Frequency Dependence of the Coercive Voltage of Ferroelectric Thin Films
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ABSTRACT Recent progress in the measuring techniques based on a combination of a quasistatic P-V analysis, conventional dynamic hysteresis measurements, and fast pulse characterization allows to determine the coercive voltage as a function of the frequency over a range of more than seven orders of magnitude. In this review, we explain the experimental techniques and present the results for the thin film systems of SrBi 2 Ta 2O9 (SBT) and Pb(ZrTi)0
3
(PZT).
Theoretical models of the correlation between the ferroelectric relaxation and the coercive voltage are discussed in the light of the new data.
INTRODUCTION Thin films of perovskite materials such as lead zirconate titanate [1] Pb(Zr,Ti)0
3
and layered
perovskite such as strontium bismuth tantalate [2-4] SrBi 2Ta 2O 9 are being intensively studied
for their application in high density FeRAM integrated circuits. The integration of these devices requires suitable simulation tools and reliable models of the transient behavior of the polarization reversal the same kind already existing for the standard silicon CMOS structures [5]. In order to replace other technologies, these materials must exhibit a sufficient fast polarization reversal ability which allows a cell access time of several nanoseconds. To develop an appropriate and sufficient model for a ferroelectric capacitor, two basic ideas are discussed in this work. First, the assumption that the polarization reversal is based on a simple transition of the spontaneous ferroelectric polarization from the direction opposite to the applied electrical field in the direction parallel to the applied field [6,7]. Second, the idea that dissipative redistribution of compensation charges or alignment of defect dipoles, of yet unidentified nature, are responsible for the deceleration of the polarization reversal. In both cases the additional opening of the P-V loop with increasing excitation frequency
291 Mat. Res. Soc. Symp. Proc. Vol. 596 © 2000 Materials Research Society
means an increase of the electric losses. For non-ferroelectric materials this loss is typically given as the tangent 6. For ferroelectric materials this measure is less suitable since the favorable ferroelectric properties also give a contribution to the value of tangent 6. For this reason the scope of this work is the frequency dependence of the coercive voltage, defined at zero polarization, which is used as a more evidential description of the switchability of the ferroelectric polarization.
EXPERIMENTS In Figure 1, the polarization loops of a tetragonal PZT (a) and of an SBT (b) thin film capacitor with varying excitation signal frequency are displayed. In both cases the saturation
polarization is not significantly affected, but the coercive voltage increases with increasing frequency. Additionally, in the left figure it can be observed that the course of the polarization during the ferroelectric switching is shifted parallel to the voltage axis with increasing frequency. 40 a
20
o0 '
SBT/Pt CSo t:B rnm A:2500pm2 25oc VirtualGround G
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