Role of interface(s) for the growth of ultra-thin amorphous oxides on Al-Si alloys

A thermodynamic analysis is presented here to predict the type of initial, amorphous oxide overgrowth (i.e., am-Al2O3 or am-SiO2) on bare Al-Si alloy substrates, which along with the bulk Gibbs free energy of oxide formation, also took into account the energies associated with both its interfaces, i.e., interface between Al-Si alloy substrate and that of the thin oxide-film and interface between the thin oxide-film and that of vacuum. For the estimation of interface energies between the alloy/oxide interfaces, macroscopic atom approach was followed, where as appropriate literature values were taken to estimate the interface energies of the thin oxide-film with that of vacuum. This developed analysis was applied against Si alloying element content at the substrate/oxide interface, the growth temperature, the oxide-film thickness (up to 1 nm) and various low-index crystallographic surfaces of the substrate. It is found that am-SiO2 overgrowth is thermodynamically preferred for a combination of lower oxide-film thickness, lower growth temperature and lower Si alloying content at the alloy/oxide interface due to the overcompensation of the lower energies of both the interfaces over the bulk Gibbs free energy. Further, it is found that for all cases, am-Al2O3 forms a more stable interface with Al-Si alloy than am-SiO2.