Influence of Buffer Layers and Excess Pb/Zr+Ti Ratios on Fatigue Characteristics of Sol-Gel-Derived Pb(Zr, Ti)O3 Thin Films

Abstract

Since fatigue of the Pb(Zr, Ti)O3 (PZT) films is closely related to the movement of defects generated due to interdiffusion of atoms through the ferroelectric/electrode interface, studies on the effect of excess Pb content, defined as R= Pb/(Zr+Ti), in the PZT films or insertion of a PZT buffer layer with a large R or of a La-bearing PZT buffer layer between the films and electrode may be effective for diminution of fatigue. Another important aspect is lowering the annealing temperature as much as possible when making the films, so as to reduce interdiffusion. In the present work, annealing temperature for making sol-gel-derived ferroelectric films was fixed at 550° C, and fatigue tests of PZT films with and without buffer layers were carried out. Fatigue tests of the PZT(52/48) films with various values of R have shown that although polarization of the PZT(52/48) film increases with R, fatigue life does not significantly improve due to an increase in fatigue rate with increasing switching cycles. Insertion of the PbTiO3 (PT) buffer layer, 4 nm thick, somewhat amplified the R dependence of ferroelectric properties of the films. Insertion of a 40-nm-thick PZT (120/52/48) buffer layer substantially increased (P* r-P∧ r), the difference between switched and nonswitched polarizations, for the PZT(100-110/52/48) or PZT(110/70/30) film in early stages of the fatigue life profile; however, an enhanced fatigue rate followed and eventually resulted in an insignificant improvement of fatigue life. AES depth profile analysis has revealed that diffusion of excess Pb in the buffer layer towards the surface removes or greatly reduces Pb deficiency which is typically generates in the vicinity of the surface of the films without buffer layers. The resulting reduction of the amount of the defects is thought to have caused substantial increase of (P* r-P∧ r) in early stages of the fatigue life profile. Insertion of double buffer layers of PZT(120/52/48) and PLZT(5/65/35), each 20 nm thick, produced a conspicuous bulge in the polarization fatigue life profile, although fatigue life was not much improved. The origin of the bulge was discussed.