Researchers from the Royal Melbourne Institute of Technology (RMIT) have introduced an ultra-thin material for semiconductors that could lead to transparent electronics.

A molten mixture of tellurium and selenium, rolled over a surface, results in the deposit of an atomically-thin sheet of beta-tellurite. As a rare "p-type" semiconductor material, the beta-tellurite transports positively charged constructs known as holes.

“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 .

What is a P-Type Semiconductor?

In a simple wire, positive and negative charges flow in opposing directions from the two terminals in the plug. The negative charge is simply an excess electron, one of the fundamental particles.

The positively charged construct, by contrast, is defined by the lack of an electron. This net positive charge, known as a hole, can also move. Electrons effectively hop into the hole, moving the positive charge along.

N-type, or negative, semiconductors have more electrons and can usually transport signals faster than p-type semiconductors; moving an extra electron along, after all, is easier than facilitating a hopping mechanism.

The RMIT team's new p-type semiconductor is ten to one hundred times faster than existing p-type oxide semiconductors., according to the project's team leader.

“In our advance, the missing link was finding the right, ‘positive’ approach,” said Daeneke.

How to Make the Ultra-Thin Material

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

In a way, it's like art.

“The process is similar to drawing: you use a glass rod as a pen and the liquid metal is your ink,” said Patjaree Aukarasereenont, a PhD student at RMIT.

The resulting ultrathin sheets measured just 1.5 nanometers thick – just a few atoms.

"The material was highly transparent across the visible spectrum, having a bandgap of 3.7 eV [electronvolts], which means that they are essentially invisible to the human eye,” said co-author Dr Ali Zavabeti.

The paper High mobility p-type semiconducting two-dimensional β-TeO2  was published in Nature Electronics in April 2021.

In a short Q&A with Tech Briefs below, Dr. Daeneke explains how his team will next try to integrate the material into the newest kinds of consumer electronics devices.

Tech Briefs: What inspired you and your team to try ultrathin beta-tellurite? And in basic terms, what is special about the material?

Dr. Torben Daeneke: The work was actually inspired by a theoretical paper that was published by Prof Haibo Zeng  (Nanjing University of Science and Technology). They predicted that beta telluride is a promising wide bandgap p-type semiconductor. When I read their paper in 2018, I thought that this material would be easy for us to make since we had just developed our liquid metal synthesis technology.

Tech Briefs: Was the material, in fact, easy to make?

Dr. Torben Daeneke: We were actually able to make the material very early on in the project. It then took us nearly two years to make devices, characterize them, and fully analyze the data to confirm that beta tellurite is indeed a unique material that can transport positively charged holes with extraordinary speeds.

The reason for this fast performance is that tellurium is in a unique place of the periodic table. As a semi-metal it can actually either behave as a metal or a non-metal. In our case we use its metallic properties, but since it is in the same group as oxygen, unique effects occur that result in a beneficial shape of the electron clouds that surround the individual atoms in the material. In the end tellurium seems to be in just the right spot of the periodic table.

The RMIT team from left, Ali Zavabeti, Patjaree Aukarasereenont, and Torben Daeneke with the transparent material.

Tech Briefs: "The process is similar to drawing: you use a glass rod as a pen and the liquid metal is your ink," said one of your fellow researchers . How difficult was this process, to create the material in this way? It seems challenging.

Dr. Torben Daeneke: The process is actually quite easy. The ultrathin material grows on the molten metal and floats on the droplet. When we draw the liquid across the substrate these thin sheets get stuck on the surface and stay there. The process is indeed similar to using a pen where the ink molecules are deposited on paper. However, you should not do this without proper safety precautions since the selenium that is used in our liquid metal ink is quite toxic. This is something we are working on as we speak. Ultimately we want to replace the selenium with a less toxic alternative.

A molten mixture of tellurium and selenium rolled over a surface deposits an atomically-thin sheet of beta-tellurite

Tech Briefs: Once you have the material, what steps are then needed to integrate it into electronics?

Dr. Torben Daeneke: At this point in time we are still working with small tellurium oxide flakes. This allowed us to created test transistors and enabled the material characterization. To fully integrate the material into electronics we will need to make larger sheets and apply the usual lithography based fabrication steps to make an integrated circuit. This is possible and we are working on it.

Tech Briefs: What’s next with this research?

Dr. Torben Daeneke: We are now upscaling the synthesis and are looking towards incorporating the material into practical devices. Ultimately we want to find partners in the semiconductors industry that are interested in the material and want to bring it to the market.

Tech Briefs: Why is transparent electronics so important? What are the most exciting applications, in your mind?

Dr. Torben Daeneke: Transparent semiconductors have many applications. The obvious ones are in creating transparent – see through – electronic devices that can be integrated into windows or displays. This would essentially lead to new consumer products and enable some technologies that we only know from sci-fi shows such as Star Trek. Other concepts include smart contact lenses that could either be used as a gadget or to help vision impaired people. More immediate applications would include the use of our material in display technology, photovoltaics, and LEDs. High performance, transparent p-type semiconductors are required in all of these products as inter-layers that can boost performance, increase efficiency, and reduce power consumption.

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