Role of Pb excess in the crystallization of lead zirconate titanate films derived via sol-gel processing
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Sol-gel spin coating of lead-titanate films differs from most processing routes, such as metalorganic chemical vapor deposition and pulsed laser deposition, in that crystallization cannot occur without a postdeposition annealing step. This work focuses on the annealing of sol-gel-derived PbZrTiO3 films on LaAlO3 substrates in attempts to identify the precise conditions necessary to grow films of quality similar to that obtained through other techniques. In particular, the effects of Pb excess (in precursor solutions), annealing times, and temperature were investigated through transmission electron microscopy and four-circle x-ray diffraction. The significance of this work is in the direct observation of the correlation between Pb excess and film crystallization. It is shown that the effects of Pb excess on the completeness of film crystallization become more dramatic at lower annealing temperatures, even while epitaxial quality is maintained.
I. INTRODUCTION
Sol-gel spin coating has been employed quite extensively as a technique for fabricating ferroelectric thin films for various device applications. Researchers commonly cite two fundamental advantages in using this method over physical vapor deposition techniques such as sputtering and pulsed laser ablation. These include the general ease of processing and good control of film stoichiometry. The former can be attributed to the fact that vacuum chambers are not required and therefore make the sol-gel technique relatively simple and inexpensive. Control of film stoichiometry can be achieved by simply preparing the precursor (starting) solutions with the desired proportions of constituent species. There has been much effort by researchers to use the sol-gel spin coating technique to consistently grow epitaxial films based on the solid solution Pb–Zr–Ti–O (PZT). Due to the large remnant polarization in this material system, it is an ideal candidate for ferroelectric, pyroelectric, and piezoelectric device applications.1,2 It is well known that the ferroelectric domain structure and epitaxial quality of the film are crucial factors for exploiting the anisotropic properties specifically because the direction of polarization is along the crystallographic c axis. As such, much effort has been focused on understanding the crystallization mechanism during annealing of sol-gel-derived PZT films, with the goal of being able to consistently grow epitaxial films on different substrates. Several authors have reported the existence of metastable fluorite phases (pyrochlore)3–7 during J. Mater. Res., Vol. 18, No. 6, Jun 2003
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post deposition crystallization, depending on the substrate being used. On platinized silicon substrates for example,5 the existence of a transient intermetallic Pt3Pb phase was revealed, and it was proposed that this phase acts as a template for the growth of (111) oriented PZT. On perovskite LaAlO3 (LAO) substrates, Kim et al. reported that metastable flourite grains form at low temperatures (approx
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