Optical SHG and RAS of molecular adsorption at Si(001) step edges

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1 Optical SHG and RAS of molecular adsorption at Si(001) step edges Robert Ehlert, Jinhee Kwon and Michael C. Downer Department of Physics, The University of Texas at Austin, Austin TX 78712, USA. OSI VI: J. Kwon reported on a common microscopic model for SHG & RAS on the oxidized vicinal Si(001) interface. J. Kwon et al., Phys. Rev. B 73, (2006) Outline Motivation SHG & RAS Results from oxidized vicinal Si(001) Clean and H-adsorbed vicinal Si(001) SBHM analysis Conclusion and Outlook β t β b γ γ t s γ l γ w β b β s OSI VII, Jackson Hole, Wyoming, July 16 th /13

2 Optical sensors provide feedback control for self directed growth of nanostructures nanostructures offer possibility to further reduce size of micro/nano-electronics 1D quantum wires, 1 molecular electronics, 2 atomic-scale memory, 3 quantum computers 4 1 McChesney, Nanotech. 13, 545 (02) 2 Kasemo, Surf. Sci. 500, 656 (02) 3 Bennewitz, Nanotech. 14, 499 (02) 4 Ladd, Phys. Rev. Lett. 89, (02) *Himpsel, F. J et al., Solid State Communications 117(3): (2001). atom by atom assembly very slow Self directed growth techniques desirable silicon stepped surfaces can be used as templates for self directed growth of nanostructures* self directed growth of nanostructures is faster but also less controlled Non-invasive in-situ sensors are needed OSI VII, Jackson Hole, Wyoming, July 16 th /13

3 Combination of SHG and RAS provides comprehensive database for bond level modeling of step surfaces Optical probes are non-invasive, non-destructive, and applicable to any transparent medium SHG High spatial and spectral resolutions Real-time mapping of fast surface dynamics ω 2ω SHG RAS measure surface-induced optical anisotropy obtain information concerning surface electronic structure and chemistry RAS ~ r ~ r ~ r ~ r ) ( reconstructed surface (broken inversion symmetry) bulk (centrosymmetry) [001] [110] Bulk Silicon ζ (anisotropic surface) (isotropic bulk) OSI VII, Jackson Hole, Wyoming, July 16 th /13

4 Optical setups to investigate samples in UHV environment tunable fs-source monochromator + PMT analyzer PEM PMT quartz Coherent MIRA Ti:Sapphire oscillator nm NOPA 520nm-780nm effusion cells polarizer strain free window Xenon lamp (1.5 to 5 ev) analyzer PMT BG39 B.S SHG ports QMS sample OSI VII, Jackson Hole, Wyoming, July 16 th /13

5 Results from previous studies of oxidized vicinal Si(001) surfaces RA-SHG J. Kwon et al., Phys. Rev. B 73, (2006) [s-in, p-out] [p-in, SHG p-out] data SHG data Applied SBHM to vicinal Si/SiO 2 interface [s-in, p-out] [p-in, reconstructed p-out] fitted using pp parameters summary of main results: relative bond strength varying upper bond angle γ t γ s γ b basis for analysis: mutually consistent SHG & RAS spectra Kramers-Kronig consistency 2 SHG polarization combinations (sp,pp) recommended * but not done yet: extend to other polarization combinations (ss,ps) * McGilp, J. F., Journal of Physics: Condensed Matter 19(1): (2007) Experimental Re[ r/r] Experimental RAS 6 o 4 o o 10 8 o 0 o photon energy (ev) OSI VII, Jackson Hole, Wyoming, July 16 th /13

6 Clean reconstructed vicinal Si(001) surface prepared in ultra high vacuum Ab-inito DFT calculation of D B step strucuture on Si(001):6º at T = 0 K (courtesy E. Pehlke) D B steps c(4x2) terraces M. Dürr et al., Phys. Rev B 63, (2001) OSI VII, Jackson Hole, Wyoming, July 16 th /13

7 OSI VII, Jackson Hole, Wyoming, July 16 th /13

8 Selective adsorption of on step-edges strongly influences SHG at λ= 1064 nm M. Dürr et al., Phys. Rev B. 63, (2001) + Single wavelength SHG sensitive to adsorption OSI VII, Jackson Hole, Wyoming, July 16 th /13 P. Kratzer et al., Phys. Rev Lett. 81, 5596 (1998)

9 Spectroscopic SH and RAS study of adsorption on step-edges + clean p-in/p-out SHG of reconstructed vicinal Si:6 o (x10 2 ) RAS p-in/p-out OSI VII, Jackson Hole, Wyoming, July 16 th /13

10 SBHM using ab-initio structure calculation fits SHG data with high fidelity SBHM * allows to fit RA-SHG with a number of distinct bond hyperpolarizabilities Input ab initio DFT calculation provides input structure for SBHM analysis complex hyperpolarizabilities β of up to 7 classes of bonds SBHM fits to RA-SHG data at selected wavelength clean Results *G. D. Powell and D. E. Aspnes et al., J. Vac. Sci. Technol. B20(4), 1699 (2002) OSI VII, Jackson Hole, Wyoming, July 16 th /13

11 Fitted hyperpolarizabilities yield linear bond polarizabilities that reproduce RAS data clean Clean β 2j *Miller s theorem : β ω = α 2 E ω ff Iˆ kk ˆˆ) α bˆ bˆ ( α 1 j j ~ r ~ r ~ r ω αω αω j j E ω in Clean OSI VII, Jackson Hole, Wyoming, July 16 th /13

12 Hyperpolarizability spectra of individual surface bonds reveals charge transfer at identifiable molecular sites step edge bonds D B step terrace bonds (2x1) terraces OSI VII, Jackson Hole, Wyoming, July 16 th /13

13 Conclusion application of SBHM to spectroscopic SHG and RAS data suggests general ability to see charge transfer from dangling bonds to back bonds. Preliminary results, limited ability to give reliable microscopic information expand data set: other polarization combinations*, broader spectral range better microscopic calculations are needed beyond SBHM * McGilp, J. F., J. Phys.Condens. Mat. 19(1): (2007) Eventually move to other adsorbates i.e. organics molecules Research group Funding NSF, Robert Welch Foundation Jinhee Kwon (PhD) Yong An (PhD) Adrian Wirth (MA) Junwei Wei (MSc) OSI VII, Jackson Hole, Wyoming, July 16 th /13