UV laser-induced photolysis of silacyclopent-3-ene: Unseparable photochemistry of reactant and product for chemical vapour deposition of Si/C/H polymer

Article abstract of DOI:10.1016/S0022-328X(98)01001-8

UV laser-induced photolysis of silacyclopent-3-ene in the gas phase is a clean extrusion of silylene yielding buta-1,3-diene. Silylene self-polymerisation and consequent deposition of Si_nH_2n agglomerates is precluded by concurrently occurring photolysis of buta-1,3-diene. The solid polymeric deposit being produced through polymerisation steps involving both H₂Si: and the products of the buta-1,3-diene photolysis makes the reaction suitable for chemical vapour deposition of Si/C/H films.

Full text of DOI:10.1016/S0022-328X(98)01001-8

Journal of Organometallic Chemistry 575 (1999) 246–250
UV laser-induced photolysis of silacyclopent-3-ene: unseparable
photochemistry of reactant and product for chemical vapour
deposition of Si/C/H polymer
a
Josef Pola a,*, Akihiko Ouchi b, Marke´ta Urbanova´ , Yoshinori Koga a, Zdene˘k Bastl c,
d
&
Jan Subrt
a Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, Roz6ojo6a Str. 135, 16502 Prague 6, Czech Republic
b National Institute of Materials and Chemical Research, AIST, MITI, Tsukuba, Ibaraki 305, Japan
c J Heyro6sky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, 18223 Prague 8, Czech Republic
d
&
Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, 25068 Rez' near Prague, Czech Republic
Received 10 August 1998
Abstract
UV laser-induced photolysis of silacyclopent-3-ene in the gas phase is a clean extrusion of silylene yielding buta-1,3-diene.
Silylene self-polymerisation and consequent deposition of SinH2n agglomerates is precluded by concurrently occurring photolysis
of buta-1,3-diene. The solid polymeric deposit being produced through polymerisation steps involving both H2Si: and the products
of the buta-1,3-diene photolysis makes the reaction suitable for chemical vapour deposition of Si/C/H films. © 1999 Elsevier
Science S.A. All rights reserved.
Keywords: Silacyclopent-3-ene; Laser-induced photolysis; Silylene; Chemical vapour deposition; Si/C/H polymer
1. Introduction
dichloro-1-silacyclopent-3-enes [1] are not involved in
the rearrrangement at all. The decomposition of the
parent silacyclopent-3-ene (SCP), which has not been
yet studied, might occur only as an extrusion of H2Si:
and thus serve as a clean source of silylene.
The H2Si: species was earlier generated and spectro-
scopically identified in the lamp, UV and IR laser
photolysis of RSiH3 (R=H, alkyl, phenyl), SinH2n+2
(n=1−3) and H3SiI compounds [7] and transiently
produced [8] in thermolysis of SinH2n+2 (n=1−4) and
RSiH3 (R=H, alkyl, phenyl, alkenyl and alkynyl) com-
pounds (e.g. Refs. [8–15]). Its gas-phase reactivity has
been the subject of intensive research (e.g. Refs. [9,16]);
it has been assessed [17] that the self-reaction into
H2SiꢀSiH2 and H3Si(H)Si: as well as the H2Si: poly-
merisation have a zero activation barrier and are ac-
Thermal or photolytic decomposition of silacy-
clopent-3-enes is known [1–3] to be the reversible pro-
cess [4] occurring via transient 2-vinyl-1-silirane [5,6]
and affording products of rearrangement and silylene
extrusion (Scheme 1).
The extent of these paths depends on the substituents
at the silicon: while the extrusion of (CH3)2Si: from
methyl-substituted silacyclopent-3-enes is accompanied
with formation of the rearranged products (silacy-
clopent-2-enes and siladienes) [2,5], the addition of
H(CH3)Si: to buta-1,3-diene (the inverse of the decom-
position of 1-methyl-1-silacyclopent-3-ene) gives only
1-methyl-1-silacyclopent-3-ene [6], and both addition of
Cl2Si: to 1,3-dienes [1] and thermal decomposition 1,1-
companied
by
disilene
and
silylsilylene
dehydrogenation. The latter steps partly occur on a hot
reactor surface [18] and impair formation of yet uncom-
* Corresponding author. Tel.: +42-2-24311498; fax: +42-2-
20920661; e-mail: pola@icpf.cas.cz.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(98)01001-8

