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Preparation of SiO2 overlayers on oxide substrates by chemical vapor deposition of Si(OC2H5)4
223
Applied Surface Science 29 (1987) 223-241 North-Holland, Amsterdam
PREPARATION BY CHEMICAL T. OKUHARA Department Received
OF SiO, OVERLAYERS ON OXIDE SUBSTRATES VAPOR DEPOSITION OF Si(OC,H,), * and J.M. WHITE
of Chemistry,
27 March
University of Texas, Austin,
1987; accepted
for publication
TX 78712, USA
23 May 1987
The deposition of SiO, onto ZrO, , TiO, , MgO, SiO, and Al IO3 by chemical vapor deposition (CVD) of Si(OC,H,), was studied by temperature programmed desorption (TPD) and Auger electron spectroscopy (AES). TPD showed that Si(OC,H,),, adsorbed at 300 K, decomposed on ZrO,, TiOz, MgO and Al,O, to give ethene and H,O during the TPD. SiOz did not adsorb Si(OC*H,), at 300 K. The decomposition of Si(OC,H,), on ZrO, decreased as the amount of SiO, grew. The Zr AES signals attenuated strongly while that of Si increased. We conclude that a SiO, thin film about 10 A thick forms on ZrO,. This thin film was stable in vacuum up to 723 K but, at 823 K, either rearranged into small clusters or formed a solid solution with the surface of ZrO,. The decomposition activity of TiOz for Si(OC,H,), did not decrease strongly as SiO, deposition proceeded. This is accounted for if SiO, clusters are produced leaving TiO, sites exposed. Over both ZrO, and TiO,, adsorption of CO2 was suppressed by the deposition of SiO,. There is no significant carbon buildup during deposition on ZrO, but on all the other oxides its accumulation is important.
1. Introduction Metal and metal oxide thin films are important in various fields [l]. For example, SiO, thin films are used for antireflection coatings and thermal stabilization of solar cells [2,3]. Silica is also used to protect pigments (including TiO,) against photodegradation and metals against corrosion. In microelectronic devices, silica thin films are also important constituents [l]. Films of chemically active oxides, such as TiO, and V,O,, have been used as photoanodes [4], semiconductive materials, sensors, and catalysts [5-81. Two main methods are used for preparing the oxide thin films: (1) impregnation or ion exchange using solutions (wet method) and (2) chemical vapor deposition (CVD) (dry method). There are several reports on thin oxide films prepared from metal alkoxides. Pettit and Brinker [3] prepared doublelayer thin films consisting of SiO, and TiO, on Si from ethanol-water solutions of ethoxides and showed that these coatings enhanced the efficiency * Permanent address: Tokyo 113, Japan.
Department
of Chemistry,
Faculty
of Engineering,
0169-4332/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
University
of Tokyo,
224
T. Okuharu, J. M. White / .SiO? on oxides by CVD of Si(OC, H5)Q
of Si-based solar c.ells. Carlson and Griffin [7] studied the photoxidation of methanol using a monolayer of V,O, on TiO, prepared by impregnation from an aqueous solution of V(OC,H,),. In CVD using metal alkoxides, Takahashi et al. [9] observed that when TiO, was deposited on glass from Ti(OC3H7)4, its crystal structure changed depending on the deposition rate. Shimogaki and Komiyama [lo] obtained TiO, films (SO-100 pm thick) having a surface area of 30-50 m2 gg’ from Ti(OC,H,),. Oxide thin film catalysts have also been obtained by CVD of alkoxides. Iwasawa and coworkers [8] reported a unique selectivity in dehydration of ethanol over Nb,O, thin films prepared from Nb(OC,H,), on SiO,. Niwa et al. [ll] showed that the pore size of ZSM-5 zeolite could be controlled by the deposition of SiO, from Si(OCH,),. In addition. V,O, on glass prepared from vanadium naphthenate [12], V,O, on sapphire from (C,H,O,),V [13], V,O, on TiO, from V,O, gas [6] and ZnO on Ag by sputtering of ZnO [14] have been reported. Interactions between oxide overlayers and bulk oxides have been reported. Roozeboom et al. [6] showed, using Raman spectroscopy, that a V,O, film reacted with bulk oxides such as ZrO, and CeO, to form solid solutions at temperatures above 873 K. Even with this work, the processes involved in the formation of oxide thin films, the thermal stability of these thin films and their chemical properties are still poorly understood. We have undertaken, using surface science methods, to study these systems. In this paper we report our first work on the deposition of SiO, on various oxides - ZrO,, TiO,, Al 203, SiO, and MgO. The SiO, was prepared by CVD of Si(OC2HS), under the controlled conditions of dosing, temperature and ambient gases that are realized in a UHV system. Dosing in UHV provides for easy and controlled variation of the film thickness. Adsorption and decomposition processes were followed by TPD (temperature programmed desorption) and Auger electron spectroscopy (AES).
2. Experimental The UHV system for TPD and AES was similar to that described elsewhere [15,16]. This system consisted of two chambers, one for dosing and reaction and the other for TPD and AES analysis. A differentially pumped and sealed rod was used to transport the sample between the chambers. The base pressures of both chambers were less than 5 X 10u9 Torr. Tetraethoxysilane (Si(OC,H,),) was introduced only into the dosing chamber. TiO, and Al,O, were purchased from Degussa; Al,O, (aluminum oxide C) had a surface area of 100 m2 gg ’ and TiO, (titanium oxide P 25, 80% anatase and 20% rutile) had a surface area of 50 m* g- ‘. MgO (30 m2 g-i) was
T. Okuhara,J.M. White/ SO, on oxides by CVD ofSi(OC2H&
225
obtained from Fischer Scientific. SrO, was obtained by calcination at 873 K of Zr(OH), prepared from zirconium nitrate, and had a surface area of 50 m2 (silica gel, M-5) and had a surface g -I. SiO, was purchased from Cab-0-Sil area of 200 m2 g-i. The samples were prepared by pressing the oxides (8-40 mg) onto a very transparent Ta mesh (1 cm2) which had been spotwelded to a Ta wire that served to mount the sample on the transfer rod and to provide for resistive heating of the Ta mesh (and the oxide) [15]. The temperature was monitored by a chromel-alumel thermocouple spotwelded to the Ta mesh. Samples were cooled to as low as 220 K by conduction to a liquid nitrogen reservoir, and were heated to as high as 800 K. Si(OC,H,), was obtained from Alfa Products (purity = 99.9999% on a metal basis), and was used without further purification. The gas was introduced into the dosing chamber through a re-entrant Pyrex tube that passed through the vacuum chamber wall using a glass-to-metal seal. The end of this tube was located about 5 mm from the plane of the sample. The liquid alkoxide source was contained in an external Pyrex tube separated from the UHV system by a greaseless stopcock. Dosing pressures were in the range of 1.5 x lop6 to 6 x 10e6 Torr. The substrate oxides were pre-evacuated in the chamber at 800 K until the desorbing gases decreased to background levels. In the case of Al,O,, treatment with 0, (1 X lop4 Torr, 773 K for 0.5 h) to remove carbon was necessary. The samples just after this pretreatment are referred to as fresh samples hereafter. Typical TPD experiments were done as follows: (1) After the samples were pretreated, Si(OC,H,), was dosed at 300 K at 6 X lop6 Torr for 1 min. (2) The sample was transferred to the analysis chamber and heated from 300 to 800 K (in some cases, 220 to 800 K) using a temperature ramp of 2 K s-l. (3) A second dose was given (6 X 10e6 Torr) at 673 K followed by cooling to 300 K in vacua and redosing at 300 K (6 X 10e6 Torr). (4) After transfer to the analysis chamber TPD was performed. The desorption of Si(OC,H,), was monitored by the masses, m/e = 208 (parent), 163 and 119. For possible decomposition products, the following were also monitored in TPD: m/e = 27 (for ethene), 18 (for H,O), 41 (for propene), 74 (for diethylether) and, in some cases, 46 (for ethanol) and 29 and 44 (for acetaldehyde). In some cases H,O and CO, adsorption-desorption were studied. H,O was introduced to the sample at 300 K and 1 X 10d6 Torr for 3 min. CO, TPD was carried out after CO, (10m5 Torr) was dosed at 250 K for 1 min. AES was performed using a single-pass cylindrical mirror analyzer with a 2 kV primary beam energy, a beam current of 15 PA, and a spot size of about 1 mm diameter [15]. The AES spectra were taken at high temperature (between 473 and 673 K) in order to minimize sample charging.
226
i? Okuhara, J.M.
white / SO,
on oxides by CVD of Si(OC, H,),
Blank tests without any dose of Si(OC,H,), to each of the oxide substrates showed no desorption of gases identified below as decomposition products (C,H, and H,O). In further blank tests with Ta mesh (no oxide), the amount of desorption and decomposition of Si(OC,H,), was negligible under the dosing and TPD conditions used in our experiments.
