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Gallium oxide shows high electron mobility

Schematic stack and the scanning electron microscopic image of the ß-(AlxGa1-x)2O3/Ga2O3 modulation-doped field effect transistor. Credit: Choong Hee Lee and Yuewei Zhang

The next generation of energy-efficient power electronics, high-frequency communication systems, and solid-state lighting, rely on materials known as wide bandgap semiconductors. Circuits based on these materials can operate at much higher power densities and with lower power losses than silicon-based circuits.

Researchers have now shown that a wide-bandgap semiconductor called gallium oxide (Ga2O3) can be engineered into nanometre-scale structures that allow electrons to move much faster within the crystal structure. With electrons that move with such ease, the team believes Ga2O3 could be a promising material for applications such as high-frequency communication systems and energy-efficient power electronics.

"Gallium oxide has the potential to enable transistors that would surpass current technology," explains Siddharth Rajan of Ohio State University, who led the research.

According to the team, Ga2O3 has one of the largest bandgaps of the wide bandgap materials being developed as alternatives to silicon, meaning it’s useful for high-power and high-frequency devices. The researchers add that it’s also unique among wide bandgap semiconductors because it can be produced directly from its molten form, which enables large-scale manufacturing of high-quality crystals.

For use in electronic devices, the electrons in the material must be able to move easily under an electric field. "That's a key parameter for any device," Rajan says. Normally, to populate a semiconductor with electrons, the material is doped with other elements. The problem is that the dopants also scatter electrons, limiting the electron mobility of the material.

To solve this, the researchers used a technique known as modulation doping, commonly used to achieve high mobility, but according to Ohio State University, never before with Ga2O3.

In their work, the researchers created a so-called semiconductor heterostructure, creating an atomically perfect interface between Ga2O3 and its alloy with aluminium, aluminium gallium oxide – two semiconductors with the same crystal structure but different energy gaps.

A few nanometers away from the interface, embedded inside the aluminium gallium oxide, was a sheet of electron-donating impurities only a few atoms thick. The donated electrons transferred into the Ga2O3, forming a 2D electron gas. But because the electrons are now also separated from the dopants in the aluminium gallium oxide by a few nanometres, they scatter much less and remain highly mobile.

Using this technique, the researchers say they reached “record mobilities”, and were able to observe Shubnikov-de Haas oscillations – a quantum phenomenon in which increasing the strength of an external magnetic field causes the resistance of the material to oscillate. These oscillations, they explain, confirm formation of the high mobility 2D electron gas and allow them to measure critical material properties.

Rajan explains that such modulation-doped structures could lead to a new class of quantum structures and electronics that harnesses the potential of Ga2O3.

Author
Bethan Grylls

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