CHINESE JOURNAL OF PHYSICS
VOL. 38, NO. 5
OCTOBER 2000
Adsorption and Desorption Studies of CF3 Cl on Si(111)(7x7) Surface
L.-C. Chou, C.-R. Wen and J. Chen
Department of Physics, National Cheng Kung University,
Tainan, Taiwan 700, R.O.C.
(Received March 29, 2000)
We have investigated the interaction of CF3 Cl with the Si(111)(7£ 7) surface at 30 K
using ultraviolet photoelectron spectroscopy (UPS), x-ray photoelectron spectroscopy (XPS)
and electron-stimulated desorption (ESD). Examination of the valence-level UPS spectra and
the Si(2p), F(1s), C(1s) and Cl(2p) core-level XPS spectra, shows that CF3 Cl is adsorbed
molecularly on the Si(111)(7£ 7) surface. The peak at -6.14 eV in the UPS spectrum was
assigned to the ionization of 5e orbital, the peak at -10.9 eV was attributed to the ionization of
the 3e orbital, and the unresolved structure in the energy range -7.5 to -10.5 eV was ascribable
to the ionization of the 5a1 , 1a2 , and 4e orbitals. The broadening of the line widths of the
photoemission peaks emitted from CF3 Cl/Si(111) is explained by Heisenberg’s uncertainty
principle. The work function change (¢ Á) with increasing coverage of adsorbed molecules
was obtained and interpreted as due to the orientation of the adsorbed molecule which has
small permanent dipole moment and polarizability. The orbital energy shifts of the 5e and 3e
orbitals of CF3 Cl, as the molecule changes from the gas-phase state to an adsorbed state, are
found to be -1.07 and -0.94 eV, respectively. The ESD mass spectrum for CF3 Cl/Si(111)(7£ 7)
of dose = 1.3 £ 1015 molecules/cm2 in the range 10-71 amu at an incident electron energy
of 250 eV was measured; it shows that F+ , CF+ and CF+3 are the major desorbing ions,
C+ , CF+2 , CCl+ , CCl+ + and CFCl+ are the minor desorbing ions. The desorption of these
positive ions are assigned to various dissociation processes via ionizing the valence orbital.
PACS. 79.60.Dp – Adsorbed layers and thin film.
PACS. 79.20.La – Photon- and electron-stimulated desorption.
I. Introduction
Adsorption of molecules on the surface and electron/photon-induced desorption of ions from
the surface have attracted many research efforts [1-9]. The binding energy shifts of the valencelevel and core-level orbitals, as the molecule changes from the gas-phase state to the adsorbed
state, can be studied by using ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron
spectroscopy (XPS). The UPS can also be employed to obtain the work function change induced
by the adsorbed molecules. By using the electron-stimulated desorption (ESD) technique, the
desorption of ions from the surface induced by the incident electrons can be measured and analyzed.
In order to gain insight into the interaction of CF3 Cl molecules with well-characterized
surfaces, we have performed UPS, XPS and ESD studies of CF3 Cl adsorbed on Si(111)(7£ 7) at
a low temperature (30 K). The system of CF3 Cl adsorbed on Si(111) was chosen because CF3 Cl
is used in the semiconductor etching process [10-13] and it has abundant experimental data of its
interactions with electrons and photons in gas phase [14-18], and silicon is the most important
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°c 2000 THE PHYSICAL SOCIETY
OF THE REPUBLIC OF CHINA
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ADSORPTION AND DESORPTION STUDIES OF ¢¢¢
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semiconductor material. On the other hand, halogenated methane such as CF3 Cl is of special
interest because of its potential to destroy the stratospheric ozone layer.
II. Experiment
The experiments were performed in an ultrahigh vacuum system (base pressure < 2£ 10¡ 10
Torr) equipped with a differentially pumped He discharge lamp, a dual anode x-ray source and
a spherical sector electron energy analyzer (VG CLAM2) for ultraviolet and X-ray photoemission spectroscopy (UPS, XPS). The system is also equipped with an argon gun (VG AG5000)
for sputtering, an electron gun (Kimball Physics ELG-2) and a quadrupole mass spectrometer
(Balzers QMG421) for electron-stimulated desorption (ESD), and a LEED optics (VG RVL900).
