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Electronic transport properties of nano-scale Si films: an ab initio study

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).

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Electronic transport properties of nano-scale Si films: an ab initio study

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  1. 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

  2. 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

  3. 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) SiO2 Charge traps Si Thickness SiO2 APS -- March Meeting 2010

  4. 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

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

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

  7. Theoretical method • Density functional theory (DFT) combined with nonequilibrium Green’s functions (NEGF)1 • Two-probe geometry under finite bias DFT  HKS NEGF Simulation Box - + Device Left lead Right lead Buffer Buffer APS -- March Meeting 2010 1Jeremy Taylor, Hong Guo and Jian Wang, PRB 63, 245407 (2001).

  8. Theoretical method • DFT: Linear Muffin-Tin Orbital (LMTO) formalism2 • Large-scale problems (~1000 atoms) • Can treat disorder, impurities, dopants and surface roughness DFT  HKS NEGF 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). APS -- March Meeting 2010

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

  10. || dimers  dimers  dimers || dimers 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) APS -- March Meeting 2010

  11. || dimers  dimers  dimers || dimers 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) APS -- March Meeting 2010

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

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

  14. 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 CB EF i VB n++ APS -- March Meeting 2010

  15. 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 CB EF i VB n++ APS -- March Meeting 2010

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

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

  18. 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

  19. 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)! APS -- March Meeting 2010

  20. 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. APS -- March Meeting 2010

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