J . Phys. Chem. 1993,97, 720-728
720
Theoretical Study of the Thermochemistry of Molecules in the Si-C-CI-H System Mark D. Allendorf and Carl F. Melius' Combustion Research Facility, Sandia National Laboratories, Livermore, California 94551 -0969 Received: September 8. 1992
Ab initio electronic structure calculations coupled with empirical corrections are used to obtain a self-consistent set of heats of formation for molecules in the series CHSiCl, (n,m = 0-3) and for selected molecules in the series (CH3),,SiH,Clp. Heats of formation are also reported for the series Cl$iSiCl, (m, n = 0 - 3 ) and for H C e S i C l Z H and H$=CH(SiC12H). Gibbs free energies as a function of temperature and standard entropies are given for all molecules in the study. Heats of formation are used to evaluate potential pathways for the decomposition of C13SiCH3, a common silicon carbide chemical vapor deposition precursor. The analysis includes the calculation of the transition state for the 1,Zelimination of HCl from Cl3SiCH3.
I. Introduction Chlorinated silanes arecommonly used by industry as reactants in a wide variety of materials synthesis processes. For example, chlorinated organosilanes are used as precursors to polydimethylsiloxanes. In addition, these compounds are used to deposit silicon carbide (Sic) by chemical vapor deposition (CVD). Methylated chlorosilanessuch as CH$X!l3 and (CHd2SiC12are of particular interest for this application.'+ Chlorosilanes such as SiH2C12, SiCl,, and SizCl~can also be used instead of silane and disilane in the chemical vapoi deposition (CVD) of silicon because homogeneous nucleation of fine particulates is less likely to occur with these reactants.] This broad interest in chlorinated silicon compounds has produced the need for a reliable base of thermodynamic data, which is an important prerequisite for the development of computationalmodels that can assist in developing new manufacturing processes. Thermodynamic data for compounds in the Si-C-Cl-H and Si&l, ( m = 1-6) systems is very limited. Heats of formation are availableonly for stable, saturated compounds.7J This makes it particularly difficult to study thegas-phasechemistry occurring during materials synthesis, since the high temperatures that are often used (typically above 1200 K for S i c deposition) produce large numbers of short-lived gas-phase species. Only a few such species have been detected, so that experimental measurements of thermodynamic properties are not often possible. Ab initio calculations can be used to provide temperaturedependentthermodynamic properties for both stable and unstable ~pecies.~We have used these methods combined with empirical bond additivity corrections (BAC's) to calculate thermodynamic data for a wide range of silicon compounds, including molecules in the Si-H-Cl,'O Si2H,,,I1Si-C-H,12 Si-H-F," and Si-N-HFI4 systems. In this paper we extend our investigations to compounds in the Si-C-Cl-H and Si2Cl, ( n = 1-6) systems to develop a self-consistent set of heats of formation and other hightemperature thermodynamic data. 11. Theoretical Methods
The theoretical methods used in these calculations have been presented in detail el~ewhere.~.'~ A general discussion is provided here, however, for the reader who is not familar with the computational technique. Molecular geometries, vibrational frequencies, and electronic energies were obtained using the Gaussian series of codes developed by Pople et al.I5 The equilibrium molecular geometry was determined using restricted HartreeFock (RHF) theory for closed-shellmolecules and unrestricted HartrebFock (UHF) .haory for open-shell molecules. A 6-3 1G* split-valence basis ,e, w:'h polarkation functions on the heavy atoms was used to ''
?-3654/58/2097-0720S04.00/0
describe the electronic wave function. Vibrational frequencies obtained at this level of theory were divided by a factor of 1.12, since they are known to be systematicallylarger than experimental values.16 To obtain accurate electronic energies, higher levels of theory incorporatingelectron correlation must be used. For this purpose, calculations employing fourth-order Mdler-Plesset perturbation theory with single, double, triple, and quadruple excitations were performed (MP4(SDTQ)), using a split-valence basis set with polarization functions on all atoms (6-31G**). The electronic energies obtained from MP4 calculations are still not sufficiently accurate to provide useful heats of formation, however. This is primarily a result of the finite basis set. Fortunately, the errors incurred are systematic and can be removed by the application of empirical correction factors. These factors are called bond additivity corrections (BAC's) in the method used here. Their values depend primarily on bond type and bond distance, but are also a function of the identity of neighboring atoms. For a molecule xk-x,-xj the BAC is given by
E,,c(Xi -
=iflkij
(1)
where
Aijand aijare empirically derived parameters that depend on the X,-Xj bond type and Rij is the bond distance (angstroms). The factor Bk in eq 4 depends on the identity of atom k. The values of Aij and aij used in these calculations are given in Table I. Neighboring-atom Bk factors used in this work are BH = 0.00, Bc = 0.31, Bsi = 0.20, and Bcl = 0.42. In the case of open-shell molecules an additional correction was made for spin contamination of the ground state by excited electronic states. The error in the electronic energy caused by this effect was estimated using the approach of Schlegel17and is given by EBAC(spins2) = E(UMP3) - E(PUMP3), where E(UMP3) is the third-order MP energy using the UHF wavefunction and E(PUMP3) is the projected UMP3 energy. This correction, though generally small (10.5kcal mol-I), may become large for molecules containing a high degree of unsaturation or low-lying electronic excited states. Closed-shell molecules that are UHF-unstable, such as SiH2, also require an additional correction. The form of the correction is EBAC(SPinUHF-unsuble) = KIJHF.IS(S + l ) , where KUHF.~ is 10.0kcal mol-' (based on the 0 1993 American Chemical Society
Thermochemistry of Molecules in the Si-C-Cl-H System
The Journal of Physical Chemistry, Vol. 97, No. 3,1993 721
TABLE I: Parametem for Bond Additivity Corrections for BAC-MP4(SDTQ) Level of Theory bond
A
ref swies
,
I
aa.b ~~
Si41 Si-C Si-H C-H
721.93 847.99
97.19 38.61
Sic14 SiH3CH3
2.0 2.5
SiH4 CH4
2.0 2.0
In A-I. * Based on related classes of compounds.
