Mechanisms in Nanoscale HEMT Devices. Through atomistic numerical simulations, we investigate how performance degradation of state-of-the-art AlGaN HEMTs is governed by an intricate coupling of the underlying thermo-electro-mechanical processes while operating at high power and/or high-temperature. The polarization induced charge density is shown to be strongly dependent on the thickness of the AlN barrier layer. This further demonstrates that the degradation in these HEMT devices is related to the reduction of the effective thickness of the AlN barrier layer, which, during operation at high device temperature, could arise from the diffusion of gate metal into the barrier material matrix. This finding has been validated using the massively parallel LAMMPS molecular dynamics tool and available experimental data. We have also demonstrated that the polarization fields alone can induce channel carriers at zero external bias and lead to a significant increase in the ON current.

Modeling droop in solid-state lighting devices : III-nitride solid-state lighting (SSL) has the potential, by 2025, to decrease electricity consumed by lighting by >50%, cut ~28 million metric tons of carbon emission annually, and benefit general illumination, transportation, communication, automobiles, imaging, agriculture, and medicine. SSL will revolutionize semiconductor market and can reestablish U.S. manufacturing leadership. The objective of this research is to computationally investigate: i) how efficiency droop and color degradation in III-nitride SSL devices are governed by an intricate interplay of crystal atomicity, built-in structural fields, and charge and phonon transport processes, and ii) how tuning the basic physical properties at nanoscale can create transformative solution paths. [Supported by NSF]

[Md. Rezaul Karim Nishat, Mayada M. Taher, and Shaikh S. Ahmed, “Million-Atom Tight-Binding Modeling of Nonpolar a-Plane InGaN Light Emitters," Journal of Computational Electronics, vol. 17, no. 4, pp. 1630–1639, 2018.
Mayada Taher and Shaikh Ahmed, “III-Nitride Multiple Disk-in-Wire Laser Structures: Effects of Crystal Orientation and Spacer Size,” Optical Materials, vol. 83, pp. 104–110, 2018.
Md Rezaul Karim Nishat, Saad M. Alqahtani, Vinay U. Chimalgi, Neerav Kharche, and Shaikh S. Ahmed, “Atomistic Modeling of Nonpolar m-Plane InGaN Disk-in-Wire Light Emitters,” Journal of Computational Electronics, vol. 16, no. 3, pp. 814–824, 2017.
Saad Mubarak Al-Qahtani, Abdulmuin Abdullah, Md. Rezaul Karim Nishat and Shaikh Ahmed, “Diameter Dependent Polarization in ZnO/MgO Disk-in-Wire Emitters: Multiscale Modeling of Optical Quantum Efficiency,” Superlattices and Microstructures, vol. 103, pp. 48–55, 2017.
Vinay Chimalgi, Md. R. K. Nishat, and Shaikh Ahmed, “Nonlinear Piezoelectricity and Efficiency Droop in Hexagonal In(Ga)N/GaN Disk-in-Wire LEDs,” Superlattices and Microstructures, vol. 84, pp. 91–98, 2015.
Shaikh Ahmed, Sasi Sundaresan, Hoon Ryu, and Muhammad Usman, “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.
Sasi Sundaresan, Vamsi Gaddipati, and Shaikh Ahmed, “Effects of Spontaneous and Piezoelectric Polarization Fields on the Electronic and Optical Properties in GaN/AlN Quantum Dots: Multimillion-Atom sp3d5s* Tight-Binding Simulations,” Int. J. Numer. Model., vol. 28, pp. 321–334, 2015.
Vinay Chimalgi, Krishna Yalavarthi, Md Rezaul Karim Nishat, and Shaikh Ahmed, “Atomistic Simulation of Surface Passivated Wurtzite Nanowires: Electronic Bandstructure and Optical Emission,” Adv. Nano Research, vol. 2, no. 3, pp. 157–172, December 2014.
Vinay Chimalgi, Neerav Kharche, and Shaikh Ahmed, “Effects of Substrate Orientation on Opto-Electronic Properties in Self-Assembled InAs/GaAs Quantum Dots,” Journal of Computational Electronics, vol. 13, pp. 1026–1032,November 2014.
Krishna Yalavarthi, Vinay Chimalgi and Shaikh Ahmed, “How Important is Nonlinear Piezoelectricity in Wurtzite GaN/InN/GaN Disk-in-Nanowire LED Structures?” Optical and Quantum Electronics, vol. 46, pp. 925–933, 2014.
Shaikh Ahmed, Mihail Nedjalkov, and Dragica Vasileska, “Comparative Study of Various Self-Consistent Event Biasing Schemes for Monte Carlo Simulations of Nanoscale MOSFETs,” Theory and Applications of Monte Carlo Simulations, no. 5, pp. 109–133, 2013.
Ky Merrill, Krishna Yalavarthi and Shaikh Ahmed, “Giant Growth-Plane Optical Anisotropy in c-Plane Wurtzite GaN/InN/GaN Dot-in-Nanowires,” Superlattices and Microstructures, vol. 52, no. 5, pp. 949–961, 2012.]

