Synthesis of polysilanes by tunneling reactions of H atoms with solid Si2H6 at 10K

Article abstract of DOI:10.1246/cl.2009.986

Tunneling reactions of H atoms with solid Si₂H₆ at 10K were investigated. The in situ and real-time reactions H + Si ₂H₆ to form silane and polysilanes were monitored using FT-IR. Quantitative analysis of gaseou

Full text of DOI:10.1246/cl.2009.986

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Chemistry Letters Vol.38, No.10 (2009)
Synthesis of Polysilanes by Tunneling Reactions of H Atoms with Solid Si H at 10 K
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6
ꢀ
1
2
2
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2
Norihito Sogoshi, Shoji Sato, Hideaki Takashima, Tetsuya Sato, and Kenzo Hiraoka
1
Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama 338-8570
2
Yamanashi University, 4 Takeda, Kofu 400-8511
(Received July 22, 2009; CL-090681; E-mail: sogoshi@chem.saitama-u.ac.jp)
Tunneling reactions of H atoms with solid Si2H6 at 10 K
were investigated. The in situ and real-time reactions H + Si2H6
to form silane and polysilanes were monitored using FT-IR.
Quantitative analysis of gaseous products was made by thermal
desorption spectrometry. Monosilane and polysilanes were de-
tected as major reaction products. The intermediate product SiH2
was suggested to play an important role for the growth of the Si–
Si network of the solid products.
The chemistry of silanes has attracted the interest of many
researchers since its basic understanding is of technological im-
portance for the formation of high quality semiconductor films
1
,2
using chemical vapor deposition. The gas-phase kinetic data
of silanes and the total reaction simulations of CVD (chemical
3
–7
vapor deposition) processes have been studied extensively.
8
In our previous work, the tunneling reactions of H atoms
with various hydrocarbons were investigated at 10 K. It was
found that the H-atom abstraction as well as recombination reac-
tions of H with radicals plays major roles in the cryogenic tun-
9
neling reactions. In our recent work, it was found that amor-
2 6
Figure 1. FT-IR spectra of 4 ML solid Si H film before and after
phous silicon was synthesized by the reaction of H with solid
SiH4 at 10 K. In the current work, we further examined the reac-
tion of H with solid Si2H6 at 10 K in order to obtain further in-
formation on the mechanism for the formation of amorphous sil-
icon at cryogenic temperature.
the reaction with H atoms for 4 h at 10 K. The upper and bottom panels
show the SiH stretching and the HSiH bending modes, respectively.
tailing extending to lower wavenumber of Si2H6 (2150–2050
ꢁ
1
ꢁ1
cm ), and a peak at ca. 860 cm designated as ‘‘polysilane.’’
The spectrum designated as ‘‘R.T.’’ corresponds to that for poly-
silane obtained after the substrate temperature was increased to
room temperature. The intermediate radical species were not de-
tected in the present experiments, possibly because they were
quickly annihilated by the reactions with H atoms sprayed over
the film and/or with the neighboring reactant, Si2H6.
9
The experimental details were described previously. Brief-
ly, a vacuum manifold equipped with a cryocooler, a discharge
tube for the generation of H atoms, and a quadrupole mass ana-
ꢁ10
lyzer was evacuated to ca. 10 Torr. The reactant, gaseous
Si2H6 (Nihon Sanso Corp., Si2H6:He = 10.5:89.5 high purity
diluted gas), was deposited on a silicon wafer substrate at 10 K.
After the deposition, the solid film was sprayed by H atoms pro-
duced by a dc-corona discharge. The temperature of the H atoms
was kept at about 27 K. The in situ and real-time observation of
tunneling reactions taking place on the solid substrate was moni-
tored by FT-IR (Mattson Instruments, Infinity Gold) using the
reflective mode. The IR signal intensities were calibrated by de-
positing known amounts of SiH4 and Si2H6 on the silicon sub-
strate. Quantitative analysis of polysilanes could not be made be-
cause their absorption coefficients are not available. After the H
