Effect of Pb Excess Content on Microstructure and Electrical Properties of Sol Gel Derived PZT Thin Films
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micro-electromechanical system (MEMS) [1-5]. Many fabrication techniques, such as sputtering, laser ablation, metallo-organic decomposition and the sol-gel process, have been used to fabricate ferroelectric PZT thin films. The sol-gel process results in films with high purity, large deposition area and easy composition control, and is one of the most promising techniques for ferroelectric thin film fabrication. In our previous work [5], we reported that fabrication of a PZT thin film by the three-step heat-treatment process resulted in good electrical properties in spite of relatively thick film deposition. However, we did not examine the optimum amount of excess Pb in that work. In most studies on PZT thin films using sol-gel or sputtering techniques, excess Pb is added. It is well known that the use of excess Pb in the starting solution compensates for the loss of Pb at the surface, and prevents the formation of a continuous layer of Pb-deficient crystal [6]. Tuttle et al. [7] found that excess Pb of up to 5 mol% increased the volume fraction of the perovskite phase and improved the ferroelectric properties of Pb(ZrxTil. x)O3 films annealed at 600'C for 30 min. Reaney et al. [8] found that the Pb/Ti ratio of PZT films decreased by 10 mol% from that of the solution after annealing at 700'C for 60 s, irrespective of the amount of excess Pb in the precursor solutions. Klee et al. [9] deduced that for PZT films fired at 700'C, a minimum Pb excess of 8% is necessary to obtain a single perovskite phase. On the other hand, our previous work showed that the addition of 20% excess Pb to the starting solution was not yet optimised [5]. The ideal amount of excess Pb for a given sample will depend upon the temperature and duration of pyrolysis and annealing. These are the reasons why we attempted to 229 Mat. Res. Soc. Symp. Proc. Vol. 596 ©2000 Materials Research Society
determine the ideal amount of excess Pb for the three-step heat-treatment process. In this work, three kinds of solutions with nominal excess Pb of -10, 0, 10 mol% were used. PZT thin films were fabricated by spin coating onto Pt/Ti/SiO2/Si substrates. The crystalline phase as well as the preferred orientation in the PZT films was examined by X-ray diffraction analysis. The microstructure and chemical composition of the films were studied by scanning electron microscopy, transmission electron microscopy and electron probe microanalysis, respectively. The dielectric constants and loss values of the films were measured at 1 kHz and the ferroelectric properties were investigated by observing the polarization P-E hysteresis loop. Based on these results, the electrical properties of the films were correlated with their microstructures. EXPERIMENTAL PROCEDURE Precursor
solutions were prepared
from lead
acetate
[Pb(CH 3 COO)2 ], zirconium-n-
propoxide [Zr(C 3 H70)4 ] and titanium tetraisopropoxide [Ti((CH3)2CHO)4]. 2-Propanol [(CH 3)2CHOH] was used as the solvent. Three kinds of solutions with nominal excess Pb of -10, 0, 10 mol% (denoted as solutions A, B a
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