heat of formation of 03) and S is the spin obtained from the UHF/6-31GZ* calculation. The BAC's for all molecules in the study are given in Table I1 with the associated bond length, spin contamination, and UHF-unstable corrections. The heat of formation at 0 K is obtained by combining the unscaled zero-point energy with the BAC-MP4(STDQ) energy. Theentropy, heat capacity, and internal energyare then calculated by applying standard expressionsfrom statisical mechanics, using the HF/6-31GZ geometries and scaled frequencies as input. The abbrevation BAC-MP4 is used here to refer to heats of formation determined from the MP4(STDQ) energies. A major source of error in the thermochemical data is the determination of the molecular heat of formation at 0 K. An estimate of this error was obtained, using the results of calculations made at lower levels of perturbation theory during the determination of the MP4(SDTQ) electronicenergyn9Bond additivity corrections are applied to the MP2, MP3, and MP4(SDQ) electronic energies to obtain W r ( 0 ) at these levels of theory, resulting in the following definition for the estimated error: error(BAC-MP4) = (1.0 kcal mol-'
+ (AHBAC-MP4 -
mBAC-MP3)2 + (MBAC-MP4 - MBAC-MP4SDQ)2 + 0*25(EBAC(spin:) Or EBAC(spin",,,))2)"2 (5) It can be seen from this expression that convergenceof A H O r ( 0 ) to a fixed value as the level of theory increases from BAC-MP2 to BAC-MP4 will result in a low value for thecalculational error. The magnitude of error(BAC-MP4) also reflects spin and UHFunstable errors which tend to increase as the molecule becomes more unsaturated. This error estimate does not account for possible inaccuracies in the values of reference heats of formation. In particular, since there are no reference data for molecules containing S i 4 double bonds, a potential systematic error exists for this class of compounds.
III. Rmults and Discussion The results of applying the BAC-MP4 method to compounds in the Si-C-Cl-H system are given in Tables 111-V. Table I11 lists the values of W r ( 0 ) at different levels of theory, from which BAC-MP4 error estimates are calculated. Computed electronic energies, vibrational frequencies, moments of inertia, and Z-matrices are not given here but are available in the supplementary material (see paragraph at the end of the paper). Table IV gives the computed values of AHOr(298) and BACMP4 error estimates, with relevant literature values for comparison. Table V lists SO(298) and values of AGor at various temperatures. A. Heats of Formation and Bond Dissociation Enthalpies. Table I11 lists the heat of formation at several levels of theory for all the compounds in this study. In almost all cases, AHor(0) (the heat of formation at 0 K)is essentially the same at all levels of theory or converges to an approximately constant value. This indicates that error estimates for the BAC-MP4 heats of formation, as measured by eq 5 , are usually small. Table IV lists standard heats of formation at 298 K ( W r ( 2 9 8 ) ) for species in this study; the calculated uncertainties are generally 12.5 kcal mol-' for molecules without a high degree of unsaturation. In cases such as CHSiCl, larger uncertainties are found, which are
*
::::I \ \
-140
I
I
I
1
2
3
I 4
n
Figure 1. Calculated heats of formation for SiCI,(C&)~-, species compared with those for SiCI,H4-, species. The slight curvature of the line for the SiCl,(CH3)4-, species is indicative of nonlocalized bonding in these molecules.
caused by either spin contamination or UHF-unstablecorrections, resulting in a large spin contribution (Table 11) to the error estimate. Trends in the heats of formation of the methylchlorosilanes areobservable in thedata given in Table IV. Chlorine substitution for methyl groups has a strong stabilizing influence on these compounds. This is consistent with earlier studies that demonstrated this effect in SiCl,H4-,compounds. Unlike the SiC1,Hc, species, however, the amount of stabilization decreases as the number of methyl groups replaced by chlorine increases. This is shown in Figure 1, in which the W r ( 2 9 8 ) is plotted for SiCl,(CH3)4-, and SiCl,H4-, (n = 04). For the latter comp o u n d ~ , the ' ~ successive replacement of hydrogen by chlorine decreases W r ( 2 9 8 ) by 42 kcal mol-' per chlorine. The amount of stabilization that results from chlorine replacement of methyl starts at 30.6 kcal mol-I for the first chlorine and decreases to 20 6 kcal mol-I for the fourth chlorine. This nonlinearity implies that bonding in these systems is not completely localized, which complicates efforts to develop group additivity methods for these compounds. Luo and Benson have addressed this issue by accounting for the ability of the chlorine ligand to r-back-bond to silicon via overlap between the filled Cl(3p) orbital and the empty Si(3d) orbital.'*Jg This produces a significant amount of double-bond character in the silicon-halide bond. It should be noted, however, that 7-back-bonding alone cannot explain the nonlinearity of stabilization by chlorine in SiCl,(CH3)4-, compounds, since r-back-bonding is possible for both SiCl,(CHs)4-n and SiC1,H4-,, compounds. This implies that Si-H bonds are relatively localized, but methyl groups can interact with chlorine ligands in a manner not available to hydrogen. Reduction of the heat of formation by chlorine substitution can also be observed in unsaturated organosilicon compounds and in organosilicon radicals. For example, chlorine substitution for hydrogen has a large stabilizing effect on silaethylenes. Replacement of each hydrogen bound to silicon in H2Si==CH2 by chlorine stabilizes the molecule by 37.5 kcal mol-', a much larger effect than observed when methyl is substituted for hydrogen.12 Heats of formation for the radical species in this study follow similar trends when chlorine is substituted for methyl. For silyl radicals (Si(CH3)3-,Cln, n = 0-3), chlorine stabilizes the molecule when it replaces methyl, but the amount of stabilizationdecreases with increasingnumbersof chlorine ligands. This trend does not extend to the silylenes (SiXnY2-,, n = &2), where substitution of chlorine for methyl decreases A P r ( 2 9 8 ) by about 34 kcal mol-' in the series Si(CH3)2, ClSiCH3, Sic12 (32.2, -2.9, and -36.2 kcal mol-', respectively). Among the radical species in this study the silylene CISiCH3 is of particular interest since silylenes are in general expected to
Allendorf and Melius
722 The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 TABLE n: Bond Additivity Correctioacl for the MP4(SDTQ) Level of Theory (kcrl mol-’)
species
spinor UHFunstable correctiona
Si-C bond lengthb (no.).
BAC
Si-CI bond length (no.)
BAC
Si-H bond length (no.)
