Abstract In order to achieve thin amorphous silicon carbide layers a two-stage process was applied. The deposition of thin layers from liquid chlorovinylsilanes was carried out under argon flow using a spin-coating-system. Afterwards, the samples were pyrolysed in a temperature range between 800 • C and 1200 • C with different hydrogen concentrations in the atmosphere. Additionally, bulk material was pyrolysed in order to characterise structural changes by transition oligomer to a-SiC:H. In this work we present studies on the structure of the layers and of bulk material, which were carried out by XRD, MAS NMR and Raman spectroscopy, depending on pyrolysis conditions. Following results were obtained: Both, silicon carbide layers and bulk material, pyrolysed at 800 • C, were amorphous. Increase of the temperature to 1200 • C leads to a partial amorphous-to-crystalline transition forming ␤-SiC. Moreover, derivations from stoichiometric SiC were observed: Free silicon was found in thin layers, whereas crystallites of graphite were detected in the bulk material. The amount of excess carbon can be influenced by addition of hydrogen to the pyrolysis atmosphere

A Hilbig

E Brendler

E Müller

G Irmer

G Roewer

G Schreiber

R Wenzel

CiteSeerX

Journal of the European Ceramic Society 25 (2005) 151–156
The microstructure of polymer-derived amorphous silicon carbide layers
A. Hilbiga,∗, E. Müllera, R. Wenzelb, G. Roewerb, E. Brendlerc, G. Irmerd, G. Schreibere
a Institute of Ceramic Materials, Gustav-Zeuner-Str. 3, Freiberg University of Mining and Technology, D-09596 Freiberg, Germany
b Institute of Inorganic Chemistry, Leipziger Str. 29, Freiberg University of Mining and Technology, D-09596 Freiberg, Germany
c Institute of Analytical Chemistry, Leipziger Str. 29, Freiberg University of Mining and Technology, D-09596 Freiberg, Germany
d Institute of Theoretical Physics, Bernhard-von-Cotta-Str. 4, Freiberg University of Mining and Technology, D-09596 Freiberg, Germany
e Institute of Physical Metallurgy, Gustav-Zeuner-Str. 5, Freiberg University of Mining and Technology, D-09596 Freiberg, Germany
Available online 11 September 2004
Abstract
In order to achieve thin amorphous silicon carbide layers a two-stage process was applied. The deposition of thin layers from liquid
chlorovinylsilanes was carried out under argon flow using a spin-coating-system. Afterwards, the samples were pyrolysed in a temperature
range between 800◦C and 1200◦C with different hydrogen concentrations in the atmosphere. Additionally, bulk material was pyrolysed in
order to characterise structural changes by transition oligomer to a-SiC:H.
d Raman
s pyrolysed
a
M hite were
d re.
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d
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t
i
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onds
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ass-
ide
ting-
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ition
.
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NMR
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In this work we present studies on the structure of the layers and of bulk material, which were carried out by XRD, MAS NMR an
pectroscopy, depending on pyrolysis conditions. Following results were obtained: Both, silicon carbide layers and bulk material,
t 800◦C, were amorphous. Increase of the temperature to 1200◦C leads to a partial amorphous-to-crystalline transition forming-SiC.
oreover, derivations from stoichiometric SiC were observed: Free silicon was found in thin layers, whereas crystallites of grap
etected in the bulk material. The amount of excess carbon can be influenced by addition of hydrogen to the pyrolysis atmosphe
2004 Elsevier Ltd. All rights reserved.
eywords:Silicon carbide; Amorphous; Polymeric route
. Introduction
Silicon carbide (SiC), due to its excellent properties such
