J . Phys. Chem. 1993,97, 2275-2283
2275
Kinetic Modeling of the Chemical Vapor Deposition of Tin Oxide from Dimethyltin Dichloride and Oxygen Carmen J. Giunta,+ David A. SMckler,+and Roy G. Gordon' Department of Chemistry, Harvard University, Cambridge, Massachusetts 02138 Received: August 7 , I992
The gas-phase kinetics of the chemical vapor deposition of tin oxide films from dimethyltin dichloride (DMT) and oxygen was modeled by a detailed mechanism involving hydrogen and methane oxidation submechanisms as well as organotin and chlorine reactions. Simulated growth profilesare presented and compared to experimental observations, with generally good agreement. The dependences of film growth profiles and of byproduct gas distributions in deposition from DMT (as reported by Strickler and Gordon) are similar in many respects to those seen by Borman and Gordon in depositions of tin oxide from tetramethyltin (TMT) and oxygen. As a result, the mechanism for deposition from DMT proposed in this paper is similar to one developed by us for deposition from TMT. The proposed mechanism is a branched chain reaction, initiated by pyrolysis of DMT and sustained by a sequence of reactions propagated by organotin radicals. The hydroxytin species (CH3)CIzSnOH is posited as a key intermediate, whose decomposition and oxidation releases branching radicals. Branching is proposed to occur when the organotin reactant is attacked by hydroxyl radicals (generated by oxidation of methyl radicals lost from the hydroxy intermediate) or chlorine atoms (also lost from (CH3)C12SnOH).
Introduction Tin oxide (SnOz)is a versatile material whose optical, electrical, and mechanical properties make it a widely used industrialcoating. It is a wide band-gap semiconductor, whose conductivity and infrared reflectivity can be dramatically enhanced by doping with fluorine (Sn02:F). Its infrared reflectivity nearly doubles the heat-insulating qualities of windows. Fluorine-doped tin oxide also transmits visible light. Such transparent conducting films are well suited for use as electrodes in solar cells, since they can carry the cell's current without blocking incident sunlight. Sn02:F films can also be fabricated for the generation of resistive heating to defrost windshields of airplanes or automobiles. Tin oxide films are sufficiently sturdy, chemically inert, and adhesive on a variety of substrates to withstand the weather and sunlight conditions typically encountered in the above applications; in fact, tin oxide is also used as a wear-resistant coating on such objects as glass bottles. The efficientfabricationof tin oxide in thin-film form has long been an area of practical research interest because of its extensive utility. In this laboratory, such research has focused on chemical vapor deposition (CVD) from organotin compounds with oxygen.lJ.4 The elucidation of elementary chemical details of such CVD oxidations has also been an area of continued interest in this laboratory, both as an aid to increased understanding (and therefore increased control) of the deposition process and as a complex and challenging problem in the area of kinetics modeling and oxidation chemi~try.3,~ Our models are based on rate-limiting chemistry in thegas phase, combined with rapid surface reactions. This paper will first summarize experimental findings by Strickler and Gordon (SG) on the oxidation of dimethyltin dichloride ((CH3)2SnC12,abbreviated hereafter as DMT). A mechanism for this CVD oxidation process will then be presented and discussed. The modeling effort described in this paper began with a model developed for the CVD oxidation of tetramethyltin (TMT) by oxygen.3 Reactions involvingchlorinated specieswere added to that mechanistic framework, and the tin reactions were only slightly modified in developing the mechanism described below. This "hierarchical" modeling procedure of building upon +
NY
Present address: Departmentof Chemistry,Le Moyne College, Syracuse, 13214-1399.
1 Present
address: Libbey-Owens-Ford Company, Toledo, OH 43605.
0022-3654f 93f 2097-2275304.00f 0
previously established reactions kept to a minimum the fitting of rate constants to CVD data. Throughout the paper, initial gas compositions are reported (unless otherwise noted) in the shorthand "mole percent DMT/mole percent oxygen"; the remainder of the gas stream was primarily helium (e.g., "2.3/20" means 2.3% DMT, 20% oxygen.) Rate constants are given in cm3, mol, s units, with activation energies in kcal mol-'. Numbering of reactions follows the scheme employed in ref 3. Summary of Experimental Observations'
SG achieved film growth rates in the hundreds of A s-I in the deposition of tin oxide films from DMT and oxygen in helium carrier gas. They employed a horizontal laminar-flow reactor and made deposits on flat substrates placed within the reactor. In depositing films much faster than did Borman and Gordon from TMT,2 they also typically employed higher temperatures and larger initial concentrations of the organotin reactant. The problem of particulatecontamination (which had restricted initial TMT concentrations to amounts usually in the tenths of mole percent) seemed to be intrinsicallyless severe in DMT oxidation than in TMT. Furthermore, whatever powder did form in the depositions from DMT was driven away from the growth surface by thermophoresis, due to the large temperature gradients employed in SG's reactor. Temperature differencesof 210-315 K were applied across the 0.6-cm height of the reactor. Substrate temperatures (Ts) in SG's reactor ranged from 793 to 893 K.The lower temperatures within this range were primarily employed in studies of the growth profile, while the highest temperatures were utilized in gas chromatographic characterization of the effluent. The differences in temperature and reactor geometry make comparisons between the DMT and TMT experimental observations difficult to interpret directly. In particular, the temperature gradients necessitated the use of a fully twodimensional modeling framework, including explicit treatment of diffusion and temperature nonuniformities-a computational expense not undergone in modeling TMT oxidation in a uniformtemperature tubular reactor. In the following summary of the DMT results, references are made to the TMT results primarily to inform the reader of the differences in the experiments; they are not intended to imply mechanistic differences. The growth profiles observed by SG are displayed in Figure 1. Peak growth was linear in DMTconcentration and only weakly Q 1993 American Chemical Society
Giunta et al.
2276 The Journal of Physical Chemistry, Vol. 97, No. IO, 1993 W calc.@5cm E4 calc.@20cm
300
observed
carbon dioxide
3
0
200
A
0
0.41120 1.2120
carbon monoxide methane ethane methyl chloride carbon oxides
6 position (cm)
2
0
two-carbon
10
8
4
1 0 ' ~i o - 2
10'~
io-'
mole fraction 150
h
v)
I
I
Figure 3. Byproduct concentrations for 1.3% DMT + 20% 0 2 at Ts= 893 K and two positions along the flow direction. Experimental data are taken from SG.
I
I
100
A
A
-
3 2.315 2.3110 2.3120
@
A 0 0
2
4 6 position (cm)
8
10
Figure 1. Growth profiles of SnO2 from DMT + 02:(a) Ts= 801 K, 20%0 2 , various DMT; (b) Ts= 793 K, 2.3% DMT, various 02.Curves depict simulation results, and symbols represent experimental points. 0
801 K
gradients were applied with different temperatures. But the energy lies in the vicinity of 20 kcal mol-', and it is undoubtedly much lower than the activation energy for peak deposition from TMT. SG's gas chromatographicinvestigationof the effluent stream measured concentrations of several key gaseous byproducts. However, due to the widely varying polarities (ranging from alkanes to hydrogen halides) and reactivities of expected byproducts, a complete accountof neither the carbon nor chlorinebudgets was possible. Among the detected carbon-containingbyproducts were found substantial quantities of both oxidized (CO) and reduced (CH4) species. It is also significant to note that SG detected CH3Clonly as a minor product. Effluent analysis data are displayed in Figure 3. Limited data suggest that the concentrationsof all the byproductsdepicted in Figure 3 increase along the flow direction. Such an increase is not surprising in light of the relative stability of the species in question. Construction of Mechanism
Y
a
A conservativeapproach was taken in constructinga mechanism for DMT oxidation. The TMT oxidation mechanism described elsewhere3was carried over as the basis of the DMT mechanism. The portion of the mechanism which described reactionsof species containing only carbon, hydrogen, and oxygen (C/H/O submechanism) was naturally retained without modification. The organotin framework of reactions were also retained with very little modification (save for changing the names of the species). Reactions involving chlorinated species (primarilyatomic C1and also HCl and C10) were added to completethe mechanism. Table I displays the mechanism and the rate constants employed. The TMT analogues of all the organotin reations D1-D17 except D9, D 12, D 16, and D 17 are closely related to analogous hydrocarbon processes or have been themselves observed. These additionalhypothesized reactions were included to explain features of tin oxide CVD which could not be adequately described with "known" chemistry; as will be seen below, these reactions also "make sense" chemically. Since alkyltin chemistry is not as well characterized as thecorrespondinghydrocarbon steps,we adopted a conservativeapproach to developing this portion of the model, relating DMT oxidation as closely as possible to similar hydrocarbon processes. Reactions were added only as required to explain experimental data or for internal model consistency. Reactions C16, C 118, and C125-reactions between chlorinated species and organotin species-are among the reactions added for internal consistency. All organotin reaction rate constants involving DMT were assigned the same rate constants as the analogous reactions in the TMT oxidation model with three exceptions: D1 was assigned an experimentally measured rate constant, and D16 and D17 were adjusted to fit observed CVD data. The pyrolyticdecomposition of DMT proceeds by (apparently sequential) loss of methyl radicals, of which the first loss is rate-
2 0
0
1 2 3 4 DMT concentration (mol O/O)
5
Figure 2. Peak growth rates of SnO2 from DMT + 20% 0 2 plotted against DMT at two substrate temperatures. Curves depict simulation results, and symbols represent experimental points.
