Crystals could help computers run on less power

Science News -- April 14, 2022 :Computers are becoming smaller and more powerful, yet they still need a lot of energy to run. Over the last decade, the quantity of energy dedicated to computers in the United States has increased considerably, and it is soon nearing that of other key industries, such as transportation.

Engineers from the University of California, Berkeley, describe a major breakthrough in the design of a component of transistors — the tiny electrical switches that form the building blocks of computers — that could significantly reduce their energy consumption without sacrificing speed, size, or performance in a study published online this week in the journal Nature. The component, known as the gate oxide, is responsible for turning the transistor on and off.

"We've been able to demonstrate that our gate-oxide technology is superior to commercially available transistors: We can essentially beat the trillion-dollar semiconductor industry," said study senior author Sayeef Salahuddin, the TSMC Distinguished professor of Electrical Engineering and Computer Sciences at UC Berkeley.

This increase in efficiency is enabled through a phenomenon known as negative capacitance, which reduces the amount of voltage required to store charge in a material. In 2008, Salahuddin predicted the presence of negative capacitance theoretically, and in 2011, he demonstrated the effect in a ferroelectric crystal for the first time.

The new research demonstrates how negative capacitance may be obtained in an engineered crystal made up of a layered stack of hafnium oxide and zirconium oxide that is compatible with modern silicon transistors. The study shows how the negative capacitance effect can greatly reduce the amount of voltage necessary to drive transistors, and hence the amount of energy spent by a computer, by introducing the material into model transistors.

"In the last 10 years, the energy used for computing has increased exponentially, already accounting for single digit percentages of the world's energy production, which grows only linearly, without an end in sight," Salahuddin said. "Usually, when we are using our computers and our cell phones, we don't think about how much energy we are using. But it is a huge amount, and it is only going to go up. Our goal is to reduce the energy needs of this basic building block of computing, because that brings down the energy needs for the entire system

Negative capacitance in real-world technology

Tens of billions of tiny silicon transistors are contained in today's computers and smartphones, each of which must be operated by applying a voltage. The gate oxide is a thin layer of material that converts the applied voltage into an electric charge, which is subsequently used to switch the transistor.

Negative capacitance can improve gate oxide performance by lowering the voltage required to create a given electrical charge. The effect, however, cannot be accomplished with just any material. Negative capacitance is achieved by carefully manipulating a material property known as ferroelectricity, which happens when a material produces an electrical field spontaneously. Previously, the phenomenon could only be produced in perovskites, which are ferroelectric materials having a crystal structure that is incompatible with silicon.

Negative capacitance can also be achieved by mixing hafnium oxide and zirconium oxide in an artificial crystal structure known as a superlattice, which results in simultaneous ferroelectricity and antiferroelectricity, according to the researchers.

Suraj Cheema, a postdoctoral researcher at UC Berkeley, said, "We found that this combination really provides us an even better negative capacitance impact, which suggests that this negative capacitance phenomenon is a lot larger than initially imagined." "Negative capacitance doesn't just occur in the conventional picture of a ferroelectric with a dielectric, which is what's been studied over the past decade. You can actually make the effect even stronger by engineering these crystal structures to exploit antiferroelectricity in tandem with ferroelectricity."

The best negative capacitance effect was reported in a superlattice structure consisting of three atomic layers of zirconium oxide sandwiched between two single atomic layers of hafnium oxide, totaling less than two nanometers in thickness. These superlattice structures may readily be integrated into advanced transistors because most state-of-the-art silicon transistors already employ a 2-nanometer gate oxide consisting of hafnium oxide on top of silicon dioxide, and zirconium oxide is also used in silicon technology.

The scientists developed and tested short channel transistors to see how well the superlattice structure would work as a gate oxide. When compared to conventional transistors, these transistors would require around 30% less voltage while keeping semiconductor industry norms and with no loss of dependability.

""One of the issues that we often see in this type of research is that we can we can demonstrate various phenomena in materials, but those materials are not compatible with advanced computing materials, and so we cannot bring the benefit to real technology," Salahuddin said. "This work transforms negative capacitance from an academic topic to something that could actually be used in an advanced transistor."

This study's co-first author is Nirmaan Shanker of UC Berkeley. Li-Chen Wang, Cheng-Hsiang Hsu, Shang-Lin Hsu, Yu-Hung Liao, Wenshen Li, Jong-Ho Bae, Steve K. Volkman, Daewoong Kwon, Yoonsoo Rho, Costas P. Grigoropoulos, Ramamoorthy Ramesh, Ramamoorthy Ramesh, Ramamoorthy Ramesh, Ramamoorthy

The Berkeley Center for Negative Capacitance Transistors (BCNCT), the DARPA Technologies for Mixed-mode Ultra Scaled Integrated Circuits (T-MUSIC) program, the University of California Multicampus Research Programs and Initiatives (UC MRPI) project, and the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under contract No. DE-AC02-05-CH11231 supported this research in part (Microelectronics Co-Design program).

Source for the story:

The University of California at Berkeley contributed the materials for this project. Kara Manke wrote the original. Please keep in mind that content may be altered for style and length.

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Journal of Engineered Crystal Structures:

Suraj S. Cheema, Nirmaan Shanker, Li-Chen Wang, Cheng-Hsiang Hsu, Shang-Lin Hsu, Yu-Hung Liao, Yu-Hung Liao, Matthew San Jose, Jorge Gomez, Wriddhi Chakraborty, Wenshen Li, Jong-Ho Bae, Steve K. Volkman, Daewoong Kwon, Yoonsoo Rho, Advanced transistors with an ultrathin ferroic HfO2–ZrO2 superlattice gate stack. Nature, 604 (7904): 65, 2022. DOI: 10.1038/s41586-022-04425-6 DOI: 10.1038/s41586-022-04425-6
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University of California - Berkeley MLA APA Chicago "Engineered crystals may be able to reduce the amount of electricity used by computers." www.sciencedaily.com/releases/2022/04/220408083839.htm>. ScienceDaily, 8 April 2022. www.sciencedaily.com/releases/2022/04/220408083839.htm>.

Wnctimes by Marjorie Farrington