Electron beam-induced borazine deposition yields intriguing insights for fabrication of miniaturized electronics

Researchers used an electron beam derived from MAX IV’s electron gun at the MAXPEEM beamline to break borazine molecules into fragments thus enabling the synthesis of amorphous boron nitride onto an otherwise inert graphene film. The results of the experiment indicate that electron beam-induced deposition of borazine actually impacts its boron nitrogen bonds. This effect had previously never been observed and could entail important insights for scientists aiming to manufacture miniature electronics by directly printing isolators onto 2D materials.

Two-dimensional materials which form single thin nano-sized sheets of atoms have emerged as a promising class of materials for the production of miniaturized electronics such as semiconductors. Building electronic materials based on such 2D materials requires the inclusion of isolator and separator parts or dielectric spacers.

Yet, this process poses a challenge since 2D materials form very strong electron (covalent) bonds and therefore are mostly chemically inactive structures. This makes it very difficult to successfully trigger direct chemical reactions in which they actually combine with other substances to form more complex, usable products. What’s more, building highly ordered defect-free 2D sheets requires very high temperatures which many components of miniaturized electronics cannot withstand.

This is where MAX IV’s electron gun came into play. Researchers at Lund University and MAX IV used the electron beam at the MAXPEEM beamline to break molecular borazine into unstable fragments. By directing the electron beam towards a 2D graphene film while exposing it to borazine gas, the scientists generated boron nitride radicals on the surface that bound to the film thus enabling the synthesis of amorphous boron nitride onto the film.

Using a multitude of experimental techniques at other synchrotron facilities – ASTRID2 and Elletra – in combination with Scanning Tunnelling Microscopy the scientist studied the shape and structure, chemical composition, and the temperature evolution of the resulting amorphous boron nitride/graphene heterostructure.

They found that following the deposition and fragmentation of borazine, an intact, amorphous boron nitride multilayer formed that was patterned onto the graphene which itself remained intact in structure.

The newly obtained heterostructure’s stability could be retained even in very high temperatures (the scientists tested up to 1,400 K), meaning it was very stable, according to Virginia Boix, a PhD student at Lund University’s Synchrotron Radiation Research Division who led the research.

“At such high temperatures we could have observed decomposition or desorption of a large part of the structure, but that was not the case,” said Boix. “For possible applications in electronics, it’s good because it means that … one doesn’t have to worry about that specific component if the device needs to undergo any temperature treatments,” said Boix.

At the same time, the experiments at MAXPEEM proved that the chemical reaction can be spatially controllable within about 50nm which is essential for the creation of nano-sized devices.

These findings suggest that electron beam-induced deposition of borazine could serve as a promising tool in the manufacturing of miniature electronics as it enables scientists to directly print dielectric structures onto 2D materials.

“The MAXPEEM Beamline is the “all-in-one” experiment, giving us structural and chemical information simultaneously and – more importantly – with spatial resolution,” said Boix. “Using the focused electron beam we could not only characterize our sample but also test that the electron-induced deposition was limited to the area exposed to the beam.”

What’s more, the study draws attention to amorphous boron nitride, an otherwise somewhat “overlooked” substance. Its counterpart, hexagonal boron nitride, a 2D material that is isostructural to graphene and commonly derived from borazine, has sparked a lot of interest in the development of 2D-based electronics. However, it is very difficult to manufacture. Amorphous boron nitride on the other hand exhibits properties very similar to its 2D counterpart but with the critical advantage that it is much easier to synthetize.

Combining amorphous boron nitride with graphene, a 2D material with particularly great potential as a building block due to its high carrier mobility – meaning electrons can pass through it and fill electron holes particularly quickly -, scientists can now create the first building blocks for 2D-based devices. Moreover, the electron beam approach could “likely be employed more broadly with other substances and surface materials,” according to the researchers’ report.

“Our studies were done on iridium as it’s a substrate that allows the synthesis of high-quality graphene and withstands high temperatures without melting… Copper or nickel are more common substrates also for 2D research and, as we saw no relevant effects from the Iridium in the electron-induced deposition of boron-nitrogen, we expect similar results to be obtained with those substrates,” said Boix.

 

Publication: 

Boix V., Struzzi C., Gallo T., et al., Area-selective Electron-beam induced deposition of Amorphous-BNx on graphene, Applied Surface Science 557, 149806 (2021), https://doi.org/10.1016/j.apsusc.2021.149806