Electric field induced domain rearrangement in potassium niobate thin films studied by in situ second harmonic generation measurements

We report a sensitive technique to study domain rearrangement in ferroelectric thin films using in situ second harmonic generation (SHG) measurements. The following film system on a MgO substrate was studied: KNbO₃(110)∥120 Å SrTiO₃(100)∥MgO(100). From x-ray diffraction, four possible domain variants in the film growth plane were determined to be KNbO₃[110]∥SrTiO₃ and MgO[001], [001], [010], or [010] denoted as variants X+, X-, Y+ and Y-, with area fractions A^X+, A^X-, A^Y+ and A^Y-, respectively, in the film growth plane. The SHG signal (532 nm) from the films generated by a transmitted fundamental beam of 1064 nm was measured and theoretically correlated to the area fractions of different domain variants in the film. The magnitude and signs of net area fractions (A^X+-A^X-) and (A^Y+-A^Y-) were determined as a function of external field applied along the MgO〈001〉 direction in the film growth plane. No remnant poling was observed in KNbO₃ films after application and removal of external fields up to 10 kV/cm at room temperature. We propose mechanisms to correlate this behavior to the pinning of domain wall motion by low angle grain boundaries in KNbO₃.