Quantum chemical study on low energy reaction path for SiH4 + O(1D) → SiO + 2H2

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applied

surface science ELSEVIER

Applied Surface Science 117/118

(1997) 151-157

Quantum chemical study on low energy reaction path for SiH, + O(‘D) --+ SiO + 2H, Akitomo Tachibana Department of Molecular

*, Ken

Sakata

Engineering, Kyoto Vniuersity, Kyoto 606-01, Japan

Abstract One of the methods to produce SiO, layer over the Si surface is the chemical vapor deposition (CVD) by using oxygen gas and silane gas. But little is known about the mechanism of this oxidation reaction not only on the surface but also in the gas phase. In this paper, we take up the gas phase reaction between singlet oxygen (‘D) and silane. Applying quantum chemical method, a low energy reaction path is found in addition to the excited state reaction path. This agrees well with the experimental observation for the vibrational energy distribution of product SiO by laser-induced fluorescence (LIF). Keywords: SiO,; CVD; Reaction;

Quantum

chemistry;

Single oxygen

1. Introduction One of the methods to produce SiO, layer over the Si surface is the chemical vapor deposition (CVD) by using oxygen gas and silane gas [l]. A number of relevant reactions have been examined [2-121 and some interesting studies have been published recently [ 13-161. But little is known about the mechanism of this oxidation reaction not only on the surface but also in the gas phase. Therefore, we have been applying quantum chemical methods to study these problems. At the first step of a series of these studies, we take up the gas phase reaction between singlet oxygen (‘D) and silane (SiH,) [17-241. This is because excited oxygen species are considered to be important in CVD reaction, especially plasma CVD reaction, while the detailed reaction mechanism is not well established. For this reaction, Koda

* Corresponding author. Tel.: + 81-75-7535932; 7535932; e-mail: [email protected]. 0169-4332/97/$17.00 Copyright PII SOl69-4332(97)00088-3

fax: + 81-75-

et al. observed SiH, and OH as the reaction products by means of infrared diode laser kinetic spectroscopy [25]. Recently, Tezaki et al. have studied this reaction in more detail by laser-induced fluorescence (LIF) [26]: (1) The overall observed rate constant is 3 X lo-” cm3 molecule-’ s- ’ , which is three times faster than that of methane. (2) A H atom was detected as one of the products. (3) A vibrationally excited (to u = 11) SiO molecule was detected as a product species, and the rate constant agrees with the pseudo-first order rate constant of 0 atom decay. Moreover, vibrational distribution of product SiO in low u regime is distinct from that in high u regime as compared with prior distribution of surprisal analysis [26]. This suggests that a low energy reaction path does exists and plays an important role in addition to the excited state reaction path. We have therefore examined this interesting reaction system SiH, + O(’ D) --+ SiO + 2H, using MP2(fc)/6-31G * * level ab initio molecular orbital calculations.

0 1997 Elsevier Science B.V. All rights reserved.

152

A. Tachibana,

K. Sakata/Applied

Sut$ace Science 117/118

2. Methods of calculation The calculations were performed with the GAUSSIAN-92 [27], -94 [28] program packages. Geometries of minima and saddle points have been optimized at Hartree-Fock level and second-order Moller-Plesset level (frozen cored) [29,30] with 631G* * basis set (denoted as HF/6-31G * * and MP2(fc)/6-3 1G * * >. For all species except for triplet and singlet oxygen atoms, restricted Hartree-Fock (RHF) calculations were performed. And for triplet oxygen atoms, unrestricted Hartree-Fock (UHF) calculations were performed. For the calculation for singlet oxygen atoms, complex orbitals were used in order to describe a closed-shell singlet atom. Electron correlation calculations were also carried out at the MP2 level. Harmonic vibrational frequencies and zero-point energies were obtained at the MP2 level for each stationary point. Zero-point energies were scaled by the factor 0.94 [31,32].

