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The electron bombardment dissociation and thermochemical stabilities of silicon tetrachloride and silicon dichloride
Journal of the Less-Common Metals, 53 (1977) 277 - 286 @ Elsevier Sequoia S.A., Lausanne -- Printed in the NetherIands
277
THE ELECTRON BOMBARDMENT DISSOCIATION AND THERMOCHEMICAL STABILITIES OF SILICON TETRACHLORIDE AND SILICON DICHLORIDE
A. OLSEN and F. R. SALE Metallurgy Department, Manchester University, Joint Metallurgy Building, Grosuenor Street, ~~la~ehester Ml ‘7HS (Gt. Britain) (Received December 17, 1976)
Summary The mass spectrum of SiCl,(g) is reported and compared with existing data. The relative abundances of the singly charged positive ions are in the order SiCl; > SiCli > SiCl+ > SiClg which indicates that silicon tetrahalide ions with an even number of electrons are more stable than ions with an odd number of electrons. The appearance potentials of all the singly charged positive ions have been determined and used to calculate the enthalpies of formation of the various species of ions. Thermal dissociation of SiCl,(g) in the analyser source of the mass spectrometer is shown to produce SiCl,(g). The appearance potentials of the ions obtained from the SiCl, are used in conjunction with tabulated values for SiCl,(g) to assess existing data for the enthalpy of formation of Sic&,(g).
Introduction The vapour deposition of silicon from silicon tetrachlo~de has been the subject of a number of studies [I - 51 and this work has been extended in recent years to include investigations of the deposition of silicon carbide [6 - 91 and mixed silicon-titanium carbide [lo]. Most of these investigations have attempted to relate the morphology of the deposits to the deposition rate and deposition temperature. Few have made a systematic study of the effects of pressure on the deposition processes. As a preliminary part of an investigation of the deposition of mixed silicon-titanium carbide using a small mass spectrometer to determine the partial pressures of the volatile species involved in the deposition reactions, it was necessary to investigate the dissociation mechanism, disintegration pattern and appearance potentials of the various ions produced from gaseous silicon tetrachloride. These data would enable the mass spectrometer to be calibrated at the various pressures of silicon tetrachloride to be used for the deposition studies, as well as
278
providing information on the electron dissociation which has been the subject of only a small amount [ll - 161.
Experimental
of silicon tetrachloride, of previous investigation
procedure
Mass spectrometer The mass spectrometer used in the study was a VG Micromass 6 fitted with a residual gas analysis source and a 60 mm 90” sector electromagnet. The resolving slit used was 0.05 mm which gave a resolution of approximately 400 at mass 40. A Faraday plate collector and amplifier was used as the ion detector. The sample inlet system to the mass spectrometer consisted of two stainless steel heated capillaries 2 m long and of internal diameter 0.3 mm. Each capillary contained an isolating needle valve and was mounted on a stainless steel cross-piece which was continuously pumped through a liquid nitrogen trap. The pressure maintained in the interspace was less than 1 Torr and was measured by a Pirani gauge. The entire inlet assembly was connected to the mass spectrometer via a calibrated glass leak. To admit a known pressure of sample to the mass spectrometer, one of the capillaries was connected to a one litre reservoir which had a mercury manometer attached to it. The reservoir and manometer were pumped separately to allow a gas to be expanded into the reservoir and then sampled continuously via the capillary. Operating conditions The mass spectrum of SiC& was obtained by continuously sampling the SiCl* from the expansion reservoir. The mass range 10 - 200 was scanned over a period of 1000 s using an accelerating voltage of 400 V, a trap current of 100 PA and an electron voltage of 60 eV. The usual background corrections were determined and applied to all data. For the measurement of appearance potentials a modification of the mass spectrometer was made in order to provide a trap current of 50 PA. This was necessary as it was observed that a 100 PA trap current was unsatisfactory at electron voltages of the order of 18 eV and below, because of electrical instability. A digital voltmeter was connected between the centre of the filament and the cage to measure the ionisation voltages. The appearance potential measurements were made by continuously sampling a known pressure of &Cl4 from the reservoir and manually selecting the individual major ions on the magnet using an electron voltage of 60 eV. The ion currents were then determined as a function of electron voltage by decreasing the electron voltage in a stepwise manner. Argon was admitted to the mass spectrometer via the second capillary, concomitantly with the &Cl,, to allow its appearance potential curve to be determined under the same experimental conditions and hence to allow the actual experimental electron voltage to be calibrated.