3. Results 3.1. TPD of Si(OC, H5)4 Mass spectra of the gas-phase Si(OC,H,), showed the following peaks: m/e = 208 (12) 193 (72) , 179 (27) 163 (47), 149 (100) 135 (26) 119 (61) 107 (25) and 79 (73) where the figures in the parentheses are relative signal intensities. Three TPD signals, m/e = 208, 163 and 119 were monitored in experiments on each oxide. Since the intensities of these three signals always maintained constant ratios during TPD from the various oxide surfaces, they always monitor desorption of the parent molecule and only the m/e = 163 signal is shown here. For the gas-phase parent molecule, the relative signal intensities for m/e = 18 (6) 27 (12) and 28 (25) were small. In TPD
Temperature/K
1. TPD of 300 K and
(a) Fresh
40 sample (after Si(OC,H,),
ZrO,. at
The
was 6 x 10Wh of Si(OC,H,),, at 773 for 1 h). (b) After K for 0.5 h.
to
T. Okuhara, J.M. White / SO, on oxides by CVD of Si(OC,H,),
227
experiments they did not follow the same profile as the parent molecule. Thus, we can clearly distinguish ethene and water from the parent molecules. Fig. 1 shows the TPD after dosing Si(OC,H,), (6 X lop6 Torr, 1 min, and 300 K) onto ZrO,. The desorbing decomposition products are mainly ethene (m/e = 27) and H,O (fig. la). Desorption of ethene started at about 500 K and peaked near 580 K. The m/e = 28 signal (parent peak of ethene) had the same profile as m/e = 27. Hz0 (m/e = 18) increased above 750 K and continued to desorb above 800 K (beyond the present experiments). The m/e = 163 signal was weak in this temperature range. No desorption of (C,H,),O (m/e = 74) and C3H, (m/e = 41) was observed. As shown in fig. lb, the TPD profile was quite different after ZrO, was exposed to Si(OC,H,)4 at 673 K and then cooled to 300 K and exposed to further Si(OC2H,),; the amount of ethene desorbed was much smaller and the peak temperature shifted to above 670 K. The desorption of Hz0 also decreased. To test for the chemisorption of ethene on fresh ZrO,, ethene (5 x lob6 Torr, 250 K and 1 min) was dosed. In subsequent TPD starting at 2.50 K, the desorption of ethene was less than l/50 that from Si(OC,H,), on fresh ZrO, (fig. la). After the TPD of ethene, carbon was not detected on ZrO, by AES. We conclude that ethene does not chemisorb on fresh zirconia to a measurable extent and that the appearance of ethene in the TPD of Si(OC,H,), is the result of reaction-limited desorption. The TPD of Hz0 from fresh ZrO, will be described in the next section. Fig. 2 shows the AES spectra of fresh ZrO,, after dosing at 300 K and TPD and after dosing at 673 K and TPD. The TPD and dosing conditions are the same as for fig. 1. The signals at 92, 113, 124 and 145 eV correspond to Zr and the signal at 510 eV corresponds to oxygen [17]. On fresh ZrO,, no carbon peak (272 eV) was observed (fig. la). After the first TPD (fig. 2b), a Si peak appeared (81 ev). The Si/O peak-to-peak height ratio was 0.05. This ratio increased to 0.38 upon the exposure to Si(OC,H,), at 673 K (fig. 2~). Significantly, no carbon peak was detectable even after the high temperature exposure to Si(OC,HS),. The TPD profile of Si(OC,HS), adsorbed on TiOz is shown in fig. 3. As for zirconia, the main desorbing gases were C,H, and H,O. In the spectra from fresh TiO, (fig. 3a), the peak temperature for ethene was 650 K, about 70 K higher than for fresh ZrO,. In contrast, the desorption of H,O became significant at lower temperatures than for zirconia and peaked at 780 K. Small peaks for m/e = 163 were observed between 600 and 700 K. TPD spectra after exposure to Si(OC,H,), at 673 K are shown in fig. 3b. Unlike the case of ZrO,, neither the intensities nor the peak positions were much different from those of fresh Tiq. Auger spectra of TiO, before and after the TPD are given in fig. 4. The Ti
228
T. Okuhara, J.M. White / SIO, on oxides by CVD of Si(OC,H,),
81: Kinetic
energylev
Fig. 2. Auger spectra of (a) fresh ZrO, (no dose), (b) after exposure to 6x 10mh Torr of Si(OC,H,), at 300 K for 1 min, and (c) after exposure lo Si(OC,Hs), at 673 K for 0.5 h. The O(KW) peak shape did not change upon the SiO, desorption.
z
C % E .5 Ei ‘Z
200 100
400
2 r”
300
-
200 loo-
300
400
500
600
700
8;O
Temperature/K
Fig. 3. TPD of Si(OC,H,), from 13 mg of TiO,. The dose was 6~10~~ Torr of Si(OC,H,),, 300 K and 1 min. (a) Fresh sample (after evacuation at 773 K for 1 h). (b) After exposure to Si(OC,H,), at 673 K for 0.5 h. The Ti and 0 AES peak shapes did not change upon SiO, deposition.
T. Okuhara, J.M. White / SO,
on oxides by CVD ofSi(OC2H5)4
)ir
a Si
w ?
/
229
.>
425
G
2
517 0 90 Kinetic
energylev
Fig. 4. Auger spectra of (a) fresh TiOz, (b) after a dose of 6 for 1 min, and (c) after exposure to Si(OC,H,)4
Si(OC,H,),
at 300 K
a
b
Temperature/K
Fig. 5. TPD of Si(OC,H,)4 from SiO, (a), Al,O, (b), and MgO (c). The sample weights were 8, 8, and 23 mg for SiO*, Al,O,, and MgO, respectively. TPD conditions: 6 X 10m6 Torr, 300 K and 1 min.
230
T. Okuhara, J.M. White / SO,
on oxides by CVD ofSi(OC,H,),
AES peaks appeared at 389 and 425 eV and O(KW) was at 517 eV. All of these are at higher energies than expected [17], indicating a small amount of surface charging. After the first TPD (fig. 4b), the ratio of Si (91 eV) to O(KW) was 0.12. After the higher temperature treatment with Si(OC,H,),, the Si/O AES ratio increased to 0.3 (fig. 4b). While the AES carbon peak was not detected on the fresh TiO, (fig. 4a), it was significant after the exposure to Si(OC,H,), at 673 K for 0.5 h (fig. 4~). Fig. 5 shows the results of TPD of Si(OC,HS), from SiO*, Al,O, and MgO after dosing at 300 K on a fresh sample. In the case of SiO, (fig. 5a), three peaks of Si(OC2H,), were observed at about 380, 580 and 640 K. The m/e = 27 signal also showed three peaks. These were small and, unlike the data of figs. 1 and 3, tracked the parent peaks. We conclude that there is no evidence for decomposition of Si(OC,H,), on SiO,. On Al,O,, there is a broad peak (450 < T < 750 K) of ethene having a maximum at 580 K (fig. 5b). The TPD profile from MgO is given in fig. 5c. Only ethene and H,O were formed. The C/O AES ratios were 0.024 and 0.08 for MgO and Al 2O, after the first TPD run, while no carbon was observed on SiO,. The Si/O AES ratios were 0.11 and 0.12 for MgO and Al,O,, respectively. During the measurement of AES for Al,O,, both the Si and the Al AES signals decreased and new peaks appeared at slightly higher kinetic energies. This change is due to reduction of SiO, and Al,O, to Si and Al by electron induced decomposition and will be described in detail elsewhere [18]. 3.2. Effect of preadsorbed
H,O on ZrO,
The effect of preadsorbed H,O on the TPD profile of Si(OC,H,), adsorbed on ZrO, is shown in fig. 6. When only H,O (1 x 10P6 Torr, 3 min) was dosed at 300 K, TPD showed two strong peaks at 540 and 610 K (fig. 6a). When Si(OC,H,), was dosed at 300 K onto ZrO, after H,O (1 X lop6 Torr, 3 min) was predosed at 300 K, two sharp peaks were also observed as shown in fig. 6b. The desorption amount of water was nearly equal to that in fig. 6a. The shape of the ethene peak was similar to that from fresh ZrO,, and the desorption amount of ethene was about 2/3 that from fresh ZrO,. AES showed no carbon deposition after the TPD from coadsorption of H,O and Si(OC,H,),. We conclude that preadsorbed water does not strongly alter the adsorption-desorption behavior of Si(OC, H, ) 4 on zirconia. 3.3. SiO, deposition on oxides Figs. 7 and 8 show how the Si/O, Zr/O and Ti/O Auger peak ratios change with dosing time at different temperatures (300 and 673 K) over ZrO, and TiO,. At the lower temperature, Si(OC,H,), was added at 300 K, and
T. Okuhara, J.M. White / SiO, on oxides by CVD of Si(OC,H,),
231
a
Temperature/K
Fig. 6. Effect of preadsorbed water on the TPD of Si(OC,HS)4 from 15 mg of ZrO,. (a) TPD of water (10e6 Torr, 3 min at 300 K). (b) TPD of Si(OC,H,)4 (6 X 10V6 Torr, 300 K, 1 xnin) after water (low6 Torr, 3 min at 300 K) was predosed.
TPD was performed from 300 to 800 K. Then AES spectra were taken at 573 K, this temperature selected to minimize charging. This procedure was repeated to generate the series of points in figs. 7 and 8. When the gas was introduced at 673 K, the sample was flash heated to 800 K and then AES spectra were taken at 573 K. The dosing pressure was 6 X 1O-6 Torr for both temperatures. As shown in fig. 7, the Si/O ratio increased rapidly as Si(OC,H,), was dosed on Zrq. It became constant after about 10 min (0.45 at 673 and 0.25 at
0+--e-%Timefmin
Fig. 7. Change of Si/O and Zr/O Auger peak ratio during dosing of Si(OC2H,), on Zr02: (0) Si/O at 673 K, (0) Si/O at 300 K (after TPD), (0) Zr/O at 673 K, (m) Zr/O at 300 K (after TPD).
232
T. Okuhara, J.M. White / SO,
on oxides by CVD
of Si(OC,H,),
15 Timelmin
Fig. 8. Change of Si/O and Ti/O Auger peak ratio during Si/O at 673 K, (0) Si/O at 300 K (after TPD), (0) Ti/O TPD).
dosing of Si(OC,H,), at 673 K, (m) Ti/O
on TiO,: (0) at 300 K (after
300 K). The rate of the SiO, deposition was higher at 673 K than at 300 K. The Zr/O ratio decreased as the Si/O ratio increased, and reached about 0.12 after a 30 min dose at 673 K. For TiO, (fig. 8), the increase of the Si/O ratio with time was faster at 300 K than at 673 K. The saturation Si/O ratios were 0.45 and 0.33 at 300 and 673 K and were reached after a 30 min dose. The Ti/O ratio decreased from 0.67 to about 0.50 (673 K) and 0.40 (300 K). The C/O AES ratio was less than 0.01 on ZrO, even after a 30 min dose at both temperatures. On the other hand, the C/O ratio on TiO, increased rapidly with the dose time (fig. 9). It reached 0.17 after 30 min at 300 K. The shape of the C(KVV) peak on TiO, resembled graphite, not carbide [19].