BAC
CI3SiCH3 C12Si(CH3)2 CISi(CH3)j
1.858 1.867 (2) 1.879 (3)
8.30 2.047 (3) 8.14 2.069 (2) 7.93 2.094
11.58 11.14 10.63
H2ClSiCH3
1.874
8.16
2.075
11.25
1.470 (2)
4.90
HC12SiCH3
1.864
8.27
2.059 (2)
11.47
1.463
4.98
HClSi(CH3)z
1.876 (2)
8.05
2.085
10.93
1.472
4.88
C12SiCHa
u 0.16
1.877
8.02
2.067 (2)
11.30
CISi(CH3)2
u 0.13
1.886 (2)
7.87
2.09 1
10.81
1.085
HClSiCH3
u0.13
1.883
7.98
2.080
11.17
1.475
CISiCH,
s 2.05
1.903
7.61
2.108
10.56
CHzSiCI3
u 0.60
1.832
8.84
CHzSiHC12 CH2SiH2CI C12Si4H2 HClSi4Hz
u 0.60 u 0.61
1.840 1.847 1.675 1.682
2.044 (2) 2.048 8.78 2.057 (2) 8.72 2.078 13.08 2.028 (2) 13.09 2.044
11.64 11.56 11.51 1.461 11.18 1.468 (2) 12.09 11.91 1.459
s 0.09
1.683 1.868
12.89 2.054 8.16
11.53
CHzSiCl
u 6.86
1.748
11.16 2.064
11.47
CHSiCI3
u 0.85
1.812
9.26
CHSiHC12 CHSiH2CI
u 0.85 u 0.87
1.818 1.828
9.26 9.16
CHSiCIZ CHSiHCl CHSiCl CSiC13
u 8.69 u 9.20 s 10.12 u -2.22
1.795 1.787 1.738 1.886
9.79 10.12 1 1.44 1.76
CSiHC12 CSiH2Cl CSiC12 CSiHCl CSiCl HCISi(CH3)CHz
u 0.18 u 0.19 u 4.09 s 10.81
1.825 1.835 1.811 1.755 1.804 1.850 1.875
s 1.58
u 6.96 u 0.61
2.038 2.043 (2) 2.056 (2) 2.073
2.056 (2) 2.072 2.130 2.036 2.038 (2) 9.10 2.048 (2) 8.98 2.064 9.42 2.053 (2) 10.95 2.036 9.70 2.089 8.58 2.088 8.08
11.77 11.65 11.53 1.458 11.30 1.465 1.468 11.51 11.31 1.466 10.07 11.84 11.80 11.71 1.458 1 1.50 1.465 (2) 11.58 12.1 1 1.462 10.93 10.87 1.470
SiSi bond length
BAC
4.85
4.99 4.92 5.02
5.03 4.95 4.92 4.94
BAC
1.085 (3) 1.085 (6) 1.086 (6) 1.088 (3) 1.087 1.085 (2) 1.085 1.086 (2) 1.085 (2) 1.086 (2) 1.088 (2) 1.085 (2) 1.086 1.085 (2) 1.087 (4) 1.085 1.086 (2) 1.086 1.089 (2) 1.076 (2)
4.41 4.41 4.40 4.38 4.39 4.41 4.41 4.40 4.41 4.40 4.39 4.41 4.40 4.41 4.39 4.41 4.40 4.40 4.37 4.49
1.077 (2) 1.077 (2) 1.075 (2) 1.075 1.076 1.075 1.077 1.085 (3) 1.076 1.077 1.072
4.48 4.48 4.50 4.50 4.49 4.50 4.48 4.4 1 4.49 4.48 4.53
1.072 1.072
4.53 4.52
1.073 1.072 1.070
4.52 4.52 4.55
C-C bond length
BAC
1.193 1.326
14.78 9.13
5.03 4.95 4.99 4.90
u 0.61
1.842 1.865
8.66 2.066 8.17 1.070
11.19 11.11
u 0.61
1.854 1.877 (2)
8.43 2.098 7.96
10.56
HCWSiC12H H2C-.CH(SiC12H)
1.815 1.850
8.87 8.34
2.048 (2) 2.059 (2)
11.70 1.457 11.48 1.462
C13 SiCH, C12SiCH2 + HCld SizC16 SizCls
1.729
1 1.45
1.997 (2)
12.86
1.594r 4.37e
u 0.28
5.82 5.70
s 7.68 s 3.31
2.459 2.425
4.34 4.74
Cl$3iSiCl
u 4.59
11.84 11.81 1 1.76 11.66 1 1.48 11.54 11.44 11.18 11.51 11.34 11.15
2.348 2.356
CI~SiSiC12 Cl3SiSiCl
2.040 (6) 2.042 2.044 (2) 2.056 (2) 2.064 (4) 2.054 (2) 2.059 2.083 2.062 2.070 2.085
2.385
5.29
-
C-H bond length (no.)
5.04 4.99
1.077 4.48 1.078 4.47 4.41 1.085 4.40 1.086 4.39 1.087 1.076 4.48 4.47 1.078 4.41 1.085 1.086 (2) 4.40 1.078 (2) 4.47 1.086 (4) 4.40 1.087 (2) 4.39 4.65 1.059 1.077 (2) . , 4.48 1.079 4.47 1.398 2.15 1.079 (2) 4.46
Thermochemistry of Molecules in the Si-C-Cl-H System
The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 723
TABLE I1 (Continued)
species CIpSiSi Cl2SiSi ClSiSiCl ClSiSi Si2 C15H
spinor UHFunstable correctiono
Si-C bond lengthb
(no.).
BAC
u 1.49 s 5.80
u 10.03
HC12SiSiC12H H2CISiSiC13 HClSiSi HSiSiCl
s 5.98 s 3.25
Si-CI bond length (no.)
BAC
2.059 (2) 2.065 2.049 (2) 2.125 (2) 2.074 2.041 2.046 (2) 2.050 (2) 2.056 (4) 2.047 (2) 2.054 2.063 2.063 2.163
11.44 11.31 11.80 10.28 11.37 11.84 11.79 11.71 11.65 11.70 11.55 11.65 11.64 9.54
Si-H bond length
(no.)
BAC
C-H bond length
Si-Si bond length
BAC
2.390
5.23 8.67 4.62 7.38 5.83
1.462
4.98
2.202 2.437 2.263 2.348
1.462 (2) 1.467 (2)
4.98 4.93
2.348 2.346
5.84 5.87
1.470 1.513
4.91 4.50
2.194
8.88
(no.)
