US team takes 2D semiconductor processing to the next level

ORNL researchers make scalable arrays of semiconductor heterojunctions for ultrathin electronics 

Arrays of semiconductor heterojunctions formed within 2D MoSe₂ by converting lithographically exposed regions to MoS₂ using pulsed laser deposition of sulphur atoms.

Researchers at the US Department of Energy's Oak Ridge National Laboratory (ORNL) have combined a novel synthesis process with commercial electron-beam lithography techniques to produce arrays of 2D semiconductor junctions in arbitrary patterns within a single, nanometer-thick semiconductor crystal.

The process relies upon transforming patterned regions of one existing, single-layer crystal into another. The researchers first grew single, nanometer-thick layers of MoSe₂ crystals on substrates and then deposited protective patterns of silicon oxide using standard lithography techniques. Then they bombarded the exposed regions of the crystals with a laser-generated beam of sulphur atoms.

The sulphur atoms replaced the selenium atoms in the crystals to form MoS₂, which has a nearly identical crystal structure. The two semiconductor crystals formed sharp junctions, the desired building blocks of electronics. The work was reported in Nature Communications.

"We can literally make any kind of pattern that we want," said Masoud Mahjouri-Samani, who co-led the study with David Geohegan. Geohegan, head of ORNL's Nanomaterials Synthesis and Functional Assembly Group at the Center for Nanophase Materials Sciences, is the principal investigator of a Department of Energy basic science project focusing on the growth mechanisms and controlled synthesis of nanomaterials.

Millions of 2D building blocks with numerous patterns may be made concurrently, Mahjouri-Samani added. In the future, it might be possible to produce different patterns on the top and bottom of a sheet. Further complexity could be introduced by layering sheets with different patterns.

Added Geohegan: "The development of a scalable, easily implemented process to lithographically pattern and easily form lateral semiconducting heterojunctions within two-dimensional crystals fulfills a critical need for 'building blocks' to enable next-generation ultrathin devices for applications ranging from flexible consumer electronics to solar energy."

Tuning the bandgap

"We chose pulsed laser deposition of sulphur because of the digital control it gives you over the flux of the material that comes to the surface," said Mahjouri-Samani. "You can basically make any kind of intermediate alloy. You can just replace, say, 20 percent of the selenium with sulphur, or 30 percent, or 50 percent." Added Geohegan, "Pulsed laser deposition also lets the kinetic energy of the sulphur atoms be tuned, allowing you to explore a wider range of processing conditions."

It is important that by controlling the ratio of sulphur to selenium within the crystal, the researchers can tune the bandgap of the semiconductors, an attribute that determines electronic and optical properties. To make optoelectronic devices such as electroluminescent displays, microchip fabricators integrate semiconductors with different bandgaps. For example, MoS₂'s bandgap is greater than that of MoSe₂. 

Applying voltage to a crystal containing both semiconductors causes electrons and "holes" (positive charges created when electrons vacate) to move from MoS₂ into MoSe₂ and recombine to emit light at the bandgap of molybdenum diselenide. For that reason, engineering the bandgaps of monolayer systems can allow the generation of light with many different colors, as well as enable other applications such as transistors and sensors, Mahjouri-Samani said.

Next the researchers will see if their pulsed laser vapourisation and conversion method will work with atoms other than sulphur and selenium. "We're trying to make more complex systems in a 2D plane - integrate more ingredients, put in different building blocks--because at the end of the day, a complete working device needs different semiconductors and metals and insulators," Mahjouri-Samani said.

To understand the process of converting one nanometer-thick crystal into another, the researchers used powerful electron microscopy capabilities available at ORNL, notably atomic-resolution Z-contrast scanning transmission electron microscopy, which was developed at the lab and is now available to scientists worldwide using the Center for Nanophase Materials Sciences. 

Employing this technique, electron microscopists Andrew Lupini and visiting scientist Leonardo Basile imaged hexagonal networks of individual columns of atoms in the nanometer-thick MoSe₂ and MoS₂ crystals.

"We could directly distinguish between sulphur and selenium atoms by their intensities in the image," Lupini said. "These images and electron energy loss spectroscopy allowed the team to characterise the semiconductor heterojunction with atomic precision."

'Patterned Arrays of Lateral Heterojunctions within Monolayer Two-Dimensional Semiconductors' by Mahjouri-Samani et al;  Nature Communications