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Computational Studies of Silicon Nanostructure Surfaces R. Q. Zhang City University of Hong Kong

Computational Studies of Silicon Nanostructure Surfaces R. Q. Zhang City University of Hong Kong “International Workshop on High-volume Experimental Data, Computational Modeling and Visualization” October 17th - 19th, 2011, Fragrant Mountain, Beijing, China. Outline. Why nanosurface

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Computational Studies of Silicon Nanostructure Surfaces R. Q. Zhang City University of Hong Kong

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  1. Computational Studies of Silicon Nanostructure Surfaces R. Q. Zhang City University of Hong Kong “International Workshop on High-volume Experimental Data, Computational Modeling and Visualization” October 17th - 19th, 2011, Fragrant Mountain, Beijing, China

  2. Outline • Why nanosurface • Surface effect on band structures • Surface doping • Surface induced thermal conductivity attenuation • Summary

  3. Outline • Why nanosurface • Surface effect on band structures • Surface doping • Surface induced thermal conductivity attenuation • Summary

  4. Si Crystal Si (100) Surface

  5. Intel's roadmap

  6. Surface of SiNWs R.Q. Zhang, et al., JCP 123, 144703 (2005); October 24, 2005 issue of VJNST Analysis + DFT <112> wire

  7. Remarkable effects of surface dihydride configurations Symmetric Canted HOMO C. S. Guo, X. B. Yang, and R. Q. Zhang, Solid State Commun., 149, 1666(2009).

  8. (111) surfaces terminated with SiH3: Symmetric, rotated, and tilted rhombus rhombus symmetric rotated tilted nanowire DDD Ma, et al, Science For both bulk and NW surface, the tilted one is the most stable. The surface states are not at the band edge. Hu Xu, R.Q. Zhang et al., Phys. Rev. B 79, 073402 (2009)

  9. SA/V ratio of nanowires:  1/a a Area Volume SA/V ratio 4aLa2L 4/a 2aLa2L 2/a L L a a

  10. SA/V ratio increase will enhance • structure change • charge transfer • boundary effect • confinement • …… • stability • excited-state property • band structure • doping effect • thermal transport

  11. Methodologies • Surface effect on band structures • => DFT, DFT/MD • Surface doping • => DFT • Surface induced thermal conductivity attenuation • => Tersoff potential based MD

  12. Outline • Why surface • Surface effect on band structures • Surface doping • Surface induced thermal conductivity attenuation • Summary

  13. Band structure tuning of SiNWs Indirect <==>direct? Si crystal <112> SiNW

  14. Indirect band structure of <112> SiNWs => quasi-direct band-gap A.J. Lu, R.Q. Zhang, et al., Nanotechnology, 19, 035708 (2008)

  15. Tuning energy band of <112> SiNWs by varying cross-section shape (111) To (110) side facet ratio A.J. Lu, R.Q. Zhang, et al., Appl. Phys. Lett., 92, 203109 (2008).

  16. Tuning energy band of <112> SiNWs by varying cross-section shape (110) (110) (111) (111) LDOS distribution determines the band gap characteristic.

  17. Tuning energy band of <112> SiNWs by atomic phosphorus adsorption Why phosphorus? - a similar atomic radius to silicon - the higher electronegativity

  18. Tuning energy band of <110> SiNWs by atomic nitrogen adsorption – metallization X. B. Yang and R. Q. Zhang, Appl. Phys. Lett., 94, 113101 (2009).

  19. Outline • Why surface • Surface effect on band structures • Surface doping • Surface induced thermal conductivity attenuation • Summary

  20. B Si h + P e - E E Ec Ec Donor level Acceptor level Ev Ev Conventional n- and p-type Si doping

  21. Temperature dependence of carry concentration

  22. Nanodevices using doped SiNWs - CMOS - Solar cells - Thermoelectrics - Sensors - …

  23. Attempted volume doping in SiNWs • Experimental observation: • Less controllable and also less effective • due to the donor deactivation • M. Diarra, et al., Phys. Rev. B 75, 045301 (2007). • M. T. Bjork, et a., Nat. Nanotechnol. 4, 103 (2009). • Theoretical: • Impurities favor the surface position and • such doping reduces the density of carriers • M. V. Fernandez-Serra, et al., PRL 96, 166805(2006).

  24. Charge transfer due to surface passivation M.X. He, R.Q. Zhang, et al., J. Theor. Comput. Chem., 8, 299–316 (2009).

  25. Surface Passivation Doping of Silicon Nanowires p q is the partial charge on a surface hydrogen atom = -0.06 |e| , a is the silicon lattice constant 5.43 Å, and D is the diameter = 100 nm . p= 1.61019 cm-3 Hole concentration due to surface doping: 1019 cm-3 C.S. Guo, R.Q. Zhang, et al., Angew. Chem. Int. Ed, 48/52, 9896(2009).

  26. Experimental verification 1: FET at Vds = - 2 V in vacuum

  27. Experimental verification 2: A p-n junction arry by surface passivation doping C.S. Guo, L.B. Luo, R.Q. Zhang, S.T. Lee, et al., Angew. Chem. Int. Ed, 48/52, 9896 (2009)

  28. Outline • Why surface • Surface effect on band structures • Surface doping • Surface induced thermal conductivity attenuation • Summary

  29. Theory z component of heat current Green-Kubo expression <autocorrelation function> Double exponential approximation (small MD simulation time + reduced cut-off artifacts)

  30. Fitting For bulk silicon with 512 atoms: The fitting parameters Ao,o, Aa anda are then used to calculate the thermal conductivity (red curve) at m.

  31. Test For bulk silicon with 512 atoms, The predicted the thermal conductivity: At 300K: ~ 82 W/mK At 1000K: ~ 15.5 W/mK Reported results: At 300K: ~85 W/mK [*] with Tersoff potential; At 1000K: 17.78 2.61W/mK [**] with Stillinger-Weber potential [*] Li X., Maute K., Dunn M. L., and Yang R., Phys. Rev. B, 81 (2010) 245318 [**] Chen J., Zhang G., and Li B., Phys. Lett. A, 374 (2010) 2392

  32. Models

  33. Surface nitrogenation induced thermal conductivity attenuation

  34. Phonon spectra along the longitude direction Si-N vibrational modes Si-Si vibrational characteristic

  35. Summary • SA/V ratio – 1/a • Surface effect on band structures • Surface doping – passivation & transfer • Thermal conductivity attenuation

  36. Acknowledgements • Co-authors: • Surface effect on band structures: H. Xu, C.S. Guo, A.J. Lu • Surface doping: C.S. Guo, X.B. Yang • Surface induced thermal conductivity attenuation: H.P. Li • Grants:RGC, CityU

  37. Thank You！

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