dependent on oxygen. The linear variation of growth with DMT extends to much higher concentrations than did the similar variation with TMT; with DMT, growth remains linear beyond 4% DMT, while powder formation with TMT makes the dependence sublinear at a much lower concentration. The SG deposition profiles appear much broader than the profiles of Borman and Gordon. This observation, however, holds in the length domain but not in the time domain: SG employed faster flow rates, which tended to spread the peak of the growth profile over a greater length of the substrate. Still, the position of the maximum deposition rate is not as easily pinpointed in SG's films as in Borman and Gordon's. The proportionality of peak growth to DMT concentration holds at higher temperatures as well, as is evident in Figure 2. The activation energy defined by the peak growths at these two temperatures is 21.9 kcal mol-'. There is some experimental scatter in the activation energy data; furthermore, activation energy is difficult to interpret, because different temperature
Chemical Vapor Deposition of Tin Oxide
The Journal of Physical Chemistry, Vol. 97, No. 10, 1993 2211
TABLE I: DMT Oxidation Mechanisma label H2* H3 H4 H5 H6* H7 H8* H9 H10 H11 H12 H13* H14 H15 H16 H17 H18 H19 H2l* H23 H24t H25 H26t H27 H291 H30 H3 1 H33t H34 H35 H36 CI c2 c3 c4
c5 C6 c7 C8 c9 c10 c11 c12 C13 C14 C15 C16 C17 C18 C19* c20 c 21 C22t C23 C24 C25 C26 C27 C28 C29 C30 C31 C32 c33 C34* c35 C36t c37 C38 c39 C40 C41 D1 D2 D3
-
reactants
products
H+H-Hz H H2O- H2 OH H H202 H2 + H02 H H202 4 H20 OH H+O-OH H + OH H2+ 0 H +OH H20 H + H02 +H2 + 0 2 H + H02 H 2 0 + 0 H H 0 2 4 OH OH 0 H + 02‘OH H + 0 2 4 H02 H2 + 0- H OH H H20 H2 + O H H2+HOz+H+H202 H2 + 0 2 4 H + H02 OH H20 0 - O H H20 H02 Hz02 + OH
+
+
+ +
4
+
+
+
+
+
+
+
- ++
+
+
+ o+o-02
+
OH + 0- H 0 2 OH + 0- HO2 H20 0 OH OH OH + OH Hz02 OH + H02 H20 + 0 2 HO2+H+02 H02 + 0 OH + 0 2 HO2 H02 H202 + 0 2 Hz02 OH + OH H202 0 H20 0 2 H202 + 0 OH H02 Hi02 OH H2O + HO2
+
+
+
-
+
+ +
+
+
+
4
A
Hydrogen Oxidation Reactions 5.4 x 10’8 6.2 x 107 4.82 X 10” 2.4 x 1013 4.7 x 10’8 4.9 x 10’ 2.2 x 1022 6.62 x 1013 1.8 X 10l2 1.69 x 1014 1.68 X 10” 6.42 X 10l8 4.33 x 1011 6.3 X lo6 3.0 x 1013 1.8 x 1014 4.6 x 109 1.8 X lo1’ 1.0 x 1014 4.5 x 1014 8.0 X 10l6 2.1 x 108 2.39 x 1019 1.54 x 1013 4.12 X lo2[ 1.75 X lo1’ 1.2 x 10” 1.2 x 1017 3.4 x 10” 3.4 x 10” 1.75 X 10l2
Hydrocarbon Oxidation Reactions CHI+H+CH~+H~ 2.2 x 104 6.9 X lo8 CHI + 0- CH3 OH CH2O + H2 CH4 OH 1.5 X lo6 1.12 x 10” CH4 + HO2 CHI H202 CH4 0 2 CHI HOJ 4.0 x 1013 1.6 X 10“ CH4 CH3O CH, + CH3OH 1.12 x 1013 CH4 CHjOO CH3 + CHjOOH CH3 H CH4 8.03 X 10I2 CH, Hz‘ CHI + H 2.9 X lo2 8.4 x 1013 CH3 0 CH20 H CH, + OH 4 CH20 + Hz 3.98 X 10l2 CHI HO2 CH4 + 02 8.3 X 10” CH3 + H202 CH4 + HO2 2.4 X 10l2 CHI 0 2 + C H 3 0 0 1.4 x 1014 CH3 + 0 2 CH@O 3.7 x 10’0 1.0 x 1013 CHI + CHI C2H6 3.6 x 1013 CHI CHjOO CH3O CH3O 5.5 x 103 CHI + CH20 CH4 + CHO 2.8 x 1014 CH3O 4 CH20 H CHjO + 0 2 CH2O + H02 6.3 X 1Olo 1.02 x 10” CH3O CH2O CH3OH CHO 1.92 X lo2* CHiOO CHI + 0 2 9.6 x 1013 C H 3 0 0 + H C H 3 0 OH 3.0 X lo1] CHIOO + H2 CH3OOH + H CH3OO + HO2 4 CH3OOH + 02 1.0 x 10” CHjOO + CHjOO CHiO CHlO + 02 4.4 x 1010 C H 3 0 0 CH2O CHiOOH C H O 2.0 x 10’2 7.9 x 1014 CHjO OH CHjOOH 2.2 x 108 CH20 + H CHO + H2 CH2O 0 CHO OH 1.78 X 10” CH20 OH CHO H20 3.4 x 109 2.0 x 10’2 CH2O HO2 CHO + H202 CHzO + 0 2 CHO HO2 2.0 x 10” CHO-CO+H 1.45 X l o i 4 CHO 0 2 CO + H02 7.58 X 10l2 co + 0- c02 6.2 x 1014 CO OH CO2 + H 3.63 X 10” 1.5 x 1014 CO H02 C02 + OH 2.5 X 10l2 co 0 2 c02 + 0 C02 H CO + OH 1.5 x 1014 c02 + 0- co + 0 2 1.7 X lo1] Organotin Reactions (CH3)2SnC12 CHJSnC12 + CHI 2.0 x 1014, (CH])2SnC12 H 1.8 x 1014 CH3C12SnCH2 + H2 9.2 x 1013 (CH])2SnC12 0 CH3C12SnCH2 + OH
+
+ + +
+
+ + +
+
+
+
4
+
+
+ +
+
+
-
+
+
+
+
-4
+
+
+
-
+
--
4
+
+ +
+
+ + + +
+
+
+
+ +
+
+
-
4
+
+
-+ + -
+
+
+
E
t
ref
0.0
-1.3 1.9
18,19 18,19 19 19 19 18,19 19,20 19 19,21 19 18.19 12, 19, 22 23 18,19 19 24 19,25 18 26 18,19 25 18,19 27 28 29 19,12 30 20 31 31 19
18.41 7.9 4.0 0.0 3.87 0.0 2.13
0.0 0.87 17.39
0.0 10.43 2.96 26.0 56.7 17.1 30.0 1.43 0.06
0.0 0.4
0.0 -0.83 49.6 -0.4 -1.5 45.5 3.97 3.97 0.32 8.75 8.48 2.44 24.6 55.2 8.8 24.6 -2.13 8.71
0.0 0.0 0.0 7.0 30.8 -1.9 -1.06
0.0 5.86 22.0 2.6 3.0 28.51
0.0 26.0 -2.0 -0.44 11.7 43.0 3.0 3.07 -0.45 11.7 38.9 19.0 0.41 3.0 0.9 23.65 48.0 26.4 52.7 58.0 9.9 7.2
0.0 0.0
-1 .o 2.8 -2.0
0.0 0.0 0.0 -0.9 -1 .o
0.0 2.0 0.0 0.0 1.3 0.0 0.0 -0.5 0.0 1.4 -0.8 0.0 -2.0 0.0 0.0 0.0
0.0 0.0 0.0 3.0 1.56 2.13
0.0 0.0 0.0 0.0 0.0
19,32 33,34 35 36 37 19
=kc4
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
38 19 34,39 19,40 41 42 43 44 45 46 19 47 47,48 19 49 19 19 12 12 19 50 19 19,51 19 19 19,52 40 53 19 54 19 26 19,26 19,26
0.0 0.0 0.0
6 b. 55 b, 34
3.12
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 2.81
0.0 0.0 0.0
-4.0 0.0 0.0 0.0 0.0 0.0 0.0 1.77
0.0 1.18
2278 The Journal of Physical Chemistry, Vol. 97, No. 10, 1993
TABLE I (Continued) label D4 D5 D6 D7 D8 D9 D10 D11 D12 D13 D14 D15 D16* D17* CI 1 c12 C13 C14 C15 C16 C17* C18 C19
c1IO Clll
c112 C113 C1 I4 C115 C116 C1 I7 C118 C119 c120 C121' C122' C123 C124 C125 C126 C127
reactants
---
-
Giunta et al.
