Realization of Detailed Kinetic Models for the Growth of II-VI Compounds: Adopting DFT Calculations and Experimental Evi
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LogOA 14.5 13.8 12.9 11.1 10.1 10.8 13.0 12.8 9.5 17.
Ea/R
3150. 3750. 8450. 7550. 28500. 12550. 18100. 14650. 8700. 2500.
Method B3LYP/3-21G** B3LYP/3-21G** B3LYP/3-2IG** B3LYP/3-21G** B3LYP/3-21 G** B3LYP/3-21G** B3LYP/6-31 +G** B3LYP/6-311 +G** B3LYP/6-31 I+G** B3LYP/6-31 I+G**
SURFACE KINETICS The realization of a surface kinetic scheme for a chemical vapor deposition system can be pursued at different levels of complexity. The most simple approach considers the events occurring at the surface as lumped reactions of an overall process where gas phase and surface species react and determine the inclusion of one or more atoms of the impinging molecule in the growing film. In this case the reactions proposed are without physical meaning and their kinetic constants can be determined only by fitting growth rates on experimental data. Differently a physically and chemically detailed approach requires to consider all the reactive events occurring at the surface as elementary steps. A systematic method of realization of a kinetic scheme would therefore require to identify all the surface species that can be present during the growth and then to make them interact with gas phase species and among themselves. This approach requires a great amount of information on the composition and reactivity of the surface, which can be obtained either through surface science experiments or quantum chemistry calculations. The
number of reaction pathways that may lead to the growth of the film is very large and physical intuition must be adopted in order to identify the key reaction steps that can determine the formation of the film and thus limit the dimension of the field of study. This is the approach we followed and the surface schemes we propose for the growth of ZnS and CdTe (reported in table 2 and 3) are the result of the consideration of many possible surface reactions. In table 2 and 3 we avoided reporting all the surface reactions we studied that were found to proceed too slowly (mainly because of too high activation energy barriers or too low frequency factors) to be active during the growth process. Table 2. Proposed surface kinetic scheme for CdTe growth (k = A.T0 -exp(-Ea/RT) in mol, cm2, s) S1 S2
Reaction Cd(CH 3)2 + " 4
[Cd(CH 3 ) 2 ]ads
[Cd(CH 3 ) 2 ]ads
[CdCH 3]ads+ CH 3
-
S3
CH3 + [CdCH 3]ads -- [Cd(CH 3)2]ads
S4
Cd(CH 3)2 + a" Te(CH 3)2 + [CdCH 3]ads --> [CdTeCH 3]ads + C2H6 [CdTeCH 3]ads --> CdTefilm + CH 3 + C7 TeCH 3 + [CdCH 3]ads - CdTefilm +2 CH 3 + a CdCH 3 + G -- [CdCH 3]ads
S5 S6 S7 S8
[Cd(CH 3 ) 2]ads --
247
LogjoA 9.6 16.0 9.1
a 0.5 0.
11.0
0. 0. 0. 0. 0.5
18.0 12.0 18.0 11.6
0.5
Ea/R 0. 16500. 0. 5000. 13500. 12500. 10000. 0.
Notes Coll. x 0.01 est. Coll.xO.001 est.
est. Fast Fast coll.
Table 3. Proposed surface kinetic scheme for ZnS growth (k = A-Tr.exp(-Ea/RT) in mol, cm 2 , s) Reaction
LogjoA
a
Ea/R
Notes
S9
Zn(CH 3)2 + a -, [Zn(CH 3)2]ads
11.5
0.5
0.
coll.
S1O Si1
[Zn(CH 3)2lads, [ZnCH3]aS + CH 3 [Zn(CH 3)2]ads -- Zn(CH 3)2 + a ThuSH + [ZnCH3]ad --+ [ZnStBu]ad
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