Adapted from a Berkeley Lab press release
In the ongoing quest to make electronic devices ever smaller and more energy efficient, researchers want to bring energy storage directly onto microchips, reducing the losses incurred when power is transported between various device components. To be effective, on-chip energy storage must be able to store a large amount of energy in a very small space and deliver it quickly when needed – requirements that can’t be met with existing technologies.
Addressing this challenge, Foundry users from Berkeley Lab and UC Berkeley have achieved record-high energy and power densities in microcapacitors made with engineered thin films of hafnium oxide and zirconium oxide, using materials and fabrication techniques already widespread in chip manufacturing. The findings, published in the journal Nature, pave the way for advanced on-chip energy storage and power delivery in next-generation electronics.
This research is part of broader efforts at Berkeley Lab to develop new materials and techniques for smaller, faster, and more energy-efficient microelectronics.
Capacitors are one of the basic components of electrical circuits but they can also be used to store energy. Unlike batteries, which store energy through electrochemical reactions, capacitors store energy in an electric field established between two metallic plates separated by a dielectric material. Capacitors can be discharged very rapidly when needed, allowing them to deliver power quickly, and they do not degrade with repeated charge-discharge cycles, giving them much longer lifespans than batteries. However, capacitors generally have much lower energy densities than batteries, meaning they can store less energy per unit volume or weight, and that problem only gets worse when you try to shrink them down to microcapacitor size for on-chip energy storage.
Here, the researchers achieved their record-breaking microcapacitors by carefully engineering thin films of HfO2-ZrO2 to achieve a negative capacitance effect. Normally, layering one dielectric material on top of another results in an overall lower capacitance. However, if one of those layers is a negative-capacitance material, then the overall capacitance actually increases. In earlier work, Salahuddin and colleagues demonstrated the use of negative capacitance materials to produce transistors that can be operated at substantially lower voltages than conventional MOSFET transistors. Here, they harnessed negative capacitance to produce capacitors capable of storing greater amounts of charge, and therefore energy.
Interested in Becoming a Foundry User?
Join our collaborative, multidisciplinary environment.
Learn more >
The crystalline films are made from a mix of HfO2 and ZrO2 grown by atomic layer deposition, using standard materials and techniques from industrial chip fabrication. Depending on the ratio of the two components, the films can be ferroelectric, where the crystal structure has a built-in electric polarization, or antiferroelectric, where the structure can be nudged into a polar state by applying an electric field. When the composition is tuned just right, the electric field created by charging the capacitor balances the films at the tipping point between ferroelectric and antiferroelectric order, and this instability gives rise to the negative capacitance effect where the material can be very easily polarized by even a small electric field.
To scale up the energy storage capability of the films, the team needed to increase the film thickness without allowing it to relax out of the frustrated antiferroelectric-ferroelectric state. They found that by interspersing atomically thin layers of aluminum oxide after every few layers of HfO2-ZrO2, they could grow the films up to 100 nm thick while still retaining the desired properties.
These high-performance microcapacitors could help meet the growing demand for efficient, miniaturized energy storage in microdevices such as Internet-of-Things sensors, edge computing systems, and artificial intelligence processors. The researchers are now working on scaling up the technology and integrating it into full-size microchips, as well as pushing the fundamental materials science forward to improve the negative capacitance of these films even more.
Read the full press release