3. Results and discussion Total energies of all species are listed in Table 1. TS denotes transition state of the reaction path as defined in Fig. 1. Reaction energies are summarized in Table 2 and the energy diagram in Fig. 1. Vibrational frequencies are listed in Table 3. 3.1. SiH, + O(‘D) + SiH,OH The title species O(‘D) is well known to insert into X-H bonds, such as H-H, C-H, N-H. Theoretically, the following insertion reactions have been studied: H, + O(‘D) [33], CH, + O(‘D) [34,35]. In the present reaction system also, we assume that the insertion reaction of O(‘D) into Si-H bond occurs and the potential surface for SiH, + O(‘D> connects to the most stable species silanol (SiH,OH) with no reaction barrier. Indeed, the reaction is extremely exothermic with reaction energy - 175.79 kcal/mol, as shown in Fig. 1. To check the accuracy of our estimation of reaction energy, we examined the O(3P) e O(‘D) splitting energy: our estimate is 49.76 kcal/mol at the MP2 level, and the experimental value is 45.37 kcal/mol [36], both being in good agreement.

(1997) 151-157

Table 1 Total energies, zero-point 3lG* * Species

energies at HF/6-31G HF/6-31G

H 0C3P) O(‘D) a 1 Hz 2 SiO (‘Cg) 30H 4 Hz0 5 SiH, 6 SiH, 7 SiH, 8 SiH,OH 9 SiH, -Hz0 10 SiH,O (‘A’) 11 SiH,OH 12 H,SiO 13 cis-HSiOH 14 trans-HSiOH TS-1 TS-2 TS-3 TS-4 TS-5 TS-6 TS-7 TS-8 TS-9 TS-10

* * and MP2/6-

* * MP2/6-31G

* * ZPE

-

(K,)

- 0.49823 - 74.78393 (K,) - 74.69685 (K,) (R,,) - 1.13133 CC,,) - 363.77884 (C,,) -75.38833 (C,,) - 76.02362 (C,, ) - 290.00263 i;;’ -290.61058 - 291.23084 (c”,, -366.14192 CC,) -366.05050 (C,) -365.50186 CC,) -365.51709 CC,,) -364.91752 (C,) - 364.93006 CC,) - 364.93042 CC,) -365.98510 (C,) -366.00397 (C,) - 366.00188 CC,) - 364.75298 (C,) b - 364.79062 (C,) -364.91567 CC,) - 364.83007 CC,) -365.99206 CC,) -365.99108 (C,) -366.00461

- 74.88004 - 74.80072 - 1.15766 - 364.04581 - 75.53209 -76.21979 - 290.08339 - 290.69825 -291.33900 - 366.42677 - 366.33540 - 365.73461 - 365.78281 - 365.19933 - 365.19430 -365.19543 - 366.29546 - 366.30880 - 366.31283 - 365.05383 -365.10063 -365.17690 - 365.12457 - 366.29759 - 366.29632 - 366.30866

6.21 1.58 5.18 12.94 7.30 13.22 19.33 23.66 24.14 16.22 17.73 11.35 12.54 12.79 21.16 21.24 21.04 8.83 8.50 11.13 9.62 22.11 21.94 21.12

Total energies are in au, and zero point energies are in kcal/mol. Zero point energies are computed at MP2/6-3 1G * * leveled and scaled by 0.94. a Using complex orbitals. bC, symmetry at HF/6-3lG* *, C, at MP2/6-3lG**.

We further assume that this silanol is decomposed into many species, as described below. 3.2. SiH, OH decomposition Thermal decomposition reaction paths of SiH,OH are considered as follows: SiH,OH

+ SiH, + OH

(1)

SiH,OH

+ SiH,O + H

(2)

SiH,OH

--) SiH,OH

SiH,OH

--f TS-1 --f SiH,-H,O

SiH,OH

+ TS-2 + truns-HSiOH

SiH,OH

+ TS-3 + H,SiO

+ H

(3) complex

(4)

+ H,

(5)

+ H,

(6)

