Stability of ultra-thin oxide overgrowths on binary Al-Si alloy substrate

摘要

This study presents a thermodynamic formalism to predict the type of ultra-thin oxide overgrowth due to dry, thermal oxidation of bare single-crystalline < AlSi > alloy substrate. The various oxide growth parameters considered in this formulation are Si alloying element content at the alloy/oxide interface, growth temperature, oxide-film thickness (up to 5 nm), and low-index crystallographic surfaces of the alloy substrate. Along with the bulk Gibbs free energies of oxide formation, this developed formalism also considered alloy/oxide interface energies and oxide surface energies. Further for estimating the alloy/oxide interface energies of the crystalline oxide overgrowths, chemical interaction energy and strain energy arising due to the anisotropic growth strain have been taken into account. Similarly, the alloy/oxide interface energies of the amorphous oxides considered contributions arising from chemical interaction, entropy, and enthalpy between the alloy substrate and oxide overgrowth. Overall, the model predicted the stability of amorphous {SiO2} and {Al2O3} at lower and higher oxide-film thicknesses, respectively, followed by phase transformation of amorphous {Al2O3} to gamma - < Al2O3 > on further thickening of the oxide film. Moreover, crystalline < SiO2 > was never found to be thermodynamically favorable for the parameters considered in this study. These thermodynamic predictions are found to be in agreement with the experimental findings.