Some of my current and past research activities and interests

Electronic Structure of 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 quantum dots through atomistic simulations of long-range strain and piezoelectric field. 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, and (2) the presence of strain which induces piezoelectric charges. Strain in self-assembled quantum dots is a long-range phenomenon and the dot states depend strongly on both the size of the strain domain and the boundary conditions. Both sources of symmetry breaking influence the energy level splitting in self-assembled quantum dots. Results show a significant dependence of the dot states and optical polarization on the size of the strain domain and the boundary conditions.

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 struc¬ture 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.

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.

Graphene nanoribbon. Electronic structures are computed with the application of magnetic field in order to investigate the quantum Hall effects and beyond within a NEGF formalism.

Novel 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.

Study of bandstructure and crystal orientation effects in novel nanoscale 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.

Noise modeling. Modeling and characterization of low frequency noise in nanoscale MOSFETs. The effects of Halo tilt angle and dose and energy of implants on the 1/f noise characteristics are the focus of this study. Modeling is being done with a full three dimensional quantum corrected Monte Carlo simulator while the experimental data are received from partners in Intel and Texas Instruments.

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.

Monte Carlo simulations. Developed a full 3D Monte-Carlo particle based simulator coupled with effective potential schemes to investigate the quantum effects occurring in recently proposed narrow-width SOI devices. From the results obtained, it is found that the two-dimensional (2D) carrier confinement in the device structure results not only in a significant increase in the threshold voltage but also in its pronounced channel width dependency.

Investigations of the impact of unintentional/discrete doping induced intrinsic parameter variations on the performance of novel MOS devices. As the silicon industry moves into the 45-nm-node regime and beyond, one of the most important challenges facing us is the increasing variability in device characteristics mainly induced by discrete/unintentional dopants. Proper treatment of Coulomb forces is essential. To treat the Coulomb (electron-ion and electron-electron) interactions properly, three different but consistent real-space molecular dynamics (MD) schemes have been implemented: the particle-particle-particle-mesh (P³M) method, the corrected Coulomb approach and the Fast Multipole Method (FMM). It is believed that the FMM algorithm has been used for the first time in the simulations of semiconductor devices. Demonstrated is the fact that unintentional doping in the channel region leads to considerable fluctuations in the device intrinsic parameters. Strong correlation between the location of the impurity/dopant atom and the magnitude of the drain current was observed and the underlying reasons were analyzed within a quantum-mechanical framework.

FIBMOS devices. Modeling and simulation of Focused Ion Beam MOSFET (FIBMOS) devices using quasi-3D Monte-Carlo particle based simulator coupled with an effective potential scheme. It is found that the short-channel effects are suppressed in the FIBMOS structures compared to their conventional counterparts.

SJTs. Modeling and simulation of Schottky Junction Transistors (SJTs) using quasi-3D Monte-Carlo particle based simulator. Gate tunneling was modeled by an Airy function approach. A nonperturbative mode of surface roughness scattering was included in the simulator. The device was found to offer higher mobility and transconductance than its conventional counterpart.

Transfer matrix formalism. Study and simulation of ballistic transport in mesoscopic devices using basically the transfer matrix formalism. Classical solution of the electrostatic confinement through 3D Poisson solver was combined with the Landauer’s formalism to calculate the on-state current quantum mechanically (treating the device as a quantum wire with a constriction).

TFEL devices. Worked on developing Hydrodynamic and Extended Drift-Diffusion models to simulate Thin Film Electro-Luminescent (TFEL) devices.

SOI device characterization and modeling. HgFET Pseudo-MOSFET (ψ-MOSFET) characterization/measurement/modeling of silicon-on-insulator (SOI) material by Four Dimensions CV Map 92-B System allowing threshold voltage, electron and hole mobility, doping density, oxide charge, interface trap density, etc. to be determined. A SIMOX wafer (p-type, 8.5-14 ohm-cm) was used in this study. 

Developing a self-consistent event-biasing scheme 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. The method is used for the simulations of subthreshold conduction in a 15 nm n-channel MOSFET. 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.

