Microstructure and electrical properties of chemical solution deposition (Pb,La)(Zr,Ti)O 3 thin films on Pt electrodes
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Microstructure and electrical properties of chemical solution deposition (Pb,La)(Zr,Ti)O3 thin films on Pt electrodes Jeffrey S. Cross,a) Mitsushi Fujiki, Mineharu Tsukada, Yasutoshi Kotaka, and Yasuyuki Goto Fujitsu Laboratories, Ltd., 10-1 Morinosato-wakamiya, Atsugi 243-0197, Japan (Received 9 April 1999; accepted 2 August 1999)
(Pb,La)(Zr,Ti)O3 (PLZT) films with thicknesses of 150 and 225 nm were prepared by the chemical solution deposition method on sputtered Pt/IrO2 coated on SiO2/Si wafers. The annealed films revealed two different microstructures: fined-grained and large-grained. The thinner film had the largest grain size and highest leakage current, whereas the thicker film had small grains and lower leakage. Atomic force microscope images showed that the thinner film had half-domed-shaped grains, which were about one-third thinner at the grain boundary triple points. These triple points also contained a nanocrystalline nonstoichiometric secondary phase, which contributed to high leakage. A model was developed showing differences in crystallization on the basis of grain growth and number of nuclei on the Pt surface. These results indicate the importance of controlling the film microstructure and its relationship to the film electrical properties.
I. INTRODUCTION
Chemical solution deposition (CSD) has become a popular method for preparing ferroelectric materials such as Pb(Zr,Ti)O3 (PZT) because of its simplicity and high reliability. Over the past several years a number of results have appeared in the literature modeling the crystallization process of PZT.1–8 Since PZT has applications in ferroelectric random-access memory (FeRAM), modeling the crystallization process is extremely important because it impacts ferroelectric properties, surface morphology, and secondary phase formation. As ferroelectric films become thinner due to requirements for lower operating voltages 艋3 V, the interfacial layer9 and grain boundaries play a larger role on the ferroelectric film properties. In general, CSD solutions for PZT films contain excess Pb in order to enhance the perovskite crystallization as well as to provide a Pb ambient environment above the film during the annealing process.10 Models by Aoki and Lefevre have proposed that excess Pb in the CSD solution is pushed out in front of the PZT grain during crystallization because excess Pb cannot be accommodated in the PZT perovskite lattice. Both of these CSD crystallization models for PZT films were developed for PZT films with thicknesses of 300–400 nm. However it is no longer clear if these models developed for thicker PZT films apply to thinner films, 400 °C.5 In that case, large perovskite grains were surrounded by a secondary phase but the films did not exhibit half-domeshaped grains. One possible reason for the inhibited heterogeneous nucleation in the case of the 150-nm-thick film has been attributed to excess oxygenation5,7 of the as-pyrolyzed film increasing the Pb oxidation state from Pb2+ to Pb4+. Excess ox
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