electronic transport properties of nano-scale si films: an ab initio study
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Electronic transport properties of nano-scale Si films: an ab initio study. Jesse Maassen , Youqi Ke, Ferdows Zahid and Hong Guo Department of Physics, McGill University, Montreal, Canada. Motivation (of transport through Si thin films). - PowerPoint PPT PresentationTRANSCRIPT
Electronic transport properties of nano-scale
Si films: an ab initio study
Jesse Maassen, Youqi Ke, Ferdows Zahid and Hong Guo
Department of Physics,
McGill University, Montreal, Canada
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University of Wisconsin-Madison
Motivation(of transport through Si thin films)
As the thickness of a film decreases, the properties of the surface can dominate.
University of Wisconsin-Madison
Motivation(of transport through Si thin films)
The main motivation for our research was the experimental work by Pengpeng Zhang et al. with silicon-on-insulators.
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Nature 439, 703 (2006)
SiO2
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SiSiO2 Vacuum
Charge traps
Used STM to image 10 nm Si film on SiO2
Surfacestates
University of Wisconsin-Madison
First-principles study of electronic transport through Si(001) nano-scale films in a two-probe geometry
Our goal
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University of Wisconsin-Madison
First-principles study of electronic transport through Si(001) nano-scale films in a two-probe geometry
Our goal
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Length
Th
ickn
ess
Surface
CurrentElectrode Electrode
Doping level(lead or channel) Orientation
University of Wisconsin-Madison
Theoretical method
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leadRightlead
• Density functional theory (DFT) combined with nonequilibrium Green’s functions (NEGF)1
• Two-probe geometry under finite bias
Buffer Buffer
NEGF
DFT
HKS
- +
Simulation Box
1 Jeremy Taylor, Hong Guo and Jian Wang, PRB 63, 245407 (2001).
University of Wisconsin-Madison
Theoretical method
DFT: Linear Muffin-Tin Orbital (LMTO) formalism2
Large-scale problems (~1000 atoms)
Can treat disorder, impurities, dopants and surface roughness
2Y. Ke, K. Xia and H. Guo, PRL 100, 166805 (2008); Y. Ke et al., PRB 79, 155406 (2009); F. Zahid et al., PRB 81, 045406 (2010).
NEGF
DFT
HKS
University of Wisconsin-Madison
System under study (surface)
Hydrogenated surface vs. clean surface
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H
Si (top)
Si
Si (top:1)
Si (top:2)
Si
H terminated [21:H] Clean [P(22)]
University of Wisconsin-Madison
Results (bulk case)
Atomic structure & bandstructure
H terminated [21:H] Clean [P(22)]
|| dimers dimers || dimers dimers
•Large gap ~0.7 eV (with local density approximation)
•Small gap ~0.1 eV (with local density approximation)
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d
imer
s
|| dimersQuickTime™ and a
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|| dimers
d
imer
s
University of Wisconsin-Madison
Results (bulk case)
Atomic structure & bandstructure
H terminated [21:H] Clean [P(22)]
|| dimers dimers || dimers dimers
•Large gap ~0.7 eV (with local density approximation)
•Small gap ~0.1 eV (with local density approximation)
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d
imer
s
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|| dimers
d
imer
s
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Results (bulk case)
Bandstructure : Direct vs. Indirect band gap
• Up to ~17nm thick, the band gap of a SiNM is direct.
• Need to calculate for thicker films.
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Band gap values with DFT
Recent development solves the “band gap” problem associated with DFT calculations.
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Results (n++- i - n++ system)
Two-probe system
Channel : intrinsic Si
Leads : n++ doped Si
21:H surface
Periodic to transport QuickTime™ and aTIFF (LZW) decompressor
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L = 3.8 nm
L = 19.2 nm
T = 1.7 nm
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University of Wisconsin-Madison
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Results (n++- i - n++ system)
Potential profile (effect of length)
Max potential varies with length
Screening length > 10nm
n++
EF
VB
i
CB
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Results (n++- i - n++ system)
Potential profile (effect of doping)
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Max potential increases with doping
Slope at interface greater with doping, i.e. better screening
n++
EF
VB
i
CB
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University of Wisconsin-Madison
Results (n++- i - n++ system)
Potential profile (effect of doping)
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Max potential increases with doping
Slope at interface greater with doping, i.e. better screening
n++
EF
VB
i
CB
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Results (n++- i - n++ system)
Conductance vs. k-points ( dimers)
Shows contribution from k-points to transport
Transport occurs near point.
Conductance drops very rapidly
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in++ n++
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TOP VIEW
University of Wisconsin-Madison
Results (n++- i - n++ system)
Conductance vs. k-points (|| dimers)
in++ n++
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Largest G near point
Conductance drops rapidly, but slower than for transport to dimers.
TOP VIEW
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Results (n++- i - n++ system)
Conductance vs. Length
Conductance has exponential dependence on length, i.e. transport = tunneling.
Large difference due to orientation.
Better transport in the direction of the dimer rows.
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Summary
Performed an ab initio study of charge transport through nano-scale Si thin films.
Expect to provide a more complete study on the influence of surface states shortly (H-passivated vs. clean)!
This method can potentially treat ~104 atoms (1800 atoms) & sizes ~10 nm (23.8 nm)!
This large-scale parameter-free modeling tool could be very useful for device and materials engineering (because of
it’s proper treatment of chemical bonding at interfaces & effects of disorder).
University of Wisconsin-Madison
Thank you !
Questions?
• Thanks to Prof. Wei Ji.• We gratefully acknowledge financial support from NSERC, FQRNT and CIFAR.• We thank RQCHP for access to their supercomputers.