Leakage current characteristics and DC resistance degradation mechanisms in Nb doped PZT films

Abstract

The correlation between defect chemistry, leakage currents, and time-dependent dielectric breakdown was studied for PbZr0.52Ti0.48O3 (PZT) films doped with 0.5, 1, 2, or 4 mol. % Nb. As the samples are nearly intrinsic (that is, close to n- to p-type transition), signatures for both hole hopping between Pb2+ and Pb3+ and electron trapping by Ti4+ were observed. For all doping levels, the dominant conduction mechanism transitioned from Poole-Frenkel emission at lower electric fields to Schottky emission at higher electric fields. The electric field for this transition diminishes from 172 to 82 kV/cm with decreasing Nb concentration. The concomitant modification of the Schottky barrier height from 1.24 to 0.95 eV with decreasing Nb concentration is attributed to Fermi level pinning via oxygen vacancies. The DC resistance degradation was controlled by Schottky emission from 250 to 400 kV/cm. It was found that the lifetime of the films increases with increasing the Nb level. The effective Schottky barrier height for 2 mol. % Nb-doped PZT films decreased from 1.12 to 0.85 eV during degradation. This is related to the movement of oxygen vacancies toward the cathode and the observation of Ti3+ near the cathode, which are proved via thermally stimulated depolarization current and electron energy loss spectroscopy, respectively. Furthermore, Schottky emission starts to control the conduction at lower electric fields after degradation as a result of oxygen vacancy accumulation near the cathode. This, in turn, decreases the potential barrier height for electron injection from the Pt electrode into the PZT films. The mechanisms for time-dependent dielectric breakdown in PZT films will thus be a strong function of the initial oxygen vacancy concentration and its distribution within the PZT films.