Timelmin
Fig. 9. Carbon
deposition
during the dosing on ZrO, and TiO,: (0) 673 K on ZrO,, ZrO, , (H) 673 K on TiO, , (0) 300 K on TiO,
(0)
300 K on
T. Okuhara, J.&f. White / SiO, on oxides by CVD of Si(OC, H,),
.. z 0
‘.._
. .._
0
0
5
Si/(Si+M+O)
10 x100/%
15
.20 in atom(M=Zr
25
233
SiOp .x_ 1
30
=35
or Ti)
Fig. 10. Change in activity for decomposition of Si(OC,H,), with the amount of 50, and TiO,. ZrO, at 673 K (0) and at 300 K (after TPD) (0), and TiO, at 673 K (m) and at 300 K (after TPD) (0). The desorption of ethene goes to zero for pure SiO, (0).
The amount of ethene in the TPD of Si(OC,H,), versus the content of Si measured by AES is shown in fig. 10. The open symbols denote 300 K exposures followed by TPD to 800 K. The closed symbols denote exposures at 673 K, flash to 800 K, cool to 300 K, dose Si(OC,H,), at 6.0 X lop6 Torr for 1 min, and then TPD. As the Si content increases, the desorption of ethene from ZrO, decreases monotonically and extrapolates to zero at a composition characteristic of pure SiO,. For TiO, it is nearly independent of the amount of Si. Assuming the ethene in the TPD monitors decomposition, fig. 10 indicates that the activity of TiO, does not change strongly as the SiO, overlayer forms, but that of ZrO, decreases. As shown in fig. 11, the Ti/O and Zr/O atomic ratios (normalized to 0.5 for fresh samples) decreased in different manners with the SiOz content. The Zr/O atomic ratio decreased monotonically as the content of Si increased but the Ti/O atomic ratio decreased slowly at first but became more rapid at higher Si. Fig. 12 shows the effect of SiO, deposition on the TPD spectra of CO,. A blank test without dosing showed negligible CO2 desorption. TPD of CO, from fresh TiO, and ZrO, yielded broad peaks of CO, (figs. 12a and 12b). When SiO, was deposited on ZrO, from Si(OC,H,), (Si/O = 0.40) the desorption of CO* was much lower than from fresh ZrO, (fig. 12a). In the case of TiO,, SiO, deposition also suppressed the adsorption of CO, (fig. 12b). Note that no clear CO, desorption peak was observed from bulk SiO, (shown by broken line in fig. 12a). Fig. 13 shows one measure of the thermal stability of the SiO, overlayers on ZrO, and TiO,. In this experiment, SiO, was deposited on ZrO, as follows: Si(OC,H,), (6 X lop6 Torr, 10 min) was introduced three times at 300 K. For TiO,, Si(OC,H,), (6 X 10e6 Tot-r, 10 min) was dosed three times at 300 K. In
234
T. Okuhara, J.M.
White / SiO, on oxides by CVD of Si(OC2 H5)4
0.1 0
z1
0 L’ 0
5
10
Si/(Si+M+O]
20
15
x100/%
25
5
30
in atom(M=Zr
or TiI
Fig. 11. Change in the Zr/O and Ti/O atomic ratios with the amount of SiO, deposited: ZrO, at 673 K (0) and at 300 K (after TPD) (0) and TiO, at 673 K (m) and at 300 K (after TPD) (III).
both cases, the samples were then flash heated to 773 K and cooled. The data of fig. 13 was then generated. The samples were evacuated for 15 min at each temperature and AES spectra were taken at 473 K. There is a significant difference between ZrO, and TiO,. The Si/O AES ratio did not change for
a
zr02
b TiO2
/’
,1--y
;’ /’
\\
,’
..
r\ 300
>\
,’ \.
,_I’
\\...
‘...__._.,-’
400
500
600
700
fSOIO
Temperature/K
Fig. 12. TPD spectra of CO1 from: (a) ZrO, and (b) TiO,. CO, (lo-’ Torr) was dosed at 250 K for 1 min. () SiO, covered ZrO, (Si/O Auger ratio = 0.42) and SiO, covered TiO, (Si/O = 0.30), (- - - - - -) fresh ZrO, and TiO,, (- - -) fresh bulk SiO,.
T. Okuhara, J.M. white / SO,
on oxides by CVD of Si(OC,H,),
235
o.5------
.o 0.4 -
zL 5 o.+-.-.-.-AA*
302
$
\
9$ 0.2 _L,-,-~-,-=-~a
A02 \
Q O.li?
t 0
I
473
573
673
773
673
Temperature/K
Fig. 13. Thermal stability of SiO, deposited on ZrO, and Ti02. SiO, was deposited on ZrO, by three doses of Si(OC,H,), at at 300 K and three doses at 473 K, each dose at 6 X 10e6 Torr for 10 min. On TiO, the exposure was done three times at 300 K and 10 min, each dose followed by TPD up to 773 K.
TiO, after the evacuation even at 823 K, but it started to decrease at 773 K and decreased abruptly at 823 K for ZrO,. As the Si/O decreased, the Zr/O increased. Deposition of SiO, by continuous exposure of Si(OC,H,), (673 K, 1.5 X 10m6 Torr) is shown in fig. 14. The order of the rate of SiO, deposition was: ZrO, > MgO = TiO, > Al,O,. For ZrO,, the Si/O ratio saturated at 0.4 after 2 h under these conditions. For the other substrates, the Si/O ratio did not saturate. The deposition of carbon was also quite different: the C/O increased rapidly on TiO,, Al,O, and MgO (after 4 h, the ratios were 0.07, 0.05 and 0.03
Dose
Time/h
Fig. 14. SiO, deposition on various oxides by continuous dosing of Si(OC2H,),. The Si(O&H,), was introduced at 673 K and 1.5 X 10e6 Torr. The weights of the samples were 40, 16, 28 and 8 mg for ZrO,, TiO,, MgO, and Al,O,, respectively.
236
T. Okuhara, J.M.
White / SO,
for TiO,, Al 2O,, and MgO, 0.005 throughout the dosing.
on oxides by CVD of Si(OC,H,),
respectively).
On ZrO,,
C/O
remained
below
4. Discussion 4.1. Decomposition
of Si(OC, H_5)4
The anchoring of various silanes on SiO, surfaces is well known [11,20-221. Hertl [22] reported that Si(CH,)(OCH,), adsorbed physically on SiO, and that no decomposition took place at room temperature, However, he found that above 363 K, the following reaction occurred: + HOSi + CH,(OCH,),Si(OSi)
(CH,O),Si(CH,)
+ CH,OH,
where HO-Si is an isolated non-hydrogen bonded hydroxyl group. Boucher et al. [21,23] compared the relative reactivity of ligands, X in (C,H,),P(CH,), Six, with OH groups on SiO, at 433 K. They found the anchoring reactivity was ordered as follows for X: Cl > OCOCH, > OCH, > OC,H,. From these findings, we expect, as observed in fig. 5, only a very weak interaction between Si(OC,H,), and SiO, surfaces and at temperatures below 300 K, little decomposition. Even if Si(OC,H,), is adsorbed physically, it is removed by evacuation at 300 K before the TPD. These conclusions are confirmed by the appearance of a m/e = 163 TPD peak at 290 K after Si(OC,H,), is dosed on SiO, at 220K. Decomposition on the other oxides was more extensive (figs. 1, 3 and 5). These oxides have acidic or basic sites [24], and we suppose that these are active for the adsorption and the decomposition of Si(OC,H,),. Imizu et al. [25] examined the interaction between Si(OCH,),(CH,),_. and MgO by IR and observed a change from molecular adsorption at room temperature to dissociation to MgO-Si-R, and methoxyl at 573 K. Our results are also consistent with mainly molecular Si(OC,H,), adsorption at 300 K on all these oxides. The following are proposed as decomposition reactions: Si(OC,H,),
+ M-O(H)-M
-+ M-O-Si(H)(OC,H,)3
+ M-O-M
+ M-0-Si(OC,H,),
+ M-OC,H,,
(Ia)
or Si(OC,H,),
+ M-OC,H,,
(lb)
M-OC,H,
--) C,H,(g)
+ M-OH,
(2)
2 M-OH
--$ M-O-M
+ H,O(g),
(3)
M-0-Si(H)(OC,H,),
--z M-0-(H)Si(OH),
M-0-(H)Si(OH),
---) M-OSiO,
+ 3 C,H,(g),
+ 2 H,O(g).