J. Pola et al. / Journal of Organometallic Chemistry 575 (1999) 246–250
247
mon saturated poly(hydridosilanes). Consequently, a-
Si:H films prepared by thermally-, plasma- and laser
photolysis-assisted decomposition of silanes (e.g. Refs.
[19–21]) are poor in H and those of the highest H
content possess 40–50 at. %H at most [22–24]. Also,
a-Si:H films produced with 185 nm light at heated
substrate contain [25] less hydrogen and have the SiH2/
SiH bond density ratio below 0.6. The dehydrogenative
steps were considered not to occur in VUV (147 nm)
gas-phase photolysis of silane or disilane in a cold
reactor [26,27], but the films obtained in this photolysis
were not characterised.
Presuming that UV photolysis of gaseous SCP in a
cold-wall reactor using high intensity laser radiation
can afford high concentrations of H2Si: and result in
deposition of rare poly(hydridosilanes) SinH2n, we ex-
amined ArF laser-induced photolysis of gaseous SCP
and report here on its mechanism and the nature of the
deposited material.
Fig. 1. UV–vis absorption spectrum of gaseous SCP (a) and of the
thin film of deposit obtained by ArF laser photolysis of SCP (b).
ESCA 3 MK II electron spectrometer), transmission
electron spectroscopy (a Philips 201 microscope) and
scanning electron microscopy (a Tesla BS 350 ultra
high vacuum instrument).
2. Experimental details
SCP was prepared using the procedure described in
the literature [28] and its purity (\98%) was checked
by gas chromatography.
Laser photolysis experiments were carried out in a
reactor which was equipped with a sleeve with rubber
septum and PTFE valve, and consisted of two orthogo-
nally positioned Pyrex tubes, one fitted with two quartz
windows and the other furnished with two NaCl win-
dows. SCP (20 Torr) was irradiated at a repetition
frequency of 10 Hz by pulses from an ArF (Lambda
Physik LPX 200 or ELI 94) laser at an incident fluence
of 10 mJ cm−2 effective on the area of 1 cm2. The
progress of the photolysis was monitored by FTIR
spectroscopy (a Shimadzu FTIR 4000 spectrometer)
3. Results and discussion
The ArF laser-induced photolysis of gaseous SCP
was achieved by tuning radiation into the absorption
band (Fig. 1a) centered at 207 nm (m=1.43×10−2
torr−1 cm−1). It results in the formation of volatile
hydrocarbons, silane (traces) and 1-silyl-1-silacyclopent-
3-ene [m/z (relative intensity): 114(19), 113(12), 85(13),
84(15), 83(100), 82(79), 81(34), 69(12), 67(14), 58(16),
57(22), 56(13), 55(46), 54(15), 53(24), 43(20), 29(16),
28(12)] and in deposition of a white solid which coats
the inside of the reactor close to the front quartz
window. The film is opaque to the 193 nm radiation
(Fig. 1b) and detrimental to the photolytic progress:
thus only 30% decomposition can be achieved by a
prolonged irradiation (ca. 6×103 pulses) utilizing both
quartz windows.
The hydrocarbon products distribution (in relative
mole %)—buta-1,3-diene (60–90), buta-1,2-diene (B
5), ethane (8–22), ethene (5–8) ethyne (1–4), propane
and methane (bothB1)—varies at different photolysis
stages (Fig. 2a). The main product being buta-1,3-diene
reveals that the major decomposition channel of SCP is
extrusion of silylene. The observation of 1-silyl-silacy-
clopent-3-ene and silane confirm this view, since these
are the products of silylene reaction with SCP and H2,
respectively. The availability of hydrogen in the system
can be explained by 1,1-H2 elimination, the path com-
using an absorption band of SCP at 670 and 840 cm−1
,
and by gas chromatography on a Gasukuro Kogyo 370
chromatograph (programmed temperature 30–150°C, a
60 m long capillary Neutra Bond-1 and 2 m long SUS
Unipak S columns) connected with a Shimadzu CR 5A
Chromatopac data processor. Some GC analyses were
performed on a Shimadzu (GC 14 A chromatograph
equipped with 1 m long Porapak P column and a
C-R5A Chromatopac data processor). Identification of
gaseous products was accomplished by means of the
gas chromatography and GC/MS technique (a Shi-
madzu QP-1000 mass spectrometer). Properties of the
deposits on KBr, quartz and Co substrates accommo-
dated in the reactor were measured by FTIR spec-
troscopy, X-ray photoelectron spectroscopy (a VG
Scheme 1.

