Chemistry Reference
In-Depth Information
Fig. 5.27
Core levels photoemission of Si2p (
a
,
c
) and C1s (
b
). Photon energies used are 215 eV
(
a
) and 400 eV (
b
,
c
). Experimental data are shown as dots, the fit curves as black lines and the
single deconvolved components as colour lines
Regarding the Si2p core level analysis, the Si
B
peak at 102.1 eV falls into the
typical BEs region of silicon oxicarbides (Önneby and Vac
1997
; Sorarù et al.
1996
),
while the Si
C
peak at 103.0 eV is related to silicon oxides. In order to better identify
the in depth distribution of the different chemical species, we analyzed the Si2p
core level with a larger in-depth sensitivity of about 3-4 nm using a 400 eV photon
(Fig.
5.27
c). With respect to Fig.
5.27
a, features showing the same BE but different
relative intensities are present (the higher width is due to the different photon used).
As a rule of thumb, by increasing the in-depth sensitivity the more external layers are
depleted while the more internal ones are enhanced. In our case, we are dealing with
a SiC core surrounded by a silicon oxicarbides structure and, finally, and external
very thin SiO
x
layer. This also suggests that the chemical treatments have effectively
etched only the more external shell, leaving a residual thin layer with a thickness
≤
2 nm thick silicon oxicarbide layer.
Regarding stoichiometry, the Si/C intensity ratio for the SiC components is
0.85
0.5 nm on a
≤
±
0.05, with a slight carbon excess, while the overall Si/C intensity ratio is
2.23
0.30, with a net silicon excess. This figures out an increasing silicon concen-
tration gradient going from the NW core towards the surface, as well as a higher
carbon stoichiometry located around the oxicarbides/SiC interface.
Both the C
B
/C
C
peaks at 284.7eV/286.2eV can be ascribed to C1s emission in
oxicarbides, typically located in a BE range of 285-289eV. However, the C
B
compo-
nent larger width suggests also the presence of sp
2
-sp
3
carbons aggregates, clusters,
typically showing a C1s emission around 285eV. The wet chemical treatment did not
remove this peak, evidencing that the carbon clusters are not surface contaminants.
These carbon aggregates are responsible of the observed carbon excess, previously
indeed located at the interface between the oxicarbide and the SiC layers.
±