(4)
T. Okuhara, J.M.
White / SO,
on oxides by CVD of Si(OC,H,),
237
Reaction (la) is well known [11,22]. In (la), the position of (H) in the products is unknown and arbitrarily located at Si but it is not important here. Reaction (lb) involves the metal cations on the oxide surface and will play an important role in our work because the number of OH groups on oxide surfaces decreases strongly with evacuation temperature [20]. After evacuation at 773 K we expect only 3.5, 2 and 0.5 OH nm-’ for Al,O,, SiO, and TiO,, respectively. Furthermore, IR studies of adsorbed pyridine show that the acid sites on Al,O, [24], ZrO, [26], and TiO, [27] are mostly of the Lewis type (M”+). Therefore, in addition to the OH groups, the surface metal cations should be considered as decomposition sites. The fact that ethene desorbed at lower temperatures than H,O means that processes (l), (2) and (3) take place sequentially during the TPD; otherwise, we would expect these two products to desorb coincidentally. Arai et al. [28] reported that ethoxyl groups start to decompose to ethene at 408 K and are completely converted after heating at 483 K. The TPD results of fig. 5b are consistent with this. In separate experiments, we found that molecular ethene does not adsorb significantly on ZrO, and that dosed H,O desorbs at lower temperature (< 650 K) than that appearing in the TPD of Si(OC,H,),. This result indicates that the reactions forming C,H, and H,O limit their appearance in the desorption spectra. Readsorption processes are not important. As a possible candidate reaction path for the formation of SiO,, eq. (4) is proposed, where ethene and H,O form around Si sites. If the hydrolysis of Si(OC,H,), took place at 300 K, the desorption of C,H,OH should be observed as for C,H,OH dosed on Al,O, [28]. Since no C,H,OH was observed and since H,O did not affect C2H, desorption (fig. 6) we conclude that water is inactive on ZrO, at 300 K. It is well known that liquid-phase hydrolysis of Si(OC,H,), at room temperature must be catalyzed by ions such as H+ and OH- [29]. Thus, in the case of ZrO,, the acidity or basicity is probably too weak to catalyze the reaction at 300 K. One very clear outcome of these experiments is the difference in the behavior of the oxides. On fresh oxides, the temperature of C,H, desorption was lowest on ZrO,, implying that ZrO, surfaces are very active for decomposition. Although the acidic and basic sites on ZrO, are weak, both are present [30]. This may be important for the decomposition. In this light it is noteworthy that ZrO, is more active than A1203 for isomerization of butene and shows a unique selectivity to 1-butene in dehydration of set-butanol [31]. Another important feature of the decomposition over ZrO, is the small amount of carbon deposition (fig. 9). This may be due to the absence of strong acid sites [24] which leads to carbon deposition from hydrocarbons. Henderson et al. reported that carbon deposition on ZrO, was quite small in the TPD of trinitrotoluene [15]. Even though the MgO surfaces are dominated by basic sites, the decomposition products are also ethene and water. Tanabe showed that mixing SiO,
238
T. Okuhara, J.M.
White / SiO, on oxldes by CVD of Si(OC2H,),
and MgO generated acid sites [24]. Moreover, Imizu et al. [25] observed the generation of acid sites (by IR of pyridine adsorption) when SiO, was deposited on MgO from Si(OCH,),(CH,),_.. In our case, acid sites generated by the decomposition of SiO, on MgO are probably active; the decomposition process on a pure MgO surface is still not clear. Finally, there is an interesting difference in the variation of the activity with the dose of Si(OC,H,), when ZrO, and TiO, are compared (fig. 10). This difference is attributed to structural variations of the SiO, and is discussed in the next section. 4.2. Structure
of SiOz overlayers on ZrOz and TiOz
SiO,, SiO and Si can be distinguished by the kinetic energy of Auger electrons [32]. In quartz, the O(KW) AES peak is at 507 eV and the Si AES peak is at 79 eV [33]. The Si transitions for SiO and Si are at 83 and 88 eV [32]. As shown in figs. 2 and 3, there is some charging; the O(KW) transition is at 510 eV for ZrO, and 517 eV for TiO,. Taking this into account the Si peaks for the oxides prepared on ZrO, and TiO, will be at 78 eV, indicating that Si exists as SiO,. We conclude that the silicon deposited by dosing Si(OC,H,), is fully oxidized during the decomposition process. We turn now to the morphology of the SiO,. 4.2.1. SiOz on ZrO, The facts that (i) the decomposition activity decreased monotonically as the Si content increased (fig. 10) and (ii) the Zr/O atomic ratio measured by AES decreased linearly as the Si content increased (fig. 11) lead to the conclusion that the SiO, film develops as a fairly uniform overlayer on zirconia. The amount of Si deposited grew rapidly as the dose time increased (fig. 7) but then the Si/O and Zr/O Auger peak ratios became constant. This means that film growth is suppressed once the surface is covered by silica. This is consistent with our observation that the decomposition of Si(OC,H,), is very inefficient on bulk silica. This model is also consistent with the suppression of CO, adsorption (fig. 12) since bulk silica did not adsorb CO,. Assuming a uniform overlayer, the thickness, x. of the SiO, film can be estimated from I = I, exp( - x/X) [34] using the following relation developed empirically by Seah et al. [35]: X = 2170E-*
+ 0.72( aE)*“,
(5)
where X is the attenuation length (in monolayers), a is the thickness of one monolayer of substrate in nm, and E is the kinetic energy of the Auger electrons in eV. When the thickness of a SiO, monolayer is assumed to be 0.27 nm, and the Zr(145 eV) AES transition [17] is used, the final SiO, layer is about 4 monolayers thick.
T. Okuhara, J.M. White / SiO, on oxides by CVD of Si(OC, H,),
239
This SiO, overlayer on ZrO, is stable up to 723 K (fig. 13) but at 773 K it decomposes probably either to small SiO, clusters or to an SiO,-ZrO, solid solution [36]. The formation of a solid solution by reaction of surface thin films and substrate oxides has been reported for V,O,-CeO, [6]. 4.2.2. SiO, on TiO, The morphology of SiOz on TiO, is apparently quite different from that on ZrO, because there are essential differences in: (i) how the decomposition activity changes with dose of Si(OC,H,), (fig. 10) and (ii) the thermal stability of the resulting oxide (fig. 13). We propose a structural model where SiOz is present as small clusters and/or a mixed metal oxide layer rather than a uniform SiOz film. In this case, TiO, is always present at the surface. This model can account for at least part of the retention of Si(OC,HS), decomposition activity. Based on the work of Tanabe [24], we also suggest that interface regions between SiO, and TiO, exhibit acidity which are active for Si(OC,H,), decomposition. These two effects together can account for the steady decomposition activity with Si(OC,H,), dose. The thin film structure on ZrO, is less stable than the particle structure on TiO,. We conclude this on the basis of the AES changes with annealing temperature (fig. 13). One possible explanation for the decreased CO, adsorption after SiO, deposition on TiO, (fig. 13) is that surface OH groups on TiO,, which induce the strong adsorption of CO, by the formation of bicarbonate species [27], are consumed during the deposition of SiO,. The atomic composition of the outermost layer of these SiO,/ZrO, and SiO,/TiO, systems is not elucidated here. ISS (ion scattering spectroscopy) experiments are now in progress to measure this. It will also be helpful to study planar oxides to complement the work reported here.
5. Summary (1) A reasonably uniform SiOz thin film having a thickness of about 10 A (4 monolayers) was formed on ZrO, by the decomposition of Si(OC,H,), during TPD after dosing at 300 K. (2) This SiO, thin film was stable up to 723 K, but started to decompose at 773 K. (3) On ZrO,, carbon deposition was minimal during the formation of SiO, thin films, while on MgO, TiO, and Al,O, carbon deposition was significant. (4) On TiO,, we propose that small clusters of SiO, and/or a mixed metal oxide layer are formed during the decomposition of Si(OC,H,),. The resulting surface retained its decomposition activity even after large doses of Si(OCzH5)4, an observation we attribute to the continued presence of TiO, at
240
T. Okuhara, J.M. White / SiO, on oxides by CVD of Si(OC2 H5).,
the surface and to acid sites generated at the interfaces between SiO, and TiO, . (5) The structure formed on TiO, is thermally stable up to at least 823 K. (6) The decomposition of Si(OC,H5), on SiO, is negligible.
Acknowledgment The authors wish to thank Dr. Sohail Akhter and Dr. Tuo Jin for many useful discussions. This work was supported in part by the Texas Advanced Technology Research Program.
References [l] [2] [3] (41 [5] [6] [7) [8] [9] [lo] [ll] [12] [13] [14] [15] 1161 [17] [18] [19] [20] 1211 [22] 1231 [24] [2S] [26] [27] 1281
Chem. Eng. News (August 11, 1986) 22. R.B. Pettit and C.J. Brinker, SPIE 562 (1985) 256. R.B. Pettit, C.J. Brinker and C.S. Ashley, Solar Cells 15 (1985) 267. Y. Takahashi, K. Tsuda and K. Sugiyama, J. Chem. Sot. Faraday I, 77 (1981) 1051. A. Vejux and P. Courtin, .I. Solid State Chem. 63 (1986) 179. F. Roozeboom, M.C. Mittelmeijer-Hazeleger, J.A. MouIijn, J. Medema, V.H.J. de Beer and P.J. Geilings, J. Phys. Chem. 84 (1980) 2783. T. Carlson and G.L. Griffin, J. Phys. Chem. 90 (1986) 5896. K. Asakura, Y. Iwasawa and H. Kuroda, Shokubai (Catalyst) 27 (1985) 371. Y. Takahashi, H. Suzuki and M. Nasu, J. Chem. Sot. Faraday I, 81 (1985) 3117. Y. Shimogaki and H. Komiyama, Chem. Letters (1986) 267. M. Niwa, M. Kato, T. Hattori and Y. Murakami, J. Phys. Chem. 90 (1986) 6233. Y. Kera, H. Imori, S. Kida and K. Degawa, Bull. Chem. Sot. Japan 57 (1984) 2069. L.A. Ryabova, I.A. Serbinov and A.S. Darevsky, J. Electrochem. Sot. 119 (1972) 427. E. Weissa and M. Foman, J. Chem. Sot. Faraday I, 82 (1986) 2025. M. Henderson, T. Jin and J.M. White, Appl. Surface Sci. 27 (1986) 127. D.D. Beck and J.M. White, J. Phys. Chem. 88 (1984) 2764. L. Davis et al., Eds., Handbook of Auger Electron Spectroscopy, 2nd ed. (Physical Electronics Industries, Eden Prairie, MN, 1976). T. Okuhara and J.M. White, submitted. D.W. Goodman, J. Vacuum Sci. Technol. 20 (1982) 522. Y. Iwasawa, in: Tailored Metal Catalysis, Ed. Y. Iwasawa (Reidel, Dordrecht, 1985). L.L. Murrell, in: Advances in Material in Catalysis, Eds. J.J. Burton and R.L. Garten (Academic Press, New York, 1971) p. 235. W. Hertl, J. Phys. Chem. 72 (1968) 1248, 3993. L.J. Boucher, A.A. Oswald and L.L. Murrell, Am. Chem. Sot. Div. Pet. Chem. Prep. 19 (1974) 55. K. Tanabe, in: Catalysis - Science and Technology, Vol. 2, Eds. J.R. Anderson and M. Boudart (Springer, Berlin, 1981) p. 231. Y. Imizu, A. Tada, K. Tanaka and I. Toyashima, Preprint of the meeting of the Catalysis Society of Japan, Tokyo, March 1987. Y. Nakano, T. Izuko, H. Hattori and K. Tanabe, J. Catalysis 57 (1979) 1. M. Primat, P. Pichat and M. Mathieu, J. Phys. Chem. 75 (1971) 1221, 1216. H. Arai, J. Take, Y. Saito and Y. Yoneda, J. Catalysis 9 (1967) 146.