products
A
Organotin Reactions (Continued) 4.8 X lo6 (CH3)2SnC12 OH CH3C12SnCH2 + H20 1.2 x 1013 (CH3)2SnC12 H02 CH3C12SnCH2 + H202 6.31 X 10" (CH3)2SnC12 CHI CH3C12SnCH2 + CH4 1.2 x IO" (CH3)2SnC12 CHjOO CH3C12SnCH2 + C H 3 0 0 H 1.7 x 1014 (CH3)2SnC12 CHjC12SnO CHjC12SnCH2 + CH3C12SnOH 1.0 x 1013 CH3SnC12 + 0 2 CH3C12SnO + 0 2.01 x 1017 CH3C12SnCH2 + 0 2 CH3ClzSnCH200 7.4 x 10'8 CH3C12SnCH200 C H J C I ~ S ~ C+H0 ~2 3.2 X 10" CH3C12SnCH200 CH3C12SnO C H 2 0 3.0 X 10" CH3C12SnO + HO2 CH3C12SnOH + 0 2 CH3C12SnO CH4 CHjC12SnOH + CH3 1.6 X 10" CH3C12SnO CH2O CH3CI2SnOH CHO 1.02 x 10" 3.0 x 107* CHCI7SnOH SnO + CH? + CI HCI 2.0 x lo"* CH;CI;SnOH + 0 2 SnO2+ CH3 + CIO + HCI
+ + + + +
--
--+ + - -
-
-
+
-
- -
+
+ +
+
+
+ + +
-
-
4
4
-
+ +
-+ +
-
+
+
-- -
0.14 19.4 9.9 19.4 7.0 15.0
2.08 0.0 0.0
0.0 10.0 0.0 8.8
+
Chlorine Reactions CI + H I - HCI H HCI + 0 2 CI + H02 CI + H202 HCI + H02 CI + CH4 HCI + CH3 CI CH2O HC1+ CHO CI + (CH3)2SnC12 HCI + CHjC12SnCH2 CI + 0 2 ClOO CI H02 CIO + OH CI CHjOO CIO + CH3O HCI + H-CI + H2 HCI + 0- CI + OH HCI OH CI + H20 HCI 0 2 Cl + HO2 CI + H202 HCI + H02 CI CHI HCI CH, HCI C H 3 0 0 CI CHjOOH HCI CHO CI + CHzO HCI + CH3C12SnO CI + CH3C12SnOH HCI C1- Cl2 + H CI + CHI CH3CI CI + CI c12 ClOO-CI+02 CIO CH4 HOC1 CH3 CIO + CH20 HOC1 + CHO CIO + (CH3)2SnC12 HOC1 + CH3C12SnCH2 C10 + H202 HOC1 + H02 CIO + H02 HOC1 + 0 2
t
25.12
+
+
E
0.0 0.0 -2.1 -2.1
0.0 0.0 0.0 0.0 0.0
3.0 27.5 21.6
2.23 X 10" 1.08 x 1013 6.62 X 1OI2 1.1 x 107 4.94 x 1013 6.8 X 10') 3.63 x 1014 2.47 X 10" 1.89 X 10" 1.16 x 1013 6.q2 X 10l2 2.25 X 10l2 1.54 x 1013 9.3 x IO" 2.34 X 10" 9.3 x 10" 1.88 X l o t 2 5.0 X I O ' O 1.0 x 1014 1.04 X l0l3 2.23 x 1014 1.62 X 10l8 6.0 X loll 6.0 X 10" 6.0 X 10" 8.04 X lolo 2.77 X 10"
0.0
0.0
4.6 -0.34 1.95 1.49 0.07 0.6 0.0 0.89 1.1 3.5 6.64 1.02 53.16 17.85 2.3 19.2 16.07 10.0 47.46 -0.9 -1.8 6.4 10.7 5.1 10.7 7.0 -1.4
0.0 0.0 0.0 1.97 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0
0.0 -0.5 0.0 -1
.o
0.0 0.0 0.0 0.0 0.0
ref b, 35 56 57 =kDs C
d b, 58 59 5 60 'kc6 =kc21 C C
11,12 11,12 11,12 13 11,12 61 62 11.12 61 e, 11, 12 11,12 13 e, 11, 12 e, 11, 12 e, 13 61
;
l1,I2 13 63 13 64 65 66 =kc123 67 e, 11, 12
Reactions common to the TMT mechanism follow the numbering of ref 3. All reactions whose labels begin with 'D" are analogous to TMT reactions labeled with 'T" in Table I of ref 3. Rate parameters of organotin reactions are the same as their TMT analogues, except where noted by an asterisk (*). All rate constants are given in cm3, mol, s units, with activation energies in kcal mol-' by k = A T exp(-E/RT), except those noted by a double dagger ($), which include a pressure-dependencefactor k = A T [ M ] exp(-EIRT). where [MI is the total gas-phase concentration in mol 6111-3. b Set equal to the rate constant reported in the cited work for the analogous neopentane reaction. kD16 and k~17were adjusted in the present work to fit CVD data (see text); kD8 = k ~which ~ , was adjusted in ref 3 to fit CVD data on TMT oxidation. Assumed fast in order to allow sufficiently short induction period for film growth. E is consistent with a strong Sn-0 bond. (Dissociation energy for the bond Sn-OH is 1 IO kcal mol-'.6* If Sn-O is as strong, or even up to 5 kcal mol-' weaker, then the rate constant has a reasonable activation energy.) See discussion in ref 3. Based on thermochemistry and the reverse rate constant reported in the listed reference(s). f Arbitrary estimate.