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Surface Science 117/118

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(19971 151-157

SiH,+O(‘D)* Tq

+ MWKomplex RHF

0.00 Fig. 1. Energy diagram

at MP2(fc)/6-3

SiH, and OH were detected by Koda and Suga by the time-resolved infrared-diode laser absorption technique [25]. H atoms were detected by Tezaki et al. by laser-induced fluorescence [26]. These species are considered to be decomposed from SiH,OH as in Eqs. (l)-(3). Energy diagrams of Fig. 1 show that the reactions of SiH, + O(’ D) leading to these products are exothermic, so that these homolytic reaction paths are considered main reaction channels. The barrier height of the Eqs. (4)-(6) is 79.91 kcal/mol (T&l), 71.10 kcal/mol (TS-21, 68.88 kcal/mol (TS-3), respectively, in agreement with higher level calculations by MP4/MC-31 lG(d,p)//MP2/63 lG(d,p) [21. 3.3. Hz SiO + SiO + 2H, The unimolecular decomposition of silanone H,SiO has been investigated by Kudo and Nagas? [3] and our group 141. A series of reaction pathways are as follows: H,SiO

-+ TS-4 -+ SiO + H,

H, SiO + TS-5 + trans-HSiOH truns-HSiOH cis-HSiOH

(7) + H,

+ TS-6 --) cis-HSiOH + TS-7 + SiO + H,

(8) (9) ( 10)

One channel is direct decomposition (Eq. (7)) through TS-4 and another channel is via rearrangement chan-

1G * * level including ZPE (scaled).

nel (Eq. (8) + Eq. (9) + Eq. (10)) through TS-5, TS-6 and TS-7. Eq. (7) has high energy barrier of 88.78 kcal/mol and hence is of little importance. While the rearrangement channel Eq. (8) -+ Eq. (9) + Eq. (10) proceeds easily as found in Fig. 1. In Eq. (9), TS-6 was found to have C, symmetry at HF/6-

Table 2 Reaction energies (kcal/mol)

a

Reaction

Activation energy

Heat of reaction

SiH, + O(‘D) + SiH,OH SiH,OH + SiH, +q3P) SiH,OH + SiH, +OH SiH,OH + SiH,O+H SiH,OH + SiH,OH+H SiH,OH --f SiH,-H,O complex SiH,OH + trans-HSiOH + H, SiH,OH + cis-HSiOH+H, SiH,OH + H,SiO+H, SiH, -H,O complex + trans-HSiOH + H, SiH,-H,O complex -+ cis-HSiOH + H, H,SiO -+ SiO+H, H 2 SiO + tram-HSiOH rruns-HSiOH --) cis-HSiOH cis-HSiOH --f SiO + H2

-

79.91 71.61 71.58 68.88 21.59

- 175.79 126.02 118.00 114.25 85.52 57.82 41.57 42.03 37.69 - 16.25

22.22

- 15.79

88.78 59.08 9.97 40.84

-6.16 -3.88 0.46 10.50

a At the MP2(fc)/6-31G tions.

-

* * level including

ZPEhcaled)

correc-

154

A. Tachibana, K. Sakara/Applied

31G* * level, while C, symmetry level [37]. 3.4. H2 S-H,

at MP2/6-3

Surface Science 117/118

IG * *

0 complex + cis- or trans-HSiOH

Although the favorable reaction path of SiH,OH is H, elimination leading to trans-HSiOH or H,SiO, yet we propose the path to the SiH,-H,O complex is considerably important. The H,Si-H,O complex has been considered a precursor of the reaction of SiH, + H,O leading to SiH,OH so far [8-lo]. Zachariah and Thang considered that in the SiH, + H,O reaction this CT complex forms at first, and then rearrangement of H atom to silanol occurs [16]. In case of singlet carben, this kind of CT complex is not on the reaction coordinate of CH, + H,O [3841], so it is considered important in case of silicon compounds. Indeed, in our present study, it is confirmed to stay on the reaction surface at HF/6of the intrinsic reaction 31G* * level calculations coordinate (IRC) starting from TS-1, as shown in Fig. 1.

