Many improvements in electronic miniaturization and efficiency can be tied to the development of capacitor technology, and minimizing the dielectric layer thickness has been the primary driver for improving capacitors. But dielectric layers are approaching the nanometer size, at which point they lose their ability to store charge. In last Sunday's Nature Materials, a team from University of California Santa Barbara and Rutgers University provided a physical basis for the loss of performance in thin-film capacitors and suggested methods that could greatly improve the energy storage capacity in next-generation devices.
Today, many capacitors are made with ferroelectric oxides like BaTiO3 and PbTiO3. Both of these materials can shift the Ti4+ ions in response to an external electric field because the Ti4+ ion is slightly too small for its place in the crystal structure. This behavior is slow compared to displacing electron clouds and bond stretching (typical electrical responses in non-ferroelectric, nonconducting materials), but it is immensely powerful in comparison. While these materials cannot respond to extremely fast changes in the electric field, they can store tens of thousands of times more energy than an ordinary dielectric material.
Capacitors are made by sandwiching ferroelectric materials between conducting metal layers. The capacitance is inversely proportional to the ferroelectric layer thickness and directly proportional to the number of layers in the device. These relationships have driven most of the development in the capacitor industry for the last several decades but now scientists are beginning to grow capacitors with thin film technology. As the layer thickness approaches the nanometer scale, however, capacitors lose the ability to store charge.
Previously, it was thought that defects at the metal/dielectric interface were at fault, but capacitors with perfect interfaces exhibited the same behavior. So, the researchers performed first-principle calculations on perfect interfaces to determine the physical origins of the property losses.
In an unsuprising result, they found that high bond strength between the ferroelectric layer and metal layer effectively clamps the displacement of ions in the ferroelectric material and limits polarizability and thus energy storage. However, when the dielectric layer terminates with a layer of oxygen ions, the bond strength is low and an enhanced ionic displacement appears in the system, leading to a dramatic increase in the device's properties.
While the property enhancement is predicted to be impressive, it has not been shown experimentally, and there are a couple of other caveats. The researchers artificially lowered the bond strength in the simulation. Further, the only current fabrication method that can reliably produce perfect, oxygen-terminated interfaces is molecular beam epitaxy (MBE). MBE is a fantastic research tool, but it can hardly be considered a production-ready technology and the ability to accurately control the bond strength between the layers is, well, conjecture at best.
That sound you hear in the background is graduate students around the world evacuating their MBE chambers and changing out the ion sources to be the first to demonstrate interface-enhanced capacitors, but I would not hold my breath waiting for one to appear in the next-generation iPhone.
Nature Materials DOI:10.1038/nmat2429