Improvement in Ferroelectric Properties of Sol-Gel Derived SrBi 2 Ta 2 O 9 thin films with seeding layers
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Improvement in ferroelectric properties of sol-gel derived SrBi2Ta2O9 thin films with seeding layers Ching-Chich Leu, Chao-Hsin Chien, Ming-Jui Yang, Ming-Che Yang, Tiao-Yuan Huang, Hung-Tao Lin* and Chen-Ti Hu* National Nano Device Laboratories, Hsinchu 30043, Taiwan, ROC * Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30043, Taiwan, ROC ABSTRACT The effects of a seeding layer, which was deposited on Pt/TiO2/SiO2/Si substrates using magnetron sputtering, on the characteristics of sol-gel-deposited strontium-bismuth-tantalate (SBT) thin films are investigated. The seeding layer serves as nucleation sites so homogeneous crystalline SBT films of bismuth-layered structure (BLS) with fine grains are successfully obtained by 750°C rapid thermal annealing in O2 ambient. The remanent polarization (2Pr) improves from 12.1 to 18.8 µC/cm2 with the addition of the seeding layer. In addition, the seeding layer also results in a lower nucleation temperature, allowing the use of 700°C annealing for 10 min to grow SBT films that are fully crystallized with BLS phase and shows good ferroelectric properties. Finally, crystallinity and microstructures of SBT films are found to be strongly dependent on the thickness of the seeding layer. Optimum Ta-seeded SBT thin film crystallized at 700 °C for 10min depicts a higher 2Pr value (12.9 µC/cm2 (@5V) than that of the un-seeded films crystallized at 750 °C for 1min.
INTRODUCTION Due to its fatigue-free and low coercive field characteristics, strontium-bismuth-tantalate (SBT) thin film has received considerable attention as a promising candidate for applications to ferroelectric nonvolatile random access memories (NVRAMs) [1]. However, SBT materials generally depict relatively low remanent polarization [2]. In order to circumvent this shortcoming, many techniques have been employed to improve the ferroelectric properties of SBT thin films, such as laser ablation method [3], chemical liquid deposition method [4], specific adhesion layer [5] and buffer or seeding layers [6,7]. For process simplicity and cost consideration, the latter two techniques are suitable and compatible with the sol-gel derived SBT system. Previously it has been reported that the nucleation and growth of sol-gel derived PZT thin films depend strongly on the underlying substrates and interface properties [8]. The effects of various seeding layers and substrate configurations on the properties of PZT thin films have also been studied [9-15]. However, there are only few reports regarding the impacts of a buffer layer on the SBT thin films. Osaka et al. have successfully grown the SBT films with desirable crystalline orientation by utilizing multi-seeding layers [6], while Shin et al. demonstrated improved electrical properties of SBT thin films with Bi2O3 buffer layers in terms of remanent polarization and leakage current [7]. Very recently we have studied the effects of titanium (Ti) and tantalum (Ta) adhesion layers, which were deposited prior to Pt bottom electrodes, on the pro
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