Insulating oxides are considered to be oxygen containing compounds that are not capable of conducting electricity. However, they can sometimes form conductive interfaces when they are layered together in an accurate manner.
The conducting electrons at the interface produce a two-dimensional electron gas (2DEG) capable of boasting exotic quantum properties, which allow the system to be potentially useful in photonics and electronics applications.
Presently, researchers at Yale University have grown a 2DEG system on gallium arsenide, creating a semiconductor that is efficient in absorbing and then emitting light. This development proves to be promising for new electronic devices interacting with light, such as new types of superconducting switches, transistors and gas sensors.
"I see this as a building block for oxide electronics," said Lior Kornblum, now of the Technion - Israel Institute of Technology, who describes the new research featuring this week in the Journal of Applied Physics, from AIP publishing.
The discovery of oxide 2DEGs happened in 2004. Researchers were surprised to discover that sandwiching together two layers of a few insulating oxides can produce conducting electrons that act like a liquid or gas near the interface between the oxides and can convey information.
Researchers have earlier observed 2DEGs with semiconductors, but oxide 2DEGs are known to have much greater electron densities, enabling them to be potential candidates for some electronic applications. Oxide 2DEGs are available with interesting quantum properties, drawing interest in their basic properties as well. For instance, the systems seem to display a mixture of superconductivity and magnetic behaviors.
Generally, it is difficult to mass-produce oxide 2DEGs since only tiny pieces of the necessary oxide crystals can be obtained, Kornblum said. However, if researchers can grow the oxides on huge, commercially available semiconductor wafers, then they will be able to scale up oxide 2DEGs for real-world applications. Growing oxide 2DEGs on semiconductors also permits researchers to improve the integration of the structures with standard electronics. According to Kornblum, allowing the oxide electrons to work together with the electrons in the semiconductor could result in new functionality and more varieties of devices.
Earlier, the Yale team grew oxide 2DEGs on silicon wafers. In the new work, they have succeeded in growing oxide 2DEGs on another vital semiconductor, gallium arsenide, which indeed established to be more challenging.
Most semiconductors react with oxygen present in the air and produce a disordered surface layer, which will have to be removed prior to growing these oxides on the semiconductor. For silicon, removal is comparatively easy - the semiconductor is heated in vacuum by researchers. This approach, however, does not work well with gallium arsenide.
As an alternative, the research team used a layer of arsenic to coat a clean surface of a gallium arsenide wafer. The arsenic enabled protecting the semiconductor's surface from the air while they also transported the wafer into an instrument that grows oxides with the help of a method known as molecular beam epitaxy. This permits one material to grow on another while also maintaining an ordered crystal structure throughout the interface.
This was followed by gently heating the wafer in order to evaporate the thin arsenic layer, thus revealing the pristine semiconductor surface beneath. The researchers then grew an oxide known as SrTiO3 on the gallium arsenide and, instantly after, another oxide layer of GdTiO3. This process developed a 2DEG between the oxides.
Gallium arsenide is considered to be one of a whole class of materials known as III-V semiconductors and this research makes way for a path to incorporate oxide 2DEGs with others.
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. This is a gateway material for other members of this family of semiconductors.