Parallel computation. 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.

Novel algorithms. The Non-Equilibrium Green’s Function (NEGF) approach is being considered as a state-of-the-art modeling tool in predicting performance and designing emerging nanoscale devices. However, accurate and reliable modeling of the future nanoscale devices requires huge computational efforts, yet the current algorithms are prohibitively expensive. Collaborated with Stanford University, NASA Ames Research Center, University of Alabama and Purdue University in the development and implementation of novel algorithms and methodologies for solving non-equilibrium Green’s functions from large sparse matrix systems. Different parallel computing methodologies using MPI and OpenMP are being studied and implemented in these applications for achieving optimum performance gain.

Modeling Nanoelectronic Materials and Devices for Harsh-Environment Applications. The objective of this research is to advance the state-of-the-art of modeling novel nanoelectronic materials and devices for harsh-environment (HE) applications, by developing (for the first time) a general and comprehensive (multiscale and multiphase) simulation platform MoHEN (Modeling Harsh-Environment Nanoelectronics) . HE electronics is a relatively new and emerging area of technology, where the use of nanoscale materials/devices (viz. SOI, SiGe, SiC, III-N, graphene) promises significant cost reduction, functional diversifications, and performance similar to the mainstream counterparts. Contemporary modeling efforts of HE nanodevices are few and disjoint–divided between materials scientists (ab initio with only ~100 atom, closed boundary) and device engineers (continuum, open boundary)–and mostly non-predictive. This research will bridge the gap between the large-size, classical semiconductor device models and the molecular level material modeling and embraces both the multiscale (atom–crystal–device) and the multiphase (growth–performance–reliability) elements within realistic boundary conditions. It would address the techniques for solving quantum mechanical (QM) non-equilibrium statistical mechanics in multimillion-atom systems with more than 10⁷ complex degrees of freedom.

Quantum Atomistic Device Simulations (QuADS). Currently working on the development of a comprehensive full three-dimensional quantum atomistic device simulator (QuADS) for nanowires, nanotubes, molecular devices, novel MOSFETs, and other nanoscale semiconductor devices. The simulator will have the capability of simulating different material systems with different geometric structures/configurations. Different schemes will be implemented for electrostatics and transport kernels nature of which range from purely classical to full quantum mechanical. Important features of the simulator: (a) Electrostatics being solved on the very atomistic level on the fundamental zincblende/diamond/graphene lattice using both finite element and finite difference schemes. (b) New algorithms for quantum transport based on non-equilibrium Green’s function such as basis-set reduction technique, contact block reduction, fast inverse using nested dissection, wavefunction decomposition, divide and conquer methodology, Sancho-Rubio algorithm and recursive algorithm. (c) Fully parallel and portable applications using Message passing Interface (MPI) and OpenMP and open source packages. (d) State-of-art graphical user interface using in-house rapid application infrastructure (RAPPTURE). (d) Open source code and application development using SVN repository system, and (e) Availability of the application through GNU public license and web-based infrastructure on www.nanohub.org.  

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.

I have been in the development team of the following nanoelectronic software. Please click on the title and you will be guided to the tool page.

1. nanoFET simulates ballistic transport properties in two-dimensional classical and novel MOSFET devices. The overall F90 & C⁺⁺ simulation framework consists of the non-equilibrium Green’s function equations solved self-consistently with Poisson’s equation. Four different algorithms have been employed ― (1) Recursive Green’s function, RGF (2) Fast Inverse using Nested Dissection, FIND (3) A parallel RGF solver, PDIV and (4) another parallel RGF solver in F90, SPIKE. A friendly GUI based on Rappture is provided. The simulations are freely available on NanoHUB.org.

2. CNTFET can currently simulate the impact of quantum mechanical size quantization and phase coherence in zigzag carbon nanotubes with both planar and coaxial exterior architectures. The package is based on non-equilibrium Greens’ function (NEGF) techniques using a pz-orbital nearest-neighbor tight binding. Full three-dimensional (3D) electrostatics has been captured by the Finite-Element-Method (FEM) of solving the Poisson Equation. Solution of this set of equations is computationally intensive. One can reduce the simulation time by using a mode-space approach instead of the real-space approach. By default the simulator solves for both electrons and holes. A friendly GUI based on Rappture is provided. The simulation is freely available on NanoHUB.org.