A variable-temperature sample holder, connected to a closed-cycle refrigerated cryostat for cooling sample down to 30 K, and equipped with wires for passing current directly through sample
for heating it up to 1100 ± C, was used for the sample cleaning and for the CF3 Cl adsorption
measurements.
The Si (111) crystal (p-type, 9.6 - ¢ cm) surface was cleaned by cycles of argon ion
bombardment (800 eV) followed by direct resistive heating to 1100 ± C. The surface structure was
checked by LEED, and the cleanliness was checked by UPS and evidenced by the characteristic
surface states in the valence region. CF3 Cl ( ¸ 99.9 % ), was obtained from a commercial source
and used without further purification. Gas exposure was made by dosing the clean Si (111)(7£ 7)
surface with the CF3 Cl gas from a gas-dosing system. The gas-dosing system is made out of a
miniature cross reservoir, a leak valve and a stainless steel tubing with microchannel plate doser
head. It is equipped with a capacitance pressure gauge (MKS-Baratron). The gas flux from the
dosing system was calibrated by standard volumetric techniques.
During the dosing of gas and the UPS, XPS, and ESD measurements, the sample was kept
with a temperature of 30 K. The results presented in this paper are for exposures in the range
of submonolayer and multilayer. The UPS and XPS spectra were measured by a spherical sector
electron analyzer (VG CLAM 2). The low kinetic energy thresholds of the secondary electron
emission of He I UPS spectra were used to measure the surface work function change (¢ Á). One
monolayer (ML) coverage was defined as the exposure at the maximum work function change
(¢ Á), which is about the dose of 0:4 £ 1015 molecules/cm2 .
The incidence angle of the electron beam was 45± from the surface, and the desorbing
positive ions were detected by a Balzers pulse-counting quadrupole mass spectrometer (QMS,
Balzer model QMG421 with off-axis secondary electron multiplier) which was positioned normal
to the surface. The sample surface was located » 3 cm from the entrance of the QMS.
III. Results and discussion
III-1. Valence-level UPS and work function change
Results of the valence-level UPS measurements of CF3 Cl adsorbed on the Si(111)(7£ 7)
surface at 30 K are shown in Fig. 1 which shows a series of spectra for various gas exposures.
The gas exposure is shown on the right of each curve in units of 1015 molecules/cm2 . The bottom
spectrum is the valence band photoelectron spectrum taken from the clean Si(111)(7£ 7) surface
which shows the characteristic dangling bond surface-state emission at about -0.4 eV. After dosing
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L.-C. CHOU, C.-R. WEN AND J. CHEN
FIG. 1. Series of He I UPS spectra of CF3 Cl
adsorbed on Si(111)(7£ 7) at 30 K for
various dose (from clean surface to dose
= 2.2 £ 1015 molecules/cm2 ). The dose
for each spectrum is given in units of
1015 molecules/cm2 and shown on the
right of the figure.
FIG. 2.
989
Series of He I UPS spectra of CF3 Cl
adsorbed on Si(111)(7£ 7) at 30 K for
various dose (from dose = 2.2 £ 1015
molecules/cm2 to dose = 5.9 £ 1015
molecules/cm2 ). The dose for each
spectrum is given in units of 1015
molecules/cm2 and shown on the right
of the figure. The He I UPS spectrum
and orbital assignments (shifted to align
the 5e orbital energy with the peak at 6.14 eV) of gas-phase CF3 Cl are given
at the bottom [19].
the clean silicon surface with CF3 Cl of different coverage a series of UPS spectra were obtained,
which show two main peaks at -6.14 and -10.9 eV, and one broad structure in the energy range -7.5
to -10.5 eV. Fig. 2 shows a series of valence-level UPS spectra, which extend the series of Fig. 1.
In order to identify these UPS spectra, we compare the spectrum taken from the clean surface
with the spectrum observed from the CF3 Cl-dosed surface (dose = 0.4 £ 1015 molecules/cm2 ).