(1997) 151-157

We propose a new reaction path of this CT complex leading directly to cis- or trans-HSiOH, not via formation of H,SiO: H,Si-H,O

complex

+ TS-8 + trans-HSiOH H,Si-H,O

+ H,

complex

--) TS-9 + cis-HSiOH

+ H,

Harmonic vibrational frequencies

Hz

4608&g) 1176(sg)

SiO OH Hz0

SiH, SiH, SiH, SiH,OH SiH,-Hz0 SiH,O SiH,OH H,SiO cis-HSiOH trans-HSiOH TS-1 TS-2 TS-3 TS-4 TS-5 TS-6 TS-7 TS-8 TS-9 TS-10

3844(sg) 1682(al), 3892(al), 4030(b2) 1078(al), 2168(al), 2169(b2) 821(al), 984(e), 2317(al), 2352(e), 2352(e) 974(t2), 1018(e), 2339(al), 2350(t2) 225(a”), 713(a’), 756($),856(a’), 93O(a’), 998(a”), 1027(a’), 1073(a’), 2318(d’), 2324(a’), 2375(a’), 3964(a’) 158, 333,475, 559, 720, 856,1061,1659,2113,2164,3841,3974 475(a”), 637(a’), 834(a’), 971(a”), 1002(a’), 1058(a’), 2343(a’), 2357(a”), 2357(a’) 276,711,812, 874,925,987,2274,2343,3953 728(bl), 728(b2), 1080(al), 1188(al), 234l(al), 2354(b2) 687(a”), 761(a’), 872(a’), 1018(a’), 2059(a’), 3909(a’) 728(a”), 83l(a’), 87O(a’), 1004(a’), 2153(a’), 3903(a’) 148Oi, 364,524,606,792,879, 1043, 1168, 1963,2270,2300,3797 136Oi, 345,632,759,806, 853,998, 1136, 1652.226.5.2369.3944 1524i(a’), 575(a”), 729(a”), 856(a’), 98O(a’), 1073(a’), 1182(a’), 1234(a”), 2053(a’), 2232(a’), 234&“), 235Ok’) 2116i(a’), 534(a”), 664(a’), 1118(a’),15lo(a’), 2359(a’) 1125i(a’), 202(a”), 817(a’), 979(a’), 209o(a’), 2217(a’) 791i, 308,904,918,2055,4077 1953&d), 946(a’), 1028(a’), 1241(a”), ISSO( 2071(d) 14431’.578,626,744,916,984, 1151, 1397, 1938, 1997,2207,3864 1402i, 551,623,698,928,961, 1125, 1413, 1964,2000,2137,3878 1338i, 396,607,727, 805,848,999,1132,1613,2274,2336,3937

a At the MP2/6-31G * *level; in cm-‘.

(12)

The products trans-HSiOH + H, and cis-HSiOH + H, are 41.57, 42.03 kcal/mol higher in energy than silanol, but the barriers of these reactions are considerably lower: 21.59 kcal/mol for TS-8, 22.22 kcal/mol for TS-9. As shown in Fig. 1, these reactions are likely to occur. Fig. 23 shows the structure of TS-9 for the transition state of Eq. (12). The imaginary frequency of this transition state is 14431’ cm-’ as shown in Table 3. The imaginary vibrational mode corresponds to H, elimination. The reaction paths are interpreted in terms of classical valence bond theory as given in Scheme 1.

Table 3 Vibrational frequencies a Species

(11)

A. Tachibana, K. Sakata/Applied

(1.692);

Surface Science 117/118

(1997) 151-157

155

“. 1.273 ‘:$1.308) 1.507 /, : (1.504)“ ; 1.678 _,*’ !: (1.682)

‘*.

@(H3-Si-0-H4)=-51.24” (-47.61”) Q(Hl-Si-0-H4)=37.42” (36.85’)

-0.21

TS-10

H2

C&Hz-Si-0-H4)=17.76” (14.31’)

Fig. 3. Optimized geometry of TS-10 at HF/6-31G details, see caption of Fig. 2.

TS-9 Fig. 2. Optimized geometry of TS-9 at HF/6-31G * * level. Bond lengths are in angstroms and bond angles and dihedral angles are in degrees. MP2 level optimized data are in parentheses. The atomic charges are in italics. Arrows show imaginary vibrational mode.