3. QuaMC (pronounced as quam-see) is a quasi three-dimensional quantum-corrected diffusive semiclassical Monte Carlo transport simulator for conventional and non-conventional MOSFET devices. A parameter-free quantum field approach has been developed and utilized quite successfully in order to capture the size-quantization effects in nanoscale MOSFETs. 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. Also in this simulator, the use of self-consistent event-biasing schemes for statistical enhancement in the Monte Carlo device simulations has been presented. The simulation is freely available on NanoHUB.org.

4. FETToy calculates the ballistic I-V characteristics for a conventional MOSFETs, Nanowire MOSFETs and Carbon Nanotube MOSFETs. Only the lowest subband is considered, but it is readily modifiable to include multiple subbands.

5. MOSCap Simulates the capacitance of bulk and dual gate capacitors for a variety of different device sizes, geometries, temperature and doping profiles.

6. MOSFET Simulates the capacitance of bulk and SOI Field Effect Transistors (FETs) for a variety of different device sizes, geometries, temperature and doping profiles. Enables the visualization of various device characteristics such as Id-Vd and Id-Vg. MOSFET lab is based on the Padre simulation tool developed by Mark Pinto, R. Kent Smith, and Ashraful Alam at Bell Labs.

7. nanoMOS 3.0 NanoMOS is a 2-D simulator for thin body (less than 5 nm), fully depleted, double-gated n-MOSFETs. A choice of five transport models is available (drift-diffusion, classical ballistic, energy transport, quantum ballistic, and quantum diffusive). The quantum transport models are based on mode-space method within an effective mass approximation. Scattering in the device can also be treated by a simple model that uses so-called Büttiker probes.

8. Schred 2.0 Calculates the envelope wavefunctions and the corresponding bound-state energies in a typical MOS (Metal-Oxide-Semiconductor) or SOS (Semiconductor-Oxide-Semiconductor) structure and a typical SOI structure by solving self-consistently the one-dimensional (1D) Poisson equation and the 1D Schrodinger equation.

Publications

Book Chapters

1. 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”, Encyclopedia of Complexity and System Science. Springer-Verlag 2008 (accepted and to appear in Dec. 2008).

2. Shaikh S. Ahmed, Dragica Vasileska, “Modeling of Narrow-Width SOI Devices: The Role of Quantum Mechanical Narrow Channel Effects on Device Performance”, 105-111, Large Scale Scientific Computing. Springer-Verlag 2004.

Major Journal Publications

1. 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.

2. S. 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”, J. High Speed Electronics, Vol 17, Issue, 3, pp 485-494, 2007.

3. D. Vasileska, H. Khan, and S. Ahmed, "Modleing Coulomb effects in nanoscale devices", Int. J. Nanoscience (Accpeted)

4. S. Li, S. Ahmed, E. Darve, and G. Klimeck, "Compute the Diagonal of Sparse Matrix Inverse using FIND Algorithm", Jour. Computational Physics (Accepted).

5. M. Usman, S. Ahmed, and G. Klimeck, "A tight binding study of self-assembled identical and non-identical InAs/GaAs coupled quantum dots", Applied Physics Letter (Submitted).

6. 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, No 9, September 2007.

7. Gerhard Klimeck, Shaikh Ahmed, Neerav Kharche, Marek Korkusinski, Muhammad Usman, Marta Prada, and Timothy B. Boykin, “Atomistic Simulation of Realistically Sized Nanodevices Using NEMO 3-D: Part II – Applications”, IEEE Transactions on Electron Devices, Vol 54, No 9, September 2007.

8. Neophytos Neophytou, Shaikh Ahmed, Gerhard Klimeck, “Non-Equilibrium Green’s Function (NEGF) Simulation of Metallic Carbon Nanotubes: The Effect of the Vacancy Defect”, Journal of Computational Electronics, Volume 6, Numbers 1-3, pp. 317-320, September, 2007.

