Flame emission spectra in the region 400-600 nm during low-pressure silane and dichlorosilane oxidation

Article abstract of DOI:10.1023/A:1019822614392

The flame emission in the region 400-600 nm during monosilane and dichlorosilane oxidation (initial pressures of 3-20 torr; T = 300 K) is caused by radical luminescence processes on the surface of aerosol microparticles of the SiO₂ formed. The

Full text of DOI:10.1023/A:1019822614392

Kinetics and Catalysis, Vol. 43, No. 4, 2002, pp. 445–452. Translated from Kinetika i Kataliz, Vol. 43, No. 4, 2002, pp. 485–492.
Original Russian Text Copyright © 2002 by Chernysh, Rubtsov, Tsvetkov.
Flame Emission Spectra in the Region 400–600 nm
during Low-Pressure Silane and Dichlorosilane Oxidation
V. I. Chernysh, N. M. Rubtsov, and G. I. Tsvetkov
Institute of Structural Macrokinetics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia
Received November 16, 2001
Abstract—The flame emission in the region 400–600 nm during monosilane and dichlorosilane oxidation (ini-
tial pressures of 3–20 torr; T = 300 K) is caused by radical luminescence processes on the surface of aerosol
microparticles of the SiO2 formed. The generation of energy by the interaction of gas-phase species with the
SiO2 surface at the initial stages of the phase formation depends on the presence of the intrinsic structural
defects Si+ and defects like Si+ implanted into SiO2. The addition of SF6 to the starting mixture results in the
appearance of emission bands due to the Si+ defects in the radical luminescence spectrum.
INTRODUCTION
debatable. Emission of this kind with spectra similar to
that presented in Fig. 1 was also observed in other oxi-
dation processes with the participation of silicon-con-
taining substances. It was suggested [18] on the basis of
the chemiluminescence spectra of the SiH4 + O3 reaction
Combustion processes involving monosilane and its
chloro derivatives are of great practical interest in view
of the problems of semiconductor technology [1], the
chemistry of semiconductor nanoparticles [2], and so
on. To control these processes, the kinetics of the oxi- that the diffuse emission in the visible region can be
dation of monosilane and its chloro derivatives by a
branched-chain mechanism [3–5] should be known.
attributed to excited H2SiO* or HSiOH* molecules. It
was shown by theoretical calculations [18] that this
emission can be due to the (A1A'–X1A1) transition in the
H2SiO* molecule. The emission of the SiH2 + O2 reac-
To elucidate the nature of the active intermediate
species and final reaction products, emission spectros-
.
copy is widely used. The emission bands of OH radi-
tion in the visible region was attributed [19] to the
.
cals (A2Σ–X2è transition) at 306.4 nm [6–8] and of
chemiluminescence of the (SiO )* radical formed in
.
SiO radicals (A1è–X1Σ transition) in the region 230–
the process
280 nm [7, 8] were identified in the chemiluminescence
.
spectra of the flame of monosilane. The bands of SiH3
radicals [9] and a broad emission band in the region
350–650 nm [8, 10] were detected in the laser magnetic
resonance spectrum (Fig. 1). The superequilibrium
concentrations of oxygen atoms [11] were determined
in flame by ESR spectroscopy, although van de Weijer
and Zwerver [8] failed to detect these atoms in the com-
OH•
bustion products by a laser-induced fluorescence tech-
.
nique. The emission of SiH2 radicals (A1B1–X1A1 tran-
.
sition) in the region 550–620 nm [12] and of SiH rad-
SiO•
2
icals (A2è–X2è transition) at 413 nm [13, 14] was
observed in a rarefied flame in the countercurrent
diffusion flows of SiH4 and molecular oxygen. Sim-
1
.
.
ilarly, the OH (A2X–X2è) [15], SiO (A1è–X1Σ), and
.
300
400
500
600
700
λ, nm
SiCl (A1B1–X2A1) radicals were detected in the spectra
of a rarefied flame of dichlorosilane (DCS) with oxygen
in the region 300–380 nm [16]. The Cl atoms were
detected in the ESR spectra [17]. However, the nature
of a broad diffuse band found in the emission spectra of
SiH4 + O2 and DCS + O2 flames in the region 350–650 nm
[16] with a maximum at ~450 nm is so far unclear and
Fig. 1. Chemiluminescence spectra during monosilane oxi-
dation: (1) preliminarily prepared mixture of 3.3%SiH +
4
29%O + 67.7% He; 723 K; 7.6 torr; spectral resolution of
2
3 nm [10]; (2) separated flows of SiH and O (1 : 3) over
4
2
the surface of a hot disk (873 K); 1 torr; spectral resolution
of 2 nm [8].
