Reactions of SiH2(X̄1A1) with H2, CH4, C2H4, SiH4 and Si2H6 at 298 K

VoIlIme

122;nu.m~

CHEMICAL

4

REACTIONS

OF SiH&%‘A,)

Gen INOUE

and M. SUZUKI

Norionol Insrirure for Enuironmenrol Received

WITH

Studies. P-0.

19 June 1985: in final form 28 Augusl

H,,

PHYSICS

CH,,

LETlJZR.5

C,H,,

Tsukubo-gokuen.

SiH,

Iboroki

13 December

AND Si,H,

1985

AT 298 K

305. Japan

1985

Reaction rate constartls of SiHI(%‘At) have been directly measured for the first time using the laser photolysis-laser-induced fluorescence method. The preparation of SiH: radical in the laser photolysis (193 m-n) or phenylsilane and the concentration of the radiczd is demonstrated by a dye laser at 580.1 nm (%(‘AI-A’B,). The rerrction raw constants or SiH, and SilHL_ are 0.001. 0.01. 0.97_ 1.1 and 5.7 xIO-‘~ cm3 molecule-’ s-l_ SiH2(X’A,) with Hz. CH,. C,H,. respectively. For BH,(A’B,(O.Z.O)). the collision-free liretime is 0.6 ps and the quenching rate cot-&ant for He is 3.8X iO-” cm’

mole.cule-’ s-‘.

I_ Introduction

SiH2+H2+SiH

The silylene radical, SiH2, has been expected to play one of the most important roles in the chemical vapor deposition (CVD) process to form hydrogenated amorphous silicon (a-Si:H) as this radical inserts in the Si-H bond on the surface and forms a new SiSi bond. In a previous paper [ 11, we reported the laserinduced fluorescence of the Sti, radical, and excitation spectra with tentative rotational assignments and a dispersed fluorescence spectrum were shown. The wavelengths of the vibronic transitions in the 550650 run region, which correspond to the lBl(On’O) + lA1(OOO) transition, agreed well with the values of the absorption measurements [2,3]. The dispersed fluorescence spectrum, ‘Bl(O$,O)+ lA,(O&O), also agreed with the hot band absorption [3] _However, the details of the rotational structure did not necessarily agree with the values obtained by Dubois [4] _Nevertheless, the general agreement of the spectra and the observation of identical spectra from different sources, the photolysis of phenylsilane and disilane and the reaction of sikne with metastable argon, confirmed it to be fluorescence from the SiH2 radical. The reaction rate constants of the Stip radical have been obtained by co-pyrolysis and products analysis. John and Pumell [5] evaluated the rate constants of the following reactions from their co-pyrolysis data of disilane and thermodynamical information: 0 009-2614/G/$ (North-HoBand

03 30 0 Elsevier Science Publishers B.V. Physics Publishing Division)

(1)

4>

SiH2 + SiH, + SizH6 ,

(2)

SiH2 + Si2H + Si3Hg _

(3)

The rate constants (in cm3 molecule-l lated to 300 K are kl

= 2.0 X 10-16,

k3 = 6.1 X lo-l2

s-1)

extrapo-

k2 = 9.5-X lo-=,

_

However, no direct measurement of the rate constants of the SiH, radical has yet been reported probably because of the mass-spectrometricaly poor detectivity of silicon hydrides. Here we report reaction rate constants of SiHz with Hz, SiH4, Si2Hg. CH4 and C2H4, and the colIision-free lifetime of the SiH~(lB,) state.

2. Experimental The photolysis and laser-induced fluorescence system was the same as before [6]. Silylene radicals were

produced by photolysis

of phenylsilane

at 193 nm by

an ArF excimer laser. The temporal change of silylene radical concentration was detected by a fluorescence intensity measurement excited at 580 nm with different delay times between the photolysis and probe 361

VoIume 122, number 4

CHEMICAL

PHYSICS LETTERS

13 Decembez 1985

Table 1 Raie cyxtants (cm3 molecuIe-1 s-l) for the reaction of SiH2(X1Ai) kompared to CH2(g1AI) at 298 I; Reactant

z, C2H4

SiI-lq

SisHs 20

40

PRESSURE

(mTorr

02

I

60

00

SiHz (1.0*0.4) (l.OiO.3) (9_7+1.2) (1.1?0.2)

CH2(:‘AI) x x x x

10-13 10-12 lo-” 10-10

(5.720.2) x lo-”

-

(i-3*0.1) (7.3*0.6) (2.4tO.4)

x 10-10 a) X lo-” a)

(3.0*0.4)

x lo-”

x lo-”

a)

a) Values from ref. [lo].