9. C. Heitzinger, C. Ringhofer, S. Ahmed, and D. Vasileska, “3D Monte-Carlo Device Simulations Using an Effective Quantum Potential Including Electron-Electron Interactions”, Journal of Computational Electronics, Volume 6, Numbers 1-3, pp. 15-18, September, 2007.

10. Neophytos Neophytou, Shaikh Ahmed, Gerhard Klimeck, “Influence of vacancies on metallic nanotube transport performance”, Applied Physics Letter, 90, 182119, 2007.

11. S. Li, S. Ahmed, and E. Darve, “Fast Inverse using Nested Dissection for NEGF”, Journal of Computational Electronics, Volume 6, Numbers 1-3, pp. 187-190, September, 2007.

12. S. Ahmed, M. Usman, C. Heitzinger, R. Rahman, A. Schliwa,  and G. Klimeck, “Atomistic Simulation of Non-Degeneracy and Optical Polarization Anisotropy in Zincblende Quantum Dots”, IEEE NEMS, 2007.

13. AKM. Ahsan and S. Ahmed, “Impact of Halo Angle on 1/f noise in Conventional MOSFET technology”, Journal of Solid State Electronics, vol. 50, pp.1705–1709, 2006.

14. 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, pp. 227–236, 2005.

15. S. Ahmed, C. Ringhofer, D. Vasileska, “Parameter-Free Effective Potential Method for Use in Particle-Based Device Simulations”, IEEE Transactions on Nanotechnology, Vol. 4, pp. 465–471, July 2005.

16. Shaikh Ahmed, Dragica Vasileska and Christian Ringhofer, “Quantum potential approach to modeling nanoscale MOSFETs”, Journal of Computational Electronics, vol. 4, pp. 57–61, 2005.

17. D. Vasileska, H. R. Khan and S. S. Ahmed, “Quantum and Coulomb Effects In Nanodevices”, International Journal of Nanoscience, Vol. 4, No. 3,  pp. 305–361, 2005, World Scientific Publishing Company.

18. J. Choi, S. Ahmed, T. Dimitrova, J. Chen, and D. K. Schroder, “The Role of the Mercury-Si Schottky-Barrier Height in ψ-MOSFETs”, IEEE Transactions on Electron Devices, vol. 51, pp. 1164–1168, 2004.

19. K. Tarik, S. Ahmed, D. Vasileska and T.J. Thornton, “Subthreshold Mobility Extraction for SOI-MESFETs”, Journal of Computational Electronics, vol. 3, pp. 243–246, 2004.

20. 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.

21. Shaikh Ahmed and Dragica Vasileska, “Inclusion of Short-Range Interactions in Monte Carlo Simulations of Nanoscale Devices”, Monte Carlo Methods and Applications, Vol. 10, No. 3-4, pp. 629–641, 2004.

22. 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.

23. S. Ahmed, C. Ringhofer, and D. Vasileska, “An effective potential approach to modeling 25 nm MOSFET devices”, Journal of Comp. Electronics, vol.2, pp. 113–117, 2003.

24. S. Ahmed and D. Vasileska, “Threshold voltage shifts in narrow-width SOI devices due to quantum mechanical size-quantization effects”, Physica E, vol. 19, pp. 48–52, 2003.

25. S. Ahmed and Dragica Vasileska, “Modelling of narrow-width SOI devices”, Semicond. Science and Tech., vol. 19, pp. 131-133, 2004.

26. S. Ahmed, C. Ringhofer, and D. Vasileska, “A thermodynamic approach to quantum potential approach to modeling of 25 nm MOSFET devices”, Superlattices and Microstructures, vol. 34, pp.311–317, 2003.

27. D. Vasileska, R. Akis, I. Knezevic, S. N. Milicic, Shaikh S. Ahmed, and D. K. Ferry, "The role of quantization effects in the operation of ultrasmall MOSFETs and SOI devices", Microelectronic Engineering, vol 63, pp. 233–237, 2002.