0023-1584/02/4304-0445$27.00 © 2002 MAIK “Nauka /Interperiodica”

446
CHERNYSH et al.
ate particles rather than SiO2. A flame with SiCl4 addi-
1
tives does not produce such an emission, although only
SiO2 is formed during combustion. The emission of the
reaction products in the countercurrent flows of O2 + N2
and N2 + H2 with SiH4 or SiCl4 additives was also con-
sidered [23]. Extra pure SiO2 powder was formed in the
flows with SiCl4 admixtures. It was found [24] that the
color of the flame of SiH4 + O2 + N2 mixtures varied
from whitish blue to bright orange with the SiH4 con-
centration. A mixture of the following composition was
used: [SiH4] = 1.6–2.1%, [O2] = 2.0–24.0%, and the
balance N2 up to a total pressure of 1 atm, T0 = 293 K.
The inhibition effect of N2, CO2, CF4, CF3Br, CF3H,
C2H4, and C6H6 gas additives on monosilane combus-
tion in oxygen at countercurrent diffusion flows was
studied in [13]. When CF3Br was added to the mixture
containing 5% SiH4, the color of the flame changed
from bright orange to whitish blue, which is typical of
a flame with a low SiH4 concentration (Fig. 2). The
intensity of flame emission near ~500 nm significantly
3
SiH•
2
400
500
600
700
λ, nm
Fig. 2. Chemiluminescence spectra during the reaction of
monosilane and oxygen (5% SiH in O ) under conditions
4
2
of countercurrent diffusion flows (1) without additives and
(2) in the presence of inhibitor CF Br (1%) [13]. (3) The
3
spectra of SiH pyrolysis products excited by a CO laser
4
2
[27] (multiphoton absorption method); 50 torr.
decreased in the presence of CF3Br additives (1%),
.
although the emission intensity of (SiO )* radicals
.
SiH2 + O2
(SiO )* + H2O + 550 kJ/mol. (I)
(A2è–X2è) remained almost unchanged. Assuming
that the continuous emission of the flame without
CF3Br additives near 500 nm is mainly determined by
the emission of solid particles, Koda and Fujiwara [13]
concluded that a decrease in the emission intensity indi-
cates a considerable decrease in the temperature of the sur-
face of particles formed, and the emission at λ < 500 nm
(see Figs. 1 and 2) cannot be assigned to a solid. A rough
approximation of the intensity of emission at 700 nm
according to Planck’s law indicates that, in the presence
of CF3Br (1%), the temperature decreased by more than
400 K, although the spatial distribution of aerosol par-
ticles remained unchanged. At the same time, the max-
imum gas temperature measured by a thermocouple
decreased by only 60 K relative to a maximal value of
1250 K. These data indicate that the emission spectra of
monosilane combustion with oxygen in the visible
region, which essentially depend on the experimental
conditions and mixture compositions, are difficult to
interpret.
However, in the well-known chemical reaction
.
Si + N2O
N2 + SiO + 635 kJ/mol,
(II)
chemiluminescence at 440 nm [20] was not observed
under any conditions. An attempt to explain the nature
of the diffuse emission was made by van de Weijer and
Zwerver [8, 21], who attributed the above chemilumi-
nescence to the emission of the oxygen molecule in a
metastable state. According to data [8], excited oxygen
molecules are formed in the gas-phase reaction
*
SiO + O + O2
SiO2 + O2 + 4.7 eV.
