
Tutorial 4c: Formation of Bandstructure in Finite Superlattices (Exercise Session)
29 Mar 2011  Online Presentations  Contributor(s): Gerhard Klimeck
How does bandstructure occur? How large does a repeated system have to be? How does a finite superlattice compare to an infinite superlattice?

Tutorial 4d: Formation of Bandstructure in Finite Superlattices (Exercise Demo)
29 Mar 2011  Online Presentations  Contributor(s): Gerhard Klimeck
Demonstration of the
PieceWise Constant Potential Barriers Tool.

Atomistic Modeling and Simulation Tools for Nanoelectronics and their Deployment on nanoHUB.org
16 Dec 2010  Online Presentations  Contributor(s): Gerhard Klimeck
At the nanometer scale the concepts of device and material meet and a new device is a new material and vice versa. While atomistic device representations are novel to device physicists, the semiconductor materials modeling community usually treats infinitely periodic structures. Two electronic structure calculation examples will illustrate the importance of atomistic disorder in realistically large systems. For strained Si quantum wells on wafermiscut SiGe substrates valley splitting is computed as a function of magnetic field. For InAs quantum dots embedded in an InGaAs strain reducing layer on top of a GaAs substrate NEMO 3D can model the nonlinear optical transition energy dependence as a function of Inconcentration.

Thermoelectric effects in semiconductor nanostructures: Role of electron and lattice properties
06 Oct 2010  Online Presentations  Contributor(s): Abhijeet Paul, Gerhard Klimeck
This presentation covers some aspects of present development in the field of thermoelectricity and focuses particularly on the silicon nanowires as potential thermoelectric materials. The electronic and phonon dispersions are calculated and used for the calculation of thermoelectric properties in these nanowires.

Nanoelectronic Modeling Lecture 41: FullBand and Atomistic Simulation of Realistic 40nm InAs HEMT
05 Aug 2010  Online Presentations  Contributor(s): Gerhard Klimeck, Neerav Kharche, Neophytos Neophytou, Mathieu Luisier
This presentation demonstrates the OMEN capabilities to perform a multiscale simulation of advanced InAsbased high mobility transistors.
Learning Objectives:
 Quantum Transport Simulator
 FullBand and Atomistic
 IIIV HEMTs
 Performance Analysis
 Good Agreement with Experiment
 Some Open Issues
 Outlook
 Improve Models (Contact)
 Investigate Scaling of Gate Length
 Scattering?

Nanoelectronic Modeling Lecture 40: Performance Limitations of Graphene Nanoribbon Tunneling FETS due to Line Edge Roughness
05 Aug 2010  Online Presentations  Contributor(s): Gerhard Klimeck, Mathieu Luisier
This presentation the effects of line edge roughness on graphene nano ribbon (GNR) transitors..
Learning Objectives:
 GNR TFET Simulation
 pz TightBinding Orbital Model
 3D SchrödingerPoisson Solver
 Device Simulation
 Structure Optimization (Doping, Lg, VDD)
 LER => Localized Band Gap States
 LER => Performance Deterioration
 Outlook and Challenges
 Ripples Scattering
 More Accurate Bandstructure Model
 Dissipative Scattering (ElectronPhonon)

Nanoelectronic Modeling Lecture 35: Alloy Disorder in Nanowires
05 Aug 2010  Online Presentations  Contributor(s): Gerhard Klimeck, Timothy Boykin, Neerav Kharche, Mathieu Luisier, Neophytos Neophytou
This presentation discusses the consequences of Alloy Disorder in unstrained strained AlGaAs nanowires
 Relationship between dispersion relationship and transmission in perfectly ordered wires
 Band folding in Si nanowires
 Tranmisison in disordered wires – relationship to an approximate bandstructre
 Reminder of the origin of bandstructure and bandstructure engineering
 Localization of wavefunctions
Learning Objectives:
 Alloy wires are NOT smooth
 “Conduction band edge” flucatuates locally
 Dispersion changes
 Transmission and Density of states show localization effects

Nanoelectronic Modeling Lecture 34: Alloy Disorder in Quantum Dots
05 Aug 2010  Online Presentations  Contributor(s): Gerhard Klimeck, Timothy Boykin, Chris Bowen
This presentation discusses the consequences of Alloy Disorder in strained InGaAs Quantum Dots
 Reminder of the origin of bandstructure and bandstructure engineering
 What happens when there is disorder?
 Concept of disorder in the local bandstructure
 Configuration noise, concentration noise, clustering
Learning Objectives:
 Devicetodevice fluctuations in nanostructures may be significant even if the shape and size of the quantum dots remain perfectly controlled.
 Configuration noise, concentration noise and clustering in perfectly size and shape controlled quantum dots can lead to optical transition fluctuations that should be experimentally relevant.

