| Bismuth layer-structured ferroelectrics (BLSFs) have been extensively studied concerning both their fundamental properties and their device applications in nonvolatile ferroelectric random access memory(NvFRAMS). However, BIT thin films exhibit unexpectedly lower 2Pr and suffer from poor fatigue endurance, failing to meet industrial requirements of NvFRAMS. To minimize or solve these problems, many research groups have performed the ion substitution in BIT to obtain large 2Pr and excellent fatigue-free properties. On the other hand, great efforts have been done to fabricate films with non–c-axis orientations, because the vector of the spontaneous polarization in BIT-based films is usually directed along a axis. In this dissertation, the effects of Hf-doping on the microstructure, ferroelectric properties and anisotropy of BIT thin films were systematically studied. Finally, due to the wide applications of ferroelectric/semiconductor integrated films, the issues have recently been of immense scientific and technological interests in the world. So, the integration of Hf-doped BIT (BTH) films with GaN was also carried out.The BTH thin films were prepared by pulsed laser deposition (PLD) method, the effect of growth parameters (the substrate temperature and the oxygen pressure)on the microstructure and ferroelectric properties of BTH thin films were studied in the dissertation and high quality ferroelectric BTH thin films were obtained under optimum conditions. Ion substitution usually leads to structure and property change in BIT thin films. Thus, the effect of Hf-doping effect was systematically researched. Compared to the BIT films, the BTH films have significantly enhanced electrical properties with 3 times larger remanent polarization (2Pr=45.7μC/cm2), 0.7 times smaller coercive field (2Ec=184 kV/cm) and better fatigue endurance (11.4% degradation). The reason for the enhanced properties of BTH film was also discussed both in the view of ion radius and Hf-O bond. Because the valence of Hf4+ ion is equal to that of Ti4+ ion, the leakage current of BTH and BIT are nearly the same. These results showed that equal-valence B-site Hf-substitution is effective to improve the electrical properties of BIT.Due to anisotropic nature of ferroelectric, it's important to find out the relationship between electric properties and film orientation of BTH. Epitaxial BTH films with c-axis and non- c-axis orientations on SrRuO3 (SRO) coated (001)- and (111)-oriented SrTiO3 (STO) substrates are fabricated by pulsed laser deposition to study the anisotropic properties of BTH. It is obvious that the (104)-oriented BTH films, with orientations close to the a axis, exhibited significantly higher remanent polarization (2Pr=45.6μC/cm2) and dielectric constant (εr=425 at 100kHz) than the (00l)-oriented BTH films (2Pr=4.5μC/cm2,εr=245 at 100kHz). It demonstrated the same electric anisotropy of BTH films as pure BIT, indicating that Hf substitution does not change the orientation dependence of electeic properties of BIT material.Many efforts have been devoted to integrating pervoskite functional oxide materials withⅢ-Ⅴsemiconductor GaN due to its potential applications in various devices with enhanced performance. BTH ferroelectric films with excellent electrical properties were epitaxially integrated with GaN semiconductor using (111) SrRuO3 (STO) / rutile (200) TiO2 as buffer layer. Electrical measurements demonstrate that the non-c axis BTH films possess a large remnant polarization (2Pr=45μC/cm2), excellent fatigue endurance (10.2% degradation after 1.1*1010 switching cycles). These results reveal that the (104)-BTH films grown on GaN template with similar electrical performance compared to these deposited on STO substrate could be obtained using SRO/TiO2 buffer layer. These results show that BTH may be a promising candidate in GaN-based ferroelectric integrated devices, such as NvFRAMS and ferroelectric field transistors. |
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