(III)
In this case, energy is transferred to the numerous
highest excited vibrational levels of the O2 molecule in
different electron states (A3Σu+ , C3∆u, and C3Σu– ). Tran-
sitions from these states to the ground state X3Σg– are
accompanied by continuous emission in the region
350–650 nm (Fig. 1). This assumption is based on the
findings of Brus and Comac [22], who observed chemi-
luminescence in the visible region during the interac-
tion of O2 with the pure Si(111) surface, which
occurred several centimeters above the surface. Brus
and Comac [22] concluded that this emission belongs
to excited oxygen molecules in a metastable state with
a lifetime of 5 × 10−4 s. However, they did not rule out
It was found [25] that upon the ignition of SiH4 + O2
mixtures ([SiH4] = 30%; ignition temperature of 373 K)
a white powder (SiO2) and H2O are the main products,
whereas in the case of the mixtures with [SiH4] > 70%,
H2 and a brown powder consisting of Si, SiO, and a
small amount of SiO2, are the main reaction products. It
was suggested that the kinetic mechanism of SiH4 oxi-
dation involves elementary reactions corresponding to the
thermal decomposition of monosilane. The time of SiH4
decomposition at 1000 K was estimated at 3 × 10−3 s. It
has been found using laser-induced fluorescence [26]
that Si atoms are formed from dichlorosilane in the bulk
during chemical vapor deposition on a support heated
to 1123 K at a total pressure of 625 torr (0.7 torr of DCS
that the emission can be caused by the desorption of
.
other particles (likely, SiO ) from the surface. Contin-
uous emission in a considerably wider spectral region
was observed in some works. For example, Chung et al.
[23] found that an orange flame, similar to the emission
of solids, can be due to the emission of SiOx agglomer-
KINETICS AND CATALYSIS Vol. 43 No. 4 2002

FLAME EMISSION SPECTRA IN THE REGION 400–600 nm
447
in He). The rate of Si formation from SiH4 in a gas
phase is one or two orders of magnitude higher than
that from DCS. The possibility of strong thermal radia-
tion on the condensation of Si particles was found [27]
in the pyrolysis of monosilane using multiphoton
absorption of CO2 laser radiation for preparing Si par-
ticles. At a pressure of 152 torr, only continuous emis-
sion due to the heat of Si condensation was observed.
However, even at 53 torr, individual emission peaks,
1
*
*
which may be attributed to the SiH2 , SiH*, and H2
species, were detected (see Fig. 2, curve 2). Compari-
son of Figs. 1 and 2 shows that the above emission
occurs in the range 350–650 nm and that the emission
beyond the range 550–650 nm is likely due to the SiOx
agglomerates and condensed Si particles. Based on
these findings, one can conclude that SiO2 aerosols,
which are the final products of oxidation of monosilane
and its derivatives, can be emitters in the range 350–
600 nm (Fig. 1). Pure SiO2 aerosol is known to have no
absorption bands near 300 nm [28]. However, in the
presence of crystal lattice defects in SiO2 aerosols, the
corresponding external sources of excitation can pro-
duce electroluminescence, photoluminescence, tribolu-
minescence, adsorption luminescence, and radical-
recombination luminescence in the visible region of the
spectrum [29, 30]. This luminescence is due to the
occurrence of SiO2 defects whose size is no greater than
the unit-cell parameter of SiO2 (a silicon–oxygen tetra-
hedron), that is, point defects (vacancies, interstitial
atoms, bond defects, and so on) [29, 30]. Figure 3 pre-
sents the photoluminescence spectra of SiO2 nanoparti-
cles (10–20 nm in diameter) prepared by the evapora-
tion of polycrystalline silicon with a laser beam in an
2
350
450
550
650
750
λ, nm
Fig. 3. Luminescence spectra: (1) photoluminescence of
SiO nanoparticles (10–20 nm in diameter) obtained by
2
evaporation of polycrystalline silicon into an atmosphere of
O (100 torr) + He (700 torr) [31]; spectral resolution of
2
1 nm; (2) triboluminescence on the mechanical grinding of
quartz glass in a vacuum [30]; spectral resolution of 15 nm;
2
S
~ 4 m /g.
sp
bottom. A prepared combustible mixture was fed into
the reactor from a storage tank through a vacuum valve.
The mixture was ignited by heating a Nichrome wire
0.3 mm in diameter using a capacitor bank (3000 µF).