)

Fig.Z.Plotof rmztionratevers!~ pn%weforthe IeaZtiOnOf SM2 with SiH~(o) and SiaHe(o) u&g 0.3 mT0r-r of phenyldlane as a precursor for SiHII and He as a buffer gas (Ptot = 1.0 Torr). The bimolecular rate constants obtained from the slope of the linear leaskquares fits are 1.1 X 10-l” and 5.7 x 1O-‘o cm3 molecule-’ s-l forSiH4 and SizHG. respectively. 3Ops, before 30 ps the observation was disturbed by luminescence caused by the photolysis laser. The rotational and vibrational distribution was therrnalized in 30~s and the laser-induced fluorescence intensity was proportional to the concentration of SiJ?i2(% lA,). So the removal rate constant obtained from fig. 2 is the reaction rate constant of SiH2(%1A1) reacting with SiH, or Si2H6. In fig. 3, the decay rates of SiH, are plotted against H2 partial pressure. The zero-pressure intercept ofthe reactant, about 80 ps, was mainly due to the flow of sample gas out of the viewing spot. All experiments were carried out with 0.3 + 0.05 mTorr phenylsilane. The typical total pressure in the

experiments was 1 Torr except for the experiment with hydrogen, for which a total pressure of 1.8 Torr was chosen because the rate constant was very small and a great amount of hydrogen was required to detect the acceleration of the decay of SiH,. A relatively small dynamic range of 20 was chosen for the decay measurement because the linearity of the experimental system was limited as the fluorescence lifetime at the pressure used was short; about 50 ns. The adjustment of the probe laser power to keep the fluorescence intensity at an appropriate level makes the dynamic range of the observation wide but this rnethod was not adapted in this case being afraid of the error caused by a limited dynamic range of laser power monitor, though a small correction for laser power fluctuations was made to the signal intensity. The reaction rate constants obtained are summerized in table 1 and the values of ICH, are tabulated for corn. parison [IO]. The disappearance rate constants of 1CH2 with SiH, were obtained here; lCH2 was produced by photolysis of ketene by a XeCl excimer laser (308 run). The removal rate constant of 1 CH2 by H2 was also measured to check our experimental system and the agreement with ref. [lo] was within experimental error.

4. Discussion

0.51

0.0

0.5 PRESSURE

1.0

I 1.5

(Torr)

Fig. 3. Plot of reaction rate of Siwz with H2 :kphenywe = 0.3 mTorr, Ptot = 2.0 Tom. The bimolecular rate mnstant is s-l_ (1.0 f 0.4) x 10-13 cm3 molecuk-

The reaction rate constants of the SiH, radical with SiH, and Si2Hs were obtained to be very large. The reaction mechanism of SiH, with Sif-& is generally accepted to be insertion in the Si-H bond, and the product is the vibrationally excited Si2H6 molecule. The fragmentation into two SiH3 radicals is endothermic [ 1 I] by 33 kcal rn01-~ if we use the AEf values of 363

Vohune

CHEMICAL

122. number 4

SiH4,SiH3andSiH20f73 [11],49.1 kcal rn01~ [5], respectively, SiH, +

sa4

+

Si2Hi +2SiH3

PHYSICS

[12] and57.9

.

