Scalable Integration of Crystalline Oxide Dielectrics with 2D Semiconductors Through Epitaxial Metal Deposition and Controlled Oxidation

Atomic layer deposition (ALD) is the established process to grow oxide dielectrics on semiconductors. ALD requires abundant nucleation sites on the substrate to enable fine control of deposition, and this creates challenges when depositing on 2D materials due to their dangling-bond-free surfaces. In prior work, we demonstrated that quasi-van der Waals (q-vdW) epitaxial growth of 3D metals on 2D materials is possible, provided that the 2D materials surfaces are sufficiently clean [1]. Here, we leverage these results to develop synthesis of epitaxial HfO₂ on 2D materials by q-vdW epitaxial growth of ultra-thin Hf metal films, followed by controlled oxidation.<br/>To control oxide phase purity and achieve a high-quality, atomically-sharp 3D/2D interface requires atomic-level understanding of the process. We use <i>in situ</i> transmission electron microscopy (TEM), reactive molecular dynamics (RMD) with ReaxFF method, and scanning near-field optical microscopy (SNOM) to study the deposition and oxidation of ultra-thin Hf on graphene. Evaporated Hf metal forms epitaxial hcp-Hf crystals on sufficiently clean graphene substrates. Subsequent oxidation proceeds through two intermediate, metastable sub-oxides - an amorphous phase (a-HfO_x) and a crystalline hexagonal phase (h-HfO_x) - before terminating at the equilibrium monoclinic phase (m-HfO₂). All crystalline phases form in epitaxial relationships with the underlying graphene, including m-HfO₂ which forms in three equivalent orientations. The h-HfO_x phase has not been described previously. We identify its crystal structure, and describe the martensitic transformation by which it converts to m-HfO₂. We use SNOM to map electrical properties on the nanoscale, yielding a quantitative comparison of the conductivity of the h-HfO_x and m-HfO₂ phases. RMD simulations using ReaxFF validate our model of oxide phase sequencing, and illustrate the role of graphene in templating the h-HfO_x intermediate phase. Simulations reveal distinct phases of Hf, HfO_x, and HfO₂ that evolve differently depending on the O₂ concentration.<br/>We then extend our work to other 2D materials, h-BN and MoS₂. Deposition of Hf on h-BN with substrate heating, followed by oxidation, results in a cubic structure that may be a boron-stabilized oxide. Similarly, deposition of Hf on MoS₂ with substrate heating results in HfS₂ that forms epitaxially on the substrate. These results highlight considerations when depositing functional layers on compound semiconductors, which may react by anion exchange mechanisms, especially when heated. In preliminary results, we also demonstrate that Hf metal can grow epitaxially on MoS₂ without formation of HfS₂, provided that the 2D material surface is sufficiently clean and cold.<br/>Our results suggest a way to integrate high-performance dielectrics with 2D materials, by deposition of Hf metal followed by controlled oxidation, achieving epitaxy at each step. By illustrating Hf oxidation sequences in atomic detail we also suggest opportunities in hafnia phase engineering.<br/><br/>[1] Reidy, K. et al., Nano Lett. 2022, 22, 14, 5849–5858.