Atomic layer deposition of HfO2 using alkoxides as precursors

We have used density functional theory to investigate the atomic layer deposition (ALD) of hafnium oxide (HfO2) thin films using metal alkoxides as both metal and oxygen precursors. in particular, we have predicted chemical mechanisms for the ALD of HfO2 using tetraethoxyl hafnium [Hf(OEt)(4)] and water. We find that the ALD half-reactions involve the formation of stable adsorbed complexes and precursor-mediated ligand-exchange mechanisms. When Hf(OEt)(4) is used as the metal precursor and water is used as the oxygen source, both the metal and oxygen ligand-exchange reactions are unactivated with respect to the reactants. However, the reactions have moderate barriers relative to the adsorbed complexes, and the removal of the ethanol byproducts requires substantial desorption energies. However, when HfCl4 is used as the metal precursor and Hf(OEt)(4) is used as the oxygen source, both the metal and oxygen half-reactions possess high activation barriers (similar to30-40 kcal/mol). Our results imply that although Hf(OEt)(4) may be a suitable metal precursor it has relatively slow kinetics when used as an ALD oxygen source. We have also investigated several alternative reaction pathways that may affect the overall growth rate, including incomplete elimination surface reactions and nongrowth ligand exchange reactions. Finally, we discuss the self-limiting nature of the half-reactions and chemical strategies for adjusting the reactivity of alkoxide ALD precursors.