Oxidation and oxidative vapor-phase etching of few-layer MoS2
Timothy N. Walter, Frances Kwok, Hamed Simchi, Haila M. Aldosari, Suzanne E. Mohney
Journal of Vacuum Science and Technology B, Vol. 35, No. 1 (January-February 2017)

Molybdenum disulfide, or MoS2, is becoming increasingly popular for researchers seeking better materials for fabricating certain electronic devices like semiconductor, nanoelectronic, flexible electronic and optoelectronic devices. Until recently, however, little was known about how to control oxidation of MoS2 for device fabrication. In a new paper in the Journal of Vacuum Science and Technology B, researchers at Penn State University used three different microscopies to investigate the chemical and morphological impacts of oxidizing and inert environments on mechanically exfoliated, "few-layer" sheets of MoS2.
 
MoS2 is a member of a group of compounds known as transition metal dichalcogenides, or TMDs. These compounds combine a transition metal such as molybdenum, tungsten or tin with one of three chalcogenide elements: sulfur, selenium or tellurium. The atoms arrange themselves into extremely thin layers, with the transition metal sandwiched between two chalcogenide sections ─ a structure that can yield some very desirable properties such as optical transparency and photon (light) emission. This makes TMD semiconductors, especially those fabricated from MoS2, solid candidates for the next generation of inexpensive and low-power electronic circuits, flexible and transparent solar cells and displays, and wearable computing devices.
 
"What makes MoS2 so popular is that the layers can be easily peeled apart, like sticky notes stacked on top of each other," said Timothy Walter, a graduate student in Suzanne Mohney's materials science and engineering group at Penn State and lead author on the JVSTB paper. "We mechanically exfoliated MoS2 into its few-layer form by peeling apart the layers with tape, the same method that was used to create the first graphene monolayers."
 
Halogen-based plasmas have traditionally been used to rapidly etch silicon-based devices, but Walter said that they don't work well with few-layer MoS2 because rapid etching is hard to control with an extremely thin structure, and halogens may damage or contaminate the material left over after the etch. "It would be like placing candles onto a birthday cake with a jackhammer," Walter said.
 
As an alternative, the researchers turned to oxidative vapor-phase plasma etching.
 
The Penn State researchers studied their few-layer MoS2 in five environments: oxygen, oxygen and water vapor, argon, argon and water vapor, and ultraviolet-generated ozone.
 
"Our experiments indicate that not only can we oxidize MoS2, we can use oxidation to etch it locally or globally, can do so at a fast or slow rate, and with appreciable control over the shape of the resulting oxide or etched pattern," Walter said. "Each of these outcomes depends on the temperature, the additional gases included with oxygen, and the length of time we conduct the oxidizing process."
 
Walter said that the research team will next use its "oxidation recipe" for fabrication to create TMD devices ─ particularly transistors ─ and then test their performance against existing ones.
 
"We expect that oxidation could help change conduction in TMD devices or make them more efficient, while oxidative etching could be a better way to make patterns in our material," he said.