Domain populations in lead zirconate titanate thin films of different compositions via piezoresponse force microscopy

Pb (Zr_xTi_1-x)O₃ (PZT) thin films with (111) texture were deposited onto commercially available Pt/Ti/SiO ₂/Si substrates via the sol-gel technique. Piezoforce microscopy (PFM) was then used to analyse the evolution of domain populations as a function of the Zr content x. Domain structures of virgin films, local piezoelectric properties of individual grains and piezoelectric histograms were studied in films with different compositions (x ≤ 0.2-0.6), which cover both the tetragonal and rhombohedral sides of the phase diagram. In films with low Zr content mainly single-domain grains were observed. As the Zr content increased, a larger fraction of polydomain grains was found. The local piezoelectric response measured inside sufficiently big grains indicated that the strongest piezoelectric effect occurs in PZT30/70 (x ≤ 0.3) films. This was attributed to two different effects: high out-of-plane polarization achieved due to the (111) texture and influence of the dielectric constant. In tetragonal films with their lower dielectric constants the electric field seen by a ferroelectric is higher as compared to other compositions, giving rise to an apparent increase of the effective piezoelectric response measured by PFM. The analysis of the domain images indicated that sol-gel derived PZT films are slightly self-polarized near the free surface. With increasing Zr/Ti ratio, the variation of domain populations resulted in reversing the sign of the average piezoelectric response at x≈0.3. It is demonstrated that PFM histograms are extremely sensitive to PZT composition and can be used as a signature of complex domain structures in ferroelectric thin films.