Fig. 3(a) and 3(b) show the UPS spectra for the clean silicon surface and CF3 Cl-dosed surface,
respectively. Fig. 3(c) indicates the difference curve (Fig. 3(b) - Fig. 3(a)), which shows the
change due to the coverage of CF3 Cl. To identify the adsorbate-induced photoemission structure,
we compare this difference curve with the gas-phase spectrum. Fig. 3(d) shows the gas-phase
CF3 Cl UPS spectrum obtained by Cvitas [19], which is shifted by 6.94 eV so that the valence
orbital 5e coincides with that of the difference curve. The outmost five valence orbitals of gasphase CF3 Cl are 5e (-13.08 eV), 5a1 (-15.2 eV), 1a2 (-15.8 eV), 4e (-16.72 eV) and 3e (-17.71
eV) [20]. These orbitals are marked at the bottom of the figure.
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ADSORPTION AND DESORPTION STUDIES OF ¢¢¢
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FIG. 3. (a) the He I UPS spectrum of clean Si(111)(7£ 7) surface; (b) the He I UPS spectrum of
CF3 Cl/Si(111) for the dose of 0.4 £ 1015 molecules/cm2 ; (c) the difference curve obtained
by subtracting Fig. 3(a) from Fig. 3(b). (d) CF3 Cl gas-phase UPS spectrum [19] .The orbital
assignments of gas-phase CF3 Cl are marked at the bottom of the gas-phase curve.
Comparing our difference curve with the gas-phase spectrum obtained by Cvitas, we observed that the peak widths of the adsorbed CF3 Cl are much larger than those of the gas-phase
CF3 Cl. Based on the Heisenberg’s uncertainty principle (¢ E ¢¢ t > h), the line width of a
photoemission peak is related to the lifetime of the hole state in the ion. Thus, the shorter lifetimes imply the larger line widths. Because the hole in the adsorbed molecule, produced by the
photoionization process, may be filled by electron from the substrate, the lifetime of the ionic
state of the adsorbed molecule is smaller than that of the gaseous molecule. It is expected that the
natural line width of a photoemission peak is larger for adsorbed CF3 Cl than for gaseous CF3 Cl.
As a result, the line widths of peaks 5e and 3e for the adsorbate are larger than those in gas-phase
as shown in Fig. 3(c) and 3(d), respectively. The broadening of the peak widths of 5a1 , 1a2 and
4e results in the umesolved structure in the energy range -7.5 to -10.5 eV. Although 5a1 , 1a2 and
4e in the energy range -7.5 to -10.5 eV can not be resolved in our spectra, the coincidence of
the energy positions and spectral shape of 5e and 3e of the adsorbed and gas-phase molecules is
characteristic of physisorbed, molecular CF3 Cl on the surface.
The work function change (¢ Á) with increasing coverage of adsorbed molecules is shown
in Fig. 4. Up to the dose of 0.4 £ 1015 molecules/cm2 , ¢ Á decreases abruptly to - 0.2 eV, and
then increases slightly with increasing coverage. Above the dose of 1.0 £ 1015 molecules/cm2 ,
¢ Á stays constant and independent of coverage for up to 3.0 £ 1015 molecules/cm2 . The variation
of work function with increasing coverage can be explained by the orientation of the adsorbate.
Scheme 1(a) shows that the CF3 Cl is aligned perpendicular to the surface with the Cl pointing
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L.-C. CHOU, C.-R. WEN AND J. CHEN
991
FIG. 4. Work function change, determined from UPS thresholds, as a function of CF3 Cl coverage.
Scheme 1. Possible arrangements of adsorbed CF3 Cl on Si(111) surface.
towards the silicon surface. Scheme 1(b) shows that the CF3 group is inclined to the surface
with two fluorines close to the silicon surface. Scheme 1(c) shows that the CF3 Cl is aligned
perpendicular to the surface with the Cl pointing towards the vacuum.
In a recent photon-stimulated desorption (PSD) studies of negative ions from submonolayer
of CF3 Cl on Si(111)(7£ 7) surface using synchrotron radiation in the photon energy range 12-35
eV [21], we observed a strong signal of F¡ ions desorbing in the direction normal to the surface
and a weak F¡ signal at 45± off the surface normal. The observation of strong F¡ signal in the
direction normal to the surface indicates that most of the adsorbed CF3 Cl molecules are oriented
with the F pointed toward vacuum (the CF3 Cl adsorbs via chlorine and the CF3 group is inclined
to the surface with two fluorines close to the surface as shown in Scheme 1(b)). On the other
hand, the observation of weak F¡ signal at 45± off the surface normal depicts that small part of
the adsorbed CF3 Cl molecules have their molecular axis aligned perpendicular to surface with the
Cl towards the surface (Scheme 1(a)).
