Power/Performance Bits: Jan. 11

Quantum dot transistors; faster spin-orbit torque writing.


Quantum dot transistors
Researchers at Los Alamos National Laboratory and University of California Irvine used quantum dots to create transistors which can be assembled into functional logic circuits.

“Potential applications of the new approach to electronic devices based on non-toxic quantum dots include printable circuits, flexible displays, lab-on-a-chip diagnostics, wearable devices, medical testing, smart implants, and biometrics,” said Victor Klimov, a physicist specializing in semiconductor nanocrystals at Los Alamos.

Colloidal semiconductor nanoparticles, also called quantum dots due to their small size and unique properties, can be processed with methods much less stringent than those needed for silicon electronics.

Quantum dot transistors have been demonstrated before, but integrating complementary n- and p-type devices within the same quantum dot layer remained a challenge. Most efforts also relied on nanocrystals based on lead and cadmium.

By depositing gold (Au) and Indium (In) contacts, researchers create two crucial types of quantum dot transistors on the same substrate, opening the door to a host of innovative electronics. (Image courtesy: Los Alamos National Laboratory)

Instead, the researchers turned to copper indium selenide (CuInSe2) quantum dots. As well as being devoid of toxic heavy metals, the material made it possible to achieve straightforward integration of n- and p-transistors in the same quantum dot layer.

The team was able to define p- and n-type transistors by applying two different types of metal contacts (gold and indium, respectively). They completed the devices by depositing a common quantum dot layer on top of the pre-patterned contacts. As a proof of concept, they created functional circuits that performed logical operations.

“This approach permits straightforward integration of an arbitrary number of complementary p- and n-type transistors into the same quantum dot layer prepared as a continuous, un-patterned film via standard spin-coating,” said Klimov.

Faster spin-orbit torque writing
Researchers from University of Lorraine, University of California Berkeley, University of California Riverside, and Université Paris-Saclay demonstrated a new technique for writing information into magnetic memory they say is nearly 100 times faster than state-of-the-art spintronic devices.

“Integrating magnetic memory directly into computer chips has been a long-sought goal,” said Jon Gorchon, a researcher at the French National Centre for Scientific Research (CNRS) working at the University of Lorraine’s L’Institut Jean Lamour in France. “This would allow local data on-chip to be retained when the power is off, and it would enable the information to be accessed far more quickly than pulling it in from a remote disk drive.”

Spin-orbit torque devices use a small magnetic film, or bit, on a metallic wire. Current flowing through the wire leads to a flow of electrons with a magnetic moment (spin) which exerts a magnetic torque on the magnetic bit, which can switch its polarity.

Current spin-orbit torque spintronics devices require current pulses of at least a nanosecond to switch the magnetic bit. This leads to the speed of the overall circuit being limited by the slow magnetic switching speed.

To address this, the researchers launched 6-picosecond-wide electrical current pulses along a transmission line into a cobalt-based magnetic bit. The magnetization of the cobalt bit was then demonstrated to be reliably switched by the spin-orbit torque mechanism.

A microscope image of the structures used to initiate the magnetization switching. (Image by K. Jhuria / UC Berkeley)

While heating by electric currents is usually unwanted, the researchers found that in this experiment, the ultrafast heating aids the magnetization reversal.

“The magnet reacts differently to heating on long versus short time scales,” said Richard Wilson, assistant professor of mechanical engineering and of materials science and engineering at UC Riverside. “When heating is this fast, only a small amount can change the magnetic properties to help reverse the magnet’s direction.”

In addition to faster switching, the team found that the energy needed in this spin-orbit torque device is almost two orders of magnitude smaller than in conventional spintronic devices that operate at much longer time scales.

“The high energy efficiency of this novel, ultrafast magnetic switching process was a big, and very welcome, surprise,” said Jeffrey Bokor, professor of electrical engineering and computer sciences at the University of California, Berkeley. “Such a high-speed, low-energy spintronic device can potentially tackle the performance limitations of current processor level memory systems, and it could also be used for logic applications.”

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