Role of Temperature, Pressure, and Surface Oxygen Migration in the Initial Atomic Layer Deposition of HfO₂ on Anatase TiO₂ (101)

The atomic layer deposition of HfO2 on a TiO2 (101) surface from tetrakis(dimethylamido)hafnium and water is investigated using a combination of in situ vacuum X-ray photoelectron spectroscopy (XPS) and time-resolved ambient pressure XPS. Precursor pressures and surface temperature are tuned as to map the space state of the deposition. In the initial stages of ALD, a reaction mechanism based on dissociative adsorption dominates over a classic ligand exchange mechanism, typically evoked when metal-amido complexes and water are used as the precursors for metal oxide ALD. Surface species, induding a dimethyl ammonium ion and an imine, are identified. It is found that they can be formed only if the active role of the TiO2(101) surface is taken into consideration. The temperature of the surface enhances the formation of these species based on an insertion reaction of a hydrogen atom, which then assists the formation of more than the expected monolayer of HfO2. A HfO2 overlayer is produced already during the first half-cycle, enabled by a reduction of the TiO2 support. Dosing water at high pressure allows hydroxyl formation, which marks the transition toward a well-described ligand exchange reaction type. From the experiments performed, we find that the ALD of HfO2 at room temperature, performed at high pressure, is mainly based on dissociation and that no side reaction occurs. These insights into the ALD reaction mechanism highlight how in situ studies can help understand how deposition parameters affect the growth of HfO2 and how the ALD model for transition metal oxide formation from amido complexes and water can be extended.