To explain why CF3 Cl induces small work function change (» -0.2 eV from clean surface to
one monolayer coverage), the permanent dipole moment (0.5 D) [22] and the polarizability (5.72
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ADSORPTION AND DESORPTION STUDIES OF ¢¢¢
VOL. 38
£ 10¡ 24 cm3 ) [23], both of which are small, should be considered. For the case of molecular axis
aligned perpendicular to surface with the Cl towards the surface (Scheme 1(a)), the polarizability
of the Cl lone pair will provide an attractive interaction orienting the CF3 Cl with the Cl towards
the substrate, the induced dipole created by the polarizability of the Cl lone pair, which interacts
with the substrate, is perpendicular to the surface with its dipole moment pointing toward vacuum.
The permanent dipole (negative end points in the direction of the fluorines) will tend to compensate
the induced dipole because its negative end is outward. As a result, the work function change due
to this molecular orientation is minor if any. However, for the case of CF3 Cl molecules oriented
with the F pointed toward vacuum (Scheme 1 (b)), the induced dipole created by the polarizability
of the Cl lone pair is perpendicular to the surface with its dipole moment pointing toward vacuum.
The permanent dipole (positive end points in the direction of the chlorine) will be inclined to the
surface. Since the induced dipole is perpendicular to the surface with its dipole moment pointing
towards vacuum, and the permanent dipole is inclined to the surface with its dipole moment
pointing in the direction of the chlorine, the work function change due to the induced dipole will
be larger than that of the permanent dipole. It is generally believed that for an adsorbate with its
dipole moment pointing towards vacuum, the surface work function will decrease. Therefore, the
work function change will be small and negative as observed in our data. The slight increase of
the work function above monolayer coverage (shown in Fig. 4) can be attributed to a less ordered
arrangement of the permanent dipoles in the second layer.
Now, we consider the binding energy shift of the molecular orbitals as CF3 Cl molecule
changes from gas-phase state to adsorbed state. The connection between the ionization energies
of the adsorbed CF3 Cl molecules (Ead ) and those of the gaseous CF3 Cl molecules (Egas ) is
reasonably approximated using [24]:
Ead = Egas ¡ (Á + ¢ Á) + Erelax + Ebond
shif t
(1)
where Á is the work function of clean Si(111)(7£ 7), ¢ Á is the work function change introduced
by the adsorbed CF3 Cl, Ebond shif t is the orbital energy shift due to the interaction of adsorbed
CF3 Cl molecule with Si surface, and Erelax is the extra-molecular relaxation energy which is due
to the interaction of the hole in the adsorbed CF3 Cl molecule, left after photoionization, with its
image in the Si substrate. Erelax is often referred to as the surface image potential which equals
(e2 =4¼ "0 )(1=4s)(1 ¡ k)=(1 + k) [25], where k, the dielectric constant of Si, is 11.9 [26] and
s is the distance (Å) of the external charge from the surface plane. The work function of clean
Si(111)(7£ 7) is 4.55 eV [27]. For one monolayer CF3 Cl, the work function change (¢ Á) equals
-0.2 eV.
Based on the measured ionization energies of adsorbed CF3 Cl molecules (Ead ), those of
gaseous CF3 Cl molecules (Egas ), the estimated Erelax , and Equation (1), we can calculate the
orbital energy shift (Ebond shif t ) for each molecular orbital. For ionization of the 5e orbital of
the adsorbed CF3 Cl, which contains the chlorine lone pair character, s is assumed to be » 2Å
(roughly the Cl-Si bond length) [28]. The Erelax of 5e orbital can be estimated to be » -1.52 eV.
By Equation (1), the Ebond shif t of 5e orbital is calculated to be » -1.07eV.
For the ionization of 3e orbital of the adsorbed CF3 Cl, which is mainly of F character, s
is assumed to be » 2Å (roughly the average distance from F atom to Si surface). The Erelax
of 3e orbital is estimated to be » -1.52 eV. Thereby, we obtain Ebond shif t of 3e orbital to be
» -0.94 eV. In fact, these small Ebond shif t of 5e and 3e orbitals are expected for the orbitals
VOL. 38
L.-C. CHOU, C.-R. WEN AND J. CHEN
993
of physisorbed molecules. Because the orbitals of 5a1 , 1a2 and 4e are not resolved in our UPS
spectra, the Ebond shif t shift of these orbitals can not be obtained from the present data.
