Above Room Temperature Ferroelectricity in Epitaxially Strained KTaO3

Technical Summary: Above Room Temperature Ferroelectricity in Epitaxially Strained KTaO3

Problem Statement
Potassium Tantalate (KTaO3) is a well-known quantum paraelectric material that remains in a cubic, non-polar ground state down to low temperatures in its bulk form. While theoretical predictions and prior studies on related materials like SrTiO3 suggest that epitaxial strain can induce ferroelectricity, high-quality epitaxial thin films of KTaO3 have historically been difficult to synthesize. This lack of high-quality films has hindered the experimental verification of theoretical predictions regarding strain-induced ferroelectricity and the exploration of KTaO3 as a platform for spintronics and superconductivity applications where switchable polarization is required. The central challenge addressed is whether epitaxial compressive strain can stabilize a ferroelectric phase in KTaO3 at temperatures above room temperature and if this induced polarization is intrinsic and switchable.

Methodology
The authors employed a multi-faceted approach combining theoretical modeling, advanced thin-film synthesis, and comprehensive structural and functional characterization:

• Theoretical Modeling: First-principles Density Functional Theory (DFT) calculations (using PBEsol functional) and thermodynamic Landau-Ginzburg-Devonshire (LGD) modeling were used to predict the phase stability of KTaO3 under various in-plane compressive strains (-0.5% to -2.1%). These models calculated transition temperatures ($T_c$) and phonon instabilities to determine the critical strain required for a polar ground state.
• Film Synthesis: High-quality KTaO3 thin films were grown using Molecular Beam Epitaxy (MBE) under adsorption-controlled conditions (K:Ta flux ratio ~10:1) on three different substrates to impose varying levels of compressive strain: SrTiO3 (001) (-2.1% strain), DyScO3 (110)o (-0.9% strain), and GdScO3 (110)o (-0.5% strain).
• Structural Characterization: X-ray diffraction (XRD), reciprocal space mapping (RSM), and rocking curves were used to assess crystalline quality, phase purity, and strain state. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was utilized to visualize atomic-scale polar displacements of potassium columns relative to tantalum columns.
• Functional Characterization:
  • Optical: Rotational anisotropy Second-Harmonic Generation (SHG) was performed to detect the breaking of inversion symmetry and determine the point group symmetry across a temperature range (4 K to 550 K).
  • Electrical: Piezoresponse Force Microscopy (PFM) was conducted on metal-insulator-metal (MIM) heterostructures (KTaO3/SrRuO3/SrTiO3) to observe ferroelectric switching and hysteresis loops, bypassing leakage current issues common in ultra-thin films.

Key Results

• Strain-Induced Phase Transition: Theoretical calculations predicted that compressive strain would drive KTaO3 from a cubic paraelectric phase to a tetragonal ferroelectric phase ($P4mm$). DFT results indicated a critical compressive strain threshold (around -0.5% to -1.0%) for this transition.
• Structural Evidence: XRD and RSM confirmed that KTaO3 films grown on SrTiO3 were coherently strained. The measured out-of-plane lattice constant (4.063 Å) for the SrTiO3 sample significantly exceeded the theoretical value for a paraelectric phase, aligning with the predicted ferroelectric contribution. HAADF-STEM revealed cooperative polar displacements of potassium columns relative to tantalum columns, with a preferential direction along the [001] axis. The average displacement magnitude was measured at 54.08 ± 19.75 pm.
• Ferroelectric Transition Temperature: SHG measurements confirmed the emergence of a polar phase. For the -2.1% strained KTaO3/SrTiO3 film, the Curie temperature ($T_c$) was observed at 475 K, well above room temperature. The -0.9% strained KTaO3/DyScO3 film showed a polar phase below 363 K. In contrast, the -0.5% strained KTaO3/GdScO3 film showed no SHG signal indicative of a polar phase down to 4 K, suggesting a strain threshold exists for stabilizing the ferroelectric state.
• Switchability: PFM measurements on KTaO3/SrRuO3/SrTiO3 capacitors demonstrated clear 180° phase reversal and butterfly-shaped amplitude curves, confirming that the induced spontaneous polarization is switchable under an applied electric field. The coercive voltage was measured at ±5.7 V.
• Quantitative Consistency: The room-temperature spontaneous polarization ($P_s$) estimated from HAADF-STEM atomic displacements ($18 \pm 6$ $\mu$C cm$^{-2}$) was consistent with phase-field simulation predictions ($\sim 19$ $\mu$C cm$^{-2}$) and the thermodynamic model.

Significance and Claims
The paper claims to demonstrate the epitaxial stabilization of a ferroelectric tetragonal polymorph in KTaO3, a material that is paraelectric in its bulk form. The primary significance lies in the experimental realization of a robust, intrinsic ferroelectric state in KTaO3 thin films with a transition temperature (475 K) significantly above room temperature.

The authors assert that this work validates the theoretical prediction that epitaxial strain can be used as a "tuning knob" to induce ferroelectricity in quantum paraelectrics. By establishing a switchable polarization in KTaO3, the study provides a new platform for investigating the interplay between ferroelectricity and other phenomena known to exist at KTaO3 interfaces, such as the two-dimensional electron gas (2DEG) and superconductivity. The results suggest that the coupling between strain and ferroelectricity in KTaO3 is highly efficient, offering a pathway to engineer functional properties for potential device applications in memory and spintronics, provided the material quality and strain control are maintained. The study emphasizes that the observation of ferroelectricity is contingent upon achieving a critical level of compressive strain, as evidenced by the lack of a polar phase in the less strained GdScO3 samples.