If you put two ordinary electrical insulators into contact with each other you get – an insulator. Hardly surprising, you might think. But do the same with a few atomic layers of one insulating oxide (ceramic) on top of another and the interface layer can suddenly become metallically conducting. This surprising discovery in 2004 opened a new paradigm in electronics, introducing ceramic materials as a possible candidate to supplement conventional silicon-based semiconductors. The new oxide conductors could even exhibit unusual behavior such asmagnetism, superconductivity or even combinations of both.
However, incorporation of oxides into electronic circuits requires mastery of the electron mobility, a quantity which describes how fast an electron moves when an electric field is applied. Very high electron mobility is needed in most electronic devices, e.g. the field-effect transistors (FET) which form the heart of most modern electronics devices. The low mobility in oxide interfaces have so far prevented their practical application.
100 times higher electron mobility
A research group at DTU Energy (at the Technical University of Denmark) is among the forerunners in the quest for developing and implementing oxide electronics. Together with international partners they have recently succeeded in creating a metallic oxide interface with a much higher mobility, around 100 times higher than the conventional oxide interfaces.
This breakthrough within oxide-based electronics has just been published in the article “Extreme mobility enhancement of two-dimensional electron gases at oxide interfaces by charge-transfer-induced modulation doping” in the prominent journal Nature Materials.
“Compared to Silicon, oxides can have a much wider range of properties. We are working on finding an alternative to conventional semiconductor materials based on multifunctional oxides; an alternative that may add more functions into electronic devices that are not yet available or possible in conventional semiconductors”, explains the main author of the article, senior researcher at DTU Energy Yunzhong Chen.
In the Nature Materials article researchers from Denmark, the Netherlands, Canada, Germany, Belgium and Israel describe how they can control the mobility at the interface formed between two insulating complex oxides using the so-called modulation doping technique, which separates the electrons from the positive donors where the electrons originate. Owing to the separation, electrons become more free and highly mobile.
The highway of electrons
“One problem of state-of-the-art oxide devices is that they intrinsically have very slow moving electrons in close proximity to the highly mobile electrons. It’s like having big trucks all over a highway, blocking the way for the speedy cars. In order to have the electrons move fast, we have to get rid of the slow ones”, says Yunzhong Chen.
To some extent a similar problem exists in conventional semiconductors, and there the solution is to use a principle called modulation doping. The breakthrough of the DTU-led research team was to figure out a way of applying the principle of modulation doping to oxide electronics. They term their new method “charge-transfer-induced modulation doping”.
“The reason we succeeded is that we took into account the intrinsic defects of oxides. We designed a new kind of buffer layer for oxides, allowing us to both remove the intrinsic defects such as oxygen vacancies and move the slow-moving electrons to the side. It is like having the big trucks drive near the border of the highway, leaving the center open for speedy cars, and it works!”, says Yunzhong Chen.
For now, the oxide interfaces are mainly of scientific interest, offering a unique opportunity to study the new physics which arise by the combination of phenomena such as magnetism, superconductivity and the quantum Hall effect in a single system.
“At present our novel strategy for improving the oxide systems can only be realized at very low temperatures. But it should be possible to do the same at higher temperatures. This will open up real-life applications of oxide electronics. Also we hope to use this principle to improve solar cells and quantum devices”, says Yunzhong Chen. He continues: “Oxide electronics are only just entering the field. It will take years of study and involvement from the industry before oxides can be integrated with or take over from silicon, but the future looks bright”.