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on oxides by CVD of Si(OC,H,),
241
[29] Bailar, Jr. al., Eds., Inorganic Chemistry New York, [30] K. Solid Acids Bases (Kodansha-Academic Tokyo-New York, [31] T. H. Sasaki K. Tanabe, Letters (1973) [32] C.C. Surface Sci. (1971) 53. B. Carriere B. Lang, Sci. 64 209. [34] Chang, in: of Solid Ed. K. (Plenum, New 1976) p. [35] M.D. and W.A. Surface Interface 1 (1979) [36] V.I. G.M. Matveyev 0.P Medhedlov-Petrossyan, of Sili(Springer, Berlin,
Applied Surface Science 29 (1987) 223-241 North-Holland, Amsterdam
PREPARATION BY CHEMICAL T. OKUHARA Department Received
OF SiO, OVERLAYERS ON OXIDE SUBSTRATES VAPOR DEPOSITION OF Si(OC,H,), * and J.M. WHITE
of Chemistry,
27 March
University of Texas, Austin,
1987; accepted
for publication
TX 78712, USA
23 May 1987
The deposition of SiO, onto ZrO, , TiO, , MgO, SiO, and Al IO3 by chemical vapor deposition (CVD) of Si(OC,H,), was studied by temperature programmed desorption (TPD) and Auger electron spectroscopy (AES). TPD showed that Si(OC,H,),, adsorbed at 300 K, decomposed on ZrO,, TiOz, MgO and Al,O, to give ethene and H,O during the TPD. SiOz did not adsorb Si(OC*H,), at 300 K. The decomposition of Si(OC,H,), on ZrO, decreased as the amount of SiO, grew. The Zr AES signals attenuated strongly while that of Si increased. We conclude that a SiO, thin film about 10 A thick forms on ZrO,. This thin film was stable in vacuum up to 723 K but, at 823 K, either rearranged into small clusters or formed a solid solution with the surface of ZrO,. The decomposition activity of TiOz for Si(OC,H,), did not decrease strongly as SiO, deposition proceeded. This is accounted for if SiO, clusters are produced leaving TiO, sites exposed. Over both ZrO, and TiO,, adsorption of CO2 was suppressed by the deposition of SiO,. There is no significant carbon buildup during deposition on ZrO, but on all the other oxides its accumulation is important.
1. Introduction Metal and metal oxide thin films are important in various fields [l]. For example, SiO, thin films are used for antireflection coatings and thermal stabilization of solar cells [2,3]. Silica is also used to protect pigments (including TiO,) against photodegradation and metals against corrosion. In microelectronic devices, silica thin films are also important constituents [l]. Films of chemically active oxides, such as TiO, and V,O,, have been used as photoanodes [4], semiconductive materials, sensors, and catalysts [5-81. Two main methods are used for preparing the oxide thin films: (1) impregnation or ion exchange using solutions (wet method) and (2) chemical vapor deposition (CVD) (dry method). There are several reports on thin oxide films prepared from metal alkoxides. Pettit and Brinker [3] prepared doublelayer thin films consisting of SiO, and TiO, on Si from ethanol-water solutions of ethoxides and showed that these coatings enhanced the efficiency * Permanent address: Tokyo 113, Japan.
Department
of Chemistry,
Faculty
of Engineering,
0169-4332/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
University
of Tokyo,
224
T. Okuharu, J. M. White / .SiO? on oxides by CVD of Si(OC, H5)Q
of Si-based solar c.ells. Carlson and Griffin [7] studied the photoxidation of methanol using a monolayer of V,O, on TiO, prepared by impregnation from an aqueous solution of V(OC,H,),. In CVD using metal alkoxides, Takahashi et al. [9] observed that when TiO, was deposited on glass from Ti(OC3H7)4, its crystal structure changed depending on the deposition rate. Shimogaki and Komiyama [lo] obtained TiO, films (SO-100 pm thick) having a surface area of 30-50 m2 gg’ from Ti(OC,H,),. Oxide thin film catalysts have also been obtained by CVD of alkoxides. Iwasawa and coworkers [8] reported a unique selectivity in dehydration of ethanol over Nb,O, thin films prepared from Nb(OC,H,), on SiO,. Niwa et al. [ll] showed that the pore size of ZSM-5 zeolite could be controlled by the deposition of SiO, from Si(OCH,),. In addition. V,O, on glass prepared from vanadium naphthenate [12], V,O, on sapphire from (C,H,O,),V [13], V,O, on TiO, from V,O, gas [6] and ZnO on Ag by sputtering of ZnO [14] have been reported. Interactions between oxide overlayers and bulk oxides have been reported. Roozeboom et al. [6] showed, using Raman spectroscopy, that a V,O, film reacted with bulk oxides such as ZrO, and CeO, to form solid solutions at temperatures above 873 K. Even with this work, the processes involved in the formation of oxide thin films, the thermal stability of these thin films and their chemical properties are still poorly understood. We have undertaken, using surface science methods, to study these systems. In this paper we report our first work on the deposition of SiO, on various oxides - ZrO,, TiO,, Al 203, SiO, and MgO. The SiO, was prepared by CVD of Si(OC2HS), under the controlled conditions of dosing, temperature and ambient gases that are realized in a UHV system. Dosing in UHV provides for easy and controlled variation of the film thickness. Adsorption and decomposition processes were followed by TPD (temperature programmed desorption) and Auger electron spectroscopy (AES).
2. Experimental The UHV system for TPD and AES was similar to that described elsewhere [15,16]. This system consisted of two chambers, one for dosing and reaction and the other for TPD and AES analysis. A differentially pumped and sealed rod was used to transport the sample between the chambers. The base pressures of both chambers were less than 5 X 10u9 Torr. Tetraethoxysilane (Si(OC,H,),) was introduced only into the dosing chamber. TiO, and Al,O, were purchased from Degussa; Al,O, (aluminum oxide C) had a surface area of 100 m2 gg ’ and TiO, (titanium oxide P 25, 80% anatase and 20% rutile) had a surface area of 50 m* g- ‘. MgO (30 m2 g-i) was
T. Okuhara,J.M. White/ SO, on oxides by CVD ofSi(OC2H&
225
obtained from Fischer Scientific. SrO, was obtained by calcination at 873 K of Zr(OH), prepared from zirconium nitrate, and had a surface area of 50 m2 (silica gel, M-5) and had a surface g -I. SiO, was purchased from Cab-0-Sil area of 200 m2 g-i. The samples were prepared by pressing the oxides (8-40 mg) onto a very transparent Ta mesh (1 cm2) which had been spotwelded to a Ta wire that served to mount the sample on the transfer rod and to provide for resistive heating of the Ta mesh (and the oxide) [15]. The temperature was monitored by a chromel-alumel thermocouple spotwelded to the Ta mesh. Samples were cooled to as low as 220 K by conduction to a liquid nitrogen reservoir, and were heated to as high as 800 K. Si(OC,H,), was obtained from Alfa Products (purity = 99.9999% on a metal basis), and was used without further purification. The gas was introduced into the dosing chamber through a re-entrant Pyrex tube that passed through the vacuum chamber wall using a glass-to-metal seal. The end of this tube was located about 5 mm from the plane of the sample. The liquid alkoxide source was contained in an external Pyrex tube separated from the UHV system by a greaseless stopcock. Dosing pressures were in the range of 1.5 x lop6 to 6 x 10e6 Torr. The substrate oxides were pre-evacuated in the chamber at 800 K until the desorbing gases decreased to background levels. In the case of Al,O,, treatment with 0, (1 X lop4 Torr, 773 K for 0.5 h) to remove carbon was necessary. The samples just after this pretreatment are referred to as fresh samples hereafter. Typical TPD experiments were done as follows: (1) After the samples were pretreated, Si(OC,H,), was dosed at 300 K at 6 X lop6 Torr for 1 min. (2) The sample was transferred to the analysis chamber and heated from 300 to 800 K (in some cases, 220 to 800 K) using a temperature ramp of 2 K s-l. (3) A second dose was given (6 X 10e6 Torr) at 673 K followed by cooling to 300 K in vacua and redosing at 300 K (6 X 10e6 Torr). (4) After transfer to the analysis chamber TPD was performed. The desorption of Si(OC,H,), was monitored by the masses, m/e = 208 (parent), 163 and 119. For possible decomposition products, the following were also monitored in TPD: m/e = 27 (for ethene), 18 (for H,O), 41 (for propene), 74 (for diethylether) and, in some cases, 46 (for ethanol) and 29 and 44 (for acetaldehyde). In some cases H,O and CO, adsorption-desorption were studied. H,O was introduced to the sample at 300 K and 1 X 10d6 Torr for 3 min. CO, TPD was carried out after CO, (10m5 Torr) was dosed at 250 K for 1 min. AES was performed using a single-pass cylindrical mirror analyzer with a 2 kV primary beam energy, a beam current of 15 PA, and a spot size of about 1 mm diameter [15]. The AES spectra were taken at high temperature (between 473 and 673 K) in order to minimize sample charging.
