One-dimensional hole gas in germanium silicon nanowire hetero-structures

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One-dimensional hole gas in germanium silicon
nanowire hetero-structures

• Linyou Cao
• Department of Materials Science and Engineering
• Drexel University
• 12/09/2005

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Motivation-Why?

• Quantum Confinement not reported in NW
• Ballistic Transport
• Conductance Quantification
• Controlled synthesis of NW offering substantial
  potential to engineer in 1-D electronic system
• Band-gap engineering in hetero-system widely used
  in semiconductor technique

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Si/Ge NW
How Si/Ge
intrinsic-Ge (i-Ge) core Chemical deposition
Vapor 5, 10, 15 nm Au cluster 30 sccm 10 GeH4
in H2 200 sccm H2 Nucleation at 315C 300Torr
for I min Growth at 280C 280Torr for 15 min
i-Si shell SiH4 (5 sccm) at 450C5 Torr for
5 min
Why Si/Ge
Lattice match, Si, 5.431 Å Ge, 5.658
Å Bandgap offset Valence band(VB) offset 0.5eV
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Chemical Vapor Deposition
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High-resolution TEM image
Structure Features

• Epitaxial growth Si lattice match with Ge,
  eliminating scattering from surface deffects
• Intrinsic silicon and gemanium eliminating
  scattering from ionized dopant
• Thin Si shell facilitating electric contact to
  Ge core and decreasing dislocation
• Circular geometry forming a channel because of
  confinement potential between Si and Ge.

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Fabrication of Devices

• Top Gated

• Back Gated

550nm Cr/Au
6nm Al2O3
2-5 nm Si /10nm Ge
50nm Ni
50nm Ni
50 nm SiO2
50 nm SiO2
n-Si Rlt0.005O.cm-1
n-Si Rlt0.005O.cm-1

• Annealing 300oC for 15 min in H2
• Electric Measurement enviroment pressurelt10-4
  Torr

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1-D Hole Gas
10-nm- Ge(core)/Si(shell)
Separate 20-nm Ge or Si

• Current increase as Vg changes from -10V to 10V
  P-type
• Core/shell structure has much larger current
  Hole accumulation

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Contact
Schottky contact Unannealed
Transparent contact Annealed
Metal
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Coulomb Blockade-Unneeded
T1.5K, Vsd0.5mV L112nm
Vg-9.38 V
Unannealed Ge/Si wire, tunnel barrier exists
between contact and silicon shell, which acts as
Coulomb Island
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Coulomb blockade-Conception
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Ballistic Transport-Conception
Electron Reservoir
Electron Reservoir
1-D conductor

• Finite conductance, which is independent to wire
  length
• No electron-phonon scattering due to ultra-high
  velocity of electron

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Ballistic Transport
L350nm,T4.7K
L170nm,T300,50,10, 4.7K

• Single-mode ballistic transport observed in Ge/Si
  at back-gate structure
• Ballistic transport at room temperature ascribed
  to reduced acoustic phonon scattering, further
  theoretical studies needed, especially
  confinement effect on phonon modes
• 0.7 structure. spontaneous spin polarization due
  to the formation of a spin gap or a localized
  spin
• Variation at conductance plateau suggestive of
  FabryPerot interferences

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Top gate

• Increases the gate coupling, to probe transport
  through more than one subband.
• Subband observed in G-Vsd (B)
• Subband spacing obtained from transcondutance as
  functions of Vg and Vsd
• Experiemental value consistent with theoretical
  calculation based on an effective mass model with
  a cylindrical confinement potential

100k
50k
10k
5k
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Conclusion

• Create a 1D hole gas system in Ge/Si core/shell
  NW heterostructures.
• Ballistic transport through individual 1D
  subbands due to confinement of carriers in the
  radial direction
• Little temperature dependence, suggesting a room
  temperature carrier mean free path on the order
  of several hundred nanometers

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Questions

• Physical model for Ge/Si,
• the effect of depletion thickness of Ge/Si??
• Effect of radial size of Ge/Si
• Effect of spin polarization??
• Theoretical Explanation for Ballistic Transport
  in Si/Ge??

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What we can do??

• 1-D Electron Gas, inverse Ge/Si??
• Controlled 1-D gas via external field, like
  Quantum Hall Effect
• Compound Semiconductor Hetero-Junction??
• Multi-layer Junction to make coupled
  hole-electron,hole-hole,electron-electron gas??
• Bipolar transistor, like Optic-electronic

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Thanks