BAC
C-C bond length
BAC
2.375
5.46
u, UHF-unstable correction; s, spin-contamination correction. In angstroms. e Number of bonds. Transition state for this reaction. Bond length and BAC for H-CI in transition state.
TABLE III: Calculated AIPdO) . . . for Si-C-CI-H Comwunds at Various Levels of Theory _ (kcal . mol-') species CIjSiCH, C12Si(CHj)2 CISi(CH3), H2CISiCH3 HCl2SiCHj HCISi(CH3)2 CI2SiCH3 CISi(CH3)l HCISiCH3 CISiCH, CH2SiC1, CH2SiHC12 CH2SiH2CI C12Si=CH2 HCISi=CHZ CI(CH,)Si=CHz CH2SiCI CHSiCI, CHSiHC12 CHSiHzCl CHSiC12 CHSiHCl CHSiCl CSiC13 CSiHC12
MP4 -79.3 -44.8 -6.6 -4.2 -41.9 -5.3 -6.8 30.6 30.8 32.2 -33.2 3.6 40.9 13.3 45.1 44.8 85.0 14.9 51.0 87.9 75.8 108.7 125.0 49.6 101.4
BACMP2 -135.8 -110.1 -80.8 -46.8 -91.5 -63.8 -51.6 -23.4 -7.3 0.6 -86.4 -42.9 1.4 -31.6 5.7 -8.9 47.2 -35.3 7.7 51.6 30.2 69.0 94.8 37.9 62.4
BACMP3 -135.5 -109.6 -80.5 -46.5 -91.1 -63.5 -50.1 -22.1 -6.2 0.8 -87.0 -43.3 1.1 -32.4 5.5
-9.3 48.1 -36.0 7.3 51.3 29.7 68.6 92.6 15.8 62.4
SDQ
BAC-MP4
SDTQ
species
MP4
BACMP2
BACMP3
SDQ
SDTQ
-135.5 -109.7 -80.7 -46.5 -91.1 -63.6 -50.2 -22.2 -6.2 0.2 -87.1 43.2 1.1 -32.6 5.2 -9.6 46.7 -36.0 7.4 51.3 29.2 68.0 90.6 10.6 62.8
-135.6 -109.8 -80.6 -46.6 -91.3 -63.6 -50.8 -22.5 -6.5 -1.2 -86.4 -42.7 1.6 -32.9 4.5 -10.1 46.6 -34.9 8.3 52.2 29.7 68.6 88.9 8.6 63.6
CSiH2CI CSiCl2 CSiHCI CSiCl HCISi(CH,)CH2 C&Si(CH,)CH2 CISi(CH&CH2 HC=CSiC12H H?C=CH(SiCl*H) C13SiCH3 C12SiCH2 HCP Si2C16 Si2C15 C12SiSiC12 Cl$3iSiCl C12SiSiCI CI3Si Si ClzSiSi ClSiSiCl ClSiSi Si2CI5H HC12SiSiC12H H2CISiSiCI3 HClSiSi HSiSiCl
137.5 125.3 158.2 172.7 39.7 0.7 38.4 25.9 -4.7 10.0
105.6 88.3 125.6 153.5 -15.6 -61.4 -32.6 -30.1 -62.5 -43.7
105.8 88.5 121.9 153.5 -16.0 -61.8 -33.0 -30.2 -63.1 -42.0
106.1 88.3 121.0 149.4 -16.1 -61.9 -33.2 -3 1.O -63.2 -41.2
106.9 119.4 145.1 -15.5 -61.2 -32.4 -30.9 -63.6 -42.6
-155.7 -87.1 -38.2 -45.8 19.0 14.5 64.6 63.8 127.4 -1 18.4 -8 1.2 -82.7 98.7 102.0
-232.7 -152.2 -92.8 -97.7 -24.0 -25.7 36.3 36.6 100.1 -188.2 -143.8 -145.3 71.2 82.8
-230.1 -149.0 -89.7 -95.4 -20.9 -24.0 35.0 38.1 101.0 -186.0 -141.8 -143.3 69.1 83.1
-230.6 -149.5 -90.6 -96.4 -22.1 -24.8 34.5 36.6 99.6 -186.3 -142.0 -143.5 68.8 81.9
-232.6 -151.7 -96.2 -99.6 -24.8 -26.4 32.4 32.8 98.6 -188.1 -143.6 -145.0 67.3 79.2
-+
BAC-MP4
88.6
Transition state for the indicated reaction.
be products of the thermal decomposition of silanes. The singlettriplet splittingin these molecules has been correlated with silylene reactivity with respect to insertion into single and double bonds.12-20,21 In an earlier paper it was shown that the magnitude of this splitting increases with the electronegativity of the substituents.12 In ClSiCH3, AHO(singlet-triplet) is 37.2 kcal mol-', which is nearly the mean of the splittings for Si(CH& and Sic&. This is consistent with localized bonding in the molecules and indicatesthat measurementof activationenergies for insertion for a few Six2 species should allow those for mixed species to be predicted. Table V gives entropies and free energies of formation at selected temperatures. Polynomial fits to the temperature dependence of the enthalpy, entropy, and heat capacity, in the format used by the NASA22and CHEMKIN23v24 thermochemical data bases, are given in Table XI1 of the supplementary material. Bond dissociation energies (BDE) for most of the species in this study are given in Table VI. As is the case with the heats of formation, patterns can be discerned that reflect the nonlocalized nature of the bonding in the compounds containing both methyl and chlorine ligands. For example, substitution of methyl
for chlorine strengthens the S i 4 1 bond by about 2.5 kcal mol-1; in contrast, hydrogen substitution has a much weaker effect and can even weaken the S i 4 1 bond. These effects are illustrated in Figure 2, which shows two different views of the surface of Si-Cl bond strengths formed from all possible combinations of H, C1, andCH3 ligands. Thedashed lines in the figurecorrespond to constant numbers of methyl groups, showing that varying the number of hydrogen and chlorine ligands typically has little effect on the Si-Cl BDE. In contrast, exchanging methyl for hydrogen with a constant number of chlorine ligands increases the S i 4 1 BDE by about 3.5 kcal mol-l (solid line, Figure 2A). Note that this case corresponds to constant d-pu back-bonding. Exchanging methyl for C1 also increases the S i 4 1 BDE, but by a smaller amount, roughly 2.3 kcal mol-l per methyl (solid line Figure 2B). The high-low-high variation of the S i 4 1 BDE in the C1Si(CH,),, (n = 1-3) series and of the Si-C BDE in the H,CSiCl, series (n = 1-3) has been observed in other related molecules (SiH,,,IO SiCI,,,lO and Si(CH3),,'2 n = 1-4) and is indicative of the added stability associated with the singlet relative to the triplet divalent state of silicon. Walsh has defined the divalentstate stabilization energy (DSSE) as DSSE = BDE(R3Si-R) -