The conductivity of the reacting mixture was measured
using a molybdenum rod 40 mm in length and 1 mm in
diameter as a single probe. Spectroscopic measure-
ments were carried out in the range 200–680 nm with
the use of a BM-25 high-aperture (aperture ratio of 1 : 4.5)
diffraction monochromator (Germany) equipped with a
device for continuous wavelength scanning. An STE-1
spectrograph (aperture ratio of 1 : 12) was used in a
number of measurements. In studies of combustion
under static conditions, two slits (1 mm) were arranged
in a focal plane at the outlet of the STE-1 spectrograph
in the calculated spectral regions to monitor chemilu-
minescence simultaneously at two wavelengths. Two
photomultiplier tubes (FEU-71 and FEU-39) were used
whose signals were fed to two channels of an S9-6 stor-
age oscilloscope. The UV absorption spectra were
recorded with the use of a DVS-25 mercury–hydrogen
lamp, which exhibits a continuous emission spectrum
in the range 200–320 nm. The lamp radiation was
passed through a quartz collimator and a reactor and
was focused with a condenser on the inlet slit of the
spectrograph. Radiation was detected with the use of an
FEU-71 photomultiplier tube. The scattering of light by
aerosol particles was analyzed using an LGN-209A
helium–neon laser (λ = 633 nm) at an angle of 90°;
detection was performed with an FEU-39 photomulti-
He + O2 or Ar + O2 atmosphere at PO = 100 torr and a
2
total pressure of 800 torr [31] and the tribolumines-
cence spectra that appeared upon mechanical grinding
of vacuum-hydrogen quartz glass in a vacuum [30]. In
.
this glass, surface ≡SiO radicals are luminescence
centers in the range 600–750 nm and =Si: radicals are
luminescence centers at 440 nm [32].
The aim of this work was to elucidate the nature of
continuous emission in the visible region (400–600 nm)
during the combustion of monosilane and its chloro
derivatives in oxygen, the integral emission in the visi-
ble region during combustion, the conductivity of a
reacting gas mixture, and the scattering of radiation
from a probing light source on a =Si: aerosol. To solve
these problems, spectroscopic measurements under
static and flow conditions were used.
EXPERIMENTAL
Kinetic measurements were carried out in a vacuum plier tube. A quartz reactor (170 cm in length and
setup, which was described elsewhere [10, 16, 33, 34] 0.9 cm in inner diameter) was used to study emission
under static conditions at 300 K and a total pressure of spectra in rarefied flames during the oxidation of SiH4
0.5 to 10.0 torr. The reaction vessel was a quartz cylin- and DCS under conditions of flame front propagation.
der 120 mm in diameter and height with a removable The reactor had optical quartz windows at the ends [10,
KINETICS AND CATALYSIS Vol. 43 No. 4 2002

448
CHERNYSH et al.
1 ms
PO , torr
2
lnI0/I
J, V
20
15
10
0.4
3
4
1
0.5
0.2
0
2
5
0
1 ms
0
2
4
6
τ, ms
1
Fig. 5. (1) Time taken to reach a maximal intensity of inte-
gral chemiluminescence in the range 400–600 nm as a func-
2
tion of pressure (a mixture of 11% DCS in O ; 300 K; initi-
2
ated ignition); (2) the limiting time of deexcitation of a par-
−4
*
ticle (possibly, O2 [8, 21, 22]), which is equal to 5 × 10 s.
5
a maximum of the integral emission in the visible
region (curves 1 and 2, respectively). The maximal val-
ues of the integral emission (curve 2), conductivity
(curve 3), and absorption at λ = 220 nm (curve 4) are
reached almost simultaneously. The time of reaching
these maxima coincides with the moment at which the
rate of increase of the intensity of scattering probing
radiation with a wavelength of 633 nm by an aerosol
(curve 5) formed in the gas phase during the reaction is
maximum. Under the experimental conditions specified
in Fig. 4, the addition of 10% SF6 produces a simulta-
neous decrease in both the conductivity and the inten-
sity of scattered radiation. Hence, the integral emission
in the visible region is certainly related to phase forma-
Time
Fig. 4. Kinetic curves for (1) emission intensity of
.
1
1
(SiO )*(A è–X Σ); (2) integral emission intensity in the
range 400–620 nm; (3) conductivity; (4) light absorption at
220 nm; and (5) radiation scattered by an aerosol at an angle
of 90° relative to the direction of a probing beam with λ =
632.8 nm. Initiated ignition of 25% DCS in O ; 4.1 torr;
2
300 K.
35]. The emission spectrum of a flame propagating
along the reactor after initiated ignition was measured
with the use of an OSA-500 optical spectrum analyzer
(Germany). The resolution of the optical system was tion in the gas phase. This relation is clearly shown in
0.4 nm per channel. The required number of spectrum Fig. 5 (curve 1). In addition, curve 2 in Fig. 5 shows the
scans (one scan is equal to 500 channels in 32 ms) was limiting time of deexcitation in the visible region of the
stored in the memory of an OSA-500 computer. The
reactor provided a number of scans at which the signal-
to-noise ratio was >25. In studies of the optical spectra,
SF6 was added to a combustible mixture (up to 10% on
a fuel basis). This additive significantly decreased the
intensity of continuous emission in the visible region of
the spectrum, which resulted from the formation of a
solid phase in the reactor volume [16, 33, 36], and
allowed us to reveal the structure of the emission. The
O2 and SF6 gases were of chemically pure grade. The
concentration of impurities in monosilane was no
higher than 0.1% according to the technical certificate;
the purity of DCS was checked by IR spectrophotome-
try [37].
