Above Room Temperature Ferroelectricity in Epitaxially Strained KTaO₃

Area of Science:

• Materials Science
• Condensed Matter Physics
• Thin Film Engineering

Background:

• Epitaxial strain is a key method for tuning material properties in thin films and heterostructures.
• Potassium tantalate (KTO) is a cubic perovskite in its bulk form, lacking ferroelectric properties.
• Engineering ferroelectricity in KTO could open new avenues for advanced electronic devices.

Purpose of the Study:

• To investigate the potential of epitaxial strain in inducing ferroelectricity in KTO thin films.
• To demonstrate the tunability of ferroelectric properties like Curie temperature and polarization via strain.
• To characterize the structural and polar nature of strained KTO films.

Main Methods:

• Epitaxial growth of KTO films on strontium titanate (STO) substrates.
• Characterization using scanning transmission electron microscopy (STEM).
• Optical second-harmonic generation (SHG) measurements.
• Fabrication and testing of metal-insulator-metal (MIM) capacitor structures.

Main Results:

• Epitaxial strain (-2.1% in-plane) induced a tetragonal polar phase in KTO films with a transition temperature of 475 K.
• STEM revealed cooperative polar displacements of K atoms relative to Ta atoms at room temperature.
• SHG confirmed a tetragonal polar point group (4mm), indicating a global polar ground state.
• Ferroelectric hysteresis was observed in MIM capacitor structures.

Conclusions:

• Epitaxially strained KTO thin films exhibit a robust intrinsic ferroelectric state.
• The Curie temperature and spontaneous polarization are controllable through epitaxial strain.
• This work establishes KTO as a highly tunable ferroelectric material for potential applications.