Effects of Rare Earth Incorporation on the Ferroelectric and Dielectric Properties of Sol-Gel Derived PbTiO 3 Films
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ABSTRACT Ferroelectric (FE) films, especially PZT films, have received increasing attention for microelectronics applications such as FE memory and in high density DRAM's. While rare earth doped PbTiO 3 ceramics has been studied for SAW and piezoelectric applications, rare earthdoped films seldom have been systematically explored. A series of sol-gel derived PbTiO 3 films with varying amounts (5-15 mole %) of rare earths (such as, Nd, Sm, Tb, Dy, Er ,Yb and La ) have been prepared using acetates and alkoxides as precursors. The solutions were spin coated onto platinized Si wafers. The effects of the type and amount of rare incorporation on the phase assembly and microstructure have been quantified. The results of dielectric characterization (e.g., dielectric constant, dissipation factor and leakage currents) and FE behaviors (viz.remanent polarization, and coercive field) are presented; these films exhibited low leakage currents ( 3E- 10 A/cm 2 ) and much higher dielectric constant ( up to 525) compared to undoped PbTiO 3 films. INTRODUCTION Lead zirconate titanate (PZT) is a widely studied ferroelectric (FE) material utilized for its FE, dielectric, electro-optic, acousto-optic, pyroelectric and piezoelectric properties in both bulk and thin film forms [1]. Recently, PZT films have attracted considerable attention for applications in non-volatile ferroelectric memory since such films exhibit switchable polarization, fast switching times and excellent fatigue and retention behaviors [e.g., 2]. Most of the attention has been focused on the morphotropic phase boundary (MPB) composition, namely PZT 53/47 ( PbZro.53Ti0.470 3). The numerous techniques used to prepare PZT films include magnetron sputtering, diode sputtering, e-beam evaporation, laser ablation, MOCVD, and sol-gel methods [e.g., 3-4]. PZT ceramics have been extensively and systematically doped with various dopants to tailor their material properties [1]. Such non-isovalent dopants can be classified into two classes, namely donors (giving rise to A and B site vacancies) and acceptors (yielding 0 site vacancies). The A vacancy additives tended to yield PZT with increased dielectric constant ( Er ), dielectric loss, piezoelectric coupling factor and bulk resistivity but lower mechanical quality factor, aging rates and coercive field, Ec. On the other hand, 0-vacancy additives resulted in PZT exhibiting lower dielectric constant, dielectric loss and bulk resistivity but higher Ec and mechanical quality factor. These effects arose from enhanced or inhibited domain wall mobilities in donor or acceptor dopants, respectively. One of the most important dopants in PZT consists of La to form PLZT (lead lanthanium zirconate titanate) and PLT (lead lanthanium titanate) compositions. The characteristics of sputtered and sol-gel PLT films have been reported [e.g., 5,6]. PLT is paraelectric at La content > 28 mole% at room temperature; and Fr increases dramatically with increasing La content since 291 Mat. Res. Soc. Symp. Proc. Vol. 346. o1994 Materials Research Society
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