limiting? (CH,),SnCl,
-
CH,SnCI,
+ CH,
(D1)
At 58 kcal mol-', the activation energy for reaction D1 is significantly less than for loss of a methyl radical from TMT (64.5 kcal mol-'). Such a lowered activation energy is a reflection of the lower Sn-C bond strength in the chlorinated molecule than in TMT: electronegativesubstituents such as C1 tend to weaken the other bonds to the central tin atom. As active radicals build up from hydrocarbon oxidation, secondary initiation of the form (CH,),SnCl,
+R
-
(CH,)Cl,SnCH,
+ RH
(reactions D2-D8 and C16) must also be considered for DMT consumption. At first, the most important of these hydrogen abstractions from DMT is attack by (CH3)C12SnO: (CH,),SnCl,
+ (CH,)CI,SnO
-
+
(CH,)Cl,SnCH, (CH,)CI,SnOH (D8)
Analogous to steps in the TMT oxidation, we propose that (CH3)C12SnCH2quickly and reversibly adds O2 (CH,)Cl,SnCH,
+ 0, e (CH,)Cl,SnCH,OO
(D10,D11)
and then undergoes @-eliminationto form (CH,)C12SnO and CH20 CI H H3CSnCOO I I
I 1
CI H
-
[: H3CA&O] CI
cHz
-
CI H3CSnO I + H&=O
(012)
I
CI
The rate constants for reactions D10 and D11 were estimated in analogy to alkyl-oxygen association reactions. Reaction D10 is, in fact, an association between oxygen and a substituted alkyl radical, for the reacting radical is carbon-based rather than tinbased. The rate constants, though far from certain, are certainly reasonable. Furthermore, k D l 0 is sufficiently large and k ~I , sufficiently small (in comparison to the estimate for &D,z) that the deposition rate was insensitive to both rate constants. The
The Journal of Physical Chemistry, Vol. 97, No. 10, 1993 2279
Chemical Vapor Deposition of Tin Oxide rate constant assigned to reaction D12 is unimportant provided it is sufficiently large. The larger size of tin and its capacity for hypervalence make the pathway more plausible for tin than for the analogous hydrocarbons. The primary reaction path of (CH3)C12Sn0was taken to be hydrogen abstraction rather than loss of a methyl radical. This a departure from analogy to hydrocarbonchemistry, but a departure well justified on the basis of the relative reluctance of heavier elements in group 14 to form doubly bonded species.’ So reactions D8,D10, and D12 are the propagating steps for the chain oxidation of DMT to (CHj)C12SnOH. This sequence converts most of the reactant DMT into the intermediate methyldichlorotin hydroxide, (CH3)C12SnOH. The reaction CH,SnCI,
-
+ 0,
(CH,)CI,SnO
+0
(D9)
provides a bridge between the primary initiation (reaction D1) and the propagationstepsjust mentioned. Reaction D9 is another departure from hydrocarbon chemistry, but (as was discussed with reference to its TMT analogue in ref 3) it is justified both by gas-phase tin chemistry and by its success in reducing the computed induction period for oxidation. The alkyltin chain described so far was not sufficiently rapid to account for the observed DMT consumption rate or (even assuming instantaneous conversion of (CHs)C12SnOH to film) tin oxide deposition rate. The other organotin reactions with crucial kineticsignificanceinvolved further reactionsof (CH3)CIlSnOH:
-
(CH,)CI,SnOH (CH,)CI,SnOH
+ 0,
SnO
SnO,
+ CH, + C1+ HCI
(D16)
+ CH, + C10 + HCI (D17)
The SnO and Sn02formed in these reactions were assumed to adhere to any surface to which they diffused. SnO was presumed to be further oxidized heterogeneously. That these tin oxides should stickefficientlyto reactor surfaces is certainly a reasonable expectation, in light of their minusculevapor pressure at reaction temperatures. That the further oxidation of SnO is rapid has also been suggested in studies of catalysis by tin oxide of the oxidation of carbon monoxide.* As was the case in the TMT mechanism, “reactions” D 16 and D17 are intended to be interpreted as sequencesof reactions (and therefore as modeling expedients) rather than as concerted elementary steps. In effect, ignorance about the reaction mechanism was confined to the chemistry of the key intermediate (CHj)C12SnOH. The importance of reactions D16 and D17 to deposition kinetics was twofold. First and most apparent, each of these reactions produced a film precursor in a fairly slow (Le,, at least partially rate-limiting) ‘step”. Second, they provided branching for the organotin oxidation chain, since the chlorine atoms they generated could readily attack the DMT reactant. Reaction D17 was also responsible for the oxygen dependence exhibited by the mechanism’s deposition rate. Loss of CH3 is a probable initial step for both of these reactions, since the Sn-C bond is the weakest of the four bonds to tin in (CH&12SnOH. CHjCl was deemed an unlikely product, both because CH3 seemed likely to be detachable separately and because CHjCl was observed by SG as only a minor byproduct. The other products of reactions D16 and D17 were selected in analogy to the reactions postulated for (CHj)3SnOH.3 Branching from the products of step D16 is quite direct. C1 is a very proficient hydrogen abstractor, and its attack on DMT can be expected to be quite rapid: C1+ (CH,),SnCl,
-
HCI
+ (CH,)CI,SnCH,
(C16) In addition, the CHj radical produced by (D16) can undergo
oxidation to OH, which also efficiently abstracts hydrogen from DMT:
+ OH
(CH,),SnCI,
-
+ H,O
(CH,)Cl,SnCH,
(D4)
The sequence for converting the relatively slow hydrogen abstractor, CH3, into the more efficient OH begins with a rapid reversible addition of oxygen:
CH,
+ 0, e,C H 3 0 0
(C15, C22)
Mass leaks out of this equilibrium when methylperoxy radicals abstract hydrogen atoms, at first from DMT or formaldehyde but later mainly from hydr~peroxy:~
-
+
CH@O + HO2 CH3OOH 02 (C25) The methylhydroperoxide formed in such an abstraction falls apart quite quickly at deposition temperatures:
+
CH3OOH CH3O O H (C28) Because the CH3 OH conversionis such a roundabout process, it contributes relatively little to branching from the products of reaction D16. Branching from the products of (D17), however, is limited to the action of CHj. C10 is not a very rapid abstractor of hydrogen atoms, nor it is likely to be converted into either OH or C1 (although it can take part in reactions which are peripheral to the methyl hydroxyl conversion). Reactions D 16 and D 17 differ from their TMT analogues not only in their products (leading to somewhat different branching effects) but also in their rate constants. As noted above, electronegative substituents bonded to tin tend to weaken the Sn-C bond. Similar to the lower activation energy for reaction D1 than for its TMT analogue, one can expect a lower activation energy for (D16) than for its TMT analogue. A lower activation energy was also assigned reaction D 17,resulting in a rate constant physicallyplausible for the initial bimolecular interaction between (CH3)C12SnOHand 02. Although there is considerable evidence for the enhancement of DMT oxidation by addition of water,I0no such pathways were considered here. Hydrolysis of Sn-CI bonds may be reasonably expectedto occur in the oxidation of a chlorinatedorganostannane, but detailed kinetic information on the role of water in individual steps is unavailable. Furthermore, under the dry oxidation conditions employed by SG,water would not be expected to form in appreciable quantities until most DMT was consumed. Because of the three chlorinated species generated in steps D16 and D17, chlorine reactions were added to the mechanism. Some, like reaction C16, directly involved DMT consumption and film growth. Others, like the reaction
-
C1+ C H 2 0
-
HCI
+ CHO
(C15) influencedthe overall reaction rate indirectly,in this case reducing the branching effect of reaction C16 by competing with it for CI atoms. The main sources of information on the chlorine subset of reactions were critical reviews compiled by NASA” and CODATA12 for use in atmospheric chemical kinetic modeling and an earlier review of higher-temperature data compiled by Baulch et a1.13 Rateconstantsfor probablereactionsofchlorinatedspecies for which no kinetic data are available (such as reaction C16) were estimated, as noted in footnotes to Table I, but were not adjusted to fit observed deposition data. As mentioned above, the kinetic equations of the mechanism described in Table I were integrated in twodimensionswith explicit treatment of diffusion. Therefore, the destruction of reactive species by collision with the walls (including that of SnO and Sn02 leading to film deposition) was treated implicitly by boundary conditions: every species was assigned a sticking probability (from 0 to 1 inclusive), and losses due to walls were computed accordingly.14 This more realistic treatment removes the necessity of listing wall losses as first-order pseudo-gas-phase
Giunta et al.