28. D. Vasileska, I. Knezevic, R. Akis, S. Ahmed, and D. K. Ferry, "The Role of Quantum Effects on the Operation of 50 nm MOSFETs, 250 nm FIBMOS Devices and Narrow-Width SOI Device Structures", Journal of Comp. Electronics, vol 1, pp. 453–457, 2002.

29. S. Ahmed and D. Vasileska, “Narrow-Width SOI Devices: Impact of Quantum Mechanical Space-Quantization Effects on Device Performance”, IEEE Nano, pp. 223–246, 2002.

Major Refereed Archival Conference Papers/Presentations

1. Muhammad Usman, Shaikh Ahmed, and Gerhard Klimeck, "Strain and Piezoelectric Effects on the Electronic Structure of Coupled InxGa1-xAs/GaAs Self-Assembled Quantum Dots", APS march meeting 2008 (submitted).

2. Shaikh Ahmed, "Intrinsic parameter variations in nanoscale devices", IUCRC, St. Luis, Dec 2007.

3. Roksana Golizadeh-Mojarad, A.N.M. Zainuddin, Shaikh S. Ahmed, Gerhard Klimeck, Supriyo Datta, “Atomistic NEGF Simulations of Carbon Nano-Ribbons in Magnetic Fields”, IWCE, UM Amherst, 2007.

4. Kurtis D. Cantley, Yang Liu, Himadri S. Pal, Tony Low, Shaikh S. Ahmed, and Mark S. Lundstrom, “Performance Analysis of III-V Materials in a Double-Gate nano-MOSFET”, IEDM, December 10-12, 2007, Hilton Washington, Washington, DC (accepted).

5. Shaikh Ahmed, Muhammad Usman, Neerav Kharche, Andrei Schliwa, and Gerhard Klimeck, “Atomistic Simulation of Non-Degeneracy and Optical Polarization Anisotropy in Pyramidal Quantum Dots”, IEEE NEMS, Bangkok, Thailand, 2007.

6. E. Ramayya, S. S. Ahmed, D. Vasileska, S. M. Goodnick and I. Knezevic, “Mobility of Electrons in Rectangular Si nanowires”, in Technical Proceeding of the 2006 Nanotech Conference, Vol. 3, pp. 13 – 15, 2006.

7. Neophytos Neophytou, Shaikh Ahmed, Diego Kienle, Mark Lundstrom, Gerhard Klimeck, “Building and Deploying Community Nanotechnology Software Tools on nanoHUB.org – Non-Equilibrium Green's Function Simulations of the Impact of Atomic Defects on the Performance of Carbon Nanotube Transistors”, 2006 APS March Meeting, Monday–Friday, March 13–17, 2006, Baltimore, MD, USA.

8. Gerhard Klimeck, Shaikh Ahmed, Marek Korkusiniski, Seungwon Lee, Faisal Saied, “Atomistic Simulations of Long-Range Strain and Close-Range Electronic Structure in Self-Assembled Quantum Dot Systems”, 2006 APS March Meeting, Monday–Friday, March 13–17, 2006, Baltimore, MD, USA.

9. Shaikh Ahmed, Dragica Vasileska, Gerhard Klimeck, Christian Ringhofer, “Efficacy of the Thermalized Quantum Potential Approach for Modeling Nanoscale Semiconductor Devices”, 2006 APS March Meeting, Monday–Friday, March 13–17, 2006, Baltimore, MD, USA.

10. Shaikh Ahmed, M. P. Anantram, Neophytos Neophytou, Marek Korkusinski, Gerhard Klimeck, “Quantum Simulations of Electronic Structure and Transport Properties in Conventional and Novel Nanoscale Devices”, 7th World Congress on Computational Mechanics, Los Angeles 2006.

11. Neophytos Neophytou, Shaikh Ahmed, M.P. Anantram and Gerhard Klimeck, “Non-Equilibrium Green’s Function (NEGF) Simulation of Metallic Carbon Nanotube Transistors: Impact of Vacancy Defect”, IWCE, Vienna, Austria, 2006.

