11 January 2018
Oxide two-dimensional electron gases integrated with GaAs, paving way for new opto devices
© Semiconductor Today Magazine / Juno PublishiPicture: Disco’s DAL7440 KABRA laser saw.
Insulating oxides are oxygen-containing compounds that do not conduct electricity but can sometimes form conductive interfaces when they are layered together precisely. The conducting electrons at the interface form a two-dimensional electron gas (2DEG), which boasts quantum properties that make the system potentially useful in electronics and photonics applications.
Yale University has now grown a 2DEG system on gallium arsenide, promising new electronic devices that interact with light, such as new kinds of transistors, superconducting switches and gas sensors (Lior Kornblum et al, ‘Oxide Heterostrutures for High Density 2D Electron Gases on GaAs’, Journal of Applied Physics, 9 January 2018; DOI: 10.1063/1.5004576). “I see this as a building block for oxide electronics,” says Lior Kornblum, now of the Technion - Israel Institute of Technology.
Oxide 2DEGs were discovered in 2004. Researchers were surprised to find that sandwiching together two layers of some insulating oxides can generate conducting electrons that behave like a gas or liquid near the interface between the oxides and can transport information.
Oxide 2DEGs have much higher electron densities than semiconductor 2DEGs, making them promising candidates for some electronic applications. They have interesting quantum properties, drawing interest in their fundamental properties as well. For example, the systems seem to exhibit a combination of magnetic behaviors and superconductivity.
Generally, it is difficult to mass produce oxide 2DEGs because only small pieces of the necessary oxide crystals are obtainable, Kornblum notes. However, if researchers can grow the oxides on large, commercially available semiconductor wafers, they can then scale up oxide 2DEGs for real-world applications. Growing oxide 2DEGs on semiconductors also allows researchers to better integrate the structures with conventional electronics. According to Kornblum, enabling the oxide electrons to interact with the electrons in the semiconductor could lead to new functionality and more types of devices.
The Yale team previously grew oxide 2DEGs on silicon wafers. In the new work, they grew oxide 2DEGs on gallium arsenide, which proved to be more challenging.
Most semiconductors react with oxygen in the air and 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 vacuum. However, this approach does not work well with GaAs.
Instead, the team coated a clean surface of a GaAs wafer with a layer of arsenic. This protects the surface from air while the wafer is transferred into a molecular beam epitaxy (MBE) system that grows oxides. 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 layer of SrTiO3 on the GaAs and, immediately after, another oxide layer of GdTiO3, forming a 2DEG between the oxides.
It is reckoned that this work opens a path to integrate oxide 2DEGs with not just GaAs but also other III-V semiconductor materials too.
“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 says. “This is a gateway material for other members of this family of semiconductors,” he believes.