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Piezoresponse force microscopy (PFM) technique has been utilized to study the evolution of domain structure with varying Pb(Zr,Ti)O3(PZT) thickness on GaN substrate. Sol-gel PZT films were deposited on the GaN/ sapphire substrate with PZT thickness of 100, 200, and 300 nm. The films exhibit ferroelectric properties that vary as a function of the film thickness. This is explained by the mechanical stress at the PZT/GaN interface. The thicker film (300 nm) is characterized by the presence of a number of oppositely polarized domains and a relatively high value of the effective piezoelectric constant. The laminar domain structure, consisting of 90° and 180° domains, has been revealed in the thinner (200 nm) PZT film. Both films show clear ferroelectric switching behavior, which is in contrast to the thinnest film (100 nm), where no switching has been observed due to mostly in-plane polarization orientation. The observed results indicate the utility of the PFM technique for characterization of the electronic properties of the PZT/GaN heterostructures.