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Ingeniously Two-Dimensional

Efficiently doping the semiconductors of tomorrow
Electronics of the future are inconceivable without two-dimensional materials. They are the great hope for high-performance and energy-efficient electronic components. At the same time, however, the unique properties of two-dimensional materials make it difficult to dope them with foreign atoms. This step is necessary to precisely adjust the electrical conductivity and to transform the material into a p- or n-type semiconductor. A team led by Dr. Slawomir Prucnal from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) with the help of Dr. Arkady Krasheninnikov’s research group at HZDR and Prof. Dietrich Zahn from the TU Chemnitz have now achieved a breakthrough, as the researchers in the scientific journal Nanoscale (DOI: 10.1039/D0NR08935D) report. The cover layer they use makes it possible to process the new materials with methods already established for conventional semiconductors. This is a huge step on the path from the laboratory to industrial manufacturing.

Conventional semiconductors, such as those in today’s home computers, mobile telephones and industrial computers are based on silicon. The technology behind them is thoroughly researched and the production processes are optimized. However, in order to satisfy the increasingly power-hungry modern electronics, silicon is reaching its limits. Alternatives must be found. Such alternatives lie in the form of ultra-thin, two-dimensional materials. The most well known is likely graphene. There are now so many more: for example, from the material class of what are known as transitional metal dichalcogenides. They all have one thing in common in that they are only a few atomic layers thick. In the case of the molybdenum diselenide studied by Slawomir Prucnal, for example, there are exactly three, which together are a hundred thousand times thinner than a human hair.

“In order for materials such as silicon or the new two-dimensional materials to function as electronic components, we need to change their internal properties,” explains Prucnal, who works in HZDR’s Department of Semiconductor Materials at the Institute of Ion Beam Physics and Materials Research. “To do so, we introduce some foreign atoms into the regular crystal structure. This is what we call doping.” In industrial processing of silicon chips, this is done by implanting ions. It is carried out by directing a beam of charged particles onto the material, some of which are built into the crystal lattice. “This is difficult with two-dimensional materials,” says Prucnal. “That is because the implanted ions must stop precisely in the few atomic layers that make up the material.”

That is why he and his team at the HZDR have provided the new materials with a special cover layer. The structure of this layer enables the scientists to control the doping process with the utmost precision. “Using ion implantation, we can now precisely incorporate the foreign atoms into the crystal lattice of the two-dimensional material so that they are electrically active there,” says Prucnal. This has a decisive advantage: thanks to the cover layer, the new materials can be processed with the same production equipment used for today’s silicon chips.

After Slawomir Prucnal and his research group demonstrated that their concept works with molybdenum diselenide—a material from the transition metal dichalcogenide class—and through doping with chlorine ions, they are currently testing other two-dimensional materials. In the next step, they want to use their method to produce a fully functional electronic circuit.”

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