Researchers at the Würzburg-Dresden Cluster of Excellence ct.qmat, along with additional collaborators, have developed a graphene-based protective film that shields quantum semiconductor layers just one atom thick from environmental influences without compromising their quantum properties. This could advance the use of these delicate atomic layers in ultrathin electronic components.
A few years ago, scientists from the Cluster of Excellence ct.qmat discovered that topological quantum materials such as indenene hold great promise for ultrafast, energy-efficient electronics. These extremely thin quantum semiconductors are composed of a single atom layer – in indenene’s case, indium atoms – and act as topological insulators, conducting electricity virtually without resistance along their edges. Experimental physicist Professor Ralph Claessen explained that producing such a single atomic layer requires sophisticated vacuum equipment and a specific substrate material. To utilize this two-dimensional material in electronic components, it would need to be removed from the vacuum environment. However, exposure to air, even briefly, leads to oxidation, destroying its revolutionary properties and rendering it useless.
“We dedicated two years to finding a method to protect the sensitive indenene layer from environmental elements using a protective coating. The challenge was ensuring that this coating did not interact with the indenene layer,” explains Cedric Schmitt, one of Claessen’s doctoral students involved in the project.
This interaction is problematic because when different types of atoms – from the protective layer and the semiconductor, for instance – meet, they react chemically at the atomic level, changing the material. This isn’t a problem with conventional silicon chips, which comprise multiple atomic layers, leaving sufficient layers unaffected and hence still functional.
“A semiconductor material consisting of a single atomic layer such as indenene would normally be compromised by a protective film. This posed a seemingly insurmountable challenge that piqued our research curiosity,” says Claessen.
The search for a viable protective layer led them to explore van der Waals materials, characterized by strong internal bonds between their atoms, while only weakly bonding to the substrate. Using sophisticated ultrahigh vacuum equipment, the team experimented with heating silicon carbide (SiC) as a substrate for indenene, exploring the conditions needed to form graphene from it.
“Silicon carbide consists of silicon and carbon atoms. Heating it causes the carbon atoms to detach from the surface and form graphene,” says Schmitt, elucidating the laboratory process. “We then vapor-deposited indium atoms, which are immersed between the protective graphene layer and the silicon carbide substrate. This is how the protective layer for our two-dimensional quantum material indenene was formed.”
For the first time, Claessen and his team at ct.qmat’s Würzburg branch successfully crafted a functional protective layer for a two-dimensional quantum semiconductor material without compromising its extraordinary quantum properties. After analyzing the fabrication process, they thoroughly tested the layer’s protective capabilities against oxidation and corrosion.
“It works! The sample can even be exposed to water without being affected in any way,” says Claessen with delight. “The graphene layer acts like an umbrella for our indenene.”
This breakthrough paves the way for applications involving highly sensitive semiconductor atomic layers. The manufacture of ultrathin electronic components requires them to be processed in air or other chemical environments. This has been made possible thanks to the discovery of this protective mechanism.
The team in Würzburg is now focused on identifying more van der Waals materials that can serve as protective layers – and they already have a few prospects in mind. The snag is that despite graphene’s effective protection of atomic monolayers against environmental factors, its electrical conductivity poses a risk of short circuits. The scientists will be working on overcoming these challenges and creating the conditions for tomorrow’s atomic layer electronics.
