AC conduction mechanism of SrBi2Ta2O9 thin films below room temperature

Ferroelectric thin films have tremendous application potential for data storage, sensors, uncooling infrared detectors, classes of tunable microwave devices, and microelectromechanical system technologies [1–4]. Carrier transport plays an essential role in determining some characteristics of the ferroelectric thin films such as fatigue and leakage to meet commercial requirements. In recent years, different transport mechanisms in ferroelectric materials, such as space charge limited conduction (SCL), thermionic emission limited conduction (TEL), and Poole-Frenkel emission limited conduction (PFL) have been reported [5–10]. Further, current-voltage (IV) characteristics may contain contributions from SCL and PFL conductions, and even TEL conduction at high direct current (dc). The latter TEL part can usually be distinguished by its dependence on the selection of electrode material. To date, most of the studies on the transport properties of ferroelectric films are based on dc measurements, such as dc IV measurements. In direct current conductivity measurements, all types of carrier contributions to conductance are superposed. Thus, the various transport mechanisms cannot be easily distinguished. It is well known, however, that each type of carrier (or mechanism) possesses an independent relaxation time. Based on this feature, it is easy to distinguish the contributions of various mechanisms to electrical transport from frequency dependent conductivity measurements. However, there has been little experimental work on alternating current (ac) transport properties of the thin films below room temperature, although data collected at a broader temperature range would be helpful in understanding the conduction mechanisms of these films. The electrical properties of ferroelectrics are known to be closely associated with their crystallographic structure. An important example is SrBi2Ta2O9 (SBT), a representative of a new class of ferroelectrics in which, the structure is based on perovskite octahedron layers (SrTa2O7)2− sandwiched between (Bi2O2)2+ layers. SBT exhibits almost no fatigue and low dc leakage on Pt electrodes at room temperature (up to 1012 cycles at saturation), accompanied by other superior properties [11–17]. These unusual properties have generated tremendous interests in the fundamental conduction mechanisms of SBT. In this letter, we report the ac transport properties from 100 Hz to 15 MHz of SBT ferroelectric thin films in the temperature range of 10 to 300 K. The dominant conduction mechanisms at different frequency ranges were analyzed. Most importantly, results indicated the existence of conduction through the perovskite octahedron layers of the SBT structure. SBT thin films were prepared by pulsed laser deposition (PLD) on heated Pt/Ti/SiO2/Si substrates (400 ◦C), followed by a post-annealing process at 750 ◦C for 1.5 h in oxygen [16] 30 nm thick Pt electrodes with an area of 2.0 × 10−4 cm2 were deposited on the top surface of the films through a shadow mask by a UHV electron beam evaporator (UMS 500p made by Balzers), giving the capacitive multilayered structure; Pt/SBT/Pt. The crystallographic structure of the SBT films was characterized by X-ray diffraction along the (008) and (115) direction. The microstructure of the films was observed by transmission electron microscopy, with the mean grain size of about 200 nm, thickness of about 200 nm, dense film morphology and abrupt interfaces. Ferroelectric properties of the thin films were determined with an RT66A ferroelectric tester from Radiant Technologies. A typical hysteresis loop showed that the remanent polarization and coercive field were 10 μC/cm2 and 57 kV/cm at room temperature, respectively. The electrical conductivity of the SBT thin films was measured by a HP4194A Impedance/Gain-Phase Analyzer in the temperature range of 10 to 300 K. These measured values of conductivity in the range between 100 Hz and 15 MHz were simulated by an equivalent circuit using an ideal capacitor shunted with an ideal resistor. The frequency dependence of the electrical conductivity is illustrated in Fig. 1 with temperature as a