Identifying intrinsic ferroelectricity of thin film with piezoresponse force microscopy

Piezoresponse force microscopy (PFM) is a powerful technique to characterize ferroelectric thin films by measuring the dynamic electromechanical response. The ferroelectricity is commonly demonstrated by the PFM hysteresis loops and a 180o phase difference of PFM images before and after poling. Such ferroelectric-like behaviors, however, recently are also found in many non-ferroelectrics. Consequently, it is still a challenge to identify intrinsic ferroelectricity in various kinds of thin films. Here, using PFM, we systematically studied the electromechanical responses in ferroelectric thin films with fast (BaTiO3) and slow (PVDF) switch dynamics, and also in the non-ferroelectric (Al2O3) thin films. It is found that both of the ac voltage (Vac) and pulsed dc voltage (Vdc) play an important role in the PFM measurement. When the Vac amplitude is higher than a explicit threshold voltage (Vc), collapse of the PFM hysteresis loops is observed for the films with fast switch dynamics. By measuring PFM hysteresis loops at various Vdc frequencies, an explicit Vc could be found in ferroelectric rather than in non-ferroelectric. The existence of an explicit Vc as well as nonvolatile behavior is proposed as an important approach to unambiguously identify intrinsic ferroelectricity in materials regardless of switch dynamics.