When molybdenum disulfide (MoS2) entered the conversation related to two-dimensional (2-D) alternatives to graphene in electronic applications, some thought that MoS2 had an edge as a transistor material. That thought was inspired by the material's intrinsic rather than engineered band gap, unlike graphene.
However, as researchers learned more about the material, clouds began to appear in the bright and sunny picture of MoS2. Among the drawbacks are less-than-ideal electron mobility and sub-threshold slope.
In collaborative research between IBM's Thomas J. Watson Research Center and Yale University, the culprit behind MoS2’s underwhelming electronic properties has been revealed. The issue turns out to be traps, an issue with which people who study semiconductors are painfully familiar. Traps are impurities or dislocations that can trap an electron or hole and hold it until a pair is completed. It took decades of research to reduce the traps' density at the dielectric-semiconductor interface to a level that allows for high performance silicon transistors.
The research, which was published in the journal Nature Communications (“Electronic transport and device prospects of monolayer molybdenum disulphide grown by chemical vapour deposition”), systematically quantifies the density-of-states and response time of band tail trapping states.
“Our work presents the first systematic understanding of the traps states in 2-D semiconductors based on transition metal dichalcogenides,” explains Tony Low, a researcher at IBM’s Nanometer Scale Science & Technology group and one of the authors of the report. “This work lays the groundwork for future engineering effort in eliminating these traps. Such concerted engineering effort is needed in order for us to harness the real potential of this new material for electronics and photonics.”
The researchers revealed that one limitation on MoS2's electron mobility was the fact that there were actually two types of charge carriers.
“Initially, we found a very low mobility,” said Fengnian Xia, a member of the research group who is an assistant professor in Yale’s electrical engineering department, in a press release. “But after careful analysis, we noticed...some carriers are trapped within the band gap, so these carriers are not really mobile. But the other carriers are in the band, where they exhibit much higher mobility.”
One of the critical features of this research is that they did not use MoS2 produced through mechanical cleavage—the so-called “Scotch-tape” method—but through chemical vapor deposition (CVD), which better lends itself to bulk production.
The researchers say that the trapped carriers are not intrinsic to MoS2 and can be addressed by improving the quality of the material.
Image: Ben Mills/Wikipedia; iStockphoto
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.
