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

One-dimensional hole gas in germanium silicon nanowire hetero-structures. Linyou Cao Department of Materials Science and Engineering Drexel University 12/09/2005. Motivation-Why?. Quantum Confinement not reported in NW Ballistic Transport Conductance Quantification

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

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  1. One-dimensional hole gas in germanium silicon nanowire hetero-structures Linyou Cao Department of Materials Science and Engineering Drexel University 12/09/2005

  2. 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

  3. Si Evac Ge Ec Si Electron injection Ge Ec Ev + + + + + + + + + + Ef + + + + + Ev 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 315˚C& 300Torr for I min Growth at 280˚C& 280Torr for 15 min i-Si shell: SiH4 (5 sccm) at 450˚C&5 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

  4. Chemical Vapor Deposition

  5. 5 nm 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.

  6. Fabrication of Devices • Back Gated • Top Gated 5~50nm Cr/Au 2-5 nm Si /10nm Ge 6nm Al2O3 50nmNi 50nm Ni 50 nm SiO2 50 nm SiO2 n-Si R<0.005Ω.cm-1 n-Si R<0.005Ω.cm-1 • Annealing: 300oC for 15 min in H2 • Electric Measurement enviroment: pressure<10-4 Torr

  7. Vg=-10V Vsd=-1V Vg=-10V Vg=0V Vg=+10V Vg=-10V Vg=0V 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

  8. Evac Ge Ec Si Electron injection Ec Ev + + + + + + + + Ef Ev Contact Schottky contact Unannealed Transparent contact Annealed Metal

  9. Coulomb Blockade-Unneeded T=1.5K, Vsd=0.5mV L=112nm Vg=-9.38 V Unannealed Ge/Si wire, tunnel barrier exists between contact and silicon shell, which acts as Coulomb Island

  10. Coulomb blockade-Conception

  11. 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

  12. Ballistic Transport L=350nm,T=4.7K L=170nm,T=300,50,10, 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 Fabry–Perot interferences

  13. 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

  14. 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

  15. 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??

  16. 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

  17. Thanks

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