【作者】 贾建峰；

【导师】 贺德衍；

【作者基本信息】 兰州大学 ， 凝聚态物理， 2006， 博士

【摘要】 钛酸锶钡[Ba_1-xSr_xTiO₃]（BST）薄膜是目前凝聚态物理领域倍受关注的铁电材料。一方面,由于它具有优异的铁电、压电、介电和热释电性能,在非挥发性铁电随机存取存储器、压电传感器、热释电传感器和超大规模随机存取寄存器中的贮存电容器等方面具有广阔的应用前景,另一方面,BST材料为简单立方的钙钛矿结构,其居里温度随组份x增大而线性的减小,因此,可以通过改变组份人为地调整材料的居里温度,从而满足不同器件应用的要求。 本论文以BST铁电薄膜为主要研究对象,首先我们发展了溶胶-凝胶（sol-gel）方法,探索了采用这种改进的sol-gel方法制备异质多层结构铁电薄膜的最佳工艺条件,成功制备出具有理想结构和优异电学性能的异质多层铁电薄膜材料。其次,我们从理论和实验两方面系统研究了BST铁电薄膜的成份与微结构对介电、铁电性能以及漏电行为的影响。通过研究发现: （1）选用LaNiO₃（LNO）作为单晶Si衬底与(Ba_xSr_1-x)TiO₃之间的过渡层和铁电薄膜电容器的底电极,在750℃、O₂气氛下热处理30 min后,(Ba_0.7Sr_0.7)TiO₃薄膜在50kHz、零偏压时的介电系数ε_r大于300。偏压为6V时,漏电流密度九小于1.2×10^-6A/cm²。实验结果表明,通过选择合适的衬底及底电极材料并优化制备工艺,采用改进的sol-gel法能够获得性能良好的铁电薄膜。理论分析还发现,Al/BST/LNO结构铁电薄膜电容的漏电流受空间电荷限制电流机制和肖特基机制共同制约; （2）随着Ni掺杂量的增加,(Ba_0.8Sr_0.2)TiO₃薄膜中的晶粒尺寸减小,电学性能发生了明显地变化。在室温条件下,薄膜在100kHz零偏压下的介电系数和10V偏压下的漏电流密度分别由未掺杂时的460和3.8×10^-4 A·cm^-2下降到0.4%Ni掺杂时的250和4.0×10^-6A·cm^-2。这表明通过适量Ni的掺杂,可以有效地调整BST薄膜的电学性能,以满足动态随机存取存贮器的应用要求; （3）两种结构异质多层铁电薄膜的漏电流密度与相应的具有同样厚度的单层BST薄膜相比降低了2—3个数量级,但MgO/(Ba_0.8Sr_0.2)TiO₃多层铁电薄膜介电系数与(Ba_0.8Sr_0.2)TiO₃单层薄膜的（>450）相比也降低了约50%,而(Ba_0.5Sr_0.5)TiO₃/Pb(Zr_0.5Ti_0.5)O₃多层铁电薄膜却能保持比较大的介电系数（>340）。实验结果显示,如果减小多层薄膜中各子层的厚度并增加重复周期数,有望在确保多层薄膜具有较高介电系数的前提下大幅度降低漏电流。更多还原

【Abstract】 As a sort of valuable ferroelectric materials, barium strontium titanate (Ba_1-xSr_x)TiO₃ （BST） thin films have recently been of immense scientific and technological interests in the world. They are an attractive candidate for a variety of integrated devices such as non-volatile ferroelectric dynamic random access memories, piezoelectric sensors, pyroelectric IR sensors and storage capacitors in ultralarge-scale integrated dynamic random access memories due to their excellent ferroelectric, piezoelectric, dielectric and pyroelectric properties. BST has the cubic perovskite structure and its Curie temperature can be decreased linearly with the increase of the value of x. Thus, we can control its Curie temperature by adjusting the component to meet the need of varied applications.In this work, BST thin films were the focus of our investigation. Firstly, we explored the fabrication of heterolayered ferroelectric thin films based on the successful preparation of homospheric ferroelectric thin films using improved sol-gel technology. The heterolayered ferroelectric thin films prepared by the technology have fine microstructure and excellent electric properties. Subsequently, we studied the influence of the components and structures of BST thin films on the dielectric, ferroelectric properties and leakage behavior via experiment and theory.Our experimental results demonstrate that:（1） As an excellent bottom electrode of ferroelectric thin film capacitance, LaNiO₃ is the optimum buffer layer in the preparation process of (Ba_1-xSr_x)TiO₃ thin film on Si substrate. The room temperature dielectric constant and leakage current density of (Ba_0.7Sr_0.3)TiO₃ thin film annealed at 750 ℃ in O₂ ambient for 30 minutes are higher than 300 （at 50 kHz and 0 V） and lower than 1.2 × 10^-6 A/cm2 （at a bias voltage of 6 V）, respectively. It indicates that ferroelectric thin films with excellent characteristics can be prepared on suitable substrates and bottom electrodes by the improved sol-gel technology. Moreover, it proves that the process of leakage in Al/BST/LNO capacitance is control ed by the Space charge limited current and Schottky emission mechanism;（2） With the increase of Ni content, not only the size of the grains in Ni-doped (Ba_0.8Sr_0.20TiO₃ thin films decreases but also the electric properties varies significantly. The room temperature dielectric constant and leakage current density of (Ba_0.8Sr_0.2)TiO₃ thinfilms change from 460 at 100 kHz, 0 V and 3.8xlO"4 A-cm"2 at 10 V （undoped） to 250 at 100 kHz, 0 V and 4.0x10"6 A-cm’2 at 10 V （0.4% Ni doped）, respectively. The results show that the introduction of Ni can significantly- change the electric properties of BST thin films to meet the need of the applicantion in DRAMs;（3） A comparative study between heterolayered ferroelectric thin films and homospheric BST thin films was carried out. It was found that the thickness of each unit layer and heterolayered interface play a significant role in affecting the electric properties of heterolayered ferroelectric thin films. Compared with homospheric （Bao.8Sro.2）TiC>3 thin films, the leakage current densities of the heterolayered MgO/（Bao.8Sro.2）TiC>3 ferroelectric thin films decrease by a magnitude of about 3 orders. However, the dielectric constants of heterolayered MgO/BST80 ferroelectric thin films are about only 50% of homospheric BST80 thin films. The leakage current densities of the heterolayered （Bao.5Sro.5）Ti03/Pb（Zro.5Tio.5）03 ferroelectric thin films decrease by a magnitude of about 2 orders compared with the homospheric BST50 thin films. Moreover, heterolayered BST5O/PZT5O ferroelectric thin films have higher dielectric constants （>340）. It suggested that the heterolayered ferroelectric thin films with superior dielectric properties and lower leakage current densities can be obtained by reducing the thickness of each unit layer and increasing the period of repetition.更多还原