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

Abstract

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 =2.18×1014 cm3 mol−1 s−1), adduct formation path will dominate followed by irreversible adduct decomposition to form amide; if A is small ( A =1.0×109 cm3 mol−1 s−1), the TMG pyrolysis will be the dominant path; if A is in mid-value ( A =1.0×1012 cm3 mol−1 s−1), both adduct formation path and TMG pyrolysis path will exist. For the reversible adduct dissociation, if A is large ( A =1.0×1014 s−1), growth rate will be higher and the pyrolysis path will dominate; if A is small ( A =9.5×109 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.