J. Phys. Chem. 1995, 99, 1466-1469
1466
Reaction of NH(alA) with Si&: Comparison with CH4 and C3Hs Atsumu Tezaki,* Kazuyo Morita, Akira Miyoshi, Tsuyoshi Sakurai, and Hiroyuki Matsui Department of Reaction Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Received: May 16, 1994; In Final Form: October 24, 1994@
By using a laser induced fluorescence technique, the reactions of NH with S i b , C h , and C3H8 are investigated. From the first-order decay rates of NH(a'A, X3Z-), the rate constants are determined to be kl = (1.36 f 0.02) x 10-lo, k2 < lo-',, and k3 = (3.26 f 0.17) x (cm3 molecule-' s-') for the reactions NH(a) SiH4 ( l ) , NH(X) Si& (2), and NH(a) CHq (3), respectively. By calibrating the LIF intensities of N H 2 and H vs NH(a) using the reaction NH(a) H2 NH2 H, product branching fractions for the above reactions are determined; that is, the yields of NH2 are found to be 0.39 f 0.07,0.86 f 0.10, and 0.20 f 0.05 for reactions 1, 3, and NH(a) C3Hs (4), respectively. The yield of H atoms is concluded to be minor, and quenching of NH(a) is found to be negligibly slow in all these reactions. Time-resolved mass spectrometric measurements indicate that the main product of reaction 1 is Hz.
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(le)
of particular concern in addition to the reaction rates, because of ihe many exothermic channels. There seems to be no preceding report on the NH S i b reaction. In contrast, NH alkanes have been studied by several researchers.16-18 The rate constants for NH(a) with alkanes are sufficiently large at room temperature, for example, 3.0 x lo-'* cm3 molecule-' s-' for NH(a) CHq, and larger for higher alkanes, but the reactions of NH(X) were found to be very slow. The yield of NH2 is considered to be approximately unity in the case of NH(a) C b . For higher alkanes, the fraction for NH2 production is much less than unity, while a substantial amount of compounds containing a C-N bond are p r o d ~ c e d . ' ~ - ~ ~ The rate constant for the reaction of O(3P), which is an isoelectronic analogue of NH(X), with S i b was reported to be about 3 x 10-13cm3molecule-' s-' at 300 K with an activation energy of 15 kJ mol-'. The rate constant for the reaction O('D) S i b was estimated to be 5 x lo-" cm3 molecule-' s-I at room temperature.' Energy distribution in the product OH of these reactions was also m e a ~ u r e d . ~ , ~ , ~ In this study, NH S i b reactions were traced in a manner similar to our previous examinations on NH reaction^.^^,^^ The rate constants for NH(a) S i b and NH(X) S i b and the branching fractions for the several products of NH(a) S i b were determined. The reactions NH(a) CHq and NH(a) C3H8 were also investigated for comparison. A time-resolved mass spectrometric measurement was conducted to detect products that are not suitable for the LIF method.
(10
11. Experimental Section
(1g)
NH(a) was generated by the photolysis of HNCO with an ArF laser (Questek 2220) at 193 nm in a quasi-static flow cell. NH(X) was prepared by quenching NH(a) with a sufficient amount of Xe in the photolysis of HNCO. The LIF setup was the same as that used in our former s t ~ d i e s . ~ A ~frequency,~~ doubled tunable dye laser (Spectra-Physics PDL-3) was used to excite NH(a, X) by the transitions NH(c'lI-a'A) (0'-0") at 325 nm and NH(A311-X3Z-) (0'"'') at 335 nm, respectively. NH2 in its vibrational ground state and in the first v2 excited state was detected at transitions of A2A1-R2B1 Z(0,9,0)'(O,O,O)" at 598 nm and A2A1-R2B1 Z(0,14,0)'-(0,1,0)" at 507 nm, respectively. SiHz was also probed similarly with a transition of A1B1-g1A1 (0,2,0)'-(O,O,O)" at 580 nm.24
I. Introduction Reactivity of silicon compounds differs from that of relevant carbon compounds in many cases, as demonstrated by the intense flammability of silane/O2 mixtures.' Some of the elementary reactions related to the silane oxidation, such as Si& O(3P, and SiH3 02,8-11 have been extensively studied in recent years. In contrast, experimental studies on the elementary reactions that yield nitrogen-containing silicon compounds are very rare thus far, despite the importance of silicon nitride as a material for thin film deposition or ultrafine powder synthesis. It is desirable to examine the detailed reaction mechanism of Si-N bond formation in relation to the silicon nitride chemical vapor deposition proce~ses.'~-'~ The possible reaction channels of the title reaction are listed below with the reaction enthalpies given by Melius and Ho.15
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(AH", /k.~mol-') NH(a'A)
+ SiH, - SiH, + NH, (-153) -H3SiNH + H (- 109) -H2SiNH, + H (-207) -NH(X3Z-) + SiH, (-152) - SiH, + NH, (-317) -H,SiNH + H, (-37 1) - HSiNH, + H, (-432)
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NH(X3Z-)
H3SiNH, (-590)
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(14 (1b) (IC) (Id)
(Ih)
+ 2H, (-201) SiNH + 2H, (-429) HSiN
(li) (lj)
+ SiH, - products
(2) In this paper, branching fractions among these products are @Abstractpublished in Advance ACS Abstracts, January 1, 1995.