s very high hardness, high abrasive wear resistance and very
igh thermal conductivity, has been successfully used in quite
ifferent forms in many industrial applications.1 In case of
iC layers, two functional aims are to distinguish: very thin
ayers for electronic devices and wear or oxidation resistant
oatings for constructive applications. The most popular way
o obtain such layers – the deposition from the gas phase –
s relatively expensive. Therefore, we obtained silicon car-
ide from a liquid phase by the conversion of organosilicon
ompounds into ceramic materials. The polymeric route is
heaper than CVD or PVD processes and also the deposition
ystem is uncomplicated and its usage very simple. A par-
icular reason for trying such an approach is as follows: For
pplications of SiC layers in the field of opto-electronics the
∗ Corresponding author. Tel.: +49 3731 392388; fax: +49 3731 393662.
E-mail address:hilbig@ikw.tu-freiberg.de (A. Hilbig).
amorphous state of silicon carbide is of special interest.2 This
state can be stabilised by hydrogen occupying dangling b
within the amorphous network of silicon carbide. During
transformation of a preceramic oligomer into polycrystal
SiC ceramics during thermal treatment, the material is p
ing through such an amorphous state.3
In our work thin amorphous hydrogenated silicon carb
layers were obtained from oligomers using a spin-coa
system and then pyrolysed to amorphous and partially
talline structures. The starting precursors were derive
a catalytic heterogeneous disproportionation reaction.4 The
polymerisation process and optimisation of the depos
and pyrolysis processes are described in detail in Refs5,6.
Additionally, we pyrolysed bulk material in order to comp
the results with these of the layers and also to apply in
tigation methods, which are not suitable for thin layers.
aim of this paper is a characterisation of the microstructu
such amorphous or semi-crystalline layers using variou
perimental techniques: Raman spectroscopy, solid state
and XRD.955-2219/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.
oi:10.1016/j.jeurceramsoc.2004.07.011
152 A. Hilbig et al. / Journal of the European Ceramic Society 25 (2005) 151–156
2. Experimental
The layers were deposited from the liquid chlorovinylsi-
lanes using a spin-coating-system, B.L.E. Laboratory Equip-
ment GmbH. The deposition was carried out under inert gas
conditions on different substrates like silicon wafers or sil-
ica. In order to obtain homogeneous layers with a thick-
ness smaller than 1m, the oligomers were dissolved in
CH2Cl2 (dichloromethane). After deposition, some layers
were heated at 800◦C under argon flow (presented in this
article are samples 38, 61 and 62) or in a gas mixture con-
taining 10% hydrogen in argon (samples 78, 81, 87 and 102)
by heating rates of 1–2◦C/min. Layers 61 and 87 were also
annealed in 10% hydrogen in argon at 1200◦C to investigate
the beginning of crystallisation. Heating rates of 3◦C/min
were applied in this case. Bulk material was pyrolysed under
the same conditions as the layers and annealed under different
hydrogen concentrations. The optimisation of the deposition
process and pyrolysis conditions are described in Ref.5.
The XRD measurements were carried out using a diffrac-
tometer TUR-M-62 HZG 4. Due to small thickness of the lay-
ers, analyses were carried out under grazing incident (about
1◦). For Raman measurements a spectrometer LABRAM 800
HR [Laser: Nd:YAG, doubled frequency (λ = 532 nm)] was
used. The MAS-NMR-measurements were carried out at a
Bruker MSL 300 at a spinning speed of 5 kHz and resonance
f
3
as
c -
btained
istic peaks of polycrystalline silicon carbide are not observ-
able in this case. After pyrolysis of the bulk material at the
same temperature (Fig. 2a) we also did not find any indi-
cation for a real crystallisation process, but a smeared in-
