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I. Montero et al. / Surface Science 528 (2003) 42–46
KLL, C KLL and O KLL peaks, marked in the
figure. The spectra (b)–(g) of Fig. 1 show the
changes in the surface composition that resulted
from the above described cleaning process by Arþ
ion bombardment. For comparison purposes, the
spectra were normalized to the oxygen signal. As
expected, a gradual increase in titanium and silicon
signals is observed. For instance, in spectrum (g)
the carbon signal has nearly disappeared, although
the oxygen is still present on the surface. The
composition of the different regions of the sample
was determined using the relative peak-to-peak
heights of the signals and the corresponding sen-
sitivity factor for the primary energy beam, Ep ¼ 3
keV [5]. Thus, the initial carbon concentration of
the surface exposed to air was estimated in 60
atomic per cent in the depth probed by AES (1
nm). The carbon concentrations corresponding to
spectra (b)–(g) of Fig. 1 were 30%, 25%, 20%, 15%,
10%, and 1%, respectively.
In order to characterize the initial excitation,
and in consequence, the local bond of the adsor-
bates that produces the desorbing ion, we have
measured the yield of the detected Hþ, Oþ, and
OHþ ions as a function of the incident electron
energy. Fig. 2(a)–(c) shows the total Hþ, Oþ, and
OHþ ion yields for titanium silicide surfaces with
approximately 1%, 10%, and 30% carbon con-
centration, respectively. It is observed that all ESD
curves present a threshold at 39 eV. However, the
shape of the ESD curves depended strongly on the
surface carbon concentration. For the sample
surface with low carbon concentration, Fig. 2(a),
the ESD curves of Hþ showed a general monoto-
nous increase with the electron energy and a broad
peak located at about 150 eV. However, the
ion yield at high electron energies decreases as
carbon concentration increases ((b) and (c)). This
decrease was accompanied by the appearance of a
second peak at high energies, at about 320 eV (b).
Finally, for the surface region rich in carbon, an
enhancement of the peak located at low electron
energies, 150 eV, was observed. Therefore, the
carbon-contaminated surface seems to desorb less
hydrogen for high incident electron energies and
more for low energies than the clean surface. A
similar behaviour was observed for the OHþ ions.
This interesting behaviour can be understood in
Fig. 2. ESD yields of Hþ, Oþ, and OHþ ions as a function of
the incident electron energy, from titanium silicide surfaces: (a)
with low carbon, 1%, (b) with medium carbon, 10%, and (c)
with high carbon concentration, 30%.
terms of the hydrogen chemisorption sites present
prior to desorption.
With the aim to identify the different contribu-
tions in the ESD curves, the Hþ ion yield curves
(b) and (c) of Fig. 2, corresponding to surfaces
with intermediate and high carbon concentration,
respectively, have been subtracted after normali-
sation. We normalized the yield curves so that
their difference, always positive, was minimal in
the first 90 eV. We selected this value because de-
sorption from Ti atoms is the only contribution
expected below this energy, i.e., we intended to
normalize to this desorption contribution and, in
this way, suppress it in the difference. Fig. 3 shows
this difference curve which resembles an ESD
curve with a threshold at approximately 100 eV,
that corresponds to the Si 2p core level, and a
broad peak at around 320 eV. This result allows us
to explain the different shapes of the ESD curves of
Fig. 2(a)–(c) as the overlapping of the signals
corresponding to desorption from titanium and
Montero
