Stress stabilization of a new ferroelectric phase incorporated into SrTaO₂N thin films

Microstructural analyses of highly stressed SrTaO₂N thin films deposited on SrTiO₃ substrates by cathodoluminescence spectroscopy revealed coexistence of ferroelectric and relaxor-ferroelectric-like phases in the films. These two phases are, respectively, associated with "trans-type" and "cis-type" anion orders, as supported by the relative difference of the band gap energies calculated by first principles calculations based on the density functional theory. The formation of the new ferroelectric phase is considered to occur upon stabilization by the high compressive residual stress stored into the film structure, with the length/size of the "trans-type" region strongly depending upon the local stress state in the film.