279
Results Mass spectra The complete mass spectrum obtained at 60 eV ionisation potential for silicon tetrachloride is given in Table 1, along with the mass spectra obtained by previous workers [ 14,151. It can be seen that a noticeable background of HCl was produced in the analyser. This was probably due to the reaction with water vapour, SiCl,(g) + 2H,O(g)
+ 4HCl(g) + SiOs(s)
even though the water vapour was present in small quantities, which produced a “normal” background in the instrument. The HCl could not be pumped out of the instrument and consequently the Cl+ ions produced from the dissociation of HCl made the appearance potential curve for Cl+ from SiCl, complex. This problem with HCl was also reported by Vought [ 111. TABLE
1
Mass spectrum
of silicon
Percentage -
m/e
abundance
Present work
._
tetrachloride
Agafonov etal. [14]
28 29 30 31.5
2.17 _ -
9.5 0.48 0.53 0.21
32 32.5 33 35 36 37
-. _
-
0.02 0.08
2.49 3.11 0.89 1.05 9.31 0.53 3.44 3.23 _ 3.28 1.22 _ _ _ 3.39 -
15.18 5.05 4.94 1.75 41.6 2.05 15.28 0.66 1.08 0.51 1.08 0.04 0.38 0.01 0.06 0.68 0.45 0.07 9.02 0.47
38 63 64 65 66 66.5 67 67.5 68 68.5 69 69.5 70 72 74 98 99
Sokolov et&. [15] 24.93 _ _ _ 54.1 _ 29.1 62.3 56.1 _ _ _ _ _ 16.7 _ 16.82 24.93
Possible
ions
280 TABLE
1 (continued)
m/e
Percentage Present
100 101 102 103 104 133 134 135 136 137 138 139 140 141 168 169 170 171 172 173 174 175 176 177 178
abundance
work
2.33
-
99.47 5.82 100 5.29 34.92 2.12 4.76 _
-
57.67 3.70 75.66 4.23 38.62 2.12 9.52
_ -
Agafonov
Sokolov [15]
et al. [14]
etal.
6.23 0.28 1.15 0.06 0.03 99.20 4.82 100 4.92 34.58 1.57 4.56 0.17 0.13 40.20 2.06 53.30 2.62 26.80 1.22 6.34 0.27 0.60 0.03 0.01
-
Possible
ions
29.1
75.2
-
28si35c137c1
9
+
100 -
56.1
-
);;;“‘;;+
)
28
.35 SI c1237c12+
-
91.8
The monoisotopic spectrum of the major ions produced from silicon tetrachloride in this work is presented in Table 2, where it is compared with other data of the previous workers [ 11, 15, 161. This monoisotopic spectrum was calculated from the complete set of experimental data, rather than from published isotopic ratios [ 171.
Appearance
potentials
and en thalpies of formation
of ions
The appearance potentials of the various singly charged positive ions of silicon tetrachloride are given in Table 3 together with the process assumed to produce the ions and the enthalpies of formation of the ions as calculated from the appearance potentials. The thermochemica.1 data used to calculate the enthalpies of formation of the ions were -641.8 kJ mol- I, -150.7 kJ mol- ’ and 121.4 kJ mol-’ for the standard enthalpies of formation of gaseous silicon tetrachloride, silicon dichloride and atomic chlorine, respectively [lS, 191. The appearance potentials were determined using the method of linear extrapolation of the ionisation efficiency curves of the ions [ll] , which are
281 TABLE
2
Monoisotopic Ion
mass spectrum
of silicon Percentage
m/e
tetrachloride abundance
This work
Si+ Cl+ SiCI+ SiC1z2+ SiCI,+ SiCla+ SiC!l*+
28 35 63 66.5 98 133 168
IO
Fig. 1. Appearance
- 37 - 65 - 68.5 - 100 - 139 - 174
14
18 electron
potential
0.86 1.33 5.26 3.06 2.27 100 75.89
22
_
Agafonov
Vought
Svec and Sparrow
etal.
1111
1161
14.0
13.8 12.2 31.1 2.2 7.8 100 60.5
[14]
4.21 -
-
23.8 1.26 6.91 100 53.4
13.0 4.9 4.1 100 56.0
__~_
_ -
26
volts
curves
for positive
ions produced
from
silicon
tetrachloride.
shown in Fig. 1. The ionisation voltage scale was calibrated by determining the ionisation efficiency curve for argon concomitant with those for the other ions. In general, the appearance potentials were reproducible to within kO.2 eV. In the cases of the ions SiCli, SiCl+ and Si+ the ionisation curves did not have the usual form of a linear portion plus a small “foot” (as shown for &Cl”,, SiCl; and Ar+); instead, these curves appeared to consist of at least two separate contributions. Values of the appearance potentials and the hypothetical associated processes are given in Table 3.
