Frequency dependence of the dielectric properties of La-doped lead zirconate titanate thin films with Pt and Ir electrodes and two different La doping levels is studied by impedance spectrometry between 10−3 and 104 Hz. A constant phase element with an impedance of Z=A0(jω)−α is introduced into the equivalent circuit, which fits very well both the real and imaginary components of the measured impedance over the full frequency range, including the resonance, using a single α (0≪α<1). Our analysis shows that α is not related to the ferroelectric properties of the films, but it has a monotonic correlation with the real dielectric constant. The close resemblance to the well-known butterfly-shaped C–V curve shown by α with the applied dc bias (α-V) suggests that α may be affected by the density of the domain walls in the film, in addition to various dielectric relaxation processes.
We have developed a method to separate the low-resistive near-interface regions (NIRs) from the high-resistive central bulk region (CBR) in a ferroelectric thin film. The NIR thickness and the CBR resistivity can thus be accurately determined. Using lanthanum-doped lead zirconate titanate films as an example, we show that the total thickness of the NIRs depends only on the electrode materials in use (Ir and Pt), while the CBR resistivity depends only on the impurity doping levels (La=1.5% and 3%). The fact that the NIR is much narrower when Pt electrodes are used instead of Ir, and that the NIR resistivity is always considerably lower than the central bulk, suggest that the NIRs is probably originated from material nonstoichiometry/defects rather than the band bending at metal/insulator interfaces.
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