Allendorf and Melius
724 The Journal of Physical Chemistry, Vol. 97, No. 3, 1993
TABLE Iv: Calculated AHor(298) for SIX-CI-H Compounds at the BAC-MP4(SDTQ) Level of Theory with Error Estimates. ( k d mol-') species BAC-MP4( SDTQ) lit. values species BAC-MPYSDTQ) lit. values ~~~~
~
ChSiCH7 CI;S~(CH~)~ CISi(CH,), H2CISiCH3 HC12SiCH3 HCISi(CH3)2 C12SiCH3 CISi(CH& HClSiCH3 ClSiCH3 CH2SiC13 CH2SiHC12 CH2SiH2CI C12Si=CH2 HCISi=CH2 CI(CH3)Si=CH2 CH2SiCI CHSiCI3 CHSiHC12 CHSiH2Cl CHSiC12 CHSiHCl CHSiCl CSiCI3 CSiHCIz CSiH2CI CSiC12 CSiHCl CSiCl HCISi(CH3)CH2 C12Si(CHp)CH2 CISi(CH3)2CH2 HC=CSiC12H H2C=CH(SiC12H)
-137.8 f 1.0 -1 13.7 f 1.0 -86.3 f 1.0 -50.1 f 1.0 -94.2 f 1.0 -68.2 f 1.0 -52.8 f 1.4 -26.2 f 1 . 1 -9.1 i 1 . 1 -2.9 f 2.8 -87.7 f 1.4 4 4 . 7 f 1.3 -0.9 f 1.2 -34.2 f 1.2 2.6 f 1.8 -13.1 i 1.3 45.7 f 3.9 -35.3 f 2.0 7.2 f 1.7 50.5 i 1.7 29.6 i 4.5 67.9 i 4.8 89.1 i 6.6 9.0 f 7.6 63.2 i 1.8 105.9 i 1.7 89.1 f 2.3 119.5 f 6.3 146.1 f 10.1 -19.2 i 1.3 -64.3 i 1.3 -37.2 f 1.4 -31.8 f 1.2 -66.3 f 1.2
-136 i 3:b-126.4C 1 1 1 f 26' -84.6 f lib-85.ld -50.2 i 2;b-51.3d -93.9 f 2b -67.4 f 2;b-68.2d -58.1' -33.1' -17.W -13.6e
-
CllSiCHq ClzSiCHz + HCI SizC16 Si2Cl~ C12SiSiC12 CI,SiSiCI C12SiSiCl CI3SiSi Cl2SiSi ClSiSiCl ClSiSi Si2ClsH HCI,SiSiCI,H H2CiSiSiCI; HClSiSi HSiSiCl Sic13 SiHC12 SiH2Cl Sic12 SiCl HSiCl SiH
Sic Si CH3 CH2 CH C CHCH2 C2H CI H
4 4 . 8 f 1.8 -232.8 f 3.4 -151.7 i 3.7 -95.7 f 9.4 -99.6 f 5.6 -24.4 f 5.4 -26.2 i 3.2 32.7 f 3.5 33.5 f 7.2 99.1 f 5.7 -189.0 f 3.0 -145.2 f 2.6 -146.6 f 2.5 67.1 f 3.9 79.1 i 5.1 -76.0 i 1.6' -33.4 i 1.4i 7.9 f l . l j -36.2 f 3.7J 37.8 i 2 . 3 15.9 f 2.Q 91.0 1.1) 171.9c 107.4 f 0.0, 34.9 f 1.2' 92.8 i 1.4' 143.4 f2.0' 171.2 i 0.0' 71.0 f 3.5' 132.2 f 6.4' 29.0 f 0.0' 52.1 i 0.0'
-228.9: -243.58 -129.5' -127.0'
-193.1h -157.8h -152.8h
-95.11
* Error estimates indicate only relative appliability of the calculational methods. See text .-r discussion. Reference 7. Reference 3 1 . d Reference 19. Reference 34. /Reference 33. 8 Reference 26. Reference 35. Reference 36. j Reference 13. Reference 24. BDE(R2Si-R). The strong stabilizinginfluenceof C1is illustrated by the increase in DSSE from 29.2 to 42.6kcal mol-' in going from Si(CH3)2 to Sic12 as the divalent state. Bonds to hydrogen in these compounds are less affected by substitution at silicon than are the Si-C and Si-Cl bonds. The BDE for Si-H bonds is essentially the same as those in the methylsilanes, with little change observed when chlorine is substituted for methyl. This illustrates the remarkable constancy of the Si-H BDE, which has been remarked upon previously.12v25 The C-H bonds in CH3groups are also essentially unaffected by substitution at silicon and are only slightly weaker than their hydrocarbon analogues. For example, BDE(H-CH2CC13) = 104.4 kcal mol-', while BDE(H-CH3SiCIp) = 102.2kcal mol-'. The strength of Si-C double bonds in C1-substituted silaethylenes decreasesdramatically with chlorine substitution at silicon. The BDEs for H 2 S i 4 H 2 , H C l S i 4 H 2 , and C12Si=CH2 are 116.9, 106.0,and 90.9kcal mol-', respectively; the double bond in the last compound is actually weaker than the S i c single bond in H3SiCH3. It is expected that the double bond in these compounds,which is weak relative to those found in hydrocarbons (BDE(C2H4) = 172 kcal mol-'), should be further weakened by chlorine substitution, since electrons in the filled 3p orbitals of chlorine can overlap with the r-antibonding orbitals centered on silicon. This can be quantified by calculating the r-bond strength D,, defined as D, = D,(Si-H) + D,(C-H) - D(H-H) + L y i h y d , which is basedon reactionsof the type CH3SiHCI2+ C@i=CH2 + H2. In this equation, L y i h y d is the hydrogenation energy of the Si< bond and D,(SiCl) and D,(C-H) are bond dissociation energiesin CHpSiHC12. In the series H2Si=CH2, HClSi=CH2, C12Si=CH2, thevaluesofD,are41.1,37.5, and 30.9 kcal mol-', respectively. From these values it is clear that the a-bond is weakened as well as the *-bond. Substracting D, from the total DBE yields 75.8,68.5,and 60.0kcal mol-', respectively, for the