tensity “hump” is already observable in the 2 region be-
tween 30◦ and 36◦, where the strongest peak of-SiC (1 1 1)
will occur after crystallisation (2θ = 35.6◦). If we interpret
this hump as a pre-form of that reflex we are able to use
the Scherrer equation for estimating the dimension of the
pre-formed coherent – but still disturbed – structure units
from the “broadening” of this “peak”. This would give a
value of about 0.9 nm. This is about 20% larger than the
diameter of one cubic unit cell of SiC. Such a unit cell
contains four [SiC4] tetrahedra connected by sharing cor-
ners. Thus, we can get a rough idea of the cluster sizes
of tetrahedra, which represent already structure units simi-
lar to the final crystal structure within the amorphous net-
work.
The Raman spectra of the layers after pyrolysis at 800◦C
in pure argon show the existence of an excess carbon with
a graphite-like structure in our materials, which was indi-
cated by the presence of the G- and D-peaks (Fig. 3a). A
band at 1598 cm−1 is assigned to ordered graphite structure
(G-peak), while the band at 1345 cm−1 – mostly called “dis-
ordered carbon” – (D-peak), originated from breathing vi-
b ce of
t layer
1 ture
h n re-
s mall
g man
s
requencies of 75.47 MHz (13C) and 56.627 MHz (29Si).
. Results and discussion
Layers obtained at 800◦C were amorphous, which w
onfirmed by XRD measurements (seeFig. 1a). Character
Fig. 1. Diffraction pattern of two SiC:H layers: (a) o after a treatment at 800◦C; (b) after pyrolysis at 1200◦C.
rations of aromatic rings in carbon clusters. The absen
hese bands after pyrolysis in 10% hydrogen in argon (
02, seeFig. 3b) indicates that already at this tempera
ydrogen is able to reduce the graphitic excess carbo
ulting from the decomposition of the precursors (very s
raphite cluster sizes below 2 nm are not visible in Ra
pectra).
A. Hilbig et al. / Journal of the European Ceramic Society 25 (2005) 151–156 153
Fig. 2. Diffraction pattern of bulk material after pyrolysis: (a) at 800◦C in a gas mixture containing hydrogen; (b) at 1200◦C in the same atmosphere; (c) at
1200◦C in pure argon.
The existence of an amount of excess carbon was also
found in the13C single pulse- and cross-polarisation-MAS-
NMR-spectra of the bulk material.Fig. 4shows two spectra of
the precursor after pyrolysis at 800◦C under argon with 10%
hydrogen. Several carbon environments can be observed. The
signal at about 17 ppm can be assigned to carbon surrounded
d at 80◦C
by silicon. The signals between 100 and 150 ppm are related
to aromatic carbon structures. The remaining hydrogen is lo-
calised in the CSi4 environments and in the aromatic region
giving rise to the signal at 135 ppm. This signal can be due to
aromatic graphite-like structures with remaining hydrogen or
connected with CH bonds. In this case they should correlateFig. 3. Raman spectra of two SiC:H layers pyrolyse 0: (a) in argon; (b) in a gas mixture containing hydrogen.
154 A. Hilbig et al. / Journal of the European Ceramic Society 25 (2005) 151–156
Fig. 4. 13C CP/MAS- and13C single pulse MAS spectra of the oligomer
after pyrolysis at 800◦C in argon + 10% hydrogen.
with the D-peak of the Raman spectra. However, the domi-
nating carbon signal at 105 ppm represents a hydrogen free
structure and is assigned to free carbon. It correlates clearly
to the graphite structure (G-peak) already observed in the
Raman spectra.
The MAS NMR spectra of the bulk material after pyrolysis
are strongly different from those of the precursors. InFig. 5,
the13C CP/MAS NMR spectrum of the starting oligomer is
shown. In the spectral region from−10 to 10 ppm methyl-
and CH2- or CH-groups connected to silicon were found.
The signal around 30 ppm is related to CH2/CH-groups, not
connected to silicon, which are formed during the reaction
of vinyl groups with each other. The low intensity signal
around 135 ppm is assigned to residual double bonds origi-