III-2. Core-level XPS
A sequence of Si(2p) core-level XPS spectra as a function of gas exposure at 30 K is
shown in Fig. 5. These spectra were taken from the same surfaces, from which the valence-level
UPS spectra (Fig. 1 and Fig. 2) were obtained. The bottom spectrum shows the Si(2p) corelevel spectrum taken from the clean Si(111)(7£ 7) surface. After dosing the clean silicon surface
with CF3 Cl of higher coverage the series of Si(2p) XPS spectra were obtained. In this series, we
observed a decrease of the Si(2p) signal with increasing coverage. This decrease is ascribable to the
attenuation of the photoemitted Si(2p) electrons by the adsorbate layers. The emission probability
of the Si(2p) electrons from CF3 Cl/Si(111) will decrease with increasing coverage, because the
substrate electrons will have larger probability of collision with the adsorbed molecules during
their transport to the outmost layer of the CF3 Cl film for higher thickness. Thus, the Si(2p) signal
will be large for the clean surface and expected to decrease with increasing adsorbate coverage.
Now, we turn to the core-level XPS spectra obtained from the adsorbed CF3 Cl. The series
of F(1s), C(1s) and Cl(2p) core-level XPS spectra as a function of CF3 Cl dose (up to 5.9 £ 1015
molecules/cm2 ) at 30 K are shown in Fig. 6, Fig. 7 and Fig. 8, respectively. These spectra were
taken from the same surfaces, from which the valence-level UPS spectra (Fig. 1 and Fig. 2) were
obtained. The increase of these core-level peaks with coverage indicates the increasing adsorption
of CF3 Cl on the Si surface with increasing dose.
FIG. 5. Series of Si(2p) core-level XPS spectra of CF3 Cl adsorbed on Si(111)(7£ 7) at 30 K for various
dose (from clean surface to dose = 5.9 £ 1015 molecules/cm2 ). The dose for each spectrum is
given in units of 1015 molecules/cm2 and shown on the right of the figure.
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ADSORPTION AND DESORPTION STUDIES OF ¢¢¢
FIG. 6. Series of F(1s) core-level XPS spectra of
CF3 Cl adsorbed on Si(111)(7£ 7) at 30
K for various dose (from clean surface
to dose = 5.9 £ 1015 molecules/cm2 ).
The dose for each spectrum is given in
units of 1015 molecules/cm2 and shown
on the right of the figure.
VOL. 38
FIG. 7. Series of C(1s) core-level XPS spectra of
CF3 Cl adsorbed on Si(111)(7£ 7) at 30
K for various dose (from clean surface
to dose = 5.9 £ 1015 molecules/cm2 ).
The dose for each spectrum is given in
units of 1015 molecules/cm2 and shown
on the right of the figure.
At the lowest-dosed surface of 0.4 £ 1015 molecules/cm2 (» 1 ML) shown in Fig. 6, the
F(1s) peak is situated at -688.5 eV (referenced to the Fermi level, and indicated by an upward
arrow). However, in the XPS measurement of gaseous CF3 Cl [29], the F(1s) peak is at -695.04 eV
(referenced to the vacuum level). In order to compare the result of the adsorbed CF3 Cl molecules
with that of the gaseous molecules, we have to take the difference in reference levels into account
and refer both binding energies to the vacuum level. The difference in these two reference levels
corresponds to the work function of CF3 Cl/Si(111) which equals to the sum of the work function
of clean Si(111)(7£ 7) (4.55 eV) and the work function shift (-0.2 eV). We thus obtained the
binding energy of the F(1s) peak, referenced to the vacuum level, for the adsorbed molecule to
be -692.85 eV. As a result, the shift of F(1s) core-level binding energy from gaseous CF3 Cl to
adsorbed CF3 Cl, both referenced to the vacuum level, is about 2.2 eV. It is well known that for a
physisorbed system, i.e. the bonding is mainly due to van der Wall’s forces, the final ionic state
is a screened state in some sense that the screening charge has piled up as a polarization charge in
the substrate, and leads to lower the bind energy of the emitting electron [30, 31]. On the other
hand, for the lowest-dosed surface of 0.4 £ 1015 molecules/cm2 which has about one monolayer
CF3 Cl coverage, the polarization effect due to electronic polarization of the surrounding molecules
is very small as compared to that in the case of multilayer coverage, it is possible to ignore the
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L.-C. CHOU, C.-R. WEN AND J. CHEN
995
FIG. 8. Series of Cl(2p) core-level XPS spectra of CF3 Cl adsorbed on Si(111)(7£ 7) at 30 K for various
dose (from clean surface to dose = 5.9 £ 1015 molecules/cm2 ). The dose for each spectrum is
given in units of 1015 molecules/cm2 and shown on the right of the figure.
