A test wafer with integrated 2D materials. Photo: Arne Quellmalz.
A test wafer with integrated 2D materials. Photo: Arne Quellmalz.

As shrinking semiconductors ever further becomes more and more challenging, the next best hope is to combine silicon semiconductors with 2D atomically thin materials such as graphene, to create circuits on an incredibly small scale. Researchers have now developed a new technique for combining these materials, which could work at an industrial scale.

The researchers from KTH Royal Institute of Technology in Stockholm, Sweden, in collaboration with colleagues at RWTH Aachen University, Universität der Bundeswehr München, AMO GmbH and Protemics GmbH in Germany, report this technique in a paper in Nature Communications.

A reliable, industrially scalable method for integrating 2D materials such as graphene with silicon semiconductors would help downscale electronics, and usher in new capabilities for sensor technology and photonics. However, the integration of 2D materials with silicon semiconductors, or a substrate with integrated electronics, is fraught with a number of challenges.

“There’s always this critical step of transferring from a special growth substrate to the final substrate on which you build sensors or components,” explains Arne Quellmalz, a researcher in photonic microsystems at KTH.

“You might want to combine a graphene photodetector for optical on-chip communication with silicon read-out electronics. But the growth temperatures of those materials is too high, so you cannot do this directly on the device substrate.”

Experimental methods for transferring grown 2D materials to desired electronics have been beset by a number of deficiencies, such as degradation of the material and its electronic transport properties or contamination of the material.

Quellmalz says the solution lies in the existing toolkit of semiconductor manufacturing, through the use a standard dielectric material called bisbenzocyclobutene (BCB), along with conventional wafer bonding equipment. “We basically glue the two wafers together with a resin made of BCB,” he says. “We heat the resin, until it becomes viscous like honey, and press the 2D material against it.”

At room temperature, the resin becomes solid and forms a stable connection between the 2D material and the wafer. “To stack materials, we repeat the steps of heating and pressing. The resin becomes viscous again and behaves like a cushion, or a waterbed, which supports the layer stack and adapts to the surface of the new 2D material.”

The researchers demonstrated the transfer of graphene and molybdenum disulfide (MoS2), as a representative of the class of 2D materials known as transition metal dichalcogenides, by stacking graphene with hexagonal boron nitride (hBN) and MoS2 to form heterostructures. All transferred layers and heterostructures were of high quality, with uniform coverage over 100mm-sized silicon wafers, and exhibited little strain in the transferred 2D materials.

This story is adapted from material from KTH Royal Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.