Influence of graded doping on the long-term reliability of Nb-doped lead zirconate titanate films

Abstract

Niobium doped lead zirconate titanate (PNZT) films show potential in piezoelectric microelectromechanical systems (piezoMEMS); their long-term reliability is important for successful application. This work describes how variations in the doping profile through the film depth can significantly increase the long-term reliability of PNZT films, while retaining excellent piezoelectric properties. Chemical solution derived {100} textured PNZT films were prepared with a Mn-doped lead zirconate titanate (PMZT) layer at the film top or bottom surface (PNZT + x mol% PMZT, or x mol% PMZT + PNZT). No significant differences were found in the baseline dielectric, ferroelectric and piezoelectric properties of the films. As the Mn-doping concentration increased, the medium time to failure (t50) of PNZT + x mol% PMZT and x mol% PMZT + PNZT films gradually increased from 14.4 h to 93.9 hours, and from 27.7 to 94.6 hours, respectively, under aggressive test conditions (200°C, 300 kV/cm) where the Mn doped layer served as the cathode. Current – voltage measurement on pristine PNZT + 2 mol% PMZT films showed Schottky emission-controlled conduction. After a 23 hours degradation period at 200°C, 300 kV/cm (field up), Poole-Frenkel conduction became dominant. The trap energy level increased from 0.45 ± 0.01 eV for PNZT to 1.27 ± 0.03 eV for the PNZT + 2 mol% PMZT film. Furthermore, Ti4+ reduction was found in degraded PNZT near the top electrode, but not in degraded PNZT + 2 mol% PMZT, as recorded by electron energy loss spectroscopy data. Electron trapping by Mn ions showed strong contributions to suppression of the conduction and improvement of long-term reliability for the PNZT film with a PMZT (top or bottom) layer.