polarization effects due to electronic polarization of the surrounding molecules. Thus, the decrease
in the binding energy of F(1s) (from -695.04 eV for gas-phase CF3 Cl to -692.85 eV for adsorbed
CF3 Cl) is ascribable to the effect of charge screening of the final-state core hole.
Similar energy shifts were observed for the C(1s) and Cl(2p) peaks in the core-level XPS
spectra (Figs. 7 and 8). At the low coverage (dose = 1.0 £ 1015 molecules/cm2 ), the C(1s) peak is
at about -294.0 eV, indicated by an upward arrow in Fig. 7. For the gaseous CF3 Cl, the C(1s) peak
is at -300.3 eV [29]. If we refer both binding energies for gaseous CF3 Cl and adsorbed CF3 Cl to
the vacuum level, the comparison of the C(1s) binding energy of gaseous CF3 Cl (-300.3 eV) and
that of adsorbed CF3 Cl (-298.35 eV) shows that the core-level binding energy shift is about 1.95
eV. This 1.95 eV energy shift for C(1s) is also attributed to the same effect of charge screening
of the final-state core hole as described above. For Cl(2p3=2 ) peak in the low coverage surface
(dose = 1.0 £ 1015 molecules/cm2 ), the peak is situated at -201.4 eV (shown by an upward arrow
in Fig. 8). However, the Cl(2p3=2 ) peak in the gaseous CF3 Cl has the peak position at -207.8
eV [32]. If we relate the binding energy of the adsorbed CF3 Cl to the vacuum level, it will be
-205.75 eV. Comparing the binding energy of the adsorbed CF3 Cl with that of gaseous CF3 Cl, we
find that the binding energy shift is 2.05 eV. This binding energy shift is also due to the effect of
charge screening.
In passing from low to high coverage, the shifts in the binding energy, from -688.5 eV to
-688.3 eV for F(1s) (Fig. 6), from -294.0 eV to -293.7 eV for C(1s) (Fig. 7), and from -201.4 eV
to -201.2 eV for Cl(2p3=2 ) (Fig. 8) were observed. These binding energy shifts can be explained
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ADSORPTION AND DESORPTION STUDIES OF ¢¢¢
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FIG. 9. Mass spectrum of positive ions desorbed from CF3 Cl/Si(111) (dose = 1.3 £ 1015 molecules/cm2 )
obtained by irradiation of the surface with 250 eV electrons.
as follows. Although, with increasing coverage, the kinetic energy of the ionized electron is
lowered by the fact that the screening effect is decreased, it will be increased by the larger
electronic polarization of the surrounding molecules because of the multilayer coverage. Therefore,
we propose that the lower core-level binding energies, observed in the multilayer, result from the
increasing effect of electronic polarization of the surrounding molecules, which is stronger than
the decreasing screening effect by the substrate.