*
spectrum of an excited molecule (likely O2 , for which
this time is <5 × 10−4 s according to [8, 21, 22]). The dif-
ference between curves 1 and 2 in Fig. 5 indicates that
experimental curve 1 cannot be explained by the deex-
*
citation of excited species (like O2 ). The experimental
relationships presented in Fig. 4 (curves 2 and 5) can be
explained by the adsorption of atoms and molecules or
the recombination of atoms on the freshly formed sur-
face of an SiO2 aerosol [30, 32, 35]. The recombination
of active species formed during combustion on the sur-
face of an SiO2 aerosol is accompanied by the
superequilibrium emission of electrons or ions (Fig. 4,
curve 3). This emission is a channel of energy relax-
ation in an excited solid at which, for example, the
energy of excited electrons exceeds the work function
of the surface. Radiative relaxation, namely radical-
recombination luminescence, is another manifestation
(simultaneous with the above process) of the formation
of electronically excited states during recombination in
RESULTS AND DISCUSSION
The kinetic data on DCS oxidation are presented in
Fig. 4. As can be seen, a maximum chemiluminescence
of (SiO )* (A1è–X1Σ) is reached 5 × 10−4 s earlier than
KINETICS AND CATALYSIS Vol. 43 No. 4 2002

FLAME EMISSION SPECTRA IN THE REGION 400–600 nm
449
the solid (Fig. 4, curve 2). The adsorption of molecules
1
or atoms (adsorption luminescence) can also produce
the emission of photons, electrons, and ions through the
interaction of sorbed particles with the point defects of
the crystal lattice (for example, on the completion of an
oxide lattice or upon the recombination of adsorbed
oxygen molecules with surface anionic vacancies).
After the occupation of surface sites with great heats of
adsorption, adsorption luminescence and adsorption
emission stopped, since adsorption at these sites is irre-
versible (see curves 2, 3, and 5 in Fig. 4). Curve 4 in
Fig. 4 also confirms the appearance of a solid SiO2
phase, which is characterized by an absorption band at
248 nm [29]. The above data indicate that the appear-
ance of a new phase (SiO2 aerosol) during the oxidation
of monosilane or DCS is accompanied by chemilumi-
nescence in the visible region of the spectrum, which is
due to the phase itself. A similar mechanism of chemi-
luminescence was observed [38] in the crystallization
of the complex Ru(bipy)3Cl2 · 6H2O from solutions
(acetonitrile and ethanol) in the presence of hydroper-
oxides. It was found that the reactions of hydroperox-
ides on the juvenile surface of the crystals formed are
responsible for the well-known phenomenon of crystal
luminescence [38]. Under these conditions, a metal ion
on the juvenile surface of a freshly formed crystal cycli-
cally changes its oxidation number to form an excited
state, which is detected by typical chemiluminescence
in the red region of the spectrum. The appearance of
luminescence on grinding quartz glass and in the con-
tact of various gases with the sample destroyed in a vac-
uum was studied [30, 32] (see Fig. 2, curve 3). New
structural defects (centers) are formed upon grinding the
quartz samples, which emit in the range 630–650 nm
and at the band with a maximum at 440 nm. These cen-
3
2
4
5
6
400
500
600
λ, nm
Fig. 6. Emission spectra in the visible region: (1) shape of
the emission spectrum of 10% SiH + O , 5.5 torr; (2) shape
4
2
of the emission spectrum of 20% DCS + O , 5.1 torr; (3)
2
spectrum of a rarefied flame of a mixture of 15.7% SiH +
4
23.2% SF + O , 5.5 torr; 293 K; initiated ignition; (4) spec-
6
2
trum of a rarefied flame of a mixture of 27.2% DCS + 15%
SF + O , 6.0 torr; 293 K; initiated ignition; (5) electrolu-
6
2
minescence spectrum of Si–SiO structures doped with ter-
2
bium; and (6) photoluminescence spectrum of a nanocrys-
talline (3–5 nm) silicon film [41]; 300 K; excitation at
337 nm; emission maximums at 390, 415, 437, and 466 nm.
probing radiation in the UV region of the spectrum
decrease in the presence of SF6 additives [16, 33, 37, 40].