The H ,Si-H,O complex forms through a donor-acceptor CT interaction between a lone pair orbital in H,O and an empty p orbital in SiH,. In this complex, the H atom bonded to the Si atom has a hydride-ion-like character (whose atomic charge is calculated to be -0.215 on the average), while a H atom bonded to a 0 atom has a proton-like character (whose atomic charge is calculated to be +0.39). Here, if the proton-like H moves over to the silylene lone pair orbital, then new sp3 bond is formed. This

* * level. For

is the 1,Zhydrogen shift reaction like the reaction path of silylene derivatives, such as dimethyl silylene. On the other hand, if the proton-like H attacks the hydride-ion-like H, then a H 2 molecule is formed. The resulting new sp2 type vacant orbital of Si is stabilized by hyperconjugation of the oxygen lone pair orbital [42]. It is interesting to observe that the barrier heights for these two reaction paths are comparable. The additional reaction path that directly leads to cis-HSiOH is found as follows: SiH,OH

-+ TS-10 + cis-HSiOH

Fig. 3 shows the structure state of Eq. (13).

CT complex

Scheme I.

+ H,

(13)

of TS-10 for transition

A. Tachibana, K. Sakata/Applied

156

3.5. General view

References

The low energy reaction paths examined in this paper are here summarized. Silanol proceeds to SiO + 2H, through the following reaction channels: (6)TS-3

+ (8)TS-5

Surface Science 117/ 118 (1997) 151-157

+ (9)TS-6

ill J.M. Jasinski, S.M. Gates, Act. Chem. Res. 24 (1991) 9. l21 M.S. Gordon, L.A. Pederson, J. Phys. Chem. 94 (19901 5527.

[31 T. Kudo, S. Nagase, J.Phys. Chem. 88 (1984) 2833. L41A. Tachibana, H. Fueno, T. Yamabe, J. Am. Chem. Sot. 108

+ (lO)TS-7

(1986) 4346.

(Channel

[51 B. Ma, H.F. Schaefer III, J. Chem. Phys. 101 (1994) 2734.

1)

l61 A. Tachibana, M. Koizumi, H. Teramae, T. Yamabe, J. Am. (5)TS-2

+ (9)TS-6

+ (lO)TS-7

(Channel

2)

Chem. Sot. 109 (1987) 1383.

[71 S. Sakai, M.S. Gordon, K. Jordan, J. Phys. Chem. 92 (1988)

(4)TS-l +

(12)TS9

--f (lO)TS-7

(Channel

3)

7053.

Bl K. Raghavachari,

(13)TS-10

+ (lO)TS-7

(Channel

4)

All channels are possible candidates as low energy processes as shown in Fig. 1. Channel 4 is probably the easiest to occur. Elimination reaction of two H, molecules can occur through these low energy processes.

4. Conclusion We studied the reaction mechanism of SiH, + O(’ D). Applying a quantum chemical method, a low energy reaction path is found in addition to the excited state reaction path. This agrees well with the experimental observation on the vibrational energy distribution of product SiO by laser-induced fluorescence (LIF).

Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan, for which the authors express their gratitude. The numerical calculations were carried out at the Computer Center of the Institute for Molecular Science (IMS) and the Data Processing Center of Kyoto University and we are grateful for their generous permission to use IBM SP2 and NEC SX-3, and FUJITSU M-1800 and VP2600 computer systems, respectively. Part of the calculations has been done using the Cerius2 software packages.

