Certain titanium-based metal oxides can form a crystal structure known as perovskite that results in a subtle internal imbalance of electric charges. This imbalance gives the material the ability to flip between two 'ferroelectric' states in response to an electric field—a promising recipe for fast, ultra-low-energy storage of digital data.
Masashi Kawasaki and colleagues from the RIKEN Center for Emergent Matter Science, in collaboration with Yusuke Kozuka and co-workers from the University of Tokyo, have now discovered a way to sweep away the stray charges that typically degrade the performance of ferroelectric materials by using an ionic liquid.
Silicon-based field-effect transistors are the basis of modern electronics. In these devices, current flow through the transistor junction is controlled by altering the electronic state of a semiconducting 'gate' between the input 'source' and output 'drain'. Current can only flow in such devices when a voltage is applied to the semiconductor. In contrast, the switchable electronic properties of ferroelectric crystals could make it possible to permanently set the polarization of the channel between source and drain terminals, allowing current to flow in the 'on' state without a constant voltage on the channel. This strategy could prove very useful for non-volatile memory applications and logic circuits.
However, ferroelectric transistors naturally attract charged adsorbents, which counteract the crystal's internal ferroelectric properties. To resolve this issue, the research team switched from a conventional gate oxide to one based on an ionic liquid—a molten organic salt that can store charge by a mechanism known as electric double-layer capacitance (Fig. 1). Applying a voltage or 'bias' to the ionic-liquid gate produced a large accumulation of ions that squeezed out the adsorbed impurities. "This cleaning function under biasing indicates that ionic liquids are an ideal medium for ferroelectric materials," says Kozuka.
The next challenge facing the researchers was to optimize the ferroelectric material, in this case barium titanate. Comparing two devices, one made by growing a crystalline layer of the ferroelectric material as a thin 'epitaxial' film and the other based on a commercial bulk crystal, they found that the thin-film transistor showed surprisingly small resistance in its 'on' state, passing 10,000 times more current than in its 'off' position. The bulk-crystal device, on the other hand, displayed an abrupt increase in resistance due to a structural phase transition, preventing transistor operation. This result, the team notes, verifies that epitaxial films are an effective way to maintain the near-metallic behavior of ferroelectric crystals.
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Ito, M., Matsubara, Y., Kozuka, Y., Takahashi, K. S., Kagawa, F., Ye, J. T., Iwasa, Y., Ueno, K., Tokura, Y. & Kawasaki, M. Electric double layer transistors with ferroelectric BaTiO3 channels. Applied Physics Letters 104, 222101 (2014). DOI: 10.1063/1.4881059