Discussion Mass spectrum A comparison of the complete mass spectrum obtained for silicon tetrachloride in this study with the previous data is given in Table 1. The
282 TABLE 3 Ion formation Process Sic&(g)
*
SiC!14+ SiCls+ + SiClz+ + SiCl+ + Si+ +
Cl 2C1 3Cl 4Cl
eV
kJ mol-’
AH, ion (kJ mol-‘)
11.4 12.3 17.7 19.4 24.8
1099.9 1186.8 1707.9 1871.9 2392.9
458.1 423.5 823.1 864.7 1265.8
SiCl,(g)
*
SiC!lz+ SiCl+ + Cl
10.2 11.8
984.2 1138.6
833.4 865.2
Si(g)
*
Si+
11.4
1099.9
1099.9
SiCl,(g)
+ Sic12
20.5
1978.0
1365.5
+ Cl+ + Cl
most abundant ion in the work of Agafonov et al. [14] is at m/e = 135 and this agrees with the present data. This m/e value is associated with the ions 3oSi35C1s and 28Si35C1237C1+.The work of Sokolov et al. [15] does not agree with the present results and gives the most abundant ion at m/e = 139 which is associated with 3oSi35C137C1~and 28Si37Clg. This finding, however, does not agree with the other published monoisotopic data. The known distribution ratio of the chlorine isotopes [17] ,35C1/37C1 = 75.4/24.6 = 3.065, which is obeyed in the present work and that of Agafonov et al. [14], is not obeyed by the data of Sokolov et al. [ 151. These last results yield the ratio 35C1/37C1= 1.86 which indicates errors of the order of a few units. The next most abundant m/e value is 133, which is associated with the ion 28Si35C1s. The relative abundance of 99.2% obtained by Agafonov for this ion is in good agreement with the present data which yield a relative abundance of 99.47%. The remaining parts of the mass spectrum are best discussed when reduced to monoisotopic presentation, so that the results of the other workers may be included [12 - 161. However, before doing this it is relevant to indicate that the present data for 35C1H+ and 37ClH+ (3.11% and 1.05%) are in agreement with the data of Agafonov (5.05% and 1.75%). This indicates that the cleanliness of the present experimental system was slightly better than that of the previous Russian work, although it would have been desirable to have obtained even lower values for these HCl peaks. Monoisotopic mass spectrum Monoisotopic data for silicon tetrachloride are more readily available than the full mass spectral data and a comparison between the present work and earlier results is given in Table 2. The data of Sokolov et al. [ 151 have not been included in this table because of the discrepancies described earlier. It can be seen that the most abundant ion obtained by all the investigators is SiCl;, followed by SiClz, SiCl+, SiCl, 2+, SiCl,‘, Cl+ and Si+ in order of abundance in this study. The four studies report the same four major singly
283
charged ions, although the lower mass ions vary in abundance with the ionisation potential used in the individual studies. The higher ionisation potentials tend to favour an increase in the abundance of the Si+ ion, as might be expected; however, this trend is not held for the next largest ion SiCl+. (The data for Cl+ has not been discussed because of the additional contributions from the HCl produced by hydrolysation in the analyser of the mass spectrometer.) Svec and Sparrow [16] report that the reaction of SiCl,(g) with water vapour could account for the smaller relative abundances of low mass ions presented by Vought [ 111; however, such a reaction will affect only the Cl+ ion and not the Si+ ion. The present data show the mass spectrum of SiCl,(g) to have the pattern where the ion intensities are in the order SiCl; > SiCli > SiCl’ > SiCla. The stability of SiCl; and SiCl+ indicates that silicon tetrahalide ions with an even number of electrons are more stable than ions with an odd number of electrons (an observation reported by Svec and Sparrow [ 161). This behaviour has been observed for carbon tetrahalides and perfluorohydrocarbons. The stability of the SiCl; ion may be attributed to partial double bond contributions which are characteristic of sp2 bonded compounds such as BFs [ 161. Appearance potentials and enthalpies of formation of ions The appearance potential curves for SiCla and SiCl+ shown in Fig. 1 possess a double contribution which can be seen to be separated at lower ionisation energies. The most likely cause of this phenomenon is the fact that a residual gas analysis source was used in the mass spectrometer. This allowed the sample gas to come into the vicinity of the filament of the source and thus to cause appreciable thermal dissociation of SiCl,(g) into SiC12(g) and atomic Cl(g). Advantage may be taken of this because the resulting mass spectra and appearance potential determinations relate to ions generated from both chlorides of silicon. Thus the different energies required to produce the same ions from the two sources may be used to evaluate the thermodynamic relationship between the chlorides. No experimental evidence was found for the thermal dissociation of SiCl, into other (lower) chlorides. The processes suggested in Table 3 for the formation of the ions involve the successive removal of chlorine atoms from the chlorides. This is in agreement with the predictions of other workers and indicates a behaviour of the halide which is similar to that of Fe(CO)s and Ni(C0)4 reported previously [22, 231. However, the situation with respect to SiCl, is not quite so clear cut, because non-stepwise or second degree dissociations involving the loss of Cl; ions also occur, although at a much lower frequency [16]. An explanation for this behaviour is that in the case of the chloride the stable fragment Cl2 may be formed by a second degree process, whereas in the case of the carbonyls the lack of the OC-CO bond makes second degree dissociations thermodynamically unlikely. The work by Svec and Sparrow [16] on the metastable ions of the silicon tetrahalides discusses the electron bombardment fragmentation of silicon tetrahalides in detail.