J. Chandrasekhar, M.J. Frisch, J. Am. Chem. Sot. 104 (1982) 3779. J. Chandrasekhar, M.S. Gordon, K. [91 K. Raghavachari, Dykema, J. Am. Chem. Sot. 106 (1984) 5853. 1101S. Su, M.S. Gordon, Chem. Phys. Lett. 204 (1993) 306. [Ill S. Nagase, T. Kudo, T. Akasaka, W. Ando, Chem. Phys. Lett. 163 (1989) 23. iI21 T. Akasaka, S. Nagase, A. Yabe, W. Ando, J. Am. Chem. Sot. 110 (1988) 6270. [131 D.J. Lucas, L.A. Curtiss, J.A. Pople, J. Chem. Phys. 99 (1993) 6697. 1141C.L. Darling, H.B. Schlegel, J. Phys. Chem. 97 (1993) 8207. D51 C.L. Darling, H.B. Schlegel, J. Phys. Chem. 98 (1994) 8910. 1161 M.R. Zachariah, W. Thang, J. Phys. Chem. 99 (1995) 5308. [ll] L. Ding, P. Marshall, J. Chem. Phys. 98 (1993) 8545. [18] C.R. Park, G.D. White, J.R. Wiesenfeld, J. Phys. Chem. 92 (1988) 152. [19] B.S. Agrawalla, D.W. Setser, J. Chem. Phys. 86 (1987) 5421. [20] 0. Horie, R. Taega, B. Reimann, N.L. Arthur, P. Potzinger, J. Phys. Chem. 95 (1991) 4393. [21] K. Sugawara, T. Nakanaga, H. Takeo, C. Matsumura, Chem. Phys. Lett. 157 (1989) 309. [22] I.R. Slagle, J.R. Bemhardt, D. Gutman, Chem. Phys. Lett. 149 (1988) 180. 1231 M. Koshi, A. Miyoshi, H. Matsui, J. Phys. Chem. 95 (1991) 9869. 1241 S. Koda, Prog. Energy Combust. Sci. 18 (1992) 513. [25] S. Koda, S. Suga, S. Tsuchiya, T. Suzuki, C. Yamada, E. Hirota, Chem. Phys. Lett. 162 (1989) 35. [26] A. Tezaki et al., private communication. [27] M.J. Frisch, G.W. Trucks, M. Head-Gordon, P.M.W. Gill, M.W. Wong, J.B. Foresman, B.G. Johnson, H.B. Schlegel, M.A. Robb, E.S. Replogle, R. Gomperts, J.L. Andres, K. Raghavachari, J.S. Binkley, C. Gonzalez, R.L. Martin, D.J. Fox, D.J. Defrees, J. Baker, J.J.P. Stewart, J.A. Pople, Gaussian 92, Gaussian Inc., Pittsburgh, PA, 1992. 1281 M.J. Frisch, J.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, M.A. Al-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, C.Y. Peng, P.A. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees,

A. Tachibana, K. Sakata/Applied

[29] [30] [31]

[32] [33] [34]

Surface Science 117/118

J. Baker, J.P. Stewart, M. Head-Gordon, C. Gonzalez, J.A. Pople, Gaussian 94, Revision B.3, Gaussian, Inc., Pittsburgh, PA, 1995. C. Meller, M.S. Plesset, Phys. Rev. 46 (1934) 618. J.S. Binkley, J.A. Pople, Int. J. Quantum Chem. 9 (1975) 229. W. Hehre, L. Radom, P.v.R. Schleyer, J.A. Pople, Ab initio Molecular Orbital Theory, Wiley-Interscience, New York, 1986. J.A. Pople, A.P. Scott, M.W. Wong, L. Radom, Israel J. Chem. 33 (1993) 345. MS. Fitzcharles, G.C. Schatz, J. Phys. Chem. 90 (1986) 3634. M. Nakajima, M. Tsuda, S. Oikawa, Chem. Pharm. Bull. 35 (1987) 941.

(1997) 151-157

157

[35] H. Arai, S. Kato, S. Koda, J. Phys. Chem. 98 (1994) 12. [36] S. Baskin, J.O. Stroner Jr., Atomic Energy Levels and Grotrian Diagrams, Vol. 1, North Holland, 1975. [37] T. Taketsugu, T. Hirano, J. Chem. Phys. 99 (1993) 9806. [38] J.A. Pople, K. Raghavachari, M.J. Frisch, J.S. Binkley, P.v.R. Schleyer, J. Am. Chem. Sot. 105 (1983) 6389. [39] L.B. Harding, H.B. Schlegel, R. Krishnan, J.A. Pople, J. Phys. Chem. 84 (1980) 3394. 1401 R.A. Eades, P.G. Gassman, D.A. Dixon, J. Am. Chem. Sot. 103 (1981) 1066. [41] S.P. Walch, J. Chem. Phys. 98 (1993) 3163. [42] B.T. Luke, J.A. Pople, M.-B. Krogh-Jespersen, Y. Apeloig, M. Kami, J. Chandrasekhar, P.v.R. Schleyer, J. Am. Chem. Sot. 108 (1986) 270.