Bandgap (and color emission) as a function of indium content

[K. Yalavarthi, SISPAD 2012]

Multiscale Design of Nanostructured Thermoelectric Coolers : Here, our objective is to deploy a multiscale simulator for thermoelectric cooler devices, where the material parameters are obtained atomistically using a combination of molecular dynamics and tight-binding simulations and then used in the system level design. In a recently published work [A. Sharmin, M. Rashid, V. Gaddipati, A. Sadeque, and S. Ahmed, “Multiscale Design of Nanostructured Thermoelectric Coolers: Effects of Contact Resistances,” Journal of Electronic Materials, in press, DOI: 10.1007/s11664-014-3520-8, November 2014], after benchmarking the simulator against a recent experimental work [I. Chowdhury, R. Prasher, K. Lofgreen, G. Chrysler, S. Narasimhan, R. Mahajan, D. Koester, R. Alley and R. Venkatasubramanian, “On-chip cooling by superlattice-based thin-film thermoelectrics”, Nature Nanotechnology, vol. 4, pp. 235–238, 2009], we carried out a detailed numerical investigation of the performance of Bi2Te3 nanowire based thermoelectric devices for hot-spot cooling. The results suggest that active hotspot cooling of as much as 23 ºC with a high heat flux is achievable using such low-dimensionality structures. However, it has been observed that thermal and electrical contact resistances, which are quite large in nanostructures, play a critical role in determining the cooling range and lead to significant performance degradation that must be addressed before these devices can be deployed in such applications. Besides applications in embedded and potable coolers, thermoelectric devices are used as power sources for remote telecommunication, navigation, and radioisotope generator for space vehicles, and show great promise in heat scavenging in vehicle exhaust system. Recently, within a sp3d5s* tight-binding scheme, we have determined the bandstructure of Bi₂Te₃ nanowire for use in high-temperature thermoelectric devices. Previously, we worked on bulk Bi₂Te₃ material (left panel of the below Figure). Right panel of the Figure shows (inset) a rectangular (atomistic) Bi₂Te₃ nanowire having dimensions of Lx = 0.8 nm Ly = 0.8 nm, and Lz = 3.045 nm, and the energy bandgap vs. nanowire dimension (side-length) plot where a decrease in bandgap with the reduction of nanowire dimension is due to the quantum-mechanical size-quantization effect. [Supported by NSF]

Software Development for Multiscale Modeling of Conventional and Emerging Devices:

This work aims 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. 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. 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 for educational purposes will soon be available on http://www.nanoHUB.org.

[Shaikh Ahmed, Mohammad Rashid, Saad Al-Qahtani, Md Rezaul Karim Nishat, Khadija Khair, Ye Wu, Abdussamad Muntahi, Mayada Taher and Abdulmuin Abdullah, “Multiscale and Multiphysics Modeling of Non-Classical Semiconductor Devices,” ICECE 2016, Proc. of 9th Int. Conference on Electrical and Computer Engineering, Dhaka, Bangladesh, December 2016. (DOI:10.1109/ICECE.2016.7853846; http://ieeexplore.ieee.org/document/7853846/)]

[Shaikh Ahmed, Krishnakumari Yalavarthi, Vamsi Gaddipati, Abdussamad Muntahi, Sasi Sundaresan, Shareef Mohammed, Sharnali Islam, Ramya Hindupur, Dylan John, And Joshua Ogden, “Quantum Atomistic Simulations of Nanoelectronic Devices using QuADS,” Nano-Electronic Devices: Semiclassical and Quantum Transport Modeling, Springer, Book Edited by D. Vasileska and S. M. Goodnick, pp. 405, 2011.]

Community nanotechnology software development: freely available on www. nanoHUB.org

The NSF Network for Computational Nanotechnology (NCN) supports the National Nanotechnology Initiative through research, simulation tools, and education and outreach. Deployment of these services to the science and engineering community is carried out via web-based services, accessible through the nanoHUB portal http://www.nanoHUB.org. The educational outreach of NCN is realized by enabling access to multimedia tutorials, which demonstrate state-of-the-art nanodevice modeling techniques, and by providing space for relevant debates and scientific events. The second purpose of NCN is to provide a comprehensive suite of nano simulation tools, which include electronic structure and transport simulators of molecular, biological, nanomechanical and nanoelectronic systems. Access to these tools is granted to users via the web browsers, without the necessity of any local installation by the remote users. The definition of specific sample layout and parameters is done using a dedicated Graphical User Interface (GUI) in the remote desktop (VNC) technology. The necessary computational resources are further assigned to the simulation dynamically by the web-enabled middleware, which automatically allocates the necessary amount of CPU time and memory. The end user, therefore, has access not only to the code, a user interface, and the computational resources necessary to run it but also to the scientific and engineering community responsible for its maintenance. The nanoHUB is currently considered one of the leaders in science gateways and cyber infrastructure. The process of web-based deployment of these tools is depicted in the following Figure. A user visits the www.nanohub.org site and finds a link to a tool. Clicking on that link will cause our middleware to create a virtual machine running on some available CPU. This virtual machine gives the user his/her own private file system. The middleware starts an application and exports its image over the Web to the user’s browser. The application looks like an Applet running in the browser. The user can click and interact with the application in real time taking advantage of high-performance distributed computing power available on local clusters at Purdue University and on the NSF TeraGrid or the open science grid. I have been a contributor in this well-established and long-term national nanotechnology initiative. In my teaching of graduate and undergraduate level courses, I make extensive use of these freely-available resources available from NCN.