284
Using the data obtained from the ionization of SiCl, produced by thermal decomposition in the source of the mass spectrometer, estimates of the standard enthalpy of formation of the dichloride can be made by considering the separate processes of formation of SiCl$ and SiCl+ ions. SiClh may be formed by either AH = 1707.9 kJ
SiCl,(g)
+ SiClg
SiCl,(g)
+ SiClz
AH =
984.2 kJ
-+ SiCl,(g) + 2Cl(g)
AH =
723.7 kJ
+ 2Cl(g)
or
Hence, SiCl,(g)
Using A@ = -641.8 kJ mol-r SiCl,(g) and A@ = 121.8 kJ mole1 Cl(g), A@ SiCl,(g) may be calculated to be -161.00 kJ mol-‘. The consistency of the experimental data may be checked by considering the formation of SiCl+: SiC14(g)
+ SiCl+
+ 3Cl(g)
AH = 1871.9 kJ
SiCls(g)
+ SiCl+
+ Cl(g)
AH = 1138.6 kJ
which yield SiCl*(g)
+ SiCl,(g) + 2Cl(g)
AH =
733.3 kJ
from which AH,” SiCl,(g) may be calculated to be -151.4 kJ mol-l. These values are in good agreement with the value of -150.7 + 20.9 kJ mol-’ quoted in the literature for A# SiCl,(g) and show this technique to be a possible means of confirming approximate values of gaseous sub-halides, provided that they can be produced by thermal dissociation in the source of a mass spectrometer. However, this technique must be used with care and sufficient reliance must be able to be placed on the determination of the appearance potentials of the various ions. Because the contributions of the ionisation energy to the kinetic energies of the fragmented ions are unknown (i.e. the appearance potential is assumed to be a measure of the chemical energies alone required to bring about the dissociation processes) the technique must always yield approximate data. The magnitude of the errors that are possible in the method can be assessed using the data of Vought [ 111 which yield a value of -247.8 kJ mol-’ for A@ SiC12(g). Estimates of the Si’v-C1 bond energy have not been made using the present data because, as indicated by Asundi et al. [ 241, this is not simply one-quarter of the dissociation energy of SiCl,(g), since the electronic configuration of silicon in the SiCl, molecule is sp3 rather than the s2p2 of the free atom. An estimate of the required excitation energy could be made. However, this would add another source of error to those caused by the necessary disregard of the transfer of kinetic energy during the electron bombardment dissociation process.
285
Conclusions The complete mass spectrum for SiCl, is shown to agree quite well with the only other complete set of data reported in the literatllre. This mass spectrum, when simplified to a monoisotopic presentation, agrees well with other monoisotopic data and gives relative abundances in the order SiCl; > SiCli > SiCl+ > SiCli for the singly charged positive ions. Hence it is indicated that silicon tetrahalide ions with an even number of electrons are more stable than those with an odd member of electrons. Appearance potential curves show that thermal dissociation of SiCl,(g) to SiCl,(g) has occurred within the mass spectrometer to a significant extent in this study. Data from the SiCl,(g) molecule have been combined with the experimental data for the SiCl,(g) molecule to assess the enthalpy of formation of SiCl,(g) to be of the order of -156.2 or 5 kJ mol-I. Although this value agrees well with the literature data the limitations of this type of assessment are shown to be very significant.
Acknowledgments Financial support from the Science Research Council is gratefully acknowledged. Thanks are also due to Professor E. Smith of the Metallurgy Department, Manchester University, for the provision of general laboratory facilities.
References 1 E. Fujii, H. Nakamura, K. Haruna and Y. Koga, J. Electrochem. Sot., 119 (1972) 1106. 2 J. N. Pring and W. Fielding, J. Chem. Sot., 95 (1909) 1497. 3 R. Hiilbling, Z. Angew. Chem., 40 (1927) 655. 4 W. Steinmaier, Philips Res. Rep., 18 (1963) 75. 5 G. K. Teal, J. R. Fisher and A. W. Treptow, J. Appl. Phys., 17 (1946) 879. 6 J. J. Nick1 and C. von Braunmuhl, J. Less-Common Met., 25 (1971) 303. 7 K. Moers, Z. Anorg. Allg. Chem., 198 (1931) 243. 8 J. T. Kendall, J. Chem. Phys., 21 (1953) 821. 9 C. F. Powell, Vapour Deposition, Wiley-Interscience,
New York,
1966,
Chap.
11.
p. 343. 10 J. J. Nickl, K. K. Schweitzer and P. Luxenberg, J. Less-Common Met., 26 (1972) 335. 11 R. H. Vought, Phys. Rev., 71 (1947) 93. 12 G. G. Devyatykh, V. G. Rachkov and I. L. Agafonov, Russ. J. Inorg. Chem., 13 (1968) 1497. 13 0. Osberghaus, Z. Phys., 128 (1950) 366. 14 I. L. Agafonov, G. G. Devyatykh and N. V. Larin, Russ. J. Inorg. Chem., 8 (1963) 810. 15 N. N. Sokolov, K. A. Andrianov and S. M. Akimov, Zh. Obshch. Khim., 25 (1955) 675. 16 H. J. Svec and G. R. Sparrow, J. Chem. Sot. A, (1970) 1162.
286 17 G. T. Seaborg and I. Perlman, Tables of Isotopes, Rev. Mod. Phys., 20 (1948) 585. 18 0. Kubaschewski, E. L. Evans and C. B. Alcock, Metallurgical Thermochemistry, 4th edn., Pergamon Press, Oxford, 1967. 19 D. D. Wagman, W. H. Evans, I. Halow, V. B. Parker, S. M. Bailey and R. H. Schumm, Natl. Bur. Stand. (U.S.) Tech. Note 270-1, 1965. 20 J. W. Warren, Nature (London), 165 (1950) 811. 21 F. P. Lossing, A. W. Tickner and W. Bryce, J. Chem. Phys., 19 (1951) 1254. 22 R. E. Winters and J. H. Collins, J. Phys. Chem., 70 (1966) 2057. 23 P. J. Clements and F. R. Sale, Metail. Trans., 7B (1976) 171. 24 R. K. Asundi, M. Karim and R. Samuel, Proc. Phys. Sot. London, 50 (1938) 581.
