OSI VIII 2009 1/11

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

Step-induced electronic resonance at vicinal Si(001)

observed by spectroscopic SHG and RASSHG and RAS

!on-invasive in-situ sensors are needed

� investigation of step-enhanced

chemical reactions

� atomic wires suitable for

transport

� “lithography” by self

assembly of nanostructures

� …

MotivationMotivation

SHG/RAS need to be applied together

Himpsel, F. J et al., Solid State Communications

117(3): 149-157 (2001).

C. Tegenkamp, Journal of Physics-Condensed

Matter 21 (2009).

J. Viernow et al., Appl. Phys. Lett. 72, 948 (1998);

OSI VIII 2009 2/11

1 2 3 4 5

-2

0

2

∆r/r [10-3]

photon energy [eV]

RAS of vicinal Si(001) surface show step induced features

S

Schmidt, W. G., F. Bechstedt, et al. PRB 63(4) 045322 (2001)

can we use SHG to see this step induced resonance and

would it provide additional information?

RAS spectra of clean and H2

adsorbed Si(001):6

Calculated optical anisotropy and step induced features

Jaloviar, S. G., J. L. Lin, et al. (1999). PRL 82(4): 791-794.

S

OSI VIII 2009 3/11

Combined SHG/RAS probe of stepped Si(001) surfaces in

UHV environment

monochromator

+ PMT

QMSsample

gas inlet valve

PEM

Xenon lamp(1.5 to 5 eV)

tun

ab

le

fs-s

ou

rce

B.S.

quartz

PM

T

BG39

analyzer

strain free

window

analyzer

polarizer

SHG ports

� Coherent MIRA

Ti:Sapphire oscillator

710-900nm

� NOPA

520nm-780nm

H2 sticking coefficient

PMT

+ H2

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

OSI VIII 2009 4/11

RAS sensitive to adsorbates but does not distinguish

terrace from step adsorption sites

� large number of dangling bonds

contamination in UHVChung, C. H., H. W. Yeom, et al. (2006). "Oxidation of Step Edges on

Si(001)-c(4 x 2)." Physical Review Letters 97(3): 036103.

Nishizawa, M., T. Yasuda, et al. (2002). "Origin of type-C defects on

the Si(100)-(2x1) surface." Physical Review B 65(16): 161302.

see also N. Witkowski et al. / Surface

Science 600 (2006) 5142–5149

24min

� surface very susceptible to contamination

� RAS is sensitive to adsorbates on

Si(001) (2x1)

� origin of features on modified surface

not well understood

250min 1200min

2 3 4 5-3

-2

-1

0

1

2 clean

after 3h in UHV

after 6h in UHV

after peforming AES

∆r/r [10-3]

photon energy

E1 E

0' E

2

� step d.b. highly reactivee.g. oxidation proceeds from step edges

OSI VIII 2009 5/11

Fourier analysis of RA-SHG clearly distinguishes step

edge and terrace adsorption sites

0 60 120 180 240 300 360

0.00

0.06

0.12

clean

1200L

1800L

2400L

SH intensity [arb. units]

azimuthal angle [degree]

0 5 10 15 20 25

-0.04

0.00

0.04

0.08

a2

a3

Fourier coefficients [arb. units]

time [hours]

0.1

0.2

0.3

0.4

a1

0 1000 2000-0.1

0.0

0.1

a2

a3

Fourier coefficients

hydrogen exposure [L]

0 1000 2000

-0.2

0.0

0.2

a1

60 180 3000.0

0.1

0.2

0.3

clean

after 3h

SH [arb. units]

azimuthal angle [degree]

0.00

0.05

0.10

0.15

after 6h

after AES

contamination H2 adsorption at step edges

OSI VIII 2009 6/11

terrace

terrace

step

SH dependence on azimuthal sample rotation Fourier analysis

� 16 λ (720-880nm)

� strong variations with azimuthal angle

� can be broken down in Fourier series

terrace

terrace

step

clean Si(001):6˚ (2x1)

after exposure to H2

Fourier analysis of SHG data yields spectra of Fourier

coefficients a0 to a4 within the available wavelength range

terrace

terrace

step

OSI VIII 2009 7/11

� ab-initio calculations for RAS have shown that

feature at 3eV is step induced*

� resembles line shape and H2 dependence of a1and a3

RA spectrum resembles line shape and H2 dependence of

step induced 1st and 3rd order Fourier coefficients

*W. G. Schmidt, F. Bechstedt and J. Bernholc, Phys. Rev. B 63, (2001).

0.1

0.3

SH [arb. units]

a1

2.8 3.3

0.0

0.1

SH photon energy [eV]

a3

1 2 3 4 5

-2

0

2

∆r/r [10-3]

photon energy [eV]

may point to common microscopic

origin of RAS/SHG

OSI VIII 2009 8/11

� ab initio DFT calculation provides

input structure for SBHM analysis� SBHM fits to RA-SHG

data at selected wavelength

Simplified Bond Hyperpolarizability Model (SBHM) using ab-

initio structure calculation fits SHG data with high fidelity

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

SBHM* enables qualitative bond level interpretation of SHG/RAS

� allows empirical fits with varying

bond hyperpolarizabilities β of

select classes of surface bonds

ResultsResults

SBHM fits data with high fidelity

ββββd.bstep

ββββrebondstep

ββββbackstep

ββββd.b.terrace

ββββdimerterrace

ββββback.terrace

H2 clean

& reproduces measured RAS**R. C. Miller. Phys. Rev. Lett. 5, 17 (1964)

-0 90 180 270 360

0.00

0.02

0.04

840nm

0.0

0.1

0.2

730nm 800nm

840nm

OSI VIII 2009 9/11

step edge bonds terrace bonds

DB step (2x1) terraces

Hyperpolarizability spectra of individual surface bonds

reveal charge transfer at identifiable molecular sites

0.1

0.6back

Re[βstep]

0

1

Re[βstep]

rebond

-1

-3

-5

d.b.

Re[βstep]

0.1

0.6

1.1

d.b.Re[β

terrace]

2.8 3.0 3.2 3.4

-0.4

0.1

0.6

Im[βstep]

back

-0.1

0.1

dimerRe[β

terrace]

0.1

0.6

dimerIm[β

terrace]

0.0

0.5

Im[βstep ]

rebond

0

1

Re[βstep]

rebond

-1

-3

-5

d.b.

Re[βstep]

OSI VIII 2009 10/11

Conclusion

� showed that SHG is sensitive to adsorbates on Si(001) (2x1):6° and

allows to distinguish specific adsorption sites

� acquired spectroscopic SHG and RAS data together on same clean

reconstructed vicinal Si(001):6˚ surfaces before and after dissociative

adsorption of H2 at DB step edges

Results reveal specific underlying connections between RAS and

SHG and allow qualitative bond level interpretation

� reconstructed RAS line shape

� charge-rich step edge dangling bond dominant contribution to SH signal

� able to “see” charge redistribute from step edge dangling bonds into back

bonds

� Fourier analysis revealed step-induced 3rd order Fourier component

shows step d.b. induced resonance, resembling RAS line shape

� SBHM:

OSI VIII 2009 11/11

NSF DMR-0706227

Robert Welch

Foundation F-1038

Funding

Research Group

Junwei Wei, Jimmy

Price, Ming Lei

Future directions

� Extend RAS/SHG study to other adsorbates, i.e. atomic hydrogen

� pump/probe

� Obtain ab-initio calculations of the nonlinear response