The rate constant of 1CH, with SiI-I, was close to be the value of JsMz, the mechanism of which is also insertion in the Si-H bond, CH, + SiH, + CH3SiH3

_

However,.their rate constants with molecular hydrogen differ considerably from each other; ‘CH.2 reacts more than one thousand times faster than SiH2. The removal rate of rCH2 includes both chemical reaction and the physical quenching to %H2 which lies 9.6-9.1 kcal mol-1 below the z lAl state. In contrast, the singlet state of SiH2 is the ground state and no physical quenching is involved in the removal process. However, the physical quenching of lCH2 by H2 is expected to be a very small part of the total removal rate, because it is twenty times larger than the value expected from the linear relation between the removal cross sections by M and E’$J (the potential energy well depth between pairs of M species) of nonreactive molecules [lo] _ Ab initio studies of the insertion of CH2 and SiI-I, in Hz have been reported by Gordon [ 131 and more recently by Grev and Schaefer [ 14]_ Their results show that the insertion of CH, has no barrier, but the SiH, insertion in H, has a barrier of 7.1 kcal mol-l. The H-H distance in the transition state of the Sil-I, + H, reaction, J -07 A, is closer to that of the hydrogen molecule, 0.733 A, than that of the silane molecule, 2.417 A, and a large strain in the transition state is expected_ No ab initio study of SiH2 or CT-I2 insertion in the Si-H bond is reported, but their large reaction rates can be reasonably ex- _ plained by the qualitative speculation of small strain in the transition state because the final Si-H bond length of SiH2 insertion, 3.16 A, is twice that of the initial Si-H bond length of SiH4.1.47 A, and that of lCH2 insertion, 2.53 A, is lengthened only 1.7 times. The reaction rate constants of SiH2 with HZ, SiH4 and Si,H, extrapolated from high-temperature measurements by John and F’umell [5] were much smaller than the values obtained here. The upper limit of the extrapolation within experimental error is 1 .S X 1 O-l1 cm3 molecule-’ s-r and this is still one order of magnitude smaller than our observation_ We believe our direct method is more reliable though the source 364

LETTERS

13 December

1985

of this serious discrepancy is not clear. The reaction rate measurement of SiH, with 0, was unsuccessful because intense chemiluminescence after the excimer irradiation to the phenyldlane and 0, mixture disturbed the observation of fluorescence. The emission builds up immediately after the excimer irradiation with a time constant of 0.8 m and it is followed by a decay with a time constant of 2~~s at 200 mTorr of oxygen, for example_ The predicted reaction is LX-I, + 02i

SiO* + H20

_

The electronic energy that the SiO molecule can carry is 109 kcal mol-l, and the Z 3Z+, 6 311 and ;? 3a states can be populated, but the origin of the emission was not studied further at the present stage.

Aclmowledgement The authors are grateful to Mitsui Tohatsu Chemicals Co. for the offer of disilane.

References G. lnoue and M. Suzti, Chem. Phys. Letters 105 (1984) 641. PI I. Dubois, G. Henberg and R.D. Verva, J. Chem. Phys. 47 (1967) 4262. PI I. Dub&, Can. J. Phyq 46 (1968) 2485. 141 1. Dubois, Deposit No. 78, Depository of Unpublished Data. National Science Library, National Research Council. Ottawa. Canada. 151 P. John and JH. Purnell. J. Cbem. Sot. Faraday I 69 (1973) 1455. I61 G. Inoue, M. SLLZ~~ and N. Wasbida, J. Chem. Phys. 79 (1983) 4730. [71 TN. Bell, K.A. Perldns and P-G. Perkins. J. Chem. Sot.

111

PI PI

Faraday Trans. I 77 (1981) 1779. JH. Meadows and H.F. Schaefer III, J. Am. Chem. Sot. 62 (1975) 541. A. Kasdan, E. Herbst and W-C. Lineberger, J. Cbem. Phys. 62 (1975)

[lo] [ll]

541.

MN-R. A&fold, M.A. Fullstone, G. Hancock and G.W. KetIey. Chem. Phys. 55 (1981) 245. S.R. Gunn and L.C. Green, J. Phys. Chem. 65 (1961)

779.

[12]

P. Potzinger and F-W. Laupe, J. Phys. Chem. 74 (1970) 719. 1131 M.S.Gordod. J. Chem. Sot. Chem. Commun. (1981) 890.

1141 RI;. Grw and H.F. Schaefer III, J. Chem. Sot. Chem. Commun. (1983) 785.