Bandstructure and effective mass



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From http://www.intel.com/technology/mooreslaw/index.htm

  • From http://www.intel.com/technology/mooreslaw/index.htm







Bandstructure and effective mass

  • Bandstructure and effective mass

    • Carrier confinement
    • Atomic scale material variations
    • Local strain variations
    • Atomistic treatment of electric and magnetic fields


Valley-splitting

  • Valley-splitting

    • Highly dependent on atomic scale thickness variations
    • Need atomistic modeling technique such as tight-binding


Motivation

  • Motivation

  • Tight-Binding Approach to Model Atomic Scale Variations

  • Summary of Results

    • Valley Degeneracies in (111) Si Quantum Wells
    • Valley-Splitting in (100) SiGe/Si/SiGe Quantum Wells
    • Transport Characteristics of AlGaAs Nanowires
    • Ultra-Scaled InAs HEMTs
  • Performance Analysis of Ultra-Scaled InAs HEMTs

    • Modeling Approach
    • Comparison to Experiments
    • Scaling Considerations
  • Summary of Contributions

  • Outlook



Motivation

  • Motivation

  • Tight-Binding Approach to Model Atomic Scale Variations

  • Summary of Results

    • Valley Degeneracies in (111) Si Quantum Wells
    • Valley-Splitting in (100) SiGe/Si/SiGe Quantum Wells
    • Transport Characteristics of AlGaAs Nanowires
    • Ultra-Scaled InAs HEMTs
  • Performance Analysis of Ultra-Scaled InAs HEMTs

    • Modeling Approach
    • Comparison to Experiments
    • Scaling Considerations
  • Summary of Contributions

  • Outlook



Objective

  • Objective

  • Resolve discrepancies in experimentally observed and theoretically predicted valley degeneracies

  • Effect of surface miscut on the electronic structure

  • Approach

  • Supercell tight-binding approach to model surface miscuts

  • Effective mass based valley-projection model to determine the directions of valley-minima of large supercells

  • Insight

  • Atomistic basis representation is essential to capture the effect of mono-atomic steps resulting from miscut



Motivation

  • Motivation

  • Tight-Binding Approach to Model Atomic Scale Variations

  • Summary of Results

    • Valley Degeneracies in (111) Si Quantum Wells
    • Valley-Splitting in (100) SiGe/Si/SiGe Quantum Wells
    • Transport Characteristics of AlGaAs Nanowires
    • Ultra-Scaled InAs HEMTs
  • Performance Analysis of Ultra-Scaled InAs HEMTs

    • Modeling Approach
    • Comparison to Experiments
    • Scaling Considerations
  • Summary of Contributions

  • Outlook





Motivation

  • Motivation

  • Tight-Binding Approach to Model Atomic Scale Variations

  • Summary of Results

    • Valley Degeneracies in (111) Si Quantum Wells
    • Valley-Splitting in (100) SiGe/Si/SiGe Quantum Wells
    • Transport Characteristics of InAlAs Nanowires
    • Ultra-Scaled InAs HEMTs
  • Performance Analysis of Ultra-Scaled InAs HEMTs

    • Modeling Approach
    • Comparison to Experiments
    • Scaling Considerations
  • Summary of Contributions

  • Outlook



Objective

  • Objective

    • Method to model bandstructure of disordered nanowires
    • Detailed understanding of transport by comparing bandstructure and transmission characteristics
  • Approach

    • Transmission: Non-equilibrium Green’s function method
    • Bandstructure: Supercell calculation and zone-unfolding


Motivation

  • Motivation

  • Tight-Binding Approach to Model Atomic Scale Variations

  • Summary of Results

    • Valley Degeneracies in (111) Si Quantum Wells
    • Valley-Splitting in (100) SiGe/Si/SiGe Quantum Wells
    • Transport Characteristics of InAlAs Nanowires
    • Ultra-Scaled InAs HEMTs
  • Performance Analysis of Ultra-Scaled InAs HEMTs

    • Modeling Approach
    • Comparison to Experiments
    • Scaling Considerations
  • Summary of Contributions

  • Outlook



Objective

  • Objective

    • Develop: a methodology to simulate ultra-scaled InAs FETs
    • Benchmark: match experimental I-Vs for “large” devices Lg = 30 - 50nm
    • Improve: device design for scaling down to 20nm node
  • Results/Impact

    • Good quantitative match to experiments
    • Performance optimization of 20nm device


Motivation

  • Motivation

  • Tight-Binding Approach to Model Atomic Scale Variations

  • Summary of Results

    • Valley Degeneracies in (111) Si Quantum Wells
    • Valley-Splitting in (100) SiGe/Si/SiGe Quantum Wells
    • Transport Characteristics of InAlAs Nanowires
    • Ultra-Scaled InAs HEMTs
  • Performance Analysis of Ultra-Scaled InAs HEMTs

    • Modeling Approach
    • Comparison to Experiments
    • Scaling Considerations
  • Summary of Contributions

  • Outlook



III-V: Extraordinary electron transport properties and high injection velocities

  • III-V: Extraordinary electron transport properties and high injection velocities

  • HEMTs: Very similar structure to MOSFETs except high-κ dielectric layer

  • Excellent to Test Performances of III-V material without interface defects

  • Every Year Devices with a Shorter Gate Length Introduced by del Alamo’s Group at MIT

  • Excellent to Test Simulation Models

    • Develop simulation tools and benchmark with experiments
    • Predict performance of ultra-scaled devices


Motivation

  • Motivation

  • Tight-Binding Approach to Model Atomic Scale Variations

  • Summary of Results

    • Valley Degeneracies in (111) Si Quantum Wells
    • Valley-Splitting in (100) SiGe/Si/SiGe Quantum Wells
    • Transport Characteristics of InAlAs Nanowires
    • Ultra-Scaled InAs HEMTs
  • Performance Analysis of Ultra-Scaled InAs HEMTs

