Characterization and aging response of the d31 piezoelectric coefficient of lead zirconate titanate thin films

The wafer flexure technique was used to characterize the $d_{31}$ coefficient of a number of sol–gel and radio frequency (rf) sputtered lead zirconate titanate (PZT) thin films with thicknesses between 0.6 and 3 μm. Typical $d_{31}$ values for well-poled 52/48 sol–gel films were found to be between −50 and −60 pC/N. The rf sputtered films possessed large as-deposited polarizations which produced $d_{31}$ coefficients on the order of −70 pC/N in some unpoled films. The subsequent poling of the material, in a direction parallel to the preferred direction increased the $d_{31}$ coefficient to values of about −85 pC/N. The aging behavior of the $d_{31}$ coefficient was also investigated. For sol–gel films the aging rate was found to be independent of poling direction and to range from 4% per decade for a 2.5 μm film to 8% per decade for a 0.6 μm film. In contrast, the aging rate of sputtered films was strongly dependent on poling direction, with maximum and minimum rates of 26% and 2% per decade recorded. These aging rates are very high in light of the limited twin wall motion in PZT films, and are believed to result from the depolarizing effects of internal electric fields in the rf sputtered films and interfacial defects in the sol–gel films.