nating from the reaction of the vinyl groups.
In the 29Si CP/MAS NMR spectrum (Fig. 6) three main
peaks around 20 ppm are related to silicon atoms connected
to a methyl-group and two chlorine atoms or to methyl, one
chlorine and an alkyl/alkene bridge to another silicon atom.
The low intensity peak at about−40 ppm is typical for lin-
ear polysilane structures (MeSiSi2C) and that one around
−65 ppm for silicon atoms bonded to three other silicon
atoms (MeSiSi3). From the13C and29Si spectra, it can be
Fig. 6. 29Si CP/MAS NMR spectrum of starting precursor.
concluded that the oligomerisation process is dominated here
by the reaction of the vinyl groups with each other.
Pyrolysis at 1200◦C leads to a partial crystallisation of
the materials, which indicates the presence of the peaks at
2Θ1 = 35,7◦ and 2Θ2 = 60◦ assigned to the cubic polytype
3 C of silicon carbide (-SiC) (Fig. 1b andFig. 2b, c). The
reflexes are broadened due to still relatively small sizes of the
SiC crystallites (about 3 nm). Diffraction pattern of layer 87
after pyrolysis at 1200◦C shows also the presence of peaks
correlated with silicon. The strongest peak at 2Θ = 52.1◦
arises from the monocrystalline silicon substrate and is ex-
cited accidentally, under the geometric diffraction conditions
chosen here (W L radiation), due to W impurities in the
Cu anode of the X-ray tube, which pass the monochromator
after the sample [Si(3 1 1,λ = W L)]. The smaller Si peaks
arise from the Cu K radiation applied here and are due to
polycrystalline silicon which was formed beside SiC during
the pyrolysis within the layer (Fig. 1b).
The occurrence of polycrystalline silicon within the layers
exhibits a significant difference to the results obtained from
bulk material under the same pyrolysis conditions (Fig. 2).
Here we can detect free carbon (graphite) beside silicon car-
F is at
1Fig. 5.
13C CP/MAS NMR spectrum of starting oligomer.
ig. 7. 13C single pulse MAS spectrum of the oligomer after pyrolys
200◦C under argon flow.
A. Hilbig et al. / Journal of the European Ceramic Society 25 (2005) 151–156 155
Fig. 8. Raman spectrum of a SiC:H layer after pyrolysis at 1200◦C in 10% hydrogen in argon.
bide, but no indication of the existence of free silicon was
found. We have to discuss two aspects responsible for these
differences. On the one hand, we must consider possible dif-
ferences in the C/Si ratio developing during the pyrolysis of
layers and bulk. Due to the small thickness of the layers,
residual monomers as well as cleaved organic side groups
may escape more quickly from the layers structure than from
bulk material during heating. It is possible, that we obtain an
amorphous Si–C–H network after pyrolysis, which is poorer
in carbon than the corresponding network of the bulk mate-
rial. On the other hand, we know from our own investigations7
that depending on the heating conditions NMR and Raman
spectra of polymers pyrolysed in different ways show differ-
ences, especially with respect to the transformation degree
of polysilanes into polycarbosilanes in the temperature range
between 300◦C and 400◦C. In case of an incomplete trans-
formation due to a high heating rate, NMR signals of [SiSi4]
or [SiSi3C] units are still detectable in the pyrolysed mate-
rial. In such cases the XRD pattern reveals also diffraction
peaks of silicon beside that ones of SiC after further anneal-
ing up to 1650◦C. Obviously, the crystallisation mechanisms
are significantly influenced by such units, which can act as
nuclei for a crystallisation of silicon. We suppose that in the
layers, due to the quickly escaping of the organic groups, the
so-called Kumada-rearrangement takes place similar incom-
p sed
b dur-
i ssing
r table
b
in
p ble
t ysed
i that
hydrogen is able to reduce the amount of excess carbon in
those materials (Fig. 2b and c).
Fig. 7shows a13C single pulse MAS NMR spectrum of a
