Phase stability of ferroelectric KNO3 switching devices during polarization aging

Abstract

The persistent observation of polarization aging during repeated room-temperature switching of metastable thin-film ferroelectric potassium nitrate capacitors has led to the conjecture that the observed polarization decay is a direct consequence of a concomitant phase III to phase II structural transformation. Appropriate large area thin-film devices have been fabricated to perform this x-ray diffraction study of the phase stability of ferroelectric KNO3 devices during in situ electrical switching. As the initial remanent polarization of 7.5 μC/cm2 decreases to <0.4 μC/cm2 in <2×106 polarization reversals, the volume fraction of III-KNO3 remains constant, and no reflections from II-KNO3 appear. After complete polarization loss, however, the initial d spacings measured for III-KNO3 have increased an average 0.4%. Examination of the line shapes during aging reveals that the observed shift is not a continuous transition, but a convolution of reflections centered at the initial and final d spacings. This characteristic modification to the original cell, here designated III′-KNO3, occurs throughout the switched volume. The simultaneous loss of polarization and appearance of the expanded unit cell suggest that III′-KNO3 either pins the polarization or is itself nonferroelectric.