248
J. Pola et al. / Journal of Organometallic Chemistry 575 (1999) 246–250
mon for decomposition of organylsilanes [8–15]. Silane
being a very minor product (its amounts are compara-
ble to those of methane) confirms that this channel is
negligible. However, amounts of 1-silyl-1-silacyclopent-
3-ene range within 10–30% photolytic progress between
35 and 45% of those of buta-1,3-diene and reveal that
the insertion of silylene into SCP is an important step.
Ethyne and methanol are efficient trapping reagents
of silylene [4], but ArF laser irradiation of SCP (20
torr) in excess of these compounds (110 torr of
methanol or 250 torr of ethyne, conditions similar to
those suitable for scavenging reactive silenes [29,30])
revealed no formation of silylene products with these
traps. The deposit formation not being impeded may
imply that the products of trapping may themselves
polymerise or be decomposed to polymer.
The amounts of the C1–C3 hydrocarbons and buta-
1,2-diene slightly increase in the course of the photoly-
sis while amounts of buta-1,3-diene diminish. The
former hydrocarbons are identical to the products of
UV photolysis of buta-1,3-diene [31–33] which takes
place via (i) isomerisation into buta-1,2-diene and sub-
sequent cleavage into CH3 and C3H3 radical, (ii) de-
composition into C2H4 and C2H2 couple, (iii)
decomposition into vinylacetylene and H2 and (iv) poly-
merisation [32,34]. They thus confirm that 1,3-butadi-
ene does not survive under photolytic conditions and
that its photolysis is a concurrent process. The products
observed in the SCP photolysis can thus be rationalised
in terms of Scheme 2.
Scheme 2.
In an effort to reduce 1,3-butadiene decomposition,
the SCP photolysis was carried out in excess of helium
or nitrogen. In excess of helium (500 torr), more buta-
1,3-diene and less hydrocarbons are obtained, which is
in accord with collisional stabilisation of buta-1,3-di-
ene. Ethane being not the major component among the
hydrocarbons (Fig. 2b) indicates that the cleavage of
buta-1,2-diene into CH3 and C3H3 radicals is somewhat
reduced. In excess of nitrogen (200 torr) (Fig. 2c),
buta-1,3-diene is not as favoured as in the excess of
helium; in this case no ethane, more ethyne and some
1,2-butadiene is produced. This is in line with less
decomposition of buta-1,2-diene. The observed changes
in relative amounts of buta-1,3-diene and of its decom-
position products are, however, rather small and indi-
cate that separation of the photochemistry of SCP and
of that of buta-1,3-diene is not possible.
This inference is in accord with the IR spectral
pattern of the deposits which is the same regardless of
the extent of the photolysis and whether or not the
photolysis is carried out in the presence of the inert gas.
In all the instances, it consists of bands at w (cm−1):
855m (w(Si–C)), 950m (l(Si–H)), 1070m (w(Si–C–Si)
and/or w(Si–O)), 2130–2150s (w(Si–H)) and 2920w
(w(C–H)) and is in accord with a polymer possessing
H2Si groups in a Si/C/H skeleton. It is known [35] that
more carbon incorporation in the Si–Si framework
shifts the w(Si–H) wavenumber to higher values and
that the films showing w(Si–H) at 2130 cm−1 corre-
spond to a-Si1−xCx:H films with carbon content x\
0.8. The absorptivity at the w(Si–H) and w(C–H)
vibrations is instructive of the distribution of H atoms
between Si and C centers [36]; the Aw(C–H):Aw(Si–H)
ratio of the deposits being 0.21–0.26 is in keeping with
roughly equal concentrations of H(C) and H(Si) atoms.
XPS analysis of the topmost (ca. 5 nm) layers of the
deposit (Fig. 3) reveals that the Si 2p core level spec-
trum is best fitted with contributions of two compo-
nents, the major at 101.1 eV belonging to a Si/C/H
polymer and the minor at 102.5 eV assignable to a
Si–O bond [37,38]. The curve fitting procedure reveals
Concurrent photolysis of buta-1,3-diene during the
SCP photolysis is harmful to achieving chemical vapour
deposition of pure SinH2n polymer. The UV photo-
polymerisation of buta-1,3-diene is judged to involve
reactions of volatile primary products of buta-1,3-diene
photo-decomposition [34], which implies that the un-
contaminated SinH2n polymer can only be deposited
when the photolysis of buta-1,3-diene has ceased.
Fig. 2. Gaseous product yield (in relative mole percent) at different
stages of SCP photolysis carried out without added gas (a) and in
excess of He (b) and N2 (c). C4H6 (“); C2H6 (ꢀ); C2H4 (ꢁ); C2H2
("); CH3CHꢀCꢀCH2 (×).