2280 The Journal of Physical Chemistry, Vol. 97, No. IO, 1993 activation energy (kcal/mol): carbon dioxide
21.8 computed
I
1
carbon monoxide methane
0
L-
CJ)
;pi,; I 4
+
1.10
ethane methyl chloride carbon oxides
computed
two-carbon
1.15
1.20 1OOO/T( K)
1.25
1.30
Figure 4. Computed Arrhenius plot of peak SnO2 deposition rate from 2.3% DMT + 20% 0 2 .
"reactions", as was done earlier in the one-dimensional treatment of TMT oxidation.3
Computational Results Satisfactoryagreement with SG's observationson the deposition of tin oxide from DMT and oxygen was obtained by treating only two rate constants, kD16 and kD17, as adjustable. Constraining kD17/kD16 to have the same value at T, = 793 K (the typical substratetemperaturefor our depositions from DMT) as obtained for kTl6/kTl7 at T = 741 K (the typical temperature for our depositions from TMT) allowed kD16 to be fit to reproduce the peakgrowth reported by SG for their deposition from 2.3% DMT and 20% 0 2 with T, = 793 K. The activation energy of reaction D16 was set at 27.5 kcal mol-' to fit the temperature effect observed upon increasing the substratetemperature from 801 to 873 K. (The activation energy of reaction D17 was fixed at 21.6 kcal mol-', the value assigned to reaction T17 in ref 3.) Deposition profiles exemplifyingconcentration dependences of the resulting simulations are shown in Figure 1. The computed peak growth rate depended linearly on the initial DMT concentration. A weak oxygen dependence, correspondingto about 0.34 order between runs at 5% and 20% 0 2 , was computed for the peak growth rate. (That is, peak growth a [O~]OO.~~.) The absolute magnitudes of the simulated growth rates also reproduced quite well the reported peak growths. (Of course, the peak deposition rate for the 2.3% TMT, 20% 0 2 run at T, = 793 K was fit to agree with observation.) The simulations agreed with experiment in producing slowly decaying deposition profiles, with broad maxima. Although thecomputed maxima were not sharply defined, they did occur somewhat later than was observed and decayed more slowly. It is worth noting that simulation of the observed peak growth was possible without consideration of powder-forming reactions. Indeed, computationswhich included powder-forming reactions in the mechanism did not allow the peak growth to remain proportional to DMT concentration. Even mild powder generation dropped peak growth to a sublinear dependence on DMT. The observation of linear DMT dependence even at higher temperatures also argues against any substantial powder formation cutting into the growth kinetics. The linearity of the simulated peak growth rates on initial DMT is illustrated in Figure 2. The computed behavior of growth profiles as a function of temperaturewas determinedin large part by fixing the activation energy of reaction D16 at 27.5 kcal mol-' to fit the temperature effect observed upon increasing the substrate temperature from 801 to 873 K. An Arrhenius plot including computed peak deposition rates at several other temperatures appears as Figure 4. The simulations produced a linear Arrhenius plot over the substrate temperature range 793-893 K. There was no sign in the computations of a "break" in the Arrhenius plot between a highly activated low-temperature region and a less highly activated
IO-' IO-^
i o - 2 IO-'
mole fraction Figure 5. Byproduct concentrations simulated for 1.3% DMT + 0 2 at Ts= 893 K and two initial 0 2 concentrations at various positions along the flow direction.
high-temperature region, such as has been frequently observed in other CVD systems. SG's report of a low activation energy in the vicinity of 20 kcal mol-' might be interpreted as evidence of a diffusion-limited deposition rate; however, this model's low activation energy of 22 kcal mol-' reflects the activation energy of the limiting kinetic steps. The computed distributionof gaseous byproducts is displayed in Figure 3. Note that the computed quantities of many of these byproducts vary along the flow direction. Some limited measurements were made by SG at earlier and later positions along the flow direction than those denoted by "observed" in Figure 3, and thesedata indicate a continuousincreasein theconcentrations of all species displayed.'s The simulated profiles for these species also display a continuous accumulation of all of these byproducts-at the specified conditions up to 20 cm (- 1 s), at any rate. The fact that hydrocarbonsas well as oxidized species show no sign of net consumption is an indication of the mildness of the oxidation conditions, at least for hydrocarbons. Carbon monoxide was computed to be the most plentiful carboncontaining byproduct. Although it was also observed to be the most abundant carbon-containing byproduct, the simulation predicted much more of it than was actually seen. Calculated methane and ethane levels were consistent with observation. Carbon dioxide levels were greatly overestimated by the computation. Methyl chloride was simulated,as well as observed, in minor quantities, although the observed levels are considerably greater than the computed ones. Experimental data on byproduct levels are not systematically available for other deposition conditions. Simulation data from a 5% 0 2 run are plotted in Figure 5, along with the 20% 0 2 data displayed previously. The fully oxidized species,C02, was present in significantly lower quantities under low-oxygen conditions, while oxygen-free species such as CH4, C2H6, and CH3Cl were present in greater quantities. This unsurprising effect is more a reflection of the processes producing these species than of the reactions consuming them. The aforementioned oxygen-free species, for example, require methyl radicals for their formation, and lower oxygen levels translateto higher methyl levels (through the near equilibrium of reactions C15 and C22). For a variety of reasons (including low vapor pressure, sensitivity to acid-catalyzed surface reactions, and general incompatibility with the columns and detectors employed in assaying the compounds reported above), no experimental observations are available for species such as DMT, water, hydrogen, formaldehyde, and hydrogen chloride. Computedconcentrations for these species are displayed in Figure 6. H20, H2, and HCl all increased continuously along the flow direction, while t he reactant, DMT, decreased, as one would expect a reactant todo. CH20, however, alsodisplayeda decline between the 5- and 20-cm points. The simulated concentration profile for
Chemical Vapor Deposition of Tin Oxide
water
The Journal of Physical Chemistry, Vol. 97, No. IO, 1993 2281
Ji r
hydrogen
I
formaldehyde 1
I
I
10"
IO-* i o - '
IO-5
mole fraction Figure6. Additional byproduct concentrationssimulated for 1.3% DMT + 02at Ts= 893 K and two initial 02concentrations at various positions along the flow direction. -9
.1 2.0x1 15x1
DMT A formaldehyde
"0.
Im carbon monoxide 0
I
m#D
D88
C
0 .-c + E 1.ox
c
8c 0
0.5~
0
0.0
position (cm) Figure 7. Simulated concentration profiles of various carbon-containing species from 1.3% DMT + 20% 02 at Ts= 893 K.
CH20 was that of an intermediate, produced in substantial quantities, and subsequently consumed. Figure 7 depicts the computed concentration profiles of three important carboncontaining species. It displays the rapid consumption of DMT. (Note the logarithmic time scale.) The concomitant rapid production of CH2O is attributable to the extreme rapidity of steps D10 and D12 in the organotin chain. Formaldehyde is extremely susceptible to hydrogen abstraction, so it is steadily consumed by free radicals produced principally in steps D 12 and D16. Once CH2O loses one H atom, it quickly gives up the other, forming CO.