Nanoelectronic Modeling Lecture 33: Alloy Disorder in Bulk
04 Aug 2010  Online Presentations  Contributor(s): Gerhard Klimeck, Timothy Boykin, Chris Bowen
This presentation discusses disorder in AlGaAs unstrained systems in bulk.
 Bandstructure of an ideal simple unit cell
 What happens when there is disorder?
 Concept of a supercell
 Band folding in a supercell
 Band extraction from the concept of approximate bandstructure
 Comparison of alloy disorder with the virtual crystal approximation
 Configuration noise, concentration noise
 How large does an alloy supercell have to be? When does the “bulk” condition occur?
Learning Objectives:
 Bandedges and bandgaps are influenced by:
 Placement / configuration disorder
 Concentration noise
 Clustering
 System size is very important
 “bulk” starts at 100,000 atoms
 => Nanostructures are not “bulk”
=> like quantum dots, nanowires, and quantum wells vary locally

Nanoelectronic Modeling Lecture 32: Strain Layer Design through Quantum Dot TCAD
04 Aug 2010  Online Presentations  Contributor(s): Gerhard Klimeck, Muhammad Usman
This presentation demonstrates the utilization of NEMO3D to understand complex experimental data of embedded InAs quantum dots that are selectively overgrown with a strain reducing InGaAs layer. Different alloy concentrations of the strain layer tune the optical emission and absorption wavelength of the quantum dots. The role of the nonlinear strain behavior ovserved in the experimental data is explored in NEMO3D. The simulation engine serves as a virtual microscope to understand the interplay of disorder, strain, and quantum dot shape.
Learning Objectives:
 Objective:
 Optical emission at 1.5μm without GaN
 Understand experimental data on QD spectra in selective overgrowth
 Approach:
 Model large structure
 60nm x 60nm x 60nm
 9 million atoms
 No changes to the published tight binding parameters
 Result:
 Match experiment remarkably well
 Strain
 change in quantum dot aspect ratio
 Quantitative model of complex system
 Studied sensitivity to experimental imperfections – small variations
 Effective mass theories provided the wrong guidance

Nanoelectronic Modeling Lecture 31a: LongRange Strain in InGaAs Quantum Dots
04 Aug 2010  Online Presentations  Contributor(s): Gerhard Klimeck
This presentation demonstrates the importance of longrange strain in quantum dots
 Numerical analysis of the importance of the buffer around the central quantum dot  local band edges – vertical and horizontal extension of the buffer
 Controlled overgrowth can tune the electron energies in the system
Learning Objectives:
 Strain is the source of the creation of the InAs QDs on GaAs
 Strain is a long range phenomenon
 Strain reaches further vertically than horizontally
 Quantum dots will grow on top of each other
 Electron wavefunctions are confined to the central quantum dots and can be computed in a smaller domain

Nanoelectronic Modeling Lecture 29: Introduction to the NEMO3D Tool
04 Aug 2010  Online Presentations  Contributor(s): Gerhard Klimeck
This presentation provides a very high level software overview of NEMO3D. The items discussed are:
 Modeling Agenda and Motivation
 TightBinding Motivation and basic formula expressions
 Tight binding representation of strainSoftware structure
 NEMO3D algorithm flow
 NEMO3D parallelization scheme – original 1D spatial decomposition
 NEMO3D scaling on parallel computes from the year 2000 til 2007
 New 1D, 2D, and 3D spatial decomposition scheme and parallel performance
 52 million atom simulation demonstration
Learning Objectives:
 Convey a broad overview of the NEMO3D simulation engine.
 Student shall learn about the algorithmic coponents of geometry construction, atom position computaion, and electronic structure calculation.
 Student shall learn the need and usefulness of parallel computers to solve the NEMO3D problems.
 Student shall learn a demonstration of a software capability and validation.

