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 in electronics and photonics applications.
Researchers at Yale University have now grown a 2DEG system on
gallium arsenide, a semiconductor that's efficient in 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 of the Technion -- Israel Institute of Technology, who
describes the new research appearing this week in the Journal of Applied Physics, from AIP publishing.
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.
Researchers have previously observed 2DEGs with semiconductors, but
oxide 2DEGs have much higher electron densities, making them promising
candidates for some electronic applications. Oxide 2DEGs 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's difficult to mass-produce oxide 2DEGs because only
small pieces of the necessary oxide crystals are obtainable, Kornblum
said. If, however, 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 successfully grew oxide 2DEGs on another important
semiconductor, 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. 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. This process formed 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 integrate 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," Kornblum said. "This is a gateway material for
other members of this family of semiconductors."
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