Orientation Selection in Sol-Gel Derived PZT Thin Films
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ORIENTATION SELECTION IN SOL-GEL DERIVED PZT THIN FILMS
G. J. NORGA and LAURA FÈ IMEC vzw, Kapeldreef 75, B 3001 Leuven, Belgium ABSTRACT Motivated by the growing impact of PZT film orientation on ferroelectric film properties as film thickness is scaled down, we present basic studies on orientation selection in sol-gel derived PZT films, using pre-annealed Pt/Ti electrode layers as a model electrode system. FTIR was used to study, on a real temperature scale, chemical reactions in the films during the initial thermal steps prior to crystallization. We found that the chemical structure of the pyrolyzed film has a much larger impact on orientation selection than has previously been realized. In addition to pyrolysis conditions, the ambient used for the crystallization step was found to play a crucial role in orientation selection. As film thickness decreases, excessive oxygen incorporation in the films is seen to result in the loss of the preferential (111) texture when crystallization is performed in air. By performing crystallizations in N2, 40 nm thick PZT films with a strongly preferential (111) orientation could be obtained. INTRODUCTION Chemical solution deposition (CSD) is an established technique to prepare high quality Pb(Zr1x,Tix)O3 (PZT) ferroelectric thin films for integration with silicon integrated circuitry. The large effect of orientation on Pr, poling behavior, and hysteresis loop abruptness has emphasized the need for methods to tightly control film orientation [1]. Due to the continued reduction of the operating voltage of silicon ICs, the reduction in the switching voltage of the films has become especially urgent in light of their application in nonvolatile memories based on destructive readout of a ferroelectric capacitor [2-4]. In our previous investigations on sol-gel derived PZT films, we uncovered the loss of preferential (111) orientation as the principal cause for hysteresis loop slanting and Pr reduction, observed in sub-100 nm PZT films on Pt electrodes [5]. This demonstrates the growing impact of film orientation on ferroelectric film properties as film thickness is scaled down. A good model system for basic studies on orientation selection in PZT is self-textured (111) Pt. Polycrystalline Pt films with a sharp (111) fiber texture are readily obtained in a reproducible way by sputtering on SiO2/Si substrates using a thin (~10 nm) Ti layer to promote adhesion. In the literature on orientation selection in PZT, there is general agreement that the two dominant PZT orientations commonly observed on (111) Pt, PZT (100) and PZT (111), can be traced to the minimal PZT surface and minimal PZT/Pt interface energy configurations of the PZT nucleus on a (111) textured Pt electrode surface, respectively [6]. Moreover, it has been proposed that the (111) orientation of the PZT nucleus on a Pt (111) surface is energetically favored over the (100) PZT orientation in case a thin crystalline TiO2 seeding layer is present [7]. For sol-gel derived PZT films on (111) textured Pt electrodes, a great variet
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