Nanoelectronic Modeling Lecture 28: Introduction to Quantum Dots and Modeling Needs/Requirements
20 Jul 2010  Online Presentations  Contributor(s): Gerhard Klimeck
This presentation provides a very high level software overview of NEMO1D.
Learning Objectives:
This lecture provides a very high level overview of quantum dots. The main issues and questions that are addressed are:
 Length scale of quantum dots
 Definition of a quantum dot
 Quantum dot examples and Applications
 Single electronics
 Need for quantum dot modeling
 Model requirements – what are the physical effects that need to be included?
 Overview of some of the existing theories and models
 Tight binding approach

Nanoelectronic Modeling Lecture 25a: NEMO1D  Full Bandstructure Effects
07 Jul 2010  Online Presentations  Contributor(s): Gerhard Klimeck
(quantitative RTD modeling at room temperature)

Nanoelectronic Modeling nanoHUB Demo 2: RTD simulation with NEGF
09 Mar 2010  Online Presentations  Contributor(s): Gerhard Klimeck
Demonstration of resonant tunneling diode (RTD) simulation using the RTD Simulation with NEGF Tool with a Hartree potential model showing potential profile, charge densities, currentvoltage characteristics, and resonance energies. Also demonstrated is a RTD simulation using a ThomasFermi potential model showing the effects of eta.

Nanoelectronic Modeling nanoHUB Demo 1: nanoHUB Tool Usage with RTD Simulation with NEGF
09 Mar 2010  Online Presentations  Contributor(s): Gerhard Klimeck
Demonstration of running tools on the nanoHUB. Demonstrated is the RTD Simulation with NEGF Tool using a simple leveldrop potential model and a more realistic device using a ThomasFermi potential model.

Nanoelectronic Modeling Lecture 25b: NEMO1D  Hole Bandstructure in Quantum Wells and Hole Transport in RTDs
09 Mar 2010  Online Presentations  Contributor(s): Gerhard Klimeck
Heterostructures such as resonant tunneling diodes, quantum well photodetectors and lasers, and cascade lasers break the symmetry of the crystalline lattice. Such break in lattice symmetry causes a strong interaction of heavy, light and splitoff hole bands. The bandstructure of holes and the transport through these states is of very current interest to the semiconductor industry. As semiconduction devices are scaled down to a nanometer level and as holes are confined to very thin triangular or square quantum wells.
A resonant tunneling diode is used as a vehicle to study the bandstructure in thin quantum wells and hole transport in heterostructures including the subband dispersion transverse to the main transport direction. Four key findings are demonstrated: (1) the heavy and light hole interaction is shown to be strong enough to result in dominant current flow off the Gamma zone center (more holes flow through the structure at an angle than straight through), (2) explicit inclusion of the transverse momentum in the current integration is needed, (3) most of the current flow is due to injection from heavy holes in the emitter, and (4) the dependence on the angle φ of the transverse momentum k is weak. Two bandstructure models are utilized to demonstrate the underlying physics: (1) independent/uncoupled heavy, light and splitoff bands, and (2) secondnearest neighbor sp3s* tightbinding model. Current–voltage (I–V ) simulations including explicit integration of the total energy E, transverse momentum k and transverse momentum angle φ are analyzed. Three independent mechanisms that generate offzonecenter current flow are identified: (1) nonmonotonic (electronlike) hole dispersion, (2) different quantum well and emitter effective masses, and (3) momentumdependent quantum well coupling strength.
The methodologies and physical mechanism explained here provide a critical guidance to the treatment of hole transport in ultrathin bodies or shallow channel transistors. Since the tight binding model intrinsically comprehends strain and crystal distortions, the methodology is immediately applicable to strain engineering methods.
Learning Objectives:
 Understand the approximate construction of hole dispersions in quantum wells from simple effective mass theories.
 Understand the consequences of band mixing in full band theories.
 Understand the correlation between transverse dispersion in a quantum well and transmission coefficents.
 Understand physical mechanisms that can cause hole transport to be highly momentum dependent.
 Appreciate the relevance to modern ultrathin body devices.

