Temperature-dependent ferroelectric and dielectric properties of Bi3.25La0.75Ti3O12 thin films
Introduction
Ferroelectric thin films are important for various technological applications such as dynamic random access memory (DRAM), micro-electro-mechanical system (MEMS), pyroelectric infrared (IR) sensors, field effect transistor (FET), and nonvolatile ferroelectric random access memory (NVFRAM) [1], [2], [3], [4]. Ferroelectric thin films of Bi-layered oxides exemplified by SrBi2Ta2O9 (SBT) and Bi3.25La0.75Ti3O12 (BLT) have attracted much attention for application because compared with conventional Pb(Zr1−xTix)O3 (PZT), they have significantly improved fatigue resistance, which is one of the key factors determining the lifetime of NVFRAM [1], [2], [3], [4], [5], [6]. Up to now, ferroelectric and dielectric properties of SBT and BLT or related Bi-layered oxide films have been widely studied at room temperature (RT) whereas their temperature dependence is rarely reported [7], [8], [9], [10], [11]. In fact, the temperature dependence of ferroelectric property is one of the important characteristics in view of not only engineering but also basic physics point [12]. For example, from view of engineering, since ferroelectrics-based devices may need to work at various temperatures, it is necessary to evaluate the ferroelectric behavior at the working temperature; from view of basic physics, it is reported that with the decrease in temperature, the potential well at ferroelectric domain boundaries, which resulted from the discontinuous polarization, will be more box-like shaped [8], so more charged defects such as oxygen vacancy can migrate to and be trapped there [13], which is related to basic ferroelectric domain switching kinetics because the trapped oxygen vacancy has great role in determining domain switching and, thus, macroscopic ferroelectric responses such as saturated polarization (Psat, which is the polarization value at a maximum electric field), coercive field (Ec) and remnant polarization (Pr, which is polarization value at a zero electric field) [14], [15], [16], [17], [18], [19], [20], [21], [22]. Actually, domain switching kinetics is the long-standing focus of studies on ferroelectrics.
Accordingly, in this article, BLT thin films have been fabricated on Pt/Ti/SiO2/Si substrates by pulsed laser deposition (PLD) and their ferroelectric and dielectric properties in a wide temperature range (80 K–RT) are reported and discussed in detail. The results may be helpful to further understand the domain switching kinetics and, thus, the macroscopic ferroelectric and dielectric properties, in ferroelectric thin films.
Section snippets
Experimental
The BLT ceramics used as PLD targets were prepared by solid-state reaction with starting materials of Bi2O3 (in excess of stoichiometric requirement by 20 mol% because of the volatile property), La2O3, and TiO2. The mixture of Bi2O3, La2O3, and TiO2 was ball milled for 24 h. The polyvinyl alcohol (PVA) was added as binder. Then the mixture was grinded and pressed into disks, the disks were at last sintered at 1050 °C for 200 min.
The PLD processes were performed using a Lambda Physik KrF excimer
Results
Fig. 1 shows the typical XRD θ–2θ scan pattern of the BLT films. The peaks are indexed according to the standard powder diffraction data of Bi4Ti3O12. It can be seen that the obtained films are polycrystalline and well crystallized into Bi-layered perovskite structure.
The surface morphology of the BLT films was recorded by SEM, as shown in Fig. 2(a). A dense, smooth, homogeneous surface without any cracks is observed. The surface is composed of rod-like and spherical grains. Further
Discussion
Ferroelectric domain switching kinetics is the long-term topic for studying. Based on the charged point defect–domain interaction model, domain switching is a consequence of the competition between field-assisted domain depinning and migrating point defects induced domain pinning [6], [9], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [30], [31]. Such competition is temperature-dependent. Firstly, charged defects may have less energy at lower temperature to overcome the attractive
Conclusions
By investigating the temperature-dependent ferroelectric and dielectric properties of the BLT films, it is experimentally revealed that the Psat decreases with decreasing temperature, whereas Pr shows a more complex temperature dependence. Based on the results, an alternative way, which can avoid the different original Vswitch, to investigate temperature-dependent ferroelectric fatigue is proposed and carried out. The measured fatigue rate is found to be linearly dependent on temperature,
Acknowledgements
This work was jointly supported by the National Nature Science Foundation of China (10874069, 50632030) and the State Key Program for Basic Research (2007CB613202), and the New Century Excellent Talents in University (NCET-08-0279).
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