226
i? Okuhara, J.M.
white / SO,
on oxides by CVD of Si(OC, H,),
Blank tests without any dose of Si(OC,H,), to each of the oxide substrates showed no desorption of gases identified below as decomposition products (C,H, and H,O). In further blank tests with Ta mesh (no oxide), the amount of desorption and decomposition of Si(OC,H,), was negligible under the dosing and TPD conditions used in our experiments.
3. Results 3.1. TPD of Si(OC, H5)4 Mass spectra of the gas-phase Si(OC,H,), showed the following peaks: m/e = 208 (12) 193 (72) , 179 (27) 163 (47), 149 (100) 135 (26) 119 (61) 107 (25) and 79 (73) where the figures in the parentheses are relative signal intensities. Three TPD signals, m/e = 208, 163 and 119 were monitored in experiments on each oxide. Since the intensities of these three signals always maintained constant ratios during TPD from the various oxide surfaces, they always monitor desorption of the parent molecule and only the m/e = 163 signal is shown here. For the gas-phase parent molecule, the relative signal intensities for m/e = 18 (6) 27 (12) and 28 (25) were small. In TPD
Temperature/K
1. TPD of 300 K and
(a) Fresh
40 sample (after Si(OC,H,),
ZrO,. at
The
was 6 x 10Wh of Si(OC,H,),, at 773 for 1 h). (b) After K for 0.5 h.
to
T. Okuhara, J.M. White / SO, on oxides by CVD of Si(OC,H,),
227
experiments they did not follow the same profile as the parent molecule. Thus, we can clearly distinguish ethene and water from the parent molecules. Fig. 1 shows the TPD after dosing Si(OC,H,), (6 X lop6 Torr, 1 min, and 300 K) onto ZrO,. The desorbing decomposition products are mainly ethene (m/e = 27) and H,O (fig. la). Desorption of ethene started at about 500 K and peaked near 580 K. The m/e = 28 signal (parent peak of ethene) had the same profile as m/e = 27. Hz0 (m/e = 18) increased above 750 K and continued to desorb above 800 K (beyond the present experiments). The m/e = 163 signal was weak in this temperature range. No desorption of (C,H,),O (m/e = 74) and C3H, (m/e = 41) was observed. As shown in fig. lb, the TPD profile was quite different after ZrO, was exposed to Si(OC,H,)4 at 673 K and then cooled to 300 K and exposed to further Si(OC2H,),; the amount of ethene desorbed was much smaller and the peak temperature shifted to above 670 K. The desorption of Hz0 also decreased. To test for the chemisorption of ethene on fresh ZrO,, ethene (5 x lob6 Torr, 250 K and 1 min) was dosed. In subsequent TPD starting at 2.50 K, the desorption of ethene was less than l/50 that from Si(OC,H,), on fresh ZrO, (fig. la). After the TPD of ethene, carbon was not detected on ZrO, by AES. We conclude that ethene does not chemisorb on fresh zirconia to a measurable extent and that the appearance of ethene in the TPD of Si(OC,H,), is the result of reaction-limited desorption. The TPD of Hz0 from fresh ZrO, will be described in the next section. Fig. 2 shows the AES spectra of fresh ZrO,, after dosing at 300 K and TPD and after dosing at 673 K and TPD. The TPD and dosing conditions are the same as for fig. 1. The signals at 92, 113, 124 and 145 eV correspond to Zr and the signal at 510 eV corresponds to oxygen [17]. On fresh ZrO,, no carbon peak (272 eV) was observed (fig. la). After the first TPD (fig. 2b), a Si peak appeared (81 ev). The Si/O peak-to-peak height ratio was 0.05. This ratio increased to 0.38 upon the exposure to Si(OC,H,), at 673 K (fig. 2~). Significantly, no carbon peak was detectable even after the high temperature exposure to Si(OC,HS),. The TPD profile of Si(OC,HS), adsorbed on TiOz is shown in fig. 3. As for zirconia, the main desorbing gases were C,H, and H,O. In the spectra from fresh TiO, (fig. 3a), the peak temperature for ethene was 650 K, about 70 K higher than for fresh ZrO,. In contrast, the desorption of H,O became significant at lower temperatures than for zirconia and peaked at 780 K. Small peaks for m/e = 163 were observed between 600 and 700 K. TPD spectra after exposure to Si(OC,H,), at 673 K are shown in fig. 3b. Unlike the case of ZrO,, neither the intensities nor the peak positions were much different from those of fresh Tiq. Auger spectra of TiO, before and after the TPD are given in fig. 4. The Ti
228
T. Okuhara, J.M. White / SIO, on oxides by CVD of Si(OC,H,),
81: Kinetic
energylev
Fig. 2. Auger spectra of (a) fresh ZrO, (no dose), (b) after exposure to 6x 10mh Torr of Si(OC,H,), at 300 K for 1 min, and (c) after exposure lo Si(OC,Hs), at 673 K for 0.5 h. The O(KW) peak shape did not change upon the SiO, desorption.
z
C % E .5 Ei ‘Z
200 100
400
2 r”
300
-
200 loo-
300
400
500
600
700
8;O
Temperature/K
Fig. 3. TPD of Si(OC,H,), from 13 mg of TiO,. The dose was 6~10~~ Torr of Si(OC,H,),, 300 K and 1 min. (a) Fresh sample (after evacuation at 773 K for 1 h). (b) After exposure to Si(OC,H,), at 673 K for 0.5 h. The Ti and 0 AES peak shapes did not change upon SiO, deposition.
T. Okuhara, J.M. White / SO,
on oxides by CVD ofSi(OC2H5)4
)ir
a Si
w ?
/
229
.>
425
G
2
517 0 90 Kinetic
energylev
Fig. 4. Auger spectra of (a) fresh TiOz, (b) after a dose of 6 for 1 min, and (c) after exposure to Si(OC,H,)4
Si(OC,H,),
at 300 K
a
b
Temperature/K
Fig. 5. TPD of Si(OC,H,)4 from SiO, (a), Al,O, (b), and MgO (c). The sample weights were 8, 8, and 23 mg for SiO*, Al,O,, and MgO, respectively. TPD conditions: 6 X 10m6 Torr, 300 K and 1 min.
230
T. Okuhara, J.M. White / SO,
on oxides by CVD ofSi(OC,H,),
AES peaks appeared at 389 and 425 eV and O(KW) was at 517 eV. All of these are at higher energies than expected [17], indicating a small amount of surface charging. After the first TPD (fig. 4b), the ratio of Si (91 eV) to O(KW) was 0.12. After the higher temperature treatment with Si(OC,H,),, the Si/O AES ratio increased to 0.3 (fig. 4b). While the AES carbon peak was not detected on the fresh TiO, (fig. 4a), it was significant after the exposure to Si(OC,H,), at 673 K for 0.5 h (fig. 4~). Fig. 5 shows the results of TPD of Si(OC,HS), from SiO*, Al,O, and MgO after dosing at 300 K on a fresh sample. In the case of SiO, (fig. 5a), three peaks of Si(OC2H,), were observed at about 380, 580 and 640 K. The m/e = 27 signal also showed three peaks. These were small and, unlike the data of figs. 1 and 3, tracked the parent peaks. We conclude that there is no evidence for decomposition of Si(OC,H,), on SiO,. On Al,O,, there is a broad peak (450 < T < 750 K) of ethene having a maximum at 580 K (fig. 5b). The TPD profile from MgO is given in fig. 5c. Only ethene and H,O were formed. The C/O AES ratios were 0.024 and 0.08 for MgO and Al 2O, after the first TPD run, while no carbon was observed on SiO,. The Si/O AES ratios were 0.11 and 0.12 for MgO and Al,O,, respectively. During the measurement of AES for Al,O,, both the Si and the Al AES signals decreased and new peaks appeared at slightly higher kinetic energies. This change is due to reduction of SiO, and Al,O, to Si and Al by electron induced decomposition and will be described in detail elsewhere [18]. 3.2. Effect of preadsorbed
H,O on ZrO,
The effect of preadsorbed H,O on the TPD profile of Si(OC,H,), adsorbed on ZrO, is shown in fig. 6. When only H,O (1 x 10P6 Torr, 3 min) was dosed at 300 K, TPD showed two strong peaks at 540 and 610 K (fig. 6a). When Si(OC,H,), was dosed at 300 K onto ZrO, after H,O (1 X lop6 Torr, 3 min) was predosed at 300 K, two sharp peaks were also observed as shown in fig. 6b. The desorption amount of water was nearly equal to that in fig. 6a. The shape of the ethene peak was similar to that from fresh ZrO,, and the desorption amount of ethene was about 2/3 that from fresh ZrO,. AES showed no carbon deposition after the TPD from coadsorption of H,O and Si(OC,H,),. We conclude that preadsorbed water does not strongly alter the adsorption-desorption behavior of Si(OC, H, ) 4 on zirconia. 3.3. SiO, deposition on oxides Figs. 7 and 8 show how the Si/O, Zr/O and Ti/O Auger peak ratios change with dosing time at different temperatures (300 and 673 K) over ZrO, and TiO,. At the lower temperature, Si(OC,H,), was added at 300 K, and
T. Okuhara, J.M. White / SiO, on oxides by CVD of Si(OC,H,),
231
a
Temperature/K
Fig. 6. Effect of preadsorbed water on the TPD of Si(OC,HS)4 from 15 mg of ZrO,. (a) TPD of water (10e6 Torr, 3 min at 300 K). (b) TPD of Si(OC,H,)4 (6 X 10V6 Torr, 300 K, 1 xnin) after water (low6 Torr, 3 min at 300 K) was predosed.
