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

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.

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 SiO 2 Charge traps Thickness

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) Nature 439, 703 (2006)

APS -- March Meeting 2010 First-principles study of electronic transport through Si(001) nano-scale films in a two-probe geometry Our goal Current Electrode

APS -- March Meeting 2010 First-principles study of electronic transport through Si(001) nano-scale films in a two-probe geometry Our goal Length Thickness Surface Current Electrode Doping level (lead or channel) Orientation

APS -- March Meeting 2010 Theoretical method Device Left lead Right lead Density functional theory (DFT) combined with nonequilibrium Green’s functions (NEGF) 1 Two-probe geometry under finite bias Buffer NEGF DFT H KS  -- ++ Simulation Box 1 Jeremy Taylor, Hong Guo and Jian Wang, PRB 63, (2001).

APS -- March Meeting 2010 Theoretical method  DFT: Linear Muffin-Tin Orbital (LMTO) formalism 2 Large-scale problems (~1000 atoms) Can treat disorder, impurities, dopants and surface roughness 2 Y. Ke, K. Xia and H. Guo, PRL 100, (2008); Y. Ke et al., PRB 79, (2009); F. Zahid et al., PRB 81, (2010). NEGF DFT H KS 

APS -- March Meeting 2010 System under study (surface)  Hydrogenated surface vs. clean surface H Si (top) Si Si (top:1) Si (top:2) Si H terminated [2  1:H] Clean [P(2  2)]

APS -- March Meeting 2010 Results (bulk case)  Atomic structure & bandstructure H terminated [2  1:H]Clean [P(2  2)] || dimers  dimers || dimers  dimers Large gap ~0.7 eV (with local density approximation) Small gap ~0.1 eV (with local density approximation)  dimers || dimers  dimers

APS -- March Meeting 2010 Results (bulk case)  Atomic structure & bandstructure H terminated [2  1:H]Clean [P(2  2)] || dimers  dimers || dimers  dimers Large gap ~0.7 eV (with local density approximation) Small gap ~0.1 eV (with local density approximation)  dimers || dimers  dimers

APS -- March Meeting 2010 Results (n ++ - i - n ++ system)  Two-probe system Channel : intrinsic Si Leads : n ++ doped Si 2  1:H surface Periodic  to transport n ++ i i L = 3.8 nm L = 19.2 nm T = 1.7 nm

APS -- March Meeting 2010 Results (n ++ - i - n ++ system)  Potential profile (effect of length) Max potential varies with length Screening length > 10nm n ++ EFEF VB i CB

APS -- March Meeting 2010 Results (n ++ - i - n ++ system)  Potential profile (effect of doping) Max potential increases with doping Slope at interface greater with doping, i.e. better screening n ++ EFEF VB i CB

APS -- March Meeting 2010 Results (n ++ - i - n ++ system)  Potential profile (effect of doping) Max potential increases with doping Slope at interface greater with doping, i.e. better screening n ++ EFEF VB i CB

APS -- March Meeting 2010 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 i n ++ n++n++   TOP VIEW

APS -- March Meeting 2010 Results (n ++ - i - n ++ system)  Conductance vs. k-points (|| dimers) i n ++ n++n++   Largest G near  point Conductance drops rapidly, but slower than for transport  to dimers. TOP VIEW

APS -- March Meeting 2010 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.

APS -- March Meeting 2010 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 ~10 4 atoms (1800 atoms) & sizes ~10 nm (23.8 nm)!

APS -- March Meeting 2010 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.