The global demand for faster, smaller and cheaper electronic devices is unrelenting. Limitations on existing semiconductor fabrication materials and processes demands that considerable effort is spent identifying new materials, characterising their properties and creating new processes to aid their transformation into devices.
Graphene and hexagonal boron nitride (hBN) are two such materials of interest, especially in combination as a conductor and dielectric, respectively. Recent research by Professor Peter Beton and his team at the University of Nottingham has focused on the formation of graphene nanoribbons (GNRs) along hBN trenches to form nanoscale circuitry.
The method for growing GNRs on hBN required nickel nanoparticles to be deposited on hBN flakes by thermal evaporation, which were subsequently annealed in a mixture of hydrogen and argon. This caused the nanoparticles to migrate along the crystal’s zigzag lattice directions and etch oriented trenches in the top layer of the hBN flake. After removing the nanoparticles with a wet chemical etch a high-temperature molecular beam epitaxy (HT-MBE) process was used to grow the GNRs, with the hBN trenches acting as a blueprint for their growth. Atomic force microscopy was used extensively to characterise the samples at each stage of preparation and growth.
Using NuNano’s silicon AFM probes (Scout 70 RAI) with an Asylum Research Cypher S instrument the Nottingham team were able to study the early growth stages of the GNRs. They were able to show that at high growth temperatures the GNR growth rate affected whether the templating hBN step is terminated with boron (B) or nitrogen (N) atoms.
Professor Peter Beton says: “My group has been using NuNano’s Scout 70 RAl AFM probes in combination with our Asylum Research Cypher AFM for the last 7 years. This has been a highly reliable combination, allowing us to readily achieve lattice resolution on hexagonal boron nitride and other 2D materials that we work with”
In Figure 1, you can see a close-up view of these etch tracks after the removal of the Ni particles using a wet chemical process. The tracks include both very shallow ones (0.36 nm, which is roughly one atomic layer) and deeper ones (1.1 nm, approximately three atomic layers). Most of the tracks are only one atomic layer deep. In figure 2 the lattice of the hBN flake is resolved with sub-nanometre imaging resolution.
Figure 1
Figure 2
Using NuNano’s conductive AFM probes (Spark 70 Pt) to undertake conductive AFM (cAFM) Nottingham were able to study the electrical performance of the fully grown GNRs. They observed the creation of a conductive network within the hBN surface and noticed signs of electrons moving between different sublattices at the interface between graphene and hBN, a phenomenon known as intervalley scattering.
Research Fellow, Dr Jonathan Bradford says: ”[Using] SPARK probes for conductive AFM [we have achieved] lattice resolution in cAFM mode, despite some challenging sample conditions”
Conclusions
Atomic force microscopy has enabled Professor Beton’s group to study the topological and electrical properties of the graphene nanoribbons at the nanoscale and enhanced their understanding not only of how the hBN and graphene interface is formed but also how this material holds great potential for use in extremely thin electronic devices and circuits.
Read the full paper here:
Graphene nanoribbons with hBN passivated edges grown by high-temperature molecular beam epitaxy, J. Bradford, T. S. Cheng, T. S. S. James, A. N. Khlobystov, C. J. Mellor, K. Watanabe, T. Taniguchi, S. V. Novikov & P. H. Beton (2023) 2D Materials, 10 035035 DOI 10.1088/2053-1583/acdefc
Related publications:
Epitaxy of boron nitride monolayers for graphene-based lateral heterostructures J. Wrigley, J. Bradford, T. James, T. S Cheng, J. Thomas, C. J. Mellor, A. N. Khlobystov, L. Eaves, C. T. Foxon & S. V. Novikov (2021) 2D Materials, 8, 3 DOI 10.1088/2053-1583/abea66
Step-flow growth of graphene-boron nitride lateral heterostructures by molecular beam epitaxy J. Bradford, T. S. Cheng, T. S. S. James, A. N. Khlobystov, C. J. Mellor, K. Watanabe, T. Taniguchi, S. V. Novikov & P. H. Beton (2023) 2D Materials, 10 035035 DOI 10.1088/2053-1583/ab89e7