TPD was performed from 300 to 800 K. Then AES spectra were taken at 573 K, this temperature selected to minimize charging. This procedure was repeated to generate the series of points in figs. 7 and 8. When the gas was introduced at 673 K, the sample was flash heated to 800 K and then AES spectra were taken at 573 K. The dosing pressure was 6 X 1O-6 Torr for both temperatures. As shown in fig. 7, the Si/O ratio increased rapidly as Si(OC,H,), was dosed on Zrq. It became constant after about 10 min (0.45 at 673 and 0.25 at
0+--e-%Timefmin
Fig. 7. Change of Si/O and Zr/O Auger peak ratio during dosing of Si(OC2H,), on Zr02: (0) Si/O at 673 K, (0) Si/O at 300 K (after TPD), (0) Zr/O at 673 K, (m) Zr/O at 300 K (after TPD).
232
T. Okuhara, J.M. White / SO,
on oxides by CVD
of Si(OC,H,),
15 Timelmin
Fig. 8. Change of Si/O and Ti/O Auger peak ratio during Si/O at 673 K, (0) Si/O at 300 K (after TPD), (0) Ti/O TPD).
dosing of Si(OC,H,), at 673 K, (m) Ti/O
on TiO,: (0) at 300 K (after
300 K). The rate of the SiO, deposition was higher at 673 K than at 300 K. The Zr/O ratio decreased as the Si/O ratio increased, and reached about 0.12 after a 30 min dose at 673 K. For TiO, (fig. 8), the increase of the Si/O ratio with time was faster at 300 K than at 673 K. The saturation Si/O ratios were 0.45 and 0.33 at 300 and 673 K and were reached after a 30 min dose. The Ti/O ratio decreased from 0.67 to about 0.50 (673 K) and 0.40 (300 K). The C/O AES ratio was less than 0.01 on ZrO, even after a 30 min dose at both temperatures. On the other hand, the C/O ratio on TiO, increased rapidly with the dose time (fig. 9). It reached 0.17 after 30 min at 300 K. The shape of the C(KVV) peak on TiO, resembled graphite, not carbide [19].
Timelmin
Fig. 9. Carbon
deposition
during the dosing on ZrO, and TiO,: (0) 673 K on ZrO,, ZrO, , (H) 673 K on TiO, , (0) 300 K on TiO,
(0)
300 K on
T. Okuhara, J.&f. White / SiO, on oxides by CVD of Si(OC, H,),
.. z 0
‘.._
. .._
0
0
5
Si/(Si+M+O)
10 x100/%
15
.20 in atom(M=Zr
25
233
SiOp .x_ 1
30
=35
or Ti)
Fig. 10. Change in activity for decomposition of Si(OC,H,), with the amount of 50, and TiO,. ZrO, at 673 K (0) and at 300 K (after TPD) (0), and TiO, at 673 K (m) and at 300 K (after TPD) (0). The desorption of ethene goes to zero for pure SiO, (0).
The amount of ethene in the TPD of Si(OC,H,), versus the content of Si measured by AES is shown in fig. 10. The open symbols denote 300 K exposures followed by TPD to 800 K. The closed symbols denote exposures at 673 K, flash to 800 K, cool to 300 K, dose Si(OC,H,), at 6.0 X lop6 Torr for 1 min, and then TPD. As the Si content increases, the desorption of ethene from ZrO, decreases monotonically and extrapolates to zero at a composition characteristic of pure SiO,. For TiO, it is nearly independent of the amount of Si. Assuming the ethene in the TPD monitors decomposition, fig. 10 indicates that the activity of TiO, does not change strongly as the SiO, overlayer forms, but that of ZrO, decreases. As shown in fig. 11, the Ti/O and Zr/O atomic ratios (normalized to 0.5 for fresh samples) decreased in different manners with the SiOz content. The Zr/O atomic ratio decreased monotonically as the content of Si increased but the Ti/O atomic ratio decreased slowly at first but became more rapid at higher Si. Fig. 12 shows the effect of SiO, deposition on the TPD spectra of CO,. A blank test without dosing showed negligible CO2 desorption. TPD of CO, from fresh TiO, and ZrO, yielded broad peaks of CO, (figs. 12a and 12b). When SiO, was deposited on ZrO, from Si(OC,H,), (Si/O = 0.40) the desorption of CO* was much lower than from fresh ZrO, (fig. 12a). In the case of TiO,, SiO, deposition also suppressed the adsorption of CO, (fig. 12b). Note that no clear CO, desorption peak was observed from bulk SiO, (shown by broken line in fig. 12a). Fig. 13 shows one measure of the thermal stability of the SiO, overlayers on ZrO, and TiO,. In this experiment, SiO, was deposited on ZrO, as follows: Si(OC,H,), (6 X lop6 Torr, 10 min) was introduced three times at 300 K. For TiO,, Si(OC,H,), (6 X 10e6 Tot-r, 10 min) was dosed three times at 300 K. In
234
T. Okuhara, J.M.
White / SiO, on oxides by CVD of Si(OC2 H5)4
0.1 0
z1
0 L’ 0
5
10
Si/(Si+M+O]
20
15
x100/%
25
5
30
in atom(M=Zr
or TiI
Fig. 11. Change in the Zr/O and Ti/O atomic ratios with the amount of SiO, deposited: ZrO, at 673 K (0) and at 300 K (after TPD) (0) and TiO, at 673 K (m) and at 300 K (after TPD) (III).
both cases, the samples were then flash heated to 773 K and cooled. The data of fig. 13 was then generated. The samples were evacuated for 15 min at each temperature and AES spectra were taken at 473 K. There is a significant difference between ZrO, and TiO,. The Si/O AES ratio did not change for
a
zr02
b TiO2
/’
,1--y
;’ /’
\\
,’
..
r\ 300
>\
,’ \.
,_I’
\\...
‘...__._.,-’
400
500
600
700
fSOIO
Temperature/K
Fig. 12. TPD spectra of CO1 from: (a) ZrO, and (b) TiO,. CO, (lo-’ Torr) was dosed at 250 K for 1 min. () SiO, covered ZrO, (Si/O Auger ratio = 0.42) and SiO, covered TiO, (Si/O = 0.30), (- - - - - -) fresh ZrO, and TiO,, (- - -) fresh bulk SiO,.
T. Okuhara, J.M. white / SO,
on oxides by CVD of Si(OC,H,),
235
o.5------
.o 0.4 -
zL 5 o.+-.-.-.-AA*
302
$
\
9$ 0.2 _L,-,-~-,-=-~a
A02 \
Q O.li?
t 0
I
473
573
673
773
673
Temperature/K
Fig. 13. Thermal stability of SiO, deposited on ZrO, and Ti02. SiO, was deposited on ZrO, by three doses of Si(OC,H,), at at 300 K and three doses at 473 K, each dose at 6 X 10e6 Torr for 10 min. On TiO, the exposure was done three times at 300 K and 10 min, each dose followed by TPD up to 773 K.
TiO, after the evacuation even at 823 K, but it started to decrease at 773 K and decreased abruptly at 823 K for ZrO,. As the Si/O decreased, the Zr/O increased. Deposition of SiO, by continuous exposure of Si(OC,H,), (673 K, 1.5 X 10m6 Torr) is shown in fig. 14. The order of the rate of SiO, deposition was: ZrO, > MgO = TiO, > Al,O,. For ZrO,, the Si/O ratio saturated at 0.4 after 2 h under these conditions. For the other substrates, the Si/O ratio did not saturate. The deposition of carbon was also quite different: the C/O increased rapidly on TiO,, Al,O, and MgO (after 4 h, the ratios were 0.07, 0.05 and 0.03
Dose
Time/h
Fig. 14. SiO, deposition on various oxides by continuous dosing of Si(OC2H,),. The Si(O&H,), was introduced at 673 K and 1.5 X 10e6 Torr. The weights of the samples were 40, 16, 28 and 8 mg for ZrO,, TiO,, MgO, and Al,O,, respectively.
236
T. Okuhara, J.M.
White / SO,
for TiO,, Al 2O,, and MgO, 0.005 throughout the dosing.
on oxides by CVD of Si(OC,H,),
respectively).
On ZrO,,
C/O
remained
below
4. Discussion 4.1. Decomposition
of Si(OC, H_5)4
The anchoring of various silanes on SiO, surfaces is well known [11,20-221. Hertl [22] reported that Si(CH,)(OCH,), adsorbed physically on SiO, and that no decomposition took place at room temperature, However, he found that above 363 K, the following reaction occurred: + HOSi + CH,(OCH,),Si(OSi)
(CH,O),Si(CH,)
+ CH,OH,
where HO-Si is an isolated non-hydrogen bonded hydroxyl group. Boucher et al. [21,23] compared the relative reactivity of ligands, X in (C,H,),P(CH,), Six, with OH groups on SiO, at 433 K. They found the anchoring reactivity was ordered as follows for X: Cl > OCOCH, > OCH, > OC,H,. From these findings, we expect, as observed in fig. 5, only a very weak interaction between Si(OC,H,), and SiO, surfaces and at temperatures below 300 K, little decomposition. Even if Si(OC,H,), is adsorbed physically, it is removed by evacuation at 300 K before the TPD. These conclusions are confirmed by the appearance of a m/e = 163 TPD peak at 290 K after Si(OC,H,), is dosed on SiO, at 220K. Decomposition on the other oxides was more extensive (figs. 1, 3 and 5). These oxides have acidic or basic sites [24], and we suppose that these are active for the adsorption and the decomposition of Si(OC,H,),. Imizu et al. [25] examined the interaction between Si(OCH,),(CH,),_. and MgO by IR and observed a change from molecular adsorption at room temperature to dissociation to MgO-Si-R, and methoxyl at 573 K. Our results are also consistent with mainly molecular Si(OC,H,), adsorption at 300 K on all these oxides. The following are proposed as decomposition reactions: Si(OC,H,),
+ M-O(H)-M
-+ M-O-Si(H)(OC,H,)3
+ M-O-M
+ M-0-Si(OC,H,),
+ M-OC,H,,
(Ia)
or Si(OC,H,),
+ M-OC,H,,
(lb)
M-OC,H,
--) C,H,(g)
+ M-OH,
(2)
2 M-OH
--$ M-O-M
+ H,O(g),
(3)
M-0-Si(H)(OC,H,),
--z M-0-(H)Si(OH),
M-0-(H)Si(OH),
---) M-OSiO,
+ 3 C,H,(g),
+ 2 H,O(g).