J. Pola et al. / Journal of Organometallic Chemistry 575 (1999) 246–250
249
Fig. 3. Fitted photoelectron spectrum of Si 2p electrons.
that an incorporation of the contribution of the a-Si:H
component at ca. 99.3 eV is not needed. These data
thus rule out the presence of (H2Si)n groupings in the
polymer and confirm that the solid deposit is produced
through polymerisation steps involving both H2Si: and
products of the buta-1,3-diene photolysis. Incorpora-
tion of oxygen in the superficial layers indicates deposit
reactivity towards ambient atmosphere.
SEM analysis reveals that the deposits have a struc-
ture of tightly bonded agglomerates of ca. 1 mm size
(Fig. 4), and TEM analysis proves that these agglomer-
ates can be described as a net of ca. 20 nm sized dots
(Fig. 5).
Fig. 5. TEM spectrum of the deposit (magnification ×100 000).
that of the a-SiC:H films deposited from other
organosilicon progenitors through their IR laser pho-
tolysis (e.g. [39,40]). The ArF laser photolysis of SCP
can be used as a low-temperature chemical vapour
deposition of a-SiC:H films possessing high adhesion
strength despite the substrates being kept at ambient
temperature.
The films are not soluble in common organic solvents
(benzene, dichloromethane, tetrahydrofuran), which in-
dicates a high degree of cross-linking. They exert excel-
lent adhesion strength to glass and metals and can be
scrubbed off the substrates with difficulty.
We conclude that the UV laser photolysis of SCP can
be described as a clean extrusion of silylene accompa-
nied by photolysis of the major product, butadiene. The
use of the SCP photolysis as a source of SinH2n polymer
is obstructed by concomittantly occurring photolysis of
buta-1,3-diene. The IR spectral pattern of the films
deposited by UV laser photolysis of SCP is similar to
Acknowledgements
The work was supported by the Grant Agency of the
Czech Republic (grant no. 203/96/1217). J.P. acknowl-
edges his STA fellowship at the National Institute of
Materials and Chemical Research at Tsukuba funded
by JISTEC.
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