Electronic transport properties of nano-scale Si films: an ab initio study Jesse Maassen, Youqi Ke,...

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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are needed to see this picture.

APS -- March Meeting 2010

Motivation(of transport through Si thin films)

As the thickness of a film decreases, the properties of the surface can dominate.




Experimental work by Pengpeng Zhang et al. at Univ. Wisconsin-Madison with Silicon-On-Insulators (SOI)

Si

SiO2

SiO2

Charge traps Thickness






Experimental work by Pengpeng Zhang et al. at Univ. Wisconsin-Madison with Silicon-On-Insulators (SOI)

Nature 439, 703 (2006)






First-principles study of electronic transport through Si(001) nano-scale films in a two-probe geometry

Our goal


are needed to see this picture.CurrentElectrode Electrode


First-principles study of electronic transport through Si(001) nano-scale films in a two-probe geometry

Our goal



Length

Th

ickn

ess

Surface

CurrentElectrode Electrode

Doping level(lead or channel) Orientation


Theoretical method


are needed to see this picture.DeviceLeft

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).


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


System under study (surface)

Hydrogenated surface vs. clean surface









H

Si (top)

Si

Si (top:1)

Si (top:2)

Si

H terminated [21:H] Clean [P(22)]


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)





d

imer

s

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|| dimers

d

imer

s


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







Potential profile (effect of length)

Max potential varies with length

Screening length > 10nm

n++

EF

VB

i

CB


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Potential profile (effect of doping)



Max potential increases with doping

Slope at interface greater with doping, i.e. better screening

n++

EF

VB

i

CB





Conductance vs. k-points ( dimers)

Shows contribution from k-points to transport

Transport occurs near point.

Conductance drops very rapidly



in++ n++



TOP VIEW



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





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.






Conclusions

Ab initio study of charge transport through nano-scale Si thin films.

More complete study on the influence of surface states to come shortly!

Large-scale parameter-free modeling tool useful for device and materials engineering (proper treatment of

chemical bonding at interfaces & effects of disorder).

Potential to treat ~104 atoms (1800 atoms) & sizes ~10 nm (23.8 nm)!


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.

Electronic transport properties of nano-scale Si films: an ab initio study Jesse Maassen, Youqi Ke,...

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