Discussion The ability of the proposed mechanism to account for so many SG's observations does not argue for the accuracy of the rate constants employed in the simulations. For example,no attempt was made to adjust the estimated expression for kCI6, but there is little doubt that adjustments in kD16 and k ~ could ~ 7 be made which would produce simulations that comparably reproduce experimental observationswith other reasonable estimates of kc16. Nor doesthe agreementbetween simulation and experiment argue for the correctness of all the mechanism's pathways. Lack of understanding of the pyrolysisor oxidation chemistry of (CHJC12SnOH has been frankly acknowledged throughout this paper. Some products other than those listed for reactions D16 and D17-perhaps SnO + CH4 2C1 for the decomposition of (CH3)ClzSnOH-might explain the observations equally well as the ones actually used. In fact, simulations in which (CH3)C12SnOH decomposition was assumed to produce SnO + CH4 + 2C1 were distinguishable from those in which it produced SnO + CH3 HCl + C1 only in the concentrations of the completely
+
+
oxidized byproducts CO2and H20and the oxygen-freebyproducts H2, CH4, and C2H6. What the mechanism presented heredoes,however, is to provide a framework within which most of the observations of SG can be discussed. The mechanism is not "infinitely flexible": there are variationson the reactions and rate constantsemployed which cannot (or at least not without extensive modificationthroughout the mechanism) explain the data. For example, one could imagine the decomposition of (CH3)CltSnOH to produce SnO + CH3Cl + HCl or SnO + CH4 C12; however, neither of these product mixes was capable of producing sufficient branching to match the observed growth rates. In both the DMT oxidation system and the TMT system upon which it was based, the hypothesized mechanisms provide a context within which such variations can be reasonablyjudged. (Several examplesof unworkablevariations on a mechanism were also discussed in ref 3 in connection with TMT oxidation.) In addition, the posited mechanisms make "chemical sense": because information besides the observations they eventually reproduced was invoked in their formulation, there are good reasons besides their ability to "fit the data" for accepting their pathways. A more detailed understanding of the oxidation kinetics of DMT, TMT, and related organotin compounds requires more detailed knowledge of the reactivity of important intermediates. Basic kinetic studies of the decomposition and oxidation of key hydroxide intermediates such as (CH3)C12SnOH and (CH3)3SnOH would greatly assist in this endeavor. Of comparableutility would be the measurementof the reaction rates of various peroxy radicals at temperatures and pressures commonly employed in CVD. Kinetic characterization of reactions involving methylperoxy and, to a lesser extent, hydroperoxy has suffered from intrinsic difficulties in measurement. Among the obstacles to measurements are the difficulty of producing the radicals "cleanly", the relativeslownessof their reactions, and the inherentlycomplicating problem of self-reactions. Uncertainties in the extinction coefficients for these species, particularly as a function of temperature, have also hindered the characterization of their reactivity.I6 Reliable rate constants for peroxy species are more crucial to modeling of oxidation under mild conditions (such as CVD) than to two larger and more activeareas of kinetics modeling, namely, atmosphericand combustion modeling. At the low temperatures prevalent in the atmosphere, peroxy radicals participate in important kinetic pathways, but not in a rate-limiting way. At the high temperaturesmost frequently encountered in combustion, peroxy radicals are too unstable thermally to play an important kinetic role. At the intermediatetemperaturesused in controlled CVD oxidations, however, peroxy radicals are key intermediates, and their reaction rates are of great interest in the further elucidation of the mechanism of all sorts of oxidations by 0 2 under mild conditions. Further "classical kinetics" studies of the type carried out by Borman and Gordon2 and SG are also in order. Obviously, the more observations one has on the variation of the entire growth profile (which represents the temporal development of the reaction) with such control parameters as temperature and composition, the more constraints one places on a mechanism. The same can be said for measurements of gaseous byproduct concentrations. Still, it is difficult to reconstruct a mechanism from product analysis alone, no matter how complete, and impossible to determineindividual rate constantsaccuratelyfrom the simulation of so complex a process. In situ detection of transient species such as peroxy radicals or the uncharacterizedorganotin radicals hypothesized to prop agate the chain oxidation would provide direct evidence for mechanistic pathways, and fundamental kinetic studies of the reactionsof such radicals are the most promising, if also the most difficult, sources of more detailed and more quantitative mechanistic understanding.
+
2282
The Journal of Physical Chemistry, Vol. 97, No. I O , 1993
Giunta et al.
TABLE II: Thermodynamic and Transport Parameters ASIPS'
A20/xoa
Ph
AHf
Sd
ref
14.6 X 11.3 x 10-5 6.85 x 10-5 6.40 x 10-5 6.90 X 6.42 x 10-5 6.96 X 6.44 x 10-5 6.47 x 10-5 6.96 x 10-5 7.02 x 10-5 6.46 x 10-5 6.44 x 10-5 6.23 x 10-5 6.22 x 10-5 6.48 x 10-5 6.50 x 10-5 6.52 X 6.27 x 10-5 5.81 x 10-5 5.81 x 10-5 5.79 x 10-5 5 . 8 2 ' ~10-5 5.81 x 10-5 5.81 x 10-5 5.89 x 10-5 5.87 x 10-5 6.38 x 10-5 6.07 x 10-5 6.36 x 10-5 6.19 x 10-5 6.09 x 10-5 6.19 x 10-5 6.20 x 10-5
4.4 x 10-5 0.9 x 10-5 5.93 x 10-5 5.30 x 10-5 6.00 X 5.33 x 10-5 6.07 x 10-5 5.35 x 10-5 5.41 x 10-5 6.07 X 6.15 x 10-5 5.38 x 10-5 5.35 x 10-5 5.06 x 10-5 5.05 x 10-5 5.41 x 10-5 5.44 x 10-5 5.47 x 10-5 5.11 x 10-5 4.45 x 10-5 4.45 x 10-5 4.43 x 10-5 4.47 x 10-5 4.45 x 10-5 4.45 x 10-5 4.57 x 10-5 4.54 x 10-5 5.27 x 10-5 4.82 x 10-5 5.25 x 10-5 5.00 x 10-5 4.85 x 10-5 4.99 x 10-5 5.02 x 10-5
1.o 0.0 0.0
52.1 0.0 -57.8 -32.6 9.3 2.5 59.6 0.0 -20.2 -17.9 34.8 4.0 -48.0 2.6 -31.3 -26.0 9.0 -26.4 -94.05
27.4 31.2 45.1 56.0 43.9 54.8 38.5 49.0 54.9 44.5 46.4 54.4 57.3 62.8 67.5 52.3 53.7 47.2 51.1
69 69 69 69 70 70 69 69 69 69 69,70 19 69 19,49 69 69 69 69 69
29.0 0.0 -22.0 24.2 23.0 -17.8 -19.6
39.5 53.3 44.6 54.1 63.0 56.6 56.0
69,70 69 69 69 69 70 69
species H H2 H20 H202 OH
HO2 0 0 2
C2H6 CHI CH3 CH3O CH,OH CH300 CH3OOH CH2O CHO
co
c02 (CH3)2SnC12 CH3CI2SnCH2 CH3C12SnCH200 CH3SnC12 CH3CI2SnO CHpC12SnOH
SnO Sn02
CI c12 HCI CIO ClOO HOC1 CHiCl
I .o
1.o 1.o 1.o 0.0 0.0 0.0 1.o 1.o 0.0 1.o 1.o 0.0 1.o 0.0 0.0 0.0 1.o 1.o 1.o 1.o 0.0 1.o 1.o 1.o 0.0 0.0 1 .o 1.o 0.0 0.0
'Prefactor" for the diffusivity D taken to depend on the temperature according to D = AT15 . D has units of cm2 s-I, so A has units of cm2 SKI K-I s. Parameters are given for 5% 02/95% He and for 20% 02/80% He mixtures. Probability of sticking to a surface which the species strikes. Enthalpy of formation in kcal mol-'. Entropy in cal mol-' K-I.
Acknowledgment. We thank the Solar Energy Research Institute (now the National Renewable Energy Laboratory) and the National Science Foundation for support. We also thank the National Center for Supercomputing Applications for computational resources. Appendix: Thermodynamic and Transport Parameters Diffusivity parameters were based on reduced mass, p, and temperature, T, according to the following functional form obtained from kinetic theory:
D = ,T',s/po.s Here, CY was taken to be a 'universal" constant, 1.3 16 X lo4 cm2 s-I a m ~ OK-l.S, , ~ extracted from diffusivitydata on common gases in air." Defining A ap4.5 yields D = AT13 for the diffusivity as a function of temperature. The reduced mass, p, of a pair of colliding molecules A and B is given by f
-1= - I + - 1 p
"B
"A
where m representsthemass. In thecaseof diffusionof a molecule A in a mixture of gases with components 1, 2, etc.-the usual situation in these simulations-the reduced mass was approximated by P
= &Pi i
where xi is the mole fraction of component i and pi the reduced mass for the pair A and i. The diffusivityparameters appropriate for 5% 0 2 / 9 5 % He and for 20% 02/80% He are listed in Table 11. The considerable difference in diffusivities at these compositions is, of course, a direct reflection of the substantial mass difference between 0 2 and He.
