First-Principles Investigation of Hydroxylated Monoclinic HfO₂ Surfaces

Atomic layer deposition (ALD) has recently gained interest because of its suitability for fabrication of conformal films with thicknesses in the nanometer range. A comparative investigation of surface hydroxylation states of different surfaces of m-HfO 2 is central to a better understanding of how precursors react on these surfaces under ALD growth conditions. We used a combined approach of density functional theory (DFT) and thermodynamics to determine the types of acidic and basic groups on different surfaces of m-HfO 2 . We find that the water adsorption energy decreases with increasing surface coverage; however, the energetics of the hydration process can be significantly different on different surfaces. Our investigation showed that under ALD working conditions the (001) surface retains a higher concentration of Brønsted acid sites compared to the thermodynamically stable (1̄11) surface because of the higher adsorption energy of water on the (001) surface. Analysis of surface species reveals that the (001) surface has hydroxyl groups which are mainly one- or twofold coordinated to one or two Hf atoms, respectively, whereas for the (1̄11) surface threefold coordinated hydroxyl groups are also populated.