Influence of Low Frequency Dielectric Dispersion on the Phase Transition for the Pulsed Laser-Deposited SrBi2Ta2O9Thin Film

Abstract

Ferroelectric bi-layered perovskite SrBi2Ta2O9 (SBT) thin films are good candidates for fatigue-free FRAM applications. SBT has excellent resistance against fatigue even with simple Pt electrodes. The dielectric and electric properties of the SBT system were widely studied in bulk materials and thin films. It is know that the composition of SBT ceramics is generally different from a stoichometry, and reported that Sr-deficient and Bi-excess ceramic and thin films show improved ferroelectric properties and changed ferroelectric phase transition temperature (Tc). 8–13) Shimakawa et al. reported values of Tc of about 300 and 400 C for powder samples of stoichometric (SrBi2Ta2O9) and nonstoichometric (Sr0:85Bi2:2Ta2O9) compositions, respectively, invoking the structural distortion to justify the observed Tc temperature shift. Shoji et al. have found Tc value close to 322 C for stoichiometric ceramic samples. Onodera et al. have also reported thermal anomalies in the specific heat at 237 and 397 C for a ceramic sample of composition SBT, and no anomalies in the dielectric constant and spontaneous polarization were reported. Thin film capacitors of SBT, PZT, and (Ba,Sr)TiO3 (BST) show remarkable low frequency dielectric dispersion below Tc. It has been shown that this phenomenon greatly affects the electrical properties of the capacitors. The frequency dependence of the measured dielectric constant and dielectric loss reflects the intrinsic property of the bulk material, the effects from the electrodes, and any internal interfacial barriers. Since the above results were mostly for oxide ferroelectric materials, it will be very interesting to study the frequency and temperature dependence of the complex dielectric constant of ferroelectric compounds such as SBT in this system. Up to now, studies of the temperature behavior of SBT thin films are scarce and issues on the phase transitions and their mechanisms are still open. The point of interest here has been to visualize how ferroelectric SBT thin film behaves under the influence of its conductivity, with regard to its dielectric characteristic. In this paper, we aim to report the dielectric constant measurements carried out on SBT thin film, as a function of the frequency below and above ferroelectric phase transition temperature. The thin film of SBT ferroelectric were synthesized on Pt/ Ti/SiO2/Si substrates using the PLD method at 540 C in oxygen pressure of 200mTorr, and then post-annealed at 800 C in an oxygen atmosphere for 1 h. We confirmed that the annealed SBT films have a highly 1⁄2115 -oriented perovskite phase by x-ray diffraction analysis. The thickness of the film observed by scanning electron microscopy was d 1⁄4 270 nm. Top electrode was used a Pt with the area of 2:8 10 4 cm, which was formed using a DC sputtering method onto SBT thin films. The complex ac impedance, Z ð!Þ, was measured between 10 1 Hz and 10 Hz at room temperature by using an Impedance Analyzer (Solatron SI1260). Polarization vs electric field (P–E) curve was observed at room temperature using a RT66A ferroelectric tester from Radiant Technology. The temperature dependence of the real (0) dielectric constant for two frequencies is shown in Fig. 1. At high frequency (100 kHz), we notice that there is a maximum in 0 at about Tc 1⁄4 295 C, which we believe corresponds to the ferroelectric phase transition as shown in inset Fig. 1, the P– E hysteresis loop is saturated and symmetric below Tc (at room temperature). But, dielectric anomaly related to phase transition appears not to at low frequency (100Hz) due to the rapid increase of 0 caused by the electrode surface capacitance, which is much larger than the bulk capacitance. This may mean that there is a coupling between the space charge and ferroelectricity. The frequency dependence of the (a) real 0 and (b) imaginary 00 parts of the complex dielectric constant on log– log scale are shown in Figs. 2(a) and 2(b), respectively. Both 0 and 00 show a strong dispersion at low frequencies. Such a strong dispersion in both the components of the complex dielectric constant appears to be a common feature in ferroelectrics associated with ionic conductivity and is referred to as low frequency dielectric dispersion (LFDD). This is in complete contrast to the effect due to dc conductivity, where the 00 is proportional to 1=!. The dispersion in the 00 of the complex dielectric constant is stronger than that in the 0 as shown in Fig. 2(b). This is because of the influence of the dc conductivity on 00. The low frequency slope of the curve log 00 vs log f is close to 1 indicating the predominance of the dc conduction in this frequency region. According to Joncher’s universal law, the complex