H2Si=CH2, HCISi=CH2, and C12Si=CH2 a-bond strengths. This is in contrast with the effect upon Si-C single bonds when chlorine is substituted for hydrogen, where the bond strength actually increases from about 2.5 kcal mol-' for every new chlorine ligand. In addition to the chlorinated organosilicon species discussed above, we have also included chlorinated disilanes in the series CISiSiCl, (n,m = 1-3) as well as a few mixed disilicon species containing both C1 and H. As in the other compounds studied, chlorineligands exert a large stabilizinginfluenceon the molecule. Comparing the heats of formation of Si2H6 and SisCl6, each chlorine on average stabilizes the disilane by 42.0kcal mol-', the same as is found for SiC1,H4-, species. The heats of formation of Si2C15Hand H2ClSiSiC13 relative to Si2C16 demonstrate that the 42.0 kcal mol-' stabilization is not affected by the number of chlorine ligands in the molecule. Thus, like the related molecules containing only one silicon atom, chlorine replacement of hydrogen results in an essentially fixed amount of thermodynamicstabilization,indicatingthat heats of formationfor higher silanes containing only silicon, hydrogen, and chlorine should be calculable within 1-3 kcal mol-' using group additivity methods. The S i 4 1 BDEs display a similar high-low-high pattern for the Si2CIn (n = 1-6) series. The silylene CI3SiSiC1, like other silylenes studied, is a ground-state singlet with a DSSE of 29.0 kcal mol-'. This represents a substantial stabilization of the divalent state, as expected, but not as large as observed for SiC14, where the DSSE is 42.6 kcal mol-'. The S i s i bonds in these compounds are marginally stronger than the one in disilane (81 kcal mol-' in Si2C16vs 76 kcal mol-' in SizH6), which is in reasonable agreement with an estimate derived from appearance potentials of 77 kcal mol-1.26 This result shows that S i S i bonds are essentially unaffected by substitution at silicon. Bonds between silicon and hydrogenareabout thesame in thechlorinated
Thermochemistry of Molecules in the Si-C-Cl-H System
TABLE V
The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 125
BAC-MP4 Thermocbemical Parameters for Si-C-CI-H Compounds at Various Temperahum (kelvin) ~
A U.00 -1 f
species
298
CI3SiCH3 C12Si(CH3)2 CISi(CH3)J H2CISiCH3 HC12SiCH3 HCISi(CH& C12SiCH3 CISi(CH3)2 HCISiCH3 CISiCH, CHzSiCI, CH2SiHC12 CH2SiH2CI C12Si=CH2 HCISi==CH2 CI(CH3)Si==CH2 CH2SiCI C H Si C13 CHSiHCl2 CHSiH2CI CHSiCl2 CHSiHCl CHSiCl CSiC13 CSiHC12 CSiH2CI CSiCl2 CSiHCl CSiCl HClSi(CH3)CH2 C12Si(CH3)CH2 CISi(CH3)2CH2 HC=CSiC12H H2C==CH(SiC12H) CI3SiCH3 C12SiCH2 + HClC Si2C16 SizCls C12SiSiC12 C13SiSiCl C12SiSiC1 CI3SiSi Cl2SiSi ClSiSiCl ClSiSi Si2ClsH HC12SiSiC12H H2CISiSiCI3 HClSiSi HSiSiCl
-137.8 -1 13.7 -86.3 -50.1 -94.2 -68.2 -52.8 -26.2 -9.1 -2.9 -87.7 -44.7 -0.9 -34.2 2.6 -13.1 45.7 -35.3 7.2 50.5 29.6 67.9 89.1 9.0 63.2 105.9 89.1 119.5 146.1 -19.1 -64.3 -37.2 -3 1.8 -66.3 44.8 -232.7 -151.7 -95.7 -99.6 -24.4 -26.2 32.7 33.5 99.1 -189.0 -145.3 -146.6 67.1 79.1
-
~
AGP
@Ob .J
298 84.5 88.7 90.8 72.0 79.6 81.6 80.8 82.7 72.9 71.0 90.2 83.3 74.2 75.2 68.5 78.4 69.4 88.4 80.9 73.1 81.1 70.6 70.3 86.8 77.5 69.9 76.7 67.7 68.1 83.8 92.2 92.9 82.0 84.7 90.4 111.1 109.1 102.1 101.5 92.4 89.4 79.9 81.8 71.4 108.0 99.8 100.6 71.3 73.0
300 -123.5 -94.3 -61.0 -38.7 -8 1.8 49.9 45.3 -12.9 -2.6 -0.4 -79.8 -38.0 5.2 -29.7 5.8 -3.2 43.9 -31.5 10.0 52.3 27.6 65.7 82.4 8.7 62.3 103.9 83.8 113.6 135.4 -6.2 -50.5 -17.2 -28.9 -54.9 -32.3 -215.6 -141.8 -91.7 -95.4 -25.5 -26.4 27.4 27.6 88.3 -174.2 -131.3 -132.9 61.0 72.5
600 -108.8 -73.9 -34.3 -26.4 -68.6 -30.4 -37.4 1.4 4.6 2.2 -72.2 -31.4 11.5 -25.1 9.4 7.4 42.4 -28.3 12.4 54.1 25.2 63.5 75.7 8.0 61.4 102.3 78.3 107.7 124.5 7.3 -36.6 3.5 -26.1 43.0 -19.4 -198.9 -132.4 -88.1 -91.9 -26.8 -26.7 22.1 21.6 77.7 -159.5 -1 17.4 -119.1 55.1 66.1
lo00 -89.0 46.1 2.1 -9.1 -50.6 -3.7 -26.4 21.1 14.9 6.6 -62.2 -22.3 20.5 -18.5 14.9 22.2 40.8 -24.3 15.7 56.7 21.9 60.6 67.1 6.7 60.3 100.5 70.9 100.1 110.3 25.8 -17.7 31.8 -22.3 -26.7 -2.0 -177.5 -120.5 -83.6 -87.4 -28.5 -27.1 15.4 14.0 64.2 -140.7 -99.0 -101.1 48.0 58.1
1500
2000
2500
-64.8 -11.9 47.3 12.3 -28.4 29.6 -12.8 45.8 27.8 12.3 -50.1 -11.2 31.6 -10.3 21.9 40.9 39.1 -19.7 19.7 60.0 17.8 57.1 56.7 5.0 58.9 98.3 61.9 90.9 93.0 48.7 5.5 66.7 -17.4 -6.5 19.5 -1 52.0 -106.5 -78.4 -82.1 -30.8 -27.7 7.3 4.6 48.0 -1 18.2 -76.9 -79.5 39.4 48.3
-38.8 23.7 93.6 35.5 -4.5 64.3 2.8 72.1 42.7 20.3 -36.1 1.8 44.6 0.0 30.9 61.3 39.6 -13.1 25.6 65.3 15.9 55.9 48.8 5.5 59.7 98.4 55.4 84.1 78.4 73.2 30.2 102.9 -10.4 15.5 43.0 -122.7 -88.4 -68.9 -72.6 -28.4 -23.7 4.1 0.2 36.7 -91.8 -5 1.O -54.1 35.6 43.2
-12.0 59.7 139.9 59.4 20.2 99.3 19.3 99.0 58.5 29.4 -21.2 15.7 58.5 11.4 41.1 82.4 41.5 -5.5 32.7 71.6 15.3 55.9 42.3 7.2 61.8 99.7 50.2 78.8 65.4 98.2 55.5 139.2 -2.5 38.2 67.4 -91.2 -67.9 -56.8 -60.4 -23.3 -16.9 3.8 -1.5 28.7 -63.2 -22.9 -26.4 34.8 41.0
In kcal mol-'. In cal mol-'K-I. Transition state.
