Think positive: the key to transparent electronics

A new study led by RMIT University could pave the way to revolutionary, transparent electronics — see-through devices that could potentially be integrated in glass, in flexible displays and in smart contact lenses, bringing to life futuristic devices that seem like the product of science fiction. Their work has been published in the journal Nature Electronics.

For several decades, researchers have sought a new class of electronics based on semiconducting oxides, whose optical transparency could enable fully transparent electronics. Oxide-based devices could also find use in power electronics and communication technology, reducing the carbon footprint of our utility networks. RMIT researchers and their collaborators have now introduced ultrathin beta-tellurite to the two-dimensional (2D) semiconducting material family, providing a solution to this decades-long search for a high-mobility, p-type oxide.

“This new, high-mobility, p-type oxide fills a crucial gap in the materials spectrum to enable fast, transparent circuits,” said team leader Dr Torben Daeneke, who led the collaboration across three nodes of the ARC Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET).

Other key advantages of the long-sought-after oxide-based semiconductors are their stability in air, less-stringent purity requirements, low costs and easy deposition. “In our advance,” said Dr Daeneke, “the missing link was finding the right ‘positive’ approach.”

Positivity has been lacking

There are two types of semiconducting materials: ‘n-type’ materials have abundant negatively charged electrons, while ‘p-type’ semiconductors possess plenty of positively charged holes. It’s the stacking together of complementary n-type and p-type materials that enables the creation of electronic devices such as diodes, rectifiers and logic circuits — the materials that serve as the building blocks of every computer and smartphone.

A barrier to oxide devices has been that while many high-performance n-type oxides are known, there is a significant lack of high-quality p-type oxides. However, in 2018 a computational study revealed that beta-tellurite (β-TeO₂) could be an attractive p-type oxide candidate, with tellurium’s peculiar place in the periodic table meaning it can behave as both a metal and a non-metal, providing its oxide with uniquely useful properties.

“This prediction encouraged our group at RMIT University to explore its properties and applications,” said Dr Daeneke, who is a FLEET associate investigator.

Liquid metal

Dr Daeneke’s team demonstrated the isolation of beta-tellurite with a specifically developed synthesis technique that relies on liquid metal chemistry. As explained by co-first author Patjaree Aukarasereenont, a FLEET PhD student at RMIT, “A molten mixture of tellurium (Te) and selenium (Se) is prepared and allowed to roll over a surface.

“Thanks to the oxygen in ambient air, the molten droplet naturally forms a thin surface oxide layer of beta-tellurite. As the liquid droplet is rolled over the surface, this oxide layer sticks to it, depositing atomically thin oxide sheets in its way.

“The process is similar to drawing: you use a glass rod as a pen and the liquid metal is your ink.”

While the desirable β-phase of tellurite grows below 300°C, pure tellurium has a high melting point, above 500°C. Therefore, selenium was added to design an alloy that has a lower melting point, making the synthesis possible.

“The ultrathin sheets we obtained are just 1.5 nm thick — corresponding to only few atoms,” said co-author Dr Ali Zavabeti. “The material was highly transparent across the visible spectrum, having a bandgap of 3.7 eV, which means that they are essentially invisible to the human eye.”

Dr Ali Zavabeti, Patjaree Aukarasereenont and Dr Torben Daeneke with transparent electronics.

Up to 100 times faster

To assess the electronic properties of the developed materials, field-effect transistors (FETs) were fabricated. According to Aukarasereenont, “These devices showed characteristic p-type switching as well as a high hole mobility (roughly 140 cm²V^-1s^-1), showing that beta-tellurite is 10 to 100 times faster than existing p-type oxide semiconductors. The excellent on/off ratio (over 10⁶) also attests the material is suitable for power-efficient, fast devices.”

Dr Zavabeti added, “The findings close a crucial gap in the electronic material library.

“Having a fast, transparent p-type semiconductor at our disposal has the potential to revolutionise transparent electronics, while also enabling better displays and improved energy-efficient devices.”

The team plans to further explore the potential of this novel semiconductor. According to Dr Daeneke, “Our further investigations of this exciting material will explore integration in existing and next-generation consumer electronics.”

Top image credit: RMIT University.

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