III-3. Electron-stimulated desorption (ESD)
The electron-stimulated desorption (ESD) technique was employed to study the positive ion
desorption yields. Fig. 9 shows the ESD mass spectrum for CF3 Cl/Si(111)(7£ 7) of dose = 1.3 £
1015 molecules/cm2 (» 3ML) in the range 10-71 amu obtained normal to the surface and at an
incident electron energy of 250 eV. This spectrum shows that F+ , CF+ and CF+
3 are the major
+ , CCl++ and CFCl+ are the minor desorbing ions.
desorbing ions; C+ , CF+
,
CC1
2
In a gas-phase CF3 Cl dissociative photoionization experiment in the photon energy range
+
+
+
+
7.5-80 eV, Zhang et al. [33] observed F+ , CF+ , CF+
3 , C , CF2 , CCl and CFCl ions, and
proposed that these positive ions are produced by the following dissociation processes via ionizing
one of the valence orbitals:
CF3 C1+ ! F+ + C + F2 + Cl (via ionization of 1e orbital),
CF3 C1+ ! CF+ + 2F + Cl (via ionization of 3a1 orbital),
CF3 C1+ ! CF+
3 + Cl (via ionization of 5e orbital),
CF3 C1+ ! C+ + 3F + Cl (via ionization of 1e orbital),
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L.-C. CHOU, C.-R. WEN AND J. CHEN
997
CF3 C1+ ! CF+
2 + F + Cl (via ionization of 3e orbital),
CF3 C1+ ! CCl+ + ?? (via ionization of 2a1 orbital),
CF3 C1+ ! CFCl+ + ?? (via ionization of 5a1 orbital),
Although in the condensed, or physisorbed state these processes can be affected by the particular
molecular environment, it is generally believed that the dissociation processes are similar in both
the condensed/physisorbed state and the gas-phase state. On the other hand, it is well known that
although ESD uses electrons as the incident source while PSD uses photons, ESD and PSD have
similar mechanisms for ion desorption. Therefore, we proposed that the desorption of positive ions
observed in the ESD from CF3 Cl/Si(111)(7£ 7) surface can be attributed to the same processes as
in the gas phase.
We have also measured the ESD mass spectrum from CF3 Cl/Si(111)(7£ 7) of dose = 0.4
£ 1015 molecules/cm2 (» 1 ML) in the range 10-71 amu at an incident electron energy of 250
eV (not shown). The spectrum indicates that F+ is the only desorbing ion which is different from
what we observed for the multilayer coverage surface (shown in Fig. 9). Since the only difference
between these two surfaces is different CF3 Cl coverages, we suppose that the aforementioned
excitations of adsorbed CF3 Cl molecules, except the excitation for the dissociation of F+ ion,
were quenched by the substrate for the one monolayer coverage surface. However, the excitations
of CF3 Cl molecules in the outmost layer for the surface of dose = 1.3 £ 1015 molecules/cm2 ,
which has multilayer coverage, is less quenched by the substrate.
IV. Conclusions
In summary, the adsorption and electron-induced desorption of CF3 Cl molecules on Si
(111)(7£ 7) surface has been studied by ultraviolet photoelectron spectroscopy (UPS), x-ray photoelectron spectroscopy (XPS) and electron-stimulated desorption (ESD). The UPS and XPS spectra
suggest that CF3 Cl molecules adsorbed molecularly on the Si (111)(7£ 7) surface.
By the analogy with the gas-phase results, we assign the peak of the adsorbate spectrum at
-6.14 eV as arising from the ionization of 5e orbital, the peak at -10.9 eV as due to the ionization
of 3e orbital, and the unresolved structure in the energy range -7.5 to -10.5 eV as arising from the
ionization of 5a1 , 1a2 , and 4e orbitals.
The broadening of line widths of the photoemission peaks emitting from CF3 Cl/Si(111) is
explained by using Heisenberg’s uncertainty principle. The work function change (¢ Á) with increasing coverage of adsorbed molecules is attributed to the orientation of the adsorbed molecules,
which has small permanent dipole moment (0.5 D). The orbital energy shifts of 5e and 3e orbitals
as CF3 Cl molecule changes from gas-phase state to adsorbed state are obtained to be -1.07 and
-0.94 eV, respectively.
The ESD mass spectrum for CF3 Cl/Si(111)(7£ 7) of dose = 1.3 £ 1015 molecules/cm2 in
the range 10-71 amu at an incident electron energy of 250 eV was measured. This spectrum shows
+
+
+
++ and CFC1+ are the
that F+ , CF+ and CF+
3 are the major desorbing ions; C , CF2 , CCl , CCl
minor desorbing ions. The desorption of these positive ions are assigned to various dissociation
processes via ionizing the valence orbitals.
Acknowledgments
This work was supported by the National Science Council, Taiwan, R.O.C. under contract
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NSC88-2112-M-006-003.
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