New BaTiO3 capacitor could enable energy-efficient microchips

The silicon-based computer chips that power our modern devices require vast amounts of energy to operate. Despite ever-improving computing efficiency, information technology is projected to consume around 25% of all primary energy produced by 2030. Researchers in the microelectronics and materials sciences communities are seeking ways to sustainably manage the global need for computing power. Non-silicon materials with enticing properties for memory and logic devices exist; but their common bulk form still requires large voltages to manipulate, making them incompatible with modern electronics. Designing thin-film alternatives that not only perform well at low operating voltages but can also be packed into microelectronic devices remains a challenge. The holy grail for reducing this digital demand is to develop microelectronics that operate at much lower voltages, which would require less energy and is a primary goal of efforts to move beyond today’s state-of-the-art CMOS (complementary metal-oxide semiconductor) devices.

Now in a new paper published in Nature Materials, a team of researchers at Lawrence Berkeley National Laboratory and UC Berkeley led by Professor Lane Martin have identified one energy-efficient route by synthesizing a thin-layer version of a well-known material whose properties are exactly what’s needed for next-generation devices. First discovered more than 80 years ago, barium titanate (BaTiO3) found use in various capacitors for electronic circuits, ultrasonic generators, transducers, and even sonar.

Crystals of the material respond quickly to a small electric field, flip-flopping the orientation of the charged atoms that make up the material in a reversible but permanent manner even if the applied field is removed. This provides a way to switch between the proverbial “0” and “1” states in logic and memory storage devices but still requires voltages larger than 1,000 millivolts (mV) for doing so.

Seeking to harness these properties for use in microchips, research team developed a pathway for creating films of BaTiO3 just 25 nanometers thin less than a thousandth of a human hair’s width whose orientation of charged atoms, or polarization, switches as quickly and efficiently as in the bulk version. Although,  BaTiO3 it was possible previously to make thin films of this material. But until now, nobody could make a film that could get close to the structure or performance that could be achieved in bulk.

Previous research efforts for synthesis attempts have resulted in films that contain higher concentrations of “defects” points where the structure differs from an idealized version of the material as compared to bulk versions. Such a high concentration of defects negatively impacts the performance of thin films. The authors developed an approach to growing the films that limits those defects. To understand what it takes to produce the best, low-defect BaTiO3 thin films, the researchers turned to a process called pulsed-laser deposition. Firing a powerful beam of an ultraviolet laser light onto a ceramic target of BaTiO3 causes the material to transform into a plasma, which then transmits atoms from the target onto a surface to grow the film.

The authors showed that their method could achieve precise control over the deposited film’s structure, chemistry, thickness, and interfaces with metal electrodes. By chopping each deposited sample in half and looking at its structure atom by atom using advanced tools at the National Center for Electron Microscopy at Berkeley Lab’s Molecular Foundry, the researchers revealed a version that precisely mimicked an extremely thin slice of the bulk. Finally, by placing a film of BaTiO3 in between two metal layers, the researchers created tiny capacitors. Applying voltages of 100 mV or less and measuring the current that emerges showed that the film’s polarization switched within two billionths of a second and could potentially be faster competitive with what it takes for today’s computers to access memory or perform calculations.

The work follows the bigger goal of creating materials with small switching voltages, and examining how interfaces with the metal components necessary for devices impact such materials.  The capacitors used currently in chips don’t hold their data unless you keep applying a voltage and current technologies generally work at 500 to 600 mV, while the thin film version developed in the study could work at 50 to 100 mV or less. Together, these measurements demonstrate a successful optimization of voltage and polarization robustness which tend to be a trade-off, especially in thin materials. Next future plans is to shrink the material down even thinner to make it compatible with real devices in computers and study how it behaves at those tiny dimensions. This work will pave the way for new first-generation electronic devices that possibly million times more efficient with much energy you save.