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A new contender for 'post silicon' electronics?

Correlated oxide transistor achieves 'colossal' switchable resistance 

Harvard researchers have engineered a quantum material called a correlated oxide to perform comparably with the best silicon switches. They achieved a reversible change in electrical resistance of eight orders of magnitude, a result the team is calling "˜colossal'.

The finding arose in a laboratory usually devoted to studying fuel cells led by Shriram Ramanathan, associate professor of Materials Science at the Harvard School of Engineering and Applied Sciences (SEAS). The researchers' familiarity with thin films and ionic transport enabled them to exploit chemistry, rather than temperature, to achieve the result.

Because the correlated oxides can function equally well at room temperature or a few hundred degrees above it, it would be easy to integrate them into existing electronic devices and fabrication methods, according to the researchers. The discovery, published in Nature Communications, suggests correlated oxides are promising semiconductors for future 3D integrated circuits as well as for adaptive, tunable photonic devices.

"Traditional silicon transistors have fundamental scaling limitations," says Ramanathan. "If you shrink them beyond a certain minimum feature size, they don't quite behave as they should."

Yet silicon transistors are hard to beat, with an on/off ratio of at least 104 required for practical use. "It's a pretty high bar to cross," Ramanathan explains, adding that until now, experiments using correlated oxides have produced changes of only about a factor of 10, or 100 at most, near room temperature. But Ramanathan and his team have crafted a new transistor, made primarily of an oxide called samarium nickelate, that in practical operation achieves an on/off ratio of greater than 105.

In future work the researchers will investigate the device's switching dynamics and power dissipation; meanwhile, this advance represents an important proof of concept. "Our orbital transistor could really push the frontiers of this field and say, you know what? This is a material that can challenge silicon," Ramanathan says.

Changing the band-gap by doping

Materials scientists have been studying the family of correlated oxides for years, but the field is still in its infancy, with most research aimed at establishing the materials' basic physical properties. "We have just discovered how to dope these materials, which is a foundational step in the use of any semiconductor," says Ramanathan.

Doping (introducing different atoms into the crystal structure of a material) typically effects how easily electrons can move through the material by increasing the number of available electrons, but this study was different. The Harvard team manipulated the band gap, the energy barrier to electron flow.

"By a certain choice of dopants-in this case, hydrogen or lithium-we can widen or narrow the band gap in this material, deterministically moving electrons in and out of their orbitals," Ramanathan says. That's a fundamentally different approach than is used in other semiconductors. The traditional method changes the energy level to meet the target; the new method moves the target itself.

During fabrication, the annealing process injects hydrogen ions into thin films of samarium nickelate (SNO) and yttrium-doped barium zirconate (BYZ). During operation, an electric field moves the charges from one layer to the other, and the influx or loss of electrons modulates the band gap in the SNO, resulting in a very dramatic change in conductivity. (Image courtesy of Jian Shi.)


In this orbital transistor, protons and electrons move in or out of the samarium nickelate when an electric field is applied, regardless of temperature, so the device can be operated in the same conditions as conventional electronics. It is solid-state, meaning it involves no liquids, gases, or moving mechanical parts. And, in the absence of power, the material remembers its present state -an important feature for energy efficiency.

"That's the beauty of this work," says Ramanathan. "It's an exotic effect, but in principle it's highly compatible with traditional electronic devices."

Quantum materials

Unlike silicon, samarium nickelate and other correlated oxides are quantum materials, meaning that quantum-mechanical interactions have a dominant influence over the material properties- and not just at small scales.

"If you have two electrons in adjacent orbitals, and the orbitals are not completely filled, in a traditional material the electrons can move from one orbital to another. But in the correlated oxides, the electrons repulse each other so much that they cannot move," Ramanathan explains. "The occupancy of the orbitals and the ability of electrons to move in the crystal are very closely tied together-or 'correlated.' Fundamentally, that's what dictates whether the material behaves as an insulator or a metal."

Ramanathan and others at SEAS have successfully manipulated the metal-insulator transition in vanadium oxide, too. In 2012, they demonstrated a tunable device that can absorb 99.75 percent of infrared light, appearing black to infrared cameras.

Similarly, samarium nickelate is likely to catch the attention of applied physicists developing photonic and optoelectronic devices.

"Opening and closing the band gap means you can now manipulate the ways in which electromagnetic radiation interacts with your material," says Jian Shi, lead author of the paper in Nature Communications. He completed the research as a postdoctoral fellow in Ramanathan's lab at Harvard SEAS and joined the faculty of Rensselaer Polytechnic Institute this fall. "Just by applying an electric field, you're dynamically controlling how light interacts with this material."

Further ahead, Researchers at the Center for Integrated Quantum Materials, established at Harvard in 2013 through a grant from the National Science Foundation, aim to develop an entirely new class of quantum electronic devices and systems that will transform signal processing and computation.

materials research to the 1950s, when transistors were newly invented and physicists were still making sense of them. "We are basically in that era for these new quantum materials," he says. "This is an exciting time to think about establishing the basic, fundamental properties. In the coming decade or so, this could really mature into a very exciting device platform."

You Zhou, a graduate student at Harvard SEAS, was co-lead author of the paper in Nature Communications. The research was supported by grants from the National Science Foundation (NSF) (CCF-0926148) and the National Academy of Sciences, as well as an NSF Faculty Early Career Development (CAREER) Award to Prof. Ramanathan (DMR- 0952794).

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