Local switching behavior and electrical polarization of ferroelectric thin films under nanoindentation

The local polarization state and switching behavior of ferroelectric thin films under external mechanical loading were investigated. A nanoindentation technique has been devised to impose local mechanical stress. The results are presented for a 700 nm thick, Pb(Zr0.30Ti0.70)O3 (PZT), film prepared by a sol–gel technique on a platinized Si substrate. A hysteric behavior was found in the local direct piezoelectric response of the ferroelectric thin films within the subcoercive stress range, and an enhanced piezoelectric activity is attributed to the effect of stress-induced domain-wall movement. Upon nanoindentation, voltage shift was observed in the Q–V hysteresis loops along the voltage axis, indicating an asymmetric switching behavior of the local polarization in the loaded films. The parameter of horizontal loop asymmetry increased and the Q–V loop shifted gradually to the positive voltage side as the indentation force increased. The changes in local switching behavior are suggested to result from a variation in residual stress state, asymmetric distribution of charged defects and asymmetric lattice distortion produced by the inhomogeneous indentation stress field.