277
THE ELECTRON BOMBARDMENT DISSOCIATION AND THERMOCHEMICAL STABILITIES OF SILICON TETRACHLORIDE AND SILICON DICHLORIDE
A. OLSEN and F. R. SALE Metallurgy Department, Manchester University, Joint Metallurgy Building, Grosuenor Street, ~~la~ehester Ml ‘7HS (Gt. Britain) (Received December 17, 1976)
Summary The mass spectrum of SiCl,(g) is reported and compared with existing data. The relative abundances of the singly charged positive ions are in the order SiCl; > SiCli > SiCl+ > SiClg which indicates that silicon tetrahalide ions with an even number of electrons are more stable than ions with an odd number of electrons. The appearance potentials of all the singly charged positive ions have been determined and used to calculate the enthalpies of formation of the various species of ions. Thermal dissociation of SiCl,(g) in the analyser source of the mass spectrometer is shown to produce SiCl,(g). The appearance potentials of the ions obtained from the SiCl, are used in conjunction with tabulated values for SiCl,(g) to assess existing data for the enthalpy of formation of Sic&,(g).
Introduction The vapour deposition of silicon from silicon tetrachlo~de has been the subject of a number of studies [I - 51 and this work has been extended in recent years to include investigations of the deposition of silicon carbide [6 - 91 and mixed silicon-titanium carbide [lo]. Most of these investigations have attempted to relate the morphology of the deposits to the deposition rate and deposition temperature. Few have made a systematic study of the effects of pressure on the deposition processes. As a preliminary part of an investigation of the deposition of mixed silicon-titanium carbide using a small mass spectrometer to determine the partial pressures of the volatile species involved in the deposition reactions, it was necessary to investigate the dissociation mechanism, disintegration pattern and appearance potentials of the various ions produced from gaseous silicon tetrachloride. These data would enable the mass spectrometer to be calibrated at the various pressures of silicon tetrachloride to be used for the deposition studies, as well as
278
providing information on the electron dissociation which has been the subject of only a small amount [ll - 161.
Experimental
of silicon tetrachloride, of previous investigation
procedure
Mass spectrometer The mass spectrometer used in the study was a VG Micromass 6 fitted with a residual gas analysis source and a 60 mm 90” sector electromagnet. The resolving slit used was 0.05 mm which gave a resolution of approximately 400 at mass 40. A Faraday plate collector and amplifier was used as the ion detector. The sample inlet system to the mass spectrometer consisted of two stainless steel heated capillaries 2 m long and of internal diameter 0.3 mm. Each capillary contained an isolating needle valve and was mounted on a stainless steel cross-piece which was continuously pumped through a liquid nitrogen trap. The pressure maintained in the interspace was less than 1 Torr and was measured by a Pirani gauge. The entire inlet assembly was connected to the mass spectrometer via a calibrated glass leak. To admit a known pressure of sample to the mass spectrometer, one of the capillaries was connected to a one litre reservoir which had a mercury manometer attached to it. The reservoir and manometer were pumped separately to allow a gas to be expanded into the reservoir and then sampled continuously via the capillary. Operating conditions The mass spectrum of SiC& was obtained by continuously sampling the SiCl* from the expansion reservoir. The mass range 10 - 200 was scanned over a period of 1000 s using an accelerating voltage of 400 V, a trap current of 100 PA and an electron voltage of 60 eV. The usual background corrections were determined and applied to all data. For the measurement of appearance potentials a modification of the mass spectrometer was made in order to provide a trap current of 50 PA. This was necessary as it was observed that a 100 PA trap current was unsatisfactory at electron voltages of the order of 18 eV and below, because of electrical instability. A digital voltmeter was connected between the centre of the filament and the cage to measure the ionisation voltages. The appearance potential measurements were made by continuously sampling a known pressure of &Cl4 from the reservoir and manually selecting the individual major ions on the magnet using an electron voltage of 60 eV. The ion currents were then determined as a function of electron voltage by decreasing the electron voltage in a stepwise manner. Argon was admitted to the mass spectrometer via the second capillary, concomitantly with the &Cl,, to allow its appearance potential curve to be determined under the same experimental conditions and hence to allow the actual experimental electron voltage to be calibrated.
