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 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

Si/Ge NW Ge Si Ge Si EcEc EcEc EvEv EvEv EfEf E vac Electron injection Lattice match, Si, Å; Ge, Å Bandgap offset Valence band(VB) offset ~0.5eV Why Si/Ge: intrinsic-Ge (i-Ge) core: Chemical deposition Vapor 5, 10, 15 nm Au cluster 30 sccm 10% GeH 4 in H sccm H 2 Nucleation at 315˚C& 300Torr for I min Growth at 280˚C& 280Torr for 15 min i-Si shell: SiH 4 (5 sccm) at 450˚C&5 Torr for 5 min How Si/Ge:

Chemical Vapor Deposition

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. 5 nm High-resolution TEM image Structure Features

Fabrication of Devices 50nm Ni n-Si R<0.005Ω.cm nm SiO 2 6nm Al 2 O 3 n-Si R<0.005Ω.cm nm SiO 2 Top Gated Top Gated 2-5 nm Si /10nm Ge 5~50nm Cr/Au  Annealing: 300 o C for 15 min in H 2  Electric Measurement enviroment: pressure<10-4 Torr Back Gated Back Gated

1-D Hole Gas 10-nm- Ge(core)/Si(shell)Separate 20-nm Ge or Si Vg=-10V Vg=0V Vg=+10V Vsd=-1V Vg=-10V Vg=0V  Current increase as Vg changes from -10V to +10V: P-type  Core/shell structure has much larger current: Hole accumulation

Ge Si EcEc EcEc EvEv EvEv EfEf E vac Electron injection Metal Contact Schottky contact Unannealed Transparent contact Annealed

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

Coulomb blockade-Conception

Ballistic Transport-Conception Electron Reservoir 1-D conductor Finite conductance, which is independent to wire length No electron-phonon scattering due to ultra-high velocity of electron

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

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 5k 10k 50k 100k

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

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??

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

Thanks