Interfaces in heterostructures have been a key point of interest in condensed-matter physics for decades owing to a plethora of distinctive phenomena—such as rectification 1, the photovoltaic effect 2, the quantum Hall effect 3 and high-temperature superconductivity 4—and their critical roles in present-day technical devices. However, the symmetry modulation at interfaces and the resultant effects have been largely overlooked. Here we show that a built-in electric field that originates from band bending at heterostructure interfaces induces polar symmetry therein that results in emergent functionalities, including piezoelectricity and pyroelectricity, even though the component materials are centrosymmetric. We study classic interfaces—namely, Schottky junctions—formed by noble metal and centrosymmetric semiconductors, including niobium-doped strontium titanium oxide crystals, niobium-doped titanium dioxide crystals, niobium-doped barium strontium titanium oxide ceramics, and silicon. The built-in electric field in the depletion region induces polar structures in the semiconductors and generates substantial piezoelectric and pyroelectric effects. 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. Our study enriches the functionalities of heterostructure interfaces, offering a distinctive approach to realizing energy transduction beyond the conventional limitation imposed by intrinsic symmetry.
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Acknowledgements M.A. acknowledges the Theo Murphy Blue-sky Awards of Royal Society. The work was partly supported by the EPSRC (UK) through grant numbers EP/M022706/1, EP/P031544/1 and EP/P025803/1. Z.M. and J.Z. acknowledge the National Natural Science Foundation of China (11772207); Natural Science Foundation of Hebei Province for Distinguished Young Scholar (A2019210204) and Shenzhen Peacock Team Program (KQTD20170810160424889). We acknowledge the discussion with H. Zhang, A. N. Iqbal and F. Zhuge; and the technical support from M. Crosbie.
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