Volume 5 Issue 5 - August 15, 2008
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: crwen@mail.ncku.edu.tw

Appl. Phys. Lett. 91, 144103 (2007)

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Site-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).
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) core-level 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
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 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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