First-principles Study Of Ferroelectric Bismuth Titanate And Conduction Mechanisms Applied In Nv-FeRAM

Nonvolatile random access memories (NvRAMs) have been regarded as the most promising memories due to its nonvolatile nature (in which the stored data will not been lost even if power is interrupted), high speed, low power consumption, low operation voltage, high repeatability and excellent radiation hardness. The key for commercial applications is the choice of ferroelectric material. Among various ferroelectric materials, ferroelectric material with perovskite Pb(Zr_xTi_1-x)O₃ (PZT) and Bi-layered structures SrBi₂Ta₂O₉ (SBT) are promising materials for NvRAMs. However, PZT contains toxic elements lead, which causes environmental pollution, and it also shows a reduction of polarization with polarity switching (fatigue). SBT, although it shows excellent endurance against polarity switching, its synthetic temperature is too high for standard LSI-device fabrication processes. Therefore, researchers are eager to find some more suitable materials.Recently, rare-earth doped Bi₄Ti₃O₁₂ (BIT) has attracted considerable attention for potential utilization because of its lead-free nature, large spontaneous polarization, low processing temperature, high Curie temperature, and excellent fatigue-free behavior. Experimentally, A site, B site and A,B sites substitution are proposed and used to improve the ferroelectric properties of BIT, and have made some achievement, but the internal mechanism of doping is not clear.In BIT device applications, leakage current often interferes and leads to device failure. Therefore, the leakage properties of BIT films have been widely investigated in the experimental and theoretical aspects. Experimentally, A,B sites substitution is considered as the most effective method to reduce the leakage current. Theoretically, some studies revealed that the leakage current is related with the volatility of Bi3+ ions and oxygen vacancies. Impurities atoms were also considered to contribute to the leakage current. A number of mechanisms were also proposed to explain the leakage behavior observed in experiment. However, the actual conduction mechanism is still controversial. The leakage current interferes with the polarization switching and makes BIT unsuitable for the ferroelectric devices when the leakage current is large. So, it is very important to make the conduction mechanism clear.In this paper, we have systematically studied the basic physical properties such as the electronic properties, and chemical bonding properties by pseudopotential project plane wave (PP-PAW) method based on the density functional theory. The main results are as follows.1)The results of total energy and density of states reveals that the stability of ferroelectric phase of BIT is determined by the distortions of the Bi2Ti3O10 block and the Bi₂O₂ layer as well as the relaxation energy, which is released by the mutual coupling between distortions of the Bi₂Ti₃O₁₂ block and the Bi₂O₂ layer. The ferroelectric property of BIT is largely determined by the strong Ti-O hybridizations, which is strengthened by the weak hybridization of Bi (A-site) and O.2)The results of the charge density and the AIM theory reveals that the ferroelectric properties and lattice distortions originates from the hybridization between Ti and O and Bi and O in BIT ,and the hybridization of Ti and O is dominant.3)The band structure and density of states of ferroelectric bismuth titanate is calculated in the framework of density functional theory. Combined with the metal induced gap state model, we calculated the Schottky barrier height of BIT on Pt electrode as high as 1.26 eV, which denotes that the Schottky effect may not be the dominant conduction mechanism in BIT.4)Compared with the analysis of experimental data, we conclude that the leakage current behavior of BIT films is dominated by bulk limited conduction mechanisms and can be reduced by doping or better processing conditions.