(4)
T. Okuhara, J.M.
White / SO,
on oxides by CVD of Si(OC,H,),
237
Reaction (la) is well known [11,22]. In (la), the position of (H) in the products is unknown and arbitrarily located at Si but it is not important here. Reaction (lb) involves the metal cations on the oxide surface and will play an important role in our work because the number of OH groups on oxide surfaces decreases strongly with evacuation temperature [20]. After evacuation at 773 K we expect only 3.5, 2 and 0.5 OH nm-’ for Al,O,, SiO, and TiO,, respectively. Furthermore, IR studies of adsorbed pyridine show that the acid sites on Al,O, [24], ZrO, [26], and TiO, [27] are mostly of the Lewis type (M”+). Therefore, in addition to the OH groups, the surface metal cations should be considered as decomposition sites. The fact that ethene desorbed at lower temperatures than H,O means that processes (l), (2) and (3) take place sequentially during the TPD; otherwise, we would expect these two products to desorb coincidentally. Arai et al. [28] reported that ethoxyl groups start to decompose to ethene at 408 K and are completely converted after heating at 483 K. The TPD results of fig. 5b are consistent with this. In separate experiments, we found that molecular ethene does not adsorb significantly on ZrO, and that dosed H,O desorbs at lower temperature (< 650 K) than that appearing in the TPD of Si(OC,H,),. This result indicates that the reactions forming C,H, and H,O limit their appearance in the desorption spectra. Readsorption processes are not important. As a possible candidate reaction path for the formation of SiO,, eq. (4) is proposed, where ethene and H,O form around Si sites. If the hydrolysis of Si(OC,H,), took place at 300 K, the desorption of C,H,OH should be observed as for C,H,OH dosed on Al,O, [28]. Since no C,H,OH was observed and since H,O did not affect C2H, desorption (fig. 6) we conclude that water is inactive on ZrO, at 300 K. It is well known that liquid-phase hydrolysis of Si(OC,H,), at room temperature must be catalyzed by ions such as H+ and OH- [29]. Thus, in the case of ZrO,, the acidity or basicity is probably too weak to catalyze the reaction at 300 K. One very clear outcome of these experiments is the difference in the behavior of the oxides. On fresh oxides, the temperature of C,H, desorption was lowest on ZrO,, implying that ZrO, surfaces are very active for decomposition. Although the acidic and basic sites on ZrO, are weak, both are present [30]. This may be important for the decomposition. In this light it is noteworthy that ZrO, is more active than A1203 for isomerization of butene and shows a unique selectivity to 1-butene in dehydration of set-butanol [31]. Another important feature of the decomposition over ZrO, is the small amount of carbon deposition (fig. 9). This may be due to the absence of strong acid sites [24] which leads to carbon deposition from hydrocarbons. Henderson et al. reported that carbon deposition on ZrO, was quite small in the TPD of trinitrotoluene [15]. Even though the MgO surfaces are dominated by basic sites, the decomposition products are also ethene and water. Tanabe showed that mixing SiO,
238
T. Okuhara, J.M.
White / SiO, on oxldes by CVD of Si(OC2H,),
and MgO generated acid sites [24]. Moreover, Imizu et al. [25] observed the generation of acid sites (by IR of pyridine adsorption) when SiO, was deposited on MgO from Si(OCH,),(CH,),_.. In our case, acid sites generated by the decomposition of SiO, on MgO are probably active; the decomposition process on a pure MgO surface is still not clear. Finally, there is an interesting difference in the variation of the activity with the dose of Si(OC,H,), when ZrO, and TiO, are compared (fig. 10). This difference is attributed to structural variations of the SiO, and is discussed in the next section. 4.2. Structure
of SiOz overlayers on ZrOz and TiOz
SiO,, SiO and Si can be distinguished by the kinetic energy of Auger electrons [32]. In quartz, the O(KW) AES peak is at 507 eV and the Si AES peak is at 79 eV [33]. The Si transitions for SiO and Si are at 83 and 88 eV [32]. As shown in figs. 2 and 3, there is some charging; the O(KW) transition is at 510 eV for ZrO, and 517 eV for TiO,. Taking this into account the Si peaks for the oxides prepared on ZrO, and TiO, will be at 78 eV, indicating that Si exists as SiO,. We conclude that the silicon deposited by dosing Si(OC,H,), is fully oxidized during the decomposition process. We turn now to the morphology of the SiO,. 4.2.1. SiOz on ZrO, The facts that (i) the decomposition activity decreased monotonically as the Si content increased (fig. 10) and (ii) the Zr/O atomic ratio measured by AES decreased linearly as the Si content increased (fig. 11) lead to the conclusion that the SiO, film develops as a fairly uniform overlayer on zirconia. The amount of Si deposited grew rapidly as the dose time increased (fig. 7) but then the Si/O and Zr/O Auger peak ratios became constant. This means that film growth is suppressed once the surface is covered by silica. This is consistent with our observation that the decomposition of Si(OC,H,), is very inefficient on bulk silica. This model is also consistent with the suppression of CO, adsorption (fig. 12) since bulk silica did not adsorb CO,. Assuming a uniform overlayer, the thickness, x. of the SiO, film can be estimated from I = I, exp( - x/X) [34] using the following relation developed empirically by Seah et al. [35]: X = 2170E-*
+ 0.72( aE)*“,
(5)
where X is the attenuation length (in monolayers), a is the thickness of one monolayer of substrate in nm, and E is the kinetic energy of the Auger electrons in eV. When the thickness of a SiO, monolayer is assumed to be 0.27 nm, and the Zr(145 eV) AES transition [17] is used, the final SiO, layer is about 4 monolayers thick.
T. Okuhara, J.M. White / SiO, on oxides by CVD of Si(OC, H,),
239
This SiO, overlayer on ZrO, is stable up to 723 K (fig. 13) but at 773 K it decomposes probably either to small SiO, clusters or to an SiO,-ZrO, solid solution [36]. The formation of a solid solution by reaction of surface thin films and substrate oxides has been reported for V,O,-CeO, [6]. 4.2.2. SiO, on TiO, The morphology of SiOz on TiO, is apparently quite different from that on ZrO, because there are essential differences in: (i) how the decomposition activity changes with dose of Si(OC,H,), (fig. 10) and (ii) the thermal stability of the resulting oxide (fig. 13). We propose a structural model where SiOz is present as small clusters and/or a mixed metal oxide layer rather than a uniform SiOz film. In this case, TiO, is always present at the surface. This model can account for at least part of the retention of Si(OC,HS), decomposition activity. Based on the work of Tanabe [24], we also suggest that interface regions between SiO, and TiO, exhibit acidity which are active for Si(OC,H,), decomposition. These two effects together can account for the steady decomposition activity with Si(OC,H,), dose. The thin film structure on ZrO, is less stable than the particle structure on TiO,. We conclude this on the basis of the AES changes with annealing temperature (fig. 13). One possible explanation for the decreased CO, adsorption after SiO, deposition on TiO, (fig. 13) is that surface OH groups on TiO,, which induce the strong adsorption of CO, by the formation of bicarbonate species [27], are consumed during the deposition of SiO,. The atomic composition of the outermost layer of these SiO,/ZrO, and SiO,/TiO, systems is not elucidated here. ISS (ion scattering spectroscopy) experiments are now in progress to measure this. It will also be helpful to study planar oxides to complement the work reported here.
5. Summary (1) A reasonably uniform SiOz thin film having a thickness of about 10 A (4 monolayers) was formed on ZrO, by the decomposition of Si(OC,H,), during TPD after dosing at 300 K. (2) This SiO, thin film was stable up to 723 K, but started to decompose at 773 K. (3) On ZrO,, carbon deposition was minimal during the formation of SiO, thin films, while on MgO, TiO, and Al,O, carbon deposition was significant. (4) On TiO,, we propose that small clusters of SiO, and/or a mixed metal oxide layer are formed during the decomposition of Si(OC,H,),. The resulting surface retained its decomposition activity even after large doses of Si(OCzH5)4, an observation we attribute to the continued presence of TiO, at
240
T. Okuhara, J.M. White / SiO, on oxides by CVD of Si(OC2 H5).,
the surface and to acid sites generated at the interfaces between SiO, and TiO, . (5) The structure formed on TiO, is thermally stable up to at least 823 K. (6) The decomposition of Si(OC,H5), on SiO, is negligible.
Acknowledgment The authors wish to thank Dr. Sohail Akhter and Dr. Tuo Jin for many useful discussions. This work was supported in part by the Texas Advanced Technology Research Program.
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T. Okuhara, J.M. White / SO,
on oxides by CVD of Si(OC,H,),
241
[29] Bailar, Jr. al., Eds., Inorganic Chemistry New York, [30] K. Solid Acids Bases (Kodansha-Academic Tokyo-New York, [31] T. H. Sasaki K. Tanabe, Letters (1973) [32] C.C. Surface Sci. (1971) 53. B. Carriere B. Lang, Sci. 64 209. [34] Chang, in: of Solid Ed. K. (Plenum, New 1976) p. [35] M.D. and W.A. Surface Interface 1 (1979) [36] V.I. G.M. Matveyev 0.P Medhedlov-Petrossyan, of Sili(Springer, Berlin,