Domain-wall pinning by grain boundaries during electric-field poling of KNbO₃ thin films

We present evidence for grain-boundary pinning of domain walls as controlling the electric field poling behavior of KNbO₃ thin films. Two epitaxial film systems are compared: (I) film on MgO (100) substrate with KNbO₃ (110)// 120A SrTiO₃ (100) //MgO(100) and (II) film on SrTiO₃ (100) substrate with KNbO₃ (110)// SrTiO₃ (100). Quantitative measurement of the area fractions of four domain variants in each film system was made using in-situ second harmonic generation measurement while applying an external electric field at room temperature. While the film system (I) showed reversible domain wall motion, the film system (II) showed some permanent poling after the removal of the external field. Transmission Electron Microscopy revealed a network of low angle grain boundaries of ≈1.5-2° misorientation along the substrate cubic axes in the growth plane of the film system (I) and also a network of 60° and 120° domain walls intersecting these grain boundaries at 45° in the film growth plane. These features were found to be absent in the film system (II). We propose that the observed electric field poling phenomena is due to the pinning of 60° and 120° domain walls by low angle grain boundaries.