279
Results Mass spectra The complete mass spectrum obtained at 60 eV ionisation potential for silicon tetrachloride is given in Table 1, along with the mass spectra obtained by previous workers [ 14,151. It can be seen that a noticeable background of HCl was produced in the analyser. This was probably due to the reaction with water vapour, SiCl,(g) + 2H,O(g)
+ 4HCl(g) + SiOs(s)
even though the water vapour was present in small quantities, which produced a “normal” background in the instrument. The HCl could not be pumped out of the instrument and consequently the Cl+ ions produced from the dissociation of HCl made the appearance potential curve for Cl+ from SiCl, complex. This problem with HCl was also reported by Vought [ 111. TABLE
1
Mass spectrum
of silicon
Percentage -
m/e
abundance
Present work
._
tetrachloride
Agafonov etal. [14]
28 29 30 31.5
2.17 _ -
9.5 0.48 0.53 0.21
32 32.5 33 35 36 37
-. _
-
0.02 0.08
2.49 3.11 0.89 1.05 9.31 0.53 3.44 3.23 _ 3.28 1.22 _ _ _ 3.39 -
15.18 5.05 4.94 1.75 41.6 2.05 15.28 0.66 1.08 0.51 1.08 0.04 0.38 0.01 0.06 0.68 0.45 0.07 9.02 0.47
38 63 64 65 66 66.5 67 67.5 68 68.5 69 69.5 70 72 74 98 99
Sokolov et&. [15] 24.93 _ _ _ 54.1 _ 29.1 62.3 56.1 _ _ _ _ _ 16.7 _ 16.82 24.93
Possible
ions
280 TABLE
1 (continued)
m/e
Percentage Present
100 101 102 103 104 133 134 135 136 137 138 139 140 141 168 169 170 171 172 173 174 175 176 177 178
abundance
work
2.33
-
99.47 5.82 100 5.29 34.92 2.12 4.76 _
-
57.67 3.70 75.66 4.23 38.62 2.12 9.52
_ -
Agafonov
Sokolov [15]
et al. [14]
etal.
6.23 0.28 1.15 0.06 0.03 99.20 4.82 100 4.92 34.58 1.57 4.56 0.17 0.13 40.20 2.06 53.30 2.62 26.80 1.22 6.34 0.27 0.60 0.03 0.01
-
Possible
ions
29.1
75.2
-
28si35c137c1
9
+
100 -
56.1
-
);;;“‘;;+
)
28
.35 SI c1237c12+
-
91.8
The monoisotopic spectrum of the major ions produced from silicon tetrachloride in this work is presented in Table 2, where it is compared with other data of the previous workers [ 11, 15, 161. This monoisotopic spectrum was calculated from the complete set of experimental data, rather than from published isotopic ratios [ 171.
Appearance
potentials
and en thalpies of formation
of ions
The appearance potentials of the various singly charged positive ions of silicon tetrachloride are given in Table 3 together with the process assumed to produce the ions and the enthalpies of formation of the ions as calculated from the appearance potentials. The thermochemica.1 data used to calculate the enthalpies of formation of the ions were -641.8 kJ mol- I, -150.7 kJ mol- ’ and 121.4 kJ mol-’ for the standard enthalpies of formation of gaseous silicon tetrachloride, silicon dichloride and atomic chlorine, respectively [lS, 191. The appearance potentials were determined using the method of linear extrapolation of the ionisation efficiency curves of the ions [ll] , which are
281 TABLE
2
Monoisotopic Ion
mass spectrum
of silicon Percentage
m/e
tetrachloride abundance
This work
Si+ Cl+ SiCI+ SiC1z2+ SiCI,+ SiCla+ SiC!l*+
28 35 63 66.5 98 133 168
IO
Fig. 1. Appearance
- 37 - 65 - 68.5 - 100 - 139 - 174
14
18 electron
potential
0.86 1.33 5.26 3.06 2.27 100 75.89
22
_
Agafonov
Vought
Svec and Sparrow
etal.
1111
1161
14.0
13.8 12.2 31.1 2.2 7.8 100 60.5
[14]
4.21 -
-
23.8 1.26 6.91 100 53.4
13.0 4.9 4.1 100 56.0
__~_
_ -
26
volts
curves
for positive
ions produced
from
silicon
tetrachloride.
shown in Fig. 1. The ionisation voltage scale was calibrated by determining the ionisation efficiency curve for argon concomitant with those for the other ions. In general, the appearance potentials were reproducible to within kO.2 eV. In the cases of the ions SiCli, SiCl+ and Si+ the ionisation curves did not have the usual form of a linear portion plus a small “foot” (as shown for &Cl”,, SiCl; and Ar+); instead, these curves appeared to consist of at least two separate contributions. Values of the appearance potentials and the hypothetical associated processes are given in Table 3.
