Current-voltage characteristics of ultrafine-grained ferroelectric Pb(Zr, Ti)O 3 thin films
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Room-temperature current-voltage dependence of ultrafine-grained ferroelectric Pb(Zr, Ti)O3 thin films has been investigated. Both strong varistor type behavior and space charge limited conduction (SCLC) were observed. Differences in the current-voltage characteristics are attributed to differences in the nature of the grain boundaries resulting from variations in processing conditions. The strong varistor type behavior is believed to be due to the presence of highly resistive grain boundaries and thus may be termed grain boundary limited conduction (GBLC). A double-depletion-layer barrier model is used to describe the origin of high resistivity of the grain boundaries. It is suggested that the barrier height varies significantly with the applied field due to the nonlinear ferroelectric polarization, and that the barrier is overcome by tunneling at sufficiently high fields. In some other cases, the resistivity of the grain boundaries is comparable to that of the grains, and therefore the intrinsically heterogeneous films degenerate into quasi-homogeneous media, to which the SCLC theory is applicable. As such, a unified grain boundary modeling reconciles different types of conduction mechanisms in the ultrafine-grained ferroelectric thin films. This grain boundary modeling also well accounts for some other dc-related phenomena observed, including abnormal current-voltage dependencies, remanent polarization effect, electrode interface effect, and unusual charging and discharging transients. In addition, many other electrical properties of the ferroelectric films may be better understood by taking the effect of grain boundaries into account.
I. INTRODUCTION It is of great significance to investigate the details of the dc conduction in ferroelectric thin films. The magnitude of leakage current is usually one of the most concerned issues for the application of these films. It is desired in many cases that leakage current be as small as possible,1 since it directly relates to the power loss of devices during use and, in some cases, significantly affects the performance of devices (e.g., the duration of charge storage in the case of dynamic random access memories).2 Resistivity degradation of the films under constant dc field stress, which leads to time-dependent dielectric breakdown (TDDB), is another important issue intrinsically related to dc conduction.3 A thorough current-voltage (I-V) study, when combined with other measurements, would depict a microscopic view of the motion of charge carriers and its couplings with effects of other extrinsic sources such as lattice defects, impurities, grain boundaries, and electrode interfaces. This would allow us to gain an insight into the nature of those extrinsic sources. Consequently, a better understanding of many other electrical properties of ferroelectric thin films, such as polarization switching, fatigue, and retention, can be a) Also
with the Department of Engineering Science and Mechanics.
1484 http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 6, Jun 1994
