| Materials

Ultrathin Semiconductor Materials “Rust” to Insulate Circuitry

These semiconductors could be made into transistors 10 times smaller than those made of silicon.

Silicon has several qualities that have led it to become the bedrock of electronics. One is that it features a very good “native” insulator — silicon dioxide, or silicon rust. Exposing silicon to oxygen during manufacturing gives chip makers a 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. Two new ultrathin materials share that trait, making them promising materials for electronics of the future.

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. The cross-section matches an overlaid color schematic on the right. (Image: Michal Mleczko)

The next generation of feature-filled and energy-efficient electronics will require computer chips just a few atoms thick. The new materials can be shrunk to functional circuits three atoms thick, and require less energy than silicon circuits, which can only be shrunk to five nanometers. The materials could be a step toward these kinds of thinner, more energy-efficient chips.

Both of the diselenides formed a 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 diselenides were shrunk to atomic thinness, the ultrathin semiconductors were shown to share another of silicon’s advantages. The energy needed to switch transistors on — a critical step in computing called the band gap — is in a perfect range. Too low and the circuits leak and become unreliable; too high and the chip takes too much energy to operate and becomes inefficient. Both materials were in the same optimal range as silicon.

Future work will refine the electrical contacts between transistors on the ultra-thin diselenide circuits, and will better control the oxidized insulators to ensure they remain as thin and stable as possible.

For more information, contact Tom Abate of Stanford Engineering at This email address is being protected from spambots. You need JavaScript enabled to view it.; 650-736-2245.