Abstract
Ferroelectric random access memory (FeRAM) application has suffered seriously from the degradation of ferroelectric thin films such as that caused by fatigue. Therefore, we have investigated the degradation of Pb(Ti,Zr)O3 (PZT) films by thermally stimulated current (TSC) measurement. The TSC has been utilized for the analysis of defects in bulk semiconductor materials. However, TSC evaluation in ferroelectric films has not been studied sufficiently because of the noise current from films and the fluctuation in the calculated trap activation energy. Therefore, we have developed and optimized a modified TSC measurement method that involves a repetition of heating and cooling cycles. Our modified method has enabled the elimination of the noise current due to pyroelectric and leakage currents and the detection of a clear TSC signal from crystalline defects. The activation energy can also be analyzed through the comparison of TSCs for several specimens subjected to various degradation treatments, such as fatigue and the introduction of lead and oxygen defects. Identical trap levels of (I) 0.75 eV and (II) 0.95 eV were observed for these films, and the latter seems to be related to the degradation since the densities of trap (II) increased in all of the degraded films. It is believed that trap (II) is attributed to Pb defects. Therefore, we propose the following degradation mechanism: polarization reversal induces crystalline defects, and the trap level of a Pb site increases with the presence of the defects. The degradation of ferroelectric properties seems to occur because of the existence of the trap.