Single-layer MoS2 or molybdenite (which is made of molybdenum and sulphur) is a semiconductor with a direct bandgap (of 1.8 eV) that might be better than indirect bandgap silicon for making certain photonic devices. It could even rival "wonder material" graphene (which does not have a bandgap at all in its pristine state) in future electronic circuits. A direct bandgap is important for making devices like light-emitting diodes, solar cells and photodetectors, and any other photonic device that exploits electron-hole pair excitation, because devices made with direct rather than indirect gap semiconductors are more efficient.

The material also has good charge mobilities of greater than 100 cm2/Vs – and perhaps even up to 500 cm2/Vs. These values compare well to state-of-the-art silicon. And that is not all: being a van der Waals solid (made up of 2D sheets that are weakly bonded to each other), it is compatible with a variety of substrates – even transparent or plastic ones. Finally, single-layer molybdenite is only about 0.65 nm thick, which means that devices made from it can suppress so-called short-channel effects, so allowing for very thin transistors.

Now, a team of researchers at IBM’s TJ Watson Research Center in Yorktown Heights are saying that it may not be all plain sailing for MoS2. The fact that the material contains large numbers of band tail states means that sample quality still needs to be improved if truly efficient devices based on this semiconductor are to see the light of day, says team leader Phaedon Avouris.

The scientists obtained their results using a battery of techniques to characterize the physical and electronic properties of MoS2 grown by chemical vapour deposition. First, they measured the AC conductance and the capacitance of MoS2 at different frequencies to determine the density-of-state and dynamics of localized electronics states in the material. They then looked at how the channel mobility of MoS2 transistors varies with temperature. Finally, they measured the so-called drain-induced-barrier lowering and the lateral breakdown field of the CVD MoS2 devices.

Understanding trap states in 2D semiconductors

“These new analyses represent the first, systematic understanding of trap states in 2D semiconductors based on transition metal dichalcogenides,” team member Wenjuan Zhu told “We often overlook the fact, but it took the semiconductor industry decades to reduce trap densities at dielectric-semiconductor interfaces to a level that allowed high-performance silicon transistors to be made. Our research now lays the groundwork for future engineering efforts to eliminate charge traps in 2D dichalcogenides so that we might harness the true potential of these promising materials in electronics and photonics applications.”

That said, current state-of-the-art wafer-scale monolayer Mo2 is already a potential alternative to organic and other thin-film materials for flexible electronics and photonics devices, including high-resolution displays, photodetectors, logic electronics and solar cells, she added.

The team, which also includes researchers at National Tsing Hua University, Yale University and the Masachussetts Institute of Technology, reports its work in Nature Communications doi:10.1038/ncomms4087.

Further reading

First light for MoS2 (Apr 2013)
MoS2 in new mobility record (Mar 2013)
MoS2 transistors get bendy (May 2013)