(399e) Investigation of Reactive Center Substituent Effects On the Primary Reaction Classes During Silicon Hydride Pyrolysis: Novel Methodology for Arrhenius Parameter Estimation

Pyrolysis of the feed gas, typically SiH₄
or Si₂H₆, at low pressures is the standard protocol for
the formation of polycrystalline silicon.  Polycrystalline silicon can be
formed as particles in the gas phase or controlled at a gas-solid interface to
form semiconductor-grade materials through chemical vapor deposition methods.  Polymerization
of silicon hydrides in the gas phase causes deposits on a growing semiconductor
surface forming point defects.  Optimizing reactor design and process
conditions plays a key role in the control of this particle formation.  A clear
understanding of the routes to polymerization will also allow for the control of
technologies where particles are intentionally formed.  These technologies
create tailored nanoparticles for optoelectronic and biophotonic applications
in which the size and crystallinity of the particles play integral roles. 

Four major reaction classes are involved during
silicon hydride pyrolysis [1].  Using high-level G3//B3LYP [2], accurate rate
constants were calculated.  Analysis was performed on 125 reactions for
silylene-silene isomerization, Si-H bond insertion/dissociation, ring closure/opening,
and hydrogen association/elimination. These reaction classes have been observed
experimentally, yet rate constants cannot be measured directly for all possible
reactions of silicon hydrides of relevant sizes and substituents.  Thus, hydrides
containing up to 10 silicon atoms, a variety of acyclic and cyclic substituents
about the reactive center, and polycyclic nature were explored.

      Benson's group additivity model was extended to predict
the Arrhenius parameters, the pre-exponential factor (A) and activation energy (E_a),
for the major reaction classes during silicon hydride pyrolysis.  The
transition state group additivity (TSGA) model, previously applied to the
pyrolysis of hydrocarbons [3], predicts rate constants with superior accuracy
compared to the currently used Evans-Polanyi relationship for E_a and
a representative value of A for each reaction class.  The TSGA method possesses
several advantages that should interest kineticists seeking parameters for the primary
reaction families in silicon hydride formation: (1) allows one to gain insight
into the effect of structural changes on the reactive center during reaction,
(2) implements more explicit values of A for each reaction class, and (3)
circumvents the need to calculate accurate enthalpies and Gibb's free energies of
reaction as required for the Evans-Polanyi correlation and subsequent
calculation of the reverse rate constant, respectively.  The knowledge of the
number and the type of substituents on the reactive center for a given reaction
class is sufficient to allow accurate kinetic parameter estimation.

Keywords: kinetics, nanoparticles, intermediates,
G3//B3LYP, group additivity

[1] Wong, H. W.; Li, X. G.; Swihart, M. T.;
Broadbelt, L. J. Journal of Physical Chemistry A 108 10122 (2004)

[2] Baboul, A.
G., Curtiss, L. A., Redfern, P. C., and Raghavachari, K.  J Chem Physics 
110 7650 (1999)

[3] Sabbe, M.
K., Reyniers, M. F., Van Speybroeck, V., Waroquier, M., Marin, G. B. Chemphyschem
9 124 (2008)