The kinetic curves of combustion are similar to the cor-
responding curves in the absence of SF6 (Fig. 4). It was
also found [33, 36] that the SF6 additives to a combus-
tible mixture lead to a decrease in the intensity of con-
tinuous emission in the visible region and to the appear-
ance of structure peculiarities in this region of the emis-
sion spectrum. Figure 6 (curves 3, 4) presents the
emission spectra measured under static conditions
(in the regime of initiated ignition) in the oxidation of
monosilane and DCS in the presence of an SF6 additive.
The shapes of the emission spectra for the combustion
of monosilane and DCS are shown in Fig. 6 (curves 1
and 2, respectively). The total pattern of the spectrum
for monosilane combustion is presented in Fig. 1. The
emission spectra of the flame obtained under conditions
of flame front propagation are similar in shape to curves 3
and 4 in Fig. 6. The emission spectra of intermediates
in the reactions of monosilane and DCS with oxygen
recorded in the presence of SF6 are almost identical in
the range 400–530 nm. The system of bands in the
wavelength range 590–660 nm is observed in the spec-
ters were attributed to the intrinsic quartz structure
.
defects ≡SiO and =SiO:. When a solid is subjected to
other impacts, for example, photoexcitation, the
appearance of these centers also manifests itself in
emission [30, 32]. To examine the possibility of lumi-
nescence due to the radiative deactivation of excited
oxygen molecules near the surface of mechanically
activated quartz powder, the spectrum of a microwave
gas discharge in oxygen was compared with the adsorp-
tion luminescence spectra of O2 on magnesium oxide
[32]. It was found that luminescence resulted from the
solid rather than a gas phase containing O2 molecules
and O atoms in ground and excited states. In addition,
the adsorption luminescence spectra and their emission
maxima on the surface of MgO and SiO2 powders differ
considerably [30, 32]. Hence, although oxygen mole-
cules in an excited state are formed in the gas phase
upon recombination of oxygen atoms on the surface
[39], their contribution to the total emission is evidently
small because of adsorption luminescence.
We found earlier that, in DCS and monosilane oxi-
dation reactions, the rate of propagation of a flame
front, the conductivity, and the absorption intensity of trum of a flame of monosilane; it can be attributed to the
KINETICS AND CATALYSIS Vol. 43 No. 4 2002

450
CHERNYSH et al.
Intensity
of dispersed SiO2 particles in the gas phase. Because of
this, the emission spectra of combustion products in the
visible region (Fig. 1 and Fig. 2, curve 2) and the photo-
and triboluminescence spectra of SiO2 particles (Fig. 3)
are almost identical. Emission at ~650 nm (~1.9 eV)
(Fig. 3, curve 2), which appears in the emission spectra
Line of DRGS-12
447 nm
of the SiH4 combustion products (Fig. 1, curve 2;
1
.
Fig. 6, curve 3), is due to the surface defect ≡SiO [29,
30]. According to [29], the absorption band at 650 nm
is due to the appearance of thermolized electrons in the
conduction band of the SiO2 oxide layer, and the exci-
tation mechanism for this band is caused by the capture
of conduction electrons by surface traps, which result
from the occurrence and dissociation of silanol groups
(Si–O–H) to form a nonbridging oxygen atom in an
excited state. The relaxation of this atom to the ground
state is accompanied by light emission at 650 nm:
2
460
480
500
520
λ, nm
≡Si–O–H + e–
(SiO )* + H
.
(IV)
Fig. 7. Spectrum of (1) a rarefied flame of DCS with O and
2
(2) the photoluminescence spectrum of combustion prod-
.
