Heterogeneous Integration of HfO₂ Resistive Random Access Memories on 2D
van der Waals Materials

In order to make use of the functionality of different material classes for future energy-efficient computing, we investigated to co-integrate HfO₂-based redox-based random access memory (ReRAM) on 2D van-der Waals materials. For that purpose, we investigated the van der Waals epitaxy of HfO₂ thin films and nanoislands on graphene and on highly oriented pyrolytic graphite (HOPG), respectively. We identified a growth window for the pulsed laser deposition of HfO₂ that preserves the graphene transferred to thermally oxidized Si wafers. This enabled us to successfully fabricate HfO₂/Ti ReRAM cells with graphene bottom electrode. Based on the growth process developed for HfO₂ thin films, we deposited HfO₂ nanoislands on HOPG. The electronic and structural properties of these well separated, crystalline HfO₂ nanoislands are investigated by scanning probe methods. The topography reveals homogeneously formed HfO₂ nanoislands with areas down to 7 nm² and a thickness of one unit cell. They exhibit several in-gap states in addition to the bulk band gap, implying bulk properties of these ultra-scaled memristive objects. Nanocrystals with a diameter of 2.7−4.5 Å are identified next to carbon vacancies in the topmost HOPG layer, indicating that carbon is incorporated into the islands at early nucleation stages. The comparison of the theoretically determined lowest-energy clusters and electronic states with the experimental results allows us to identify the structure of the most relevant HfO₂ sub-nanometer crystals formed during the first nucleation steps and to assign the in-gap states to the observed carbon incorporation during heterogeneous integration [1].<br/>[1] N. Schmidt, K. Z. Rushchanskii, U.Trstenjak, R. Dittmann, and S. Karthäuser, ACS Appl. Nano Mater. (2022)