Discussion Mass spectrum A comparison of the complete mass spectrum obtained for silicon tetrachloride in this study with the previous data is given in Table 1. The
282 TABLE 3 Ion formation Process Sic&(g)
*
SiC!14+ SiCls+ + SiClz+ + SiCl+ + Si+ +
Cl 2C1 3Cl 4Cl
eV
kJ mol-’
AH, ion (kJ mol-‘)
11.4 12.3 17.7 19.4 24.8
1099.9 1186.8 1707.9 1871.9 2392.9
458.1 423.5 823.1 864.7 1265.8
SiCl,(g)
*
SiC!lz+ SiCl+ + Cl
10.2 11.8
984.2 1138.6
833.4 865.2
Si(g)
*
Si+
11.4
1099.9
1099.9
SiCl,(g)
+ Sic12
20.5
1978.0
1365.5
+ Cl+ + Cl
most abundant ion in the work of Agafonov et al. [14] is at m/e = 135 and this agrees with the present data. This m/e value is associated with the ions 3oSi35C1s and 28Si35C1237C1+.The work of Sokolov et al. [15] does not agree with the present results and gives the most abundant ion at m/e = 139 which is associated with 3oSi35C137C1~and 28Si37Clg. This finding, however, does not agree with the other published monoisotopic data. The known distribution ratio of the chlorine isotopes [17] ,35C1/37C1 = 75.4/24.6 = 3.065, which is obeyed in the present work and that of Agafonov et al. [14], is not obeyed by the data of Sokolov et al. [ 151. These last results yield the ratio 35C1/37C1= 1.86 which indicates errors of the order of a few units. The next most abundant m/e value is 133, which is associated with the ion 28Si35C1s. The relative abundance of 99.2% obtained by Agafonov for this ion is in good agreement with the present data which yield a relative abundance of 99.47%. The remaining parts of the mass spectrum are best discussed when reduced to monoisotopic presentation, so that the results of the other workers may be included [12 - 161. However, before doing this it is relevant to indicate that the present data for 35C1H+ and 37ClH+ (3.11% and 1.05%) are in agreement with the data of Agafonov (5.05% and 1.75%). This indicates that the cleanliness of the present experimental system was slightly better than that of the previous Russian work, although it would have been desirable to have obtained even lower values for these HCl peaks. Monoisotopic mass spectrum Monoisotopic data for silicon tetrachloride are more readily available than the full mass spectral data and a comparison between the present work and earlier results is given in Table 2. The data of Sokolov et al. [ 151 have not been included in this table because of the discrepancies described earlier. It can be seen that the most abundant ion obtained by all the investigators is SiCl;, followed by SiClz, SiCl+, SiCl, 2+, SiCl,‘, Cl+ and Si+ in order of abundance in this study. The four studies report the same four major singly
283
charged ions, although the lower mass ions vary in abundance with the ionisation potential used in the individual studies. The higher ionisation potentials tend to favour an increase in the abundance of the Si+ ion, as might be expected; however, this trend is not held for the next largest ion SiCl+. (The data for Cl+ has not been discussed because of the additional contributions from the HCl produced by hydrolysation in the analyser of the mass spectrometer.) Svec and Sparrow [16] report that the reaction of SiCl,(g) with water vapour could account for the smaller relative abundances of low mass ions presented by Vought [ 111; however, such a reaction will affect only the Cl+ ion and not the Si+ ion. The present data show the mass spectrum of SiCl,(g) to have the pattern where the ion intensities are in the order SiCl; > SiCli > SiCl’ > SiCla. The stability of SiCl; and SiCl+ indicates that silicon tetrahalide ions with an even number of electrons are more stable than ions with an odd number of electrons (an observation reported by Svec and Sparrow [ 161). This behaviour has been observed for carbon tetrahalides and perfluorohydrocarbons. The stability of the SiCl; ion may be attributed to partial double bond contributions which are characteristic of sp2 bonded compounds such as BFs [ 161. Appearance potentials and enthalpies of formation of ions The appearance potential curves for SiCla and SiCl+ shown in Fig. 1 possess a double contribution which can be seen to be separated at lower ionisation energies. The most likely cause of this phenomenon is the fact that a residual gas analysis source was used in the mass spectrometer. This allowed the sample gas to come into the vicinity of the filament of the source and thus to cause appreciable thermal dissociation of SiCl,(g) into SiC12(g) and atomic Cl(g). Advantage may be taken of this because the resulting mass spectra and appearance potential determinations relate to ions generated from both chlorides of silicon. Thus the different energies required to produce the same ions from the two sources may be used to evaluate the thermodynamic relationship between the chlorides. No experimental evidence was found for the thermal dissociation of SiCl, into other (lower) chlorides. The processes suggested in Table 3 for the formation of the ions involve the successive removal of chlorine atoms from the chlorides. This is in agreement with the predictions of other workers and indicates a behaviour of the halide which is similar to that of Fe(CO)s and Ni(C0)4 reported previously [22, 231. However, the situation with respect to SiCl, is not quite so clear cut, because non-stepwise or second degree dissociations involving the loss of Cl; ions also occur, although at a much lower frequency [16]. An explanation for this behaviour is that in the case of the chloride the stable fragment Cl2 may be formed by a second degree process, whereas in the case of the carbonyls the lack of the OC-CO bond makes second degree dissociations thermodynamically unlikely. The work by Svec and Sparrow [16] on the metastable ions of the silicon tetrahalides discusses the electron bombardment fragmentation of silicon tetrahalides in detail.