sample pyrolysed at 1200◦C under argon flow. The line nar-
rowing for the CSi4 peak (in comparison with the same signal
in Fig. 4) is connected with the beginning of crystallisation
of SiC, which is in agreement with the XRD results. The in-
tensity of the SiC-signal is lower than that of free carbon,
which is related to the very long relaxation time of carbon in
crystalline SiC, especially of-SiC.
The last figure shows a comparison of the Raman spectra
of layer 102 and layer 110 (Fig. 8). Layer 110 was pyrolysed
in the same atmosphere as layer 102, but at a higher temper-
ature of 1200◦C. Although after the treatment up to 800◦C,
free carbon was not detectable in layer 102, we find the D- and
G-peaks after treatment at 1200◦C, which can be interpreted
by the growth of carbon clusters at higher temperature, using
excess carbon, which was distributed within the amorphous
network, and was released only at higher temperatures due
to the crystallisation of SiC.
Expected Raman bands of silicon carbide at 795 cm−1 are
not observable, but we know from former investigations7,8
that the sizes of SiC crystallites, which are detectable by Ra-
man spectroscopy, required higher annealing temperatures
than applied here, above 1500◦C. Non characteristic Raman
b
4
sta-
b ute.
A ial
w tionletely like in case of the bulk material, which was pyroly
y high heating rates. The silicon peaks are disappearing
ng a longer high temperature treatment due to a progre
eaction of silicon with excess carbon which is also detec
y Raman spectroscopy.
A reflex at 2Θ = 26.5◦ in Fig. 2c of the sample pyrolysed
ure argon at 1200◦C is related to graphite. It is remarka
hat this reflex is not detectable in case of materials pyrol
n the gas mixture containing hydrogen, which suggestsand is known for amorphous SiC:H.
. Conclusions
Amorphous silicon carbide layers and bulk material
ilised by hydrogen (a-SiC:H) were obtain by polymer ro
fter pyrolysis at 800◦C, both layers and bulk mater
ere amorphous, which was confirmed by X-ray diffrac
156 A. Hilbig et al. / Journal of the European Ceramic Society 25 (2005) 151–156
and MAS NMR spectroscopy. The materials contain also an
amount of excess carbon which can be influenced by an addi-
tion of hydrogen to the pyrolysis atmosphere and is depend-
ing on the geometric dimensions of the sample. Therefrom,
drastic differences of the content of free carbon in the amor-
phous networks of thin layers in comparison to bulk material
result. Annealing at 1200◦C leads to partial crystallisation
processes by forming-SiC in all materials. Additionally,
free silicon was found in thin layers, whereas the crystallites
of graphite were detected in the bulk material.
Acknowledgement
The authors thank Deutsche Forschungsgemeinschaft
(DFG) for financial support of this work.
References
1. Müller, E. and Martin, H. P.,J. Prakt. Chem., 1997,339, 401.
2. Krause, M., Topic, M., Stiebig, H. and Wagner, H.,Phys. Stat. Sol.
(a), 2001,185(1), 121.
3. Roewer, R., Herzog, U., Trommer, K., Müller, E. and Fr̈uhauf, S.,
High-Performance Non-Oxide Ceramics. I, Vol 101, ed. M. Jansen.
Springer, Berlin, Heidelberg, New York, 2002, p. 92.
4. Richter, R., Roewer, G., B̈ohme, U., Busch, K., Babonneau, F., Martin,
H.-P. et al., Appl. Organomet. Chem., 1997,11, 71.
5. Müller, E., Hilbig, A., Wenzel, R., Trommer, K., Roewer, G., Sciurova,
O. et al., Adv. Eng. Mater., 2002,4, 880.
6. Trommer, K., Herzog, U., Schulze, N. and Roewer, G.,Main Group
Metal Chem., 2001,24, 425.
7. Martin, H.-P., Irmer, G. and M̈uller, E.,J. Eur. Ceram. Soc., 1998,18,
193.
8. Martin, H.-P., Thesis, TU Freiberg, 1994.


The microstructure of polymer-derived amorphous silicon carbide layers

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The microstructure of polymer-derived amorphous silicon carbide layers

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