Insulating oxides are oxygen-containing compounds that do not conduct electricity but can sometimes form conductive interfaces when they're layered together precisely. The conducting electrons at the interface form a two-dimensional electron gas (2DEG), which boasts exotic quantum properties that make the system potentially useful for electronics and photonics applications.
Researchers at Yale University have now grown a 2DEG system on gallium arsenide, a semiconductor that's efficient at absorbing and emitting light. This development is promising for new electronic devices that interact with light, such as new kinds of transistors, superconducting switches and gas sensors.
"I see this as a building block for oxide electronics," said Lior Kornblum, now at Technion – Israel Institute of Technology. He and his colleagues describe their research in a paper in the Journal of Applied Physics.
Oxide 2DEGs were first discovered in 2004. Researchers were surprised to find that sandwiching together two layers of certain insulating oxides can generate conducting electrons that behave like a gas or liquid near the interface between the oxides. These conducting electrons can also transport information.
Researchers had previously observed 2DEGs with semiconductors, but oxide 2DEGs have much higher electron densities, making them promising candidates for some electronic applications. Oxide 2DEGs also have interesting quantum properties: for example, the systems seem to exhibit a combination of magnetic behaviors and superconductivity.
"The ability to couple or to integrate these interesting oxide two-dimensional electron gases with gallium arsenide opens the way to devices that could benefit from the electrical and optical properties of the semiconductor."Lior Kornblum, Technion – Israel Institute of Technology
Generally, it's difficult to mass-produce oxide 2DEGs because only small pieces of the necessary oxide crystals are obtainable, Kornblum said. If, however, researchers could grow the oxides on large, commercially available semiconductor wafers, they would be able to scale up oxide 2DEGs for real-world applications. Growing oxide 2DEGs on semiconductors would also allow researchers to better integrate the structures with conventional electronics. According to Kornblum, getting the oxide electrons to interact with the electrons in the semiconductor could lead to new functionality and more types of devices.
The Yale team had previously succeeded in growing oxide 2DEGs on silicon wafers. In the new work, they successfully grew oxide 2DEGs on another important semiconductor, gallium arsenide, which proved to be more challenging.
Most semiconductors react with oxygen in the air to form a disordered surface layer, which must be removed before growing these oxides on the semiconductor. For silicon, removal is relatively easy – researchers heat the semiconductor in a vacuum. This approach, however, doesn't work well with gallium arsenide.
Instead, the research team coated a clean surface of a gallium arsenide wafer with a layer of arsenic. The arsenic protected the semiconductor's surface from the air while they transferred the wafer into an instrument that grows oxides using a method called molecular beam epitaxy. This allows one material to grow on another while maintaining an ordered crystal structure across the interface.
Next, the researchers gently heated the wafer to evaporate the thin arsenic layer, exposing the pristine semiconductor surface beneath. They then grew an oxide called SrTiO3 on the gallium arsenide and, immediately after, another oxide layer of GdTiO3, forming a 2DEG between the oxides.
Gallium arsenide is but one of a whole class of materials called III-V semiconductors, and this work opens a path to integrating oxide 2DEGs with other members of this class.
"The ability to couple or to integrate these interesting oxide two-dimensional electron gases with gallium arsenide opens the way to devices that could benefit from the electrical and optical properties of the semiconductor," Kornblum said. "This is a gateway material for other members of this family of semiconductors."
This story is adapted from material from the American Institute of Physics, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.