12. Shaikh Ahmed, Muhammad Usman, Clemens Heitzinger, Rajib Rahman, Andrei Schliwa, and Gerhard Klimeck “Symmetry breaking and fine structure splitting in self-assembled zincblende quantum dots: atomistic simulations of long-range strain and piezoelectric field”, ICPS 2006, Vienna, Austria, July 24-28 2006 .

13. Gerhard Klimeck, Rick Kennel, Michael McLennan, Stephen Clark, Clemens Heitzinger, Shaikh S. Ahmed, Wei Qiao, David Ebert,  Sebastien Goasguen, Krishna Madhavan, “nanoHUB.org – A fully operational Science  Gateway for the nano Science Community”, The Second IEEE/ACM International Workshop on High Performance Computing for Nano-science and Technology (HPCNano06), Nov. 13, 2006, Tampa, Florida, USA (Invited).

14. S. Ahmed, M. Usman, C. Heitzinger, R. Rahman, A. Schliwa,  and G. Klimeck, “Atomistic Simulation of Non-Degeneracy and Optical Polarization Anisotropy in Zincblende Quantum Dots”, The 2nd Annual IEEE International Conference on Nano/Micro Engineered and Molecular Systems (IEEE-NEMS), Bangkok, Thailand, Jan 16-19, 2007.

15. Gerhard Klimeck, Shaikh Ahmed, Clemens Heitzinger, Neerav Kharche, Muhammad Usman, Mathieu Luisier, Raesong Kim, Neophytos Neophytou, Michael McLennan, and Timothy B. Boykin, “Quantum Dot, Nanowire, and Bandstructure Modeling, and Deployment On Nanohub.Org”, International Workshop Tera- and Nano-Devices: Physics and Modeling, October 16-19, 2006, Aizu, Japan (Invited).

16. Shaikh S. Ahmed, Marek Korkusinski, Faisal Saied, Haiying Xu, Seungwon Lee, Mohamed Sayeed, Sebastien Goasguen and Gerhard Klimeck, “Large Scale Simulation in Nanostructures with NEMO3-D on Linux Clusters”, The 6th LCI International Conference on Linux Clusters: The HPC Revolution 2005, April 26-28, 2005, The Carolina Inn, University of North Carolina, Chapel Hill, North Carolina, USA.

17. Shaikh Ahmed, “Building and Deploying Community Nanotechnology Software Tools on nanoHUB.org – Atomistic Simulations of Multimillion-Atom Quantum Dot Nanostructures”, I-light Symposium 2005, Indiana University, September 2005.

18. Dragica Vasileska, Shaikh Ahmed, Christian Ringhofer, “Quantum Effects Incorporation into Monte Carlo Device Simulators for Modeling Nano-Scale Devices”, 2nd Annual Conference on Foundations of Nanoscience: Self-Assembled Architectures and Devices (Fnano05), Snowbird Cliff Lodge, Snowbird, UT, April 24 –April 28, 2005.

19. H. Khan, S.S. Ahmed, D. Vasileska, “Examination of the Effects of Unintentional Doping on the Operation of FinFETs with Monte Carlo Simulation Integrated with Fast Multipole Method (FMM)”, 2005 NSTI Nanotech Conference & Trade Show, Anaheim, May 8-12, 2005.

20. S. Ahmed and D. Vasileska, “The Influence of Unintentional Doping on nanoscale MOSFET Operation”, IVth IMACS Seminar on Monte Carlo Methods MCM, 15–19 September 2003, Berlin, Germany.  

21. S. S. Ahmed, R. Akis and D. Vasileska, "Quantum Effects in SOI Devices: A Scattering matrix calculation based on Landauer’s formalism", 4^th International Conference on Modeling and Simulation of Microsystems, San Juan, Puerto Rico, USA, pp. 518–521, April 22–25, 2002.

22. S.S. Ahmed and D. Vasileska, “Quantum effects in narrow-width SOI devices”, MSED Proc., Barcelona, Spain, September 2003.