≡SiO + H + hν(~1.9 eV).
ucts in the gas phase 0.5 s after completion of combustion:
(1) 22% DCS + O + 6% SF , 10 torr; 300 K; initiated igni-
2
6
tion; (2) the same conditions; a DRGS-12 mercury–helium
This fact indicates that the concentration of the Si–
O–H groups on the surface of SiO2 can be monitored by
the band intensity at 650 nm [43]. This conclusion is in
agreement with the findings of Ratnov et al. [44], who
found by IR spectrophotometry that the SiO2 films
obtained by DCS oxidation contain hydrogen atoms
and hydroxyl groups in significantly smaller amounts
than in the case of monosilane oxidation. The broad
emission band near 440 nm (Figs. 1 and 3) can be
assigned to the intrinsic defect of quartz, a two-coordi-
nated silicon atom in the =Si: radical [29–31, 43]. Note
that, in a study of the photoluminescence of strongly
oxidized films of porous silicon stored in air, a weakly
pronounced structure with the maxima at 417, 435, and
465 nm was found in the continuous emission spectrum
in the range 400–500 nm [45]. A similar structure was
observed in both the blue photoluminescence spectrum
of silicon oxide films implanted with silicon [46] and in
the photoluminescence spectrum of nanocrystalline sil-
icon films containing no SiHn, Si–O–H, and SiO groups
(Fig. 6, curve 6) [41]. According to Filippov et al. [45],
surface defects of the Si+ type stabilized not only by
trapped free carriers but also by molecular oxygen or
water can play an important role in the blue photolumi-
nescence spectra. Thus, it is believed that the manifes-
tation of structure peculiarities in the emission spectra
of rarefied flames during oxidation of monosilane and
DCS in the presence of SF6 additives at 432, 457, 481,
and 513 nm (Fig. 6, curves 3 and 4) is due to the appear-
ance of Si+ impurity ions in the SiO2 aerosol formed.
The above spectra are similar to the electrolumines-
cence spectra of quartz glass doped with terbium
(Fig. 6, curve 5) [29]. The presence of defects in the
structure of SiO2 formed upon the combustion of
lamp as a source of excitation.
.
emission of the radical (SiH2 )* (A1B1–X1A1): (020)–
(000), (030)–(010), and (020)–(010) transitions, which
correspond to the bands at 580, 589, and 614 nm,
respectively [33, 39]. In both cases, the absorption band
near 413 nm was assigned to the emission of the radical
(SiH )*(A1è–X1Σ+) [42]. According to calculated data
[18], the bands at 432, 467, 486, and 513 nm (see
Fig. 6) were attributed to the emission of the electroni-
cally excited species H2SiO*(A1A'–X1A1) [33].
.
However, according to Figs. 4 and 5, the experimen-
tally identified emission in the visible region is due to
phase formation in the course of combustion. Figure 7
presents the emission spectrum of a rarefied flame of
DCS with O2 (curve 1) and the photoluminescence
spectrum (curve 2) of a SiO2 aerosol formed in the
course of combustion (a DRGS-12 mercury–helium
lamp as a probing radiation source; an OSA-500 optical
spectrum analyzer; a mixture of 22% DCS + O2 + 6%
SF6; P = 10 torr; T = 300 K; static conditions; initiated
ignition), which was measured 0.5 s after the comple-
tion of combustion. Note that, 3 s after the completion
of combustion, the intensity of this photoluminescence
spectrum sharply decreased and became comparable
with a noise level of OSA-500. Figure 7 presents the
photoluminescence spectrum of a SiO2 aerosol in the
range 450–520 nm, where the effect of the scattered
emission lines of the DRGS-12 lamp was minimal. The
accordance between the spectra (Fig. 7, curves 1 and 2)
confirms the fact that emission observed in the visible
region is due to radical luminescence on the formation monosilane in a threefold excess of oxygen was found
KINETICS AND CATALYSIS Vol. 43 No. 4 2002

FLAME EMISSION SPECTRA IN THE REGION 400–600 nm
451
.
[47]; the paramagnetic centers ≡Si and centers as 17. Vartanyan, A.A., Cand. Sci. (Chem.) Dissertation, Cher-
nogolovka: Inst. of Chemical Physics, 1991.
anionic vacancies with trapped electrons were detected
by ESR spectroscopy. Based on published data and our
experimental results, we can conclude that emission in
the visible region observed during the combustion of
monosilane and DCS is due to adsorption luminescence
and radical-recombination luminescence on the surface
defects of a dispersed SiO2 aerosol formed. The fine
structure of this spectrum (Fig. 7, curves 1 and 2) and
the character of its time decay depend on local crystal
fields, which depend on the surrounding of the Si+ atom
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23. Chung, S.L., Tsai, M.S., and Lin, H.D., Combust. Flame,
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ACKNOWLEDGMENTS
This work was supported by the Russian Foundation
for Basic Research (project nos. 02-03-32993 and
00-03-32979).
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