Resolving the Nucleation Stage in Atomic Layer Deposition of Hafnium Oxide on Graphene

Abstract

The integration of two-dimensional (2D) materials with functional non-2D materials such as metal oxides is of key importance for many applications, but underlying mechanisms for such non-2D/2D interfacing remain largely elusive at the atomic scale. To address this, we here investigate the nucleation stage in atomic layer deposition (ALD) of the important metal oxide HfO2 on chemical vapor deposited graphene using atomically resolved and element specific scanning transmission electron microscopy (STEM). To avoid any deleterious influence of polymer residues from pre-ALD graphene transfers we employ a substrate-assisted ALD process directly on the as grown graphene still remaining on its Cu growth catalyst support. Thereby we resolve at the atomic scale key factors governing the integration of non-2D metal oxides with 2D materials by ALD: Particular to our substrate-assisted ALD process we find a graphene-layer-dependent catalytic participation of the supporting Cu catalyst in the ALD process. We further confirm at high resolution the role of surface irregularities such as steps between graphene layers on oxide nucleation. Employing the energy transfer from the scanning electron beam to in situ crystallize the initially amorphous ALD HfO2 on graphene, we observe HfO2 crystallization to non-equilibrium HfO2 polymorphs (cubic/tetragonal). Finally our data indicates a critical role of the graphene's atmospheric adventitious carbon contamination on the ALD process whereby this contamination acts as an unintentional seeding layer for metal oxide ALD nucleation on graphene under our conditions. As atmospheric adventitious carbon contamination is hard to avoid in any scalable 2D materials processing, this is a critical factor in ALD recipe development for 2D materials coating. Combined our work highlights several key mechanisms underlying scalable ALD oxide growth on 2D materials.