Strain Effects on Bulk <001> Ge Valence Band

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Strain Effects on Bulk lt001gt Ge Valence Band

• EEL6935 Computational Nanoelectronics
• Fall 2006
• Andrew Koehler

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Outline

• Motivation
• Background
• Strain
• Germanium
• Simulation Results and Discussion
• Summary
• References

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Motivation

• Moores Law
• 0.7X linear scale factor
• 2X increase in density / 2 years
• Higher performance (30 / 2 years)
• Approaching Fundamental Limits
• No Exponential is Forever
• What is the solution?

Ultimate CMOS Current CMOS
Energy kTln(2) kT(104105)
Channel Length 1 nm 100 nm
Density 1014/cm2 109/cm2
Power 107 W/cm2 100 W/cm2
Speed 0.01 ps 1 ps
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Solution Novel Materials
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History of Strain

• 1954 Piezoresistance in silicon was first
  discovered by C. S. Smith
• (resistance change due to applied stress)
• 1980s Thin Si layers grown on relaxed
  silicongermanium (SiGe) substrates
• 1990s High-stress capping layers deposited on
  MOSFETs were investigated as a technique to
  introduce stress into the channel
• 1990s SiGe incorporated in the source and drain
  areas
• 2002 Intel uses strained Si in P4 processor

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What is Strain?

• Stress Limit of Force/Area as Area approaches
  zero
• Strain Fractional change in length of an
  object Distortion of a structure caused by
  stress

Normal Stress Component
Normal Strain Component
Shear Stress Component
Shear Strain Component
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What is Strain?
Elastic Stiffness Coefficients (1011N/cm2)
c11 c12 c44
Si 1.657 0.639 0.7956
Ge 1.292 0.479 0.670
Compliance Coefficients (10-11cm2/N)
s11 s12 s44
Si 0.768 -0.214 1.26
Ge 0.964 -0.260 1.49
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Strain Effect on Valence Band
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History of Germanium

• 1959 First germanium hybrid integrated
• circuit demonstrated.
• - Jack Kilby, Robert Noyce
• 1960 High purity silicon began replacing
• germanium in transistors, diodes,
• and rectifiers
• 2000s Germanium transistors are still used in
  some stompboxes by musicians who wish to
  reproduce the distinctive tonal character of the
  "fuzz"-tone from the early rock and roll era.
• 2000s Germanium is being discussed as a possible
  replacement of silicon???

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Why Did Si Replace Ge?

• Germaniums limited availability
• High Cost
• Impossible to grow a stable oxide that could
• Passivate the surface
• Be used as an etch mask
• Act as a high-quality gate insulator

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Novel Materials to the Rescue

• High-k Dielectric
• Used as gate oxide
• eliminate the issue that germaniums native oxide
  is not suitable for nanoelectronics
• Atomic Layer Deposition (ALD)
• HfO2
• ZrO2
• SrTiO3, SrZrO3 and SrHfO3
• ALD WN/LaAlO3/AlN gate stack

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Ge vs Other Semiconductors

• nMOS GaAs is the best material
• pMOS Ge is the best material

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Future of Ge in Nanoelectronics

• Researchers Believe
• Combination of a Ge pMOS with a GaAs nMOS could
  be a manufacturable way to further increase the
  CMOS performance.
• Current Problems
• Passivation of interface states
• Reduction of diode leakage
• Availability of high-quality germanium-on-insulato
  r substrates

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k p method

• k p method was introduced by Bardeen and Seitz
• Kanes model takes into account spin-orbit
  interaction
• ?nk(r) eikrunk(r)
• unk(rR) unk(r) Bloch function
• n refers to band
• k refers to wave vector
• Useful technique for analyzing band structure
  near a particular point k0

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k p method

• Schrodinger equation
• Written in terms of unk(r)

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Unstressed Band Structures
Silicon Germanium
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Biaxial Compression 1 GPa
Silicon Germanium
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Longitudinal Compression 1 GPa
Silicon Germanium
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Band Splitting
Biaxial Compression Longitudinal Compression
Ge
Ge
Si
Si
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Silicon Mass Change

• Longitudinal Compression
• In-Plane Out-of-Plane

80
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Germanium Mass Change

• Longitudinal Compression
• In-Plane Out-of-Plane

90
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Summary

• Strain
• Germanium
• Strained Germanium Compared to Silicon
• Unstressed
• Band Splitting
• Biaxial Compression
• Longitudinal Compression
• Mass Change - Longitudinal Compression
• In-Plane
• Out-of-Plane

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References

• C. S. Smith, Piezoresistance effect in germanium
  and silicon, Phys. Rev., vol. 94, no. 1, pp.
  4249, Apr. 1954.
• R. People, J. C. Bean, D. V. Lang, A. M. Sergent,
  H. L. Stormer, K. W. Wecht, R. T. Lynch, and K.
  Baldwin, Modulation doping in GexSi1-x/Si
  strained layer heterostructures, Appl. Phys.
  Lett., vol. 45, no. 11, pp. 12311233, Dec. 1984.
• S. Gannavaram, N. Pesovic, and C. Ozturk, Low
  temperature (800 ?C) recessed junction selective
  silicon-germanium source/drain technology for
  sub-70 nm CMOS, in IEDM Tech. Dig., 2000, pp.
  437440.
• S. E. Thompson and et al., "A Logic
  Nanotechnology Featuring Strained-Silicon," IEEE
  Electron Device Lett., vol. 25, pp. 191-193,
  2004.
• S. E. Thompson and et al., "A 90 nm Logic
  Technology Part I - Featuring Strained Silicon,"
  IEEE Trans. Electron Devices, 2004.
• W. A. Brantley, "Calculated Elastic Constants for
  Stress Problem Associated with Semiconductor
  Devices," J. Appl. Phys., vol. 44, pp. 534-535,
  1973.
• Semiconductor on NSM, URL http//www.ioffe.rssi.ru
  /SVA/NSM/Semicond/.
• O. Madelung, ed., Data in Science and Technology
  Semiconductors-Group IV elements and III-V
  Compounds (Springer, Berlin, 1991).

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• THANK YOU