Nanoelectronic Modeling Lecture 23: NEMO1D  Importance of New Boundary Conditions
09 Mar 2010  Online Presentations  Contributor(s): Gerhard Klimeck
One of the key insights gained during the NEMO1D project was the development of new boundary conditions that enabled the modeling of realistically extended Resonant Tunneling Diodes (RTDs). The new boundary conditions are based on the partitioning of the device into emitter and collector reservoirs which are assumed to be in local equilibrium with a local quasi Fermi level and a central nonequilibrium region. In the reservoirs the electrostatic potential generally varies spatially due to nonuniform doping and possibly heterostructures. The introduction of an empirical scattering relaxation rate in the reservoirs enabled the modeling of phasebreaking and relaxation in the equilibrium reservoirs and the elimination of unrealistically narrow resonance states. With these new boundary conditions one can reduce dramatically the spatial region in which the nonequilibrium problem is being computed. This allowed for the efficient simulation of scattering effects inside the central RTD under nonequilibrium conditions at low temperature, and avoided the need to compute explicitly the computation of the equilibrating scattering in the high electron density contacts.
The presentation closes with the challenge that the boundary conditions alone are not sufficient to completely explain the valley current of resonant tunneling diodes. It leads into the discussion of incoherent scattering inside the central RTD for the next lecture.
Learning Objectives:
 Comprehension of the major concept of device partition into reservoirs and central nonequilibrium region
 Conprehension of the associated reduction in computational cost due to device partitioning
 Comprehension of the physical effects of relaxation in the reservoirs and the broadening of the resonance states

Nanoelectronic Modeling Lecture 24: NEMO1D  Incoherent Scattering
09 Mar 2010  Online Presentations  Contributor(s): Gerhard Klimeck
Incoherent processes due to phonons, interface roughness and disorder had been suspected to be the primary source of the valley current of resonant tunneling diodes (RTDs) at the beginning of the NEMO1D project in 1994. The modeling tool NEMO was created at Texas Instruments to fundamentally understand the valley current in RTDs. With the common understanding that scattering is the source of the valley current and with the early successes in NEGF significant resources were invested to model incoherent scattering. A full NEGF transport model implemented in NEMO1D enabled an analysis of various scattering mechanisms. Important incoherent scattering mechanisms that affect the operation of a GaAs/AlGaAs RTD are alloy disorder, interface roughness, acoustic and polar optical phonon scattering. A thorough analysis of each of these scattering mechanisms has shown that the effects of alloy and acoustic phonon scattering are small compared to those of interface roughness and polar optical phonon scattering. It is found from the analysis performed with NEMO1D tool that incoherent scattering affects the valley current of the RTD particularly at low temperatures. These scattering effects are, however not strong enough to explain the valley current in high performance, high temperature devices. Two other key elements are needed to explain the valley current in RTDs: 1) scattering in the contact/emitter and 2) the proper modeling of excited states through full band material representations.
This presentation provides an overview of the physical scattering mechanisms and tries to convey some intuition of what is to be expected from these scattering mechanisms. Quantitative agreement of NEMO1D simulations with experimental data at low temperatures proves that NEMO1D indeed models the critical scattering mechanisms inside the central RTD properly. Experimental data for the same device at room temperature that scattering is not enough to expain the valley current at room temperature.
Learning Objectives:
 Overview scattering mechanisms inside a resonant tunneling diode, polar optical phonons, acoustic phonons, interface roughness, and alloy disorder.
 Demonstrate that NEMO1D can model scattering quantitatively at low temperatures and match experimental data.
 Demonstrate that scattering is not enough to explain room temperature data.

Nanoelectronic Modeling Lecture 26: NEMO1D 
09 Mar 2010  Online Presentations  Contributor(s): Gerhard Klimeck
NEMO1D demonstrated the first industrial strength implementation of NEGF into a simulator that quantitatively simulated resonant tunneling diodes. The development of efficient algorithms that simulate scattering from polar optical phonons, acoustic phonons, alloy disorder, and interface roughness were critical in testing the theory towards its general capability to deliver quantitative matches to experimental data for low temperature devices. That quantitative agreement at low temperature devices and disagreement at room temperature led to a significant conclusion on the importance of full bandstructure models for devices which have material and potential variations on the order of 5nm.
This presentation oveviews the computational flow of the various scattering models implemented in NEMO1D: single sequential scattering, multiple sequential scattering, multiple sequential scattering at coupled energies, and selfconsistent first Born approximations. For the derivations of the equations and further detail I just refer here to the Journal of Applied Physics publication in 1997 [1].
This presentation is NOT intended to teach anyone NEGF. It is merely a computational flow overview. For true NEGF teaching material I refer to Datta’s NEGF topic page on nanoHUB [2]
Learning Objectives:
 Understand the general concept of sequential scattering, multiple sequential scattering, and selfconsistent first Born approximation
 Appreciate the complexity of of the the flow of computational objects in a large scale simulation engine
 Roger Lake, Gerhard Klimeck, R. Chris Bowen and Dejan Jovanovic, "Single and multiband modeling of quantum electron transport through layered semiconductor devices", J. of Appl. Phys. 81, 7845 (1997).
 Supriyo Datta maintains an excellent web page on nanoHUB.org which contains tutorials, OnLine seminars, Ph.D. theses, and tool examples.

