Adding a Schottky junction to certain crystals can turn them into piezoelectric or pyroelectric crystals, even if they were unaffected by deformation or heat before, according to the the University of Warwick, where the research was done.

UofWarwick-gold_and_crystal-Schottky

The subjects were ‘centrosymmetric’ semiconductors.

• niobium-doped strontium titanium oxide crystals
• niobium-doped titanium dioxide crystals
• niobium-doped barium strontium titanium oxide (BSTO) ceramics
• silicon

“Generally, the properties of these crystals are determined by two factors: the intrinsic properties of the elements that the crystal consists of, and how those elements are arranged to form that crystal, which we call its symmetry,” said Dr Mingmin Yang, who was on the Warwick team, and is now at the Riken Institute in Japan.
“In physics, those materials are rather boring,” said Warwick physicist Professor Marin Alexe. “From the point-of-view of functionality, symmetry is not the greatest thing you want to have. You want to break the symmetry in such a way that you would get new effects. Materials with broken symmetry are rich in functionalities.”

To break the symmetry in this case, noble metal (gold or platinum) Schottky contacts, chosen for their high thermodynamic work function, were deposited onto the material surface. “but copper, silver, gold, iridium or platinum would also be good options”, according to Warwick. Ohmic counter-electrodes were aluminium.

The built-in electric field in the depletion region created by the junction induces polar structures in the semiconductors and generates substantial piezoelectric and pyroelectric effects,” according to the paper ‘Piezoelectric and pyroelectric effects induced by interface polar symmetry’, which describes the work in Nature. In particular, the pyroelectric coefficient and figure of merit of the interface are over one order of magnitude larger than those of conventional bulk polar materials.

Gold-Nb:BSTO was the junction that showed the large pyroelectric. Its 5.3mC/(m2K) room-temperature coefficient compares with 230μC/(m2K) for the conventional ferroelectric crystal lithium tantalate (LiTaO3) used in pyroelectric detectors.

“The observed effects give the technique great potential for use in sensors, which require high sensitivity, or in technologies relying on energy conversion,” according to Warwick. “As a piezoelectric effect, the crystals can harvest energy, or work as an actuator or transducer. With the pyroelectric effect, they can work as a sensor or in infra-red imaging.”

The work is published here: ‘Interface Polar Symmetry Induced Piezoelectric and Pyroelectric Effects’

DOI: 10.1038/s41586-020-2602-4

Part funding came from the Engineering and Physical Sciences Research Council and the Royal Society.

The same team previously looked into breaking symmetry by mechanical means.

Image: Atomic model of a Au-SrTiO3 Schottky interface.