Strain engineering of KTaO₃ grown by Suboxide Molecular-Beam Epitaxy

Strain-engineering is a powerful means to tune the polar, structural, and electronic instabilities of ferroelectrics. KTaO₃ is an incipient ferroelectric, with a very large spin-orbit coupling, in which highly anisotropic superconductivity emerges near a polar instability in electron doped samples[1, 2]. Growth of high-quality epitaxial films provides an opportunity to use epitaxial strain to finely tune electronic and polar instabilities in KTaO₃. Using a molecular beam of the suboxides TaO₂ emanating from effusion cells containing Ta₂O₅ combination with a molecular beam of potassium emanating from an indium-potassium intermetallic in an oxidant (~10% O₃ + 90% O₂) background pressure of 1x10^–6 Torr, KTaO₃ films are grown under conditions of excess potassium in an absorption-controlled regime. Biaxial strains ranging from to are imposed on the commensurately strained KTaO₃ films by growing them upon GdScO₃, TbScO₃, DyScO₃ and SrTiO₃ substrates, all with the perovskite structure. To probe dielectric and ferroelectric properties, KTaO₃ films were grown on conductive substrates i.e., Nb-doped SrTiO₃ as well as metal-insulator-metal capacitors made with symmetric perovskite electrodes. Reciprocal-space mapping shows the epitaxial thin films are coherently strained to the underlying perovskite substrates provided the films are sufficiently thin. Cross-sectional scanning transmission electron microscopy does not show any extended defects and confirms that the films have an atomically abrupt interface with the substrate. X-ray diffraction rocking curves (full width at half maximum < 30 arc sec on all the above substrates) are the narrowest reported to date for KTaO₃ films grown by any technique. Temperature- and polarization-dependent second harmonic generation measurements show evidence of strain-dependent broken inversion symmetry leading to polar order in KTaO₃ thin films. SIMS measurements confirm that the films are free of indium contamination.

References

[1] K. Ueno, S. Nakamura, H. Shimotani, H. T. Yuan, N. Kimura, T. Nojima, H. Aoki, Y. Iwasa, and M. Kawasaki, Nat. Nanotechnol. 6, 408 (2011).

[2] F. Y. Bruno, S. M. Walker, S. Riccò, A. d. l. Torre, Z. Wang, A. Tamai, T. K. Kim, M. Hoesch, M. S. Bahramy, and F. Baumberger. Adv. Electron. Mater. 5, 1800860 (2019).