Oxidation State Discrimination in the Atomic Layer Deposition of Vanadium Oxides

We describe the use of a vanadium 3+ precursor for atomic layer deposition (ALD) of thin films that span the common oxidation states of vanadium oxides. Self-limiting surface synthesis of V₂O₃, VO₂, and V₂O₅ are realized through four distinct reaction mechanisms accessed via judicious choice of oxygen ALD partners. In situ quartz crystal microbalance and quadrupole mass spectrometry were used to study the reaction mechanism of the vanadium precursor with O₃, H₂O₂, H₂O/O₂, and H₂O₂/H₂. A clear distinction between nonoxidative protic ligand exchange and metal oxidation is demonstrated through sequential surface reactions with different nonmetal precursors. This synergistic effect provides greater control of the resultant metal species in the film, as well as reactive surface species during growth. In an extension of this approach, we introduce oxidation state control through reducing equivalents of H₂ gas. When H₂ is dosed after H₂O₂ during growth, amorphous films of VO₂ are deposited that are readily crystallized with a low temperature anneal. These VO₂ films show a temperature dependent Raman spectroscopy response in the expected range and consistent with the well-known phase-change behavior of VO₂.