Nanoelectronic Modeling Lecture 27: NEMO1D 
09 Mar 2010  Online Presentations  Contributor(s): Gerhard Klimeck
This presentation provides a very high level software overview of NEMO1D. The items discussed are:
 User requirements
 Graphical user interface
 Software structure
 Program developer requirements
 Dynamic I/O design for batch and GUI
 Resonance finding algorithm
 Inhomogeneous energy meshing
 Information flow, code modularity
 Code documentation system
 Revision control system
Learning Objectives:
 Convey the complexity of a large software package in its various components –
 User requirements
 Graphical user interface requirements and examples
 Software structure
 Program developer requirements
 Dynamic I/O design for batch and GUI
 Resonance finding algorithm – numerical and analytic advantages
 Inhomogeneous energy meshing – computational savings
 Information flow, code modularity
 Code documentation system
 Revision control system

Nanoelectronic Modeling Lecture 22: NEMO1D  Motivation, History and Key Insights
07 Feb 2010  Online Presentations  Contributor(s): Gerhard Klimeck
The primary objective of the NEMO1D tool was the quantitative modeling of high performance Resonant Tunneling Diodes (RTDs). The software tool was intended for Engineers (concepts, fast turnaround, interactive) and Scientists (detailed device anaysis). Therefore various degrees of sohphistication have been built into the tool which allow the users to trade off accuracy and completeness of the models against computation time and memory usage.
The Nanoelectronic Modeling tool (NEMO) is a 1D device design tool for the quantum mechanical simulation of electron (and hole) states in semiconductor heterostructures. A variety of material systems such as GaAs, InP and Si can presently be analysed. A graphical user interface enables the simple enrty of the heterostructure, the entry of the simulation parameters, the simulation control, and the analysis of the data. The code consists presently of approximately 255,000 lines of code written in C, FORTRAN, F90 and yacc.
The four key modeling aspects that resulted in the accurate modeling of RTDs are:
 Proper treatment of extended contacts. Contacts typically contain resonance states which modify the injection of carriers into the central RTD structure.
 Proper treatment of the quantum mechanical charging in the central RTD AND the contacts.
 Proper treatment of the material bandstructure properties, such as nonparabolicity, bandwarping, and GammaX transistions, and
 at low temperatures the proper treatement of electron scattering due to optical phonons, acoustic phonons, and interface roughness...
NEMO was developed at the Applied Research Laboratory of Raytheon (formerly known as the Central Research Lab of Texas Instruments) with U.S. government funding. The tool was delivered to the U.S. government and it was available to the U.S. research community.
Learning Objectives:
General NEMO 1D modeling challenge – understanding valley current.
Overview of the stateofthe art knowledge of resonant tunneling diode simulation before the NEMO project in 1994
High level overview of alternative modeling methodologies available in 1994
Key simulation results for room temperature, high performance RTDs
Software overview
Stateoftheart knowledge in 1998 / 2000

Nanoelectronic Modeling Lecture 21: Recursive Green Function Algorithm
07 Feb 2010  Online Presentations  Contributor(s): Gerhard Klimeck
The Recursive Green Function (RGF) algorithms is the primary workhorse for the numerical solution of NEGF equations in quasi1D systems. It is particularly efficient in cases where the device is partitioned into reservoirs which may be characterized by a nonHermitian Hamiltonian and a central device region which is Hermitian. Until now (2009) it also appears to be the only scalable algorithm that enables the rapid computation of incoherent transport with NEGF.

Nanoelectronic Modeling: Exercises 13  Barrier Structures, RTDs, and Quantum Dots
27 Jan 2010  Online Presentations  Contributor(s): Gerhard Klimeck
Exercises:
 Barrier Structures
Uses: PieceWise Constant Potential Barrier Tool
 Resonant Tunneling Diodes
Uses: Resonant Tunneling Diode Simulation with NEGF
• Hartree calculation
• Thomas Fermi potential
 Quantum Dots
Uses: Quantum Dot Lab
• pyramidal dot

Nanoelectronic Modeling Lecture 20: NEGF in a Quasi1D Formulation
27 Jan 2010  Online Presentations  Contributor(s): Gerhard Klimeck, Samarth Agarwal, Zhengping Jiang
This lecture will introduce a spatial discretization scheme of the Schrödinger equation which represents a 1D heterostructure like a resonant tunneling diode with spatially varying band edges and effective masses.