This approximationfor D neglectseffects of molecularpolarity and size (volume) but provides an easily evaluable formula of sufficient accuracy. Uncertainities inherent in the diffusivity are less than those of most, if not all, of the rate constants in this mechanism. Furthermore, simulation results are not strongly dependent upon diffusivity values, even those of film precursors. Thermodynamic data were employed in computingsome reverse reaction rate constants and in determining reaction energetics. References in Table I1 relate to the thermodynamic parameters. Heats of formation are given in kcal mol-' and entropies in cal mol-' K-I. Sticking probabilities of 0.0 were assigned to "stable" species and 1.O to 'reactive" species (on the assumption that they would also be stable or reactive with respect to a wall).
References and Notes (1) (a) Strickler, D. A. Ph.D. Thesis, Harvard University, 1989. (b) Strickler, D. A.; Gordon, R. G. Manuscript in preparation. (2) (a) Borman, C. G.; Gordon, R. G. J. Elecrrochem.Soc. 1989,136, 3820. (b) Borman, C. G. Ph.D. Thesis, Harvard University, 1984. (3) Zawadzki, A. G.; Giunta, C. J.; Gordon, R.G. J . Phys. Chsm. 1992, 96, 5364. (4) (a) Gordon, R. G.; Proscia, J.; Ellis, F. B.. Jr.; Delahoy, A. Sol. Energv Mater. 1989, 18, 263. (b) Proscia, J. W. Ph.D. Thesis, Harvard University, 1988. ( 5 ) Bienstock, S.Ph.D. Thesis, Harvard University, 1980. (6) (a) Price, S.J. W.; Trotman-Dickenson, A. F. Trans. Faraday Soc. 1958,54, 1630. (b) Johnson, R.P.; Price, S.J. W. Can. J . Chem. 1972,50, 50. (7) Ebsworth, E. A. V. Volatile Silicon Compounds; Pergamon: New York, 1963. (8) Fuller, M. J.; Warwick, M. E. J . Caral. 1973, 29, 441. (9) Hydroperoxy is itself rapidly produced once H, CHO,or CH,O is formed. (See reactions H13, C20, and C35.) H atoms are principally formed in the decomposition of C H O or CHjO (reactions C19 and C34). CHO
-
results when any radical abstracts hydrogen from formaldehyde, and CH,O is formed in the methyl hydroxy pathway from the decomposition of CH,OOH (reaction C28).
Chemical Vapor Deposition of Tin Oxide
The Journal of Physical Chemistry, Vol. 97, No. 10, 1993 2283
(IO) (a) Adachi, K.; Mizuhashi, M. Proc. Electrochem. Soc. 1987,874, (42) Derived from reverse rate constant (kc,) and thermochemical 999. (b) McCurdy, R. J. Personal communication, 1989. (c) Hydrolysis of information: kC J = kcrKcl3, AclJ = Acr exp(AScl j/R), ECIJ = E ~+JAHCIJ. chlorinatedorganotin compoundsis well-known in solutionchemistry to produce AHCIJ= 2.5 - 17.9 - 34.8 + 32.6 = -17.6 kcal mol I, ASel, = 54.8 + 44.5 stannoxanes; see: Okawara, R.; Wada, M. Ado. Organomer. Chem. 1967,5, - 46.4 - 56.0 = -3.1 cal mol I K I, AclJ = 1.12 X 10" exp(-3.1/R) = 2.4 137. X IO'? cmJ mol I s I, and E,,, = 24.6 - 17.6 = 7.0 kcal mol I . ( 1 1 ) DeMore,W.B.;Margitan,J.J.;Molina,M.J.;Watson,R.T.;Golden, (43) C o b s , C. J.; Hippler, H.; Luther, K.; Ravishankara, A. R.; Troe, J. D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R. J. Phys. Chem. 1985.89, 4332. Chemical Kinetics and Photochemical Datafor Use in StratosphericModeling, (44) Reference 12 recommends Evaluation Number 7; Jet Propulsion Laboratory: Pasadena, CA, 1985;JPL Pub 85-37. log F, (12) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr.; Kerr. log k = log 1 + ko/kJ. A.; Troe, J. J. Phys. Chem. Ref. Dara 1989, 18, 881. 1 (log k 0 / k J 2 (13) Baulch, D. L.; Duxbury, J.; Grant, S. J.; Montague, D. C. J . Phys. withko=8X IO >I(T/300) J3[M]cm3moleculeIs l,k,=2.2X IO '?(T/300) Chem. Ref. Dara 1981, 10 (Suppl. I ) . cm' molecule I s I , and F, = 0.27. We fit the atmospheric-pressure curve to (14) Further computational detail can be found in: Giunta, C. J.; ChappleArrhenius form over 500-750 K. Sokol, J. D.; Gordon, R. G. J. Electrochem. Soc. 1990, 137, 3237. Giunta, (45) SIagle et al. (Slagle, 1. R.; Gutman, D.; Davies, J. W.; Pilling, M.J. C. J. Ph.D. Thesis, Harvard University, 1989. J . Phys. Chem. 1988, 92, 2455) report ko = 8.76 X IO 'T 7 0 3 exp(-1390/ (15) Actually, the sampling port was at a fixed position, but the flow rate T)[M] cm' molecule-ls-',k , = 1.50 X I O 7 T lXexp(-329/RT) cm'molecule was varied so as to put the port before, at, or after a given residence time. s I, and F, = 0.381 exp(-T/73.2) + 0.619 exp(-T/I 180) for thesamestandard pressure-dependent parametrization as used for in footnote 44. We fit the (16) Ravishankara. A. R. Annu. Rev. Phys. Chem. 1988, 39, 367. atmospheric-pressure data to Arrhenius form over 500-750 K. (17) CRC Handbook of Chemistry and Physics, 69th ed.; Weast, R. C., Ed.; CRC: Boca Raton, FL, 1977; p F-48. (46) Parkes, D. A. Inf. J . Chem. Kinet. 1977, 9, 451. (18) Cohen, N.; Westberg, K. J. Phys. Chem. Ref. Data 1983, 12, 531. (47) Zaslonko, 1. S.; Mukoseev, Y. K.; Tyurin, A. N. Kinet. Caral. (Engl. kll? is the value for N? bath gas. Transl.) 1988, 29, 244; Kinet. Karol. 1988, 29, 283. (19) Tsang. W.; Hampson, R. F. J. Phys. Chem. Ref. Dara 1986, 15, (48) Gutman, D.; Sanders, N.; Butler, J. E. J . Phys. Chem. 1982,86,66. 1087. (49) Khachatryan et al. (Khachatryan, L. A,; Niazyan, 0.M.; Mantashyan, (20) Baulch, D. L.; Drysdale, D. D.; Horne, D. G.; Lloyd, A. C. Eualuated A. A.; Vedeneev, V. I.; Teitel'boim, M. A. Inr. J . Chem. Kinet. 1982,14,123 1 ) Kinetic Data for High Temperature Reactions; Butterworths: London, 1972; report Kcl, = 10-5.7y exp(28.51/RT) atm I. Combining this result with kcl! Vol. I . yieldskc??= kcls/R'TKcls -2.55 X 1024TJ/[RTXIO 5J9exp(28.51/RT)] (21) Sridharan, U. C.; Qiu, L. X.;Kaufman, F. J . Phys. Chem. 1982,86, and kc?>= 1.92 X 1028T-4exp(-28.51/RT) cml mol-' s - I . 