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Just Like Silicon, Only Better

15 August 2017

Bands of black and white reveal alternating layers of hafnium diselenide and its insulator. (Image credit: Michal Mleczko)Bands of black and white reveal alternating layers of hafnium diselenide and its insulator. (Image credit: Michal Mleczko)The next generation of electronics is going to call for computer chips just a few atoms thick -- which is why it’s going to need alternatives to silicon.

Fortunately, electrical engineers at Stanford have identified two ultrathin semiconductor materials that share, or even exceed, some of silicon’s desirable traits.

“Engineers have been unable to make silicon transistors thinner than about five nanometers, before the material properties begin to change in undesirable ways,” said Eric Pop, an associate professor of electrical engineering at Stanford. By contrast, the new materials --hafnium diselenide and zirconium diselenide -- can be shrunk to functional circuits just three atoms thick, or about two-thirds of a nanometer.

Pop, co-author of a new paper on the subject in the journal Science Advances, notes that silicon has become the bedrock of electronics thanks to several qualities. One is that, unlike many other semiconductors, it is a very good native insulator: When exposed to oxygen during manufacturing, it “rusts” in a way that insulates its tiny circuitry. The new materials not only share this quality, but also form “high-K” insulators that enable lower power operation than is possible with silicon.

The amount of energy needed to switch transistors on (also known as the “band gap”) is another important measure for a semiconductor: If it’s too low, the circuits leak and become unreliable; if it’s too high, the chip becomes inefficient. Like silicon, the Stanford researchers discovered, the two diselenides operate in an optimal range.

The combination of thinner circuits and high-K insulation means that the new materials could be made into transistors 10 times smaller than anything possible with silicon today – translating to much longer battery life and, if integrated with silicon, much more complex functionality.

The researchers will next work on refining the electrical contacts between transistors on their ultrathin circuits; better controlling the oxidized insulators for stability; and, ultimately, integrating other materials and scaling up to working wafers, complex circuits and eventually complete systems.

“There’s more research to do, but a new path to thinner, smaller circuits – and more energy-efficient electronics – is within reach,” Pop said.

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