0022-3654/95/2099-1466$09.00/0
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0 1995 American Chemical Society
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Reaction of NH(a'A) with S i b
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J. Phys. Chem., Vol. 99, No. 5, 1995 1467
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Timeips
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3 4 5 6 [SiR]I 1Onmolecule.cmJ
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Figure 2. Decay rate of NH(X) as a function of S a concentration. [HNCO] = 1 mTorr and [Xe] = 80 mTorr in 10 Torr total pressure
maintained by He.
1 (a) CFq+He buffer
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[SiH4]/1014m01ecule.cm'3 Figure 1. (a) LIF intensities of NH(a) as a function of time after the photolysis of HNCO. [HNCO] = 1 mTorr in 7 Torr (1 Torr = 133.32 Pa) total pressure maintained by He. (b) Pseudo-fust-order decay rate of NH(a) as a function of Si& concentration. H atom was detected by a vacuum-W LIF at the Lyman-a transition, where the probe light was generated in a Kr cell by a frequency tripling of an excimer-laser-pumped dye laser (Lambda Physik LPD3002/ LPXl 1Oi) at around 364.8 nm. The vacuum-W fluorescence was monitored by a solar-blind photomultiplier (Hamamatsu R972). A mass spectrometer was also used to search for other products. The apparatus used in this work has been described in detail elsewhere." A Pyrex flow cell of 1.6 cm i.d. was equipped in a vacuum chamber of a quadrupole mass spectrometer (Anelva TE-600s). HNCO was irradiated by the ArF laser to initiate the reaction of NH(a). The gas in the cell was continuously sampled through a 100 pm pinhole located at the side wall into an ionization chamber. The sample gas was ionized by electron impact with low energies (-10 eV) and detected by a secondary electron multiplier. The signal was recorded with a multichannel scaler with a gate width of 50 or 100 ps. All the experiments were carried out at room temperature (295 f 3 K). The HNCO was synthesized by heating a mixture of potassium cyanate and stearic acid in vacuo and purified by distillations. Its purity was checked by means of a FT-IR spectrometer. HR-grade S i b (99.99995%, Nippon-Sanso), which was diluted to 1%in He by the supplier, was used without further purification. H2, Xe, CF4, C&, C3Hg, and He were also used as supplied.
III. Results A. Overall Rate Constants. As shown in Figure 1, the LIF intensity of NH(a) exhibited single-exponential decay in the presence of Si&, and the rate constant for NH(a) S i b (1) was determined to be kl = (1.36 f 0.02) x cm3 molecule-' s-l (error limit is 2a; including only statistical deviations) from the slope of the pseudo-fist-order decay rate vs [Sib]. On the other hand, NH(X) showed no dependence of the decay rate on the Si& concentration, as shown in Figure
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b l pJH(a)+H2]
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Time@ Figure 3. LIF intensities of NH2 (O,O,O)after the photolysis of HNCO with Hz or S a in the buffer gas of (a) 1 Torr of CFd 5 Torr of He; (b) 6 Torr He. [HNCO] = 5 mTorr; [H2] = 0.48 Torr (a1 and bl); [ S a ] = 13 mTorr (a2 and b2). Solid circles show the time profies of total [NH2] expected from the NH(a) decay rate.
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2. From the uncertainty of the decay rate, the upper limit of the rate constant for NH(X) S i b (2) was evaluated to be cm3molecule-' s-'. In the same manner, the rate constant for NH(a) C b (3) was obtained as k3 = (3.26 f 0.17) x cm3 molecule-' s-', which is in good agreement with the recent publication,18i.e., (3.0 f0.3) x cm3 molecule-'
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S-1.
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Apparently, NH(X) S i b is much slower than O(3P) Si&. On the other hand, NH(a) S i b is very fast, i.e., near the collision frequency. Since the reported rate constant of O(lD) S i b is 5 x lo-" cm3 molecule-' s-l? this may be a rare case that an NH(a) reaction is faster than the relevant reaction of O(*D). B. Branching Fraction for N H 2 Production in Reaction 1. A reference reaction of NH(a) with H2 (5) was used to determine the branching fraction for (la):
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NH(a)
+ H2-NHz + H
(5)
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Since the branching fraction for the NHz H channel is regarded to be unity at low pressures,22it is possible, in principle, to obtain the branching fraction, k&l, by comparing the LIF intensity of N H 2 in (1) with that in (5), as shown in Figure 3. Here, the following procedures were required to achieve a reliable measurement.
Tezaki et al.
1468 J. Phys. Chem., Vol. 99,No. 5, 1995 TABLE 1: Rate Constant and Product Branching Fraction for the Reaction of NH(alA) Reactants (R)
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R
k,lcm3 molecule-'
@H
SKI
Si&
(1.36 i: 0.02) x 0.39f 0.07 0.14 f 0.04 (3.26 f 0.17) x lo-'* 0.86 i: 0.10 S0.22 (3.0 f 0.3) x lo-'* C3H8 0.20 f 0.05
