Above Room Temperature Ferroelectricity in Epitaxially Strained KTaO₃.

Abstract

Epitaxial strain is a powerful means to engineer emergent phenomena in thin films and heterostructures. Here, we demonstrate that <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>KTaO</mi> <mn>3</mn></msub> </math> , a cubic perovskite in bulk form, can be epitaxially strained into a highly tunable ferroelectric. <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>KTaO</mi> <mn>3</mn></msub> </math> films grown commensurate to <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>SrTiO</mi> <mn>3</mn></msub> </math> (001) substrates experience an in-plane strain of -2.1 % that transforms the cubic structure into a tetragonal polar phase with a transition temperature of <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mn>475</mn> <mspace></mspace> <mi>K</mi></mrow> </math> , consistent with our thermodynamic calculations. We show that the Curie temperature and the spontaneous electric polarization can be systematically controlled with epitaxial strain. Scanning transmission electron microscopy reveals cooperative polar displacements of the potassium columns with respect to the neighboring tantalum columns at room temperature. Optical second-harmonic generation results are described by a tetragonal polar point group ( <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mn>4</mn> <mi>m</mi> <mi>m</mi></mrow> </math> ), indicating the emergence of a global polar ground state. We observe a ferroelectric hysteresis response using metal-insulator-metal capacitor test structures. The results demonstrate a robust intrinsic ferroelectric state in epitaxially strained <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>KTaO</mi> <mn>3</mn></msub> </math> thin films.