    • Modeling Approach
    • Comparison to Experiments
    • Scaling Considerations
  • Summary of Contributions

  • Outlook



Intrinsic device

  • Intrinsic device

    • Near gate contact
    • Self consistent 2D Schrodinger-Poisson
    • Electrons injected from all contacts
  • Extrinsic source/drain contacts

    • Series resistances RS and RD


Motivation

  • Motivation

  • Tight-Binding Approach to Model Atomic Scale Variations

  • Summary of Results

    • Valley Degeneracies in (111) Si Quantum Wells
    • Valley-Splitting in (100) SiGe/Si/SiGe Quantum Wells
    • Transport Characteristics of InAlAs Nanowires
    • Ultra-Scaled InAs HEMTs
  • Performance Analysis of Ultra-Scaled InAs HEMTs

    • Modeling Approach
    • Comparison to Experiments
    • Scaling Considerations
  • Summary of Contributions

  • Outlook







Motivation

  • Motivation

  • Tight-Binding Approach to Model Atomic Scale Variations

  • Summary of Results

    • Valley Degeneracies in (111) Si Quantum Wells
    • Valley-Splitting in (100) SiGe/Si/SiGe Quantum Wells
    • Transport Characteristics of InAlAs Nanowires
    • Ultra-Scaled InAs HEMTs
  • Performance Analysis of Ultra-Scaled InAs HEMTs

    • Modeling Approach
    • Comparison to Experiments
    • Scaling Considerations
  • Summary of Contributions

  • Outlook









Motivation

  • Motivation

  • Tight-Binding Approach to Model Atomic Scale Variations

  • Summary of Results

    • Valley Degeneracies in (111) Si Quantum Wells
    • Valley-Splitting in (100) SiGe/Si/SiGe Quantum Wells
    • Transport Characteristics of InAlAs Nanowires
    • Ultra-Scaled InAs HEMTs
  • Performance Analysis of Ultra-Scaled InAs HEMTs

    • Modeling Approach
    • Comparison to Experiments
    • Scaling Considerations
  • Summary of Contributions

  • Outlook



(111) Si quantum wells: Explained 2-4 valley degeneracy breaking (APL 2009)

  • (111) Si quantum wells: Explained 2-4 valley degeneracy breaking (APL 2009)

  • Miscut (100) SiGe/Si/SiGe quantum wells: Provided qualitative and quantitative understanding of valley splitting (APL 2007)



AlGaAs and SiGe nanowires: Provided understanding of transmission coefficients by employing zone-unfolding method (TNANO 2007, JCE 07)

  • AlGaAs and SiGe nanowires: Provided understanding of transmission coefficients by employing zone-unfolding method (TNANO 2007, JCE 07)

  • InAs HEMTs: Demonstrated quantitative agreement between experiments and simulations. Performance optimizations for ultra-scaled HEMTs (IEDM 09)



Motivation

  • Motivation

  • Tight-Binding Approach to Model Atomic Scale Variations

  • Summary of Results

    • Valley Degeneracies in (111) Si Quantum Wells
    • Valley-Splitting in (100) SiGe/Si/SiGe Quantum Wells
    • Transport Characteristics of InAlAs Nanowires
    • Ultra-Scaled InAs HEMTs
  • Performance Analysis of Ultra-Scaled InAs HEMTs

    • Modeling Approach
    • Comparison to Experiments
    • Scaling Considerations
  • Summary of Contributions

  • Outlook



Valley degeneracies in (110) Si QWs

  • Valley degeneracies in (110) Si QWs

    • Both 4 and 2 fold valley degeneracies are reported in experiments
    • Flat (110) => 2 fold degenerate
    • Miscut (110) => 4 fold degenerate
  • Effect of Ge concentration on valley splitting in (100) SiGe/Si/SiGe QWs

    • Disorder in SiGe reduces valley splitting and sensitivity to Ge concentration


Supercell approach and zone-unfolding

  • Supercell approach and zone-unfolding

    • Electronic structure of rough nanowires and QWs
    • Hole transport in SiGe pMOS devices
  • III-V MOSFETs



Advisors:

  • Advisors:

    • Professor Gerhard Klimeck
    • Professor Timothy Boykin
  • Committee members:

    • Professor Mark Lundstrom
    • Professor Supriyo Datta
    • Professor Ronald Reifenberger
  • Dr. Mathieu Luisier

  • Klimeck Group Members and Labmates









InAs Channel Scaling:

  • InAs Channel Scaling:

  • Better electrostatic control

    • lower SS
    • larger ION/IOFF ratio
  • Increase of transport m*

    • reduced vinj, higher Ninv => higher ION
  • Increase of gate leakage current

    • ION/IOFF ratio saturates


InAlAs Insulator Scaling:

  • InAlAs Insulator Scaling:

  • Better electrostatic control (due to larger Cox)

  • Increase of gate leakage current

    • larger IOFF
    • larger SS
    • smaller ION/IOFF ratio




Electrons tunnel from gate into InAs channel

  • Electrons tunnel from gate into InAs channel

  • Tunneling barriers

    • InAlAs and InGaAs
    • Position dependent barriers
  • Current crowding at edges (due to lower tunneling barriers)

  • Barriers modulated by ΦM



Characteristics:

  • Characteristics:

  • Same Gate Overdrive

    • same thermionic current (source to drain)
  • Gate Fermi levels shifted by ΔΦM

    • different tunneling barrier height
  • ΦM =4.7 eV

    • tunnel through InAlAs only
    • larger Ig
  • ΦM =5.1 eV

    • tunnel through InAlAs and InGaAs
    • lower Ig


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