OSI VII, Jackson Hole, Wyoming, July 16th 2007 1/13

Robert Ehlert, Jinhee Kwon and Michael C. DownerDepartment of Physics, The University of Texas at Austin, Austin TX 78712, USA.

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

MotivationSHG & RASResults from oxidized vicinal Si(001)CleanClean and HH--adsorbedadsorbed vicinal Si(001)SBHM analysisConclusion and Outlook

Outline

OSI VI: J. Kwon reported on a common microscopic model for SHG & RAS on the oxidizedoxidized vicinal Si(001) interface. J. Kwon et al., Phys. Rev. B 73, 195330(2006)

γs

γt

γl

γw

βsβt

βb

βb

OSI VII, Jackson Hole, Wyoming, July 16th 2007 2/13

Optical sensors provide feedback control for self directed growth of nanostructures

Non-invasive in-situ sensors are needed

1D quantum wires,1molecular electronics,2atomic-scale memory,3quantum computers4

1McChesney, Nanotech. 13, 545 (02)2Kasemo, Surf. Sci. 500, 656 (02)3Bennewitz, Nanotech. 14, 499 (02)4Ladd, Phys. Rev. Lett. 89, 017901 (02)

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

nanostructures offer possibility to further reduce size of micro/nano-electronics

atom by atom assembly very slowSelf directed growth techniques desirable

*Himpsel, F. J et al., Solid State Communications 117(3): 149-157 (2001).

OSI VII, Jackson Hole, Wyoming, July 16th 2007 3/13

Optical probes are non-invasive, non-destructive, and applicable to any transparent medium

bulk(centrosymmetry)

reconstructed surface(broken inversion symmetry)

(isotropic bulk)

(anisotropic surface)[001]

[110]ζ

Bulk Silicon

ωSHG

2ωRAS

||~r⊥r~

)~~( || ⊥− rr

• High spatial and spectral resolutions • Real-time mapping of fast surface dynamics

SHG RAS• measure surface-induced optical

anisotropy• obtain information concerning

surface electronic structure and chemistry

Combination of SHG and RAS provides comprehensive database for bond level modeling of step surfaces

OSI VII, Jackson Hole, Wyoming, July 16th 2007 4/13

Optical setups to investigate samples in UHV environment

monochromator

+ PMT

QMSsample

effusion cells

PEM

Xenon lamp(1.5 to 5 eV)

tuna

ble

fs-s

ourc

e

B.S

quartz

PMT

PMT

BG39

analyzerstrain freewindow

analyzer

polarizer

SHG ports

Coherent MIRA Ti:Sapphire oscillator 710-900nmNOPA520nm-780nm

OSI VII, Jackson Hole, Wyoming, July 16th 2007 5/13

Results from previous studies of oxidizedoxidized vicinal Si(001) surfaces

relative bond strengthvarying upper bond angle

summary of main results:

2.8 3.0 3.2 3.4 photon energy (eV)

8o10o

0o

4o

6o

Exp

erim

enta

l Re[∆

r/r]

1.5

0.0

-1.5

Experimental RAS

Applied SBHM to vicinal Si/SiO2 interface

J. Kwon et al., Phys. Rev. B 73, 195330(2006) [p-in, p-out] SHG data[p-in, p-out] fitted

RA-SHG[s-in, p-out] SHG data[s-in, p-out] reconstructed using pP parameters

mutually consistent SHG & RAS spectrabasis for analysis:

recommended* but not done yet:extend to other polarization combinations (sS,pS)

*McGilp, J. F., Journal of Physics: Condensed Matter 19(1): 016006 (2007)

γsγt

γb

Kramers-Kronig consistency2 SHG polarization combinations (sP,pP)

OSI VII, Jackson Hole, Wyoming, July 16th 2007 6/13

(courtesy E. Pehlke)Ab-inito DFT calculation of DB step strucuture on Si(001):6º at T = 0 K

Clean reconstructedClean reconstructed vicinal Si(001) surface prepared in ultra high vacuum

M. Dürr et al., Phys. Rev B 63, 121315 (2001)

c(4x2) terraces

DB steps

OSI VII, Jackson Hole, Wyoming, July 16th 2007 7/13

H2

H2 H2 H2

H2

H2

OSI VII, Jackson Hole, Wyoming, July 16th 2007 8/13

Single wavelength SHG sensitive to H2 adsorption

P. Kratzer et al., Phys. Rev Lett. 81, 5596 (1998)

Selective adsorption of H2 on step-edges strongly influences SHG at λ= 1064 nm

+ H2

M. Dürr et al., Phys. Rev B. 63, 121315 (2001)

OSI VII, Jackson Hole, Wyoming, July 16th 2007 9/13

+ H2

SHG of reconstructed vicinal Si:6o

cleanp-in/p-out

H2

Spectroscopic SH and RAS study of H2adsorption on step-edges

RAS(x

102 )

p-in/p-out

OSI VII, Jackson Hole, Wyoming, July 16th 2007 10/13

ab initio DFT calculation provides input structure for SBHM analysis

SBHM fits to RA-SHG data at selected wavelength

SBHM using ab-initio structure calculation fits SHG data with high fidelity

H2

clean

*G. D. Powell and D. E. Aspnes et al., J. Vac. Sci. Technol. B20(4), 1699 (2002)

SBHM* allows to fit RA-SHG with a number of distinct bond hyperpolarizabilities

InputInput

complex hyperpolarizabilities β of up to 7 classes of bonds

ResultsResults

OSI VII, Jackson Hole, Wyoming, July 16th 2007 11/13

ωω α injjj

jff EbbkkIE ⋅⋅−∝ ∑ ˆˆ)ˆˆˆ( 1

*Miller’s theorem :

β2j||

j1α

||||||2

||2 ωωωω αααβ ∆=

rrr

~~~110011 −

H2

Clean

Clean

H2

clean

H2

Fitted hyperpolarizabilities yield linear bond polarizabilities that reproduce RAS data

OSI VII, Jackson Hole, Wyoming, July 16th 2007 12/13

step edge bonds terrace bondsDB step (2x1) terraces

Hyperpolarizability spectra of individual surface bonds reveals charge transfer at identifiable molecular sites

OSI VII, Jackson Hole, Wyoming, July 16th 2007 13/13

Conclusion

expand data set: other polarization combinations*, broader spectral rangebetter microscopic calculations are needed beyond SBHM

application of SBHM to spectroscopic SHG and RAS data suggests general ability to “see” charge transfer from dangling bonds to back bonds.

Research group

NSF, Robert Welch FoundationFunding

Preliminary results, limited ability to give reliable microscopic information

*McGilp, J. F., J. Phys.Condens. Mat. 19(1): 016006 (2007)

Eventually move to other adsorbates i.e. organics molecules

Adrian Wirth (MA)

Junwei Wei(MSc)

Jinhee Kwon (PhD)

Yong An(PhD)