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

Transcription

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.

3 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. Nature 439, 703 (2006) Used STM to image 10 nm Si film on SiO 2 Charge traps Surface states SiO 2 SiO 2 Si Vacuum

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

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

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

7 Theoretical method DFT: Linear Muffin-Tin Orbital (LMTO) formalism 2 Large-scale problems (~1000 atoms) Can treat disorder, impurities, dopants and surface roughness DFT H KS ρ NEGF 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).

8 System under study (surface) Hydrogenated surface vs. clean surface H terminated [2 1:H] Clean [P(2 2)] H Si (top) Si Si (top:1) Si (top:2) Si

9 Results (bulk case) Atomic structure & bandstructure H terminated [2 1:H] Clean [P(2 2)] dimers dimers dimers dimers dimers dimers dimers dimers Large gap ~0.7 ev (with local density approximation) Small gap ~0.1 ev (with local density approximation)

10 Results (bulk case) Atomic structure & bandstructure H terminated [2 1:H] Clean [P(2 2)] dimers dimers dimers dimers dimers dimers dimers dimers Large gap ~0.7 ev (with local density approximation) Small gap ~0.1 ev (with local density approximation)

11 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.

12 Band gap values with DFT Recent development solves the band gap problem associated with DFT calculations.

13 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 n ++ i n ++ L = 3.8 nm n ++ n ++ i L = 19.2 nm

14 Results (n ++ - i - n ++ system) Potential profile (effect of length) Max potential varies with length Screening length > 10nm CB E F n ++ i VB

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 E F n ++ i VB

16 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 E F n ++ i VB

17 Results (n ++ - i - n ++ system) Conductance vs. k-points ( dimers) Shows contribution from k-points to transport TOP VIEW i n ++ n ++ Transport occurs near Γ point. Conductance drops very rapidly

18 Results (n ++ - i - n ++ system) Conductance vs. k-points ( dimers) TOP VIEW i n ++ n ++ Largest G near Γ point Conductance drops rapidly, but slower than for transport to dimers.

19 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.

20 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 ~10 4 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).

21 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.