Fatigue-free La-modified Pb(Zr, Ti)O 3 capacitors using aseed layer
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The development of lead zirconate titanate (PZT)-based capacitors using common Pt electrodes has been a long-time goal of ferroelectric random access memories (FRAM). In this work, a series of Pb1−xLax(Zr0.55Ti0.45)O3 capacitors (for 0.01 艋 x 艋 0.05) having fatigue-free characteristics have been grown on Pt/ Ti/SiO2/Si substrates. Typically 2–3 mol% La-modified PZT capacitors fabricated at 580 °C by applying a PZT seed layer exhibited fatigue-free behavior up to 6.5 × 1010 switching cycles, a low coercive field of 50–55 kV/cm, and a stable charge retention profile with time, all of which assure their suitability for the future nonvolatile FRAM.
There is a great need to find suitable capacitors to meet the present trend in nonvolatile ferroelectric random access memories (FRAM) toward lower operating voltages.1,2 In this challenging search to find suitable materials, once again, we consider lead zirconate titanate (PZT)-based perovskites as potential candidates for FRAM1–4 and microelectromechanical systems5 applications because of their high Pr values, easy switching characteristics, and dramatic domain refinement up to a nanometer size, in addition to relatively low processing temperatures and low costs. However, some serious problems related to the reliability must be overcome before the practical implementations. These difficulties are mainly arising from the poor fatigue resistance of PZT after a certain number of switching cycles. To solve the problems associated with the electrical fatigue without using electrically leaky and expensive oxide electrodes,6 –9 in this study we have focused on the intrinsic oxygen vacancies, which are known to play a key role in the ferroelectric degradation of capacitors in conjugation with film/electrode interface and domain mobility.10,11 The concentration of oxygen vacancies can be substantially reduced by the Pb-site vacancies (VPb⬙) under the condition of donor-doping. This can be predicted by considering the following intrinsic defect reaction in Pb-based perovskites: PbO ⳱ VPb⬙ + VO¨ with Kint ⳱ [VPb⬙][VO¨]. However, Kint increases rapidly with temperature. Therefore, one should reduce the processing temperature to suppress the formation of oxygen vacancies even under the donor-doping typically using
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http://journals.cambridge.org
J. Mater. Res., Vol. 17, No. 8, Aug 2002
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lanthanum (La). The concentrations of oxygen vacancies that are primarily responsible for the electrical fatigue can further be reduced by annealing La-modified PZT films in an oxygen-rich atmosphere. Therefore, one of the efficient ways of suppressing the formation of oxygen vacancies could be low-temperature processing in an oxygen-rich atmosphere. However, the processing temperature of La-doped PZT (PLZT) is known to be substantially higher (艌700 °C) than that of undoped PZT film (艋600 °C).12 In view of these requirements, one of the feasible approaches is to insert a suitable thin seed layer
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