Degradation of Ferroelectric Pb(Zr,Ti)O 3 Under Reducing Conditions
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DEGRADATION OF FERROELECTRIC Pb(Zr,Ti)O3 UNDER REDUCING CONDITIONS Yuichi SHIMAKAWA* and Yoshimi KUBO Fundamental Research Laboratories, NEC Corporation, Tsukuba 305-8501, Japan * [email protected] ABSTRACT The degradation mechanism of Pb(Zr,Ti)O3 (PZT) under reducing conditions was revealed. In contrast to the significant decrease in the sample weight of SrBi2Ta2O9 due to decomposition, the weight of Pb(Zr,Ti)O3 samples change little during H2-annealing at typical process temperatures. Although no apparent changes were detected by thermogravimetric and x-ray diffraction measurements after annealing, the PZT material did actually change. The main mechanism of the degradation is introduction of oxygen defects into the materials. The oxygen defects produce a donor level within the band gap, and light absorption by this donor level accounts for the change in sample appearance from white to black. Different endurance behaviors of materials against reducing conditions were also found: PbZrO3 decomposes through oxygen dissociation more easily than PbTiO3. Oxygen atoms in PbZrO3 are more weakly bonded to Zr than to Ti in PbTiO3 because of a significant anti-bonding component in the hybridized orbitals. INTRODUCTION Ferroelectric PbZr1-xTixO3 (PZT) and SrBi2Ta2O9 (SBT) are leading candidate materials for use in FeRAM applications, and thin-film capacitors based on them have been intensively studied. Recently, however, serious challenges in integrating the oxide-ferroelectric capacitors into a standard LSI have emerged [1-3]. In thin-film capacitors, for example, leakage current significantly increases and ferroelectric polarization severely decreases after the device fabrication process, which usually includes an annealing step at 350-550 °C in a H2-containing reducing atmosphere. Although many papers have reported on the degraded properties of oxide thin-film capacitors [1-5], few studies on the mechanisms of the degradation during fabrication have been done. In our previous papers on SBT ferroelectric oxides, we demonstrated that solid-state chemistry analysis of bulk ceramic samples helps us better understand the fabrication process for FeRAM devices [6,7]. With this approach, we can see material degradation in exaggerated forms; thus, we can investigate such degradation quantitatively. In the present study, we applied a similar solid-state analysis to PZT ferroelectric compounds, and we revealed a possible degradation mechanism of PZT under a reducing atmosphere [8]. Material stability of PZT perovskites against H2-annealing was also investigated and the results were compared with that of SBT. EXPERIMENTAL RESULTS AND DISCUSSION Ceramic powder samples of PZT were prepared by solid-state reaction. Pb(Zr0.48Ti0.52)O3 was synthesized from a mixture of PbTiO3 (PTO) and PbZrO3 (PZO). Changes in the sample during annealing were traced by measuring changes in the sample weight by a thermogravimetric (TG) analysis system. Argon gas containing 4% H2 was used for the reducing conditions, whereas 100% O2 was used for the o
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