ReaxFF studies of surface fluorination of alumina and etching of alumina/aluminum metal heterostructures under gas-phase hydrogen fluoride exposure

The self-limiting surface conversion reactions of alumina and alumina/aluminum metal heterostructures under HF exposure are studied at 1250 and 750 K, respectively, using a newly developed Al/O/H/F ReaxFF force field. Simulations reveal that surface termination and HF concentration significantly influence the fluorination behavior of alumina. The 100% Al-terminated α-Al2O3 (0001) exhibits the highest degree of fluorination at equilibrium, while the O-rich surface shows negligible surface conversion. Increasing HF concentration on the Al-rich surface reveals the maximum surface fluorination, resulting in a 1.38 Å decrease in alumina thickness. During the fluorination of Al-terminated alumina, HF dissociates to form hydroxyl groups and fluorides. Subsequently, absorbed HF leads to H2O formation by transferring hydrogen to hydroxyls, removing surface oxygen and leaving surface Al unsaturated for further fluorination. As the solid AlFx layer thickens, conversion of the underlying surface relies on hydrogen diffusion from HF adsorbed on AlFx to the interfacial hydroxyls. This process ceases when the AlFx layer is thick enough to hinder any effective hydrogen diffusion. In heterostructures, Al atoms from the aluminum layer diffuse into the alumina, forming Al-enriched oxide layers that consume hydrogen and form volatile AlHx, along with solid AlFx, which significantly reduces the reliance on hydrogen diffusion for continued reaction. Under rarefied HF exposure, completely different reaction mechanisms emerge for alumina and heterostructures. Alumina quickly reaches equilibrium with minimal -F and -OH formation, while heterostructures predominately form AlHx and AlFx.