atom irradiation, the gaseous products were analyzed by thermal
desorption spectrometry (TDS) using a quadrupole mass spec-
trometer (Leda mass, Microvision 300D).
Figure 2 shows the relationship between the amounts of the
reaction products and the reaction time. The amounts of Si2H6
and SiH4 are displayed as equivalent monolayers (ML). It should
be noted that the initial decrease of Si2H6 is accounted for by the
increases of the products, SiH4 and polysilane. The formation of
SiH4 as a major product strongly suggests the occurrence of re-
action (1).
0
ꢁ1 10
)
Si2H6 þ H ! SiH4 þ SiH3 (ꢀH ¼ ꢁ64 kJ mol
The possible competing reaction (2):
ð1Þ
0
ꢁ1 10
)
Si2H6 þ H ! Si2H5 þ H2 (ꢀH ¼ ꢁ64 kJ mol
ð2Þ
Figure 1 shows the FT-IR spectra of the 4 ML (equivalent
monolayers) thick solid Si2H6 film before and after 4 h of reac-
tion with H atoms at 10 K. The intensities of peaks for Si2H6 (ca.
2165, ca. 2145, ca. 940, ca. 840, and ca. 820 cm ) decreased
with the reaction time. These decreases were accompanied by
has almost the same enthalpy change as that for reaction (1).
Dobbs et al. made the theoretical calculations on Si H + H
2
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1
1
for three configurations of the transient states. They predicted
that the barrier (E ) for the ‘‘frontside’’ attack to a Si–Si bond
ꢁ
1
a
ꢁ1
ꢁ1
by a H atom is E ¼ 3:0 kcal mol
a
(1 kcal mol = 4.184
kJ mol ) for reaction (1). They further reported that the barrier
height of H atom abstraction for reaction (2) is even lower as
ꢁ1
concomitant increases of monosilane (SiH4, ca. 2187(ꢀ1), ca.
ꢁ
1 9
2
191(ꢀ3), and ca. 900(ꢀ4) cm ), other products with a broad
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.10 (2009)
987
Figure 2. The relationship between the amounts of the reaction
products and the reaction time. The amounts of Si2H6 and SiH4 were
displayed as equivalent monolayers (ML).
ꢁ
1
2
.4 kcal mol . Because the reactions at 10 K should proceed via
tunneling processes, the reactions with lower barriers may take
place more favorably. The formation of SiH4 clearly indicates
that reaction (1) is one of the major processes for the reaction
H + Si2H6. The discrepancy between the theoretical predict
and our result exists possibly because the reactions (1) and (2)
competes, but the Si2H5 produced by reaction (2) is quickly an-
nihilated by the recombination H þ Si2H5 ! Si2H6.
Figure 3. TDS spectra of the Si2H6 film reacted with H atoms for
In Figure 2, the amount of SiH4 shows a steep initial in-
crease that is followed by a gradual decrease. The decrease in
SiH4 should be attributed to its conversion to polysilane. The
gradual decrease in polysilane after about 50 min should be
due to the growth of a Si–Si bond network, i.e., decrease in
the IR absorption for Si–H bonds. The formation of SiH4 as a
major product in Figure 2 indicates that the rate-controlling
step for the formation of polysilane is the reaction of H with
SiH4 but not the reaction H + Si2H6. As for the hydrogen
4
h at 10 K. Panel (a) shows the mass signals tuned for the ions con-
taining 1 and 2 Si atoms. Panel (b) shows the mass signals for the ions
containing 3–5 Si atoms.
In summary, the cryogenic synthesis of polysilane by the
tunneling reaction of H with solid Si2H6 at 10 K is investigated.
It was found that the major reaction channel is the cleavage of
the Si–Si bond of Si2H6, i.e., reaction (1). The highly reactive in-
termediate product SiH2 inserts to Si–Si and also to Si–H bonds
of neighboring reactants, resulting in the growth of Si–Si bond
networks for polysilane and ultimately for amorphous silicon.
1
2
abstraction from SiH4, a theoretical study by Yu et al. predict-
ed Ea ¼ 5:54 kcal mol
ꢁ
1
for SiH4 þ H ! SiH3 þ H2, which
ꢁ
1
is significantly higher than Ea ¼ 3:0 kcal mol
for Si2H6 þ
H ! Si2H5 þ H2. The considerably smaller bond energy of
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1
7
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Published on the web (Advance View) September 12, 2009; doi:10.1246/cl.2009.986