Microstructural development in sol-gel derived lead zirconate titanate thin films: The role of precursor stoichiometry a
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Microstructural development in sol-gel derived lead zirconate titanate thin films: The role of precursor stoichiometry and processing environment M. J. Lefevre and J. S. Speck Materials Department, University of California, Santa Barbara, Santa Barbara, California 93106-5050
R. W. Schwartz, D. Dimos, and S. J. Lockwood Sandia National Laboratories, Albuquerque, New Mexico 87185 (Received 21 November 1995; accepted 21 March 1996)
The role of precursor stoichiometry and local firing environment on the microstructural development of sol-gel derived lead zirconate titanate (PZT) thin films was investigated. Typically, excess Pb is added to films to compensate for PbO volatilization during heat treatment. Here, it is shown that the use of stoichiometric precursors with either a PbO atmosphere powder or a PbO overcoat during the crystallization heat treatment is an attractive and viable alternative method for control of film stoichiometry. Using these approaches, we have fabricated single phase perovskite thin films with microstructures and electrical properties (Pr , 36 mCycm2 and Ec , 45 kV ycm) comparable to those of films using optimized solution chemistries and excess Pb additions. The potential advantage of increasing PbO partial pressure, or activity, during firing versus excess Pb additions is discussed from the standpoint of a proposed crystallization scenario based on the kinetic competition between Pb loss and the nucleation and growth rates of the perovskite phase.
I. INTRODUCTION
Ferroelectric thin films have recently received considerable attention because of their potential use in a range of device applications, including decoupling capacitors, volatile and nonvolatile memories, optical data storage, and other electro-optic applications (e.g., waveguides, switches, and modulators).1 The Pbbased perovskites, such as lead zirconate titanate [PZT; Pb(Zrx Ti12x )O3 ], have many properties that make them attractive for such applications. Since many of these applications require integration of the ferroelectric film with semiconductor devices, solutionderived ferroelectric thin films are of particular interest because of their ease of fabrication, excellent control of stoichiometry, low crystallization temperatures, and compatibility with existing integrated circuit fabrication technology. Typically, sol-gel derived PZT films first crystallize upon heating to a fluorite structure at intermediate temperatures before transforming to the perovskite structure, which is the useful phase for electrical applications.2 It has been suggested by others that this transitory fluorite phase can be avoided by rapid thermal processing, which essentially bypasses the temperature range where it is kinetically stable.3 In fact, the formation of this transitory fluorite can be advantageous because it reduces the driving force for the subsequent transformation to perovskite, thereby promoting heterogeneous nucleation of this phase at the bottom electrode. Therefore, any 2076
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