New ultrathin semiconductor materials exceed some of silicon’s ‘secret’ powers

The next generation of feature-filled and energy-efficient electronics will require computer chips just a few atoms thick. For all its positive attributes, trusty silicon can’t take us to these ultrathin extremes.

In this greatly enlarged cross-section of an experimental chip, the bands of black and white reveal alternating layers of hafnium diselenide – an ultrathin semiconductor material – and the hafnium dioxide insulator. (Image credit: Michal Mleczko)

Now, electrical engineers at Stanford have identified two semiconductors – hafnium diselenide and zirconium diselenide – that share or even exceed some of silicon’s desirable traits, starting with the fact that all three materials can “rust.”

“It’s a bit like rust, but a very desirable rust,” said Eric Pop, an associate professor of electrical engineering, who co-authored with post-doctoral scholar Michal Mleczko a paper that appears in the journal Science Advances.

The new materials can also be shrunk to functional circuits just three atoms thick and they require less energy than silicon circuits. Although still experimental, the researchers said the materials could be a step toward the kinds of thinner, more energy-efficient chips demanded by devices of the future.

Silicon’s strengths

Silicon has several qualities that have led it to become the bedrock of electronics, Pop explained. One is that it is blessed with a very good “native” insulator, silicon dioxide or, in plain English, silicon rust. Exposing silicon to oxygen during manufacturing gives chip-makers an easy way to isolate their circuitry. Other semiconductors do not “rust” into good insulators when exposed to oxygen, so they must be layered with additional insulators, a step that introduces engineering challenges. Both of the diselenides the Stanford group tested formed this elusive, yet high-quality insulating rust layer when exposed to oxygen.

Not only do both ultrathin semiconductors rust, they do so in a way that is even more desirable than silicon. They form what are called “high-K” insulators, which enable lower power operation than is possible with silicon and its silicon oxide insulator.

As the Stanford researchers started shrinking the diselenides to atomic thinness, they realized that these ultrathin semiconductors share another of silicon’s secret advantages: the energy needed to switch transistors on – a critical step in computing, called the band gap – is in a just-right…

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