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Effects of reaction-kinetic parameters on modeling reacting pathways in GaN MOVPE growth

In the modeling of the reaction-transport process in GaN MOVPE
growth, the selections of kinetic parameters (activation energy E a and pre-exponential factor A ) for gas-phase reactions are very uncertain, which cause deviations
in both chemical reaction path and growth rate. In this study, the
deviations of kinetic parameters in the modeling of GaN MOVPE growth
by previous researchers are summarized and the sources of the deviations
are discussed. On this basis, numerical simulation of the reaction-transport
process for GaN MOVPE growth in a vertical rotating disk reactor is
carried out, with variation of the groups of kinetic parameters in
different reactions, such as the pre-exponential factors in adduct
formation and reversible dissociation, the pre-exponential factor
and activation energy in irreversible adduct decomposition into amide,
and the pre-exponential factor in the trimer formation. By comparisons
of the molar concentrations of the major Ga-containing species and
the corresponding growth rates, the effects of kinetic parameters
on reaction paths in GaN MOVPE growth are determined. It is found
that, for the adduct formation, if A is large ( A =2.18×10 14 cm 3 mol −1 s −1 ), adduct formation path will dominate followed
by irreversible adduct decomposition to form amide; if A is small ( A =1.0×10 9 cm 3 mol −1 s −1 ), the
TMG pyrolysis will be the dominant path; if A is
in mid-value ( A =1.0×10 12 cm 3 mol −1 s −1 ), both adduct formation
path and TMG pyrolysis path will exist. For the reversible adduct
dissociation, if A is large ( A =1.0×10 14 s −1 ), growth rate will be higher and the
pyrolysis path will dominate; if A is small ( A =9.5×10 9 s −1 ), both adduct
formation path and TMG pyrolysis path will provide reaction precursors.
For the irreversible adduct decomposition into amide, the activation
energy E a will determine the reaction
paths. If E a is small ( E a =32 kcal mol −1 ), the adduct formation
path will dominate; if E a is large ( E a =49 kcal mol −1 ), both the
adduct formation and the TMG pyrolysis exist. The main conclusions
are obtained as follows: (1) different pre-exponential factors and
activation energies lead to different reaction paths and growth rates,
and the TMG pyrolysis path can increase the growth rate more effectively
than the adduct formation path. (2) When the trimer formation is not
considered, the predicted growth rates are close to the experimental
value. The reason is that the transport-limited growth depends on
the transport rate of Ga-containing species. Although the concentrations
of Ga-containing species are different in different reaction paths,
the total amount of Ga-containing species arriving on the substrate
does not change much. The above fact gives well explanation why different
previous researchers using different kinetic parameters for simulation
obtained the similar growth rates with the experiment values. (3)
When the trimer formation is considered and the pre-exponential factor
for the trimer formation is varied, it is found that the trimer contribution
to the growth rate is small due to the repulsive force on large molecules
by thermophoretic force. The more the trimer is formed, the larger
the growth rate error is. The simulation results have clarified some
problems existed in the simulation of GaN MOVPE growth and improved
the understanding for GaN growth reaction paths. The results of this
study may not be applicable to the horizontal reactor, but it also
has the guiding significance for the basic principle of the reaction-transport
simulation.