284
Using the data obtained from the ionization of SiCl, produced by thermal decomposition in the source of the mass spectrometer, estimates of the standard enthalpy of formation of the dichloride can be made by considering the separate processes of formation of SiCl$ and SiCl+ ions. SiClh may be formed by either AH = 1707.9 kJ
SiCl,(g)
+ SiClg
SiCl,(g)
+ SiClz
AH =
984.2 kJ
-+ SiCl,(g) + 2Cl(g)
AH =
723.7 kJ
+ 2Cl(g)
or
Hence, SiCl,(g)
Using A@ = -641.8 kJ mol-r SiCl,(g) and A@ = 121.8 kJ mole1 Cl(g), A@ SiCl,(g) may be calculated to be -161.00 kJ mol-‘. The consistency of the experimental data may be checked by considering the formation of SiCl+: SiC14(g)
+ SiCl+
+ 3Cl(g)
AH = 1871.9 kJ
SiCls(g)
+ SiCl+
+ Cl(g)
AH = 1138.6 kJ
which yield SiCl*(g)
+ SiCl,(g) + 2Cl(g)
AH =
733.3 kJ
from which AH,” SiCl,(g) may be calculated to be -151.4 kJ mol-l. These values are in good agreement with the value of -150.7 + 20.9 kJ mol-’ quoted in the literature for A# SiCl,(g) and show this technique to be a possible means of confirming approximate values of gaseous sub-halides, provided that they can be produced by thermal dissociation in the source of a mass spectrometer. However, this technique must be used with care and sufficient reliance must be able to be placed on the determination of the appearance potentials of the various ions. Because the contributions of the ionisation energy to the kinetic energies of the fragmented ions are unknown (i.e. the appearance potential is assumed to be a measure of the chemical energies alone required to bring about the dissociation processes) the technique must always yield approximate data. The magnitude of the errors that are possible in the method can be assessed using the data of Vought [ 111 which yield a value of -247.8 kJ mol-’ for A@ SiC12(g). Estimates of the Si’v-C1 bond energy have not been made using the present data because, as indicated by Asundi et al. [ 241, this is not simply one-quarter of the dissociation energy of SiCl,(g), since the electronic configuration of silicon in the SiCl, molecule is sp3 rather than the s2p2 of the free atom. An estimate of the required excitation energy could be made. However, this would add another source of error to those caused by the necessary disregard of the transfer of kinetic energy during the electron bombardment dissociation process.
285
Conclusions The complete mass spectrum for SiCl, is shown to agree quite well with the only other complete set of data reported in the literatllre. This mass spectrum, when simplified to a monoisotopic presentation, agrees well with other monoisotopic data and gives relative abundances in the order SiCl; > SiCli > SiCl+ > SiCli for the singly charged positive ions. Hence it is indicated that silicon tetrahalide ions with an even number of electrons are more stable than those with an odd member of electrons. Appearance potential curves show that thermal dissociation of SiCl,(g) to SiCl,(g) has occurred within the mass spectrometer to a significant extent in this study. Data from the SiCl,(g) molecule have been combined with the experimental data for the SiCl,(g) molecule to assess the enthalpy of formation of SiCl,(g) to be of the order of -156.2 or 5 kJ mol-I. Although this value agrees well with the literature data the limitations of this type of assessment are shown to be very significant.
Acknowledgments Financial support from the Science Research Council is gratefully acknowledged. Thanks are also due to Professor E. Smith of the Metallurgy Department, Manchester University, for the provision of general laboratory facilities.
References 1 E. Fujii, H. Nakamura, K. Haruna and Y. Koga, J. Electrochem. Sot., 119 (1972) 1106. 2 J. N. Pring and W. Fielding, J. Chem. Sot., 95 (1909) 1497. 3 R. Hiilbling, Z. Angew. Chem., 40 (1927) 655. 4 W. Steinmaier, Philips Res. Rep., 18 (1963) 75. 5 G. K. Teal, J. R. Fisher and A. W. Treptow, J. Appl. Phys., 17 (1946) 879. 6 J. J. Nick1 and C. von Braunmuhl, J. Less-Common Met., 25 (1971) 303. 7 K. Moers, Z. Anorg. Allg. Chem., 198 (1931) 243. 8 J. T. Kendall, J. Chem. Phys., 21 (1953) 821. 9 C. F. Powell, Vapour Deposition, Wiley-Interscience,
New York,
1966,
Chap.
11.
p. 343. 10 J. J. Nickl, K. K. Schweitzer and P. Luxenberg, J. Less-Common Met., 26 (1972) 335. 11 R. H. Vought, Phys. Rev., 71 (1947) 93. 12 G. G. Devyatykh, V. G. Rachkov and I. L. Agafonov, Russ. J. Inorg. Chem., 13 (1968) 1497. 13 0. Osberghaus, Z. Phys., 128 (1950) 366. 14 I. L. Agafonov, G. G. Devyatykh and N. V. Larin, Russ. J. Inorg. Chem., 8 (1963) 810. 15 N. N. Sokolov, K. A. Andrianov and S. M. Akimov, Zh. Obshch. Khim., 25 (1955) 675. 16 H. J. Svec and G. R. Sparrow, J. Chem. Sot. A, (1970) 1162.
286 17 G. T. Seaborg and I. Perlman, Tables of Isotopes, Rev. Mod. Phys., 20 (1948) 585. 18 0. Kubaschewski, E. L. Evans and C. B. Alcock, Metallurgical Thermochemistry, 4th edn., Pergamon Press, Oxford, 1967. 19 D. D. Wagman, W. H. Evans, I. Halow, V. B. Parker, S. M. Bailey and R. H. Schumm, Natl. Bur. Stand. (U.S.) Tech. Note 270-1, 1965. 20 J. W. Warren, Nature (London), 165 (1950) 811. 21 F. P. Lossing, A. W. Tickner and W. Bryce, J. Chem. Phys., 19 (1951) 1254. 22 R. E. Winters and J. H. Collins, J. Phys. Chem., 70 (1966) 2057. 23 P. J. Clements and F. R. Sale, Metail. Trans., 7B (1976) 171. 24 R. K. Asundi, M. Karim and R. Samuel, Proc. Phys. Sot. London, 50 (1938) 581.