disilanes as in disilane itself (88.4 kcal mol-' in Si& vs 89.4 kcal mol-' in S ~ ~ C I S H ) . B. MetbyltrichlorosilaneDecomposition. An important motivation for this study is to provide data required to evaluate the importance of organosilicon compounds in the CVD of S i c from chlorosilanes. A common precursor in this process is methyltrichlorosilane (Cl3SiCH3; MTS). Table VI1 lists reaction enthalpies for several possible decomposition pathways for MTS. As expected from the BDE's discussed earlier, the least favorable pathways from a thermodynamic point of view are reactions 3 and 6,which both involve the breaking of strong Si-Cl bonds. An energetically more favorable path is the simple fission of the Si-C bond to produce a methyl radical and a trichlorosilyl radical (reaction 1). The two lowest energy paths, the extrusion of SiC12 (reaction 4) and the 1,2-eliminationof HCl to give the silaethylene Cl*Si=CH* (reaction 5 ) , share the same enthalpy of reaction. The six reactions can be subdivided into two categories: simple bond fission (reactions 1-3) and elimination/insertion reactions (reactions 4-6). The former have activation energiescomparable to the bond strength but also high A-factors (typically L 10'6 s - I ) . ~ ~ The latter type often have activation energies lower than
those for bond fission but also have lower A-factors, typically 101L1015s-I, which is characteristic of their tight transition states.27 On this basis it is possible to remove some of these reactions from consideration since their rates are likely to be very slow. Thus, reaction 6,with both a high activation energy and an anticipated low A-factor, can be eliminated. Given A-factors comparable to reaction 1, reactions 2 and 3 will also play minor roles in the decomposition process. For the lowest energy pathways (reactions 4 and 5 ) to compete effectively with reaction 1, they must have substantially lower activation barriers since their A-factors are at least a factor of 10 times smaller. We thus performed BAC-MP4 calculations to determine the transition-state energies for these two paths. Attempts to determine W r ( 2 9 8 ) for the transition state of reaction 4 (which can be used to calculate the activation barrier for the reaction at infinite pressure) were unsuccessful, however, probably owing to a large activation barrier that leads to the breakage of the weakest bond in the molecule (the S i 4 bond) instead of elimination of SiC12. Instead, a lower limit to this energy was determined from the heat of reaction (8 1.5 kcal mol-'
Allendorf and Melius
726 The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 TABLE VI: Calculated Bond Dissociation Enthalpies at 298 K (kcal mol-') for Selected Molecules bond
BDE 111.4
CISiC13°
78.9
ClSiCICH3
87.3 86.9 106.4 18.5 110.1 81.1 84.9 100.5 102.4 104.1
CISi(CH3)2 CISiHCH3 CISiCH3 CISi(CH&CH2 CI-Si2C15 CISiClSiCI3 CISiC12SiC12 ClSiClSiCl2 CISiSiCI3 CISiC12SiCl
bond CISiC12CHj CISiH2CH3 ClSiCH3HCl ClSiCl2CH2 CISiHCICH2 CISiH2CH2 CISiCICH2 CISiHCH2 CISiCH2
CISiCI(CH,)CH2 CISiC12Si CISiSiC12 ClSiClSiCl ClSiClSi ClSiSiCl ClSiSi
BDE
bond
(a) Si-CI Bonds 114.0 CI-SiCI(CH3)2 112.2 CI-SiH(CH3)2 114.1 CI-SiC12CH 82.5 76.3 CI-SiHCICH 70.6 CI-SiH2CH 108.9 CI-SiCICH 112.2 ClSiCH 57.4
80.1 87.9 86.1 86.9 95.4 94.6 75.7
CI-SiH(CHj)CH2 ClSiHSi ClSiSiH
BDE
bond
BDE
116.5 115.5
CISi(CH3)I
118.4
93.9 89.7 83.9 88.5
CISiCl2C ClSiHClC CISiH2C ClSiClC CISiHC ClSiC
109.0 85.3 90.3 86.0 94.4 54.8
HSiC12C HSiHClC HSiClC HSiSiCl
78.0 65.1 18.1 72.1
CSiC13 CSiHCl2 CSiH2Cl
23.4 14.0 73.2
H3CSiHCICH3 CSiC12 CSiHCl
94.0 46.0 67.6
92.1 104.3 103.8 102.8
CSiCl
62.9
ClSiSi
64.3
74.5 85.3 73.3
(b) Si-H Bonds 93.1
H-SiHCICH3
58.3 89.4
HSiClCH3 HSiC12SiCI3 H-CH2SiCI3 H-CH2SiCH3C12 H-CH2Si(CH3)2Cl H-CH2SiHCI(CH3) H-CH2SiH2CI H-CH2SiC12 H-CH2SiHCI H-CH2SiCH3CI H-CH2SiCI
HSiC12CH3 HSiC12CH2 HSiHCICH2 HSiCICH2
102.2 101.6 101.2 101.2 101.3 70.6 63.8 65.2 100.6
H-CHSiCI3
96.7 95.1 92.9 89.6 95.9 51.5 59.8 58.2 63.9 75.5 64.9 56.7 59.0 93.7
H2CSiC13 H2CSiHC12 H2CSiH2CI
H-CHSiC12 H-CHSiHCI H-CHSiCl
H3CSi(CH3)2CI H2CSiC12 H2CSiHCI H2CSiCH3CI H2CSiCl
HSiCI(CH3)2 HSiC12CH HSiHClCH HSiClCH HSiClSi (c) C-H Bonds 104.5 H-CSiCI3 93.5 62.6 55.6 95.2
115.9 117.4
H-CSiC12
95.5 H-CSiCl (d) Si-C Bonds 104.5 HCSiC13 103.5 HCSiHC12 101.6 HCSiH2Cl 95.0 90.9 106.0 103.0 84.9
H3C-Si(CH3)lb HCSiCl2 HC-SiHCI HC-SiCI HzCSiCH3Clz H2CSi(CH3)2CI H2C-SiHCH3CI
94.1 74.5 69.5 73.3 84.1 96.5
111.6 109.1 102.7 102.8 100.8 93.8 77.6 91.4
130.0 103.3
(e) Si-Si Bonds 80.8 19.6 78.5 78.4 39.5 23.4 61.3
C13SiSi C12SiSiCI
57.5 26.0
C12SiSi ClSiSiCl
38.5 42.0
HClSiSi
56.2
HSi-SiCI
49.7
46.0
Taken from ref 13. Taken from ref 12.
