Epitaxial Formation of Ultrathin HfO₂ on Multilayer Graphene by Sequential Oxidation

We demonstrate the formation of epitaxial, ultrathin hafnia (HfO₂) on graphene. Monoclinic hafnia (m-HfO₂) forms as the end of a series of sequential oxidation reactions. Starting from Hf metal grown epitaxially on graphene, oxidation leads first to an amorphous suboxide (a-HfO_x), then to a crystalline, hexagonal suboxide (h-HfO_x) in epitaxial relationship with the substrate, and finally to m-HfO₂ that is also epitaxial. We use scanning transmission electron microscopy to characterize the epitaxial relationships and to investigate the structure of h-HfO_x. We propose a series of displacive transformations that relate the different crystalline phases and are consistent with the observed epitaxial relationships with the graphene substrate. ReaxFF-based reactive molecular dynamics simulations confirm our model of the oxide phase sequencing, and illustrate the role of graphene in promoting oxide crystallization. Our results suggest a way to achieve heteroepitaxial integration of high-performance, crystalline dielectrics with two-dimensional (2D) semiconductors with an atomically sharp interface, and are also relevant to hafnia phase engineering.