23. C. Heitzinger, S. Ahmed, C. Ringhofer, and D. Vasileska, “On the Influence of the Number of Moments in the Boltzmann Transport Equation Compared to the Hydrodynamic Transport Model”, Proc. 34th European Solid-State Dev. Res. Conference ESSDERC, September 2004, Leuven, Belgium.

24. S. S. Ahmed, R. Akis and D. Vasileska, "Modeling of Narrow-Width SOI devices", 2002 IEEE Si Nanoelectronics Workshop, Honolulu, Hawaii, USA, June 9-10, 2002.

25. S. S. Ahmed, and D. Vasileska, “Narrow-Width SOI Devices: Impact of Quantum Mechanical Space-Quantization Effects on Device Performance”, IEEE-NANO 2002, Arlington, Virginia, USA, August 26–28, 2002.

26. S. S. Ahmed, and D. Vasileska, “Quantum mechanical narrow-channel effect in SOI devices”, in the Proc. of the 4th Int. Symp. on Nanostructures and Mesoscopic Systems  (NanoMes), Tempe Mission Palms Hotel, Tempe, Arizona, USA, February 17–21, 2003.

27. S. Ahmed, C. Ringhofer, and D. Vasileska, “Effective potential approach to modeling of 25 nm MOSFET devices”, Sixth International Conference on New Phenomena in Mesoscopic Systems (NPMS-6) and Fourth International Conference on Surfaces and Interfaces of Mesoscopic Devices (SIMD-4), Maui, Hawaii, USA, December 1–5, 2003.

28. C. Heitzinger, S. Ahmed, C. Ringhofer, and D. Vasileska, “Efficient Simulation of the Full Coulomb Interaction in Three Dimensions”, Proc. 9th International Workshop on Computational Electronics (IWCE 10), Purdue University, 2004.

29. S. Ahmed and D. Vasileska, “Threshold voltage shifts in narrow-width SOI devices due to quantum mechanical size-quantization effects”, Technical Proceedings of the Nanotechnology Conference and Trade Show, San Fransisco, California, USA, pp. 222–225, Feb. 23–27, 2003.

30. C. Ringhofer, D. Vasileska, S. Ahmed, “A thermodynamic quantum potential approach”, Workshop on Quantum and Many-Body Effects in Nanoscale Devices, Arizona State University, Tempe, Arizona, October 24–25, 2003. 

31. S. Ahmed and D. Vasileska, “Modelling of narrow-width SOI devices”, 13th International Conference on Nonequilibrium Carrier Dynamics in Semiconductors (HCIS-13), July 28-August 01, 2003, Italy.

32. C. Heitzinger, S. Ahmed, C. Ringhofer, and D. Vasileska, “Accurate Three-Dimensional Simulation of Electron Mobility Including Electron-Electron and Electron-Dopant Interactions”, Proc. 206th Meeting of the Electrochem Soc. ECS, October 2004, Honolulu, HI, USA.

33. S. Ahmed and D. Vasileska, “Coulomb Effects on Nanoscale MOSFET Operation”, Fourth IEEE Conference on Nanotechnology, August 17-19, 2004, Munich, Germany.

34. S. Ahmed, C. Ringhofer, and D. Vasileska, “Quantum potential for use in particle based simulations”, Proc. 9th International Workshop on Computational Electronics (IWCE 9), 25–28 May 2003, Italy.

35. S. S. Ahmed, and D. Vasileska, “Modeling of Narrow-Width SOI Devices: The Impact of Quantum Mechanical Size Quantization Effects and Unintentional Doping on Device Operation”, 62^nd Device Research Conference DRC, University of Notre Dame Notre Dame, Indiana, USA, June 21-23, 2004.

36. C. Heitzinger, S. Ahmed, C. Ringhofer, and D. Vasileska, “On the Efficient Simulation of Electron-Electron Interactions in Nanoscale MOSFETs”, Proc. Trends in Nanotechnology, TNT September 2004, Segovia, Spain.

37. T. Khan, S. Ahmed, T. Thornton, D. Vasileska, “Subthreshold mobility modeling of SOI MESFETs”, IWCE 2004, Purdue University, West Lafayette, USA, October 2004.

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