fromTableVI1) and the activation barrier for thereverse reaction, Ea(4).The value of Ea(-4),which is expected to be significant
on the basis of calculations2' and experiments,28-30 can be estimated by comparison with similar reactions. The only similar reaction for which the temperature dependence of the rate is known is SiF2 + C12 SiFzC12, with an activation barrier of 8 kcal Since the C-Cl bond is considerably stronger than the Cl-Cl bond (83.8 kcal mol-' vs 58.0 kcal mol-]), Ea(4)is likely to be significantly larger than this. We thus estimate Ea(4) = Ea(-4) + A W , > 90 kcal mol-], making it unlikely that
-
reaction 4 competes significantly with reaction 1 at deposition temperatures unless the A-factor for the forward reaction is 21015
S-1.
For reaction 5, a calculation of the transition-state geometry and energetics (see Tables 11-V for BAC's and thermodynamic properties) shows that the reaction proceeds by dissociation of C1 followed by H abstraction from the methyl group. A value of 93.0 kcal mol-' was obtained for E,(S). The size of this activation barrier is sufficiently large that, even with an A-factor as large as 1015 s-I, the rate of reaction 5 would be a factor of
Thermochemistry of Molecules in the Si-C-CI-H System
'--I
I
'--I
I
Number of CI Ligands (4 - m - n)
Figure2. Twoviews of the surface formed by the Si-CI bond dissociation energics for all possible compounds in the series SiHn(CH3)mC14-m-n (n, m = 0-3). Dashedlinescorrespondtoconstantnumbersofmethylgroups. A, top: The solid line in this figure corresponds to exchanging methyl for hydrogen with a constant number of chlorine ligands. This case corresponds to constant d-pa back-bonding. B, bottom: Another view of the same surface. The solid line corresponds to replacement of methyl by chlorine with a constant number of hydrogen ligands.
TABLE MI: Enthalpies of Reaction for Pathways in the
1.
2. 3. 4. 5. 6.
---
AHr' (kcal mol-')
bond fission reactions C13SiCH3 CH3 + Sic13 CI3SiCH3 CI3SiCH2 H CI3SiCH3 C12SiCH3 + CI elimination/insertion reactions
CI3SiCH3 CI3SiCH3 CI3SiCH3
+
CH3CI + Sic12 C12Si=CH2+ HCI CH3SiCI + CIz
96.7
102.2 114.0 81.5
81.5 134.9
2.4 smaller than that of reaction 1 at 1300 K, which is lower temperature bound for S i c CVD. Thus, we estimate that a maximum of 30% of the MTS would decompose by this route (with the remainder of the MTS decomposing through reaction 1). The combined kinetic and thermodynamic analysis described above indicates that the most favorable pathway for the gasphase decomposition of MTS is Si-C bond fission. Since S i c deposition typically uses hydrogen as the carrier gas, most of the methyl radical should be converted to methane through the reaction CH3 H2 CHI + H. The reactivity of the siliconcontaining product of reaction 1, SiCl3, is unfortunately not well characterized. One possibility for reaction is the abstraction of chlorine from SiC13 by the methyl radical formed in reaction 1 (Sic13 + CH3 Sic12 + CH3Cl). The heat of reaction for this process is -15.1 kcal mol-', which partially reflects the stability of the divalent state of silicon, as discussed above. The reactivity of Sic12 is expected to be relatively low with species such as Hz,
+
-
-
HCl, and CH4 that are expected to be present in high concentrations in the gas phase of a S i c CVD p r o c e ~ s . Thus, ~ ' ~ ~we~ ~ ~ ~ predict that substantial concentrations of this radical could be present during the CVD of Sic. The analysis also suggests that, under the most favorable circumstances(A-factor for reaction 5 1 1015s-l), an organosilicon compound (C12Si=CH2) could constitute as much as 30% of the initial productsof decomposition. This has significant implications for the analysis of gas-phase chemistry in S i c CVD systems, since the identity of the species most responsible for carbon deposition is currently unknown. Unsaturated organosilicon compounds such as C12Si=CH2 may have much higher surface reactivities (reactive sticking coefficients > 0.1) than other more abundant carbon-containingcompoundssuch as methane (reactive sticking coefficient lo4). Thus, our results underscore the need for experimental measurementsof MTS decompositionrates and product species concentrations in order to assess their importance to the transport of carbon to a growing S i c surface. C. Comparison with Literature Values. Experimental data and other theoretical predictionsfor comparison with these results are much more limited than in the case of previously studied compounds in the Si
