Above Room Temperature Ferroelectricity in Epitaxially Strained KTaO3

TL;DR

This work tackles the challenge of inducing ferroelectricity in the cubic quantum paraelectric KTaO3 by applying epitaxial compressive strain to yield a polar tetragonal ground state. The authors combine first-principles DFT and Landau-Ginzburg-Devonshire thermodynamics with high-quality MBE growth of coherently strained KTaO3 thin films and multi-modal characterization (XRD, SHG, STEM, PFM) to reveal a tunable, above-room-temperature ferroelectric state. They demonstrate polar displacements and switchable polarization in KTaO3 thin films, with SHG confirming polar symmetry and PFM showing ferroelectric switching in capacitive structures; the strain-dependence aligns with theoretical phase boundaries for P4mm and Amm2 polar phases. The findings establish KTaO3 as a robust, tunable ferroelectric platform in thin-film form, enabling exploration of strain-modulated oxide interfaces and potential device applications in ferroelectric–superconducting and spintronic systems.

Abstract

Epitaxial strain is a powerful means to engineer emergent phenomena in thin films and heterostructures. Here, we demonstrate that KTaO3, a cubic perovskite in bulk form, can be epitaxially strained into a highly tunable ferroelectric. KTaO3 films grown commensurate to SrTiO3 (001) substrates experience an in-plane strain of -2.1 % that transforms the cubic structure into a tetragonal polar phase with transition temperature of 475 K, consistent with our thermodynamic calculations. We show that the Curie temperature and the spontaneous electric polarization can be system- atically 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 (4mm), indicating the emergence of a global polar ground state. We observe a ferroelectric hysteresis response, using metal-insulator-metal capacitor test structures. The results demon- strate a robust intrinsic ferroelectric state in epitaxially strained KTaO3 thin films.

Figures (5)

• Figure 1: Thermodynamic and DFT investigation of the epitaxial strain control of ferroelectricity in KTaO3. a Expected shift in $T_c$ of (001) KTaO3, under epitaxial strain. The experimental values of $T_c$ are from second-harmonic generation (SHG) experiments (Fig. 4 and Fig. S11). The components of the spontaneous polarization are given with respect to the pseudocubic axes of KTaO3. b Lowest phonon frequencies of the structures with the $P4/mmm$, $P4mm$, and $Amm2$ symmetries as a function of epitaxial strain, with imaginary phonon frequencies presented as negative. Tetragonal structure with the P4/mmm symmetry starts to have imaginary phonon frequency beyond 0.5% compressive (tensile) strain with the $A_{2u}$ ($E_u$) symmetry. Freezing in this phonon mode leads to a ferroelectric phase transition to a structure with the $P4mm$ ($Amm2$) symmetry.
• Figure 2: X-Ray diffraction of a 9.3 nm thick film of KTaO3 grown on SrTiO3 (001).a$\theta$-2$\theta$ scan showing the 001 reflection of KTaO3 and SrTiO3. Symmetric Laue fringes indicate a well-defined film thickness, indicative of an abrupt interface between film and substrate (asterisks * denotes substrate reflections). b X-ray reciprocal space mapping of the same 9.3 nm thick KTaO3 film, confirming that the film is commensurately strained to the SrTiO3 substrate. c Overlaid rocking curves of the 001 KTaO3 and SrTiO3 peaks, showing comparable FWHMs, indicating low out-of-plane mosaicity ($\Delta\omega \approx$ 0.008°). d Measured out-of-plane lattice constant of KTaO3 thin films on various substrates against the expected lattice constants from thermodynamic analysis of the paraelectric (blue) and the ferroelectric (orange) ground states. The range in the calculated out-of-plane lattice constant is due to the range in the reported sets of coefficients (Table S2-3).
• Figure 3: Probing the polar distortions at the atomic scale in epitaxially strained KTaO3. a HAADF-STEM image of the KTaO3 film grown on SrTiO3(001). b The interface structure is uniform throughout the sample, indicating a coherent strain between the film and the substrate. No extended defects are observed, which suggests high-quality epitaxial growth. c Displacement vector of the difference between the centeroid of four tantalum columns and potassium columns overlaid with HAADF-STEM image. d Tracing of the magnitude and direction of the displacements of atomic columns. e Polar histogram, based on more then 20,000 analyzed columns, shows the overall direction of displacements for the KTaO3 thin film grown on a SrTiO3 substrate. f Histogram of the magnitude of the polar displacements in potassium columns, based on the 20,000 columns analyzed.
• Figure 4: Emergence of the polar P4mm space group in epitaxially strained KTaO3. a Schematic of SHG setup in reflection geometry. b Polar plots of SHG intensity (radius) versus fundamental polarization (azimuth). c Normal and oblique incidence SHG intensity with temperature (10-550 K), shown for the 9.3 nm thick KTaO3 film grwon on SrTiO3 and a 18 nm thick KTaO3 film grown on DyScO3.
• Figure 5: Switching of spontaneous electric polarization in epitaxially strained KTaO3. a Thermodynamic calculation of the polarization as a function of epitaxial strain and temperature. b Schematic of the epitaxial heterostructure used for PFM measurements.c PFM switching using the KTaO3/SrRuO3/SrTiO3 sample showing a upward build-in field. d Magnitude of the piezo response.