4569. (50) Benson,S. W.;ONeal,H. E. KineticDataonCasPhaseUnimolecular (22) Baulch, D. L.; Cox, R. A.; Hampson, R. F.,Jr.; Kerr, J. A.; Troe, Reactions; US.National Bureau of Standards: Washington, DC, 1970. J.; Watson, R. T. J . Phys. Chem. Ref. Data 1980, 9, 295. (51) Klemm, R. B.; Skolnik, E. G.;Michael, J. V. J . Chem. Phys. 1990, (23) Sutherland, J. W.; Michael, J. V.; Pirraglia, A. N.; Nesbitt, F. L.; 72, 1256. Klemm, R. B. Symp. (Int.) Combust., [Proc.], 2Ist 1986, 929. (52) Baldwin, R. R.; Fuller, A. R.; Longthorn, D. J . Chem. Soc.,Faraday (24) Based on reverse rate constant (ktlq) and thermochemistry: k ~ =~ 7 Trans. I 1974, 70, 1257. kiivKiiiJ. Ail17= Aiio exp(AS1~~7/R), Eti17= EHY + AHtiii, M H=I2.5 ~+ (53) Timonen, R. S.;Ratajczak, E.;Gutman, D. J . Phys. Chem. 1988,92, 52.1 - 0 - 0 = 54.6 kcal mol I, A&{17 = 54.8 + 27.4 - 31.2 - 49.0 = 2.0 cal 651. mol I K I, A l I l 7= 6.62 X 1013 exp(Z.O/R) = 1.8 X 1014 cm3 mol-' s 1, and Etl17 = 2.13 + 54.6 = 56.7 kcal mol I . (54) Ravishankara et al. (Ravishankara, A. R.; Thompson, R. L. Chem. (25) Dougherty, E. P.; Rabitz, H. J . Chem. Phys. 1980, 72, 6571. Phys. Lett. 1983, 99, 377) report k = exp(-30.03 + 1.22 X IO '7') cml molecule-' s I at 100 Torr, which we fit to a simple Arrhenius function over (26) Baulch, D. L.; Drysdale, D. D.; Duxbury, J.; Grant, S.Evaluated the temperature range 500-750 K. Thesame laboratory (Hynes, A. J.; Wine, Kinetic Data for High Temperature Reactions; Butterworths: London, 1976; P. H.; Ravishankara, A. R. J . Geophys. Res. 1986, 91, 11815) reported a Vol. 3. kll?, is recommended for O?bath gas. pressure dependence of k = 1.47 X IO "(1 + 0.59P) cm' molecule I s 1 ( P (27) Zellner, R.; Ewig, F.; Paschke, R.; Wagner, G.J . Phys. Chem. 1988, in atm) in an extremely limited temperature range near room temperature. 92, 4 184. Combining the pressure dependence from the later paper with the refit simple (28) Mozurkewich, M. J . Phys. Chem. 1986, 90, 2216. Arrhenius temperature dependence of the earlier yielded our preferred rate (29) Derived from reverse rateconstant and thermochemical information: constant. kll?o = klilIKIw/R'T, A I I X= (AlilJ/RV exp ( U H ? P / R ) E, H ~=QE H I+~ (55) Baker, R. R.; Baldwin, R. R.; Walker, R. W. Combust. Flame 1976, AHtl?o, AHll?q = 52.1 + 0 - 2.5 = 49.6 kcal mol I, ASH?^ = 27.4 + 49.0 27, 147. 54.8 = 21.6cal mol I K I , = (6.42 X 101"T-'/R'T)exp(21.6/R)) = 4.12 (56) Walker, R. W. In Reaction Kinetics; Specialists Periodical Reports; X 10?lT? cmJ mol I S I, and Ell?q = 0 + 49.6 = 49.6 kcal mol I. Chemical Society: London, 1975; Vol. 1, p 161. (30) Reference 12recommends2.2X IO I3exp(600/T)+ 1.9X 10-31[N?] (57) Kerr, J. A.; Parsonage, M. J. Eualuared Kinetic Dara on Gas Phase exp(98O/T) cm3 molecule I s 1, which we fit to Arrhenius form over 500-750 Hydrogen Transfer Reactions of Methyl Radicals; Butterworths: London, K. 1976. (31) Wine et al. (Wine, P. H.; Nicovich, J. M.; Thompson, R. J.; Ravishankara, A. R. J. Phys. Chem. 1983,87, 3948) report k = 6.8 X 10" (58) Xi, Z.; Han, W.-J.; Bayes, K. D. J . Phys. Chem. 1988, 92, 3450. exp(-2000/T) cm' mol I s I for reactions H34 and H35 combined. We have (59) Baldwin et al. (Baldwin. R. R.; Hisham, M. W. M.; Walker, R. W. assigned half of this rate to kl13,and half to kllir, which is consistent with the J . Chem. SOC.,Faraday Trans. 1 1982,78,1615) report neopentaneanalogue recommendationsof Wineet al. andof Alberset al. (Albers, E. A.; Hoyermann, of the equilibrium constant kDlo/kDlI. We combined thisquilibrium constant K.; Wagner, H. G.; Wolfrum, J. Symp. (Int.) Combust., [Proc.], 13th 1971, with kDlo. 81). (60) Set q u a l to the rate constant reported in ref 19 for CHlO + H?O (32) Warnatz, J. Combust. Sci. Technol. 1983, 34, 177. reaction. (33) Sutherland, J. W.; Michael, J. V.; Klemm, R. B. J . Phys. Chem. (61) Estimated using the method of Alfassi, Z. 9.;Benson, S.W.Int. J. 1986, 90, 5941. Chem. Kinet. 1973, 5, 879. (34) Herron, J. T. J. Phys. Chem. Ref. Data 1988, 17, 967. (62) Estimated to be independent of T (in CVD range) at half its 300 K (35) Baulch, D. L.; Bowers, M.; Malcom, D. G.;Tuckerman, R. T. J . value from ref 1 I . Phys. Chem. Ref. Data 1986, 15, 465. (63) Cross-combination rule: kAB= ( 2 k A A k ~ ~ )where l l ? , the k's represent (36) Baldwin, R. R.; Jones, P. N.; Walker, R. W. J . Chem. Soc., Faraday combination reactions among radicals A and 9. Trans. 2 1988. 84. 199. (64) Reference 11 reports the equilibrium constant ktl7/kcl??. We (37) Walker, R. W. In Reaction Kinetics; Specialists Periodical Reports; combined this equilibrium constant with kc 17. Chemical Society: London, 1975; Vol. 1, p 161. Table 4 lists for RH + 01 R + HO?: A = 4 X 1Ol"dmJ mol I s I and E = AH, Our thermochemical (65) Reference 11 reports A; we took E to q u a l AH. data give AH'c = 34.8 2.5 - 0 + 17.9 = 55.2 kcal mol I. (66) Reference 1 1 reports A; we took E to q u a l 1 kcal mol I more than (38) Reference 19 reports k, = 2 X IO q T cm' molecule I s I and log the lower limit E recommended in the same reference. (klk..) = 0.275 - 8.75 X IO 'T- 7.10 X IO xT? for 1 atm. We fit the I-atm (67) In analogy to reactions C13, CIS, and C124, kc 1~ was set q u a l to rate constant to Arrhenius form over the temperature range 500-750 K. krixkcii/kcic. (39) Slagle, 1. R.; Sarzydski, D.; Gutman, D. J. Phys. Chem. 1987, 91, (68) Jackson, R. A. J. Organomel. Chem. 1979, 166. 17. 4375. (69) Benson,S. W. Thermochemical Kinetics, 2nd ed.; Wiley: New York, (40) Westbrook, C. K. Combust. Sci. Techno/. 1983, 34, 201. 1976. (41) Derived from reverse rate constant (ktr) and thermochemical information: k c l ? =k c 5 K c 1 2 , A c IACcexp(A&lz/R),Ecl?= ?= Ecr+AHcl: (70) Chase, M. W., Jr.; Davies. C. A.; Downey, J. R., Jr.; Frurip, D. J.; = 0 (see footnote 37), ASc.,. = 49.0 + 44.5 - 54.8 - 46.4 =: -7.7 cal mol McDonald, R. A.; Syverud, A. N. JANAF Thermochemical Tables, 3rd ed.; K I . and A t l : = 4.0 X loll exp(-7.7/R) = 8.3 X 1011 cmJ mol I s I. J . Phys. Chem. ReJ Dara 1985, 14 (Suppl. I).
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