Preparation And Electrical Characterization Of Heterolayered Ferroelectric Thin Films

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.