Manufacturing Bits: Jan. 14

MoS2 FETs
Two-dimensional materials are gaining steam in the R&D labs. The 2D materials include graphene, boron nitride (BN) and the transition-metal dichalcogenides (TMDs). One TMD, molybdenum diselenide (MoS2), is an attractive material for use in future field-effect transistors (FETs).

MoS2 has several properties, including a non-zero band gap, atomic scale thickness and pristine interfaces. A so-called few-layer (FL) FET based on MoS2 has good electrostatics, but the technology also has questionable mobilities.

The University of California at Santa Barbara and the University of Notre Dame have devised a high-performance FL-MoS2 FET with good mobilities. The device has a contact resistance of about 0.8 kΩ.μm, which is close to the value of CMOS-based metal-silicon contacts, according to researchers. The top-gated FL-MoS2 is a 5nm structure with a high drive current of 24 μA/μm without source/drain doping.

For a few-layer MoS2 (5L and 8L), the mobilities are about 55 cm2/Vs, which is nearly four times higher than previous efforts. A 1L MoS2 has a direct band gap of 1.8 eV, according to researchers. Bilayer and bulk (> ~5 L) have an indirect band gap of 1.5 eV and 1.2 eV, respectively. “It clearly shows that bulk MoS2 has higher DOS than that of 1L MoS2,’’ according to researchers. “Few-layer (5L-15L) MoS2 FETs show better potential for high-performance digital circuits due to their small contact resistances and high mobilities.”

Peaks And Valleytronics
SLAC, Stanford University and Berkeley Lab have demonstrated the ability to grow sheets of material based on MoS2 technology. With the technology, MoS2 could propel the development of a new class of thin-film and light-based electronics. It could also evolve into a new technology called valleytronics.

Valleytronics involves channeling the charge carriers into valleys. One of the emerging technologies in the industry, valleytronics is also a way to utilize the quantum number of an electron to encode information. In this application, the material used is diamond.

Meanwhile, to make the MoS2 sheets, researchers heated molybdenum and selenium in a vacuum chamber at Lawrence Berkeley National Laboratory’s Advanced Light Source (ALS). The ALS is a source of ultraviolet and soft x-ray light based a synchrotron technology.

This diagram shows a single layer of MoSe2 thin film (green and yellow balls) grown on a layer of graphene (black balls) that has formed on the surface of a silicon carbide substrate. Scientists who made the material and measured details of its electronic structure discovered it’s a natural fit for making thin, flexible light-based electronics. (Yi Zhang/Stanford Institute for Materials and Energy Sciences and Advanced Light Source, Berkeley Lab)

The two elements are combined in the ALS. Then, by using molecular beam epitaxy, the materials enable films that are one to eight atomic layers thick. Researchers also reported the first direct observation of indirect to direct band gap in monolayer MoS2 samples by using angle-resolved photoemission spectroscopy. The discovery of a ~180 meV at the valence band could enable a new class of spintronic devices.

“Based on tests at the ALS and at Stanford, now we can say MoS2 has possible applications in photoelectronic devices, such as light detectors and solar cells,” said Yi Zhang, a postdoctoral researcher, on SLAC’s Web site.

Yongtao Cui, a Stanford postdoctoral researcher, added: “This field is still in the initial stage of development. People have these applications in mind, but as research goes along they may discover new aspects of these materials, and possibly new applications.”

Mixing 2D Materials
The Department of Energy’s Oak Ridge National Laboratory and the University of Tennessee put a new twist on 2D materials. Researchers combined two compounds—graphene and boron nitride—into a single layer at only one atom thick.

Researchers used epitaxy to combine the materials into heterostructures. Initially, researchers grew graphene on a copper foil. Then, they etched the graphene to create clean edges. Next, researchers grew boron nitride using CVD. All told, the boron nitride took on the crystallography of the graphene.

“The graphene piece acted as a seed for the epitaxial growth in two-dimensional space, so that the crystallography of the boron nitride is solely determined by the graphene,” said Gong Gu, a researcher at the University of Tennessee.

“People call graphene a wonder material that could revolutionize the landscape of nanotechnology and electronics. Indeed, graphene has a lot of potential, but it has limits. To make use of graphene in applications or devices, we need to integrate graphene with other materials,” said An-Ping Li of Oak Ridge.

“If we want to harness graphene in an application, we have to make use of the interface properties, since as Nobel laureate Herbert Kroemer once said ‘the interface is the device,’” Li said on Oak Ridge’s Web site. “By creating this clean, coherent, 1-D interface, our technique provides us with the opportunity to fabricate graphene-based devices for real applications.”