Optical SHG and RAS of molecular adsorption at Si(001 ...
Transcript of Optical SHG and RAS of molecular adsorption at Si(001 ...
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).
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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
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H2
H2 H2 H2
H2
H2
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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
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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
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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
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step edge bonds terrace bondsDB step (2x1) terraces
Hyperpolarizability spectra of individual surface bonds reveals charge transfer at identifiable molecular sites
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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)