The piezoelectric properties and polarity distribution of preferred c-axis-oriented ZnO films have been investigated using piezoresponse force microscopy (PFM). Simultaneous imaging of the morphology, as well as the magnitude and phase of the out-of-plane and in-plane piezoresponse, was performed by PFM on the ZnO films. The out-of-plane piezoresponse with two primary antiparallel spontaneous polarization orientations (upward and downward), which is perpendicular to the surface of the film, has been found, and the spontaneous polarization orientation of most grains is upward. Meanwhile, the in-plane piezoresponse has also been detected in some of the grains. In addition, the polarization orientation of the individual grains is discussed and determined by analyzing phase images of the out-of-plane and in-plane piezoresponse.

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