Growth of (111)-oriented Pb(Zr,Ti)O 3 layers on nanocrystalline RuO 2 electrodes using the sol-gel technique
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The influence of the RuO2 bottom electrode microstructure on the texture and ferroelectric properties of sol-gel PZT in RuO2/PZT/RuO2/Pt capacitors is discussed. For high growth temperatures, RuO2 has a columnar microstructure and a mixed (100)/(110) texture, attributed to epitaxial growth on Pt(111) and TiO2 (110) grains of the preannealed Pt / Ti layer. The mixed orientation of RuO2 resulted in a mixed PZT texture and slanted ferroelectric hysteresis characteristic. As the RuO2 growth temperature was reduced, the microstructure of the RuO2 layer turned to fine-grained equiaxed, with a grain size of approximately 10 nm, and PZT developed a sharp (111) texture (I111/I100 > 100) resulting in a rectangular hysteresis characteristic and a remanent polarization >30 C/cm2. The emergence of a strong (111) fiber texture in PZT on fine-grained RuO2 shed new light on the orientation selection mechanisms in sol-gel-prepared PZT layers. Possible applications of this phenomenon for high-density ferroelectric memories with a stacked layout are briefly discussed.
I. INTRODUCTION
The presence of a bistable, electrically switchable dipole in the unit cell of ferroelectric oxides makes them natural candidates for use as a memory material. Of growing technological importance in this context are thin, polycrystalline films of Pb(Zr,Ti)O3 (PZT), employed as capacitor dielectric in nonvolatile memories (FERAM) based on the destructive readout of a ferroelectric capacitor (FECAP).1 The inherent anisotropy of ferroelectrics results in a significant dependence of the ferroelectric properties on thin film orientation, making the development of deposition schemes for high-quality, oriented layers an important challenge in view of the future high-density FERAMs. For PZT compositions on the tetragonal side of the morphotropic phase boundary, the largest Pr is in principle afforded by (001)oriented films. However, tensile stresses in the PZT film, caused by the large thermal expansion coefficient of PZT compared to Si, lead to partial transformation of (001) (c-axis) into (100)/(010) domains, which do not contribute to Pr. Absence of such ferroelastic effects in (111)oriented PZT layers results in a high remanent polarization and abrupt switching behavior,2 making (111) the preferred orientation for FERAM applications of tetragonal PZT. a)
Address all correspondence to this author. e-mail: [email protected] b) Currently with Philips Semiconductors, Nijmegen, The Netherlands. 828
http://journals.cambridge.org
J. Mater. Res., Vol. 16, No. 3, Mar 2001 Downloaded: 15 Mar 2015
In-depth studies3–5 on texture selection in sol-gelderived PZT layers have yielded several useful approaches for the achievement of (111)-oriented layers on Pt. However, the strong ferroelectric fatigue of PZT on Pt makes the development of an equivalent approach for conductive oxide electrodes an important priority. Unfortunately, the attainment of (111)-oriented layers on RuO2, IrO2, or (La,Sr)CoO3 has turned out to be more difficult than on Pt. Recently, however, Maeder
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