Research Express@NCKU - Articles Digest
Research Express@NCKU Volume 5 Issue 5 - August 15, 2008
[ http://research.ncku.edu.tw/re/articles/e/20080815/1.html ]
Continuous-time photoelectron spectroscopy
for monitoring monochromatic soft x-ray
photodissociation of CF3Cl adsorbed on Si
(111)-7×7
L.-C. Chou, W.-M. Chuang, W.-C. Tsai, S.-K. Wang, Y.-H. Wu, and
Ching-Rong Wen*
Department of Physics, National Cheng Kung University
*Email:
[email protected]
Appl. Phys. Lett. 91, 144103 (2007)
S
ite-specific chemical bond scission of molecules adsorbed on solid surface by
core-electron excitation using monochromatic soft x-ray is a promising approach
to the control of surface chemical reactions due to the high intensity and energy
tunability of soft x-ray synchrotron radiation (SR) source. Understanding the
basic mechanisms responsible for the photochemical reactions of adsorbate on a
semiconductor surface has become a very important research work, and
development of advanced characterization techniques are crucial.
Photoelectron spectroscopy (PES) has been extensively employed as a probe to
investigate the electronic structure and chemical bonding of adsorbates. For high-intensity soft x rays,
especially produced by third-generation SR sources, and/or the molecules with high photolysis cross
sections the decay of the adsorbate concentration by beam damage due to PES itself is not negligible. As
a result, a dramatic change in a series of PES spectra, measured continually during irradiation of probe
light, will be observed. Monochromatic SR can therefore be used as a soft x-ray light source in the
photon-induced dissociation of adsorbed molecules and also as a probe for studying the chemical states
of the adsorbed molecules and produced surface species. The sequential PES spectra can be employed to
show the variation of the surface chemical bonding structure during irradiation of soft-x-ray photons. In
the present work, we name this method continuous-time PES. The advantage of using this technique for
such experiments in the photochemical processing is the possibility of “real-time”monitoring the
photodissociation of adsorbed molecules.
In order to gain insight into the monochromatic soft x-ray SR-excited microscopic reaction of adsorbed
fluorochlorocarbon molecules with well-characterized semiconductor surfaces, in the present study we
employed continuous-time PES to monitor the dissociation of adsorbed CF3Cl molecules at two photon
energies of 240 and 730 eV [near the Cl(2p) and F(1s) edges] and attempt to deduce the photolysis cross
section as a function of energy.
Results of the continuous-time Cl(2p) core-level PES measurements are shown in Fig. 1(a) through a
series of spectra that differ only by the amount of time the surface was exposed to 240 eV photons. The
first Cl(2p) PES spectrum (spectrum 1) indicates two resonances at -202.9 and -201.3 eV, while the last
Cl(2p) PES spectrum (spectrum 84) shows two resonances at -200.8 and -199.2 eV. These sequential Cl
(2p) PES spectra can be decomposed to two Cl(2p) components as shown in Fig. 1(b).
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Research Express@NCKU - Articles Digest
FIG. 1. (a) Series of Cl(2p) core-level PES spectra of CF3Cl adsorbed on Si(111)-7×7 at 30 K as a function
of photon exposure using 240 eV photons. (b) Curve-fitting results of the first, fifth, and last Cl(2p) corelevel PES spectrum in a series of 84 spectra. The dots indicate the raw data with secondary-electron
background subtracted. The solid line in spectrum 1 (F1) and that in spectrum 84 (F2) show the Cl(2p)
lines fitted by a single spin-orbital-splitting pairs of Cl(2p1/2) and Cl(2p3/2) from adsorbed CF3Cl
molecules and produced SiCl surface species, respectively. The thin solid lines in spectrum 5 indicate the
component of adsorbed CF3Cl molecules (F1) and that of produced SiCl surface species (F2), and the
thick solid line shows the sum of the components.
It is possible to obtain the photolysis cross section of the adsorbed CF3Cl from Fig. 1(a). Since the feature
with two peaks at -202.9 and -201.3 eV is attributed to the photoemission from the adsorbed CF3Cl
molecules via the excitation of Cl(2p) core level, the area of this feature should be proportional to the
concentration of CF3Cl molecules on the surface. The photolysis cross section of CF3Cl adsorbed on Si
(111)-7×7 irradiated by 240 eV photons is found to be ~1.3×10-17cm2.
Figure 2(a) shows a series of F(1s) core-level PES spectra of CF3Cl adsorbed on Si(111)-7×7 at 30 K for
various photon exposures using 730 eV photons. The first F(1s) PES spectrum (spectrum 1) indicates one
resonance at -688.5 eV. After inspection of the shapes of the sequential F(1s) PES spectra in Fig. 2(a), we
found that it is very difficult to identify the number of components required to do the curve fitting.
However, if we are only interested in the photodissociation of adsorbed CF3Cl molecules instead of the
formation of the new surface species, it will be very useful to employ the difference curves obtained by
subtracting the first F(1s) PES spectrum (spectrum 1) from the subsequent F(1s) PES spectra (spectrum
2 to 50). The obtained difference curves can be fitted using an algorithm that performed a nonlinear
least-square analysis on the function
y=-F1+F2+F3,
where Fi (i=1-3) is the Voigt function, and -F1 represents the function corresponding to the F(1s) PES
curve contributed by the decrease of adsorbed CF3Cl molecules and F2 and F3 are those from the created
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Research Express@NCKU - Articles Digest
surface species. A typical curve-fitting result is shown in Fig. 2(b).
It is possible to obtain the photolysis cross section from the variation of the component area of -F1 with
photon exposure. The photolysis cross section of CF3Cl adsorbed on Si(111)-7×7 irradiated by 730-eV
photons is found to be ~1.7×10-17 cm2.
FIG 2. (a) Series of F(1s) core-level PES spectra of CF3Cl adsorbed on Si(111)-7×7 at 30 K as a function of
photon exposure using 730 eV photons. (b) Typical curve-fitting result of a difference curve. Top: the
first and 31st PES spectrum. Middle: the difference curve obtained by subtracting the first PES spectrum
from the 31st PES spectrum. (The dots indicate the difference of the raw data of these two spectra. The
solid line indicates the total fit of curve-fitting result). Bottom: the solid lines indicate the component
peaks of curve-fitting result.
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